Variation In Nitrogen Use Efficiency Research Articles

Spatial-temporal variability in nitrogen use efficiency: Insights from a long-term experiment and crop simulation modeling to support site specific nitrogen management

Within-field soil heterogeneity can lead to large variation in nitrogen use efficiency (NUE). Crop simulation models provide a multi-faceted approach to management considering both soil and plant interactions. However, research using crop models for investigating within field variation in NUE is limited, in part because of challenges quantifying spatially variable soil model parameters. Here soil apparent electrical conductivity (ECa) and measured soil properties were used to map spatial variations in soil characteristics across a Long-Term Experiment in Norfolk, England. The relationship between plot ECa across the 3 ha experiment and agronomic data across three different nitrogen rates (0, 110, and 220 kg N ha-1) over five wheat years (2010–2020) was quantified. The Sirius crop model was parameterized for two soils representing the extremes of ECa. Sirius was validated using recorded plot data. Site-specific optimal nitrogen and associated leaching risks were simulated across 29 years of weather data. Variation in soil properties had significant impact on measured NUE. At 220 kg N ha-1 mean observed yields across 5 years ranged from 9.0 to 10.7 t ha-1 and grain protein from 11.6% to 11% on the low EC and high EC plots, respectively. On average fertiliser grain N recovery was 19.7 kg N ha-1 lower on the low ECa plots. Sirius simulated the variation in yield, grain protein and grain N recovery to a good level of accuracy with RRMSE of 19.5%, 15.4% and 19.5%, respectively. Simulated optimal nitrogen on the low EC soils was on average 12 kg N ha-1 lower, with >1 in 4 years with optimal nitrogen <200 kg N ha-1. Our work demonstrated that using a combination of proximal soil EC scans and targeted soil sampling we can optimize the data requirements for model parameterisation to support site-specific N management.

Mining natural genetic variations for nitrogen use efficiency utilizing nested synthetic hexaploid wheat introgression libraries

Wheat is grown on more than 240 million hectares globally, more than any other commercial crop. Variation for nitrogen-use efficiency (NUE) in wheat germplasm is very important as efficiency of nitrogen fertilizers use can have significant effect on overall consumption of N. In the present study we used nested synthetic hexaploid wheat introgression libraries to detect genetic variation associated with NUE and related traits. The libraries were genotyped with 9474 SNP markers and used for genome-wide association mapping. Significant phenotypic variation was observed for all measured and derived traits. We detected 10 Quantitative trait locus (QTL) and 19 marker-trait associations (MTAs) possibly involved in improving NUE. Of these, 5 QTL and 8 MTAs detected under nitrogen-limited conditions have potential for use in breeding to increase nitrogen-deficiency tolerance. Nitrate transporter genes collocated with detected MTAs showed significant changes in expression of TaNRT2 genes in response to N-starvation and N-recovery. The identified promising breeding lines with stable yield, better NUE and acceptable protein content may constitute an important genetic resource in improving NUE of modern wheat varieties.

Soil nitrogen dynamics drive regional variation in nitrogen use efficiency in rice: A multi‐scale study

AbstractNortheast and East China account for ~36% of the Chinese rice cultivation area, yet considerable spatial disparities in nitrogen use efficiency (NUE) exist between these regions. The underlying causes remain poorly understood. Herein, we conducted a case study in two sample sites from two regions, Wuchang and Changshu, using multi‐scale evidence chains spanning macro‐ and micro‐ processes to identify the determinants of spatial NUE variability. Field studies showed higher NUE (partial factor productivity, PFP, and agronomic efficiency, AE) but lower ammonia volatilisation in Wuchang paddy soil. By separating edaphic factors from climatic conditions, soil replacement pot studies between Wuchang and Changshu revealed that both apparent (AE and recovery efficiency, RE) and 15N‐traced NUE were higher, whereas 15N fertilizer losses were lower in Wuchang‐soil than Changshu‐soil irrespective of site, suggesting soil type contributed to differences in soil N retention capacity and NUE of the soils. Process‐scale results showed that greater rates of gross N mineralizsation (13‐fold higher), gross nitrification (93% higher), and denitrification (52% higher) in Changshu‐soil compared to Wuchang‐soil corresponded to functional gene relative abundance, signifying larger reactive N losses and reduced soil N retention capacity. Microbial community analysis suggested that the differential N transformations were caused by differences in ammonia‐oxidizing archaea (AOA) family nitrososphaeraceae and Nitrospira‐like nitrite‐oxidizing bacteria (NOB). This highlights the importance of specific efficiency‐enhanced strategies tailored to the edaphic characteristics of cropping regions, such as increasing soil N retention capacity using enhanced‐efficiency fertilizer in East China, while implementing conservation management strategies in Northeast China.Highlights Field‐scale studies revealed higher NUE and lower NH3 volatilisation in Wuchang site. Soil replacement pot studies showed higher NUE in Wuchang‐soil regardless of sites. Process‐scale results unravelled high N losses and low soil N retention in Changshu‐soil. Varying NUE partially correlated with Nitrososphaeraceae AOA and Nitrospira‐like NOB.

Linear dependency of winter wheat yield and nitrogen use efficiency on the pre-anthesis temperature in the lower reach of the Yangtze River

Global warming is expected to enhance the nitrogen (N) absorption capability of crops, which causes a change in nitrogen fertilizer input. However, the impacts of warming and nitrogen application on crop growth, yield and nitrogen use efficiency (NUE) are still unclear. Here, we conducted a 4-year field experiment of nighttime warming before winter wheat anthesis to explore the response of winter wheat yield to warming and the variation in nitrogen use efficiency under different nitrogen application levels (N150: 150 kg·ha−1, N225: 225 kg·ha−1, and N375: 375 kg·ha−1). The results showed that warming shortened the winter wheat vegetative growth period (VGP) by 8.3 days, while it prolonged the reproductive growth period (RGP) by 6.7 days. However, there was no significant difference in the growth period of winter wheat between nitrogen treatments. Meanwhile, warming increased winter wheat yield by increasing the number of effective spikes and grains per spike. Every 1 °C increase in pre-anthesis temperature increased winter wheat yield by 45.48, 55.78 and 60.88 g·m−2 for the N150, N225 and N375 treatments, respectively. There was no significant difference in yield between the N225 and N375 treatments, but the nitrogen partial factor productivity (PFPN) and the nitrogen utilization efficiency (NUtE) of N225 were significantly higher than those of N375. PFPN and NUtE were positively correlated with pre-anthesis air temperature under different nitrogen treatments. Multiple linear regression showed that a 1.0 °C increase enhanced NUtE by 11.62%, 9.75% and 7.85% for the N150, N225 and N375 treatments, respectively, and enhanced the PFPN by 7.77%, 8.94% and 9.81% respectively. Therefore, we suggest that 225 kg·ha−1 is an appropriate amount of nitrogen application in the lower reach of the Yangtze River under future climate warming.

Diversified crop sequences to reduce soil nitrogen mining in agroecosystems

Nitrogen (N) is the primary driver of increased global food supply, but has environmental consequences from both under- and over-fertilisation. While over-fertilisation and reactive nitrogen release onto the environment is widespread in North America and Europe, under-fertilisation and soil mining prevail in South American croplands, calling for novel nitrogen-balancing strategies. The encroachment of soybean-centric, over-simplified cropping systems has eroded ecosystem services in South America. Here we compare the current soybean-centric system in Argentina with seven crop sequences in two long-term experiments started in 2008. Our aim was to identify alternative, more diverse crop sequences to maintain productivity and profitability with a close to neutral apparent N balance of the agroecosystem in two contrasting soils, i.e., a Mollisol and a Vertisol. Crop sequences combined locally adapted crops – soybean, maize and wheat for grain, field pea as cover crop – in a range from monocultures to complex sequences including all four crops. Crop sequence returned a 2.2–3.1-fold variation in productivity (from 4.7 to 10.9 Mg ha-1 in the Mollisol and from 3.4 to 9.9 Mg ha-1 in the Vertisol), 1.5-fold variation in profitability (from 0.92 to 2.14), variation in nitrogen balance from soil mining at − 35 kg N ha-1 year-1 to excess at 17 kg N ha-1 year-1, and a variation in nitrogen use efficiency at crop sequence level (NUEs) from 0.7 to 1.2 in both soil types. High soybean proportion reduced the NUEs and grain productivity of crop sequences. More complex crop sequences, i.e. including three/four crops, showed an N surplus and a similar grain yield than maize monoculture in both soils. The inclusion of maize into crop sequences with high cropping intensity increased both yield and NUEs. We identify new crop sequences that meet three conditions: high productivity and profitability, a close-to-neutral nitrogen balance, and a high nitrogen-use efficiency. These insights allow for alternatives to the current, unsustainable trajectories of simplified soybean-based systems that also avoid the path of over-fertilisation followed by cropping systems elsewhere.

Clonal differences in nitrogen use efficiency and macro-nutrient uptake in young clonal cocoa (Theobroma cacao L.) seedlings from Indonesia

Nitrogen (N) is one of the most required nutrients in cocoa production. Unlike in many other crops, there is limited knowledge on genotypic variation in nitrogen use efficiency (NUE) in clonal cocoa. We therefore evaluated the effect of nitrogen application on cocoa NUE in 10 clonal cocoa scions that were top-grafted on 4-month old rootstock seedlings of cocoa clone MCC 02. Grafted seedlings received either 0 or 2.3 g/plant of N at the beginning of the experiment, which lasted for five months and was arranged in a nursery of the Indonesian Coffee and Cocoa Research Institute (ICCRI) in Jember, Indonesia. A factorial randomized block design was used with four replications. We found that clones significantly affected rootstock stem diameter and dry biomass both in roots and shoots. Nitrogen application decreased rootstock stem diameter and root biomass but increased shoot-root ratio. Following the N application, we observed significantly higher N concentration and uptake both in roots and shoots. This higher N concentration and uptake contributed to significantly lower NUE in both roots and shoots. We did not observe clonal differences in N concentration, uptake, NUE in either roots or shoots and ANR (Apparent Nitrogen Recovery). Nitrogen application influenced P, K, Mg, and Ca uptake in both roots and shoots. There was a clonal effect on P, K and Mg uptake and on P and Mg use efficiency in both roots and shoots. This implies that the studied clones vary in their P and Mg nutrient use efficiency but not in NUE.

Temporal and spatial variations in nitrogen use efficiency of crop production in China

The low value of nitrogen use efficiency (NUE) (around 30%) of crop production in China highlights the necessity to adopt reasonable N managements in national scale. After the implementation of ‘National Soil Testing and Formulated Fertilization’ program in 2005, many field experiments have reported an increase of NUE for crop productions in China. This has prompted discussion regarding the extent to which NUE in crop production has been improved. Here, we analyzed the temporal and spatial changes in NUE (crop N uptake/total N input) and cumulative synthetic and non-synthetic N fertilizer recovery efficiency of crop production in China during 1980–2014, and evaluated the relationship between NUE and economic growth (purchasing power parity, PPP) at national and provincial scale. The results showed that the overall NUE of crop production in China clearly increased from 35 to 42% during 2003–2014, and an increase in NUE was further evidenced by increases in cumulative recovery efficiency of both synthetic and non-synthetic N fertilizer. The relationship between NUE and PPP can be described by an environmental Kuznets curve at the national scale, with NUE first decreasing then increasing with PPP. However, this relationship exhibited large spatial variation: 1) In economically developed (e.g., Guangdong and Zhejiang) and undeveloped provinces (e.g., Yunnan and Guizhou), NUE generally decreased and then remained at low levels (20–35%) as PPP increased. 2) In major agricultural provinces with high (e.g., Shandong and Jiangsu) or intermediate levels (e.g., Hunan and Hebei) of economic development, a pronounced increasing trend in NUE with PPP was observed. These results highlight the necessity of developing region-oriented N management strategies to further increase the NUE of crop production in China, particularly in the economically developed and undeveloped provinces.

Nitrogen Uptake Efficiency, Mediated by Fine Root Growth, Early Determines Temporal and Genotypic Variations in Nitrogen Use Efficiency of Winter Oilseed Rape.

Maintaining seed yield under low N inputs is a major issue for breeding, which requires thoroughly exploiting the genetic diversity of processes related to Nitrogen Use Efficiency (NUE). However, dynamic analysis of processes underlying genotypic variations in NUE in response to N availability from sowing to harvest are scarce, particularly at the whole-plant scale. This study aimed to dynamically decipher the contributions of Nitrogen Uptake Efficiency (NUpE) and Nitrogen Utilization Efficiency (NUtE) to NUE and to identify traits underlying NUpE genetic variability throughout the growth cycle of rapeseed. Three experiments were conducted under field-like conditions to evaluate seven genotypes under two N conditions. We developed NUE_DM (ratio of total plant biomass to the amount of N available) as a new proxy of NUE at harvest, valid to discriminate genotypes from the end of inflorescence emergence, and N conditions as early as the beginning of stem elongation. During autumn growth, NUpE explained up to 100% of variations in NUE_DM, validating the major role of NUpE in NUE shaping. During this period, under low N conditions, up to 53% of the plant nitrogen was absorbed and NUpE genetic variability resulted not from differences in Specific N Uptake but in fine-root growth. NUtE mainly contributed to NUE_DM genotypic variation during the reproductive phase under high-N conditions, but NUpE contribution still accounted for 50–75% after flowering. Our study highlights for the first time NUpE and fine-root growth as important processes to optimize NUE, which opens new prospects for breeding.

Assessing genotypic variation for nitrogen use efficiency and associated traits in Brazilian maize hybrids grown under low and high nitrogen inputs

Even though nitrogen (N) is the nutrient required in the largest amount for maize production and is one of the major limiting factors of maize production, we were not aware of any information about the N-use efficiency of maize hybrids currently cropped in Brazil. For this, we assessed the genotypic variation of a set of 114 maize hybrids currently commercialized and planted in Brazil for N-use traits and investigated the relationships among these traits. The hybrids were evaluated over two growing seasons (2015/2016 and 2016/2017) and under two contrasting N inputs (high N with 200 kg ha−1 of N, and low N with 20 kg ha−1 of N as urea). The mean genotypic values of N uptake amount traits were reduced by around 20%, grain N concentration (GNC) by 9.51%, and GY by 12.30%, whereas N-utilization efficiency increased by 12.28% due to N stress. We found a large genotypic variation among maize hybrids for GY and most N-use traits, but due to the influence of N stress, we found a lower or non-genotypic variation for most tested traits under N stress. The Brazilian hybrids showed similar performances for N remobilization and N content in grain under low N input (LN), and for N accumulation during the post-silking stage under both N inputs. The $${\hat{\text{h}}}_{{{\overline{\text{X}}}}}^{2}$$ values were much higher under high N input (HN) than under LN for almost all traits, except for stover N concentration. While there was no evidence that GNC was a clear indicator for an improved GY, mainly under LN, breeding for maize hybrids with decreased GNC will lead to greater N-utilization by them. Under LN, the N accumulation until flowering is very important in determining GY highest-yielding hybrids, whereas under HN, the development of the highest-yielding hybrids seems to be dependent on N allocation from stover to the grain and N-utilization. We also concluded that N stress environments must be included in the screening of maize hybrids for a general adaptation to variation in N level. Finally, we have identified the maize hybrids BM709PRO2 (20), P3456H (64), and SHS7939PRO2 (95) as being well adapted under both N conditions; therefore, they have the potential to be grown in a wide range of soil N.

Physiological parameters and nitrogen use contribution to yield in rice (Oryza sativa L.)

A field experiment was conducted to study the genetic variation for nitrogen use efficiency and yield related traits in nine crosses of rice genotypes. Based on the nitrogen use efficiency and yield performance, six rice genotypes viz., MTU-1001, WGL-2395, MTU-1010, Pothana, Bhadrakali and JGL-1798 were selected and nine crosses were made. From each cross six generations i.e., P1, P2, F1, F2, B1 and B2 were generated and analyzed in an experimental trial conducted in rabi, 2012-13. Hybrids showed superior performance. Among the crosses, MTU-1001 X JGL-1798 followed by MTU-1010 X JGL-1798 recorded maximum SCMR values, photosynthetic rate, nitrogen use efficiency and grain yield. The cultivars with high uptake efficiency had higher nitrogen contents than cultivars with low uptake efficiency from nitrogen application. Therefore, selection of hybrids with high uptake efficiency could reduce the losses of nitrogen and facilitates increased nitrogen uptake and yield.

Characterizing Variation in Nitrogen Use Efficiency in Wheat Genotypes Using Proximal Canopy Sensing for Sustainable Wheat Production

Global nitrogen use efficiency (NUE) for cereal production is marginal and is estimated to be about 33%. Remote sensing tools have tremendous potential for improving NUE in crops through efficient nitrogen management as well as the identification of high-NUE genotypes. The objectives of this study were (i) to identify and quantify the variation in NUE across 24 winter wheat genotypes (Triticum aestivum L.) and (ii) to determine if the normalized difference vegetation index (NDVI) could characterize the variability in NUE across wheat genotypes. This study was conducted in 2010 and 2011 in the semi-arid climate of Northeastern Colorado across dryland and irrigated conditions. Our results indicate significant variation in the NUE among genotypes across two irrigation conditions. We observed a strong relationship between the NDVI and NUE—as PFP (partial factor productivity) and PNB (partial nitrogen balance)—across the 24 wheat genotypes under dryland conditions (average R2 for PFP and PNB = 0.84) at Feekes growth stage 11.1, for site year II. However, poor association was observed under irrigated conditions (average R2 for PFP and PNB = 0.29) at Feekes growth stage 3 to 4 for site year II. This study demonstrates the potential and limitations of active canopy sensing to successfully characterize the variability in NUE across wheat genotypes.

Small-Scale Variation in Nitrogen Use Efficiency Parameters in Winter Wheat as Affected by N Fertilization and Tillage Intensity

Limited information exists on how tillage and nitrogen (N) fertilization affects small-scale variation in nitrogen use efficiency (NUE) and crop performance. In a two-year field study under temperate conditions, we investigated how tillage (NT, no-tillage; CT, conventional tillage) and N fertilization affected the small-scale variation in NUE and winter wheat performance (grain yield, Gw; grain protein concentration, GPC). A randomized complete block design with three replications was used. Within each tillage plot (12 × 35 m2), N rates (0, 50, 100, 150, 200, 250 kg N ha−1) were completely randomized within each of four groups of microplots (1.5 × 1.5 m2). Early-season soil mineral N (Nmin) was also monitored in both years. At rates &lt; 150 kg N ha−1, NT was not competitive with CT in terms of Gw and NUE. Gw and aboveground plant N were not correlated with Nmin prior to application of N fertilizer. NT usually led to larger spatial heterogeneity of Nmin, Gw, and NUE. The small-scale variability of Gw, GPC, NUE, and N supply decreased with increasing N fertilization rates under both tillage systems. Significant increases in Gw and GPC were observed with increasing N rates, whereas NUE decreased slightly with increasing N rates in both NT and CT. The overall moderate spatial variation in Nmin, Gw, and NUE did not justify site-specific N fertilization in these small fields, with the exception of the stony within-plot positions, which were not responsive to rates of N &gt; 50 kg N ha−1.

Are We Getting Better in Using Nitrogen?: Variations in Nitrogen Use Efficiency of Two Cereal Crops Across the United States

AbstractSpatial variation and temporal trajectory of crop nitrogen use efficiency (NUE) have important implications for nitrogen management and environmental conservation. Previous studies have examined cross‐nation divergences in crop NUE but often overlooked its spatial heterogeneity and cross‐crop differences at subnational scales. We examined the relationship between state‐level NUE and nitrogen fertilizer use for two major fertilizer‐consuming crops, corn and winter wheat, which account for over half of national N fertilizer use in the United States. Since 1970, as N fertilizer use rates have changed, the responses of crop yield and NUE have exhibited large temporal and spatial variations. It is evident that NUE of corn begins to decline when N fertilizer application rate exceeds ~150 kg N ha−1 yr−1, and that yield response of winter wheat slows down with annual N fertilizer input above ~50 kg N ha−1 yr−1. State‐level NUE in both crops has risen in recent decades, which could potentially reduce N loss from agricultural production. Across the United States, some major corn‐producing states demonstrate a shift from an increasing trend of NUE during the period 1970 to 1999 to a decreasing trend after 2000, whereas winter wheat‐producing states present an opposite pattern. Furthermore, this study indicates that annual dynamics of N surplus in corn is closely tied with grain yields, while that in winter wheat significantly correlates with N fertilizer input. A larger proportion of N loss would be anticipated if no further increase in corn yield was obtained or fertilizer use kept rising in winter wheat.

Variation in nitrogen use efficiency of winter wheat

Optimization of nitrogen fertilization to specific cultivar requirements is a major objective for improvement of trade-offs between grain yield, environmental sustainability and maximum profitable production. The aim of this study was to assess the effects of nitrogen fertilization on grain yield, and nitrogen use efficiency of modern wheat cultivars in different growing seasons. The trials with eight winter wheat cultivars and seven top-dressing nitrogen treatments were carried out in three successive growing seasons under rain-fed conditions of the southern Pannonian plain. The results from our study showed a significant variation in grain yield and nitrogen use efficiency among winter wheat cultivars when grown under different environmental and soil nitrogen level conditions. On average, grain yield ranged from 4961 to 6375 kg ha-1 among winter wheat cultivars. Increase of N soil level resulted in significant grain yield increase and nitrogen use efficiency decrease compared to the control. In 2015/16, 2016/17, and 2017/18 growing seasons grain yield of winter wheat cultivars reached plateau at 156, 175, and 128 nitrogen soil level, respectively. Significant influence of cultivar by nitrogen fertilization interaction indicated that it is necessary to adjust nitrogen fertilization to each cultivar. Moreover, notable weather variability between different growing seasons is a major limiting factor for optimal nitrogen fertilizer application in winter wheat production under conditions of the Pannonian plain.

Response of nutrients in submergence-prone rainfed lowland rice ecosystem with different flood histories

Site specific nutrient management approach was used to understand response of rice (variety Swarna Sub-1) yields to applied nutrients in soils with different flood histories of Bihar. Trials on nutrient omission plot technique (NOPT) were conducted in four sites for nitrogen, phosphorus, potassium and zinc with four replications at each site. Per cent response of yield as compared to nutrient omission plots ranged from 36.4 to 77.2%, 10.9 to 37.2%, 5.8 to 17.0% and 4.3 to 32.9% with addition of N, P, K and Zn respectively. Differences in response of applied nutrients were found due to difference in submergence condition and soil-nutrient status. The variation in nitrogen use efficiency was from 24.1 to 37.9%, phosphorus use efficiency from 22.1 to 33.6%, potassium use efficiency from 65.5 to 71.8% and zinc use efficiency from 2.72 to 3.93%. It may be concluded that soil nutrient contributions and level of submergence affected nutrient use efficiency and crop yield response to applied nutrients.

Associations of NAM-A1 alleles with the onset of senescence and nitrogen use efficiency under Western Australian conditions

Wheat grain yield and protein content are significantly influenced by the onset of senescence and the duration of the grain filling phase. The onset of senescence also affects Nitrogen use efficiency (NUE) through interacting pathways involving N accumulation and translocation of N into the grains. The objective of this study was to relate variation in NUE and its components with two groups of the NAM-A1 gene alleles; (i) early onset of senescence in cultivars carrying the NAM-A1a allele, (ii) delayed onset of senescence in cultivars carrying the Non-NAM-A1a allele (b, c, d) in wheat cultivars grown under Western Australia conditions. A field trial was carried out over two seasons examining 19 cultivars under different N rates and time of N application. The Normalized Difference Vegetation Index was utilized to determine the onset of senescence after anthesis. The early onset of senescence results in high grain yield, harvest index, and NUE due to improvements in the N utilization ability. Accelerating the onset of senescence results in a short grain filling period leading to grain maturity before the onset of unfavourable summer conditions. The function of alleles of NAM-A1 gene in controlling senescence hence the NUE is highly regulated by environmental conditions. This study concluded that the function of NAM-A1a allele induces the onset of senescence with a positive effect on the NUE and its components under Western Australian conditions.

Identifying oilseed rape varieties with high yield and low nitrogen fertiliser requirement

Reducing the fertiliser nitrogen (N) requirement of oilseed rape (Brassica napus L.) whilst maintaining yield will be beneficial for both growers and the environment. The aim of this paper was to identify differences in fertiliser N requirement between varieties and investigate simple in-field methods to rapidly identify varieties that can maintain high yield whilst requiring lower optimal N rates (‘HYLO’ varieties) to aid breeding for lower N fertiliser requirement. Four field experiments were conducted in the UK during seasons 2011-12 or 2012-13 on a range of soil types. Each experiment included 21 elite open pollinated and hybrid varieties. Each variety was grown at seven levels of N fertiliser ranging from zero to super-optimal (340 or 400 kg N/ha). Measurements of seed yield and seed oil content were combined to calculate gross output yield, i.e. seed yield adjusted for oil content. Linear plus exponential yield response curves were fitted to calculate the economically optimum N rate (Nopt) of each variety and the associated nitrogen use efficiency (NUE) was calculated at the Nopt. Across varieties, gross output yield responses to optimum N fertiliser, compared with yields at nil N rates, were significant, ranging from 1.3 to 2.7 t/ha between sites. Nopt ranged from 117 to 286 kg N/ha between sites giving experimental average yields at Nopt (Yopt) of 3.0 to 4.6 t/ha. Varietal yield ranges at Nopt within each experiment ranged from 0.6 to 1.7 t/ha, and there was a positive trend for yield to increase with date of variety introduction by 0.24 t/ha per decade. Significant variety x N fertiliser rate interactions were measured for gross output yield in two out of four experiments and significant varietal differences in Nopt were measured in one experiment ranging from 195 kg N/ha to 296 kg N/ha. There was no correlation between Nopt and Yopt indicating that improving varietal yield potential will not necessarily increase N requirement. Significant varietal differences in NUE were measured at Nopt ranging from 13.5 to 15.9 kg of gross output yield per kg of N supply from soil residues and fertiliser. Seed with low N and high oil content, and high biomass harvest index could be useful indicators of high NUE. However, no single trait correlated consistently with varietal variation in NUE. The yield difference between sub- and super-optimal N rates was positively correlated with Nopt. Further research is required to develop new experimental approaches for measuring yield at more than one N level on small experimental plot areas and to investigate a wider range of genotypes.

Investigating Genetic Progress and Variation for Nitrogen Use Efficiency in Spring Wheat

Improved N use efficiency (NUE) increases wheat (Triticum aestivum L.) yields and reduces N losses in the environment. We investigated genetic variation and correlations among agronomic and NUE traits in Canada Western Red Spring wheat cultivars to further improve NUE. Trials were conducted for 3 yr at two locations in Alberta, Canada, under two levels of N (200 and ~50 kg ha−1). Genotype × environment interaction was significant for traits associated with vegetative growth, and genotype × N fertilizer treatment interaction was significant for important yield and NUE traits. There were significant positive correlations between total dry matter and N uptake efficiency (NUpE) in the high (r = 0.74, P &lt; 0.05) and low (r = 0.83, P &lt; 0.05) N treatments. The effect of dwarfing Rht‐1b allele was more prominent under high N treatment in increasing NUE. However, cultivars with Rht‐1b allele showed inconsistent results for NUpE, indicating that Rht alleles might have pleiotropic effects on N uptake. Grain yield, NUE, and N utilization efficiency (NUtE) exhibited genetic improvement over time only under high N treatment. Our results indicated that grain yield increased mainly due to improved harvest index (HI), suggesting improvement in C assimilation rather than N partitioning efficiency. Nitrogen use efficiency may further be improved by intercrossing cultivars with high HI, N harvest index, and NUtE and those with good NUpE, while using total dry matter production as a selection criterion.

Nitrogen use efficiency in spring wheat: genotypic variation and grain yield response under sandy soil conditions

SUMMARYAgricultural practices are likely to lower nitrogen (N) fertilization inputs for economic and ecological limitation reasons. The objective of the current study was to assess genotypic variation in nitrogen use efficiency (NUE) and related parameters of spring wheat (Triticum aestivumL.) as well as the relative grain yield performance under sandy soil conditions. A sub-set of 16 spring wheat genotypes was studied over 2 years at five N levels (0, 70, 140, 210 and 280 kg N/ha). Results indicated significant differences among genotypes and N levels for grain yield and yield components as well as NUE. Genotypes with high NUE exhibited higher plant biomass, grain and straw N concentration and grain yield than those with medium and low NUE. Utilization efficiency (grain-NUtE) was more important than uptake efficiency (total NUpE) in association with grain yield. Nitrogen supply was found to have a substantial effect on genotype; Line 6052 as well as Shandawel 1, Gemmiza 10, Gemmiza 12, Line 6078 and Line 6083 showed higher net assimilation rate, more productive tillers, increased number of spikes per unit area and grains per spike, extensive N concentration in grain and straw, heavier grains, higher biological yield and consequently maximized grain yield. The relative importance of NUE-associated parameters such as nitrogen agronomic efficiency, nitrogen physiological efficiency and apparent nitrogen recovery as potential targets in breeding programmes for increased NUE genotypes is also mentioned.

Genetic variability of nitrogen use efficiency in rainfed upland rice

In sub-Saharan Africa and in Madagascar, upland rice is mostly grown for subsistence by resource-poor farmers in low-input production systems with low yields. In this context, soil nitrogen availability is a major limiting factor. To determine the appropriate breeding strategy to improve nitrogen use efficiency (NUE), the genetic variability and the level of G×N interaction of NUE need to be evaluated. NUE and its components are complex quantitative traits and the study of their relationship with other simpler traits should help understand the mechanisms involved in NUE and identify ways to improve it. A study was conducted at mid-altitude in Madagascar using 13 adapted tropical japonica rice varieties over three cropping seasons with two contrasted levels of applied N (high N with 90–120kgNha−1 as urea and low N without mineral fertilizer) in rainfed upland conditions. Year×N and Year×G interactions were significant due to contrasted rainfall distributions across the three cropping seasons. Agronomic efficiencies of N fertilization were low (11.5kg grain kg−1N in the best year) because of N loss through leaching or volatilization. Our results also suggest gaseous loss of N by plants between flowering and harvest, particularly under the high N treatment. The experiment revealed significant genetic variability for NUE, nitrogen uptake efficiency (NUPE) and nitrogen utilization efficiency (NUTE) in both high N and low N treatments but a low level of G×N interaction. The variation in NUPE accounted for more of the variation in NUE than the variation in NUTE. There was no correlation between NUPE and NUTE, either under high N or under low N. We found no negative relationship between grain yield and grain N concentration under low N. The relationship between NUE and agronomic and N-related traits differed from one year to the next, illustrating the plasticity of the contribution of the different agronomic traits to NUE as a function of the contrasted climatic conditions (particularly the rainfall distribution pattern). However, two traits, the number of panicles per m2 and the harvest index were consistently positively correlated with NUE. The difference between total N uptake at flowering and at harvest was positively correlated with NUE, particularly under the low N treatment.