More N fertilizer, more maize, and less alfalfa: maize benefits from its higher N uptake per unit root length

Root plasticity is fundamental to soil nutrient acquisition and maximizing production. Different soil nitrogen (N) levels affect root development, aboveground dry matter accumulation, and N uptake. This phenotypic plasticity is well documented for single plants and specific monocultures but is much less understood in intercrops in which species compete for the available nutrients. Consequently, the study tested whether the plasticity of plant roots, biomass and N accumulation under different N levels in maize/alfalfa intercropping systems differs quantitatively. Maize and alfalfa were intercropped for two consecutive years in large soil-filled rhizoboxes and fertilized with 6 different levels of N fertilizer (0, 75, 150, 225, 270, and 300 kg ha-1). Root length, root surface area, specific root length, N uptake and yield were all increased in maize with increasing fertilizer level, whereas higher N rates were supraoptimal. Alfalfa had an optimal N rate of 75-150 kg ha-1, likely because the competition from maize became more severe at higher rates. Maize responded more strongly to the fertilizer treatment in the second year when the alfalfa biomass was much larger. N fertilization contributes more to maize than alfalfa growth via root plasticity responses. Our results suggest that farmers can maximize intercropping yield and economic return by optimizing N fertilizer management.

Exploring the genetic landscape of nitrogen uptake in durum wheat: genome-wide characterization and expression profiling of NPF and NRT2 gene families

Nitrate uptake by plants primarily relies on two gene families: Nitrate transporter 1/peptide transporter (NPF) and Nitrate transporter 2 (NRT2). Here, we extensively characterized the NPF and NRT2 families in the durum wheat genome, revealing 211 NPF and 20 NRT2 genes. The two families share many Cis Regulatory Elements (CREs) and Transcription Factor binding sites, highlighting a partially overlapping regulatory system and suggesting a coordinated response for nitrate transport and utilization. Analyzing RNA-seq data from 9 tissues and 20 cultivars, we explored expression profiles and co-expression relationships of both gene families. We observed a strong correlation between nucleotide variation and gene expression within the NRT2 gene family, implicating a shared selection mechanism operating on both coding and regulatory regions. Furthermore, NPF genes showed highly tissue-specific expression profiles, while NRT2s were mainly divided in two co-expression modules, one expressed in roots (NAR2/NRT3 dependent) and the other induced in anthers and/ovaries during maturation. Our evidences confirmed that the majority of these genes were retained after small-scale duplication events, suggesting a neo- or sub-functionalization of many NPFs and NRT2s. Altogether, these findings indicate that the expansion of these gene families in durum wheat could provide valuable genetic variability useful to identify NUE-related and candidate genes for future breeding programs in the context of low-impact and sustainable agriculture.

Evaluating the influence of straw mulching and intercropping on nitrogen uptake, crop growth, and yield performance in maize and soybean

Introduction

Intercropping and straw mulching are sustainable agricultural practices that can positively affect crop growth and development, especially together.

Methods

A split-plot experimental design was used to investigate the effects of intercropping and straw mulching on crop growth, crop yield, nitrogen uptake, and photosynthetic characteristics. The main plot focused on three planting patterns: soybean monoculture (S), maize monoculture (M), and maize/soybean intercropping (I). The subplot structure consisted of four levels of straw mulching (0, 4.8, 7.2, 9.6 t ha-1).

Results

Interaction and variance analyses showed that straw mulching, intercropping, and their interaction had significant effects on plant height, stem diameter, leaf area index, chlorophyll content, nitrogen uptake, photosynthetic characteristics, and crop yield. Based on two-year averages for maize and soybean, the net photosynthetic rate (Pn) was up to 51.6% higher, stomatal conductance (Sc) was up to 44.0% higher, transpiration rate (Tr) was up to 46.6% higher, and intercellular carbon dioxide concentration (Ci) was up to 25.7% lower relative to no mulching. The maximum increases of Pn, Sc, and Tr of intercropped maize were 15.48%, 17.28%, and 23.94%, respectively, and the maximum Ci was 17.75% lower than that of monoculture maize. The maximum increase of Pn, Sc, and Tr of monoculture soybean was 24.58%, 16.90%, and 17.91%, respectively, and the maximum Ci was 13.85% lower than that of intercropped soybean. The nitrogen uptake of maize and soybean in the mulching treatment was 24.3% higher than that in the non-mulching treatment; the nitrogen uptake of intercropped maize was 34.2% higher than that of monoculture maize, and the nitrogen uptake of monoculture soybean was 15.0% higher than that of intercropped soybean. The yield of maize and soybean in the mulching treatment was 66.6% higher than that in the non-mulching treatment, the maize yield under intercropping was 15.4% higher than that under monoculture, and the yield of monoculture soybean was 9.03% higher than that of intercropped soybean.

Discussion

The growth index and photosynthesis of crops are important parts of yield formation. The results of this study confirmed that straw mulching, intercropping, and their interaction can ultimately increase crop yield by improving crop growth, nitrogen uptake, and photosynthesis. This result can be used as the theoretical basis for the combined application of these measures in agriculture.

Grain nitrogen content and productivity of rice and maize under variable doses of fertilizer nitrogen

The rice-maize system is a dominant cropping system of south Asia and consumes a considerable amount of fertilizer. The indiscriminate use of fertilizer particularly nitrogen (N) is degrading the soil health and polluting the environment. Lower N-use efficiency is a major problem and needs to be improved for higher yield, lower cost of cultivation and better environment. The grain quality is also altered by the N-application as N is a major constituent of protein. Studies on the effect of N-application on grain N-content is still lacking. We hypothesised that optimization of N application would result in economising N dose, improving yield and NUE and improving grain quality. Under that context, a field experiment was conducted with different doses of fertilizer N for rice and maize. Fertilizer N was applied at the rate of 0, 40, 80, 120, 160, 200 and 240 kg ha^-1 (N0-N240). An increase in grain yield was observed up to 80 and 160 kg ha^-1 for rice and maize, respectively. The N content of grain increased with N rates and a significant increase was noted in N200 (1.42%) being at par with N240 (1.49%) but significantly higher than others by 13-32%. With an increase of each kilogram of N, the grain N content increased by 14 and 20 μg (microgram) for rice and maize, respectively. The leaf N content registered a decreasing trend with the progress of the crop growth for both rice and maize. The agronomic efficiency (AE) of N initially increased with an increase in the rate of fertilizer N followed by a decrease with higher doses of N. Unlike the AE, the partial factor productivity (PFP) of N decreased gradually with an increase in the rate of fertilizer N. The chlorophyll content of flag leaves also registered an increasing trend with an increasing rate of fertilizer N. On the surface soil (0-15 cm), the treatments which received lower (N0, N40) and higher (N240) fertilizer N recorded a comparatively higher total soil N than other treatments. The mean NUE was 0.42 and 0.75 for rice and maize, respectively. The study suggests an economic fertilizer N rate of 165 and 167 kg N ha^-1, for rice and maize, respectively. It also concludes that the grain N content can be altered by N-application rates though more research is needed.

Rice yield penalty and quality deterioration is associated with failure of nitrogen uptake from regreening to panicle initiation stage under salinity

In recent years, the development and utilization of saline land for rice cultivation have effectively expanded grain productivity. Rice is a salt-sensitive crop, and the increasing salinity problem threatens rice yield and quality. Therefore, we conducted open field experiments to study the effect of salinity on different growth stages of rice. Irrigating saline treatment was conducted at three different growth stages: irrigating saline from the regreening stage to the panicle initiation stage (S1), irrigating saline from the panicle initiation stage to the flowering stage (S2), and irrigating saline from the flowering stage to the maturity stage (S3). Each treatment period lasted for about 30 days. At the same time, irrigating saline water from the regreening stage to the maturity stage (S4) treatment was added in 2022 to explore the performance of salt stress during the whole growth period of rice. Based on the treatment of these different saline irrigation growth periods, three saline concentrations were incorporated, including salinity 0‰ (T1), 3‰ (T2), and 6‰ (T3) concentrations. No irrigating saline during the whole growth period was also used as a control (CK). The results indicated that rice grain yield and quality were most sensitive to saline treatment during S1 among the three stress periods. At the S1 stage, salinity mainly reduced the nitrogen uptake, resulting in stunted plant growth, reducing tillering, yield, and yield components, and deteriorating the rice quality. Compared to the control, IE_N (grain yield over the total amount of N uptake in plants at maturity) was more sensitive at the S1 stage than S2 and S3 stages under salinity. Furthermore, the findings of our study suggest that under salinity, rice growth is not only directly affected by the higher sodium (Na⁺) content in plants, but the higher concentration of Na⁺ reduced the ability of plants to uptake nitrogen. Thus, more attention should be paid to the field management of the S1 stage, the most sensitive stage during rice cultivation in salinized areas. It is necessary to avoid salt damage to rice during this period and ensure irrigation with precious freshwater resources.

Contrasting effects of nitrogen and phosphorus additions on nitrogen competition between coniferous and broadleaf seedlings

Nitrogen (N) is a major element limiting plant growth and metabolism. Nitrogen addition can influence plant growth, N uptake, and species interactions, while phosphorus (P) addition may affect N acquisition. However, knowledge of how nutrient availability influences N uptake and species interactions remains limited and controversial. Here, pot experiments were conducted for 14 months, in which conifers (Pinus massoniana and Pinus elliottii) and broadleaved trees (Michelia maudiae and Schima superba) were planted in monoculture or mixture, and provided additional N and P in a full-factorial design. Nitrogen addition increased the biomass, but P addition did not significantly affect the biomass of the four subtropical species. Combined N and P (NP) addition had no additive effect on plant biomass over N addition. Total plant biomass was significantly positively correlated to root traits (branching intensity and root tissue density) and leaf traits (net photosynthetic rate, stomatal conductance, and transpiration rate), but negatively correlated to root diameter in response to nutrient addition. Plant uptake rates of NH₄⁺ or NO₃^- were not altered by N addition, but P or NP additions decreased NH₄⁺ uptake rates and increased NO₃^- uptake rates. Neighboring conifers significantly inhibited NH₄⁺ and NO₃^- uptake rates of the two broadleaf species, but neighboring broadleaves had no effects on the N uptake rates of pine species. The effects of nutrient additions on interspecific interactions differed among species. Nitrogen addition altered the interaction of P. elliottii and M. maudiae from neutral to competition, while P addition altered the interaction of P. massoniana and M. maudiae from neutral to favorable effects. Increasing nutrient availability switched the direction of interspecific interaction in favor of pines. This study provides insights into forest management for productivity improvement and optimizing the selection of broadleaf species regarding differences in soil fertility of subtropical plantations.

Alfalfa growth and nitrogen fixation constraints in salt-affected soils are in part offset by increased nitrogen supply

INTRODUCTION

In China, alfalfa (Medicago sativa L.) is often grown on marginal land with poor soil fertility and suboptimal climate conditions. Soil salt stress is one of the most limiting factors for alfalfa yield and quality, through its inhibition of nitrogen (N) uptake and N fixation.

METHODS

To understand if N supply could improve alfalfa yield and quality through increasing N uptake in salt-affected soils, a hydroponic experiment and a soil experiment were conducted. Alfalfa growth and N fixation were evaluated in response to different salt levels and N supply levels.

RESULTS AND DISCUSSION

The results showed that salt stress not only significantly decreased alfalfa biomass, by 43%-86%, and N content, by 58%-91%, but also reduced N fixation ability and N derived from the atmosphere (%Ndfa) through the inhibition of nodule formation and N fixation efficiency when the salt level was above 100 mmol Na₂SO₄ L^-1. Salt stress also decreased alfalfa crude protein by 31%-37%. However, N supply significantly improved shoot dry weight by 40%-45%, root dry weight by 23%-29%, and shoot N content by 10%-28% for alfalfa grown in salt-affected soil. The N supply was also beneficial for the %Ndfa and N fixation for alfalfa with salt stress, and the increase reached 47% and 60%, respectively. Nitrogen supply offset the negative effects on alfalfa growth and N fixation caused by salt stress, in part through improving plant N nutrition status. Our results suggest that optimal N fertilizer application is essential to alleviate the loss of growth and N fixation in alfalfa in salt-affected soils.

Exogenous calcium promotes growth of adzuki bean (Vigna angularis Willd.) seedlings under nitrogen limitation through the regulation of nitrogen metabolism

Plants exhibit lower nitrogen use efficiency (NUE) under N-limitation conditions. Although the function of calcium (Ca) has been widely studied in plants, it remains to be explored whether regulation of nitrate uptake and reduction is needed. A hydroponics experiment on adzuki beans (Vigna angularis Willd.) was used as a test material to determine the interactions between Ca and three levels of nitrogen supply. The height of the plant, the leaf area per plant, the biomass of the plant, the morphology of the roots, the hydraulic conductivity of the roots, the level of gas exchange, and the level of N metabolism of the adzuki beans were evaluated. Furthermore, RT-qPCR was conducted to explore the expression of genes related to nitrate transporter responses to Ca under N-limitation stress conditions. The rate of accumulation of N in plant tissue increased with the application of Ca. However, plant biomass, photosynthetic parameters, and root activity peaked for Ca²⁺ supply under N-marginal conditions. Further investigation revealed that the activities of nitrate reductase and glutamine synthetase were relatively high. The transcription of the nitrate transporter (VaNRT1.1; VaNRT2.5) was up-regulated in the roots of the Ca-treated plants. Both N-marginal conditions and N deficiency inhibit N absorption and utilization. The favorable effects of Ca on seedling growth and N metabolism under N-marginal conditions were more significant than those under N-deficiency conditions. The supply of Ca²⁺ is optimal, as it increases NUE by enhancing photosynthesis, N-metabolizing enzyme activities, and NO₃ uptake and transport under N-marginal conditions.

Magnesium Fertilization Increases Nitrogen Use Efficiency in Winter Wheat (Triticum aestivum L.)

Wheat fertilized with Mg, regardless of the method of application, increases nitrogen fertilizer (N_f) efficiency. This hypothesis was tested in 2013, 2014, and 2015. A two-factorial experiment with three doses of Mg (i.e., 0, 25, and 50 kg ha^-1) and two stages of Mg foliar fertilization (without; BBCH 30; 49/50; 30 + 49/50) was carried out. Foliar vs. in-soil Mg fertilization resulted in a comparable grain yield increase (0.5-0.6 t ha^-1). The interaction of both fertilization systems increased the yield by 0.85-0.9 t ha^-1. The booting/heading phase was optimal for foliar fertilization. Mg accumulation by wheat fertilized with Mg increased by 17% compared to the NPK plot. The recovery of foliar Mg was multiple in relation to its dose. The recovery of the in-soil Mg applied ranged from 10 to 40%. The increase in yield resulted from the effective use of N taken up by wheat. In 2014 and 2015, this amount was 21-25 kg N ha^-1. The increase in yield resulted from the extended transfer of N from vegetative wheat parts to grain. Mg applied to wheat, irrespective of the method, increased the efficiency of the N taken up by the crop. Mg fertilization resulted in higher N_f productivity, as indicated by the increased nitrogen apparent efficiency indices.

Agronomic Strategies to Improve N Efficiency Indices in Organic Durum Wheat Grown in Mediterranean Area

Organic farming systems are often constrained by limited soil nitrogen (N) availability. Here we evaluated the effect of foliar organic N and sulphur (S), and selenium (Se) application on durum wheat, considering N uptake, utilization efficiency (NUtE), grain yield, and protein concentration as target variables. Field trials were conducted in 2018 and 2019 on two old (Cappelli and old Saragolla) and two modern (Marco Aurelio and Nadif) Italian durum wheat varieties. Four organic fertilization strategies were evaluated, i.e., the control (CTR, dry blood meal at sowing), the application of foliar N (CTR + N) and S (CTR + S), and their joint use (CTR + NS). Furthermore, a foliar application of sodium selenate was evaluated. Three factors-variety, fertilization strategies and selenium application-were arranged in a split-split-plot design and tested in two growing seasons. The modern variety Marco Aurelio led to the highest NUtE and grain yield in both seasons. S and N applications had a positive synergic effect, especially under drought conditions, on pre-anthesis N uptake, N translocation, NUtE, and grain yield. Se treatment improved post-anthesis N uptake and NUtE, leading to 17% yield increase in the old variety Cappelli, and to 13% and 14% yield increase in Marco Aurelio and Nadif, mainly attributed to NUtE increase. This study demonstrated that the synergistic effect of foliar applications could improve organic durum wheat yields in Mediterranean environments, especially on modern varieties.

The N uptake-associated physiological processes at late growth stage in wheat (Triticum aestivum) under N deprivation combined with deficit irrigation condition

Elucidating physiological mechanisms underlying the plant N uptake benefits breeding of high N use efficiency (NUE) crop cultivars. In this study, we investigated the growth and N uptake-associated processes in wheat under N deprivation and deficit irrigation, using two contrasting NUE cultivars. Compared with sufficient-N (SN), deficient-N (DN) treatment reduced plant biomass, N accumulation, and yields in two cultivars (high NUE Shinong 086 and N deprivation-sensitive Jimai 585), suggesting that N deprivation negatively regulates plant growth and N uptake. Shinong 086 was better on growth and N uptake-associated traits than Jimai 585 due to the improved root biomass across soil profile, which was consistent with the decrease of available N contents in soil layers. These results suggested that the improved root system architecture (RAS) enhances plant acquirement for soil N under N- and water-deprivation condition, contributing to the plant N uptake and yield formation capacities. Transcriptome investigation revealed that numerous genes were differentially expressed (DE) in the N-deprived Shinong 086 plants, which involve the regulation of complicate biochemical pathways. These results suggested that the modified RAS and N uptake in high NUE plants are accomplished underlying the regulation of numerous DE genes. TaWRKY20, a gene in ZFP transcription factor family, was functionally characterized for the role in mediating plant N uptake. Overexpression of it conferred plants improved growth and N uptake under DN due to its regulation on TaNRT2.1 and TaNRT2.2, two nitrate transporter genes. Our investigation provides insights in high NUE mechanisms in wheat under N deprivation.

Mechanism and role of seeded native grasses to immobilize nitrogen on harvested blanket peat forests for protection of water courses

Forest harvesting activities on peatlands have long been associated with nutrient leaching and deterioration of downstream water quality. This study aims to assess the effect of grass seeding practice on harvested blanket peatlands to immobilize N and reduce its export to water courses. First, a plot-scale field experiment was conducted by seeding with two grass species (Holcus lanatus and Agrostis capillaris) to study the N uptake potential from a harvested area. Secondly, a simulated rainfall experiment was conducted to study the effect of these grasses on reducing N leaching from surface peat using laboratory flume approach. In the end, the role of seeded grasses in removing N from nutrient-rich throughflow water was assessed using simulated overland flow experiment. The results showed that the seeded grasses had the potential to uptake over 30 kg ha^-1 of N in the first year after seeding on harvested peatlands, whereas it takes over 2.5 years to establish the same level of N uptake by natural re-vegetation (non-grassed). In the simulated rainfall experiment, the inorganic N (NH₄⁺-N and NO₃^--N) leaching in surface runoff from grassed flumes was 72% lower (453 mg m^-2) than non-grassed flumes (1643 mg m^-2). In the simulated overland flow experiment, the N retention by grassed flumes was significantly higher (98%) as compared to non-grassed flumes (70%) in the simulated overland flow experiment. Comparatively higher concentrations of NH₄⁺-N and NO₃^--N in soil porewaters of non-grassed flumes suggest that this N retention by non-grassed flumes is less sustainable and is likely to be leached in runoff in subsequent flow events. The results from all three experiments in this study suggest that seeded grasses are a major sink of N on harvested blanket peatland forests. Immobilization of N onsite using the grass seeding and mini-buffer practice could be an efficient and a feasible mean of reducing N export from harvested blanket peatland forests in order to protect the sensitive water courses. However, the sustainability of retention and immobilization of N by grasses needs to be studied further in long-term field-scale experiments on multiple peatland sites.

Effects of Elevated Carbon Dioxide and Chronic Warming on Nitrogen (N)-Uptake Rate, -Assimilation, and -Concentration of Wheat

The concentration of nitrogen (N) in vegetative tissues is largely dependent on the balance among growth, root N uptake, and N assimilation. Elevated CO₂ (eCO₂) plus warming is likely to affect the vegetative-tissue N and protein concentration of wheat by altering N metabolism, but this is poorly understood. To investigate this, spring wheat (Triticum aestivum) was grown for three weeks at two levels of CO₂ (400 or 700 ppm) and two temperature regimes (26/21 or 31/26 °C, day/night). Plant dry mass, plant %N, protein concentrations, NO₃⁻ and NH₄⁺ root uptake rates (using ¹⁵NO₃ or ¹⁵NH₄), and whole-plant N- and NO₃^--assimilation were measured. Plant growth, %N, protein concentration, and root N-uptake rate were each significantly affected only by CO₂, while N- and NO₃⁻-assimilation were significantly affected only by temperature. However, plants grown at eCO₂ plus warming had the lowest concentrations of N and protein. These results suggest that one strategy breeding programs can implement to minimize the negative effects of eCO₂ and warming on wheat tissue N would be to target the maintenance of root N uptake rate at eCO₂ and N assimilation at higher growth temperatures.

N metabolism performance in Chenopodium quinoa subjected to drought or salt stress conditions

Currently it is estimated that the 20% of total cultivated land is affected by salt. Besides, drought events will increase worldwide. These factors are affecting plant growth and crop production compromising food security. Within this context, quinoa (Chenopodium quinoa) is becoming an alternative pseudocereal for food supply due to its capacity to grow under harsh environmental conditions. Besides, it is being proposed as key model species to study the physiological processes that permit this tolerance, although how N metabolism responds has been barely studied. This paper addresses, on one hand, the response of quinoa's N metabolism (N uptake, translocation, reduction and assimilation) under the forthcoming climatic conditions and, on the other hand, the comparison of the effects of both stresses when plants have similar relative water content and photosynthetic rates. Under mild salt stress (120 and 240 mM NaCl) N assimilation is not affected, while the N uptake is favored. Under severe salt stress (500 mM NaCl), N uptake is reduced, decreasing leaf nitrate and protein concentration; nevertheless, leaf free amino acids are maintained -to perform osmotic adjustment-. N uptake rate is more affected under drought than under severe salt; furthermore, under severe salt stress, quinoa allocates more nitrogen to roots to finely regulate NO₃^- and Cl^- uptake, while under drought it allocates more to leaves to ensure photosynthesis. These results indicate that quinoa's N metabolism is tolerant to drought and salt stress, although the strategies of this species for coping with the aforementioned stresses are different.

Air warming and CO2 enrichment increase N use efficiency and decrease N surplus in a Chinese double rice cropping system

Effectiveness of N might be modified in rice cultivation under future climate change with elevated atmospheric CO₂ concentration ([CO₂]). At present, limited information is available to understand how plant N uptake and N use efficiency respond to elevated [CO₂] and/or temperature in Chinese double rice cropping systems. A four-year field experiment was therefore conducted using open-top chambers with varying [CO₂] (ambient, ambient +60 μmol mol^-1) and varying temperature (ambient, ambient +2 °C) in Hubei Province, Central China. Compared with ambient conditions, elevated [CO₂] increased plant N uptake and N use efficiency, as measured by fertilizer N recovery efficiency (NRE), N agronomic efficiency (NAE), N physiological efficiency (NPE) and apparent system N use efficiency (NUE_sys), in both early rice and late rice. CO₂ enrichment tended to decrease soil mineral N concentration since more N was assimilated by plants. Elevated temperature led to lower plant N uptake and decreased NRE and NAE in early rice, due to a reduction in grain yield induced by heat injury. In contrast, warming increased plant N uptake and N use efficiency in late rice as no heat stress existed. Warming tended to increase soil mineral N concentration in early rice but had negligible effects in late rice. When elevated [CO₂] and temperature were combined, the positive effects of CO₂ enrichment for N utilization were able to compensate for the negative effects of warming in early rice, while the interaction was synergetic in late rice. Hence, co-elevation of [CO₂] and temperature led to higher N use efficiency (64.6% for NUE_sys across four years) and decreased annual N surplus by 28.6-36.5 kg N ha^-1 compared with ambient conditions. Our findings confirm that CO₂ enrichment and air warming can improve N use efficiency at both crop level and system level in Chinese double rice cultivation.

Substituting organic manure for compound fertilizer increases yield and decreases NH3 and N2O emissions in an intensive vegetable production systems

Substituting organic manure for compound fertilizer may play an important role in regulating the nitrogen (N) cycle and consequently affecting crop yield in agroecosystems. However, how substituting different organic manures for compound fertilizer affects crop yield and ammonia (NH₃) and nitrous oxide (N₂O) emissions in the vegetable system during the life-cycle production (including storage and field application) remains poorly elucidated. Thus, we conducted a greenhouse experiment to investigate the effects of substituting organic manure species, i.e., stored swine manure fertilizer (SS), swine manure covered by straw (CS), stored swine fertilizer mixed with biochar (BS), and stored swine manure fertilizer with void expansion (OS) for compound fertilizer (FC) on rapeseed yield and NH₃ and N₂O emissions in a rapeseed-cropping system in China. The results showed that the total gaseous N losses (NH₃ and N₂O) were 1.6, 1.4 and 1.1 times higher in SS, CS and OS than FC, respectively. However, total gaseous N losses in BS was 0.9 times less than FC. Compared with FC, rapeseed yield and N uptake in SS and CS were decreased by 17.2-20.2% and 16.0%-28.1%, respectively, but which were increased by 7.3% and 54.1% in BS, respectively. In addition, OS decreased rapeseed yield by 17.2%, but increased N uptake by 8.5%. Therefore, the effects of substituting organic manure for compound fertilizer on rapeseed yield, N uptake, NH₃ and N₂O varied regarding different organic manure species. Adopting stored swine fertilizer mixed with biochar might be a sound management practice to reduce gaseous N losses and enhance N uptake and yield in intensive vegetable production systems.

Legumes in catch crop mixtures: Effects on nitrogen retention and availability, and leaching losses

Catch crop (CC) mixtures of non-legumes (nL) and legumes (L) have been promoted as a strategy to achieve two different goals: to decrease the risk of nitrate leaching and to enhance the nitrogen supply to the subsequent crop. To investigate if two-component mixtures of nL + L have advantages over pure nL stands experiments were carried out over a two year period (2013-2015) at two contrasting field sites in Denmark. Nitrogen (N) uptake by the CCs was measured by aboveground biomass sampling, and N leaching by ceramic suction cups. When grown in pure stands, white clover (Trifolium repens) on coarse sand and common vetch (Vicia sativa) on sandy loam were less effective at reducing N leaching than perennial ryegrass (Lolium perenne) and fodder radish (Raphanus sativus). When the proportion of the nL + L in mixtures was similar or favored the nL, leaching was not significantly different from the nL in the pure stand. However, during one of the years on the sandy loam L (vetch) almost outperformed nL (fodder radish), resulting in N leaching from nL + L similar to L. The yield of the following spring barley was only significantly different from the yield in the plots with previously bare soil in one of the years on the coarse sandy soil. It is concluded that in nL + L mixtures L can take over and thereby lower the effect of the CCs on N leaching while not necessarily enhancing the N supply for the subsequent crop.

Moderate water stress does not inhibit nitrogen remobilization, allowing fast growth in high nitrogen content Quercus variabilis seedlings under dry conditions

Remobilization of stored nitrogen (N) plays an important role in the early growth of deciduous trees in spring. Several environmental factors can modulate N remobilization, but whether water stress is one such factors is unknown. This study analyzes how the size of N storage in Quercus variabilis Blume seedlings interacts with water stress to affect N remobilization, uptake and new growth. This information is important for improving success of forest tree plantations under dry spring conditions. During the first growing season, we produced seedlings with distinct N content by applying two fall N fertilization rates (12 or 24 mg N per seedling) using 15N-enriched fertilizer. At the beginning of the second growing season, a new experiment was started where seedlings were transplanted into larger pots and subjected to two watering levels (85 or 40% of field capacity). The plants were sampled at 4 weeks (T1), 8 weeks (T2) and 12 weeks (T3) after transplanting. Low watering reduced the growth of high and low N seedlings, but high N seedlings showed greater growth than low N seedlings. During bud burst and initial shoot elongation (T1), restricted watering, which induced a moderate water stress, did not affect the amount of N remobilized from roots, the major source of stored N source at this growth stage. This suggests that high N storage can partially counteract the negative effect of moderate water stress on early growth. At T1, water stress did not affect N uptake, and high N content seedlings absorbed significantly less soil N than did low N content seedlings. At T3, in contrast, water stress was the main determinant for N uptake, with drought-stressed plants showing lower uptake than well-watered plants. We conclude that moderate drought does not inhibit N remobilization from the major storage organ at early growth stages in spring, and that increasing N storage of planted seedlings through fall fertilization can mitigate the negative effect of moderate spring drought on growth.

Differential carbon allocation to nitrogen-rich patches in Poa annua precedes root proliferation but has no immediate benefit to N uptake

Nutrients are heterogeneously distributed in the soil environment. Plants have evolved a variety of mechanisms to maximise their ability to compete for limited resources, with differential root growth considered among the more important mechanisms. Despite the significant costs of root growth, little data is available regarding carbon (C) allocation to roots growing in heterogeneous conditions. Here, we investigate the allocation of recently assimilated C in Poa annua plants growing in uniform or heterogeneous nutrient conditions. In the first experiment we grew plants in split-root boxes, providing N either equally between the two chambers (0.5 mg/0.5 mg, 8 mg/8 mg) or with one side receiving more N (0.5 mg/8 mg, 8 mg/0.5 mg), and quantified C allocation and N uptake using ¹³CO₂ and K¹⁵NO₃. Where N was supplied equally to the two chambers, C was allocated equally to the roots irrespective of the total N supply. However, the ¹³C label was preferentially allocated (60:40) to high-N roots in the unequal treatments. N uptake was a function of local supply and was not affected by N supply to the roots in the other chamber. C allocation had no discernible effect on N uptake. In the second experiment, we tested whether differential N supply would lead to increased root growth in the high-N side. In this experiment, equal amounts of N were supplied to all plants as ammonium, with half receiving an equal distribution to the two root chambers (50/50), while the other half received an unequal supply (94/6). While no difference in root growth was noted in 50/50 plants, a 60:40 mass allocation was noted from day six onwards in plants receiving the 94/6 N supply. Despite increased root growth in the high-N side, the plants receiving the 94/6 treatment could not achieve the same shoot mass or N concentration as the 50/50 plants. No difference in total C allocation to the roots between treatments was noted in the first experiment, and no difference in total root mass between treatments was found in the second experiment, suggesting that root C supply was source-limited, while allocation to specific roots was strongly influenced by sink strength. Differential C allocation appears to be an important pre-requisite for increased root growth in N-rich patches.

Low 15N Natural Abundance in Shoot Tissue of Brachiaria humidicola Is an Indicator of Reduced N Losses Due to Biological Nitrification Inhibition (BNI)

The tropical forage grass Brachiaria humidicola (Bh) suppresses the activity of soil nitrifiers through biological nitrification inhibition (BNI). As a result, nitrate (NO 3 - ) formation and leaching are reduced which is also expected to tighten the soil nitrogen (N) cycle. However, the beneficial relationship between reducedNO 3 - losses and enhanced N uptake due to BNI has not been experimentally demonstrated yet. Nitrification discriminates against the 15N isotope and leads to 15N depletedNO 3 - , but 15N enrichedNH 4 + in soils. Leaching of 15N depletedNO 3 - enriches the residual N pool in the soil with 15N. We hypothesized that altered nitrification andNO 3 - leaching due to diverging BNI magnitudes in contrasting Bh genotypes influence soil 15N natural abundance (δ15N), which in turn is reflected in distinct δ15N in Bh shoot biomass. Consequently, high BNI was expected to be reflected in low plant δ15N of Bh. It was our objective to investigate under controlled conditions the link between shoot value of δ15N in several Bh genotypes and leachedNO 3 - amounts and shoot N uptake. Additionally, plant 15N and N% was monitored among a wide range of Bh genotypes with contrasting BNI potentials in field plots for 3 years. We measured leaf δ15N of young leaves (regrown after cutback) of Bh and combined it with nitrification rates (NRs) of incubated soil to test whether there is a direct relationship between plant δ15N and BNI. Increased leachedNO 3 - was positively correlated with higher δ15N in Bh, whereas the correlation between shoot N uptake and shoot δ15N was inverse. Field cultivation of a wide range of Bh genotypes over 3 years decreased NRs in incubated soil, while shoot δ15N declined and shoot N% increased over time. Leaf δ15N of Bh genotypes correlated positively with NRs of incubated soil. It was concluded that decreasing plant δ15N of Bh genotypes over time reflects the long-term effect of BNI as linked to lowerNO 3 - formation and reducedNO 3 - leaching. Accordingly, a low δ15N in Bh shoot tissue verified its potential as indicator of high BNI activity of Bh genotypes.

Genetic variation in N-use efficiency and associated traits in Indian wheat cultivars

Nitrogen (N) fertilizer represents a significant cost for the grower and may also have environmental impacts through nitrate leaching and N2O (a greenhouse gas) emissions associated with denitrification. The objectives of this study were to quantify the genetic variability in N-use efficiency (NUE) in Indian spring wheat cultivars and identify traits for improved NUE for application in breeding. Twenty eight bread wheat cultivars and two durum wheat cultivars were tested in field experiments in two years in Maharashtra, India. Detailed growth analysis was conducted at anthesis and harvest including dry matter (DM) and N partitioning. Senescence of the flag leaf was assessed from a visual score every 3-4 days from anthesis to complete flag-leaf senescence and fitted against thermal time to estimate the onset and end of post-anthesis senescence. Grain yield (GY) was reduced under low N (LN) by an average of 1.46 t ha-1 (-28%). Significant N × genotype level interaction was observed for grain yield and NUE. Above-ground N uptake at harvest was reduced from 162 kg N ha-1 under high N (HN) to 85 kg N ha-1 under low N (LN) conditions, while N-utilization efficiency (grain DM yield per unit crop N uptake at harvest; NUtE) increased from 32.7 to 44.6 kg DM kg-1 N. Genetic variation in GY under LN related mainly to variation in N uptake at harvest rather than NUtE; and the N × genotype effect for GY was mainly explained by the interaction for N uptake at harvest. Averaging across years, the linear regression of onset of flag-leaf senescence on GY amongst cultivars was significant under both HN (R2 0.16. p < 0.05) and LN (R2 0.21, p < 0.05) conditions. Onset of flag-leaf senescence was positively associated with N uptake at anthesis under HN (R2 0.34, p < 0.001) and LN (R2 0.22, p < 0.01) conditions. Flag-leaf senescence timing was not associated with post-anthesis N uptake. It is concluded that increased N accumulation at anthesis was correlated with flag-leaf senescence timing and that N accumulation at anthesis is an important trait for enhancing grain yield and NUE of wheat grown under low to moderate N supply in India.

Trichoderma-Based Biostimulants Modulate Rhizosphere Microbial Populations and Improve N Uptake Efficiency, Yield, and Nutritional Quality of Leafy Vegetables

Microbial inoculants such as Trichoderma-based products are receiving great interest among researchers and agricultural producers for their potential to improve crop productivity, nutritional quality as well as resistance to plant pathogens/pests and numerous environmental stresses. Two greenhouse experiments were conducted to assess the effects of Trichoderma-based biostimulants under suboptimal, optimal and supraoptimal levels of nitrogen (N) fertilization in two leafy vegetables: Iceberg lettuce (Lactuca sativa L.) and rocket (Eruca sativa Mill.). The yield, nutritional characteristics, N uptake and mineral composition were analyzed for each vegetable crop after inoculation with Trichoderma strains T. virens (GV41) or T. harzianum (T22), and results were compared to non-inoculated plants. In addition, the effect of the Trichoderma-based biostimulants on microbes associated with the rhizosphere in terms of prokaryotic and eukaryotic composition and concentration using DGGE was also evaluated. Trichoderma-based biostimulants, in particular GV41, positively increased lettuce and rocket yield in the unfertilized plots. The highest marketable lettuce fresh yield was recorded with either of the biostimulant inoculations when plants were supplied with optimal levels of N. The inoculation of rocket with GV41, and to a lesser degree with T22, elicited an increase in total ascorbic acid under both optimal and high N conditions. T. virens GV41 increased N-use efficiency of lettuce, and favored the uptake of native N present in the soil of both lettuce and rocket. The positive effect of biostimulants on nutrient uptake and crop growth was species-dependent, being more marked with lettuce. The best biostimulation effects from the Trichoderma treatments were observed in both crops when grown under low N availability. The Trichoderma inoculation strongly influenced the composition of eukaryotic populations in the rhizosphere, in particularly exerting different effects with low N levels in comparison to the N fertilized plots. Overall, inoculations with Trichoderma may be considered as a viable strategy to manage the nutrient content of leafy horticulture crops cultivated in low fertility soils, and assist vegetable growers in reducing the use of synthetic fertilizers, developing sustainable management practices to optimize N use efficiency.

Optimizing the nitrogen application rate for maize and wheat based on yield and environment on the Northern China Plain

Optimizing the nitrogen (N) application rate can increase crop yield while reducing the environmental risks. However, the optimal N rates vary substantially when different targets such as maximum yield or maximum economic benefit are considered. Taking the wheat-maize rotation cropping system on the North China Plain as a case study, we quantified the variation of N application rates when targeting constraints on yield, economic performance, N uptake and N utilization, by conducting field experiments between 2011 and 2013. Results showed that the optimal N application rate was highest when targeting N uptake (240kgha^-1 for maize, and 326kgha^-1 for wheat), followed by crop yield (208kgha^-1 for maize, and 277kgha^-1 for wheat) and economic income (191kgha^-1 for maize, and 253kgha^-1 for wheat). If environmental costs were considered, the optimal N application rates were further reduced by 20-30% compared to those when targeting maximum economic income. However, the optimal N rate, with environmental cost included, may result in soil nutrient mining under maize, and an extra input of 43kgNha^-1 was needed to make the soil N balanced and maintain soil fertility in the long term. To obtain a win-win situation for both yield and environment, the optimal N rate should be controlled at 179kgha^-1 for maize, which could achieve above 99.5% of maximum yield and have a favorable N balance, and at 202kgha^-1 for wheat to achieve 97.4% of maximum yield, which was about 20kgNha^-1 higher than that when N surplus was nil. Although these optimal N rates vary on spatial and temporal scales, they are still effective for the North China Plain where 32% of China's total maize and 45% of China's total wheat are produced. More experiments are still needed to determine the optimal N application rates in other regions. Use of these different optimal N rates would contribute to improving the sustainability of agricultural development in China.

Nitrogen Nutrition of Fruit Trees to Reconcile Productivity and Environmental Concerns

Although perennial fruit crops represent 1% of global agricultural land, they are of a great economic importance in world trade and in the economy of many regions. The perennial woody nature of fruit trees, their physiological stages of growth, the root distribution pattern, and the presence of herbaceous vegetation in alleys make orchard systems efficient in the use and recycling of nitrogen (N). The present paper intends to review the existing literature on N nutrition of young and mature deciduous and evergreen fruit trees with special emphasis to temperate and Mediterranean climates. There are two major sources of N contributing to vegetative tree growth and reproduction: root N uptake and internal N cycling. Optimisation of the use of external and internal N sources is important for a sustainable fruit production, as N use efficiency by young and mature fruit trees is generally lower than 55% and losses of fertilizer N may occur with the consequent economic and environmental concern. Organic alternatives to mineral N fertilizer like the application of manure, compost, mulching, and cover crops are scarcely used in perennial fruit trees, in spite of the fact that society’s expectations call for more sustainable production techniques and the demand for organic fruits is increasing.

A New Role for SAG12 Cysteine Protease in Roots of Arabidopsis thaliana

Senescence associated gene (SAG) 12, which encodes a cysteine protease is considered to be important in nitrogen (N) allocation to Arabidopsis thaliana seeds. A decrease in the yield and N content of the seeds was observed in the Arabidopsis SAG12 knockout mutants (sag12) relative to the wild type (Col0) under limited nitrogen nutrition. However, leaf senescence was similar in both lines. To test whether SAG12 is involved in N remobilization from organs other than the leaves, we tested whether root N could be used in N mobilization to the seeds. Root architecture, N uptake capacity and 15N partitioning were compared in the wild type and sag12 under either high nitrogen (HN) or low nitrogen (LN) conditions. No differences in root architecture or root N uptake capacity were observed between the lines under HN or LN. However, under LN conditions, there was an accumulation of ¹⁵N in the sag12 roots compared to the wild type with lower allocation of ¹⁵N to the seeds. This was accompanied by an increase in root N protein contents and a significant decrease in root cysteine protease activity. SAG12 is expressed in the root stele of the plants at the reproductive stage, particularly under conditions of LN nutrition. Taken together, these results suggest a new role for SAG12. This cysteine protease plays a crucial role in root N remobilization that ensures seed filling and sustains yields when nitrogen availability is low.

Silicon Promotes Growth of Brassica napus L. and Delays Leaf Senescence Induced by Nitrogen Starvation

Silicon (Si) is the second most abundant element in soil and has several beneficial effects, especially in plants subjected to stress conditions. However, the effect of Si in preventing nitrogen (N) starvation in plants is poorly documented. The aim of this work was to study the effect of a short Si supply duration (7 days) on growth, N uptake, photosynthetic activity, and leaf senescence progression in rapeseed subjected (or not) to N starvation. Our results showed that after 1 week of Si supply, Si improves biomass and increases N uptake and root expression of a nitrate transporter gene. After 12 days of N starvation, compared to -Si plants, mature leaf from +Si plants showed a high chlorophyll content, a maintain of net photosynthetic activity, a decrease of oxidative stress markers [hydrogen peroxide (H₂O₂) and malondialdehyde (MDA)] and a significant delay in senescence. When N-deprived plants were resupplied with N, a greening again associated with an increase of photosynthetic activity was observed in mature leaves of plants pretreated with Si. Moreover, during the duration of N resupply, an increase of N uptake and nitrate transporter gene expression were observed in plants pretreated with Si. In conclusion, this study has shown a beneficial role of Si to alleviate damage associated with N starvation and more especially its role in delaying of leaf senescence.

Sphagnum can 'filter' N deposition, but effects on the plant and pore water depend on the N form

The ability of Sphagnum moss to efficiently intercept atmospheric nitrogen (N) has been assumed to be vulnerable to increased N deposition. However, the proposed critical load (20kgNha(-1)yr(-1)) to exceed the capacity of the Sphagnum N filter has not been confirmed. A long-term (11years) and realistic N manipulation on Whim bog was used to study the N filter function of Sphagnum (Sphagnum capillifolium) in response to increased wet N deposition. On this ombrotrophic peatland where ambient deposition was 8kgNha(-1)yr(-1), an additional 8, 24, and 56kgNha(-1)yr(-1) of either ammonium (NH4(+)) or nitrate (NO3(-)) has been applied for 11years. Nutrient status of Sphagnum and pore water quality from the Sphagnum layer were assessed. The N filter function of Sphagnum was still active up to 32kgNha(-1)yr(-1) even after 11years. N saturation of Sphagnum and subsequent increases in dissolved inorganic N (DIN) concentration in pore water occurred only for 56kgNha(-1)yr(-1) of NH4(+) addition. These results indicate that the Sphagnum N filter is more resilient to wet N deposition than previously inferred. However, functionality will be more compromised when NH4(+) dominates wet deposition for high inputs (56kgNha(-1)yr(-1)). The N filter function in response to NO3(-) uptake increased the concentration of dissolved organic N (DON) and associated organic anions in pore water. NH4(+) uptake increased the concentration of base cations and hydrogen ions in pore water though ion exchange. The resilience of the Sphagnum N filter can explain the reported small magnitude of species change in the Whim bog ecosystem exposed to wet N deposition. However, changes in the leaching substances, arising from the assimilation of NO3(-) and NH4(+), may lead to species change.

The Genetics of Nitrogen Use Efficiency in Crop Plants

In the past 50 years, the application of synthetic nitrogen (N) fertilizer to farmland resulted in a dramatic increase in crop yields but with considerable negative impacts on the environment. New solutions are therefore needed to simultaneously increase yields while maintaining, or preferably decreasing, applied N to maximize the nitrogen use efficiency (NUE) of crops. In this review, we outline the definition of NUE, the selection and development of NUE crops, and the factors that interact with NUE. In particular, we emphasize the challenges of developing crop plants with enhanced NUE, using more classical genetic approaches based on utilizing existing allelic variation for NUE traits. The challenges of phenotyping, mapping quantitative trait loci (QTLs), and selecting candidate genes for NUE improvement are described. In addition, we highlight the importance of different factors that lead to changes in the NUE components of nitrogen uptake efficiency (NUpE) and nitrogen utilization efficiency (NUtE).

Cascading effects of belowground predators on plant communities are density-dependent

Soil food webs comprise a multitude of trophic interactions that can affect the composition and productivity of plant communities. Belowground predators feeding on microbial grazers like Collembola could decelerate nutrient mineralization by reducing microbial turnover in the soil, which in turn could negatively influence plant growth. However, empirical evidences for the ecological significance of belowground predators on nutrient cycling and plant communities are scarce. Here, we manipulated predator density (Hypoaspis aculeifer: predatory mite) with equal densities of three Collembola species as a prey in four functionally dissimilar plant communities in experimental microcosms: grass monoculture (Poa pratensis), herb monoculture (Rumex acetosa), legume monoculture (Trifolium pratense), and all three species as a mixed plant community. Density manipulation of predators allowed us to test for density‐mediated effects of belowground predators on Collembola and lower trophic groups. We hypothesized that predator density will reduce Collembola population causing a decrease in nutrient mineralization and hence detrimentally affect plant growth. First, we found a density‐dependent population change in predators, that is, an increase in low‐density treatments, but a decrease in high‐density treatments. Second, prey suppression was lower at high predator density, which caused a shift in the soil microbial community by increasing the fungal: bacterial biomass ratio, and an increase of nitrification rates, particularly in legume monocultures. Despite the increase in nutrient mineralization, legume monocultures performed worse at high predator density. Further, individual grass shoot biomass decreased in monocultures, while it increased in mixed plant communities with increasing predator density, which coincided with elevated soil N uptake by grasses. As a consequence, high predator density significantly increased plant complementarity effects indicating a decrease in interspecific plant competition. These results highlight that belowground predators can relax interspecific plant competition by increasing nutrient mineralization through their density‐dependent cascading effects on detritivore and soil microbial communities.

Nitrogen yield advantage from grass-legume mixtures is robust over a wide range of legume proportions and environmental conditions

Current challenges to global food security require sustainable intensification of agriculture through initiatives that include more efficient use of nitrogen (N), increased protein self-sufficiency through homegrown crops, and reduced N losses to the environment. Such challenges were addressed in a continental-scale field experiment conducted over 3 years, in which the amount of total nitrogen yield (Ntot ) and the gain of N yield in mixtures as compared to grass monocultures (Ngainmix ) was quantified from four-species grass-legume stands with greatly varying legume proportions. Stands consisted of monocultures and mixtures of two N2 -fixing legumes and two nonfixing grasses. The amount of Ntot of mixtures was significantly greater (P ≤ 0.05) than that of grass monocultures at the majority of evaluated sites in all 3 years. Ntot and thus Ngainmix increased with increasing legume proportion up to one-third of legumes. With higher legume percentages, Ntot and Ngainmix did not continue to increase. Thus, across sites and years, mixtures with one-third proportion of legumes attained ~95% of the maximum Ntot acquired by any stand and had 57% higher Ntot than grass monocultures. Realized legume proportion in stands and the relative N gain in mixture (Ngainmix /Ntot in mixture) were most severely impaired by minimum site temperature (R = 0.70, P = 0.003 for legume proportion; R = 0.64, P = 0.010 for Ngainmix /Ntot in mixture). Nevertheless, the relative N gain in mixture was not correlated to site productivity (P = 0.500), suggesting that, within climatic restrictions, balanced grass-legume mixtures can benefit from comparable relative gains in N yield across largely differing productivity levels. We conclude that the use of grass-legume mixtures can substantially contribute to resource-efficient agricultural grassland systems over a wide range of productivity levels, implying important savings in N fertilizers and thus greenhouse gas emissions and a considerable potential for climate change mitigation.

Nitrogen yield advantage from grass-legume mixtures is robust over a wide range of legume proportions and environmental conditions

Current challenges to global food security require sustainable intensification of agriculture through initiatives that include more efficient use of nitrogen (N), increased protein self-sufficiency through homegrown crops, and reduced N losses to the environment. Such challenges were addressed in a continental-scale field experiment conducted over 3 years, in which the amount of total nitrogen yield (Ntot ) and the gain of N yield in mixtures as compared to grass monocultures (Ngainmix ) was quantified from four-species grass-legume stands with greatly varying legume proportions. Stands consisted of monocultures and mixtures of two N2 -fixing legumes and two nonfixing grasses. The amount of Ntot of mixtures was significantly greater (P ≤ 0.05) than that of grass monocultures at the majority of evaluated sites in all 3 years. Ntot and thus Ngainmix increased with increasing legume proportion up to one-third of legumes. With higher legume percentages, Ntot and Ngainmix did not continue to increase. Thus, across sites and years, mixtures with one-third proportion of legumes attained ~95% of the maximum Ntot acquired by any stand and had 57% higher Ntot than grass monocultures. Realized legume proportion in stands and the relative N gain in mixture (Ngainmix /Ntot in mixture) were most severely impaired by minimum site temperature (R = 0.70, P = 0.003 for legume proportion; R = 0.64, P = 0.010 for Ngainmix /Ntot in mixture). Nevertheless, the relative N gain in mixture was not correlated to site productivity (P = 0.500), suggesting that, within climatic restrictions, balanced grass-legume mixtures can benefit from comparable relative gains in N yield across largely differing productivity levels. We conclude that the use of grass-legume mixtures can substantially contribute to resource-efficient agricultural grassland systems over a wide range of productivity levels, implying important savings in N fertilizers and thus greenhouse gas emissions and a considerable potential for climate change mitigation.