Soil Nitrogen Supply Research Articles

Soil Nitrogen Supply: Linking Plant Available N to Ecosystem Functions and Productivity

Nitrogen (N), a common chemical element in the atmosphere (78% of our atmosphere) yet less common within the Earth’s crust (less than 2%), is a crucial nutrient for life [...]

Effects of Irrigation and Nitrogen Application on Soil Nutrients in Triploid Populus tomentosa Stands

Irrigation and nitrogen application directly affect the availability and distribution of soil nutrients. Understanding the response of soil nutrients to long-term water–fertilizer coupling conditions is helpful to improve the management and use efficiency. Irrigation was divided into three gradient levels, which accounted for 45%, 60%, and 75% (W1, W2, and W3) of the field water holding capacity. Based on pure nitrogen, four levels of nitrogen application were set: 0.0, 101.6, 203.2, and 304.8 kg·hm−2 (N0, N1, N2, and N3). We measured tree height and diameter at breast height (DBH), and we analyzed the chemical properties of the soil at 0–40 cm depth, from 2007 to 2020. The ranges of DBH, tree height, individual volume, and stand volume were 5.80–25.25 cm, 6.10–16.47 m, 0.01–0.37 m3, and 11.76–481.47 m3·hm−2, respectively. The contents of organic matter, total nitrogen, available phosphorus, and available potassium in the soil ranged from 8.60 g·kg−1 to 18.72 g·kg−1, from 0.21 g·kg−1 to 0.79 g·kg−1, from 8.09 mg·kg−1 to 47.05 mg·kg−1, and from 90 mg·kg−1 to 322 mg·kg−1, respectively. Soil pH value decreased rapidly at a rate of 0.31 units per year for the first five years. Irrigation and nitrogen application, and their interaction, had significant (p < 0.01) effects on soil total nitrogen, available phosphorus, available potassium, and nitrate-nitrogen. We suggest maintaining the field water holding capacity above 60%, with a nitrogen application rate of 203.2 kg·hm−2, to save water, maintain soil fertility, and optimize soil nitrogen supply. Our study aimed to achieve scientific and accurate fertilization of Populus tomentosa stands over different periods, to alleviate the decline of soil fertility, and to improve the utilization rate of water and fertilizer through long-term nutrient monitoring.

Carbon, Nitrogen, Phosphorus, and Extracellular Soil Enzyme Responses to Different Land Use

Extracellular soil enzymes play a key role in soil organic matter decomposition and nutrient cycling. However, it is not fully understood how these enzymes respond to different land use. Long-term research studies were used to evaluate how diversified management practices affect extracellular enzymes driving C cycling [phenol oxidases (PO), peroxidases (PP), α-glucosidase (AG), β-glucosidase (BG), cellobiohydrolase (CB), β-1,-4-Nacetylglucosaminidase (NAG)], N cycling [leucine aminopeptidase (LAP)], and P cycling [phosphomonoesterase (PME)]. The soil pH, contents of total organic C, total N, mehlich-3 P, soil respiration and soil nitrogen supply capacity were also measured. Different land use included tillage frequency, tillage regimes, mineral N fertilization, crop rotations and liquid dairy manure. Compared to medium or high tillage frequency, low tillage frequency increased total organic C and total N and soil respiration as well as NAG and PME activities, whereas it decreased soil nitrogen supply, mehlich-3 P, and soil pH, as well as PO, PP, AG, BG, CB, and LAP activities. Non till was associated with lower PP and PO activities than moldboard plow. Nitrogen fertilization decreased soil pH and PO activity but increased PME activity. Barley (Hordeum vulgare) in rotation with forage increased total organic C, total N, soil nitrogen supply and soil respiration by 31, 21, 44, and 33%, respectively, in comparison with barley in monoculture. The application of liquid dairy manure increased soil pH, total N and soil nitrogen supply and soil enzyme activities (AG, BG, NAG) in comparison to the mineral N fertilizer. When principal component analysis was performed, soil pH, PO, PP, CB, LAP, and PME were grouped in the first component, which explained the highest variance. This is the core group controlling the C, N, and P cycling. The activities of C, N, and P acquiring enzymes, soil nitrogen supply and soil respiration were related to changes in soil total C and N, and extractable P contents across a broad range of management practices. Increased PO and PP activities reflect total C decline.

Combined nitrogen and phosphorus management based on nitrate nitrogen threshold for balancing crop yield and soil nitrogen supply capacity

An appropriate combined nitrogen (N) and phosphorus (P) fertilization strategy is essential for obtaining sustained higher grain yields while maintaining soil fertility. In this study, a long-term split-plot design farmland experiment (initiated in 2009) with five N fertilizer rates combined with four P fertilizer rates was established during 2016–2019 to determine an appropriate nitrate-N (NO3-N) threshold in an intensive managed wheat–maize double cropping system. A fertilization strategy was then proposed based on the NO3-N threshold to balance the crop yields and soil nitrogen supply capacity. The results showed that N fertilizer increased the accumulated NO3-N, and the combined application of phosphate fertilizer with each N rate reduced the accumulated NO3-N to different degrees. The residual soil NO3-N reached a steady-state soil N pool balance after long-term application of N at 150–225 kg ha−1 combined with P at 60–120 kg ha−1. The residual NO3-N threshold in the root zone (0–100 cm soil layer) was determined as about 100 kg ha−1 at the crop harvest to maintain the N supply capacity and prevent leaching into the deep soil (>100 cm soil layer). The fertilization guidelines are 154 kg ha−1 for N fertilizer and 106 kg ha−1 for P fertilizer in the winter wheat season, and 162 kg ha−1 for N fertilizer and 122 kg ha−1 for P fertilizer in the summer maize growing season based on the NO3-N safety threshold. The optimized fertilizer strategy reduced the fertilizer application rate by 67 kg N ha−1 per year and the residual NO3-N by 34.2 % in the deep soil, while only reducing the average yield by 3.1 % across the crops and years. These findings provide a basis for sustainably balancing grain yields and soil nitrogen supply capacity as well as preventing nitrate pollution of farmland.

Response of plant functional traits to nitrogen enrichment under climate change: A meta-analysis

Soil nitrogen (N) supply is essential in influencing plant functional traits and regulating plant morphological and physiological performances. The effects of N on plants can be altered by complex environmental changes. However, conflicting results have been reported on the co-effects of N and climatic variables on plant performance, which may be attributed to differences in experiment setting and approach, e.g., ecosystem, duration, plant type, and fertilizer form. To elucidate the general response of plant performance to increasing soil N availability under climate change, a global meta-analysis was conducted to synthesize 380 publications studying interactions of N enrichment and four climatic variables (e.g., elevated atmospheric CO2 (eCO2), drought, precipitation, and warming) on performance-related traits (e.g., size, nutrient, and fitness). Results showed that N enrichment increased shoot and root size, nutrient, and fitness of terrestrial plants. The synergistic interactions of N × eCO2 and antagonistic interactions of N × drought were found on plant overall performance (mainly on plant size), indicating that the N effects can be aggregated by eCO2 and mitigated by drought. The co-effects of N and climatic variables on plant overall performance rely on experiment approach, duration, ecosystem type, or plant functional type. Synergistic interactions of N × eCO2 and antagonistic interactions of N × drought, N × precipitation, and N × warming on plant overall performance were found mainly in greenhouse experiments and short-term experiments (duration ≤ one year), but not in the field or longer-term experiments. The results highlighted that N effects on plant performance were not isolated, but can be modified by climate changes. These findings can improve the future modeling predictions of plant performance under complex climate change and provide a fundamental basis for N management strategies to optimize plant performance in production, N nutrient, and reproduction while enabling sustainability of plant production systems.

Growth and photosynthetic responses to ozone of Siebold's beech seedlings grown under elevated CO2 and soil nitrogen supply

Ozone (O3) is a phytotoxic air pollutant, the adverse effects of which on growth and photosynthesis are modified by other environmental factors. In this study, we examined the combined effects of O3, elevated CO2, and soil nitrogen supply on Siebold's beech seedlings. Seedlings were grown under combinations of two levels of O3 (low and two times ambient O3 concentration), two levels of CO2 (ambient and 700 ppm), and three levels of soil nitrogen supply (0, 50, and 100 kg N ha−1 year−1) during two growing seasons (2019 and 2020), with leaf photosynthetic traits being determined during the second season. We found that elevated CO2 ameliorated O3-induced reductions in photosynthetic activity, whereas the negative effects of O3 on photosynthetic traits were enhanced by soil nitrogen supply. We observed three-factor interactions in photosynthetic traits, with the ameliorative effects of elevated CO2 on O3-induced reductions in the maximum rate of carboxylation being more pronounced under high than under low soil nitrogen conditions in July. In contrast, elevated CO2-induced amelioration of the effects of O3 on stomatal function-related traits was more pronounced under low soil nitrogen conditions. Although we observed several two- or three-factor interactions of gas and soil treatments with respect to leaf photosynthetic traits, the shoot to root dry mass (S/R) ratio was the only parameter for which a significant interaction was detected among seedling growth parameters. O3 caused a significant increase in S/R under ambient CO2 conditions, whereas no similar effects were observed under elevated CO2 conditions. Collectively, our findings reveal the complex interactive effects of elevated CO2 and soil nitrogen supply on the detrimental effects of O3 on leaf photosynthetic traits, and highlight the importance of taking into consideration differences between the responses of CO2 uptake and growth to these three environmental factors.

Biochar Application Increases Labile Carbon and Inorganic Nitrogen Supply in a Continuous Monocropping Soil

Biochar is an effective method for increasing soil carbon (C) sequestration and nitrogen (N) supply under continuous monocropping. To investigate the impact of biochar placement methods on soil C and N, a one-year field experiment with five treatments was conducted including control, mineral fertilizers only (F), biochar hole placement (BFH; biochar applied to the soil layer at 5–10 cm) + F, biochar band placement (BFB; biochar applied to the soil layer at 15–20 cm) + F, and biochar band and hole placement + F (BFBH). The results showed that, regardless of the placement method, biochar application increased soil total organic C (TOC) and C pool management index by 6.9–39.7% and 4.1–36.1%, respectively, especially for dissolved organic C (DOC; 6.9–51.3%), readily oxidizable C (ROC; 2.4–46.4%), and microbial biomass C (MBC; 10.4–41.7%). Single biochar placement methods significantly influenced DOC, MBC, and ROC contents of both soil layers in the rank order of BFH ≈ BFBH > BFB at 0–15 cm and BFB ≈ BFBH > BFH at 15–30 cm. Soil TN and microbial biomass N (MBN) mainly accumulated at the site of biochar placement. The increased soil TOC:TN and MBC:N ratios under biochar treatments promoted inorganic N immobilization and reduced the loss of ammonium N and nitrate N (NO3−-N) through leaching at the early stage of tobacco growth. Biochar-adsorbed N was remobilized at a later period (vigorous growth stage and maturity), possibly causing the slow decrease in NO3−-N content. Additionally, soil C and N pools were significantly influenced by the main effects of soil layer and growth stage. Overall, biochar application increased soil C and N pools and inorganic N supply through N remobilization. However, the increased labile organic C content and microbial activity may prevent C sequestration in biochar-amended soils.

Rapid Detection of Different Types of Soil Nitrogen Using Near-Infrared Hyperspectral Imaging.

Rapid and accurate determination of soil nitrogen supply capacity by detecting nitrogen content plays an important role in guiding agricultural production activities. In this study, near-infrared hyperspectral imaging (NIR-HSI) combined with two spectral preprocessing algorithms, two characteristic wavelength selection algorithms and two machine learning algorithms were applied to determine the content of soil nitrogen. Two types of soils (laterite and loess, collected in 2020) and three types of nitrogen fertilizers, namely, ammonium bicarbonate (ammonium nitrogen, NH4-N), sodium nitrate (nitrate nitrogen, NO3-N) and urea (urea nitrogen, urea-N), were studied. The NIR characteristic peaks of three types of nitrogen were assigned and regression models were established. By comparing the model average performance indexes after 100 runs, the best model suitable for the detection of nitrogen in different types was obtained. For NH4-N, R2p = 0.92, RMSEP = 0.77% and RPD = 3.63; for NO3-N, R2p = 0.92, RMSEP = 0.74% and RPD = 4.17; for urea-N, R2p = 0.96, RMSEP = 0.57% and RPD = 5.24. It can therefore be concluded that HSI spectroscopy combined with multivariate models is suitable for the high-precision detection of various soil N in soils. This study provided a research basis for the development of precision agriculture in the future.

Nitrate Uptake from an Aquifer by Two Plantation Forests: Plausibility Strengthened by Process-Based Modelling

Forest plantations can access water from some unconfined aquifers that also contain nitrate at concentrations that could support hydroponic culture, but the separate effects of such additional water and nitrogen availability on tree growth have not hitherto been quantified. We demonstrate these effects using simulation modelling at two contrasting sites supporting Eucalyptus globulus Labill. or Pinus radiata D.Don plantations. The APSIM Eucalyptus and Pinus models simulated plantation growth within 2% of observed growth where the water table was at 4 m depth for eucalypts (height 28 m, MAI 32 m3 ha−1 year−1) and at 23 m for pines (height 37 m, MAI 20 m3 ha−1 year−1). In simulations without an aquifer, observed growth could only be matched using unrealistically high surface soil nitrogen (N) supply, suggesting this is an unlikely mechanism. Simulated aquifer N concentrations, evapotranspiration, and net N mineralization and leaching (emergent properties of modelling) were similar to measured values. These results strengthen the plausibility that aquifer N uptake by plantations could be contributing to tree growth. This hypothesis warrants further research that quantifies these processes at multiple sites. Simulations included growth of herbaceous and tree weed species, and pasture, which demonstrated the utility of the process-based APSIM modelling framework for dynamically simulating carbon, water and N of plantations and other mixed-species systems.

Poređenje florističkog sastava brdske bukove šume i veštačkih podignutih sastojina smrče području Kosmaja

The paper presents the comparative characteristics of the floristic composition in the association of the submontane beech forest (Helleboro odori-Fagetum moesiacae Soo & Borhidi 1960.) and artificially established spruce stands (Picea abies (L.) Karst.) in the area of Kosmaj. According to the obtained results, the cover in the shrub and ground flora layers of the artificially established spruce stands increased significantly compared to the autochthonous beech forests, but the floristic diversity decreased. Regarding the spectrum of floral elements, a higher percentage of mesophilic (51%) and frigophilic plants (11%) was observed in the artificially established spruce stands compared to the beech forests. Significant differences could also be observed regarding the spectrum of life forms since there were twice as many phanerophytes in the artificially established spruce stands (48%) than in the beech forests (23%). This resulted from the open canopy of artificially established spruce stands and the influx of a greater amount of light, which made conditions favourable for an abundant shrub layer to develop in this community. A comparative study of the mean plant indicator values showed that in the artificially established spruce stands, the mean values of the ecological factors for moisture, light and soil nitrogen supply increased, while the value of the ecological factors for temperature and soil acidity decreased compared to the beech forests.

Genetic analysis of maize grain yield components and physiological determinants under contrasting nitrogen availability

Context Most maize breeding is conducted under high-input conditions, with nitrogen supply being crucial due to its impact on yield. Aims This study aimed to investigate broad-sense heritability, and general and specific combining ability variances of physiological traits defining grain yield under contrasting soil nitrogen supply. Methods A six-parent full diallel cross was analysed under high (fertilised with 200 kg N/ha) and low (unfertilised control) nitrogen supply in two seasons. We measured kernel number per plant and kernel weight, the associated traits of plant growth during the critical and grain-filling periods, and source–sink relationships in both periods. Key results Heritabilities of traits ranged from 0.54 to 0.88, and general surpassed specific combining ability for most traits. At low nitrogen (1) the relative importance of general combining ability estimated by Baker’s ratio increased across traits (low nitrogen: 0.90 vs high: 0.85) because the decrease in combining ability variance was larger for specific than general (–78% vs −39%), and (2) source–sink relationship during grain filling had the highest Baker’s ratio (0.96) and heritability (0.78). Plant growth rates during the critical period and kernel number increased substantially at high nitrogen (40 and 34%, respectively), and they had the highest heritability (0.79 and 0.88) and Baker’s ratio (>0.90). Conclusions Low nitrogen environments increased the relative importance of general combining ability effects, and high yield can be obtained by improving the source–sink relationship during grain filling, whereas high nitrogen increased yield by improving plant growth rate during the critical period and kernel number. Implications Knowledge of source–sink relationship during effective filling period, plant growth during the critical period and kernel number may result in a more targeted selection program.

Comparing Soil vs. Foliar Nitrogen Supply of the Whole Fertilizer Dose in Common Wheat

Late-season N application through foliar spraying is recognized as an efficient agronomic practice for improving grain quality in common wheat, although the major part of N is still supplied by soil fertilization. This study assessed the impact of various N doses entirely applied by repeated foliar sprayings on wheat growth, yield and quality, in comparison with conventional soil fertilization management with a recommended dose of 160 kg N ha−1 as ammonium nitrate (C-M). Doses of 96, 104 and 120 kg N ha−1 as both UAN (urea-ammonium-nitrate) and urea applied by foliar spraying were evaluated in a 2-year field trial in Northern Italy in a silty loam soil with 1.7% organic matter. Here, it was demonstrated that the canopy greenness was similar in all treatments, with slight grain yield increases by the lowest foliar N dose vs. C-M. The higher N foliar doses mainly improved the grain protein content and both high- and low-molecular-weight glutenin subunits (HMW-GS, LMW-GS), particularly with urea. It is concluded that in our fertile soil, managing N fertilization exclusively through foliar spraying is feasible without compromising grain yield and ameliorating quality at the same time. Improved nutrient use efficiency and beneficial environmental effects are also expected by reducing the nitrogen load on the agricultural fields by 25–40%.

Divergent Pathways of Nitrogen-Fixing Trees through Succession Depend on Starting Nitrogen Supply and Priority Effects.

AbstractNitrogen-fixing trees are a major potential source of nitrogen in terrestrial ecosystems. The degree to which they persist in older forests has considerable implications for forest nitrogen budgets. We characterized nitrogen-fixing tree abundance across stand age in the contiguous United States and analyzed a theoretical model to help understand competitive outcomes and successional trajectories of nitrogen-fixing and nonfixing trees. Nitrogen-fixing tree abundance is bimodal in all regions except the northeastern United States, even in older forests, suggesting that competitive exclusion (including priority effects) is more common than coexistence at the spatial scale of our analysis. Our model analysis suggests conditions under which alternative competitive outcomes are possible and when they are transient (lasting decades or centuries) versus persistent (millennia). Critically, the timescale of the feedbacks between nitrogen fixation and soil nitrogen supply, which is thought to drive the exclusion of nitrogen-fixing trees through succession, can be long. Therefore, the long transient outcomes of competition are more relevant for real forests than the long-term equilibrium. Within these long-term transients, the background soil nitrogen supply is a major determinant of competitive outcomes. Consistent with the expectations of resource ratio theory, competitive exclusion is more likely at high and low nitrogen supply, while intermediate nitrogen supply makes coexistence or priority effects possible. However, these outcomes are modified by the nitrogen fixation strategy: obligate nitrogen fixation makes coexistence more likely than priority effects, compared with perfectly facultative fixation. These results advance our understanding of the successional trajectories of nitrogen-fixing trees and their effects on ecosystem development in secondary succession.

The analysis of environmental space conditions for bryoflora forming in artificial pine communities of the Buzuluksky pine forest and the Krasnosamarsky forest

The paper deals with ecological conditions and peculiarities of bryoflora of artificial pine communities of the Buzuluksky pine forest and the Krasnosamarsky forest. The authors have revealed that the pine plantings of the Buzuluksky pine forest are characterized by a lower soil moisture and nitrogen supply than similar communities of the Krasnosamarsky forest in accordance with the criteria of D.N. Tsyganov scales. In addition, all communities belonging to the same territory show differences in the acidity and salt regime of soils as well as in moisture contrasts. The differences seem to be the result of a specific manifestation of parallel multidirectional successional processes (afforestation and steppe formation). For the bryofloras of the studied artificial pine communities, a significant similarity level (Czekanowski-Srensen index of 0,7) and close values of the projective cover and vitality of bryophytes have been shown. However, the substrate preferences of bryophytes are significantly different: 85% of the species grow on soil in the Buzuluksky pine forest, while 52% of the species grow on soil and 39% of the species grow on decaying wood in the Krasnosamarsky forest. In addition, more intensive sporulation of bryophytes has been noted in artificial pine communities of the Krasnosamarsky forest and the share of dominant species in the projective cover is 47,7% there, and their share is 83,4% in the Buzuluksky pine forest. The authors have suggested that genetic polymorphism should underlie the revealed differences in the species similarity of bryofloras. Its manifestation is influenced, among other things, by the stage of ecological succession of plant communities in which bryophytes grow. It is the result of the implementation of strategies for bryophytes adaptation to existing conditions and reflects the stage of bryoflora formation.

Sensitivity of grassland carbon pools to plant diversity, elevated CO2, and soil nitrogen addition over 19 years

Whether the terrestrial biosphere will continue to act as a net carbon (C) sink in the face of multiple global changes is questionable. A key uncertainty is whether increases in plant C fixation under elevated carbon dioxide (CO2) will translate into decades-long C storage and whether this depends on other concurrently changing factors. We investigated how manipulations of CO2, soil nitrogen (N) supply, and plant species richness influenced total ecosystem (plant + soil to 60 cm) C storage over 19 y in a free-air CO2 enrichment grassland experiment (BioCON) in Minnesota. On average, after 19 y of treatments, increasing species richness from 1 to 4, 9, or 16 enhanced total ecosystem C storage by 22 to 32%, whereas N addition of 4 g N m-2 ⋅ y-1 and elevated CO2 of +180 ppm had only modest effects (increasing C stores by less than 5%). While all treatments increased net primary productivity, only increasing species richness enhanced net primary productivity sufficiently to more than offset enhanced C losses and substantially increase ecosystem C pools. Effects of the three global change treatments were generally additive, and we did not observe any interactions between CO2 and N. Overall, our results call into question whether elevated CO2 will increase the soil C sink in grassland ecosystems, helping to slow climate change, and suggest that losses of biodiversity may influence C storage as much as or more than increasing CO2 or high rates of N deposition in perennial grassland systems.

Selenium uptake and grain nutritional quality are affected by nitrogen fertilization in rice (Oryza sativa L.).

The effects of selenium in rice grain composition depend on the soil nitrogen supply. Selenium and nitrogen have the potential to modify rice grain composition; however, it is unclear how the combined effect of Se and nitrogen affects the grain nutritional quality of rice. In our study, grain Se concentration was positively associated with the increased availability of nitrogen in soil. The accumulation of Se in grain of rice plants treated with Se combined with nitrogen was accompanied by an increase in expression of NRT1.1B, a rice nitrate transporter and sensor, in root. Moreover, Se potentiates the response of nitrogen supply in expression of sulfate transporter OsSULTR1.2, phosphate transporter OsPT2 and silicon transporter OsNIP2.1 in root, thereby increasing root Se uptake capacity. The combination of Se with high nitrogen increased the concentrations of protein, carbohydrates, Se, Mo and Mg, but decreased concentrations of Fe, Mn, Cu and Zn in grain. Overall, our results revealed that many of the effects of Se in rice grain composition are due to a shift in the nitrogen status of the plant.

Modeling biomass and yield production based on nitrogen accumulation in soybean grown in upland fields converted from paddy fields in Japan

ABSTRACT Crop models can help in identifying constraints to crop production and enhancing crop yield. Because one of the major constraints is the availability of nitrogen for seed production, evaluation of nitrogen accumulation is important for soybean crop models. In the present study, we developed a soybean crop model to evaluate biomass production and nitrogen accumulation of Japanese soybean cultivars grown in upland fields converted from paddy fields. The model simplified the effects of leaf nitrogen deficit on biomass production by reducing only the leaf area while maintaining leaf nitrogen concentration, and we introduced a zero-order reaction kinetics model to estimate soil nitrogen supply. The proposed model was observed to simulate the accumulation of aboveground biomass and nitrogen content in both nodulating and non-nodulating soybean cultivars until the mid-seed filling period. The model accounts for 94% of the observed variations in aboveground biomass and nitrogen content and for 69% of the observed variation in seed biomass. The normalized root mean square errors of aboveground biomass, nitrogen content, and seed biomass are 19.3%, 19.6%, and 20.2%, respectively. Because the model does not include the effects of soil water status on pod formation and nitrogen fixation, seed biomass was overestimated in some cases. However, our model quantified the effects of changes in the soil nitrogen supply and biological nitrogen fixation on soybean production and will be useful for identifying and eliminating production constraints in Japanese soybeans.

Vital roles of soil microbes in driving terrestrial nitrogen immobilization.

Nitrogen immobilization usually leads to nitrogen retention in soil and, thus, influences soil nitrogen supply for plant growth. Understanding soil nitrogen immobilization is important for predicting soil nitrogen cycling under anthropogenic activities and climate changes. However, the global patterns and drivers of soil nitrogen immobilization remain unclear. We synthesized 1350 observations of gross soil nitrogen immobilization rate (NIR) from 97 articles to identify patterns and drivers of NIR. The global mean NIR was 8.77±1.01mgNkg-1 soilday-1 . It was 5.55±0.41mgNkg-1 soil day-1 in croplands, 15.74±3.02mgNkg-1 soilday-1 in wetlands, and 15.26±2.98mgNkg-1 soilday-1 in forests. The NIR increased with mean annual temperature, precipitation, soil moisture, soil organic carbon, total nitrogen, dissolved organic nitrogen, ammonium, nitrate, phosphorus, and microbial biomass carbon. But it decreased with soil pH. The results of structural equation models showed that soil microbial biomass carbon was a pivotal driver of NIR, because temperature, total soil nitrogen, and soil pH mostly indirectly influenced NIR via changing soil microbial biomass. Moreover, microbial biomass carbon accounted for most of the variations in NIR among all direct relationships. Furthermore, the efficiency of transforming the immobilized nitrogen to microbial biomass nitrogen was lower in croplands than in natural ecosystems (i.e., forests, grasslands, and wetlands). These findings suggested that soil nitrogen retention may decrease under the land use change from forests or wetlands to croplands, but NIR was expected to increase due to increased microbial biomass under global warming. The identified patterns and drivers of soil nitrogen immobilization in this study are crucial to project the changes in soil nitrogen retention.

Earthworm contributions to soil nitrogen supply in corn-soybean agroecosystems in Quebec, Canada

Accurately quantifying the soil nitrogen (N) supply in crop fields is essential for enabling environmentally sustainable and economically profitable crop production. It requires using field-based methods to account for the contribution of soil biota, including earthworms, to N mineralization in temperate agroecosystems. The direct contribution of earthworms to the soil N cycle is the N they release throughout their life and after death, and it can be estimated using the secondary production method. This study was conducted in 2014 and 2015 in two adjacent fields with annual corn-soybean rotation in Ste-Anne-de-Bellevue, Quebec, Canada. The cumulative biomass of Aporrectodea spp. in two no-till corn-soybean agroecosystems was determined, and the direct N flux from these earthworms was estimated during the corn and soybean phases of the rotation. Secondary production was estimated by sampling earthworms biweekly during April–June and September–November and inferring the change in earthworm biomass between sampling dates using a size frequency calculation. The N flux was calculated as the sum of the N released through excretion, during periods when earthworms were active, and from mortality. The secondary production of the Aporrectodea population was estimated to be 8–43 g ash-free dry weight m–2 year–1, and the N flux was 22–105 kg N ha–1 year–1. The N flux was higher at the early vegetative growth stage, which is a period of high N demand for corn. These findings suggest that refining the N fertilization recommendation by accounting for soil N supplied by earthworms could potentially reduce fertilizer costs and environmental N losses.

Increased soil nitrogen supply enhances root-derived available soil carbon leading to reduced potential nitrification activity

Nitrogen (N) immobilisation by heterotrophic microorganisms is critical for reducing N losses from soils and ensuring a long-term supply of N to plants in grassland ecosystems. The supply of carbon (C) available to soil microbes may stimulate heterotrophic N immobilisation by reducing the availability of ammonium to autotrophic nitrifiers and, hence, for nitrification activity. The main source of available C to soils is rhizodeposition, but its effects on nitrification activity remain unclear as rhizodeposition differs between plant species and varying N availabilities. The aim of this work was to investigate the role of root-derived C on nitrification activity for five different grassland plant species. Cichorium intybus (chicory), Lolium perenne (perennial ryegrass), Plantago lanceolata (ribwort plantain), Raphanus raphanistrum and R. sativus (wild and cultivated radish), and an unplanted control were grown for nine weeks under controlled environmental conditions and treated either with a low (no urea-N) or a high rate of additional N (550 kg urea-N ha−1). Plant biomass, water-extractable C concentration and ammonia-oxidising bacteria (AOB) abundance increased in the planted high N treatments. The high N addition to planted soils resulted in increased C available for microbial activity and led to decreased potential nitrification activity compared to those for the low N treatments. An increase in water-extractable C concentration was associated with a decrease in potential nitrification activity, suggesting that the increase in available C for microbial activity may have stimulated heterotrophic NH4+ uptake and thus N immobilisation.This study highlights that N addition can be used to manipulate root-derived available C and, with the tight coupling of soil C and N cycling processes, can be used to identify management practices that will promote N retention and reduce losses from grassland soils.