Optimizing irrigation and nitrogen addition to balance grassland biomass production with greenhouse gas emissions: A mesocosm study

Achieving a balance between greenhouse gas mitigation and biomass production in grasslands necessitates optimizing irrigation frequency and nitrogen addition, which significantly influence grassland productivity and soil nitrous oxide emissions, and consequently impact the ecosystem carbon dioxide exchange. This study aimed to elucidate these influences using a controlled mesocosm experiment where bermudagrass (Cynodon dactylon L.) was cultivated under varied irrigation frequencies (daily and every 6 days) with (100 kg ha-1) or without nitrogen addition; measurements of net ecosystem carbon dioxide exchange, ecosystem respiration, soil respiration, and nitrous oxide emissions across two cutting events were performed as well. The findings revealed a critical interaction between water-filled pore space, regulated by irrigation, and nitrogen availability, with the latter exerting a more substantial influence on aboveground biomass growth and ecosystem carbon dioxide exchange than water availability. Moreover, the total dry matter was significantly higher with nitrogen addition compared to without nitrogen addition, irrespective of the irrigation frequency. In contrast, soil nitrous oxide emissions were observed to be significantly higher with increased irrigation frequency and nitrogen addition. The effects of nitrogen addition on soil respiration components appeared to depend on water availability, with autotrophic respiration seeing a significant rise with nitrogen addition under limited irrigation (5.4 ± 0.6 μmol m-2 s-1). Interestingly, the lower irrigation frequency did not result in water stress, suggesting resilience in bermudagrass. These findings highlight the importance of considering interactions between irrigation and nitrogen addition to optimize water and nitrogen input in grasslands for a synergistic balance between grassland biomass production and greenhouse gas emission mitigation.

General reversal of N-decomposition relationship during long-term decomposition in boreal and temperate forests

Decomposition of dead organic matter is fundamental to carbon (C) and nutrient cycling in terrestrial ecosystems, influencing C fluxes from the biosphere to the atmosphere. Theory predicts and evidence strongly supports that the availability of nitrogen (N) limits litter decomposition. Positive relationships between substrate N concentrations and decomposition have been embedded into ecosystem models. This decomposition paradigm, however, relies on data mostly from short-term studies analyzing controls on early-stage decomposition. We present evidence from three independent long-term decomposition investigations demonstrating that the positive N-decomposition relationship is reversed and becomes negative during later stages of decomposition. First, in a 10-y decomposition experiment across 62 woody species in a temperate forest, leaf litter with higher N concentrations exhibited faster initial decomposition rates but ended up a larger recalcitrant fraction decomposing at a near-zero rate. Second, in a 5-y N-enrichment experiment of two tree species, leaves with experimentally enriched N concentrations had faster decomposition initial rates but ultimately accumulated large slowly decomposing fractions. Measures of amino sugars on harvested litter in two experiments indicated that greater accumulation of microbial residues in N-rich substrates likely contributed to larger slowly decomposing fractions. Finally, a database of 437 measurements from 120 species in 45 boreal and temperate forest sites confirmed that higher N concentrations were associated with a larger slowly decomposing fraction. These results challenge the current treatment of interactions between N and decomposition in many ecosystems and Earth system models and suggest that even the best-supported short-term controls of biogeochemical processes might not predict long-term controls.

Soil respiration and its response to climate change and anthropogenic factors in a karst plateau wetland, southwest China

It is important to investigate the responses of greenhouse gases to climate change (temperature, precipitation) and anthropogenic factors in plateau wetland. Based on the DNDC model, we used meteorological, soil, and land cover data to simulate the soil CO2 emission pattern and its responses to climate change and anthropogenic factors in Guizhou, China. The results showed that the mean soil CO2 emission flux in the Caohai Karst Plateau Wetland was 5.89 ± 0.17 t·C·ha-1·yr-1 from 2000 to 2019, and the annual variation showed an increasing trend with the rate of 23.02 kg·C·ha-1·yr-1. The soil total annual mean CO2 emissions were 70.62 ± 2.04 Gg·C·yr-1 (annual growth rate was 0.28 Gg·C·yr-1). Caohai wetland has great spatial heterogeneity. The emissions around Caohai Lake were high (the areas with high, middle, and low values accounted for 3.07%, 70.96%, and 25.97%, respectively), and the emission pattern was characterized by a decrease in radiation from Caohai Lake to the periphery. In addition, the cropland and forest areas exhibited high intensities (7.21 ± 0.15 t·C·ha-1·yr-1 and 6.73 ± 0.58 t·C·ha-1·yr-1, respectively) and high total emissions (54.97 ± 1.16 Gg·C·yr-1 and 10.24 ± 0.88 Gg·C·yr-1, respectively). Croplands and forests were the major land cover types controlling soil CO2 emissions in the Caohai wetland, while anthropogenic factors (cultivation) significantly increased soil CO2 emissions. Results showed that the soil CO2 emissions were positively correlated with temperature and precipitation; and the temperature change had a greater impact on soil respiration than the change in precipitation. Our results indicated that future climate change (increased temperature and precipitation) may promote an increase in soil CO2 emissions in karst plateau wetlands, and reasonable control measures (e.g. returning cropland to lakes and reducing anthropogenic factors) are the keys to controlling CO2 emissions.

Spatial and temporal patterns of above- and below- ground biomass over the Tibet Plateau grasslands and their sensitivity to climate change

The sensitivity of grassland above- (AGB, gC m-2) and below-ground biomass (BGB, gC m-2) to climate has been shown to be significant on the Tibetan Plateau, however, the spatial patterns and sensitivity of biomass with altitudinal change needs to be quantitated. In this study, large data sets of AGB and BGB during the peak growth season, and the corresponding geographical and climate conditions in the grasslands of the Tibetan Plateau between 2001 and 2020 were analyzed, and modelled using a Cubist regression trees algorithm. The mean values for AGB and BGB were 61.3 and 1304.3 gC m-2, respectively, for the whole region over the two decades. There was a significant change in spatial AGB of 64.8 % on the Plateau (P < 0.05, with areas where AGB increased being twice as large as areas where AGB decreased), while BGB did not change significantly in majority the of the region (≥ 90.1 %, P > 0.05). In general, the areas where AGB showed positive partial correlations with precipitation were larger than the areas where AGB had positive correlations with temperature (P < 0.05). However, these trends varied depending on the climatic conditions: in the wetter regions, temperature had a greater effect on the size of the areas with positive AGB responses than precipitation (P < 0.05), while precipitation had a greater effect on the size of areas with positive BGB changes than temperature (P < 0.05). In the drier areas, however, precipitation affected the AGB response significantly compared to temperature (P < 0.05), while temperature influenced the BGB response greater than precipitation (P < 0.05). The response and sensitivity of grassland biomass to temperature and precipitation varied according to the altitude of the Plateau: the response and sensitivity were stronger and more sensitive at medium altitudes, and weak at the higher or lower altitudes. Likely, this phenomenon was resulted from the natural selection of plants to maintain the efficient use of resources during un-favourable and stressed conditions for maximum plant development and growth. These findings will help assess the ecological consequences of global climate change for the grasslands of the Tibetan Plateau, particularly in those regions with highly variable altitudes.

Earthworms and plants can decrease soil greenhouse gas emissions by modulating soil moisture fluctuations and soil macroporosity in a mesocosm experiment

Earthworms can stimulate microbial activity and hence greenhouse gas (GHG) emissions from soils. However, the extent of this effect in the presence of plants and soil moisture fluctuations, which are influenced by earthworm burrowing activity, remains uncertain. Here, we report the effects of earthworms (without, anecic, endogeic, both) and plants (with, without) on GHG (CO2, N2O) emissions in a 3-month greenhouse mesocosm experiment simulating a simplified agricultural context. The mesocosms allowed for water drainage at the bottom to account for the earthworm engineering effect on water flow during two drying-wetting cycles. N2O cumulative emissions were 34.6% and 44.8% lower when both earthworm species and only endogeic species were present, respectively, and 19.8% lower in the presence of plants. The presence of the endogeic species alone or in combination with the anecic species slightly reduced CO2 emissions by 5.9% and 11.4%, respectively, and the presence of plants increased emissions by 6%. Earthworms, plants and soil water content interactively affected weekly N2O emissions, an effect controlled by increased soil dryness due to drainage via earthworm burrows and mesocosm evapotranspiration. Soil macroporosity (measured by X-ray tomography) was affected by earthworm species-specific burrowing activity. Both GHG emissions decreased with topsoil macropore volume, presumably due to reduced moisture and microbial activity. N2O emissions decreased with macropore volume in the deepest layer, likely due to the presence of fewer anaerobic microsites. Our results indicate that, under experimental conditions allowing for plant and earthworm engineering effects on soil moisture, earthworms do not increase GHG emissions, and endogeic earthworms may even reduce N2O emissions.

Microbial carbon use efficiency promotes global soil carbon storage

Soils store more carbon than other terrestrial ecosystems1,2. How soil organic carbon (SOC) forms and persists remains uncertain1,3, which makes it challenging to understand how it will respond to climatic change3,4. It has been suggested that soil microorganisms play an important role in SOC formation, preservation and loss5-7. Although microorganisms affect the accumulation and loss of soil organic matter through many pathways4,6,8-11, microbial carbon use efficiency (CUE) is an integrative metric that can capture the balance of these processes12,13. Although CUE has the potential to act as a predictor of variation in SOC storage, the role of CUE in SOC persistence remains unresolved7,14,15. Here we examine the relationship between CUE and the preservation of SOC, and interactions with climate, vegetation and edaphic properties, using a combination of global-scale datasets, a microbial-process explicit model, data assimilation, deep learning and meta-analysis. We find that CUE is at least four times as important as other evaluated factors, such as carbon input, decomposition or vertical transport, in determining SOC storage and its spatial variation across the globe. In addition, CUE shows a positive correlation with SOC content. Our findings point to microbial CUE as a major determinant of global SOC storage. Understanding the microbial processes underlying CUE and their environmental dependence may help the prediction of SOC feedback to a changing climate.

Climate change stress alleviation through nature based solutions: A global perspective

Global climate change stress has greatly influenced agricultural crop production which leads to the global problems such as food security. To cope with global climate change, nature based solutions (NBS) are desirable because these lead to improve our environment. Environmental stresses such as drought and salinity are big soil problems and can be eradicated by increasing soil organic matter which is directly related to soil organic carbon (SOC). SOC is one of the key components of the worldwide carbon (C) cycle. Different types of land use patterns have shown significant impacts on SOC stocks. However, their effects on the various SOC fractions are not well-understood at the global level which make it difficult to predict how SOC changes over time. We aim to investigate changes in various SOC fractions, including mineral associated organic carbon (MAOC), mineral associated organic matter (MAOM), soil organic carbon (SOC), easily oxidized organic carbon (EOC), microbial biomass carbon (MBC) and particulate organic carbon (POC) under various types of land use patterns (NBS), including cropping pattern, residue management, conservation tillages such as no tillage (NT) and reduced tillage (RT) using data from 97 studies on a global scale. The results showed that NT overall increased MAOC, MAOM, SOC, MBC, EOC and POC by 16.2%, 26.8%, 24.1%, 16.2%, 27.9% and 33.2% (P < 0.05) compared to CT. No tillage with residue retention (NTR) increased MAOC, MAOM, SOC, MBC, EOC and POC by 38.0%, 29.9%, 47.5%, 33.1%, 35.7% and 49.0%, respectively, compared to CT (P < 0.05). RT overall increased MAOC, MAOM, SOC, MBC, EOC and POC by 36.8%, 14.1%, 25.8%, 25.9, 18.7% and 16.6% (P < 0.05) compared to CT. Reduced tillage with residue retention (RTR) increased MAOM, SOC and POC by 14.2%, 36.2% and 30.7%, respectively, compared to CT (P < 0.05). Multiple cropping increased MAOC, MBC and EOC by 14.1%, 39.8% and 21.5%, respectively, compared to mono cropping (P < 0.05). The response ratios of SOC fractions (MAOC, MAOM, SOC, MBC, EOC and POC) under NT and RT were mostly influenced by NBS such as residue management, cropping pattern along with soil depth, mean annual precipitation, mean annual temperature and soil texture. Our findings imply that when assessing the effects of conservation tillage methods on SOC sequestration, SOC fractions especially those taking part in driving soil biological activities, should be taken into account rather than total SOC. We conclude that conservation tillages under multiple cropping systems and with retention of crop residues enhance soil carbon sequestration as compared to CT in varying edaphic and climatic conditions of the world.

Integrative Alternative Tactics for Ixodid Control

Simple Summary

Hard ticks are important for economic and health reasons, and control has mainly relied upon use of synthetic acaricides. Contemporary development of resistance and concerns relating to health and environmental safety have elicited exploration into alternative tactics for hard tick management. Some examples of alternative tactics involve biological control, desiccant dusts, growth regulators, vaccines, cultural methods, and ingested medications.

Abstract

Ixodids (hard ticks), ectoparasitic arthropods that vector the causal agents of many serious diseases of humans, domestic animals, and wildlife, have become increasingly difficult to control because of the development of resistance against commonly applied synthetic chemical-based acaricides. Resistance has prompted searches for alternative, nonconventional control tactics that can be used as part of integrated ixodid management strategies and for mitigating resistance to conventional acaricides. The quest for alternative control tactics has involved research on various techniques, each influenced by many factors, that have achieved different degrees of success. Alternative approaches include cultural practices, ingested and injected medications, biological control, animal- and plant-based substances, growth regulators, and inert desiccant dusts. Research on biological control of ixodids has mainly focused on predators, parasitoid wasps, infective nematodes, and pathogenic bacteria and fungi. Studies on animal-based substances have been relatively limited, but research on botanicals has been extensive, including whole plant, extract, and essential oil effects on ixodid mortality, behavior, and reproduction. The inert dusts kaolin, silica gel, perlite, and diatomaceous earth are lethal to ixodids, and they are impervious to environmental degradation, unlike chemical-based toxins, remaining effective until physically removed.

Soil carbon stocks in temperate grasslands differ strongly across sites but are insensitive to decade-long fertilization

Enhancing soil carbon (C) storage has the potential to offset human-caused increases in atmospheric CO₂ . Rising CO₂ has occurred concurrently with increasing supply rates of biologically limiting nutrients such as nitrogen (N) and phosphorus (P). However, it is unclear how increased supplies of N and P will alter soil C sequestration, particularly in grasslands, which make up nearly a third of non-agricultural land worldwide. Here, we leverage a globally distributed nutrient addition experiment (the Nutrient Network) to examine how a decade of N and P fertilization (alone and in combination) influenced soil C and N stocks at nine grassland sites spanning the continental United States. We measured changes in bulk soil C and N stocks and in three soil C fractions (light and heavy particulate organic matter, and mineral-associated organic matter fractions). Nutrient amendment had variable effects on soil C and N pools that ranged from strongly positive to strongly negative, while soil C and N pool sizes varied by more than an order of magnitude across sites. Piecewise SEM clarified that small increases in plant C inputs with fertilization did not translate to greater soil C storage. Nevertheless, peak season aboveground plant biomass (but not root biomass or production) was strongly positively related to soil C storage at seven of the nine sites, and across all nine sites, soil C covaried with moisture index and soil mineralogy, regardless of fertilization. Overall, we show that site factors such as moisture index, plant productivity, soil texture, and mineralogy were key predictors of cross-site soil C, while nutrient amendment had weaker and site-specific effects on C sequestration. This suggests that prioritizing the protection of highly productive temperate grasslands is critical for reducing future greenhouse gas losses arising from land use change.

Management of phosphorus nutrient amid climate change for sustainable agriculture

Nutrients are essential for plant growth and development and influence overall agricultural production. Phosphorus (P) is a major nutrient required for many physiological and biochemical functions of a plant. Phosphate rock is the major source of phosphate fertilizer but is becoming increasingly limited in both developing and developed countries. The resources of phosphate rock need to be conserved, and import dependency on phosphate fertilizer needs to be minimized; this will help increase the availability of phosphate fertilizer over the next 300 yr. Climate change creates new challenges in the management of nutrients including P, affecting the overall production of crops. The availability, acquisition, and translocation of P are influenced by the fluctuation of temperatures, pH, drought, and elevated CO₂ . Both lower and higher soil temperatures reduce uptake and translocation of P. High soil pH affects P concentration and decreases the rate of plant P uptake. Low soil pH decreases the activity of soil microorganisms, the rate of transpiration, and P uptake and utilization. Elevated CO₂ decreases P uptake from soil by the plants. Future research is needed on chemical, molecular, microbiological, and physiological aspects to improve the understanding on how temperature, pH, drought, and elevated CO₂ affect the availability, acquisition, and transport of P by plants. Better P management strategies are required to secure the P supply to ensure long-term protection of soil fertility and to avoid environmental impacts such as eutrophication and water pollution, ensuring sustainable food production.

Positive Effects of Legumes on Soil Organic Carbon Stocks Disappear at High Legume Proportions Across Natural Grasslands in the Pyrenees

Soil is the largest terrestrial carbon pool, making it crucial for climate change mitigation. Soil organic carbon (SOC) is suggested to depend on biodiversity components, but much evidence comes from diversity-function experiments. To disentangle the relationships of plant guild diversity with SOC storage (kg m−2) at broad spatial scales, we applied diversity-interaction models to a regional grassland database (n = 96) including wide environmental conditions and management regimes. The questions were: (1) Are the effects of plant guilds on SOC stocks in natural grasslands consistent with those found in experimental systems? (2) Are plant guild effects on SOC stocks independent of each other or do they show interactive—synergistic or antagonistic—effects? (3) Do environmental variables, including abiotic and management, modify guild effects on SOC stocks? Among our most novel results we found, legume effects on grassland SOC vary depending on legume proportion consistently across broad spatial scales. SOC increased with legume proportion up to 7–17%, then decreased. Additionally, these effects were strengthened when grasses and forbs were codominant. Grazing intensity modulated grass proportion effects on SOC, being maximum at relatively high intensities. Interpreting our results in terms of existing contrasted ecological theories, we confirmed at broad spatial scales and under wide-ranging environmental conditions the positive effects of plant guild diversity on SOC, and we showed how legumes exert a keystone effect on SOC in natural grasslands, probably related to their ability to fix inorganic N. Niche complementarity effects were illustrated when codominance of forbs and grasses at optimum legume proportions boosted SOC storage, whereas grass dominance increased SOC stocks at medium–high grazing intensities. These findings can facilitate the preparation of regional and local strategies to ameliorate the soil capacity to absorb carbon.

Soil respiration variation along an altitudinal gradient in the Italian Alps: Disentangling forest structure and temperature effects

On the mountains, along an elevation gradient, we generally observe an ample variation in temperature, with the associated difference in vegetation structure and composition and soil properties. With the aim of quantifying the relative importance of temperature, vegetation and edaphic properties on soil respiration (SR), we investigated changes in SR along an elevation gradient (404 to 2101 m a.s.l) in the southern slopes of the Alps in Northern Italy. We also analysed soil physicochemical properties, including soil organic carbon (SOC) and nitrogen (N) stocks, fine root C and N, litter C and N, soil bulk densities and soil pH at five forest sites, and also stand structural properties, including vegetation height, age and basal area. Our results indicated that SR rates increased with temperature in all sites, and 55–76% of SR variability was explained by temperature. Annual cumulative SR, ranging between 0.65–1.40 kg C m^-2 yr^-1, decreased along the elevation gradient, while temperature sensitivity (Q10) of SR increased with elevation. However, a high SR rate (1.27 kg C m^-2 yr^-1) and low Q10 were recorded in the mature conifer forest stand at 1731 m a.s.l., characterized by an uneven-aged structure and high dominant tree height, resulting in a nonlinear relationship between elevation and temperature. Reference SR at 10°C (SR_ref) was unrelated to elevation, but was related to tree height. A significant negative linear relationship was found between bulk density and elevation. Conversely, SOC, root C and N stock, pH, and litter mass were best fitted by nonlinear relationships with elevation. However, these parameters were not significantly correlated with SR when the effect of temperature was removed (SR_ref). These results demonstrate that the main factor affecting SR in forest ecosystems along this Alpine elevation gradient is temperature, but its regulating role can be strongly influenced by site biological characteristics, particularly vegetation type and structure, affecting litter quality and microclimate. This study also confirms that high elevation sites are rich in SOC and more sensitive to climate change, being prone to high C losses as CO₂. Furthermore, our data indicate a positive relationship between Q10 and dominant tree height, suggesting that mature forest ecosystems characterized by an uneven-age structure, high SR_ref and moderate Q10, may be more resilient.

Spatial biases of information influence global estimates of soil respiration: How can we improve global predictions?

Soil respiration (Rs), the efflux of CO₂ from soils to the atmosphere, is a major component of the terrestrial carbon cycle, but is poorly constrained from regional to global scales. The global soil respiration database (SRDB) is a compilation of in situ Rs observations from around the globe that has been consistently updated with new measurements over the past decade. It is unclear whether the addition of data to new versions has produced better-constrained global Rs estimates. We compared two versions of the SRDB (v3.0 n = 5173 and v5.0 n = 10,366) to determine how additional data influenced global Rs annual sum, spatial patterns and associated uncertainty (1 km spatial resolution) using a machine learning approach. A quantile regression forest model parameterized using SRDBv3 yielded a global Rs sum of 88.6 Pg C year^-1 , and associated uncertainty of 29.9 (mean absolute error) and 57.9 (standard deviation) Pg C year^-1 , whereas parameterization using SRDBv5 yielded 96.5 Pg C year^-1 and associated uncertainty of 30.2 (mean average error) and 73.4 (standard deviation) Pg C year^-1 . Empirically estimated global heterotrophic respiration (Rh) from v3 and v5 were 49.9-50.2 (mean 50.1) and 53.3-53.5 (mean 53.4) Pg C year^-1 , respectively. SRDBv5's inclusion of new data from underrepresented regions (e.g., Asia, Africa, South America) resulted in overall higher model uncertainty. The largest differences between models parameterized with different SRDVB versions were in arid/semi-arid regions. The SRDBv5 is still biased toward northern latitudes and temperate zones, so we tested an optimized global distribution of Rs measurements, which resulted in a global sum of 96.4 ± 21.4 Pg C year^-1 with an overall lower model uncertainty. These results support current global estimates of Rs but highlight spatial biases that influence model parameterization and interpretation and provide insights for design of environmental networks to improve global-scale Rs estimates.

Adaptive multi-paddock grazing enhances soil carbon and nitrogen stocks and stabilization through mineral association in southeastern U.S. grazing lands

Grassland soils are a large reservoir of soil carbon (C) at risk of loss due to overgrazing in conventional grazing systems. By promoting regenerative grazing management practices that aim to increase soil C storage and soil health, grasslands have the potential to help alleviate rising atmospheric CO₂ as well as sustain grass productivity across a vast area of land. Previous research has shown that rotational grazing, specifically adaptive multi-paddock (AMP) grazing that utilizes short-duration rotational grazing at high stocking densities, can increase soil C stocks in grassland ecosystems, but the extent and mechanisms are unknown. We conducted a large-scale on-farm study on five "across the fence" pairs of AMP and conventional grazing (CG) grasslands covering a spectrum of southeast United States grazing lands. We quantified soil C and nitrogen (N) stocks, their isotopic and Fourier-transform infrared spectroscopy signatures as well as their distribution among soil organic matter (SOM) physical fractions characterized by contrasting mechanisms of formation and persistence in soils. Our findings show that the AMP grazing sites had on average 13% (i.e., 9 Mg C ha^-1) more soil C and 9% (i.e., 1 Mg N ha^-1) more soil N compared to the CG sites over a 1 m depth. Additionally, the stocks' difference was mostly in the mineral-associated organic matter fraction in the A-horizon, suggesting long-term persistence of soil C in AMP grazing farms. The higher N stocks and lower ¹⁵N abundance of AMP soils also point to higher N retention in these systems. These findings provide evidence that AMP grazing is a management strategy to sequester C in the soil and retain N in the system, thus contributing to climate change mitigation.

Real-time monitoring of rhizosphere nitrate fluctuations under crops following defoliation

BACKGROUND

Management regime can hugely influence the efficiency of crop production but measuring real-time below-ground responses is difficult. The combination of fertiliser application and mowing or grazing may have a major impact on roots and on the soil nutrient profile and leaching.

RESULTS

A novel approach was developed using low-cost ion-selective sensors to track nitrate (NO₃^-) movement through soil column profiles sown with the forage crops, Lolium perenne and Medicago sativa. Applications of fertiliser, defoliation of crops and intercropping of the grass and the legume were tested. Sensor measurements were compared with conventional testing of lysimeter and leachate samples. There was little leaching of NO₃^- through soil profiles with current management practices, as monitored by both methods. After defoliation, the measurements detected a striking increase in soil NO₃^- in the middle of the column where the greatest density of roots was found. This phenomenon was not detected when no NO₃^- was applied, and when there was no defoliation, or during intercropping with Medicago.

CONCLUSION

Mowing or grazing may increase rhizodeposition of carbon that stimulates soil mineralization to release NO₃^- that is acquired by roots without leaching from the profile. The soil columns and sensors provided a dynamic insight into rhizosphere responses to changes in above-ground management practices.

The Syringate O-Demethylase Gene of Sphingobium sp. Strain SYK-6 Is Regulated by DesX, while Other Vanillate and Syringate Catabolism Genes Are Regulated by DesR

Syringate and vanillate are the major metabolites of lignin biodegradation. In Sphingobium sp. strain SYK-6, syringate is O demethylated to gallate by consecutive reactions catalyzed by DesA and LigM, and vanillate is O demethylated to protocatechuate by a reaction catalyzed by LigM. The gallate ring is cleaved by DesB, and protocatechuate is catabolized via the protocatechuate 4,5-cleavage pathway. The transcriptions of desA, ligM, and desB are induced by syringate and vanillate, while those of ligM and desB are negatively regulated by the MarR-type transcriptional regulator DesR, which is not involved in desA regulation. Here, we clarified the regulatory system for desA transcription by analyzing the IclR-type transcriptional regulator desX, located downstream of desA Quantitative reverse transcription (RT)-PCR analyses of a desX mutant indicated that the transcription of desA was negatively regulated by DesX. In contrast, DesX was not involved in the regulation of ligM and desB The ferulate catabolism genes (ferBA), under the control of a MarR-type transcriptional regulator, FerC, are located upstream of desA RT-PCR analyses suggested that the ferB-ferA-SLG_25010-desA gene cluster consists of the ferBA operon and the SLG_25010-desA operon. Promoter assays revealed that a syringate- and vanillate-inducible promoter is located upstream of SLG_25010. Purified DesX bound to this promoter region, which overlaps an 18-bp inverted-repeat sequence that appears to be essential for the DNA binding of DesX. Syringate and vanillate inhibited the DNA binding of DesX, indicating that the compounds are effector molecules of DesX.IMPORTANCE Syringate is a major degradation product in the microbial and chemical degradation of syringyl lignin. Along with other low-molecular-weight aromatic compounds, syringate is produced by chemical lignin depolymerization. Converting this mixture into value-added chemicals using bacterial metabolism (i.e., biological funneling) is a promising option for lignin valorization. To construct an efficient microbial lignin conversion system, it is necessary to identify and characterize the genes involved in the uptake and catabolism of lignin-derived aromatic compounds and to elucidate their transcriptional regulation. In this study, we found that the transcription of desA, encoding syringate O-demethylase in SYK-6, is regulated by an IclR-type transcriptional regulator, DesX. The findings of this study, combined with our previous results on desR (encoding a MarR transcriptional regulator that controls the transcription of ligM and desB), provide an overall picture of the transcriptional-regulatory systems for syringate and vanillate catabolism in SYK-6.

The Grain for Green Project May Enrich the Mercury Concentration in a Small Karst Catchment, Southwest China

The Chinese project, better known as the Grain for Green Project (GGP), has changed the land-use type in the karst area of Puding county, Guizhou province, southwest China, and this study is aimed at evaluating the Hg distribution and determining factors in soils after the land-use change. A total of ten soil profiles were selected in the typical karst region, and the land-use types were divided into native vegetation land (NVL), farmland (FL), and abandoned farmland (AFL). Total Hg concentration under different land-use types increased in the order: NVL (average 63.26 μg∙kg−1) < FL (average 71.48 μg∙kg−1) < AFL (average 98.22 μg∙kg−1). After agricultural abandonment for four to five years with a cover of native vegetation in the AFL, a higher concentration of Hg compared to the other two land-use types indicate that the Hg accumulation in soil results from vegetation restoration of AFL due to land-use change. Soil organic carbon (SOC) and macro-aggregates were highly correlated to Hg concentration in this study. Macro-aggregates can provide a stable condition for Hg due to the thin regolith and high porosity in the karst region. A high proportion of macro-aggregates can reduce the mobility of Hg in the karst area. Intense tillage can significantly reduce the formation of macro-aggregates in FL, but the macro-aggregates in AFL were recovered as well as those in NVL, resulting in the accumulation of Hg.

Effects of Experimental Throughfall Exclusion on Soil Respiration in a Continental Coniferous Stand, South Korea

Severe droughts and changing precipitation patterns could alter the biogeochemical properties of the soil, affecting soil carbon cycles in forest ecosystems. A throughfall exclusion (TFE) experiment was conducted in a continental climate coniferous stand in Gangwon Province, Korea, to examine the effects of excluding rainfall on total soil respiration (SR), heterotrophic soil respiration (HR), autotrophic soil respiration (AR), sapling diameter growth, and soil bacterial communities from July 2016 to October 2017. The soil water content (SWC) was significantly decreased by the exclusion of the throughfall, resulting in changes in the bacterial communities, and subsequently a decrease in HR. Although AR did not present significant differences between the control and TFE plots, the rate of sapling growth was significantly lower in the TFE plots compared with that in the control plots. An exponential function relating SR to soil temperature accounted for 0.61% and 0.82% of the variance in SR in the control and TFE plots, respectively (Q10 = 2.48 and 2.86, respectively). Furthermore, a multivariate nonlinear model based on soil temperature and SWC explained 0.89% and 0.88% of the variance in SR in the control and TFE plots, respectively. When soil temperature was high, SR showed high fluctuations due to SWC variation. However, when SWC was low, we detected relatively small fluctuations in SR due to soil temperature. The results of this study show that the activity of soil microbial and root respiration during the growing season may be lower under future drought conditions.

Interaction of abiotic factor on soil CO2 efflux in three forest communities in tropical deciduous forest from India

Environmental factors along with soil physico-chemical properties play a significant role on the diurnal trend of soil CO₂ efflux. Soil CO₂ efflux in Indian tropical forests is poorly studied. We studied the soil CO₂ efflux in a representative tropical deciduous forest at Katerniaghat Wildlife Sanctuary (KWLS), Uttar Pradesh. The three forest communities namely dry mixed (DMF), Sal mixed (SMF), and Teak plantation (TPF) were selected for measuring soil CO₂ efflux in the summer season during April to May 2017 using automated LI-COR 8100 soil CO₂ flux system. Soil physico-chemical parameters were also studied in the three abovementioned forest communities. We also measured the different microclimatic variables at forest understorey in all three communities during the summer season. Total day time soil CO₂ efflux of 826.70, 1089.24, and 828.94 (μmolCO₂ m^-2d^-1) was observed in TPF, SMF, and DMF respectively. Soil CO₂ efflux observed significant differences (P < 0.01) among the three forest communities studied for the summer season in tropical deciduous forest of Terai Himalaya. Average soil CO₂ efflux rate (μmol CO₂ m^-2 s^-1) of 4.06 ± 0.36, 5.03 ± 0.45, and 4.37 ± 0.79 was observed in TPF, SMF, and DMF, respectively, which is positively correlated with total organic carbon (TOC) and water holding capacity (WHC) among soil physico-chemical variables. Among microclimatic variables, soil temperature (ST, °C) and air temperature (AT, °C) observed strong positive correlation with day time soil CO₂ efflux in all three communities. Significant increase in soil CO₂ flux was observed with increasing air and soil temperature (AT and ST) in DMF and SMF. Maximum TOC of 19.23 g Kg^-1 was observed in SMF among all communities in the summer season. The result showed that soil CO₂ efflux is closely associated with TOC, WHC, AT, and ST for Indian deciduous forest ecosystems.

Non-rainfall Moisture: A Key Driver of Microbial Respiration from Standing Litter in Arid, Semiarid, and Mesic Grasslands

Abstract

Models assume that rainfall is the major moisture source driving decomposition. Non-rainfall moisture (NRM: high humidity, dew, and fog) can also induce standing litter decomposition, but there have been few measurements of NRM-mediated decomposition across sites and no efforts to extrapolate the contribution of NRM to larger scales to assess whether this mechanism can improve model predictions. Here, we show that NRM is an important, year-round source of moisture in grassland sites with contrasting moisture regimes using field measurements and modeling. We first characterized NRM frequency and measured NRM-mediated decomposition at two sites in the Namib Desert, Namibia (hyper-arid desert), and at one site in Iowa, USA (tallgrass prairie). NRM was frequent at all sites (85–99% of hours that litter was likely to be wet were attributed to NRM) and tended to occur in cool, high-humidity periods for several hours or more at a time. NRM also resulted in CO2 release from microbes in standing litter at all sites when litter became sufficiently wet (> 5% gravimetric moisture for fine litter and > 13% for coarse), and significantly contributed to mass loss, particularly in the western Namib site that received almost no rain. When we modeled annual mass loss induced by NRM and rain and extrapolated our characterization of NRM decomposition to a final semiarid site (Sevilleta, New Mexico), we found that models driven by rainfall alone underestimated mass loss, while including NRM resulted in estimates within the range of observed mass loss. Together these findings suggest that NRM is an important missing component in quantitative and conceptual models of litter decomposition, but there is nuance involved in modeling NRM at larger scales. Specifically, temperature and physical features of the substrate emerge as factors that affect the microbial response to litter wetting under NRM in our sites, and require further study. Hourly humidity can provide an adequate proxy of NRM frequency, but site-specific calibration with litter wetness is needed to accurately attribute decomposition to periods when NRM wets litter. Greater recognition of NRM-driven decomposition and its interaction with other processes like photodegradation is needed, especially since fog, dew, and humidity are likely to shift under future climates.

Regulation of vanillate and syringate catabolism by a MarR-type transcriptional regulator DesR in Sphingobium sp. SYK-6

Vanillate and syringate are major intermediate metabolites generated during the microbial degradation of lignin. In Sphingobium sp. SYK-6, vanillate is O demethylated to protocatechuate by LigM; protocatechuate is then catabolized via the protocatechuate 4,5-cleavage pathway. Syringate is O demethylated to gallate by consecutive reactions catalyzed by DesA and LigM, and then gallate is subjected to ring cleavage by DesB. Here, we investigated the transcriptional regulation of desA, ligM, and desB involved in vanillate and syringate catabolism. Quantitative reverse transcription-PCR analyses indicated that the transcription of these genes was induced 5.8–37-fold in the presence of vanillate and syringate. A MarR-type transcriptional regulator, SLG_12870 (desR), was identified as the gene whose product bound to the desB promoter region. Analysis of a desR mutant indicated that the transcription of desB, ligM, and desR is negatively regulated by DesR. Purified DesR bound to the upstream regions of desB, ligM, and desR, and the inverted repeat sequences similar to each other in these regions were suggested to be essential for DNA binding of DesR. Vanillate and syringate inhibited DNA binding of DesR, indicating that these compounds are effector molecules of DesR. The transcription of desA was found to be regulated by an as-yet unidentified regulator.

Soil respiration of four forests along elevation gradient in northern subtropical China

BACKGROUND AND AIMS

Soil respiration is the second-largest terrestrial carbon (C) flux, and soil temperature and soil moisture are the main drivers of temporal variation in soil respiration and its components. Here, we quantified the contribution of soil temperature, soil moisture, and their intersection on the variation in soil respiration and its components of the evergreen broad-leaved forests (EBF), mixed evergreen and deciduous broad-leaved forests (MF), deciduous broad-leaved forests (DBF), and subalpine coniferous forests (CF) along an elevation gradient.

METHODS

We measured soil respiration of four types of forests along the elevation gradient in Shennongjia, Hubei China based on the trenching experiments. We parameterized the relationships between soil respiration and soil temperature, soil moisture, and quantified the intersection of temperature and moisture on soil respiration and its components.

RESULTS

Total soil respiration (R S), heterotrophic respiration (R H), and autotrophic respiration (R A) were significantly correlated with soil temperature in all four forests. The Q 10 value of soil respiration significantly differed among the four types of forest, and the Q 10 was 3.06 for EBF, 3.75 for MF, 4.05 for DBF, and 4.49 for CF, respectively. The soil temperature explained 62%-81% of the variation in respiration, while soil temperature and soil moisture together explained 91%-97% of soil respiration variation for the four types of forests. The variation from the intersection of soil temperature and moisture were 12.1%-25.0% in RS, 1.0%-7.0% in R H, and 17.1%-19.6% in R A, respectively.

CONCLUSIONS

Our results show that the temperature sensitivity (Q 10) of soil respiration increased with elevation. The intersection between soil temperature and soil moisture had strong effects on soil respiration, especially in R H. We demonstrated that the intersection effects between soil temperature and soil moisture on soil respiration were essential to understand the response of soil respiration and its components to climate change.

Effects of Soil Aggregate Stability on Soil Organic Carbon and Nitrogen under Land Use Change in an Erodible Region in Southwest China

Soil aggregate stability can indicate soil quality, and affects soil organic carbon (SOC) and soil organic nitrogen (SON) sequestration. However, for erodible soils, the effects of soil aggregate stability on SOC and SON under land use change are not well known. In this study, soil aggregate distribution, SOC and SON content, soil aggregate stability, and soil erodibility were determined in the soils at different depths along the stages following agricultural abandonment, including cropland, abandoned cropland, and native vegetation land in an erodible region of Southwest China. Soil aggregation, soil aggregate stability, and SOC and SON content in the 0–20 cm depth soils increased after agricultural abandonment, but soil texture and soil erodibility were not affected by land use change. Soil erodibility remained in a low level when SOC contents were over 20 g·kg⁻¹, and it significantly increased with the loss of soil organic matter (SOM). The SOC and SON contents increased with soil aggregate stability. This study suggests that rapidly recovered soil aggregate stability after agricultural abandonment promotes SOM sequestration, whereas sufficient SOM can effectively maintain soil quality in karst ecological restoration.

Measuring Regional Atmospheric CO2 Concentrations in the Lower Troposphere with a Non-Dispersive Infrared Analyzer Mounted on a UAV, Ogata Village, Akita, Japan

We have developed a simple measuring system prototype that uses an unmanned aerial vehicle (UAV) and a non-dispersive infrared (NDIR) analyzer to detect regional carbon dioxide (CO2) concentrations and obtain vertical CO2 distributions. Here, we report CO2 measurement results for the lower troposphere above Ogata Village, Akita Prefecture, Japan (about 40° N, 140° E, approximately −1 m amsl), obtained with this UAV system. The actual flight observations were conducted at 500, 400, 300, 200, 100, and 10 m above the ground, at least once a month during the daytime from February 2018 to February 2019. The raw CO2 values from the NDIR were calibrated by two different CO2 standard gases and high-purity nitrogen (N2) gas (as a CO2 zero gas; 0 ppm). During the observation period, the maximum CO2 concentration was measured in February 2019 and the minimum in August 2018. In all seasons, CO2 concentrations became higher as the flight altitude was increased. The monthly pattern of observed CO2 changes is similar to that generally observed in the Northern Hemisphere as well as to surface CO2 changes simulated by an atmospheric transport model of the Japan Meteorological Agency. It is highly probable that these changes reflect the vegetation distribution around the study area.

Towards Quantifying Carrion Biomass in Ecosystems

The decomposition of animal biomass (carrion) contributes to the recycling of energy and nutrients through ecosystems. Whereas the role of plant decomposition in ecosystems is broadly recognised, the significance of carrion to ecosystem functioning remains poorly understood. Quantitative data on carrion biomass are lacking and there is no clear pathway towards improved knowledge in this area. Here, we present a framework to show how quantities derived from individual carcasses can be scaled up using population metrics, allowing for comparisons among ecosystems and other forms of biomass. Our framework facilitates the generation of new data that is critical to building a quantitative understanding of the contribution of carrion to trophic processes and ecosystem stocks and flows.

Spatial-Temporal Changes of Soil Respiration across China and the Response to Land Cover and Climate Change

Soil respiration (Rs) plays an important role in the carbon budget of terrestrial ecosystems. Quantifying the spatial and temporal variations in Rs in China at the regional scale helps improve our understanding of the variations in terrestrial carbon budgets that occur in response to global climate and environmental changes and potential future control measures. In this study, we used a regional-scale geostatistical model that incorporates gridded meteorological and pedologic data to evaluate the spatial Rs variations in China from 2000 to 2013. We analysed the relationship between Rs and environmental factors, and suggest management strategies that may help to keep the terrestrial carbon balance. The simulated results demonstrate that the mean annual Rs value over these 14 years was 422 g/m2/year, and the corresponding total amount was 4.01 Pg C/year. The Rs estimation displayed a clear spatial pattern and a slightly increasing trend. Further analysis also indicated that high Rs values may occur in areas that show a greater degree of synchronicity in the timing of their optimal temperature and moisture conditions. Moreover, cultivated vegetation exhibits higher Rs values than native vegetation. Finally, we suggest that specific conservation efforts should be focused on ecologically sensitive areas where the Rs values increase significantly.

Interspecific variation in responses to microclimate by terrestrial isopods: implications in relation to climate change

The importance of considering species-specific biotic interactions when predicting feedbacks between the effects of climate change and ecosystem functions is becoming widely recognised. The responses of soil animals to predicted changes in global climate could potentially have far-reaching consequences for fluxes of soil carbon, including climatic feedbacks resulting from increased emissions of carbon dioxide from soils. The responses of soil animals to different microclimates can be summarised as norms of reaction, in order to compare phenotypic differences in traits along environmental gradients. Thermal and moisture reaction norms for physiological, behavioural and life history traits of species of terrestrial isopods differing in their morphological adaptations for reducing water loss are presented. Gradients of moisture reaction norms for respiratory rates and thermal reaction norms for water loss, for a species from the littoral zone were steeper than those for species from mesic environments. Those for mesic species were steeper than for those from xeric habitats. Within mesic species, gradients of thermal reaction norms for aggregation were steeper for Oniscusasellus than for Porcellioscaber or Armadilliumvulgare, and moisture reaction norms for sheltering and feeding behaviours were steeper for Philosciamuscorum than for either P.scaber or A.vulgare. These differences reflect differences in body shape, permeability of the cuticle, and development of pleopodal lungs. The implications of differences between different species of soil animals in response to microclimate on the possible influence of the soil fauna on soil carbon dynamics under future climates are discussed. In conclusion a modelling approach to bridging the inter-disciplinary gap between carbon cycling and the biology of soil animals is recommended.

SCFSen: A Sensor Node for Regional Soil Carbon Flux Monitoring

Estimation of regional soil carbon flux is very important for the study of the global carbon cycle. The spatial heterogeneity of soil respiration prevents the actual status of regional soil carbon flux from being revealed by measurements of only one or a few spatial sampling positions, which are usually used by traditional studies for the limitation of measurement instruments, so measuring in many spatial positions is very necessary. However, the existing instruments are expensive and cannot communicate with each other, which prevents them from meeting the requirement of synchronous measurements in multiple positions. Therefore, we designed and implemented an instrument for soil carbon flux measuring based on dynamic chamber method, SCFSen, which can measure soil carbon flux and communicate with each other to construct a sensor network. In its working stage, a SCFSen node measures the concentration of carbon in the chamber with an infrared carbon dioxide sensor for certain times periodically, and then the changing rate of the measurements is calculated, which can be converted to the corresponding value of soil carbon flux in the position during the short period. A wireless sensor network system using SCFSens as soil carbon flux sensing nodes can carry out multi-position measurements synchronously, so as to obtain the spatial heterogeneity of soil respiration. Furthermore, the sustainability of such a wireless sensor network system makes the temporal variability of regional soil carbon flux can also be obtained. So SCFSen makes thorough monitoring and accurate estimation of regional soil carbon flux become more feasible.

Taxonomic and Functional Responses of Soil Microbial Communities to Annual Removal of Aboveground Plant Biomass

Clipping, removal of aboveground plant biomass, is an important issue in grassland ecology. However, few studies have focused on the effect of clipping on belowground microbial communities. Using integrated metagenomic technologies, we examined the taxonomic and functional responses of soil microbial communities to annual clipping (2010-2014) in a grassland ecosystem of the Great Plains of North America. Our results indicated that clipping significantly (P < 0.05) increased root and microbial respiration rates. Annual temporal variation within the microbial communities was much greater than the significant changes introduced by clipping, but cumulative effects of clipping were still observed in the long-term scale. The abundances of some bacterial and fungal lineages including Actinobacteria and Bacteroidetes were significantly (P < 0.05) changed by clipping. Clipping significantly (P < 0.05) increased the abundances of labile carbon (C) degrading genes. More importantly, the abundances of recalcitrant C degrading genes were consistently and significantly (P < 0.05) increased by clipping in the last 2 years, which could accelerate recalcitrant C degradation and weaken long-term soil carbon stability. Furthermore, genes involved in nutrient-cycling processes including nitrogen cycling and phosphorus utilization were also significantly increased by clipping. The shifts of microbial communities were significantly correlated with soil respiration and plant productivity. Intriguingly, clipping effects on microbial function may be highly regulated by precipitation at the interannual scale. Altogether, our results illustrated the potential of soil microbial communities for increased soil organic matter decomposition under clipping land-use practices.

Distinct responses of soil respiration to experimental litter manipulation in temperate woodland and tropical forest

Global change is affecting primary productivity in forests worldwide, and this, in turn, will alter long‐term carbon (C) sequestration in wooded ecosystems. On one hand, increased primary productivity, for example, in response to elevated atmospheric carbon dioxide (CO₂), can result in greater inputs of organic matter to the soil, which could increase C sequestration belowground. On other hand, many of the interactions between plants and microorganisms that determine soil C dynamics are poorly characterized, and additional inputs of plant material, such as leaf litter, can result in the mineralization of soil organic matter, and the release of soil C as CO₂ during so‐called “priming effects”. Until now, very few studies made direct comparison of changes in soil C dynamics in response to altered plant inputs in different wooded ecosystems. We addressed this with a cross‐continental study with litter removal and addition treatments in a temperate woodland (Wytham Woods) and lowland tropical forest (Gigante forest) to compare the consequences of increased litterfall on soil respiration in two distinct wooded ecosystems. Mean soil respiration was almost twice as high at Gigante (5.0 μmol CO₂ m⁻² s⁻¹) than at Wytham (2.7 μmol CO₂ m⁻² s⁻¹) but surprisingly, litter manipulation treatments had a greater and more immediate effect on soil respiration at Wytham. We measured a 30% increase in soil respiration in response to litter addition treatments at Wytham, compared to a 10% increase at Gigante. Importantly, despite higher soil respiration rates at Gigante, priming effects were stronger and more consistent at Wytham. Our results suggest that in situ priming effects in wooded ecosystems track seasonality in litterfall and soil respiration but the amount of soil C released by priming is not proportional to rates of soil respiration. Instead, priming effects may be promoted by larger inputs of organic matter combined with slower turnover rates.

Bacterial catabolism of lignin-derived aromatics: New findings in a recent decade: Update on bacterial lignin catabolism

Lignin is the most abundant phenolic polymer; thus, its decomposition by microorganisms is fundamental to carbon cycling on earth. Lignin breakdown is initiated by depolymerization catalysed by extracellular oxidoreductases secreted by white-rot basidiomycetous fungi. On the other hand, bacteria play a predominant role in the mineralization of lignin-derived heterogeneous low-molecular-weight aromatic compounds. The outline of bacterial catabolic pathways for lignin-derived bi- and monoaryls are typically composed of the following sequential steps: (i) funnelling of a wide variety of lignin-derived aromatics into vanillate and syringate, (ii) O demethylation of vanillate and syringate to form catecholic derivatives and (iii) aromatic ring-cleavage of the catecholic derivatives to produce tricarboxylic acid cycle intermediates. Knowledge regarding bacterial catabolic systems for lignin-derived aromatic compounds is not only important for understanding the terrestrial carbon cycle but also valuable for promoting the shift to a low-carbon economy via biological lignin valorisation. This review summarizes recent progress in bacterial catabolic systems for lignin-derived aromatic compounds, including newly identified catabolic pathways and genes for decomposition of lignin-derived biaryls, transcriptional regulation and substrate uptake systems. Recent omics approaches on catabolism of lignin-derived aromatic compounds are also described.

Plant litter chemistry alters the content and composition of organic carbon associated with soil mineral and aggregate fractions in invaded ecosystems

Through the input of disproportionate quantities of chemically distinct litter, invasive plants may potentially influence the fate of organic matter associated with soil mineral and aggregate fractions in some of the ecosystems they invade. Although context dependent, these native ecosystems subjected to prolonged invasion by exotic plants may be instrumental in distinguishing the role of plant-microbe-mineral interactions from the broader edaphic and climatic influences on the formation of soil organic matter (SOM). We hypothesized that the soils subjected to prolonged invasion by an exotic plant that input recalcitrant litter (Japanese knotweed, Polygonum cuspidatum) would have a greater proportion of plant-derived carbon (C) in the aggregate fractions, as compared with that in adjacent soil inhabited by native vegetation that input labile litter, whereas the soils under an invader that input labile litter (kudzu, Pueraria lobata) would have a greater proportion of microbial-derived C in the silt-clay fraction, as compared with that in adjacent soils that receive recalcitrant litter. At the knotweed site, the higher C content in soils under P. cuspidatum, compared with noninvaded soils inhabited by grasses and forbs, was limited to the macroaggregate fraction, which was abundant in plant biomarkers. The noninvaded soils at this site had a higher abundance of lignins in mineral and microaggregate fractions and suberin in the macroaggregate fraction, partly because of the greater root density of the native species, which might have had an overriding influence on the chemistry of the above-ground litter input. At the kudzu site, soils under P. lobata had lower C content across all size fractions at a 0-5 cm soil depth despite receiving similar amounts of Pinus litter. Contrary to our prediction, the noninvaded soils receiving recalcitrant Pinus litter had a similar abundance of plant biomarkers across both mineral and aggregate fractions, potentially because of the higher surface area of soil minerals at this site. The plant biomarkers were lower in the aggregate fractions of the P. lobata-invaded soils, compared with noninvaded pine stands, potentially suggesting a microbial co-metabolism of pine-derived compounds. These results highlight the complex interactions among litter chemistry, soil biota, and minerals in mediating soil C storage in unmanaged ecosystems; these interactions are particularly important under global changes that may alter plant species composition and hence the quantity and chemistry of litter inputs in terrestrial ecosystems.

The temperature sensitivity of soil organic carbon decomposition is greater in subsoil than in topsoil during laboratory incubation

The turnover of soil organic carbon (SOC) in cropland plays an important role in terrestrial carbon cycling, but little is known about the temperature sensitivity (Q₁₀) of SOC decomposition below the topsoil layer of arable soil. Here, samples of topsoil (0–20 cm) and subsoil (20–40 cm) layers were obtained from paddy fields and upland croplands in two regions of China. Using a sequential temperature changing method, soil respiration rates were calculated at different temperatures (8 °C to 28 °C) and fitted to an exponential equation to estimate Q₁₀ values. The average SOC decomposition rate was 59% to 282% higher in the topsoil than in the subsoil layer because of higher labile carbon levels in the topsoil. However, Q₁₀ values in the topsoil layer (5.29 ± 1.47) were significantly lower than those in the subsoil layer (7.52 ± 1.84). The pattern of Q₁₀ values between the topsoil and subsoil was significantly negative to labile carbon content, which is consistent with the carbon quality-temperature hypothesis. These results suggest that the high temperature sensitivity of SOC decomposition in the subsoil layer needs to be considered in soil C models to better predict the responses of agricultural SOC pools to global warming.

Soil-soil solution distribution coefficient of soil organic matter is a key factor for that of radioiodide in surface and subsurface soils

We investigated the vertical distribution of the soil-soil-solution distribution coefficients (K_d) of ¹²⁵I, ¹³⁷Cs, and ⁸⁵Sr in organic-rich surface soil and organic-poor subsurface soil of a pasture and an urban forest near a spent-nuclear-fuel reprocessing plant in Rokkasho, Japan. K_d of ¹³⁷Cs was highly correlated with water-extractable K⁺. K_d of ⁸⁵Sr was highly correlated with water-extractable Ca²⁺ and SOC. K_d of ¹²⁵I^- was low in organic-rich surface soil, high slightly below the surface, and lowest in the deepest soil. This kinked distribution pattern differed from the gradual decrease of the other radionuclides. The thickness of the high-¹²⁵I^-K_d middle layer (i.e., with high radioiodide retention ability) differed between sites. K_d of ¹²⁵I^- was significantly correlated with K_d of soil organic carbon. Our results also showed that the layer thickness is controlled by the ratio of K_d-OC between surface and subsurface soils. This finding suggests that the addition of SOC might prevent further radioiodide migration down the soil profile. As far as we know, this is the first report to show a strong correlation of a soil characteristic with K_d of ¹²⁵I^-. Further study is needed to clarify how radioiodide is retained and migrates in soil.

Evaluation of factors affecting soil carbon sequestration services of stormwater wet retention ponds in varying climate zones

The carbon sequestration services of stormwater wet retention ponds were investigated in four different climates: U.S., Northern Sweden, Southern Sweden, and Singapore, representing a range of annual mean temperatures, growing season lengths and rainfall depths: geographic factors that were not statistically compared, but have great effect on carbon (C) accumulation. A chronosequence was used to estimate C accumulations rates; C accumulation and decomposition rates were not directly measured. C accumulated significantly over time in vegetated shallow water areas (0-30cm) in the USA (78.4gCm^-2yr^-1), in vegetated temporary inundation zones in Sweden (75.8gCm^-2yr^-1), and in all ponds in Singapore (135gCm^-2yr^-1). Vegetative production appeared to exert a stronger influence on relative C accumulation rates than decomposition. Comparing among the four climatic zones, the effects of increasing rainfall and growing season lengths (vegetative production) outweighed the effects of higher temperature on decomposition rates. Littoral vegetation was a significant source to the soil C pool relative to C sources draining from watersheds. Establishment of vegetation in the shallow water zones of retention ponds is vital to providing a C source to the soil. Thus, the width of littoral shelves containing this vegetation along the perimeter may be increased if C sequestration is a design goal. This assessment establishes that stormwater wet retention ponds can sequester C across different climate zones with generally annual rainfall and lengths of growing season being important general factors for C accumulation.

Potential of temperate agricultural soils for carbon sequestration: A meta-analysis of land-use effects

Restoring depleted soil organic carbon (SOC) stocks of arable land to remove carbon from the atmosphere and offset fossil fuel emissions is a promising strategy for the mitigation of climate change. In agroecosystems conservational tillage practices and the abandonment of formerly plowed fields (ex-arable land) are shown to have the highest potential to sequester SOC. Nevertheless reported sequestration rates vary and the effects of environmental site conditions remain poorly understood. Our results are based on a meta-analysis of 273 paired SOC estimates from 65 publications which included only mineral soils from the temperate zone. SOC stocks of ex-arable grasslands with an average of 14years since abandonment were 18% larger compared to the SOC of arable land. Likewise, SOC stocks of never-plowed grassland plots were 11% larger than the SOC stocks of abandoned fields. The average sequestration rate was 0.72t Cha(-1)yr(-1). Semi-arid and sub-humid climate as well as low initial SOC stocks positively affected proportional SOC gains suggesting that the recovery of carbon stocks is not limited by low primary production. Therefore, the northward shift of cultivation areas in the temperate zone will lead to the abandonment of soils with high SOC recovery potential. However, if native soils are opened up elsewhere to compensate for yield losses due to abandonment the surplus of SOC in ex-arable land can easily be overcompensated by cultivation losses.

Contribution of soil respiration to the global carbon equation

Soil respiration (Rs) is the second largest carbon flux next to GPP between the terrestrial ecosystem (the largest organic carbon pool) and the atmosphere at a global scale. Given their critical role in the global carbon cycle, Rs measurement and modeling issues have been well reviewed in previous studies. In this paper, we briefly review advances in soil organic carbon (SOC) decomposition processes and the factors affecting Rs. We examine the spatial and temporal distribution of Rs measurements available in the literature and found that most of the measurements were conducted in North America, Europe, and East Asia, with major gaps in Africa, East Europe, North Asia, Southeast Asia, and Australia, especially in dry ecosystems. We discuss the potential problems of measuring Rs on slope soils and propose using obliquely-cut soil collars to solve the existing problems. We synthesize previous estimates of global Rs flux and find that the estimates ranged from 50 PgC/yr to 98 PgC/yr and the error associated with each estimation was also high (4 PgC/yr to 33.2 PgC/yr). Using a newly integrated database of Rs measurements and the MODIS vegetation map, we estimate that the global annual Rs flux is 94.3 PgC/yr with an estimation error of 17.9 PgC/yr at a 95% confidence level. The uneven distribution of Rs measurements limits our ability to improve the accuracy of estimation. Based on the global estimation of Rs flux, we found that Rs is highly correlated with GPP and NPP at the biome level, highlighting the role of Rs in global carbon budgets.