Rethinking sources of nitrogen to cereal crops

Rhizosphere priming promotes plant nitrogen acquisition by microbial necromass recycling

Nitrogen availability in the rhizosphere relies on root-microorganism interactions, where root exudates trigger soil organic matter (SOM) decomposition through the rhizosphere priming effect (RPE). Though microbial necromass contribute significantly to organically bound soil nitrogen (N), the role of RPEs in regulating necromass recycling and plant nitrogen acquisition has received limited attention. We used 15N natural abundance as a proxy for necromass-N since necromass is enriched in 15N compared to other soil-N forms. We combined studies using the same experimental design for continuous 13CO2 labelling of various plant species and the same soil type, but considering top- and subsoil. RPE were quantified as difference in SOM-decomposition between planted and unplanted soils. Results showed higher plant N uptake as RPEs increased. The positive relationship between 15N-enrichment of shoots and roots and RPEs indicated an enhanced necromass-N turnover by RPE. Moreover, our data revealed that RPEs were saturated with increasing carbon (C) input via rhizodeposition in topsoil. In subsoil, RPEs increased linearly within a small range of C input indicating a strong effect of root-released C on decomposition rates in deeper soil horizons. Overall, this study confirmed the functional importance of rhizosphere C input for plant N acquisition through enhanced necromass turnover by RPEs.

Breeding progress of nitrogen use efficiency of cereal crops, winter oilseed rape and peas in long-term variety trials

KEY MESSAGE

Grain yield and NUE increased over time while nitrogen yield did not drop significantly despite reduced nitrogen input. Selection for grain and nitrogen yield is equivalent to selection for NUE. Breeding and registration of improved varieties with high yield, processing quality, disease resistance and nitrogen use efficiency (NUE) are of utmost importance for sustainable crop production to minimize adverse environmental impact and contribute to food security. Based on long-term variety trials of cereals, winter oilseed rape and grain peas tested across a wide range of environmental conditions in Germany, we quantified long-term breeding progress for NUE and related traits. We estimated the genotypic, environmental and genotype-by-environment interaction variation and correlation between traits and derived heritability coefficients. Nitrogen fertilizer application was considerably reduced between 1995 and 2021 in the range of 5.4% for winter wheat and 28.9% for spring wheat while for spring barley it was increased by 20.9%. Despite the apparent nitrogen reduction for most crops, grain yield (GYLD) and nitrogen accumulation in grain (NYLD) was increased or did not significantly decrease. NUE for GYLD increased significantly for all crops between 12.8% and 35.2% and for NYLD between 8% and 20.7%. We further showed that the genotypic rank of varieties for GYLD and NYLD was about equivalent to the genotypic rank of the corresponding traits of NUE, if all varieties in a trial were treated with the same nitrogen rate. Heritability of nitrogen yield was about the same as that of grain yield, suggesting that nitrogen yield should be considered as an additional criterion for variety testing to increase NUE and reduce negative environmental impact.

Plant-soil synchrony in nutrient cycles: Learning from ecosystems to design sustainable agrosystems

Redesigning agrosystems to include more ecological regulations can help feed a growing human population, preserve soils for future productivity, limit dependency on synthetic fertilizers, and reduce agriculture contribution to global changes such as eutrophication and warming. However, guidelines for redesigning cropping systems from natural systems to make them more sustainable remain limited. Synthetizing the knowledge on biogeochemical cycles in natural ecosystems, we outline four ecological systems that synchronize the supply of soluble nutrients by soil biota with the fluctuating nutrient demand of plants. This synchrony limits deficiencies and excesses of soluble nutrients, which usually penalize both production and regulating services of agrosystems such as nutrient retention and soil carbon storage. In the ecological systems outlined, synchrony emerges from plant-soil and plant-plant interactions, eco-physiological processes, soil physicochemical processes, and the dynamics of various nutrient reservoirs, including soil organic matter, soil minerals, atmosphere, and a common market. We discuss the relative importance of these ecological systems in regulating nutrient cycles depending on the pedoclimatic context and on the functional diversity of plants and microbes. We offer ideas about how these systems could be stimulated within agrosystems to improve their sustainability. A review of the latest advances in agronomy shows that some of the practices suggested to promote synchrony (e.g., reduced tillage, rotation with perennial plant cover, crop diversification) have already been tested and shown to be effective in reducing nutrient losses, fertilizer use, and N2 O emissions and/or improving biomass production and soil carbon storage. Our framework also highlights new management strategies and defines the conditions for the success of these nature-based practices allowing for site-specific modifications. This new synthetized knowledge should help practitioners to improve the long-term productivity of agrosystems while reducing the negative impact of agriculture on the environment and the climate.

Aridity creates global thresholds in soil nitrogen retention and availability

Identifying tipping points in the relationship between aridity and gross nitrogen (N) cycling rates could show critical vulnerabilities of terrestrial ecosystems to climate change. Yet, the global pattern of gross N cycling response to aridity across terrestrial ecosystems remains unknown. Here, we collected 14,144 observations from 451 15 N-labeled studies and used segmented regression to identify the global threshold responses of soil gross N cycling rates and soil process-related variables to aridity index (AI), which decreases as aridity increases. We found on a global scale that increasing aridity reduced soil gross nitrate consumption but increased soil nitrification capacity, mainly due to reduced soil microbial biomass carbon (MBC) and N (MBN) and increased soil pH. Threshold response of gross N production and retention to aridity was observed across terrestrial ecosystems. In croplands, gross nitrification and extractable nitrate were inhibited with increasing aridity below the threshold AI ~0.8-0.9 due to inhibited ammonia-oxidizing archaea and bacteria, while the opposite was favored above this threshold. In grasslands, gross N mineralization and immobilization decreased with increasing aridity below the threshold AI ~0.5 due to decreased MBN, but the opposite was true above this threshold. In forests, increased aridity stimulated nitrate immobilization below the threshold AI ~1.0 due to increased soil C/N ratio, but inhibited ammonium immobilization above the threshold AI ~1.3 due to decreased soil total N and increased MBC/MBN ratio. Soil dissimilatory nitrate reduction to ammonium decreased with increasing aridity globally and in forests when the threshold AI ~1.4 was passed. Overall, we suggest that any projected increase in aridity in response to climate change is likely to reduce plant N availability in arid regions while enhancing it in humid regions, affecting the provision of ecosystem services and functions.

Balancing potato yield, soil nutrient supply, and nitrous oxide emissions: An analysis of nitrogen application trade-offs

Potato has been promoted as a national key staple food to alleviate pressure on food security in China. Appropriate nitrogen (N) application rate is prerequisite and is crucial for increasing yield, improving fertilizer efficiency, and reducing N losses. In the present study, we determined the optimum N application rates by analyzing field trial data from the main potato producing areas of China between 2004 and 2020. We considered the equilibrium relationships between potato yield, N uptake, partial N balance (PNB), and N2O emission under different soil indigenous N supply (INS) scenarios. The results showed that N rate, INS, and their interactions all significantly affect potato yield and nutrient uptake increment. On average, N application increased potato yield and N uptake by 29.5 % and 56.7 %, respectively. The relationship between N rate and yield increment was linear-plateau, while the relationship between N rate and N uptake increment was linear-linear. Soil INS accounted for 63.5 % of total potato N requirement. Potato yield increment and nutrient uptake increment were exponentially negatively correlated with INS and had a significant parabolic-nonlinear relationship with the interaction of N fertilizer application rate and INS. PNB was negatively correlated with fertilizer N supply intensity as a power function. Based on our analysis, a N application rate of 166 kg N ha-1 was found to be sufficient when the target yield was <34 t ha-1. However, when the target yield reached 40, 50 and 60 t ha-1, the recommended N application rate increased to 182, 211, and 254 kg N ha-1, respectively, while ensuring N2O emissions low with an emission factor of 0.2 %. Our findings will help guide potato farming toward cleaner production without compromising environmental benefit.

Global mean nitrogen recovery efficiency in croplands can be enhanced by optimal nutrient, crop and soil management practices

An increase in nitrogen (N) recovery efficiency, also denoted as N use efficiency (NUEr), is crucial to reconcile food production and environmental health. This study assessed the effects of nutrient, crop and soil management on NUEr accounting for its dependency on site conditions, including mean annual temperature and precipitation, soil organic carbon, clay and pH, by meta-regression models using 2436 pairs of observations from 407 primary studies. Nutrient management increased NUEr by 3.6-11%, crop management by 4.4-8%, while reduction in tillage had no significant impact. Site conditions strongly affected management induced changes in NUEr, highlighting their relevance for site-specific practices. Data driven models showed that the global mean NUEr can increase by 30%, from the current average of 48% to 78%, using optimal combinations of nutrient (27%), crop (6.6%) and soil (0.6%) management. This increase will in most cases allow to reconcile crop production with acceptable N losses to water. The predicted increase in NUEr was below average in most high-income regions but above average in middle-income regions.

High N availability decreases N uptake and yield under limited water availability in maize

Water and nitrogen (N) are the most limiting factors to plant productivity globally, but we lack a critical understanding of how water availability impacts N dynamics in agricultural systems. Plant N requirements are particularly uncertain when water is limited because of the interactive effect of water and N on plant growth, N demand, and plant uptake. We investigated impacts of N application and water availability on plant growth and N movement, including above and belowground growth, water productivity, N productivity, N uptake, N recovery, and greenhouse gas emissions within a semi-arid system in northeastern Colorado, USA. Moderately high soil N availability depressed grain yield and shoot growth under both limited and full water availability, despite no indication of physical toxicity, and came with additional risk of deleterious N losses. Under low N availability, plant N concentrations in aboveground tissues showed greater recovery of N than what was applied in the low N treatments under both full and limited water availability. This enhanced recovery underscores the need to better understand both plant soil foraging and processes governing resource availability under these conditions. Finally, limited water availability reduced N uptake across all N treatments and left 30% more soil nitrate (NO3-) deep in the soil profile at the end of the season than under full water availability. Our results show that plant N needs are not linearly related to water use and emphasize the need for an integrated understanding of water and N interactions, plant foraging for these resources, and the dynamics of processes that make N available to plants.

Influence of anthropogenic nitrogen inputs and legacy nitrogen change on riverine nitrogen export in areas with high agricultural activity

Increased riverine nitrogen (N) concentrations due to human activities is one of the leading causes of water quality decline, worldwide. Therefore, quantitative information about the N exported from watershed to the river (TN exports) is essential for defining N pollution control practices. This paper evaluated the changes in net anthropogenic N inputs (NANI) and the N stored in land ecosystems (legacy N) in the Jianghan Plain (JHP) from 1990 to 2019 and their impacts on TN exports. Moreover, an empirical model was developed to estimate TN exports, trace its source, and predict its future variations in 2020-2035 under different scenarios. According to the results, NANI exhibited a rise-decrease-rise-decrease M-shaped trend, with N fertilizer application being the dominant driver for NANI change. In terms of the NANI components, non-point-source was the primary N input form (96%). Noteworthy is that the correlation between NANI and TN exports became weaker over time, and large differences in changing trends were observed after 2014. A likely cause for this abnormal trend was that the accumulation of N surplus in soil led to N saturation in agricultural areas. Legacy N was also an important source of TN exports. However, the contribution of legacy N has rarely been considered when defining N pollution control strategies. An empirical model, incorporating legacy N, agricultural irrigation water use, and cropland area ratio, was developed. Based on this model, legacy N contributed a large proportion (15-31%). Furthermore, the results of future predictions indicated that legacy N had a larger impact on future TN exports changes compared to other factors, and increased irrigation water would increase rather than decrease TN exports. Therefore, an integrated N management strategy considering the impact of NANI, legacy N, and irrigation water use is crucial to control N pollution in areas with intensive agriculture.

Genetic resources and precise gene editing for targeted improvement of barley abiotic stress tolerance

Abiotic stresses, predominately drought, heat, salinity, cold, and waterlogging, adversely affect cereal crops. They limit barley production worldwide and cause huge economic losses. In barley, functional genes under various stresses have been identified over the years and genetic improvement to stress tolerance has taken a new turn with the introduction of modern gene-editing platforms. In particular, clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) is a robust and versatile tool for precise mutation creation and trait improvement. In this review, we highlight the stress-affected regions and the corresponding economic losses among the main barley producers. We collate about 150 key genes associated with stress tolerance and combine them into a single physical map for potential breeding practices. We also overview the applications of precise base editing, prime editing, and multiplexing technologies for targeted trait modification, and discuss current challenges including high-throughput mutant genotyping and genotype dependency in genetic transformation to promote commercial breeding. The listed genes counteract key stresses such as drought, salinity, and nutrient deficiency, and the potential application of the respective gene-editing technologies will provide insight into barley improvement for climate resilience.

Casein as protein and hydrolysate: Biostimulant or nitrogen source for Nicotiana tabacum plants grown in vitro?

In contrast to inorganic nitrogen (N) assimilation, the role of organic N forms, such as proteins and peptides, as sources of N and their impact on plant metabolism remains unclear. Simultaneously, organic biostimulants are used as priming agents to improve plant defense response. Here, we analysed the metabolic response of tobacco plants grown in vitro with casein hydrolysate or protein. As the sole source of N, casein hydrolysate enabled tobacco growth, while protein casein was used only to a limited extent. Free amino acids were detected in the roots of tobacco plants grown with protein casein but not in the plants grown with no source of N. Combining hydrolysate with inorganic N had beneficial effects on growth, root N uptake and protein content. The metabolism of casein-supplemented plants shifted to aromatic (Trp), branched-chain (Ile, Leu, Val) and basic (Arg, His, Lys) amino acids, suggesting their preferential uptake and/or alterations in their metabolic pathways. Complementarily, proteomic analysis of tobacco roots identified peptidase C1A and peptidase S10 families as potential key players in casein degradation and response to N starvation. Moreover, amidases were significantly upregulated, most likely for their role in ammonia release and impact on auxin synthesis. In phytohormonal analysis, both forms of casein influenced phenylacetic acid and cytokinin contents, suggesting a root system response to scarce N availability. In turn, metabolomics highlighted the stimulation of some plant defense mechanisms under such growth conditions, that is, the high concentrations of secondary metabolites (e.g., ferulic acid) and heat shock proteins.

Nitrogen retention potentials of magnesium oxide- and sepiolite-modified biochars and their impacts on bacterial distribution under nitrogen fertilization

Mitigating the loss and negative impacts of reactive N from fertilized soils remains a global environmental challenge. To optimize N retention by biochar, bamboo and pig manure biochars were modified as MgO- and sepiolite-biochar composites and characterized. Novel soil application of the modified biochars and their raw forms were comparatively evaluated for N-retention in a fertilized soil leached for 90 days in a column experiment. Changes in N-cycling-related enzyme and bacterial structure were also reported after 90 days. Results revealed low leaching losses of NH₄⁺, which reduced over time across all the treatments. However, while sole fertilizer (F) increased the initial and cumulative NO₃^- leached from the soil, the MgO-bamboo biochar (MgOBF) and sepiolite-bamboo biochar (SBF) treatments reduced leachate NO₃^- by 22.1 % and 10.5 % compared to raw bamboo biochar (BBF) treatment. However, 15.5 % more NO₃^- was leached from the MgO-pig manure biochar-treated soil (MgOPF) compared to its raw biochar treatment (PMBF) after 90 days. Dissolved organic N leached was reduced by 9.2 % and 0.5 % in MgOBF and SBF, as well as 15.4 % and 40.5 % in MgOPF and SPF compared to their respective raw forms. The total N of the biochars, adjustment of surface charges, cation exchange capacity, surface area, pore filling effects, and the formation of potential MgN precipitates on the modified-biochar surfaces regulated N leaching/retention. In addition, the modified biochar treatments reduced the hydrolysis of urea and stimulated some nitrate-reduction-related bacteria crucial for NO₃^- retention. Hence, unlike the raw biochar and MgOPF treatments, MgOBF, SBF, and SPF hold promise in mitigating inorganic-N losses from fertilized soils while improving the soil's chemical properties.

Opening the door to nitrogen fixation in oxygenic phototrophs

Conveying biological nitrogen fixation (BNF) to photosynthetic species may be the next agricultural revolution, yet poses major engineering challenges. Liu et al. created a diazotrophic strain of a previously non-nitrogen-fixing species, the cyanobacterium Synechocystis sp. PCC 6803, and uncovered critical aspects of nitrogen fixation in an oxygenic species.

Fertilizer 15N balance in a soybean-maize-maize rotation system based on a 41-year long-term experiment in Northeast China

Global awareness of the need to enhance crop production and reduce environmental issues associated with nitrogen (N) fertilizer has increased. However, studies on how the N fate changed with manure addition are still limited. To explore efficient fertilization management for an improved grain yield, N recovery efficiency, and reduced N residual in the soil or that unaccounted for, a field 15N micro-plot trial in a soybean-maize-maize rotation was conducted to evaluate the effect of fertilization regimes on soybean and maize yields and the fertilizer N fate in the plant-soil system during 2017-2019 within a 41-year experiment in Northeast China. Treatments included chemical N alone (N), N and phosphorus (NP), N, P, and potassium (NPK), and those combined with manure (MN, MNP, and MNPK). Application of manure increased grain yield, on average, by 153% for soybean (2017) and 105% and 222% for maize (2018 and 2019) compared to no manure, with the highest at MNPK. Crop N uptake and that from labeled ¹⁵N-urea also benefited from manure addition, mainly partitioned to grain, and the average ¹⁵N-urea recovery was 28.8% in the soybean season with a reduction in the subsequent maize seasons (12.6%, and 4.1%). Across the three years, the fertilizer ¹⁵N recovery ranged from 31.2-63.1% (crop) and 21.9-40.5% (0-40 cm soil), with 14.6-29.9% unaccounted for, including N losses. In the two maize seasons, manure addition significantly increased the residual ¹⁵N recovery in crop attributed to the enhancing ¹⁵N remineralization, and reduced that in soil and unaccounted for compared to single chemical fertilizer, with MNPK performing the best. Therefore, applying N, P, and K fertilizers in the soybean season and NPK combined with manure (13.5 t ha^-1) in the maize seasons is a promising fertilization management strategy in Northeast China and similar regions.

Nitrate leaching losses and the fate of 15N fertilizer in perennial intermediate wheatgrass and annual wheat - A field study

Perennial grains, such as the intermediate wheatgrass (Thinopyrum intermedium) (IWG), may reduce negative environmental effects compared to annual grain crops. Their permanent, and generally larger, root systems are likely to retain nitrogen (N) better, decreasing harmful losses of N and improving fertilizer N use efficiency, but there have been no comprehensive N fertilizer recovery studies in IWG to date. We measured fertilizer N recovery with stable isotope tracers in crop biomass and soil, soil N mineralization and nitrification, and nitrate leaching in IWG and annual wheat in a replicated block field experiment. Nitrate leaching was drastically reduced in IWG (0.1 and 3.1 kg N ha^-1 yr^-1) in its third and fourth year since establishment, compared with 5.6 kg N ha^-1 yr^-1 in annual wheat and 41.0 kg N ha^-1 yr^-1 in fallow respectively. There were no differences in net N mineralization or nitrification between IWG and annual wheat, though there was generally more inorganic N in the soil profile of annual wheat. More ¹⁵N fertilizer was recovered in the straw and all depths of the roots and soils in IWG than annual wheat. However, annual wheat recovered much more ¹⁵N fertilizer in the seeds compared to IWG, which had lower grain yields. ¹⁵N-labeled fertilizer contributed little (<3 %) to nitrate-N in leachate, highlighting the role of soil microbes in regulating loss of current year fertilizer N. The large reduction in nitrate leaching demonstrates that perennial grains can reduce harmful nitrogen losses and offer a more sustainable alternative to annual grains.

Advancing the science and practice of ecological nutrient management for smallholder farmers

Soil degradation is widespread in smallholder agrarian communities across the globe where limited resource farmers struggle to overcome poverty and malnutrition. This review lays out the scientific basis and practical management options for an ecologically based approach to sustainably managing soil fertility, with particular attention to smallholder subsistence systems. We seek to change the trajectory of development programs that continue to promote inorganic fertilizers and other high input strategies to resource constrained smallholders, despite ample evidence that this approach is falling short of food security goals and contributing to resource degradation. Ecological nutrient management (ENM) is an agroecological approach to managing the biogeochemical cycles that govern soil ecosystem services and soil fertility. The portfolio of ENM strategies extends beyond reliance on inorganic fertilizers and is guided by the following five principles: (1) Build soil organic matter and other nutrient reserves. (2) Minimize the size of N and P pools that are the most susceptible to loss. (3) Maximize agroecosystem capacity to use soluble, inorganic N and P. (4) Use functional and phylogenetic biodiversity to minimize bare fallows and maximize presence of growing plants. (5) Construct agroecosystem and field scale mass balances to track net nutrient flows over multiple growing seasons. Strategic increases in spatial and temporal plant species diversity is a core ENM tactic that expands agroecosystem multifunctionality to meet smallholder priorities beyond soil restoration and crop yields. Examples of ENM practices include the use of functionally designed polycultures, diversified rotations, reduced fallow periods, increased reliance on legumes, integrated crop-livestock production, and use of variety of soil amendments. These practices foster soil organic matter accrual and restoration of soil function, both of which underpin agroecosystem resilience. When ENM is first implemented, short-term yield outcomes are variable; however, over the long-term, management systems that employ ENM can increase yields, yield stability, profitability and food security. ENM rests on a solid foundation of ecosystem and biogeochemical science, and despite the many barriers imposed by current agricultural policies, successful ENM systems are being promoted by some development actors and used by smallholder farmers, with promising results.

Investigation of the direct effect of a precision Ascophyllum nodosum biostimulant on nitrogen use efficiency in wheat seedlings

Reduction in the greenhouse gas (GHG) emissions and nitrogen (N) pollution of ground water by improving nitrogen use efficiency (NUE) in crops has become an intensively investigated research topic in pursuit of a more sustainable future. Although, distinct solutions have been proposed there are only a few reports documenting the detailed interplay between observed plant growth dynamics and changes in plant N related transcriptional and biochemical changes. It was previously demonstrated that the application of a formulated biostimulant (PSI-362) derived from Ascophyllum nodosum (ANE) improves N uptake in Arabidopsis thaliana and in barley. In this study, the effect of PSI-362 on the growth dynamics of wheat seedlings was evaluated at different biostimulant and N supplementation rates. Wheat grown on N deficient MS medium was also analysed from the first hour of the treatment until the depletion of the nutrients in the medium 9 days later. During this time the biomass increase measured for PSI-362 treated plants versus untreated controls was associated with increased nitrate uptake, with surplus N assimilated by the biomass in the form of glutamate, glutamine, free amino acids, soluble proteins, and chlorophyll. Phenotypical and biochemical analysis were supported by evaluation of differential expression of genetic markers involved in nitrate perception and transport (TaNRT1.1/NPF6.3), nitrate and nitrite reduction (TaNR1 and TaNiR1) and assimilation (TaGDH2, TaGoGAT, TaGS1). Finally, a comparative analysis of the precision biostimulant PSI-362 and two generic ANEs demonstrated that the NUE effect greatly differs depending on the ANE formulation used.

Winter cover crops increased nitrogen availability and efficient use during eight years of intensive organic vegetable production

Efficient use of nitrogen (N) is essential to protect water quality in high-input organic vegetable production systems, but little is known about the long-term effects of organic management on N mass balances. We measured soil N and tabulated N inputs (organic fertilizers, compost, irrigation water, atmospheric deposition, cover crop seed, vegetable transplant plugs and fixation by legume cover crops) and exports in harvested crops (lettuce, broccoli) over eight years to calculate soil surface and soil system N mass balances for the Salinas Organic Cropping Systems study in Salinas, CA. Our objectives were to 1) quantify the long-term effects of compost, cover crop frequency and cover crop type on soil N, cover crop and vegetable crop N uptake, and yield, and 2) tabulate N balances to assess the effects of these factors on N export in harvested crops, soil N storage and potential N loss. Results show that across all systems only 13 to 23% of N inputs were exported in harvest. Annual compost applications increased soil N stocks but had little effect on vegetable N uptake or yield, increasing the cumulative soil system N balance surplus over eight years by 999 kg ha^-1, relative to the system receiving organic fertilizers alone. Annually planted winter cover crops increased N availability, crop uptake and export; however, biological N fixation by legumes negated the positive effect of increased harvest exports on the balance surplus in the legume-rye cover cropped system. Over eight years, rye cover crops improved system performance and reduced the cumulative N surplus by 384 kg ha^-1 relative to the legume-rye mixture by increasing N retention and availability without increasing N inputs. Reduced reliance on external compost inputs and increased use of annually planted non-legume cover crops can improve efficient N use and cropping system yield, consequently improving environmental performance.

Effects of different cropping systems on ammonia nitrogen load in a typical agricultural watershed of South China

The excessive application of agricultural irrigation water and chemical fertilizer has increased crop yields to help meet the demand for food, but it has also led to major water environment problem, i.e. non-point source (NPS) pollution, which needs to be addressed to achieve sustainable development targets. Although numerous studies have focused on the control and reduction of agricultural NPS pollution from the perspective of irrigation and fertilizer, the effects of different cropping systems on NPS pollution (ammonia nitrogen (NH₃-N)) in the Dongjiang River Basin (DRB) were seldom assessed. Specifically, variation in the NH₃-N load was simulated and analyzed at the annual and semi-annual scales under ten different cropping systems using the Soil and Water Assessment Tool (SWAT) model, which was calibrated and validated with satisfactory statistical index values in the DRB. The results indicated that the NH₃-N load decreased, distinctly increased, slightly decreased when sweet potato, peanut, and rice were planted, respectively. Compared with mono-cropping, crop rotation could reduce the NH₃-N load, and the planting sequence of crops could affect the NH₃-N load to a certain extent. Planting peanuts in spring would dramatically increase NH₃-N load. To evaluate NH₃-N pollution, a critical threshold of NH₃-N emission (5.1 kg·ha^-1·year^-1) was proposed. Meeting the NH₃-N emission threshold cannot be achieved by altering the cropping system alone; additional measures are needed to reduce agricultural NPS pollution. This study facilitates the development of cropping systems and provides relevant information to aid the sustainable development of agriculture in the DRB.

Crop Domestication, Root Trait Syndromes, and Soil Nutrient Acquisition in Organic Agroecosystems: A Systematic Review

Selecting crops that express certain reproductive, leaf, and root traits has formed detectable, albeit diverse, crop domestication syndromes. However, scientific and informal on-farm research has primarily focused on understanding and managing linkages between only certain domestication traits and yield. There is strong evidence suggesting that functional traits can be used to hypothesize and detect trade-offs, constraints, and synergies among crop yield and other aspects of crop biology and agroecosystem function. Comparisons in the functional traits of crops vs. wild plants has emerged as a critical avenue that has helped inform a better understanding of how plant domestication has reshaped relationships among yield and traits. For instance, recent research has shown domestication has led important economic crops to express extreme functional trait values among plants globally, with potentially major implications for yield stability, nutrient acquisition strategies, and the success of ecological nutrient management. Here, we present an evidence synthesis of domestication effects on crop root functional traits, and their hypothesized impact on nutrient acquisition strategies in organic and low input agroecosystems. Drawing on global trait databases and published datasets, we show detectable shifts in root trait strategies with domestication. Relationships between domestication syndromes in root traits and nutrient acquisition strategies in low input systems underscores the need for a shift in breeding paradigms for organic agriculture. This is increasingly important given efforts to achieve Sustainable Development Goal (SDG) targets of Zero Hunger via resilient agriculture practices such as ecological nutrient management and maintenance of genetic diversity.

Nitrogen inputs best predict farm field nitrate leaching in the Willamette Valley, Oregon

Nitrate leaching is an important yet difficult to manage contribution to groundwater and surface water contamination in agricultural areas. We examine 14 farm fields over a four year period (2014-2017) in the southern Willamette Valley, providing 53 sets of annual, field-level agricultural performance metrics related to nitrogen (N), including fertilizer inputs, crop harvest outputs, N use efficiency (NUE), nitrate-N leaching and surplus N. Crop-specific nitrate-N leaching varied widely from 10 kg N ha-1yr-1 in hazelnuts to >200 kg N ha-1yr-1 in peppermint. Averaging across all sites and years, most leaching occurred during fall (60%) and winter (32%). Overall NUE was 57%. We used a graphical approach to explore the relationships between N inputs, surplus, crop N harvest removal and NUE by crop type. The blueberry site had high inputs and surplus, peppermint had high inputs but also high crop N removal and NUE and thus lower surplus, and most wheat crops had high NUE and evidence of using soil N. Annual N surplus was not well correlated with leaching, and leaching varied more by crop type and inputs. Grass seed and hazelnuts, which are dominant crop types in the southern Willamette Valley, were intermediate in terms of NUE, leaching and surplus. Of all performance metrics, N input was most closely aligned with field-level crop N harvest and nitrate leaching, therefore optimizing N inputs may well inform local efforts to reduce groundwater nitrate contamination.

A holistic framework integrating plant-microbe-mineral regulation of soil bioavailable nitrogen

Soil organic nitrogen (N) is a critical resource for plants and microbes, but the processes that govern its cycle are not well-described. To promote a holistic understanding of soil N dynamics, we need an integrated model that links soil organic matter (SOM) cycling to bioavailable N in both unmanaged and managed landscapes, including agroecosystems. We present a framework that unifies recent conceptual advances in our understanding of three critical steps in bioavailable N cycling: organic N (ON) depolymerization and solubilization; bioavailable N sorption and desorption on mineral surfaces; and microbial ON turnover including assimilation, mineralization, and the recycling of microbial products. Consideration of the balance between these processes provides insight into the sources, sinks, and flux rates of bioavailable N. By accounting for interactions among the biological, physical, and chemical controls over ON and its availability to plants and microbes, our conceptual model unifies complex mechanisms of ON transformation in a concrete conceptual framework that is amenable to experimental testing and translates into ideas for new management practices. This framework will allow researchers and practitioners to use common measurements of particulate organic matter (POM) and mineral-associated organic matter (MAOM) to design strategic organic N-cycle interventions that optimize ecosystem productivity and minimize environmental N loss.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10533-021-00793-9.

Reducing Nitrogen Input in Barley Crops While Maintaining Yields Using an Engineered Biostimulant Derived From Ascophyllum nodosum to Enhance Nitrogen Use Efficiency

Intensive agricultural production utilizes large amounts of nitrogen (N) mineral fertilizers that are applied to the soil to secure high crop yields. Unfortunately, up to 65% of this N fertilizer is not taken up by crops and is lost to the environment. To compensate these issues, growers usually apply more fertilizer than crops actually need, contributing significantly to N pollution and to GHG emissions. In order to combat the need for such large N inputs, a better understanding of nitrogen use efficiency (NUE) and agronomic solutions that increase NUE within crops is required. The application of biostimulants derived from extracts of the brown seaweed Ascophyllum nodosum has long been accepted by growers as a sustainable crop production input. However, little is known on how Ascophyllum nodosum extracts (ANEs) can influence mechanisms of N uptake and assimilation in crops to allow reduced N application. In this work, a significant increase in nitrate accumulation in Arabidopsis thaliana 6 days after applying the novel proprietary biostimulant PSI-362 was observed. Follow-up studies in barley crops revealed that PSI-362 increases NUE by 29.85-60.26% under 75% N input in multi-year field trials. When PSI-362 was incorporated as a coating to the granular N fertilizer calcium ammonium nitrate and applied to barley crop, a coordinated stimulation of N uptake and assimilation markers was observed. A key indicator of biostimulant performance was increased nitrate content in barley shoot tissue 22 days after N fertilizer application (+17.9-72.2%), that was associated with gene upregulation of root nitrate transporters (NRT1.1, NRT2.1, and NRT1.5). Simultaneously, PSI-362 coated fertilizer enhanced nitrate reductase and glutamine synthase activities, while higher content of free amino acids, soluble protein and photosynthetic pigments was measured. These biological changes at stem elongation stage were later translated into enhanced NUE traits in harvested grain. Overall, our results support the agronomic use of this engineered ANE that allowed a reduction in N fertilizer usage while maintaining or increasing crop yield. The data suggests that it can be part of the solution for the successful implementation of mitigation policies for water quality and GHG emissions from N fertilizer usage.

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 ± 1.01 mg N kg^-1  soil day^-1 . It was 5.55 ± 0.41 mg N kg^-1  soil day^-1 in croplands, 15.74 ± 3.02 mg N kg^-1  soil day^-1 in wetlands, and 15.26 ± 2.98 mg N kg^-1  soil day^-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.

Different quantification approaches for nitrogen use efficiency lead to divergent estimates with varying advantages

Nitrogen use efficiency (NUE) is a key indicator with which to study nitrogen cycles and inform nitrogen management. However, different quantification approaches may result in substantially divergent NUE values even for the same production system or for the same experimental plot. Based on our investigation of the differences between and connections among the three principal approaches for NUE quantification, we offer recommendations for choosing the appropriate approach and call for long-term observations to assess the impacts of management practices.

Tracing plant–environment interactions from organismal to planetary scales using stable isotopes: a mini review

Natural isotope variation forms a mosaic of isotopically distinct pools across the biosphere and flows between pools integrate plant ecology with global biogeochemical cycling. Carbon, nitrogen, and water isotopic ratios (among others) can be measured in plant tissues, at root and foliar interfaces, and in adjacent atmospheric, water, and soil environments. Natural abundance isotopes provide ecological insight to complement and enhance biogeochemical research, such as understanding the physiological conditions during photosynthetic assimilation (e.g. water stress) or the contribution of unusual plant water or nutrient sources (e.g. fog, foliar deposition). While foundational concepts and methods have endured through four decades of research, technological improvements that enable measurement at fine spatiotemporal scales, of multiple isotopes, and of isotopomers, are advancing the field of stable isotope ecology. For example, isotope studies now benefit from the maturation of field-portable infrared spectroscopy, which allows the exploration of plant–environment sensitivity at physiological timescales. Isotope ecology is also benefiting from, and contributing to, new understanding of the plant–soil–atmosphere system, such as improving the representation of soil carbon pools and turnover in land surface models. At larger Earth-system scales, a maturing global coverage of isotope data and new data from site networks offer exciting synthesis opportunities to merge the insights of single-or multi-isotope analysis with ecosystem and remote sensing data in a data-driven modeling framework, to create geospatial isotope products essential for studies of global environmental change.

Nitrogen and Stem Development: A Puzzle Still to Be Solved

High crop yields are generally associated with high nitrogen (N) fertilizer rates. A growing tendency that is urgently demanding the adoption of precision technologies that manage N more efficiently, combined with the advances of crop genetics to meet the needs of sustainable farm systems. Among the plant traits, stem architecture has been of paramount importance to enhance harvest index in the cereal crops. Nonetheless, the reduced stature also brought undesirable effect, such as poor N-uptake, which has led to the overuse of N fertilizer. Therefore, a better understanding of how N signals modulate the initial and late stages of stem development might uncover novel semi-dwarf alleles without pleiotropic effects. Our attempt here is to review the most recent advances on this topic.

The undefined N source might be overestimated by 15 N tracer trials

Based on data from previous ¹⁵ N-tracer trials, Yan et al. (Global Change Biology, 26, 191-199; 2020) accounted the sources of cereal crop nitrogen (N) from fertilizer inputs (from both current and previous seasons) and possible uptake of nonfertilizer N inputs, leaving a significant part of N source undefined. We argue that the undefined N source does not reflect the net change in soil N stock and might be overestimated for three reasons: (a) higher yield in ¹⁵ N tracer trials; (b) a low residue return rate is assumed; (c) underestimated contribution by fertilizer inputs from previous seasons due to the lack of long-term trials.

Improved soil biological health increases corn grain yield in N fertilized systems across the Corn Belt

Nitrogenous fertilizers have nearly doubled global grain yields, but have also increased losses of reactive N to the environment. Current public investments to improve soil health seek to balance productivity and environmental considerations. However, data integrating soil biological health and crop N response to date is insufficient to reliably drive conservation policy and inform management. Here we used multilevel structural equation modeling and N fertilizer rate trials to show that biologically healthier soils produce greater corn yields per unit of fertilizer. We found the effect of soil biological health on corn yield was 18% the magnitude of N fertilization, Moreover, we found this effect was consistent for edaphic and climatic conditions representative of 52% of the rainfed acreage in the Corn Belt (as determined using technological extrapolation domains). While N fertilization also plays a role in building or maintaining soil biological health, soil biological health metrics offer limited a priori information on a site's responsiveness to N fertilizer applications. Thus, increases in soil biological health can increase corn yields for a given unit of N fertilizer, but cannot completely replace mineral N fertilization in these systems. Our results illustrate the potential for gains in productivity through investment in soil biological health, independent of increases in mineral N fertilizer use.