15N Loss Research Articles

Investigating drivers of free-living diazotroph activity in paddy soils across China

Microbially mediated N fixation is widespread in rice paddy ecosystems and crucial in maintaining soil fertility. However, our understanding of the factors determining the distribution of free-living diazotrophic microorganisms that perform this process in paddy fields is limited. This study investigated the spatial distribution and factors influencing presence and potential activity of free-living microorganisms capable of N2 fixation in addition to dissimilatory nitrate reduction to ammonium (DNRA), anaerobic ammonium oxidation (anammox), and denitrification in 50 paddy soils across China. Using 15N isotope tracing in laboratory incubations and microbial community analysis via metagenomics, we demonstrate that paddy soils may represent a previously underappreciated hotspot for N2 fixation with mean potential rates of 24.4 ± 17.8 nmol N g−1 h−1, 10-fold higher than DNRA (2.55 ± 0.4 nmol N g−1 h−1), and could counterbalance a portion of N2 losses through anammox and denitrification (9.24 ± 1.1 nmol N g−1 h−1). Site longitude and organic carbon (C) concentrations, as well as the diazotrophic community composition, were the dominant abiotic and biotic factors accounting for regional variations in potential N2 fixation rates. The N2 metabolic pathways predicted from the metagenome-assembled genomes (MAGs) revealed significant co-occurrence of the diazotroph marker gene nifH with denitrification-associated genes (nirS/K and nosZ) and organic C oxidation-related genes (yiaY and galM). Furthermore, enzymes involved in organic C oxidation, particularly glycoside hydrolases and glycosyltransferases, were not only phenotypically correlated with free-living N2 fixation rates but were also identified in nifH-containing MAGs, indicating the heterotrophic capabilities of diazotrophs in paddy soils. Collectively, our results underscore the substantial contribution of free-living N2 fixation to soil N fertility in paddy fields, and highlight the importance of coupling organic C oxidation with nitrate reduction to enhance N2 fixation.

The Combined Application of Urea and Fulvic Acid Regulates Apple Tree Carbon and Nitrogen Metabolism and Improves Anthocyanin Biosynthesis

Improving apple peel color has been an important objective in apple production. To better understand the effect and mechanism of the combined application of urea and FA (fulvic acid) regulation of anthocyanin biosynthesis, a field experiment was performed in 2022 and 2023, respectively, under five treatments of urea + FA (CK, urea only; FA50, urea + 50 kg ha−1 FA; FA100, urea + 100 kg ha−1 FA; FA150, urea + 150 kg ha−1 FA; FA200, urea + 200 kg ha−1 FA), using isotope (13C and 15N) marking to analyze the changes in carbon (C) and nitrogen (N) nutrient distribution as well as anthocyanin biosynthesis in fruits. We observed that, under FA application conditions, anthocyanin content in the peel was elevated in both years, with increases of 15.98~52.88% in 2022 and 15.93~52.94% in 2023. The best promotion effects were observed under FA150 treatment. Apart from the expression levels of anthocyanin biosynthesis-related genes and transcription factors in the apple peel, this positive effect on anthocyanin content induced by FA addition was also found to be associated with the optimization of C and N distribution in leaves and fruits. On the one hand, the application of FA not only enhanced leaf photosynthetic-related indexes (such as Pn, Gs, and Rubisco activity) and influenced (increased) S6PDH, SPS, and SS activities in leaves, but also elevated fruit sugar metabolism-related enzyme (SDH, SS-c, AI, and NI) activity and upregulated fruit stalk sugar transporter (MdSOT1, MdSOT3, MdSUT1 and MdSUT4) gene expression, which ultimately promoted the synthesis and the leaf to fruit transport of photosynthates, thus promoting 13C-photosynthate accumulation in fruits. On the other hand, FA application elevated leaves’ N metabolism-related enzyme (GS and GOGAT) activity and optimized 15N distribution in leaves and fruits. Moreover, we also observed that FA application altered the fate of N fertilizer in apple orchards, showed an elevation in apple tree 15NUE and soil 15N residuals and showed a decrease in soil 15N loss. In summary, the appropriate application of FA150 (urea + 150 kg ha−1) synergistically optimized C and N nutrient distribution, and promoted anthocyanin biosynthesis in apple trees.

Determining N2O and N2 fluxes in relation to winter wheat and sugar beet growth and development using the improved 15N gas flux method on the field scale

AbstractThe objectives of this field trial were to collect reliable measurement data on N2 emissions and N2O/(N2O + N2) ratios in typical German crops in relation to crop development and to provide a dataset to test and improve biogeochemical models. N2O and N2 emissions in winter wheat (WW, Triticum aestivum L.) and sugar beet (SB, Beta vulgaris subsp. vulgaris) were measured using the improved 15N gas flux method with helium–oxygen flushing (80:20) to reduce the atmospheric N2 background to < 2%. To estimate total N2O and N2 production in soil, production-diffusion modelling was applied. Soil samples were taken in regular intervals and analyzed for mineral N (NO3− and NH4+) and water-extractable Corg content. In addition, we monitored soil moisture, crop development, plant N uptake, N transformation processes in soil, and N translocation to deeper soil layers. Our best estimates for cumulative N2O + N2 losses were 860.4 ± 220.9 mg N m−2 and 553.1 ± 96.3 mg N m−2 over the experimental period of 189 and 161 days with total N2O/(N2O + N2) ratios of 0.12 and 0.15 for WW and SB, respectively. Growing plants affected all controlling factors of denitrification, and dynamics clearly differed between crop species. Overall, N2O and N2 emissions were highest when plant N and water uptake were low, i.e., during early growth stages, ripening, and after harvest. We present the first dataset of a plot-scale field study employing the improved 15N gas flux method over a growing season showing that drivers for N2O and N2O + N2 fluxes differ between crop species and change throughout the growing season.

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.

Lysimeter-based full fertilizer 15N balances corroborate direct dinitrogen emission measurements using the 15N gas flow method

AbstractThe 15N gas flux (15NGF) method allows for direct in situ quantification of dinitrogen (N2) emissions from soils, but a successful cross-comparison with another method is missing. The objectives of this study were to quantify N2 emissions of a wheat rotation using the 15NGF method, to compare these N2 emissions with those obtained from a lysimeter-based 15N fertilizer mass balance approach, and to contextualize N2 emissions with 15N enrichment of N2 in soil air. For four sampling periods, fertilizer-derived N2 losses (15NGF method) were similar to unaccounted fertilizer N fates as obtained from the 15N mass balance approach. Total N2 emissions (15NGF method) amounted to 21 ± 3 kg N ha− 1, with 13 ± 2 kg N ha− 1 (7.5% of applied fertilizer N) originating from fertilizer. In comparison, the 15N mass balance approach overall indicated fertilizer-derived N2 emissions of 11%, equivalent to 18 ± 13 kg N ha− 1. Nitrous oxide (N2O) emissions were small (0.15 ± 0.01 kg N ha− 1 or 0.1% of fertilizer N), resulting in a large mean N2:(N2O + N2) ratio of 0.94 ± 0.06. Due to the applied drip fertigation, ammonia emissions accounted for < 1% of fertilizer-N, while N leaching was negligible. The temporal variability of N2 emissions was well explained by the δ15N2 in soil air down to 50 cm depth. We conclude the 15NGF method provides realistic estimates of field N2 emissions and should be more widely used to better understand soil N2 losses. Moreover, combining soil air δ15N2 measurements with diffusion modeling might be an alternative approach for constraining soil N2 emissions.

Hybrid pathways of denitrification drive N2O but not N2 emissions from an acid-sulphate sugarcane soil

AbstractAcid-sulphate sugarcane soils in the subtropics are known hot-spots for nitrous oxide (N2O) emissions, yet the reduction of reactive N2O to non-reactive dinitrogen (N2) via specific pathways remains a major uncertainty for nitrogen (N) cycling and loss from these soils. This study investigated the magnitude and the N2O:N2 partitioning of N2O and N2 losses from a subtropical acid-sulphate soil under sugarcane production using the 15N gas flux method, establishing the contribution of hybrid (co- and chemo-denitrification) and heterotrophic denitrification to N2O and N2 losses. Soils were fertilised with potassium nitrate, equivalent to 25 and 50 kg N ha−1, watered close to saturation then incubated over 30 days. An innovative, fully automated incubation system coupled to an isotope-ratio mass-spectrometer enabled real time analysis of 15N2O and 15N2 at sub-diel resolution. Peak losses of N2O and N2 reached 6.5 kg N ha−1 day−1, totalling > 50 kg of N2O+N2-N ha−1. Emissions were dominated by N2, accounting for more than 57% of N2O+N2 losses, demonstrating that the reduction of N2O to N2 proceeded even under highly acidic conditions. Over 40% of N2O, but only 2% of N2 emissions, were produced via hybrid pathways. These findings demonstrate hybrid pathways are generally limited to N2O production, likely driven by high organic matter content and low soil pH, promoting both biotic, and abiotic nitrosation. Regardless of the underlying process, the magnitude of the N2O emissions demonstrates the environmental, but also the potential agronomic significance, of hybrid pathways of N2O formation for N loss from fertilised acid-sulphate soils.

Increasing winter wheat yield and nitrogen utilization, and reducing residual soil nitrogen in semi-humid areas: A study matching deep fertilizer application with regional water scenarios

The deep fertilizer application (DF) has been recommended as an effective strategy for promoting crop growth and yield formation. However, the controversial discussion revolves on the challenge of effectively using DF to simultaneously increase crop nitrogen (N) uptake and minimize residual nitrate contents to enhance N utilization efficiency. Based on a precipitation manipulation experiment conducted during 2019–2022 in a semi-humid area, we examined how annual precipitation amount during wheat growing season (125 mm = dry year; 200 mm = normal year; 275 mm = wet year) and the soil water storage before sowing (500 mm, SWSS500; 400 mm, SWSS400; 300 mm, SWSS300) variability affected the N uptake, N utilization efficiency, yield, and residual soil nitrate content in wheat fields treated with N fertilization at different depths (conventional surface fertilization depth of 5 cm: D5; 15 cm: D15; 25 cm: D25; 35 cm: D35). A complementary field experiment was also performed under natural rainfall condition using the 15N tracer technique to quantify the fate of fertilizer-N and 15N use efficiency (15NUE). 15N analyses showed that DF (D15, D25 and D35) significantly increased 15NUE by improving wheat 15N uptake (5.8–15.3%) and reducing potential 15N loss (9.6–12.3%) compared with D5, but the residual soil 15N in the 0–1.0 m layer increased (13.2–26.3%) as the fertilization depth increased (p < 0.05). The simulated precipitation experiment showed that compared with D5, DF significantly increased wheat N uptake, N utilization efficiency and yield by 1.4–16.4%, 1.4–14.0% and 3.3–21.5%, respectively, in the dry and normal years. In the wet year, N uptake, N utilization efficiency and yield under D15 increased by 1.9–9.8%, 2.1–11.1% and 2.2–13.4% compared to other fertilization depth treatments (p < 0.05). SWSS500 and SWSS400 significantly increased the N uptake (9.8–65.0%) and yield (0.6–69.3%) under all fertilization depth treatments irrespective of the annual precipitation amount than SWSS300 (p < 0.05). DF increased the residual NO3–-N concentration (0–2.0 m soil layer) by 2.2–7.3% compared to D5 across all annual precipitation amounts and SWSS levels, and D35 increased the maximum downward movement of NO3–-N to 1.4 m under wet year within SWSS500. DF at the depth of 14–20 cm effectively improved wheat N uptake and yield as well as minimizing the residual soil NO3–-N content. Applying moderate DF can enhance sustainable wheat production with high efficiency and yields in semi-humid regions.

Long-term fermented organic fertilizer application reduce urea nitrogen-15 loss from plastic shed agricultural soils

Continuous application of fermented organic fertilizer can improve soil quality, while the performance of nitrogen (N) in the improved soils is rarely investigated. To investigate the fate of applied N in the soils with organic management history, the 15NH2CO15NH2 (15N abundance of 19.6 %) was employed as the exogenous N source to conduct an experiment in the Chinese cabbage and tomato rotation system under plastic shed condition. The cultivated soils have received 15-year of effective microorganism (EM) fermented organic fertilizer (EM-OF), N, P, K inorganic fertilizer (NPK-IF) and no fertilizer (NoF). The 15N use by cabbage and tomato, the soil 15N forms, as well as the 15N distribution were observed. Results showed that the 15N use efficiency of cabbage in the EM-OF, NPK-IF and NoF soils were 55.1 %, 37.3 % and 26.6 % respectively, showing significant (p ≤ 0.05) differences. The succeeding crop tomato could take up the soil residual 15N, and the highest 15N reuse efficiency was 7.1 % that detected in the NoF soil. The total 15N loss (6.0 %) from the rotation system was the lowest in the EM-OF soil, compared to that in the NPK-IF and NoF soils. It was concluded that the long-term fermented organic fertilizer applied soils can reduce urea 15N loss from plastic shed agriculture, mainly through improving the in-season crop 15N use efficiency.

Denitrification Losses in Response to N Fertilizer Rates—Integrating High Temporal Resolution N2O, In Situ 15N2O and 15N2 Measurements and Fertilizer 15N Recoveries in Intensive Sugarcane Systems

AbstractDenitrification is a key process in the global nitrogen (N) cycle, causing both nitrous oxide (N2O) and dinitrogen (N2) emissions. However, estimates of seasonal denitrification losses (N2O + N2) are scarce, reflecting methodological difficulties in measuring soil‐borne N2 emissions against the high atmospheric N2 background and challenges regarding their spatio‐temporal upscaling. This study investigated N2O + N2 losses in response to N fertilizer rates (0, 100, 150, 200, and 250 kg N ha−1) on two intensively managed tropical sugarcane farms in Australia, by combining automated N2O monitoring, in situ N2 and N2O measurements using the 15N gas flux method and fertilizer 15N recoveries at harvest. Dynamic changes in the N2O/(N2O + N2) ratio (&lt;0.01 to 0.768) were explained by fitting generalized additive mixed models (GAMMs) with soil factors to upscale high temporal‐resolution N2O data to daily N2 emissions over the season. Cumulative N2O + N2 losses ranged from 12 to 87 kg N ha−1, increasing non‐linearly with increasing N fertilizer rates. Emissions of N2O + N2 accounted for 31%–78% of fertilizer 15N losses and were dominated by environmentally benign N2 emissions. The contribution of denitrification to N fertilizer loss decreased with increasing N rates, suggesting increasing significance of other N loss pathways including leaching and runoff at higher N rates. This study delivers a blueprint approach to extrapolate denitrification measurements at both temporal and spatial scales, which can be applied in fertilized agroecosystems. Robust estimates of denitrification losses determined using this method will help to improve cropping system modeling approaches, advancing our understanding of the N cycle across scales.

Higher lime rates for greater nitrogen recovery: A long-term no-till experiment labeled with 15N

Soil acidity limits crop growth and yield all over the world. Low grain yields is usually associated with poor soil fertility; however, little attention has been given to the nitrogen-based fertilizer use efficiency in soils managed with lime. Given the current scenario of uncertainties regarding the availability and prices of fertilizers, our study aimed to understand how maize intercropped with ruzigrass and soybean plants develop in long-term soils managed with lime rates, and what the fate of the 15N–labeled ammonium sulfate [(15NH4)2SO4] applied in the soil-plant system. The treatments consisted of four dolomitic lime rates applied to the soil surface [control, half the recommended lime rate (½ RLR), full recommended lime rate (1 RLR) and double the recommended lime rate (2 RLR)]. The higher lime rate (2 RLR) improved fertility, carbon and nitrogen stocks in the soil profile, and grain and/or stover production of maize, ruzigrass and soybean. As a consequence, maize and ruzigrass recovered a high amount of 15N-fertilizer. On the other hand, soybean recovered less 15N-fertilizer, regardless of treatment, but a greater amount was found in acidic soils. At the end of the maize and soybean growth cycles, our results showed that in 2 RLR-amended soil, the 15N unrecovered was 71% lower than control. Finally, our results suggested that the use of low lime rates (½ RLR) may increase the 15N losses potential to deep layers, whereas low amounts of 15N were found in the subsoil when higher lime rates were applied. Soil acidity management through higher lime rates leads, over time, to increased soil fertility, resulting in a favorable environment for plant growth and the use of nitrogen fertilizers. In this way, it is possible to obtain a more productive and less costly agricultural system, and with less potential to pollute the environment.

Fate of 15N-labelled urea as affected by long-term manure substitution

Quantifying the fate of fertilizer nitrogen (N) is essential to develop more sustainable agricultural fertilization practices. However, the fate of chemical fertilizer N, particularly in long-term manure substitution treatment regimes, is not fully understood. The present study aimed to investigate the fate of 15N-labelled urea in a chemical fertilizer treatment (CF, 240 kg 15N ha−1) and N manure 50 % substitution treatment (1/2N + M, 120 kg 15N ha−1 + 120 kg manure N ha−1) in two continuous crop seasons, based on a 10-year long-term experiment in the North China Plain (NCP). The results showed that manure substitution greatly enhanced 15N use efficiency (15NUE) (39.9 % vs. 31.3 %) and suppressed 15N loss (6.9 % vs. 7.5 %) compared with the CF treatment in the first crop. However, the N2O emissions factor in the 1/2N + M treatment was increased by 0.1 % (0.5 kg 15N ha−1 for CF vs. 0.4 kg 15N ha−1 for 1/2N + M) compared with the CF treatment, although N leaching and NH3 volatilization rates decreased by 0.2 % (10.8 kg 15N ha−1 for CF vs. 5.1 kg 15N ha−1 for 1/2N + M) and 0.5 % (6.6 kg 15N ha−1 for CF vs. 2.8 kg 15N ha−1 for 1/2N + M), respectively. In which, only NH3 volatilization presented significantly difference between treatments. It is important to note that in the second crop, the residual 15N in soil (0–20 cm) remained mostly in the soil for the CF (79.1 %) and the 1/2N + M treatment (85.3 %), and contributed less to crop N uptake (3.3 % vs. 0.8 %) and leached losses (2.2 % vs. 0.6 %). This proved that manure substitution could enhance the stabilization of chemical N. These results suggested that long-term manure substitution effectively increases NUE, suppresses N loss, and improves N stabilization in soil, but negative impacts such as N2O emissions due to climate change should be investigated further.

Plant availability and leaching of 15N‐labelled mineral fertilizer residues retained in agricultural soil for 25 years: A lysimeter study

AbstractBackgroundA high use‐efficiency of fertilizer N remains essential to sustain high crop productivity with low environmental impact. However, little is known on the long‐term lability of mineral fertilizer N.AimsTo quantify crop uptake and leaching of 15N‐labelled mineral fertilizer that has been retained in an agricultural soil for 25–30 years in crops with variable growing season.MethodsA field plot received 15N‐labelled mineral fertilizers over a period of 5 years and was then kept under arable cropping for 12 years. After relocation to 16 lysimeters, the topsoil grew set‐aside grassland for the next 13 years. Then crop uptakes and leaching losses of 15N remaining in soil was tested over a 2‐year period by either converting set‐aside grass to production grassland, or by replacing it with spring barley (+/− autumn cover crop) or vegetation‐free fallow. All treatments received unlabelled mineral N fertilizers.ResultsCrop uptake and leaching of 15N were generally highest in the first test year after termination of the set‐aside. The leaching of residual 15N in soil declined in the order: vegetation‐free soil (4.7%), spring barley (1.9%), spring barley + cover crop (0.7%) and production grassland (0.2%). Corresponding losses for the second leaching period were 2.7%, 0.9%, 0.4% and 0.06%. There was a fixed relationship between leaching losses of 15N and total N.ConclusionsAfter residing in soil for 25–30 years, the lability of labelled mineral N fertilizer residues appeared slightly higher than the lability of bulk soil N. Autumn vegetation was crucial for reducing leaching losses.

Fate and recovery of nitrogen applied as slow release brown coal-urea in field microcosms: 15N tracer study.

The over-use of synthetic nitrogen (N) fertilisers for crop production can cause environmental pollution through leaching and gaseous losses, resulting in low N use efficiency (NUE). Previous work has shown that brown coal (BC) combined with urea can slow down the fertiliser-N release to better synchronise soil N supply with crop N demand. The study aimed to evaluate the impact of granulated BC-urea (BCU) applied to sweet corn on NUE, fate and recovery of fertiliser-N using an 15N tracer technique. In this in-field microcosm study, 10 atom percent enriched 15N-labelled urea (46% N) and BCU (20% N) were applied as N fertilisers at rates of 90 or 180 kg N ha-1. On average, BCU fertiliser reduced the urea-derived 15N losses as nitrous oxide (N2O) by 64%, ammonia (NH3) by 73% and downward movement of total N by 59% compared to urea. Reduced losses of applied BCU fertiliser-15N were associated with significantly increased microbial immobilisation, soil retention and availability of fertiliser-15N to plants for longer periods of time, compared with urea. As a result, BCU enhanced cob yield by an average of 23%, 15N uptake by 21% and fertiliser NUE by 21% over urea. The plant recovery of fertiliser-15N was significantly higher from BCU (59%) than the recovery from urea (38%). Moreover, mining of native soil-N was lower when the N-fertiliser source was BCU cf. urea, suggesting that BCU could be used as a more N-efficient alternative to urea in cropping systems.

Aboveground herbivory does not affect mycorrhiza-dependent nitrogen acquisition from soil but inhibits mycorrhizal network-mediated nitrogen interplant transfer in maize.

Arbuscular mycorrhizal fungi (AMF) are considered biofertilizers for sustainable agriculture due to their ability to facilitate plant uptake of important mineral elements, such as nitrogen (N). However, plant mycorrhiza-dependent N uptake and interplant transfer may be highly context-dependent, and whether it is affected by aboveground herbivory remains largely unknown. Here, we used 15N labeling and tracking to examine the effect of aboveground insect herbivory by Spodoptera frugiperda on mycorrhiza-dependent N uptake in maize (Zea mays L.). To minimize consumption differences and 15N loss due to insect chewing, insect herbivory was simulated by mechanical wounding and oral secretion of S. frugiperda larvae. Inoculation with Rhizophagus irregularis (Rir) significantly improved maize growth, and N/P uptake. The 15N labeling experiment showed that maize plants absorbed N from soils via the extraradical mycelium of mycorrhizal fungi and from neighboring plants transferred by common mycorrhizal networks (CMNs). Simulated aboveground leaf herbivory did not affect mycorrhiza-mediated N acquisition from soil. However, CMN-mediated N transfer from neighboring plants was blocked by leaf simulated herbivory. Our findings suggest that aboveground herbivory inhibits CMN-mediated N transfer between plants but does not affect N acquisition from soil solutions via extraradical mycorrhizal mycelium.

Fertilizer 15N Fates of the Coastal Saline Soil-Wheat Systems with Different Salinization Degrees in the Yellow River Delta

In order to clarify the fates of fertilizer N in coastal saline soil-wheat systems with different salinization degrees, this study was conducted to determine the 15N uptake rates in various parts of wheat plant at maturity stage and the residual 15N in three different saline soils and the 15N loss of soil-wheat systems by using the 15N-labeled urea N tracing method in the Yellow River Delta. The results showed that: (1) The increase of soil salinity from 0.2% to 1% promoted the wheat plant to absorb N from soil and not from fertilizer and significantly inhibited the dry matter mass accumulation and 15N uptakes of each wheat parts and whole plant, but especially increased the total N concentration of wheat roots, stems, leaves, and grains. The aggravation of soil salinity significantly enhanced the distribution ratios of 15N uptakes and Ndffs in the wheat roots, stems, and leaves to depress the salt stress. (2) The 15N residues were mainly concentrated in the 0~20 cm saline soil layer and decreased as the soil profile deepened from 0 to 100 cm; the 15N residues decreased in the 0~40 cm soil profile layer and accumulated in the 40~100 cm with the increase of soil salinization degrees significantly. (3) The fates of 15N applied to the coastal saline soil-wheat system were wheat uptakes 1.53~13.96%, soil residues 10.05~48.69%, losses 37.35~88.42%, with the lowest 15N uptake and utilization in the three saline soils, the highest residual rate in lightly saline soils, and the highest loss in moderately and heavily saline soils. The increase of soil salinity inhibits wheat uptakes and soil residues and intensifies the losses from fertilizer 15N. Therefore, the fate of fertilizer N losses significantly increased as the degree of soil salinity increased. The conventional N management that was extremely inefficient for more N loss should be optimized to enhance the N efficiency and wheat yield of the coastal saline soil-wheat system in the Yellow River Delta.

Hole fertilization in the root zone facilitates maize yield and nitrogen utilization by mitigating potential N loss and improving mineral N accumulation

Reducing environmental impacts and improving N utilization are critical to ensuring food security in China. Although root-zone fertilization has been considered an effective strategy to improve nitrogen use efficiency (NUE), the effect of controlled-release urea (CRU) applied in conjunction with normal urea in this mode is unclear. Therefore, a 3-year field experiment was conducted using a no-N-added as a control and two fertilization modes (FF, furrow fertilization by manual trenching, i.e., farmer fertilizer practice; HF: root-zone hole fertilization by point broadcast manually) at 210 kg N ha–1 (controlled-release:normal fertilizer=5:5), along with a 1-year in-situ microplot experiment. Maize yield, NUE and N loss were investigated under different fertilization modes. The results showed that compared with FF, HF improved the average yield and N recovery efficiency by 8.5 and 22.3% over three years, respectively. HF had a greater potential for application than FF treatment, which led to increases in dry matter accumulation, total N uptake, SPAD value and LAI. In addition, HF remarkably enhanced the accumulation of 15N derived from fertilizer by 17.2% compared with FF, which in turn reduced the potential loss of 15N by 43.8%. HF increased the accumulation of N in the tillage layer of soils at harvest for potential use in the subsequent season relative to FF. Hence, HF could match the N requirement of summer maize, sustain yield, improve NUE and reduce environmental N loss simultaneously. Overall, root-zone hole fertilization with blended CRU and normal urea can represent an effective and promising practice to achieve environmental integrity and food security on the North China Plain, which deserves further application and investigation.

Different soil particle size changes the 15N retention in soil and 15N utilization by maize

Improving soil structure is key to improving soil quality and nitrogen utilization efficiency (NUE) in global cropping systems. Soil sieving is a direct means of dividing soil activity into components with different practical functional significance. In order to have a more intuitive understanding of nutrient turnover and NUE under different soil particle sizes, we identified the key soil particle sizes that affect N cycling in cropping systems. An in-situ field experiment was conducted in Mollisol for three years using the 15N isotope tracer technique to examine the effect of soil structure on maize growth, nutrient uptake, and 15NUE. We artificially destructed the soil structure by sieving it into four particle size classes: (i) unsieved soil (CK), (ii) <0.5 mm size (A), (iii) 0.5–2 mm size (B), and (iv) 2–5 mm size (C). Then each year, the physical properties of the soil, 15N loss and retention rate (15NRR), the 15NUE, 15N absorption, and distribution, as well as maize growth and yield, were measured. The results showed that the 0.5–2 mm size (B) and 2–5 mm size (C) improved soil physical properties and increased the uptake of 15N by maize (especially in the leaves and grains), thereby increasing maize yield. The B and C particle sizes had lower soil 15NRR and higher alkaline hydrolysable N content than the other treatments in the depth. We concluded that the B sieving treatment is the key aggregate fraction to increasing maize 15NUE and yield and minimizing the negative effects on soil N retention capacity. Furthermore, the B treatment is key to affecting crop nutrient absorption and utilization.

Amplitude and frequency of wetting and drying cycles drive N2 and N2O emissions from a subtropical pasture

This study investigated the effects of irrigation frequency on N2 and N2O emissions from an intensively managed pasture in the subtropics. Irrigation volumes were estimated to replace evapotranspiration and were applied either once (low frequency) or split into four applications (high frequency). To test for legacy effects, a large rainfall event was simulated at the end of the experiment. Over 15 days, 7.9 ± 2.7 kg N2 + N2O-N ha−1 was emitted on average regardless of irrigation frequency, with N2O accounting for 25% of overall N2 + N2O. Repeated, small amounts of irrigation produced an equal amount of N2 + N2O losses as a single, large irrigation event. The increase in N2O emissions after the large rainfall event was smaller in the high-frequency treatment, shifting the N2O/(N2O + N2) ratio towards N2, indicating a treatment legacy effect. Cumulative losses of N2O and N2 did not differ between treatments, but higher CO2 emissions were observed in the high-frequency treatment. Our results suggest that the increase in microbial activity and related O2 consumption in response to small and repeated wetting events can offset the effects of increased soil gas diffusivity on denitrification, explaining the lack of treatment effect on cumulative N2O and N2 emissions and the abundance of N cycling marker genes. The observed legacy effect may be linked to increased mineralisation and subsequent increased dissolved organic carbon availability, suggesting that increased irrigation frequency can reduce the environmental impact (N2O), but not overall magnitude of N2O and N2 emissions from intensively managed pastures.

Earthworms promote the transfer of 15N-urea to lettuce while limit appreciably increase 15N losing to environment

Earthworm activities not only increase nitrogen (N) uptake by crops, but also lead to N losing to environment. Thus, the present study examined the transformation of 15N-labeled urea with and without earthworms (Metaphire guillelmi) in a soil-lettuce system. We evaluated lettuce 15N uptake, 15N losses including N2O emission, NH3 volatilization and leaching, as well as 15N remaining in soil. Results showed that 15N-urea uptakes by lettuce significantly increased from 33.07% to 42.72% with earthworm presence. However, little difference was found on the total amounts of leaching and gaseous losses (N2O emission and NH3 volatilization) from 15N-urea between the treatment with and without earthworms (4.04 and 5.38%, respectively). Most of the 15N-urea remained in the soil, accounting for 48.44% and 60.65% of the 15N-urea in soil with and without earthworm presence. We conclude that earthworms enhanced the transfer of 15N-urea to lettuce without appreciably increasing the 15N-urea loss from soil to the environment.

Interaction between soil and fertiliser nitrogen drives plant nitrogen uptake and nitrous oxide (N2O) emissions in tropical sugarcane systems

AimsHigh nitrogen (N) fertiliser inputs in intensive sugarcane systems drive productivity but also significant emissions of nitrous oxide (N2O), a potent greenhouse gas. Fertiliser and soil N availability for both plant N uptake and N2O emissions across different N rates remain unknown, hindering efficient N management. This study investigated the contribution of fertiliser and soil N and their interaction to plant N uptake and N2O emissions in two intensively managed tropical sugarcane systems.MethodsHigh temporal resolution N2O measurements were combined with 15N recoveries across four N fertiliser rates, (100, 150, 200 and 250 kg N ha− 1) in soil, plant and N2O emissions.ResultsCumulative N2O emissions ranged from 0.3 to 4.1 kg N ha− 1, corresponding to emission factors ranging from 0.7 to 2.4%. Native soil N accounted for > 60% of cumulative N2O emissions and total plant N uptake. Fertiliser N addition increased N2O emissions from native soil N compared to the unfertilised control, highlighting the interaction between fertiliser and soil N, which determined the overall magnitude but also the response of total N2O emissions to N rates dependent on the site conditions. Overall fertiliser 15N loss responded exponentially to N rates with 50% of applied N fertiliser permanently lost even at the recommended N rate.ConclusionsThe interaction between fertiliser and soil N and its contribution to N uptake and N2O emissions demonstrate the importance of integrating soil fertility management with N fertiliser rate recommendations for sugarcane systems to maintain crop productivity and reduce environmental impacts.