Nitrogen Application Timing and Levels Affect the Fate and Budget of Fertilizer Nitrogen in the Apple-Soil System

This study aimed to determine the effects of the nitrogen (N) application period and level on the fate of fertilizer N and the contribution of N absorption and translocation to apple organ N. Two N application periods (labeled by the 15N tracer technique in spring and summer, represented by SP and SU, respectively) and three N levels (N0, MN, and HN) were used to determine the physiological indexes and aboveground, root, and soil 15N content of 4-year-old dwarf ('Red Fuji'/M9T337) and arborized ('Red Fuji'/Malus hupehensis Rehd.) apple trees. The results showed that HN led to shoot overgrowth, which was not conducive to the growth of the apple root system (root length, root tips, root surface area, and root volume) or the improvement of root activity. The contribution of soil N to apple organ N accounted for more than 50%, and the contribution of N application in summer to fruit N was higher than that in spring. Under HN treatment, the proportion of soil N absorbed by trees decreased, while that of fertilizer N increased; however, the highest proportion was still less than 50%, so apple trees were highly dependent on soil N. Under MN treatment, fertilizer N residue was similar to soil N consumption, and soil N fertility maintained a basic balance. Under HN treatment, fertilizer N residue was significantly higher than soil N consumption, indicating that excessive N application increased fertilizer N residue in the soil. Overall, the 15N utilization rate of arborized trees (17.33-22.38%) was higher than that of dwarf trees (12.89-16.91%). A total of 12.89-22.38% of fertilizer 15N was absorbed by trees, 30.37-35.41% of fertilizer 15N remained in the soil, and 44.65-54.46% of fertilizer 15N was lost. The 15N utilization rate and 15N residual rate of summer N application were higher than those of spring N application, and the 15N loss rate was lower than that of spring N application. High microbial biomass N (MBN) may be one of the reasons for the high N utilization rate and the low loss rate of N application in summer.

Nitrogen acquisition strategy and its effects on invasiveness of a subtropical invasive plant

Introduction

Preference and plasticity in nitrogen (N) form uptake are the main strategies with which plants absorb soil N. However, little effort has been made to explore effects of N form acquisition strategies, especially the plasticity, on invasiveness of exotic plants, although many studies have determined the effects of N levels (e.g. N deposition).

Methods

To address this problem, we studied the differences in N form acquisition strategies between the invasive plant Solidago canadensis and its co-occurring native plant Artemisia lavandulaefolia, effects of soil N environments, and the relationship between N form acquisition strategy of S. canadensis and its invasiveness using a 15N-labeling technique in three habitats at four field sites.

Results

Total biomass, root biomass, and the uptakes of soil dissolved inorganic N (DIN) per quadrat were higher for the invasive relative to the native species in all three habitats. The invader always preferred dominant soil N forms: NH4 + in habitats with NH4 + as the dominant DIN and NO3 - in habitats with NO3 - as the dominant DIN, while A. lavandulaefolia consistently preferred NO3 - in all habitats. Plasticity in N form uptake was higher in the invasive relative to the native species, especially in the farmland. Plant N form acquisition strategy was influenced by both DIN levels and the proportions of different N forms (NO3 -/NH4 +) as judged by their negative effects on the proportional contributions of NH4 + to plant N (f NH4 +) and the preference for NH4 + (β NH4 +). In addition, total biomass was positively associated with f NH4 + or β NH4 + for S. canadensis, while negatively for A. lavandulaefolia. Interestingly, the species may prefer to absorb NH4 + when soil DIN and/or NO3 -/NH4 + ratio were low, and root to shoot ratio may be affected by plant nutrient status per se, rather than by soil nutrient availability.

Discussion

Our results indicate that the superior N form acquisition strategy of the invader contributes to its higher N uptake, and therefore to its invasiveness in different habitats, improving our understanding of invasiveness of exotic plants in diverse habitats in terms of utilization of different N forms.

Threshold responses of soil gross nitrogen transformation rates to aridity gradient

The responses of soil nitrogen (N) transformations to climate change are crucial for biome productivity prediction under global change. However, little is known about the responses of soil gross N transformation rates to drought gradient. Along an aridity gradient across the 2700 km transect of drylands on the Qinghai-Tibetan Plateau, this study measured three main soil gross N transformation rates in both topsoil (0-10 cm) and subsoil (20-30 cm) using the laboratorial 15 N labeling. The relevant soil abiotic and biotic variables were also determined. The results showed that gross N mineralization and nitrification rates steeply decreased with increasing aridity when aridity was less than 0.5 but just slightly decreased with increasing aridity when aridity was larger than 0.5 at both soil layers. In topsoil, the decreases of the two gross rates were accompanied by the similar decreased patterns of soil total N content and microbial biomass carbon with increasing aridity (p < .05). In subsoil, although the decreased pattern of soil total N with increasing aridity was still similar to the decreases of the two gross rates (p < .05), microbial biomass carbon did not change (p > .05). Instead, bacteria and ammonia oxidizing archaea abundances decreased with increasing aridity when aridity was larger than 0.5 (p < .05). With an aridity threshold of 0.6, gross N immobilization rate increased with increasing aridity in wetter region (aridity < 0.6) accompanied with an increased bacteria/fungi ratio, but decreased with increasing aridity in drier region (aridity > 0.6) where mineral N and microbial biomass N also decreased at both soil layers (p < .05). This study provided new insight to understand the differential responses of soil N transformation to drought gradient. The threshold responses of the gross N transformation rates to aridity gradient should be noted in biogeochemical models to better predict N cycling and manage land in the context of global change.

Silicon Supply Improves Nodulation and Dinitrogen Fixation and Promotes Growth in Trifolium incarnatum Subjected to a Long-Term Sulfur Deprivation

In many crops species, sulfur (S) deprivation negatively affects growth, seed yield quality and plant health. Furthermore, silicon (Si) is known to alleviate many nutritional stresses but the effects of Si supply on plants subjected to S deficiency remain unclear and poorly documented. The objective of this study was to evaluate whether Si supply would alleviate the negative effects of S deprivation on root nodulation and atmospheric dinitrogen (N₂) fixation capacity in Trifolium incarnatum subjected (or not) to long-term S deficiency. For this, plants were grown for 63 days in hydroponic conditions with (500 µM) or without S and supplied (1.7 mM) or not with Si. The effects of Si on growth, root nodulation and N₂ fixation and nitrogenase abundance in nodules have been measured. The most important beneficial effect of Si was observed after 63 days. Indeed, at this harvest time, a Si supply increased growth, the nitrogenase abundance in nodules and N₂ fixation in S-fed and S-deprived plants while a beneficial effect on the number and total biomass of nodules was only observed in S-deprived plants. This study shows clearly for the first time that a Si supply alleviates negative effects of S deprivation in Trifolium incarnatum.

Sources of nitrous oxide emissions from hydroponic tomato cultivation: Evidence from stable isotope analyses

Introduction

Hydroponic vegetable cultivation is characterized by high intensity and frequent nitrogen fertilizer application, which is related to greenhouse gas emissions, especially in the form of nitrous oxide (N₂O). So far, there is little knowledge about the sources of N₂O emissions from hydroponic systems, with the few studies indicating that denitrification could play a major role.

Methods

Here, we use evidence from an experiment with tomato plants (Solanum lycopersicum) grown in a hydroponic greenhouse setup to further shed light into the process of N₂O production based on the N₂O isotopocule method and the ¹⁵N tracing approach. Gas samples from the headspace of rock wool substrate were collected prior to and after ¹⁵N labeling at two occasions using the closed chamber method and analyzed by gas chromatography and stable isotope ratio mass spectrometry.

Results

The isotopocule analyses revealed that either heterotrophic bacterial denitrification (bD) or nitrifier denitrification (nD) was the major source of N₂O emissions, when a typical nutrient solution with a low ammonium concentration (1-6 mg L^-1) was applied. Furthermore, the isotopic shift in ¹⁵N site preference and in δ¹⁸O values indicated that approximately 80-90% of the N₂O produced were already reduced to N₂ by denitrifiers inside the rock wool substrate. Despite higher concentrations of ammonium present during the ¹⁵N labeling (30-60 mg L^-1), results from the ¹⁵N tracing approach showed that N₂O mainly originated from bD. Both, ¹⁵N label supplied in the form of ammonium and ¹⁵N label supplied in the form of nitrate, increased the ¹⁵N enrichment of N₂O. This pointed to the contribution of other processes than bD. Nitrification activity was indicated by the conversion of small amounts of ¹⁵N-labeled ammonium into nitrate.

Discussion/Conclusion

Comparing the results from N₂O isotopocule analyses and the ¹⁵N tracing approach, likely a combination of bD, nD, and coupled nitrification and denitrification (cND) was responsible for the vast part of N₂O emissions observed in this study. Overall, our findings help to better understand the processes underlying N₂O and N₂ emissions from hydroponic tomato cultivation, and thereby facilitate the development of targeted N₂O mitigation measures.

Fungus Pichia kudriavzevii XTY1 and heterotrophic nitrifying bacterium Enterobacter asburiae GS2 cannot efficiently transform organic nitrogen via hydroxylamine and nitrite

Heterotrophic nitrification is a process of organic nitrogen degradation completed by the participation of heterotrophic nitrifying microorganisms, which can accelerate the nitrogen transformation process. However, the current research mainly focuses on heterotrophic nitrifying bacteria and their ammonium degradation capacities. And there is little accumulation of research on fungi, the main force of heterotrophic nitrification, and their capacities to transform organic nitrogen. In this study, novel heterotrophic nitrifying fungus (XTY1) and bacterium (GS2) were screened and isolated from upland soil, and the strains were identified and registered through GenBank comparison. After 24 h single nitrogen source tests and ¹⁵N labeling tests, we compared and preliminarily determined the heterotrophic nitrification capacities and pathways of the two strains. The results showed that XTY1 and GS2 had different transformation capacities to different nitrogen substrates and could efficiently transform organic nitrogen. However, the transformation capacity of XTY1 to ammonium was much lower than that of GS2. The two strains did not pass through NH₂OH and NO₂ ^- during the heterotrophic nitrification of organic nitrogen, and mainly generated intracellular nitrogen and low N₂O. Other novel organic nitrogen metabolism pathways may be existed, but they remain to be further validated.

Deepened snow loosens temporal coupling between plant and microbial N utilization and induces ecosystem N losses

Seasonal differences in plant and microbial nitrogen (N) acquisition are believed to be a major mechanism that maximizes ecosystem N retention. There is also a concern that climate change may interrupt the delicate balance in N allocation between plants and microbes. Yet, convincing experimental evidence is still lacking. Using a ¹⁵ N tracer, we assessed how deepened snow affects the temporal coupling between plant and microbial N utilization in a temperate Mongolian grassland. We found that microbial ¹⁵ N recovery peaked in winter, accounting for 22% of the total ecosystem ¹⁵ N recovery, and then rapidly declined during the spring thaw. By stimulating N loss via N₂ O emission and leaching, deepened snow reduced the total ecosystem ¹⁵ N recovery by 42% during the spring thaw. As the growing season progresses, the ¹⁵ N released from microbial biomass was taken up by plants, and the competitive advantage for N shifted from microbes to plants. Plant ¹⁵ N recovery reached its peak in August, accounting for 17% of the total ecosystem ¹⁵ N recovery. The Granger causality test showed that the temporal dynamics of plant ¹⁵ N recovery can be predicted by microbial ¹⁵ N recovery under ambient snow but not under deepened snow. In addition, plant ¹⁵ N recovery in August was positively correlated with and best explained by microbial ¹⁵ N recovery in March. The lower microbial ¹⁵ N recovery under deepened snow in March reduced plant ¹⁵ N recovery by 73% in August. Together, our results provide direct evidence of seasonal differences in plant and microbial N utilization that are conducive to ecosystem N retention; however, deepened snow disrupted the temporal coupling between plant-microbial N use and turnover. These findings suggest that changes in snowfall patterns may significantly alter ecosystem N cycling and N-based greenhouse gas emissions under future climate change. We highlight the importance of better representing winter processes and their response to winter climate change in biogeochemical models when assessing N cycling under global change.

Post-anthesis Relationships Between Nitrogen Isotope Discrimination and Yield of Spring Wheat Under Different Nitrogen Levels

Wheat grain yield and nitrogen (N) content are influenced by the amount of N remobilized to the grain, together with pre-anthesis and post-anthesis N uptake. Isotopic techniques in farmed areas may provide insight into the mechanism underlying the N cycle. 15N-labeled urea was applied to microplots within five different fertilized treatments 0 kg ha-1 (N1), 52.5 kg ha-1 (N2), 105 kg ha-1 (N3), 157.5 kg ha-1 (N4), and 210 kg ha-1 (N5) of a long-term field trial (2003-2021) in a rainfed wheat field in the semi-arid loess Plateau, China, to determine post-anthesis N uptake and remobilization into the grain, as well as the variability of 15N enrichment in aboveground parts. Total N uptake was between 7.88 and 29.27 kg ha-1 for straw and 41.85 and 95.27 kg ha-1 for grain. In comparison to N1, N fertilization increased straw and grain N uptake by 73.1 and 56.1%, respectively. Nitrogen use efficiency (NUE) and harvest index were altered by N application rates. The average NUE at maturity was 19.9% in 2020 and 20.01% in 2021; however, it was usually higher under the control and low N conditions. The amount of 15N excess increased as the N rate increased: N5 had the highest 15N excess at the maturity stage in the upper (2.28 ± 0.36%), the middle (1.77 ± 0.28%), and the lower portion (1.68 ± 1.01%). Compared to N1, N fertilization (N2-N5) increased 15N excess in the various shoot portions by 50, 38, and 35% at maturity for upper, middle, and lower portions, respectively. At maturity, the 15N excess remobilized to the grain under N1-N5 was between 5 and 8%. Our findings revealed that N had a significant impact on yield and N isotope discrimination in spring wheat that these two parameters can interact, and that future research on the relationship between yield and N isotope discrimination in spring wheat should take these factors into account.

Sample Preparation, Data Acquisition, and Data Analysis for 15N-Labeled and Nonlabeled Monoterpene Indole Alkaloids in Catharanthus roseus

Recent approaches developed in metabolomics using liquid chromatography-tandem mass spectrometry (LC-MS/MS) enabled us to assign a part of specialized metabolites in plants. However, the approaches are not good enough for the rest of the metabolites, which are still unknown. To characterize the unknown metabolites, more appropriate and precise approaches need to be developed. Here, a procedure to analyze ¹⁵N-labeled and nonlabeled LC-MS/MS data for identification of monoterpene indole alkaloids was developed.

Tracking Animal-Dispersed Seedlings Using 15N Xylem Injection Method

Although various seed-marking methods have been developed for seed dispersal, it remains difficult to track the actual patterns of seed dispersal and seedling recruitment. Thus, new labeling methods that accurately track seedling establishment along with seed movement would help us better understand seed dispersal. Here, we developed a new nondestructive method using ¹⁵N xylem injection to track seed dispersal and seedling recruitment based on the enriched isotopic signals in the mature seeds. Our results first showed that xylem injection of ¹⁵N successfully enriched ¹⁵N both in the acorns and seedlings of Quercus variabilis. By marking acorns and seedlings with ¹⁵N stable isotopes, we successfully tracked seedlings established from acorns dispersed by seed-eating animals in the field. Our xylem ¹⁵N injection caused little alteration to seeds and showed no significant effects on seed selection by seed-eating animals as well as seed germination and seedling establishment, verifying the validity of the ¹⁵N xylem injection method to track seedling establishment. Our xylem ¹⁵N injection method is expected to be a powerful tool for tracking seed dispersal and seedling recruitment mediated by seed-eating animals in seed dispersal ecology.

Canopy processing of N deposition increases short-term leaf N uptake and photosynthesis, but not long-term N retention for aspen seedlings

Forest canopies can retain nitrogen (N) from atmospheric deposition. However, most empirical and modeling studies do not consider the processing of the N deposited in the canopy. To assess whether N deposition through canopy will alter the plant's N uptake and retention, we conducted a 3-yr mesocosm experiment by applying (¹⁵ NH₄ )₂ SO₄ solution to aspen sapling canopies or directly to the soil. We found that ¹⁵ N-NH₄⁺ applied to the canopy was directly taken up by leaves. Compared with the soil N application, the canopy N application resulted in higher photosynthesis but lower N retention of the plant-soil system in the first growing season. Plant biomass, N concentration, and leaf N resorption were not significantly different between the canopy and soil N applications. The partitioning of retained ¹⁵ N among plant components and soil layers was similar between the two treatments 3 yr after the N application. Our findings indicated that the canopy N processing could alter leaf N supply and photosynthesis in the short term but not N retention in the long term. Under natural conditions, the chronic N deposition could continuously refill the canopy N pool, causing a sustained increase in canopy carbon uptake. Canopy N processing needs to be considered for accurately predicting the impact of N deposition.

Mature conifers assimilate nitrate as efficiently as ammonium from soils in four forest plantations

Conifers are considered to prefer to take up ammonium (NH₄⁺ ) over nitrate (NO₃^- ). However, this conclusion is mainly based on hydroponic experiments that separate roots from soils. It remains unclear to what extent mature conifers can use nitrate compared to ammonium under field conditions where both roots and soil microbes compete for nitrogen (N). We conducted an in situ whole mature tree nitrogen-15 (¹⁵ N) labeling experiment (¹⁵ NH₄⁺ vs ¹⁵ NO₃^- ) over 15 d to quantify ammonium and nitrate uptake and assimilation rates in four 40-yr-old monoculture coniferous plantations (Pinus koraiensis, Pinus sylvestris, Picea koraiensis and Larix olgensis, respectively). For the whole tree, ¹⁵ NO₃^- contributed 39% to 90% to total ¹⁵ N tracer uptake among four plantations during the study period. At day 3, the ¹⁵ NO₃^- accounted for 77%, 64%, 62% and 59% by Larix olgensis, Pinus koraiensis, Pinus sylvestris and Picea koraiensis, respectively. Our study indicates that mature coniferous trees assimilated nitrate as efficiently as ammonium from soils even at low soil nitrate concentration, in contrast to the results from hydroponic experiments showing that ammonium uptake dominated over nitrate. This implies that mature conifers can adapt to increasing availability of nitrate in soil, for example, under the context of globalization of N deposition and global warming.

Radicle pruning by seed-eating animals helps oak seedlings absorb more soil nutrient

Although radicle pruning has well been observed in plant–animal interactions, research has not been conducted to determine how radicle pruning by seed‐eating animals regulates nutrition mobilization of cotyledonary reserves and absorption of soil nutrients. We used stable nitrogen isotopes to test how acorns of early‐germinating oak species (Quercus variabilis, Q. aliena, and Q. mogolica) trade off nutrients in the cotyledons and those in the soil in response to radicle pruning by seed‐eating rodents. Radicle pruning by rodents resulted in root branching in the 3 early‐germinating oak species. Moreover, radicle pruning increased shoot dry weight and substantially reduced the root‐to‐shoot ratio of oak species. Corresponding to the decreased dry weight of roots and root‐to‐shoot ratio, the dry weight of the remnant cotyledons was higher after radicle pruning in the 3 oak species. We provided first evidence that radicle pruning by seed‐eating animals improved seedling performance of early‐germinating oaks by increasing absorption of nutrients from soil. The results indicate that early‐germinating oak seedlings trade off nutrition budget by altering nutrient absorption from soil and reserve mobilization from cotyledons in response to radicle pruning by seed‐eating animals. Our study provided new insight into the nutrition allocation mechanism of young seedlings in response to radicle pruning by seed‐eating animals, reflecting a mutualistic interaction between early‐germinating oak and food‐hoarding animals.

High Temperatures During the Seed-Filling Period Decrease Seed Nitrogen Amount in Pea (Pisum sativum L.): Evidence for a Sink Limitation

Higher temperatures induced by the on-going climate change are a major cause of yield reduction in legumes. Pea (Pisum sativum L.) is an important annual legume crop grown in temperate regions for its high seed nitrogen (N) concentration. In addition to yield, seed N amount at harvest is a crucial characteristic because pea seeds are a source of protein in animal and human nutrition. However, there is little knowledge on the impacts of high temperatures on plant N partitioning determining seed N amount. Therefore, this study investigates the response of seed dry matter and N fluxes at the whole-plant level (plant N uptake, partitioning in vegetative organs, remobilization, and accumulation in seeds) to a range of air temperature (from 18.4 to 33.2°C) during the seed-filling-period. As pea is a legume crop, plants relying on two different N nutrition pathways were grown in glasshouse: N2-fixing plants or NO3 --assimilating plants. Labeled nitrate (15NO3 -) and intra-plant N budgets were used to quantify N fluxes. High temperatures decreased seed-filling duration (by 0.8 day per °C), seed dry-matter and N accumulation rates (respectively by 0.8 and 0.032 mg seed-1 day-1 per °C), and N remobilization from vegetative organs to seeds (by 0.053 mg seed-1 day-1 per °C). Plant N2-fixation decreased with temperatures, while plant NO3 - assimilation increased. However, the additional plant N uptake in NO3 --assimilating plants was never allocated to seeds and a significant quantity of N was still available at maturity in vegetative organs, whatever the plant N nutrition pathway. Thus, we concluded that seed N accumulation under high temperatures is sink limited related to a shorter seed-filling duration and a reduced seed dry-matter accumulation rate. Consequently, sustaining seed sink demand and preserving photosynthetic capacity of stressed plants during the seed-filling period should be promising strategies to promote N allocation to seeds from vegetative parts and thus to maintain crop N production under exacerbated abiotic constraints in field due to the on-going climate change.

Variation of 13C and 15N enrichments in different plant components of labeled winter wheat (Triticum aestivum L.)

Information on the homogeneity and distribution of ¹³carbon (¹³C) and nitrogen (¹⁵N) labeling in winter wheat (Triticum aestivum L.) is limited. We conducted a dual labeling experiment to evaluate the variability of ¹³C and ¹⁵N enrichment in aboveground parts of labeled winter wheat plants. Labeling with ¹³C and ¹⁵N was performed on non-nitrogen fertilized (-N) and nitrogen fertilized (+N, 250 kg N ha^-1) plants at the elongation and grain filling stages. Aboveground parts of wheat were destructively sampled at 28 days after labeling. As winter wheat growth progressed, δ ¹³C values of wheat ears increased significantly, whereas those of leaves and stems decreased significantly. At the elongation stage, N addition tended to reduce the aboveground δ ¹³C values through dilution of C uptake. At the two stages, upper (newly developed) leaves were more highly enriched with ¹³C compared with that of lower (aged) leaves. Variability between individual wheat plants and among pots at the grain filling stage was smaller than that at the elongation stage, especially for the -N treatment. Compared with those of ¹³C labeling, differences in ¹⁵N excess between aboveground components (leaves and stems) under ¹⁵N labeling conditions were much smaller. We conclude that non-N fertilization and labeling at the grain filling stage may produce more uniformly ¹³C-labeled wheat materials, whereas the materials were more highly ¹³C-enriched at the elongation stage, although the δ ¹³C values were more variable. The ¹⁵N-enriched straw tissues via urea fertilization were more uniformly labeled at the grain filling stage compared with that at the elongation stage.

Biochar prepared at different pyrolysis temperatures affects urea-nitrogen immobilization and N2O emissions in paddy fields

Background

Food safety has become a major issue, with serious environmental pollution resulting from losses of nitrogen (N) fertilizers. N is a key element for plant growth and is often one of the most important yield-limiting nutrients in paddy soil. Urea-N immobilization is an important process for restoring the levels of soil nutrient depleted by rice production and sustaining productivity. The benefits of biochar application include improved soil fertility, altered N dynamics, and reduced nutrient leaching. However, due to high variability in the quality of biochar, the responses of N loss and rice productivity to biochar amendments, especially those prepared at different pyrolysis temperatures, are still unclear. The main objectives of the present study were to examine the effects of biochar prepared at different pyrolysis temperatures on fertilizer N immobilization in paddy soil and explore the underlying mechanisms.

Methods

Two biochar samples were prepared by pyrolysis of maize straw at 400 °C (B400) and 700 °C (B700), respectively. The biochar was applied to paddy soil at three rates (0, 0.7, and 2.1%, w/w), with or without N fertilization (0, 168, and 210 kg N ha^–1). Pot experiments were performed to determine nitrous oxide (N₂O) emissions and ¹⁵N recovery from paddy soil using a ¹⁵N tracer across the rice growing season.

Results

Compared with the non-biochar control, biochar significantly decreased soil bulk density while increasing soil porosity, irrespective of pyrolysis temperature and N fertilizer level. Under B400 and B700, a high biochar rate decreased N loss rate to 66.42 and 68.90%, whereas a high N level increased it to 77.21 and 76.99%, respectively. Biochar also markedly decreased N₂O emissions to 1.06 (B400) and 0.75 kg ha⁻¹ (B700); low-N treatment caused a decrease in N₂O emissions under B400, but this decrease was not observed under B700. An application rate of biochar of 2.1% plus 210 kg ha⁻¹ N fertilizer substantially decreased the N fertilizer-induced N₂O emission factor under B400, whereas under B700 no significant difference was observed. Biochar combined with N fertilizer treatment decreased rice biomass and grain yield by an average of 51.55 and 23.90 g pot^–1, respectively, but the yield reduction under B700 was lower than under B400.

Conclusion

Irrespective of pyrolysis temperature, biochar had a positive effect on residual soil ¹⁵N content, while it negatively affected the ¹⁵N recovery of rice, N₂O emissions from soil, rice biomass, and grain yield in the first year. Generally, a high application rate of biochar prepared at high or low pyrolysis temperature reduced the N fertilizer-induced N₂O emission factor considerably. These biochar effects were dependent on N fertilizer level, biochar application rate, and their interactions.

Fate and transport of urea-N in a rain-fed ridge-furrow crop system with plastic mulch

A better understanding of the fate and transport of fertilizer nitrogen (N) is critical to maximize crop yields and minimize negative environmental impacts. Plastic film mulching is widely used in drylands to increase soil water use efficiency and crop yields, but the effects on fertilizer N use efficiency need to be evaluated. A field experiment with ¹⁵N-urea (260 kg N ha^-1) was conducted to determine the fate and transport of fertilizer N in a ridge-furrow system with plastic film mulched ridge (Plastic), compared with a flat system without mulching (Open). In the Plastic, the ¹⁵N-urea was applied to the ridge only (Plastic-Ridge), or to the furrow only (Plastic-Furrow). Maize grain yield and net economic benefit for Plastic were significantly higher (by 9.7 and 8.5%, respectively) than those for Open. Total plant ¹⁵N uptake was 72.5% greater in Plastic compared with Open, and ¹⁵N was allocated mostly to the grain. Losses of the applied urea-N were 54.5% lower in Plastic and much more residual ¹⁵N was recovered in 0-120 cm soil compared with Open (42.7 and 26.8% of applied ¹⁵N, respectively). Lateral N movements from furrow to ridge and from ridge to furrow were observed and attributed to lateral movement of soil water due to microtopography of ridges and furrows and uneven soil water and heat conditions under mulching and plant water uptake. The ridges were the main N fertilizer source for plant uptake (96.5 and 3.5% of total N uptake in Plastic from ridge and furrow, respectively) and the furrow was the main source of N losses (78.6 and 21.4% of total N losses in Plastic from furrow and ridge, respectively). Gas emissions, especially ammonia volatilization was probably the main N loss in furrow. Thus, appropriately localized N application - into the ridges, and management strategies should be designed for Plastic to maximize N use efficiency by crops, decrease N gas losses and maintain sustainable agricultural systems in drylands.

Labeling Maize (Zea mays L.) Leaves with 15 NH4+ and Monitoring Nitrogen Incorporation into Amino Acids by GC/MS Analysis

The human body contains approximately 3.2% nitrogen (N), mainly present as protein and amino acids. Although N exists at a high concentration (78%) in the air, it is not readily available to animals and most plants. Plants are however able to take up both nitrate (NO₃^- ) and ammonium (NH₄⁺ ) ions from the soil and convert them to amino acids and proteins, which are excellent sources for all animals. Most N is available as the stable isotope ¹⁴ N, but a second form, ¹⁵ N, is present in very low concentrations. ¹⁵ N can be detected in extracts of plants by gas chromatography followed by mass spectrometry (GC/MS). In this protocol, the methods are described for tracing the pathway by which plants are able to take up ¹⁵ N-labeled nitrate and ammonium and convert them into amino acids and proteins. A protocol for extracting and quantifying amino acids and ¹⁵ N enrichment in maize (Zea mays L.) leaves labeled with ¹⁵ NH₄⁺ is described. Following amino acid extraction, purification, and separation by GC/MS, a calculation of the ¹⁵ N enrichment of each amino acid is carried out on a relative basis to identify any differences in the dynamics of amino acid accumulation. This will allow a study of the impact of genetic modifications or mutations on key reactions involved in primary nitrogen and carbon metabolism. © 2018 by John Wiley & Sons, Inc.

Elevated CO2 Increases Nitrogen Fixation at the Reproductive Phase Contributing to Various Yield Responses of Soybean Cultivars

Nitrogen deficiency limits crop performance under elevated CO2 (eCO2), depending on the ability of plant N uptake. However, the dynamics and redistribution of N2 fixation, and fertilizer and soil N use in legumes under eCO2 have been little studied. Such an investigation is essential to improve the adaptability of legumes to climate change. We took advantage of genotype-specific responses of soybean to increased CO2 to test which N-uptake phenotypes are most strongly related to enhanced yield. Eight soybean cultivars were grown in open-top chambers with either 390 ppm (aCO2) or 550 ppm CO2 (eCO2). The plants were supplied with 100 mg N kg-1 soil as 15N-labeled calcium nitrate, and harvested at the initial seed-filling (R5) and full-mature (R8) stages. Increased yield in response to eCO2 correlated highly (r = 0.95) with an increase in symbiotically fixed N during the R5 to R8 stage. In contrast, eCO2 only led to small increases in the uptake of fertilizer-derived and soil-derived N during R5 to R8, and these increases did not correlate with enhanced yield. Elevated CO2 also decreased the proportion of seed N redistributed from shoot to seeds, and this decrease strongly correlated with increased yield. Moreover, the total N uptake was associated with increases in fixed-N per nodule in response to eCO2, but not with changes in nodule biomass, nodule density, or root length.

Allocation of Nitrogen and Carbon Is Regulated by Nodulation and Mycorrhizal Networks in Soybean/Maize Intercropping System

Soybean/maize intercropping has remarkable advantages in increasing crop yield and nitrogen (N) efficiency. However, little is known about the contributions of rhizobia or arbuscular mycorrhizal fungi (AMF) to yield increases and N acquisition in the intercropping system. Plus, the mechanisms controlling carbon (C) and N allocation in intercropping systems remain unsettled. In the present study, a greenhouse experiment combined with ¹⁵N and ¹³C labeling was conducted using various inoculation and nutrient treatments. The results showed that co-inoculation with AMF and rhizobia dramatically increased biomass and N content of soybean and maize, and moderate application of N and phosphorus largely amplified the effect of co-inoculation. Maize had a competitive advantage over soybean only under co-inoculation and moderate nutrient availability conditions, indicating that the effects of AMF and rhizobia in intercropping systems are closely related to nutrient status. Results from ¹⁵N labeling showed that the amount of N transferred from soybean to maize in co-inoculations was 54% higher than that with AMF inoculation alone, with this increased N transfer partly resulting from symbiotic N fixation. The results from ¹³C labeling showed that ¹³C content increased in maize shoots and decreased in soybean roots with AMF inoculation compared to uninoculated controls. Yet, with co-inoculation, ¹³C content increased in soybean. These results indicate that photosynthate assimilation is stimulated by AM symbiosis in maize and rhizobial symbiosis in soybean, but AMF inoculation leads to soybean investing more carbon than maize into common mycorrhizal networks (CMNs). Overall, the results herein demonstrate that the growth advantage of maize when intercropped with soybean is due to acquisition of N by maize via CMNs while this crop contributes less C into CMNs than soybean under co-inoculation conditions.

Allocation of Nitrogen and Carbon Is Regulated by Nodulation and Mycorrhizal Networks in Soybean/Maize Intercropping System

Soybean/maize intercropping has remarkable advantages in increasing crop yield and nitrogen (N) efficiency. However, little is known about the contributions of rhizobia or arbuscular mycorrhizal fungi (AMF) to yield increases and N acquisition in the intercropping system. Plus, the mechanisms controlling carbon (C) and N allocation in intercropping systems remain unsettled. In the present study, a greenhouse experiment combined with ¹⁵N and ¹³C labeling was conducted using various inoculation and nutrient treatments. The results showed that co-inoculation with AMF and rhizobia dramatically increased biomass and N content of soybean and maize, and moderate application of N and phosphorus largely amplified the effect of co-inoculation. Maize had a competitive advantage over soybean only under co-inoculation and moderate nutrient availability conditions, indicating that the effects of AMF and rhizobia in intercropping systems are closely related to nutrient status. Results from ¹⁵N labeling showed that the amount of N transferred from soybean to maize in co-inoculations was 54% higher than that with AMF inoculation alone, with this increased N transfer partly resulting from symbiotic N fixation. The results from ¹³C labeling showed that ¹³C content increased in maize shoots and decreased in soybean roots with AMF inoculation compared to uninoculated controls. Yet, with co-inoculation, ¹³C content increased in soybean. These results indicate that photosynthate assimilation is stimulated by AM symbiosis in maize and rhizobial symbiosis in soybean, but AMF inoculation leads to soybean investing more carbon than maize into common mycorrhizal networks (CMNs). Overall, the results herein demonstrate that the growth advantage of maize when intercropped with soybean is due to acquisition of N by maize via CMNs while this crop contributes less C into CMNs than soybean under co-inoculation conditions.

Expression of denitrification genes in response to a waterlogging event in a Fluvisol and its relationship with large nitrous oxide pulses

The contributions of large N2 O pulses following waterlogging to the annual cumulative N2 O productions were significant in a Fluvisol. To uncover the mechanisms underlying these large N2 O pulses, a Fluvisol sampled from an agricultural field in Japan was subjected to waterlogging during incubation. Larger N2 O emissions were observed in intact soil cores when compared to emissions from sieved soils, indicating the importance of soil properties. The most important factor controlling the magnitude of the N2 O pulses after waterlogging was the soil moisture prior to waterlogging. The major pathway for N2 O production was denitrification. Quantitative PCR and quantitative RT-PCR analyses showed that the denitrification genes (nirS, nirK, and nosZ) correlated with N2 O emissions at the mRNA level but not at the DNA level. The change in denitrification gene mRNA levels was more prominent in the 0- to 1-cm soil compared with the 1- to 3-cm soil. Water-soluble and hot-water-soluble carbon contents also showed the highest amount in the 0- to 1-cm soil. These indicate that there was a strong variation in soil microbial properties over very small changes in soil depth, and this variation is important in determining the magnitude of N2 O emissions.

Anthropogenic nitrogen deposition in boreal forests has a minor impact on the global carbon cycle

It is proposed that increases in anthropogenic reactive nitrogen (Nr ) deposition may cause temperate and boreal forests to sequester a globally significant quantity of carbon (C); however, long-term data from boreal forests describing how C sequestration responds to realistic levels of chronic Nr deposition are scarce. Using a long-term (14-year) stand-scale (0.1 ha) N addition experiment (three levels: 0, 12.5, and 50 kg N ha(-1)  yr(-1) ) in the boreal zone of northern Sweden, we evaluated how chronic N additions altered N uptake and biomass of understory communities, and whether changes in understory communities explained N uptake and C sequestration by trees. We hypothesized that understory communities (i.e. mosses and shrubs) serve as important sinks for low-level N additions, with the strength of these sinks weakening as chronic N addition rates increase, due to shifts in species composition. We further hypothesized that trees would exhibit nonlinear increases in N acquisition, and subsequent C sequestration as N addition rates increased, due to a weakening understory N sink. Our data showed that understory biomass was reduced by 50% in response to the high N addition treatment, mainly due to reduced moss biomass. A (15) N labeling experiment showed that feather mosses acquired the largest fraction of applied label, with this fraction decreasing as the chronic N addition level increased. Contrary to our hypothesis, the proportion of label taken up by trees was equal (ca. 8%) across all three N addition treatments. The relationship between N addition and C sequestration in all vegetation pools combined was linear, and had a slope of 16 kg C kg(-1)  N. While canopy retention of Nr deposition may cause C sequestration rates to be slightly different than this estimate, our data suggest that a minor quantity of annual anthropogenic CO2 emissions are sequestered into boreal forests as a result of Nr deposition.

Rapid nitrogen transfer in the Sorghum bicolor-Glomus mosseae arbuscular mycorrhizal symbiosis

We have recently identified two genes coding for ammonium transporters (AMT) in Sorghum bicolor that were induced in roots colonized by arbuscular mycorrhizal (AM) fungi. To improve our understanding of the dynamics of ammonium transport in this symbiosis, we studied the transfer of soil-ammonium-derived (15)N to S. bicolor plants via the Glomus mosseae fungal mycelium in compartmented microcosms. The (15)NH (4+)-containing hyphal compartment was inaccessible to the roots in the plant compartment. (15)N label concentrations significantly increased in plant roots and leaves already 48 h after exposure of the AM fungus to the (15)NH (4+) substrate, attesting an efficient symbiotic N transfer between the symbiotic partners and further highlighting that AM symbiosis represents an important component of plant nitrogen nutrition.