Tansley Review No. 95 15 N natural abundance in soil-plant systems

Biochar and peat amendments affect nitrogen retention, microbial capacity and nitrogen cycling microbial communities in a metal and polycyclic aromatic hydrocarbon contaminated urban soil

Soil contaminants may restrict soil functions. A promising soil remediation method is amendment with biochar, which has the potential to both adsorb contaminants and improve soil health. However, effects of biochar amendment on soil-plant nitrogen (N) dynamics and N cycling microbial guilds in contaminated soils are still poorly understood. Here, a metal- and polycyclic aromatic hydrocarbon (PAH) contaminated soil was amended with either biochar (0, 3, 6 % w/w) and/or peat (0, 1.5, 3 % w/w) in a full-factorial design and sown with perennial ryegrass in an outdoor field trial. After three months, N and the stable isotopic ratio δ15N was measured in soil, roots and leaves, along with microbial responses. Aboveground grass biomass decreased by 30 % and leaf N content by 20 % with biochar, while peat alone had no effect. Peat in particular, but also biochar, stimulated the abundance of microorganisms (measured as 16S rRNA gene copy number) and basal respiration. Microbial substrate utilization (MicroResp™) was altered differentially, as peat increased respiration of all carbon sources, while for biochar, respiration of carboxylic acids increased, sugars decreased, and was unaffected for amino acids. Biochar increased the abundance of ammonia oxidizing archaea, while peat stimulated ammonia oxidizing bacteria, Nitrobacter-type nitrite oxidizers and comB-type complete ammonia oxidizers. Biochar and peat also increased nitrous oxide reducing communities (nosZI and nosZII), while peat alone or combined with biochar also increased abundance of nirK-type denitrifiers. However, biochar and peat lowered leaf δ15N by 2-4 ‰, indicating that processes causing gaseous N losses, like denitrification and ammonia volatilization, were reduced compared to the untreated contaminated soil, probably an effect of biotic N immobilization. Overall, this study shows that in addition to contaminant stabilization, amendment with biochar and peat can increase N retention while improving microbial capacity to perform important soil functions.

Global distribution and drivers of relative contributions among soil nitrogen sources to terrestrial plants

Soil extractable nitrate, ammonium, and organic nitrogen (N) are essential N sources supporting primary productivity and regulating species composition of terrestrial plants. However, it remains unclear how plants utilize these N sources and how surface-earth environments regulate plant N utilization. Here, we establish a framework to analyze observational data of natural N isotopes in plants and soils globally, we quantify fractional contributions of soil nitrate (fNO3-), ammonium (fNH4+), and organic N (fEON) to plant-used N in soils. We find that mean annual temperature (MAT), not mean annual precipitation or atmospheric N deposition, regulates global variations of fNO3-, fNH4+, and fEON. The fNO3- increases with MAT, reaching 46% at 28.5 °C. The fNH4+ also increases with MAT, achieving a maximum of 46% at 14.4 °C, showing a decline as temperatures further increase. Meanwhile, the fEON gradually decreases with MAT, stabilizing at about 20% when the MAT exceeds 15 °C. These results clarify global plant N-use patterns and reveal temperature rather than human N loading as a key regulator, which should be considered in evaluating influences of global changes on terrestrial ecosystems.

Lipid biomarkers and stable isotopes uncover paleovegetation changes in extremely species-rich forest-steppe ecosystems, Central Europe

The historical development of the vegetation of semi-dry grasslands in Central Europe is not satisfactorily understood. Long-term continuity of open vegetation or, conversely, deep-past forest phases are considered possible sources of the current extreme species diversity of these ecosystems. We aimed to reveal the trajectory of paleovegetation development in these ecosystems through detailed analysis of terrestrial in-situ soil geoarchives. We measured the bulk soil carbon and nitrogen contents, lipid molecular distribution, and compound-specific stable carbon and hydrogen isotopic signatures of mid- and long-chain n-alkanes extracted from soil and modern plant material tissues (i.e., deciduous and Pinus leaves and grass/herbaceous species). The C23-C33 n-alkane homologues were identified in soils with different abundances. Normally, C27 and C29 n-alkanes were the most abundant homologues in tree-leaf samples, while grass-derived n-alkanes were mostly C31 and C33 homologues. Soils were largely dominated by C29 and C31 n-alkanes. Odd-numbered C27-C33 soil n-alkane δ13C values ranged from -36.2‰ to -23.2‰, whereas their δ2H values showed a wider range of variability that fluctuated from -224‰ to -172‰. Molecular distribution in combination with radiocarbon analysis of soil organic matter (SOM) and δ13C and δ2H values of n-alkanes revealed a large contribution of C3 trees (both deciduous and coniferous trees/pine trees) as the main source of n-alkanes between the late Pleistocene and early Holocene (ca 15,000-8200 calibrated year before present/cal year BP). A clear shift toward more grassy/herbaceous vegetation was observed from the early Holocene (ca 11,700-8200 cal year BP) onwards. Distribution patterns of lipids and soil geochemical parameters showed that plants are the main source of SOM and that biodegradation and kinetic isotope fractionation are not the main reasons for 13C enrichment in soil profiles. Past C3 vegetation shifts as well as paleoclimate changes (i.e., aridity) can have played a role in the observed 13C depth profiles.

Decades-long organic tea production is distinguished by N deficiency: Evidence from soil and tea δ15N data

To investigate the possibility of identifying commercial organic teas from conventional teas based on their isotopic signatures, we sampled tea leaves and soil samples from three tea gardens in Pu'er, China, that underwent decades of certified organic cultivation and compared them with adjacent conventional gardens. We found that long-term organic tea cultivation increased the soil organic carbon and soil pH but significantly decreased the total N content of tea. Higher δ15N values were observed in the organic teas, but significant overlap existed with non-organic teas. The lower N content of the organic tea and contrasting pattern between the organic tea δ15N and soil δ15N suggested that the decline of the N availability could potentially act as a robust characteristic for discriminating between organic and non-organic tea cultivation systems. Further analysis implies that combining tea and soil N content with δ15N value is a promising approach to organic tea identification.

Ectomycorrhizal fungi integrate nitrogen mobilisation and mineral weathering in boreal forest soil

Tree growth in boreal forests is driven by ectomycorrhizal fungal mobilisation of organic nitrogen and mineral nutrients in soils with discrete organic and mineral horizons. However, there are no studies of how ectomycorrhizal mineral weathering and organic nitrogen mobilisation processes are integrated across the soil profile. We studied effects of organic matter (OM) availability on ectomycorrhizal functioning by altering the proportions of natural organic and mineral soil in reconstructed podzol profiles containing Pinus sylvestris plants, using 13CO2 pulse labelling, patterns of naturally occurring stable isotopes (26Mg and 15N) and high-throughput DNA sequencing of fungal amplicons. Reduction in OM resulted in nitrogen limitation of plant growth and decreased allocation of photosynthetically derived carbon and mycelial growth in mineral horizons. Fractionation patterns of 26Mg indicated that magnesium mobilisation and uptake occurred primarily in the deeper mineral horizon and was driven by carbon allocation to ectomycorrhizal mycelium. In this horizon, relative abundance of ectomycorrhizal fungi, carbon allocation and base cation mobilisation all increased with increased OM availability. Allocation of carbon through ectomycorrhizal fungi integrates organic nitrogen mobilisation and mineral weathering across soil horizons, improving the efficiency of plant nutrient acquisition. Our findings have fundamental implications for sustainable forest management and belowground carbon sequestration.

Nitrogen Isotopes Reveal High NOx Emissions from Arid Agricultural Soils in the Salton Sea Air Basin

Air quality management commonly aims to mitigate emissions of oxides of nitrogen (NO x ) from combustion, reducing ozone and particulate matter pollution. Despite such efforts, regulations have recently proven ineffective in rural areas like the Salton Sea Air Basin of Southern California, which routinely violates air quality standards. With $2 billion in annual agricultural sales and low population density, air quality in the region is likely influenced by year-round farming. We conducted NO x source apportionment using nitrogen stable isotopes of ambient NO 2 , which indicate a substantial contribution of soil-emitted NO x . The soil source strength was estimated based on the mean δ 15 N-NO x from each emission category in the California Air Resources Board's NO x inventory. Our annual average soil emission estimate for the air basin was 11.4 ± 4 tons/d, representing ~ 30% of the extant NO x inventory, 10× larger than the state's inventory. Therefore, the impact of soil NO x in agricultural regions must be re-evaluated.

Soil organic matter turnover: Global implications from δ13C and δ15N signatures

The turnover and residence time of carbon (C) and nitrogen (N) in soil is a fundamental parameter reflecting the rates of soil organic matter (SOM) transformation and the contribution of soils to greenhouse gases fluxes. Based on the global database of the stable isotope composition of C (δ13C) and N (δ15N) depending on soil depth (171 profiles), we assessed С and N turnover and related them to climate, biome types and soil properties. The 13C and 15N discrimination between the litter horizon and mineral soil was evaluated to explain the key processes of litter transformation. The 13C and 15N discrimination by microbial utilization of litter and SOM, as well as the continuous increase of δ13C and δ15N with depth, enabled to assess C and N turnover within SOM. N turnover was two times faster than that of C, which reflects i) repeated N recycling by microorganisms accelerating N turnover, ii) C loss as CO2 and input of new C atoms to cycling, which reduces the C turnover within soil, and iii) generally slower turnover of N free persistent organic compounds (e.g. lignin, suberin, cellulose) compared to the N containing compounds (e.g. amino acids, ribonucleic acids). An increase in temperature and precipitation accelerated C and N turnover because: i) higher microbial activity and SOM decomposition rate, ii) larger soil moisture and fast diffusion of dissolved organics towards exoenzymes, iii) downward transport of 13C-enriched organic matter (e.g. sugars, amino acids), and iii) leaching of 15N-depleted nitrates from the topsoil into subsoil and losses from the whole soil profile. Temperature accelerates SOM turnover stronger than precipitation. The temperature increase by 10 °C accelerates the C and N turnover for 40 %. SOM turnover is boosted by decreasing C/N ratio because: i) SOM with a high C/N ratio originated from litter is converted to microbially-derived SOM in mineral soil characterized by a low C/N ratio; ii) litter with a low C/N ratio is decomposed faster than litter with a high C/N; iii) microbial carbon-use efficiency increases with N availability. The biome type affects SOM decomposition by i) climate: slower turnover under wet and cold conditions, and ii) by litter quality: faster utilization of leaves than needles. Thus, the fastest C turnover is common under evergreen forests and the lowest under mixed and coniferous ones, whereas temperature and C/N ratio are the main factors controlling SOM turnover. Concluding, the assessment of SOM turnover by δ13C and δ15N approach showed two times faster N turnover compared to C, and specifics of SOM turnover depending on the biomes as well as climate conditions.

Large Seasonal Variation in Nitrogen Isotopic Abundances of Ammonia Volatilized from a Cropland Ecosystem and Implications for Regional NH3 Source Partitioning

Ammonia (NH3) volatilization from agricultural lands is a main source of atmospheric reduced nitrogen species (NHx). Accurately quantifying its contribution to regional atmospheric NHx deposition is critical for controlling regional air nitrogen pollution. The stable nitrogen isotope composition (expressed by δ15N) is a promising indicator to trace atmospheric NHx sources, presupposing a reliable nitrogen isotopic signature of NH3 emission sources. To obtain more specific seasonal δ15N values of soil NH3 volatilization for reliable regional seasonal NH3 source partitioning, we utilized an active dynamic sampling technique to measure the δ15N-NH3 values volatilized from maize cropping land in northeast China. These values varied from -38.0 to -0.2‰, with a significantly lower rate-weighted value observed in the early period (May-June, -30.5 ± 6.7‰) as compared with the late period (July-October, -8.5 ± 4.3‰). Seasonal δ15N-NH3 variations were related to the main NH3 production pathway, degree of soil ammonium consumption, and soil environment. Bayesian isotope mixing model analysis revealed that without considering the seasonal δ15N variation in soil-volatilized NH3 could result in an overestimate by up to absolute 38% for agricultural volatile NH3 to regional atmospheric bulk ammonium deposition during July-October, further demonstrating that it is essential to distinguish seasonal δ15N profile of agricultural volatile NH3 in regional source apportionment.

Soil δ15N spatial distribution is primarily shaped by climatic patterns in the semiarid Caatinga, Northeast Brazil

Soil nitrogen isotopic composition (δ15Nsoil) is an invaluable tool as it integrates nitrogen (N) transformations in soils. In addition to serving as a baseline to understand the N cycle, spatial representations of δ15Nsoil across landscapes (or isoscapes) is a multi-purpose tool useful to investigate, for example, plant-microbe interactions, animal migration and forensics. We investigate the climatic and edaphic controls of δ15Nsoil utilising data from 29 geographical locations sampled across the semiarid Brazilian Caatinga biome. The sampling covered a mean annual precipitation (PA) gradient ranging from 0.51 to 1.36 m a-1 and eight soil types originating from three different geological origins. Our data show that the combination of higher aridity and lower seasonality (ψ) leads to higher values of δ15Nsoil. Moreover, soil total carbon had a positive relationship with δ15Nsoil, appearing within the best-supported models according to the information-theoretic approach undertaken here. The contribution to the plant communities by the Fabaceae trees expressed as their basal area was not related to δ15Nsoil values, suggesting that the magnitude of biological N fixation in the Caatinga is not large enough to be reflected in the soil. In addition, considering PA in a categorical fashion, i.e., 'high' (> 0.8 m a-1) and 'low' PA (< 0.8 m a-1), we found that, within the wetter category, δ15Nsoil was positively related to several soil properties (i.e., clay content, effective cation exchange capacity, exchangeable calcium, silt content, pHH2O, total phosphorus and sum of bases) and negatively related to sand content. Our study provides new insights into the functioning of semiarid ecosystems from a pedo-isotopic perspective and contributes to the overall understanding of the N cycle in the Caatinga region, with the potential to support the development of new conceptualisation of biogeochemical process and testing of global models that simulate N and C cycles.

Tree-ring δ15N as an indicator of nitrogen dynamics in stands with N2-fixing Alnus rubra

Tree-ring δ15N may depict site-specific, long-term patterns in nitrogen (N) dynamics under N2-fixing species, but field trials with N2-fixing tree species are lacking and the relationship of temporal patterns in tree-ring δ15N to soil N dynamics is controversial. We examined whether the tree-ring δ15N of N2-fixing red alder (Alnus rubra Bong.) would mirror N accretion rates and δ15N of soils and whether the influence of alder-fixed N could be observed in the wood of a neighboring conifer. We sampled a 27-year-old replacement series trial on south-eastern Vancouver Island, with red alder and coastal Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) planted in five proportions (0/100, 11/89, 25/75, 50/50 and 100/0) at a uniform stem density. An escalation in forest floor N content was evident with an increasing proportion of red alder, equivalent to a difference of ~750 kg N ha-1 between 100% Douglas-fir versus 100% alder. The forest floor horizon also had high δ15N values in treatments with more red alder. Red alder had a consistent quadratic fit in tree-ring δ15N over time, with a net increase of $\sim$1.5‰, on average, from initial values, followed by a plateau or slight decline. Douglas-fir tree-ring δ15N, in contrast, was largely unchanged over time (in three of four plots) but was significantly higher in the 50/50 mix. The minor differences in current leaf litter N content and δ15N between alder and Douglas-fir, coupled with declining growth in red alder, suggests the plateau or declining trend in alder tree-ring δ15N could coincide with lower N2-fixation rates, potentially by loss in alder vigor at canopy closure, or down-regulation via nitrate availability.

Legume effects in a native community invaded by alien Asteraceae in a multi-species comparison

Facilitation has been a long-neglected type of interaction but received more attention recently. Legumes are commonly involved in facilitative interactions due to their nitrogen fixation. Facilitative interactions are so far underappreciated yet potentially important for biological invasions, especially given increasing numbers of alien species. In a common garden experiment using 30 annual Asteraceae species (neophytes, archaeophytes, plus some natives), grown in communities with or without legume presence, we measured functional traits and fitness in focal Asteraceae, as well as nitrogen characteristics of Asteraceae and two native community phytometer species. We investigated how legume presence affects relationships between trait and nitrogen concentration and Asteraceae fitness; and whether mechanisms of facilitation in legume presence and its effects on aboveground performance differ among native phytometer, neophyte, and archaeophyte Asteraceae using the δ¹⁵N natural abundance method. Lower specific leaf area was associated with higher aboveground biomass and seed production, with a stronger effect in legume absence. Nitrogen concentration had a positive relationship with biomass, but did not generally increase seed production. Our results hint at N facilitation for the native grass phytometer Festuca rupicola when growing in legume presence, whereas the forb Potentilla argentea and 27 alien Asteraceae species did not indicate facilitative effects. Intriguingly, direct legume facilitation in native phytometer species was only detected when growing with archaeophytes neighbors, not with neophytes. This hints at varied mechanisms of competition for nitrogen between natives and alien species of different residence time and deepens the understanding of altered facilitative leguminous effects in alien species presence.

Response of soil health indicators to dung, urine and mineral fertilizer application in temperate pastures

Healthy soils are key to sustainability and food security. In temperate grasslands, not many studies have focused on soil health comparisons between contrasting pasture systems under different management strategies and treatment applications (e.g. manures and inorganic fertilisers). The aim of this study was to assess the responses of soil health indicators to dung, urine and inorganic N fertiliser in three temperate swards: permanent pasture not ploughed for at least 20 years (PP), high sugar ryegrass with white clover targeted at 30% coverage reseeded in 2013 (WC), and high sugar ryegrass reseeded in 2014 (HG). This study was conducted on the North Wyke Farm Platform (UK) from April 2017 to October 2017. Soil health indicators including soil organic carbon (SOC, measured by loss of ignition and elemental analyser), dissolved organic carbon (DOC), total nitrogen (TN), C:N ratio, soil C and N bulk isotopes, pH, bulk density (BD), aggregate stability, ergosterol concentration (as a proxy for fungi biomass), and earthworms (abundance, mass and density) were measured and analysed before and after application of dung and N fertilizer, urine and N fertiliser, and only N fertiliser. The highest SOC, TN, DOC, ergosterol concentration and earthworms as well as the lowest BD were found in PP, likely due to the lack of ploughing. Differences among treatments were observed due to the application of dung, resulting in an improvement in chemical indicators of soil health after 50 days of its application. Ergosterol concentration was significantly higher before treatment applications than at the end of the experiment. No changes were detected in BD and aggregate stability after treatment applications. We conclude that not enough time had passed for the soil to recover after the ploughing and reseeding of the permanent pasture, independently of the sward composition (HG or WC). Our results highlight the strong influence of the soil management legacy in temperate pasture and the positive effects of dung application on soil health over the short term. In addition, we point out the relevance of using standardised methods to report soil health indicators and some methodological limitations.

Stable C and N isotope variation during anaerobic digestate composting and in the compost-amended soil-plant system

Although the use of composts derived from anaerobic digestates as soil amendments is likely to increase in the future, there is little information concerning the fate of their C and N compounds after their incorporation into soil. This work assesses C and N concentrations and the associated changes in δ¹⁵N and δ¹³C during the composting processes of cattle and pig slurry anaerobic digestates. In addition, the compost effect on C and N fractions and plant uptake were studied during a six-month pot experiment with rosemary plants. The results did not show δ¹³C and δ¹⁵N isotopic discrimination during composting, indicating a previous stabilization of cattle manure and pig slurry during the anaerobic digestion. This fact was also confirmed by the low C losses during the composting processes (1.2-fold and 1.05-fold for the composting piles with cattle and pig slurry anaerobic digestates, respectively). After soil addition, the composts augmented N values (from 0.41 g kg^-1 to around 0.56 g kg^-1 in low dose and 0.68 g kg^-1 in high dose compost amended soils) and δ¹⁵N soil values (increases in the range of 50%-156%), but showed only slight differences in C and δ¹³C values compared to unfertilised control and inorganic fertilized soils. Moreover, the rosemary leaves of the plants grown on the compost amended soils presented higher N and δ¹⁵N abundance than control and inorganic fertilized plants. We conclude that δ¹⁵N abundance of anaerobic digestate composts is useful to discern its N uptake and could thus be a useful tool to detect whether organic or mineral fertiliser types were used for agricultural production.

Alterations of ecosystem nitrogen status following agricultural land abandonment in the Karst Critical Zone Observatory (KCZO), Southwest China

Background

Secondary succession after agricultural land abandonment generally affects nitrogen (N) cycle processes and ecosystem N status. However, changes in soil N availability and NO₃ ^- loss potential following secondary succession are not well understood in karst ecosystems.

Methods

In the Karst Critical Zone Observatory (KCZO) of Southwest China, croplands, shrub-grass lands, and secondary forest lands were selected to represent the three stages of secondary succession after agricultural land abandonment by using a space-for-time substitution approach. The contents and ¹⁵N natural abundance (δ ¹⁵N) of leaves, soils, and different-sized aggregates at the three stages of secondary succession were analyzed. The δ ¹⁵N compositions of soil organic nitrogen (SON) in aggregates and soil to plant ¹⁵N enrichment factor (EF = δ ¹⁵N_leaf -δ ¹⁵N_soil), combined with soil inorganic N contents and δ ¹⁵N compositions were used to indicate the alterations of soil N availability and NO₃ ^-loss potential following secondary succession.

Results

Leaf N content and SON content significantly increased following secondary succession, indicating N accumulation in the soil and plant. The δ ¹⁵N values of SON also significantly decreased, mainly affected by plant δ ¹⁵N composition and N mineralization. SON content in macro-aggregates and soil NH₄ ⁺ content significantly increased while δ ¹⁵N values of NH₄ ⁺ decreased, implying increases in SON stabilization and improved soil N availability following secondary succession. Leaf δ ¹⁵N values, the EF values, and the (NO₃ ^--N)/(NH₄ ⁺-N) ratio gradually decreased, indicating reduced NO₃ ^- loss following secondary succession.

Conclusions

Soil N availability improves and NO₃ ^- leaching loss reduces following secondary succession after agricultural land abandonment in the KCZO.

Multi-isotope zooarchaeological investigations at Abri du Maras: The paleoecological and paleoenvironmental context of Neanderthal subsistence strategies in the Rhône Valley during MIS 3

The exploitation of mid- and large-sized herbivores (ungulates) was central to hominin subsistence across Late Pleistocene Europe. Reconstructing the paleoecology of prey-taxa is key to better understanding procurement strategies, decisions and behaviors, and the isotope analysis of faunal bones and teeth found at archaeological sites represent a powerful means of accessing information about past faunal behaviors. These isotope zooarchaeological approaches also have a near-unique ability to reveal environmental conditions contemporary to the human activities that produced these remains. Here, we present the results of a multi-isotope, multitissue study of ungulate remains from the Middle Paleolithic site of Abri du Maras, southern France, providing new insights into the living landscapes of the Rhône Valley during MIS 3 (level 4.2 = 55 ± 2 to 42 ± 3 ka; level 4.1 = 46 ± 3 to 40 ± 3 ka). Isotope data (carbon, nitrogen) reveal the dietary niches of different ungulate taxa, including the now-extinct giant deer (Megaloceros). Oxygen isotope data are consistent with a mild seasonal climate during level 4.2, where horse (Equus), bison (Bison), and red deer (Cervus elaphus) were exploited year-round. Strontium and sulfur isotope analyses provide new evidence for behavioral plasticity in Late Pleistocene European reindeer (Rangifer) between level 4.2 and level 4.1, indicating a change from the migratory to the sedentary ecotype. In level 4.1, the strong seasonal nature of reindeer exploitation, combined with their nonmigratory behavior, is consistent with a seasonally restricted use of the site by Neanderthals at that time or the preferential hunting of reindeer when in peak physical condition during the autumn.

Traceability of nitrate polluted hotspots in plain river networks of the Yangtze River delta by nitrogen and oxygen isotopes coupling bayesian model

The adverse effects of increased nitrate (NO₃^-) pollution especially from the non-point source on the hydrosphere and anthroposphere are becoming more prominent. The non-point-derived NO₃^- in the rivers supplying the upstream threatens the aquatic ecosystem of Taihu Lake. Here, dual-stable isotopes (δ¹⁵N and δ¹⁸O) of NO₃^- were applied to the Bayesian model (SIAR) for quantitative source identification of reactive nitrogen (Nr) in a mixed agricultural and urban region along the complex river network of the Yangze River delta. The results showed that the NO₃^- concentrations in the rivers ranged from 1.09 to 4.44 mg L^-1 and decreased from the highly urbanized areas to the lakeside rural areas. The specific isotopic characteristics of four sources (atmospheric deposition, AD; chemical fertilizer, CF; manure and sewage, MS; and soil leachate, SL) by the SIAR isotope model indicated that the MS source made the greatest contribution (46.56%) to the total NO₃^- load, followed by SL (27.86%), CF (23.77%), and AD (1.81%). The highly urbanized areas and the hybrid areas, which contained a mix of industrialized, populated, and agricultural areas, were identified as hotspot areas with heavy Nr pollution, responsible for spatial patterns of δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-. These hotspot areas were characterized by a less well-developed sewage pipeline system with high Nr emissions from cash crops. The changes in wastewater treatment level, the agricultural production structure, and meteorological changes were the main factors of spatial variation of Nr concentration and source in the upstream Taihu Lake Basin. The variation in Nr concentration across Taihu Lake would respond to these anthropogenic-driven Nr loads. These findings suggest that MS was the predominant source had the strongest effect on the overall riverine NO₃^- source which was the primary problem that needed to be solved.

Significant contributions of combustion-related sources to ammonia emissions

Atmospheric ammonia (NH₃) and ammonium (NH₄⁺) can substantially influence air quality, ecosystems, and climate. NH₃ volatilization from fertilizers and wastes (v-NH₃) has long been assumed to be the primary NH₃ source, but the contribution of combustion-related NH₃ (c-NH₃, mainly fossil fuels and biomass burning) remains unconstrained. Here, we collated nitrogen isotopes of atmospheric NH₃ and NH₄⁺ and established a robust method to differentiate v-NH₃ and c-NH₃. We found that the relative contribution of the c-NH₃ in the total NH₃ emissions reached up to 40 ± 21% (6.6 ± 3.4 Tg N yr^-1), 49 ± 16% (2.8 ± 0.9 Tg N yr^-1), and 44 ± 19% (2.8 ± 1.3 Tg N yr^-1) in East Asia, North America, and Europe, respectively, though its fractions and amounts in these regions generally decreased over the past decades. Given its importance, c-NH₃ emission should be considered in making emission inventories, dispersion modeling, mitigation strategies, budgeting deposition fluxes, and evaluating the ecological effects of atmospheric NH₃ loading.

Global distribution and climate sensitivity of the tropical montane forest nitrogen cycle

Tropical forests are pivotal to global climate and biogeochemical cycles, yet the geographic distribution of nutrient limitation to plants and microbes across the biome is unresolved. One long-standing generalization is that tropical montane forests are nitrogen (N)-limited whereas lowland forests tend to be N-rich. However, empirical tests of this hypothesis have yielded equivocal results. Here we evaluate the topographic signature of the ecosystem-level tropical N cycle by examining climatic and geophysical controls of surface soil N content and stable isotopes (δ¹⁵N) from elevational gradients distributed across tropical mountains globally. We document steep increases in soil N concentration and declining δ¹⁵N with increasing elevation, consistent with decreased microbial N processing and lower gaseous N losses. Temperature explained much of the change in N, with an apparent temperature sensitivity (Q₁₀) of ~1.9. Although montane forests make up 11% of forested tropical land area, we estimate they account for >17% of the global tropical forest soil N pool. Our findings support the existence of widespread microbial N limitation across tropical montane forest ecosystems and high sensitivity to climate warming.

The Effects of Elevated Tropospheric Ozone on Carbon Fixation and Stable Isotopic Signatures of Durum Wheat Cultivars with Different Biomass and Yield Stability

Tropospheric ozone (O₃) enrichment caused by human activities can reduce important crop yields with huge economic loss and affect the global carbon cycle and climate change in the coming decades. In this study, two Italian cultivars of durum wheat (Claudio and Mongibello) were exposed to O₃ (80 ppb, 5 h day^-1 for 70 consecutive days), with the aim to investigate the changes in yield and biomass, ecophysiological traits, and stable carbon and nitrogen isotope values in plants, and to compare the stable isotope responses under environmental stressors. Both cultivars showed a relative O₃ tolerance in terms of photosynthetic performance, but in cultivar Mongibello, O₃ was detrimental to the grain yield and plant biomass. The δ¹³C values in the leaves of plants identified that the impact of O₃ on CO₂ fixation by RuBisCO was dominant. The δ¹⁵N value showed significant differences between treatments in both cultivars at seven days from the beginning of the exposure, which could be considered an early indicator of ozone pollution. Under increasingly frequent extreme climates globally, the relationships among stable isotope data, ecophysiological traits, and agronomic parameters could help breed future cultivars.

Applying the 15N labelling technique to material derived from a landfill simulation experiment to understand nitrogen cycle processes under aerobic and anaerobic conditions

Reactive nitrogen (N) species, such as ammonium (NH₄⁺), nitrate (NO₃) and gaseous nitrous oxide (N₂O), are released into the environment during the degradation of municipal solid waste (MSW), causing persistent environmental problems. Landfill remediation measures, such as in-situ aeration, may accelerate the degradation of organic compounds and reduce the discharge of ammonium via leachate. Nonetheless, the actual amount of N in the waste material remains relatively constant and a coherent explanation for the decline in leachate ammonium concentrations is still lacking. Hence, the present study aimed to elucidate the dynamics of N and its transformation processes during waste degradation. To this end, the gross rates of organic N mineralization and nitrification were measured using ¹⁵N pool dilution in waste material derived from a landfill simulation reactor (LSR) experiment. The results revealed a high potential for N mineralization and nitrification, the latter of which declined with the diminishing amount of extractable ammonium (after aeration). The analysis of the concentration and isotopic composition of N₂O formed confirmed incomplete denitrification as the main source for N₂O. Moreover, the natural abundance of ¹⁵N was investigated in various waste N pools to verify the conclusions drawn from the ¹⁵N tracing experiment. δ¹⁵N values of total waste N increased during aeration, indicating that nitrification is the major driver for N losses from aerated waste. The application of stable isotopes thereby allowed unprecedented insights into the complex N dynamics in decomposing landfill waste, of their response to aeration and their effect on hydrological versus gaseous loss pathways.

Urban form and scale shaped the agroecology of early 'cities' in northern Mesopotamia, the Aegean and Central Europe

Agricultural extensification refers to an expansive, low-input production strategy that is land rather than labour limited. Here, we present a robust method, using the archaeological proxies of cereal grain nitrogen isotope values and settlement size, to investigate the relationship between agricultural intensity and population size at Neolithic to Bronze/Iron Age settlement sites in northern Mesopotamia, the Aegean and south-west Germany. We conclude that urban form-in particular, density of occupation-as well as scale shaped the agroecological trajectories of early cities. Whereas high-density urbanism in northern Mesopotamia and the Aegean entailed radical agricultural extensification, lower density urbanism in south-west Germany afforded more intensive management of arable land. We relate these differing agricultural trajectories to long-term urban growth/collapse cycles in northern Mesopotamia and the Aegean, on the one hand, and to the volatility of early Iron Age elite power structures and urban centralization in south-west Germany, on the other.

Climate and vegetation and their impact on stable C and N isotope ratios in bat guano

Cave guano deposits represent a relatively untapped paleoecological archive that can provide information about past vegetation, climate, and bat diet over several millennia. Recent research suggests that carbon isotope values (δ13C) measured in guano accumulations from insectivorous bats reflect the relative abundance of C3 and C4 plants on the landscape while nitrogen isotope values (δ15N) may reflect precipitation amount. Together, these proxies can provide useful information for restoration practitioners seeking to understand how plant species composition has changed over time in relation to climate and land use. To better calibrate these proxies for use in restoration, we compared instrumental records of precipitation and satellite imagery of vegetation with isotope values measured in modern bat guano from Mary Lawson Cave, a large limestone cavern located in central Missouri. Mary Lawson Cave hosts a maternity colony of insectivorous gray bats (Myotis grisescens), and as such, contains significant guano accumulations. In the fall of 2018, we collected a 60 cm long guano core that dates to 1999 cal AD at its base. Guano core δ13C values decrease from the base toward the surface (from ~-26 to -27‰) whereas δ15N values increase toward the surface even after accounting for ammonia volatilization (from ~3 to 5‰). Presently, the landscape around Mary Lawson Cave is dominated by a deciduous forest and pasture. Given that the land cover has changed very little over this period, the decline in δ13C values toward the present likely reflects a shift in land management on farms and/or increases in invasive C3 species. Rainfall amounts from nearby Lebanon, Missouri, are significantly positively correlated with guano δ15N values, a relationship that is notably opposite that observed previously in soil and plants. We argue that heavy fertilizer application and significant grazing intensity could lead to the accumulation of large pools of excess labile nitrogen which would be vulnerable to leaching during precipitation events. The relationship between guano δ15N values and precipitation may differ for materials from less agriculturally impacted locations or periods and should be extended into the past with caution.

Natural 15N abundance as an indicator of nitrogen utilization efficiency in rice under alternate wetting and drying irrigation in soils with high clay contents

The ¹⁵N natural abundance is an effective indicator of nitrogen dynamics in plants. The impact of different irrigation regimes as a function of varied soil clay contents on stable nitrogen isotope abundance (δ¹⁵N) in rice remains unknown. Therefore, the response of δ¹⁵N and nitrogen utilization efficiency (NUE) of rice to different combinations of alternate wetting and drying irrigation (AWD) and clay contents were investigated. The study included three AWD regimes, viz. I100, (100 % saturation, 30 mm flooded), I90 (90 % saturation, 30 mm flooded) and I70 (70 % saturation, 30 mm flooded), and three soil clay content treatments, viz. 40 % (S40), 50 % (S50), and 60 % (S60) clay content. Compared with I100, I90 and I70 with high clay content (S60) significantly increased the crack volumes and N leaching losses and reduced the total N accumulation and different forms of NUE of rice plants. The values of δ¹⁵N in above-ground organs and soil were greatly increased by I90 and I70 irrigation regimes compared to I100. An increasing trend of organs δ¹⁵N from root to shoot was found for all three irrigation regimes. Significant negative relationships were found between (i) N partial factor productivity (PFP) and grain ¹⁵N, (ii) PFP and leaf ¹⁵N, and (iii) N harvest index (NHI) and leaf ¹⁵N. These significant negative relationships might contribute to the increased N losses and changed N allocation under AWD with high clay contents. Hence, it is suggested that cracks should be taken into consideration in rice cultivation. Moreover, δ¹⁵N may serve as an effective indicator of NUE in rice grown under AWD irrigation with high clay contents as well as an indirect indicator for assessing the N loss in agro-ecosystems.

Elevational shifts in foliar-soil δ15 N in the Hengduan Mountains and different potential mechanisms

The natural abundance of stable nitrogen isotopes (δ¹⁵ N) provides insights into the N dynamics of terrestrial ecosystems, the determination of which is considered an effective approach for gaining a better understanding ecosystem N cycling. However, there is currently little information available regarding the patterns and mechanisms underlying the variation in foliar-soil δ¹⁵ N among mountain ecosystems. In this study, we examined the determinants of foliar-soil δ¹⁵ N in association with N transportation rates along an elevational gradient in the Hengduan Mountains. Despite the relatively high levels of available N produced from high N fixation and mineralization, we detected the lowest levels of foliar δ¹⁵ N at 3500 m a.s.l., reflecting the stronger vegetation N limitation at medium high elevations. The enhanced vegetation N limitation was driven by the combined effects of higher microbial immobilization and inherent plant dynamic (the shifts of δ¹⁵ N in vegetation preference, including vegetation community) with changing climate along the elevational gradient. Unexpectedly, we established that soil δ¹⁵ N was characterized by an undulating rise and uncoupled correlation with foliar δ¹⁵ N with increasing elevation, thereby indicating that litter input might not be a prominent driver of soil δ¹⁵ N. Conversely, soil nitrification and denitrification were found to make a more pronounced contribution to the pattern of soil δ¹⁵ N along the elevational gradient. Collectively, our results serve to highlight the importance of microbial immobilization in soil N dynamics and provide novel insights that will contribute to enhancing our understanding of N cycling as indicated by foliar-soil δ¹⁵ N along elevational gradients.

Co-Inoculation with Azospirillum brasilense and Bradyrhizobium sp. Enhances Nitrogen Uptake and Yield in Field-Grown Cowpea and Did Not Change N-Fertilizer Recovery

This study was designed to investigate the effects of Azospirillum brasilense and Bradyrhizobium sp. co-inoculation coupled with N application on soil N levels and N in plants (total N, nitrate N-NO₃⁻ and ammonium N-NH₄⁺), photosynthetic pigments, cowpea plant biomass and grain yield. An isotopic technique was employed to evaluate ¹⁵N fertilizer recovery and derivation. Field trials involved two inoculations—(i) single Bradyrhizobium sp. and (ii) Bradyrhizobium sp. + A. brasilense co-inoculation—and four N fertilizer rates (0, 20, 40 and 80 kg ha⁻¹). The co-inoculation of Bradyrhizobium sp. + A. brasilense increased cowpea N uptake (an increase from 10 to 14%) and grain yield (an average increase of 8%) compared to the standard inoculation with Bradyrhizobium sp. specifically derived from soil and other sources without affecting ¹⁵N fertilizer recovery. There is no need for the supplementation of N via mineral fertilizers when A. brasilense co-inoculation is performed in a cowpea crop. However, even in the case of an NPK basal fertilization, applied N rates should remain below 20 kg N ha⁻¹ when co-inoculation with Bradyrhizobium sp. and A. brasilense is performed.

Utility of carbon and nitrogen stable isotopes for inferring wild bee (Hymenoptera: Apoidea) use of adjacent foraging habitats

Isotope analysis has proven useful for understanding diets of animals that are difficult to track for extended periods. Bees are small yet highly mobile and often forage from multiple habitats. However, current methods of assessing diet are limited in scope. Efficient methods of tracking bee diets that integrate across life stages, distinguish habitat use, and are sensitive to taxonomic differences will inform conservation strategies. We evaluated the utility of stable isotope analysis for estimating contributions of adjacent habitats to bees’ diets. We also investigated taxonomic variation in bee and flower isotope composition. We measured natural abundance of carbon and nitrogen stable isotopes in two body regions from three wild bee genera, as well as in 25 species of flowers that likely comprised their diets. Bee ∂13C and ∂15N varied with habitat and taxonomic groups (conflated with month), but did not match spatial or seasonal trends in their food plants. Flower ∂13C was lowest in the forest and in April–June, as expected if driven by water availability. However, bee ∂13C was elevated in the spring, likely from overwintering nutritional stress or unpredictable food availability. Bumble bees (Bombus) were enriched in ∂15N compared to others, possibly reflecting differences in larval feeding. Bee diet mixing models had high variation and should be interpreted with caution. Models estimated similar habitat contributions to diets of spring Andrena and overwintered Bombus queens. Summer Bombus queens and workers were indistinguishable. Sweat bees (Halictus) were estimated to use comparatively more field flowers than others. Overall, taxon more strongly influenced isotope composition than either foraging habitat or month, likely because of associated differences in sociality and timing of annual activity. Future studies seeking to reveal bee diets by isotope analysis may gain better resolution in more isotopically distinct habitats, in conjunction with controlled feeding or isotope labeling experiments.

Changes in forest nitrogen cycling across deposition gradient revealed by δ15N in tree rings

Tree rings provide valuable insight into past environmental changes. This study aimed to evaluate perturbations in tree ring width (TRW) and δ¹⁵N alongside soil acidity and nutrient availability gradients caused by the contrasting legacy of air pollution (nitrogen [N] and sulphur [S] deposition) and tree species (European beech, Silver fir and Norway spruce). We found consistent declines of tree ring δ¹⁵N, which were temporarily unrelated to the changes in the TRW. The rate of δ¹⁵N change in tree rings was related to the contemporary foliar carbon (C) to phosphorus (P) ratio. This observation suggested that the long-term accumulation of ¹⁵N depleted N in tree rings, likely mediated by retained N from deposition, was restricted primarily to stands with currently higher P availability. The shifts observed in tree-ring δ¹⁵N and TRW suggest that acidic air pollution rather than changes in stand productivity determined alteration of N and C cycles. Stable N isotopes in tree rings provided helpful information on the trajectory of the N cycle over the last century with direct consequences for a better understanding of future interactions among N, P and C cycles in terrestrial ecosystems.

Foliar stable isotope ratios of carbon and nitrogen in boreal forest plants exposed to long-term pollution from the nickel-copper smelter at Monchegorsk, Russia

Long-term exposure to primary air pollutants, such as sulphur dioxide (SO2) and nitrogen oxides (NOx), alters the structure and functions of forest ecosystems. Many biochemical and biogeochemical processes discriminate against the heavier isotopes in a mixture; thus, the values of δ13C and δ15N (i.e. the ratio of stable isotopes 13C to 12C and that of 15 N to 14 N, respectively) may give insights into changes in ecosystem processes and identify the immediate drivers of these changes. We studied sources of variation in the δ13C and δ15N values in the foliage of eight boreal forest C3 plants at 10 sites located at the distance of 1-40 km from the Monchegorsk nickel-copper smelter in Russia. From 1939‒2019, this smelter emitted over 14,000,000 metric tons (t) of SO2, 250,000 t of metals, primarily nickel and copper, and 140,000 t of NOx. The δ13C value in evergreen plants and the δ15N value in all plants increased near the smelter independently of the plant mycorrhizal type. We attribute the pollution-related increase in the foliar δ13C values of evergreen species mainly to direct effects of SO2 on stomatal conductance, in combination with pollution-related water stress, which jointly override the potential opposite effect of increasing ambient CO2 concentration on δ13C values. Stomatal uptake of NOx and root uptake of 15N-enriched organic N compounds and NH4+ may explain the increased foliar δ15N values and elevated foliar N concentrations, especially in the evergreen trees (Pinus sylvestris), close to Monchegorsk, where the soil inorganic N supply is reduced due to the impact of long-term SO2 and heavy metal emissions on plant biomass. We conclude that, despite the uncertainties in interpreting δ13C and δ15N responses to pollution, the Monchegorsk smelter has imposed and still imposes a great impact on C and N cycling in the surrounding N-limited subarctic forest ecosystems.

Age-Related Changes in Water and Nitrogen Utilization in Crop Trees and Understory Vegetation in a Hinoki Cypress Plantation Forest in Kochi City, Southern Japan

Age-related changes in water and nitrogen utilization of crop and understory vegetation in a hinoki cypress plantation forest were investigated from the age of 21 to 46 years in Kochi City, southern Japan. Nitrogen concentration in the leaf litter of hinoki cypress showed a decreasing trend with forest age. The leaf δ15N of hinoki cypress was related to a quadratic function and increased from the age of 21 to 26 years and then decreased to the age of 46 years. These results suggest that older hinoki cypress trees utilize soil nitrogen sources with lower δ15N values, and the competition for soil nitrogen with understory vegetation should be stronger. Carbon isotope discrimination (Δ13C) of hinoki cypress decreased from the age of 21 to 30 years and then increased to the age of 46 years. In contrast, the intrinsic water-use efficiency (iWUE) of hinoki cypress increased from the age of 21 to 36 years and then decreased to the age of 46 years. These findings suggest that hinoki cypress trees in the earlier time increased their iWUE by reducing stomatal opening. In the earlier time, the stomatal opening of understory vegetation increased due to higher soil water availability with decreasing stand density of crop trees. In the later time, the iWUE of hinoki cypress decreased due to lower photosynthetic capacity with nitrogen limitation. These results suggest that the increase in the iWUE of hinoki cypress in response to elevated atmospheric carbon dioxide levels should be smaller in the later time because of stronger competition with understory vegetation for soil nitrogen resources.

Depth profiles of soil organic carbon isotopes across a lithosequence: implications for drivers of soil δ13C vertical changes

To addresshow parent materials are affecting organic carbon dynamics in a soil profile, soils from a lithosequence comprising six parent lithologies under a rangeland ecosystem have been explored at three depth intervals for soil organic carbon (SOC) content and its ¹³C depth trends. Studied parent materials ranged from metamorphic (foliated: FM and non-foliated: NFM) to sedimentary (clastic carbonate: CCS) to plutonic (intermediate: IP, felsic: FP and intermediate felsic: IFP) geological contexts. The relationship between SOC concentration and its isotopic signatures to a depth of 50 cm in FM, NFM, FP and IFP profiles was well described by the kinetic fractionation of SOC during biodegradation. For CCS and IP lithologies, strong divergence from the Rayleigh equation was observed suggesting that the ¹³C enrichments in these soils resulted from both mixing different SOC pools and isotope fractionation related to the C mineralization. Results suggest that SOC across the lithosequence goes through different isotopic evolutions resulting from different ¹³C-enriched inputs and pedogenic properties as described by the extended Rayleigh equation (0 ≤ β_C≤ 0.80). These are presumably caused by the bedrock lithology implying that parent material affects C storage and dynamics.

Lack of coordination between stomatal and vein traits provides functional benefits to the dioecious tropical tree Myrsine coriacea

Climate change will affect the distribution of many tropical plant species. However, the understanding of how dioecious tropical species cope with different environmental conditions is still limited. To address this issue, we investigated how secondary trait attributes in populations of the dioecious tropical tree Myrsine coriacea change along an altitudinal gradient. Eighty individual plants (40 male and 40 female) were selected among seven natural populations. Leaf variation in morphological and stomatal traits, and carbon and nitrogen isotopic compositions were analyzed. Female plants had greater isotopic leaf carbon composition (δ¹³ C) and nitrogen content than male plants, increasing their carboxylation capacity. Plants of both sexes had smaller stomata, greater water-use efficiency (greater δ¹³ C), and greater nitrogen isotopic composition (δ¹⁵ N) at higher altitudes. They also showed lower δ¹⁵ N and had greater carbon: nitrogen ratios at lower altitudes. There was a lack of coordination between stomatal and vein traits, which was compensated for by variation in specific leaf areas. This mechanism was essential for increasing plant performance under the limiting conditions found by the species at higher altitudes.

Water levels primarily drive variation in photosynthesis and nutrient use of scrub Red Mangroves in the southeastern Florida Everglades

We investigated how mangrove-island micro-elevation (i.e., habitat: center vs edge) affects tree physiology in a scrub mangrove forest of the southeastern Everglades. We measured leaf gas exchange rates of scrub Rhizophora mangle L. trees monthly during 2019, hypothesizing that CO2 assimilation (Anet) and stomatal conductance (gsw) would decline with increasing water levels and salinity, expecting more considerable differences at mangrove-island edges than centers, where physiological stress is greatest. Water levels varied between 0 and 60 cm from the soil surface, rising during the wet season (May-October) relative to the dry season (November-April). Porewater salinity ranged from 15 to 30 p.p.t., being higher at mangrove-island edges than centers. Anet maximized at 15.1 μmol m-2 s-1, and gsw was typically <0.2 mol m-2 s-1, both of which were greater in the dry than the wet season and greater at island centers than edges, with seasonal variability being roughly equal to variation between habitats. After accounting for season and habitat, water level positively affected Anet in both seasons but did not affect gsw. Our findings suggest that inundation stress (i.e., water level) is the primary driver of variation in leaf gas exchange rates of scrub mangroves in the Florida Everglades, while also constraining Anet more than gsw. The interaction between inundation stress due to permanent flooding and habitat varies with season as physiological stress is alleviated at higher-elevation mangrove-island center habitats during the dry season. Freshwater inflows during the wet season increase water levels and inundation stress at higher-elevation mangrove-island centers, but also potentially alleviate salt and sulfide stress in soils. Thus, habitat heterogeneity leads to differences in nutrient and water acquisition and use between trees growing in island centers versus edges, creating distinct physiological controls on photosynthesis, which likely affect carbon flux dynamics of scrub mangroves in the Everglades.

Is halophyte species growing in the vicinity of the shrimp ponds a promising agri-aquaculture system for shrimp ponds remediation in New Caledonia?

Plant culture integration within aquaculture activities is a topic of recent interest with economic and environmental benefits. Shrimp farming activities generate nutrient-rich waste trapped in the sediments of farming ponds or release in the mangrove area. Thus, we investigate if the halophytes species naturally growing around the pond can use nitrogen and carbon from shrimp farming for remediation purposes. Halophyte biomasses and sediments influenced by shrimp farm effluents, were collected in two farms in New-Caledonia. All samples were analyzed for their C and N stable isotopic composition and N content. Higher δ¹⁵N values were found in plants influenced by shrimp farm water thus evidenced their abilities to take nutrient derived from shrimp farming. Deep root species Chenopodium murale, Atriplex jubata, Suaeda australis and Enchylaena tomentosa appears more efficient for shrimp pond remediation. This work demonstrates that halophytes cultivation in shrimp ponds with sediments, could be effective for the pond's remediation.

High atmospheric wet nitrogen deposition and major sources in two cities of Yangtze River Delta: Combustion-related NH3 and non-fossil fuel NOx

High ammonia (NH₃) and nitrogen oxide (NO_x) emissions are related to serious air pollution in urban areas and the negative impacts of excessive reactive nitrogen (N) deposition on many ecosystems. However, whether there is a relationship between N deposition rates and their sources with urbanization or not remains unclear in many areas. Here, we investigated the deposition rates of ammonium (NH₄⁺), nitrate (NO₃^-), dissolved organic N, and water-insoluble particular N from July 2017 to June 2018 at two urban and two suburban sites in the Yangtze River Delta (YRD). The δ¹⁵N values of precipitation NH₄⁺ and NO₃^- were measured, and major sources were analyzed using a Bayesian isotope mixing model. Wet N deposition rates were higher in Yangzhou (developing city, 20.3-22.7 kg N ha^-1 yr^-1) than those in Nanjing (developed city, 19.4-20.5 kg N ha^-1 yr^-1), and were higher at urban sites (20.4-22.5 kg N ha^-1 yr^-1) than those at suburban sites (18.7-20.3 kg N ha^-1 yr^-1). δ¹⁵N values of precipitation NH₄⁺ increased with an increase in precipitation pH because ambient acidity affects the equilibrium isotope fractionation between NH₃ and NH₄⁺ and wet scavenging coefficients of NH₃ and particulate NH₄⁺. For NH₄⁺, combustion-related NH₃ sources (62%-65% with 5.5-6.4 kg N ha^-1 yr^-1, including coal combustion, vehicle exhaust, and biomass burning) contributed more than volatilization NH₃ sources (35%-38% with 2.9-3.9 kg N ha^-1 yr^-1, including fertilizer application and waste volatilization). For NO₃^-, non-fossil fuel NO_x sources (50%-63% with 3.4-4.1 kg N ha^-1 yr^-1, including biomass burning and microbial N cycle) were comparable to fossil fuel NO_x sources (37%-50% with 2.4-3.4 kg N ha^-1 yr^-1, including coal combustion and vehicle exhaust). This study evidenced high N deposition rates and the importance of combustion-related NH₃ emissions and non-fossil fuel NO_x emissions in city areas of the YRD.

Biochar-N fertilizer interaction increases N utilization efficiency by modifying soil C/N component under N fertilizer deep placement modes

The situation of imbalance application of nitrogenous fertilizers in maize production is a serious issue in China, and excessive nitrogen (N) application is hazardous to sustainable agricultural production and environment. In this experiment, two biochar levels (C0: 0, C1: 2 %), three different N rates (N1: 50, N2: 100, and N3: 200 mg kg^-1), and two fertilization methods (T: traditional N fertilizer application mode and D: deep N fertilizer placement mode) were set up to study the response of different treatments on maize yield, N uptake, and N use efficiency. Herein, we found that fresh and dry biomasses were increased by 292 % and 283 % under C1N3 treatment with the deep application of N fertilizer compared to the control treatment (without nitrogen fertilizers and biochar). According to structural equation modeling (SEM), soil physical and chemical properties, N component and C component in different soil layers were associated with biochar and N fertilizer treatment, especially at 20-40 depth. The combination of N fertilizer and biochar application promoted the effects of biochar on the improving NUE of plants. The biochar alleviated the loss of soil nitrogen (from 52.00 to 25.94 %) under traditional N fertilizer application. Overall, excessive input of N fertilizer not only promotes the growth of crops but also causes a waste of resources and environmental pollution. We suggest that combined application of biochar and N fertilizer could significantly reduce N loss, and improve root growth and N uptake, resulting in improving NUE by improving soil environment (pH, SOM, EC) and adjusting soil C/N component.

Biochar compound fertilisers increase plant potassium uptake 2 years after application without additional organic fertiliser

Biochar compound fertilisers (BCFs) are an emerging technology that combine biochar with nutrients, clays and minerals and can be formulated to address specific issues in soil-plant systems. However, knowledge of BCF performance over consecutive crops and without re-application is limited. This study aims to assess the residual effect of organic BCFs soil-plant nutrient cycling 2 years after application and without additional fertiliser inputs. We applied BCFs and biochar with organic fertiliser amendments and established a crop of ginger and a second crop of turmeric (Curcuma longa) without re-application or additional fertilisation. All treatment formulations included bamboo-biochar and organic fertiliser amendments; however, two novel BCFs were formulated to promote agronomic response in an intensive cropping system. We report here on the effect of treatments on soil and plant macronutrient and micronutrient cycling and turmeric growth, biomass and yield at harvest. Both BCFs (enriched (10 t ha^-1) and organo-mineral biochar (8.6 t ha^-1) increased foliar K (+155% and +120%) and decreased foliar Mg (-20% and -19%) concentration compared with all other treatments, suggesting antagonism between K and Mg. Plants were limited for K, P and B at harvest but not N, Ca or Mg. Foliar K was dependent on the biochar formulation rather than the rate of application. Biochar-clay aggregates increased K retention and cycling in the soil solution 2 years after application. Clay blended BCFs reduced K limitation in turmeric compared to biochar co-applied with organic amendments, suggesting these blends can be used to manage organic K nutrition. All formulations and rates of biochar increased leaf biomass and shoot-to-root ratio. Novel BCFs should be considered as an alternative to co-applying biochar with organic fertiliser amendments to decrease application rates and increase economic feasibility for farmers. Applying BCFs without re-application or supplementary fertiliser did not provide sufficient K or P reserves in the second year for consecutive cropping. Therefore, supplementary fertilisation is recommended to avoid nutrient deficiency and reduced yield for consecutive organic rhizome crops.

Post-silking 15N labelling reveals an enhanced nitrogen allocation to leaves in modern maize (Zea mays) genotypes

Nitrogen (N) metabolism is a major research target for increasing productivity in crop plants. In maize (Zea mays L.), yield gain over the last few decades has been associated with increased N absorption and utilization efficiency (i.e. grain biomass per unit of N absorbed). However, a dynamical framework is still needed to unravel the role of internal processes such as uptake, allocation, and translocation of N in these adaptations. This study aimed to 1) characterize how genetic enhancement in N efficiency conceals changes in allocation and translocation of N, and 2) quantify internal fluxes behind grain N sources in two historical genotypes under high and low N supply. The genotypes 3394 and P1197, landmark hybrids representing key eras of genetic improvement (1990s and 2010s), were grown under high and low N supply in a two-year field study. Using stable isotope ¹⁵N labelling, post-silking nitrogen fluxes were modeled through Bayesian estimation by considering the external N (exogenous-N) and the pre-existing N (endogenous-N) supply across plant organs. Regardless of N availability, P1197 exhibited greater exogenous-N accumulated in leaves and cob-husks. This response was translated to a larger amount of N mobilized to grains (as endogenous-N) during grain-filling in this genotype. Furthermore, the enhanced N supply to leaves in P1197 was associated with increased post-silking carbon accumulation. The overall findings suggest that increased N utilization efficiency over time in maize genotypes was associated with an increased allocation of N to leaves and subsequent translocation to the grains.

Atmospheric nitrogen enrichment changes nutrient stoichiometry and reduces fungal N supply to peatland ericoid mycorrhizal shrubs

Peatlands store one third of global soil carbon (C) and up to 15% of global soil nitrogen (N) but often have low plant nutrient availability owing to slow organic matter decomposition under acidic and waterlogged conditions. In rainwater-fed ombrotrophic peatlands, elevated atmospheric N deposition has increased N availability with potential consequences to ecosystem nutrient cycling. Here, we studied how 14 years of continuous N addition with either nitrate or ammonium had affected ericoid mycorrhizal (ERM) shrubs at Whim Bog, Scotland. We examined whether enrichment has influenced foliar nutrient stoichiometry and assessed using N stable isotopes whether potential changes in plant nutrient constraints are linked with plant N uptake through ERM fungi versus direct plant uptake. High doses of ammonium alleviated N deficiency in Calluna vulgaris and Erica tetralix, whereas low doses of ammonium and nitrate improved plant phosphorus (P) nutrition, indicated by the lowered foliar N:P ratios. Root acid phosphatase activities correlated positively with foliar N:P ratios, suggesting enhanced P uptake as a result of improved N nutrition. Elevated foliar δ¹⁵N of fertilized shrubs suggested that ERM fungi were less important for N supply with N fertilization. Increases in N availability in peat porewater and in direct nonmycorrhizal N uptake likely have reduced plant nitrogen uptake via mycorrhizal pathways. As the mycorrhizal N uptake correlates with the reciprocal C supply from host plants to the soil, such reduction in ERM activity may affect peat microbial communities and even accelerate C loss via decreased ERM activity and enhanced saprotrophic activity. Our results thus introduce a previously unrecognized mechanism for how anthropogenic N pollution may affect nutrient and carbon cycling within peatland ecosystems.

Determination of the nitrogen isotope enrichment factor associated with ammonification and nitrification in unsaturated soil at different temperatures

For nitrogen (N) migration and transformation from unsaturated soil to groundwater, the N stable isotope (δ¹⁵N) was modified due to the isotope fractionation effect. To quantitatively evaluate the N cycle in groundwater systems, the determination of isotope fractionation is decisive. In this research, for the first time, incubation experiments were conducted to quantitatively investigate the N isotope enrichment factor (ϵ_p/s) associated with ammonification in unsaturated soil. Under weak isotopic fractionation, the Rayleigh function cannot be directly applied during ammonification. Thus, we proposed a different method calculating the ϵ_p/s values during ammonification, which were -0.03‰ for 15 °C and -2.34‰ for 30 °C. Moreover, for the first time, experimental equipment is presented to explore the isotopic fractionation effects under the co-occurrence of nitrification and volatilization. The results indicated that the isotope effect of volatilization during nitrification can be ignored in this study, and the ϵ_p/s values during nitrification were -10.59 and -6.81‰ at 15 and 30 °C, respectively. This work provides a novel arrangement determining the crucial parameters for identifying nitrate pollution sources in groundwater systems.

Long-term nitrogen isotope dynamics in Encelia farinosa reflect plant demographics and climate

While plant δ¹⁵ N values have been applied to understand nitrogen (N) dynamics, uncertainties regarding intraspecific and temporal variability currently limit their application. We used a 28 yr record of δ¹⁵ N values from two Mojave Desert populations of Encelia farinosa to clarify sources of population-level variability. We leveraged > 3500 foliar δ¹⁵ N observations collected alongside structural, physiological, and climatic data to identify plant and environmental contributors to δ¹⁵ N values. Additional sampling of soils, roots, stems, and leaves enabled assessment of the distribution of soil N content and δ¹⁵ N, intra-plant fractionations, and relationships between soil and plant δ¹⁵ N values. We observed extensive within-population variability in foliar δ¹⁵ N values and found plant age and foliar %N to be the strongest predictors of individual δ¹⁵ N values. There were consistent differences between root, stem, and leaf δ¹⁵ N values (spanning c. 3‰), but plant and bulk soil δ¹⁵ N values were unrelated. Plant-level variables played a strong role in influencing foliar δ¹⁵ N values, and interannual relationships between climate and δ¹⁵ N values were counter to previously recognized spatial patterns. This long-term record provides insights regarding the interpretation of δ¹⁵ N values that were not available from previous large-scale syntheses, broadly enabling more effective application of foliar δ¹⁵ N values.

Tracing the sources of phosphorus along the salinity gradient in a coastal estuary using multi-isotope proxies

Eutrophication in coastal water has compromised ecosystem services. Identification of phosphorus (P) sources and their load contributions are required for the development of effective nutrient management plans. In this research, multi-isotope proxies were applied to track P sources and evaluate their relative contributions in Love Creek, a coastal estuary in Delaware. The isotope values of carbon (ca. -22‰), nitrogen (ca.+6‰), and phosphate oxygen (ca.+18‰) of agricultural soils under different agricultural practices are generally similar even though their concentrations are distinctly different from forest soils (δ¹³C: ca. -27‰; δ¹³N: ca.+2‰; δ¹⁸O_P: ca.+22‰). Comparison of these parameters among potential land sources (agricultural soils, forest soils, septic wastes, and plant debris) and sink (colloids in water) revealed that the plant debris and soils from forest sources are likely dominant sources of P in freshwater sites. The contribution of terrestrial P sources gradually decreased along the salinity gradient and agricultural soil sources gradually dominanted in the saline water portion of the creek. The variations of P loads due to weather-related discharge, changing land use and activities, and seasons were high and reflected the limitation of accurate estimation of sources. Overall, these results provide improved insights into potential sources and biogeochemical processes in the estuary, which are expected to be useful for water quality monitoring programs.

Crossing a critical threshold: Accelerated and widespread land use changes drive recent carbon and nitrogen dynamics in Vichuquén Lake (35°S) in central Chile

Global afforestation/deforestation processes (e.g., Amazon deforestation and Europe afforestation) create new anthropogenic controls on carbon cycling and nutrient supply that have not been fully assessed. Here, we use a watershed-lake dynamics approach to investigate how human-induced land cover changes have altered nutrient transference during the last 700 years in a mediterranean coastal area (Vichuquén Lake). We compare our multiproxy reconstruction with historical documentation and use satellite images to reconstruct land use/cover changes for the last 45 years. Historical landscape changes, including those during the indigenous settlements, Spanish conquest, and the Chilean Republic up to mid-20th century did not significantly alter sediment and nutrient fluxes to the lake. In contrast, the largest changes in the lake-watershed system occurred in the mid-20th century and particularly after the 1980s-90s and were characterized by a large increase in total nitrogen and organic carbon fluxes as well as negative shifts in sediment δ¹⁵N and δ¹³C values. This shift was coeval with the largest land cover transformation in the Vichuquén watershed, as native forests nearly disappeared while anthropogenic tree plantations expanded up to 60% of the surface area.

Conservation with elevated elephant densities sequesters carbon in soils despite losses of woody biomass

Nature conservation and restoration in terrestrial ecosystems is often focused on increasing the numbers of megafauna, expecting them to have positive impacts on ecological self-regulation processes and biodiversity. In sub-Saharan Africa, conservation efforts also aspire to protect and enhance biodiversity with particular focus on elephants. However, elephant browsing carries the risk of woody biomass losses. In this context, little is known about how increasing elephant numbers affects carbon stocks in soils, including the subsoils. We hypothesized that (1) increasing numbers of elephants reduce tree biomass, and thus the amount of C stored therein, resulting (2) in a loss of soil organic carbon (SOC). If true, a negative carbon footprint could limit the sustainability of elephant conservation from a global carbon perspective. To test these hypotheses, we selected plots of low, medium, and high elephant densities in two national parks and adjacent conservancies in the Namibian component of the Kavango Zambezi Transfrontier Area (KAZA), and quantified carbon storage in both woody vegetation and soils (1 m). Analyses were supplemented by the assessment of soil carbon isotopic composition. We found that increasing elephant densities resulted in a loss of tree carbon storage by 6.4 t ha^-1 . However, and in contrast to our second hypothesis, SOC stocks increased by 4.7 t ha^-1 with increasing elephant densities. These higher SOC stocks were mainly found in the topsoil (0-30 cm) and were largely due to the formation of SOC from woody biomass. A second carbon input source into the soils was megaherbivore dung, which contributed with 0.02-0.323 t C ha^-1  year^-1 to ecosystem carbon storage in the low and high elephant density plots, respectively. Consequently, increasing elephant density does not necessarily lead to a negative C footprint, as soil carbon sequestration and transient C storage in dung almost compensate for losses in tree biomass.

A suite of rare microbes interacts with a dominant, heritable, fungal endophyte to influence plant trait expression

Endophytes are microbes that live, for at least a portion of their life history, within plant tissues. Endophyte assemblages are often composed of a few abundant taxa and many infrequently observed, low-biomass taxa that are, in a word, rare. The ways in which most endophytes affect host phenotype are unknown; however, certain dominant endophytes can influence plants in ecologically meaningful ways-including by affecting growth and immune system functioning. In contrast, the effects of rare endophytes on their hosts have been unexplored, including how rare endophytes might interact with abundant endophytes to shape plant phenotype. Here, we manipulate both the suite of rare foliar endophytes (including both fungi and bacteria) and Alternaria fulva-a vertically transmitted and usually abundant fungus-within the fabaceous forb Astragalus lentiginosus. We report that rare, low-biomass endophytes affected host size and foliar %N, but only when the heritable fungal endophyte (A. fulva) was not present. A. fulva also reduced plant size and %N, but these deleterious effects on the host could be offset by a negative association we observed between this heritable fungus and a foliar pathogen. These results demonstrate how interactions among endophytic taxa determine the net effects on host plants and suggest that the myriad rare endophytes within plant leaves may be more than a collection of uninfluential, commensal organisms, but instead have meaningful ecological roles.