Higher N-use Efficiency Research Articles

Interaction between plant density and nitrogen management strategy in improving maize grain yield and nitrogen use efficiency on the North China Plain

SUMMARYUnderstanding the physiological mechanisms of biomass accumulation and partitioning in the grain, and the nitrogen (N) uptake associated with different plant densities and N management strategies, is essential for achieving both high yield and N use efficiency (NUE) in maize plants. A field experiment was conducted in 2013 and 2014, using five rates of N application and three plant densities (6·0, 7·5 and 9·0 plants/m2) in Quzhou County on the North China Plain (NCP). The objective was to evaluate whether higher plant density can produce more biomass allocated to the grain to achieve higher grain yield and to determine the optimal N management strategies for different plant densities. The highest grain yield and NUE were achieved in the 7·5 plants/m2 treatment; both the sub-optimal (6·0 plants/m2) and supra-optimal (9·0 plants/m2) plant densities resulted in diminished yield and NUE. Compared to 6·0 plants/m2, the 7·5 plants/m2 treatment displayed higher biomass accumulation during the grain-filling period and also exhibited more biomass allocated to kernels with similar total biomass accumulation compared with the 9·0 plants/m2 treatment, which contributed to its higher grain yield. The N uptake in the 7·5 plants/m2 treatment was similar to that in the 9·0 plants/m2 treatment up to pre-silking. However, the post-silking N uptake of the 7·5 plants/m2 treatment was 66·4 kg/ha, which was 29·1% higher than that of the 9·0 plants/m2 treatment. Furthermore, the highest maize grain yield was achieved in the 0·7 × optimal N rate (ONR × 0·7), ONR and ONR × 1·3 treatments for 6·0, 7·5 and 9·0 plants/m2, respectively, which suggests that different N management strategies are needed for different plant densities. In conclusion, selecting a planting density of 7·5 plants/m2 with an in-season root zone N management is a potentially effective strategy for achieving high grain yield and high NUE for maize production on the NCP.

Integrated manure management to reduce environmental impact: II. Environmental impact assessment of strategies

Manure management contributes to adverse environmental impacts through losses of nitrogen (N), phosphorus, and carbon (C). In this study, we aimed to assess the potential of newly designed strategies for integrated manure management (IS) to reduce environmental impact. An important aspect of the strategies was that they prevented pollution swapping. Life cycle assessment was used to compute climate change (CC), fossil fuel depletion (FFD), terrestrial acidification (TA), marine eutrophication (ME), particulate matter formation (PMF), N use efficiency (NUE), and phosphorus over application rate (POA), relative to the crop demand for N. We applied the IS to North West European practice (Ref) and included the Dutch current situation of progressive manure management (NL) to illustrate the potential of the IS to reduce environmental impact. Manure management in Ref included production and management of liquid and solid dairy cattle manure applied to maize and grass, and liquid pig manure applied to wheat. A Monte Carlo uncertainty simulation was done to assess the effect of variation in N and C losses and N uptake by crops on the comparison with Ref, IS, and NL. Results showed that the IS reduced all environmental impacts in all manure product and crop combinations and more than doubled the NUE (70% compared with maximum 33% in Ref). Main causes were: segregation of pig and dairy cattle urine and feces inside the housing system reduced methane (CH4) and ammonia (NH3) emissions; addition of zeolite to solid dairy cattle manure reduced NH3 emission; sealed storages in all IS reduced volatilization of N and C; bio-energy production from the feces reduced the production of fossil electricity and heat; and finally N emissions in the field were reduced by ammonia emission reducing application techniques and improved application management (tillage, field traffic en synchronization of manure product application with crop demand). Compared with the Ref, NL had lower TA, PMF, POA, and higher NUE, except for solid cattle manure applied to grass. This result indicates that the Dutch regulations to reduce NH3 emissions were successful, but that CC can be improved. Compared with NW EU practice, IS reduced environmental impact up to 185% for CC, up to >700% for FFD, up to 96% for TA, up to 99% for ME, up to 100% for PMF, up to 110% for POA and more than doubled the NUE. We concluded that the designed IS avoid pollution swapping in the entire manure management system.

A large and deep root system underlies high nitrogen-use efficiency in maize production.

Excessive N fertilization results in low N-use efficiency (NUE) without any yield benefits and can have profound, long-term environmental consequences including soil acidification, N leaching and increased production of greenhouse gases. Improving NUE in crop production has been a longstanding, worldwide challenge. A crucial strategy to improve NUE is to enhance N uptake by roots. Taking maize as a model crop, we have compared root dry weight (RDW), root/shoot biomass ratio (R/S), and NUE of maize grown in the field in China and in western countries using data from 106 studies published since 1959. Detailed analysis revealed that the differences in the RDW and R/S of maize at silking in China and the western countries were not derived from variations in climate, geography, and stress factors. Instead, NUE was positively correlated with R/S and RDW; R/S and NUE of maize varieties grown in western countries were significantly greater than those grown in China. We then testified this conclusion by conducting field trials with representative maize hybrids in China (ZD958 and XY335) and the US (P32D79). We found that US P32D79 had a better root architecture for increased N uptake and removed more mineral N than Chinese cultivars from the 0-60 cm soil profile. Reported data and our field results demonstrate that a large and deep root, with an appropriate architecture and higher stress tolerance (higher plant density, drought and N deficiency), underlies high NUE in maize production. We recommend breeding for these traits to reduce the N-fertilizer use and thus N-leaching in maize production and paying more attention to increase tolerance to stresses in China.

Plant invasive success associated with higher N-use efficiency and stoichiometric shifts in the soil–plant system in the Minjiang River tidal estuarine wetlands of China

The tidal estuarine wetlands of China are rich in plant diversity, but several human-driven processes, such as species invasion, can affect the biogeochemical cycles of these ecosystems, and by changing soil conditions can inhibit the regeneration of native vegetation. We seasonally analyzed the carbon (C), nitrogen (N) and phosphorus (P) concentrations in soils and in leaves, stems and roots of the invasive species Spartina alterniflora and of the native species Cyperus malaccensis var. brevifolius Boeckeler. This latter species was analyzed both in natural non-invaded stands and in stands that had been invaded by Spartina but from which it had been removed and replaced by Cyperus. The aim was to investigate the effect of plant invasion, subsequent removal and replanting with a native species on C, N and P stoichiometry of the plant–soil system in the tidal wetlands of the Minjiang River. C and N concentrations averaged across seasons did not differ significantly among the plant species. P concentration was lower in the stems of Spartina than in the stems of the native species Cyperus but was not significantly different in the roots of the two species. The soil C and N concentrations were higher in the Spartina stand than in the Cyperus stand, whereas the soil P concentrations were not significantly different. The invasive species had a higher N-resorption capacity, N:P ratios in stem and roots, biomass, absolute growth and biomass N and had a lower relative growth rate and litter production than the native species. After the removal of the invasive plants, the regenerating native plants have a higher capacity to resorb N and lower relative growth rates. All these traits show that a conservative strategy and a high N-use efficiency and internal plant control of the N in the ecosystem underlie the invasive success of Spartina in this N-limited wetland. Relative growth rate was associated with lower plant N:P ratios, whereas absolute growth rate was associated with higher nutrient-use efficiency and lower C and N turnover and storage capacities in the biomass. Changes in soil properties produced by the establishment of an invasive plant can condition the later regeneration of native plants.

Contribution of legumes to the availability of soil nitrogen and its uptake by wheat in Mediterranean environments of central Chile

The absorption and partitioning of N is determined by its availability and demand during the various phenological stages of the crop. In two experiments conducted in rainfed areas of the Mediterranean climate region of central Chile, legume-wheat (Triticum aestivum L.) rotations were compared to oat (Avena sativa L.)-wheat systems (with and without applications of N fertilizer). The objective was to assess the contribution of legumes to the availability of soil N and N uptake by the wheat crop following grain legumes. N mineralization and uptake, N use efficiency (NUE), and N uptake efficiency (NUpE) by wheat were evaluated at different phenological stages in 2010. Synchronization existed between accumulated soil N mineralization and N uptake by the wheat. N uptake by wheat was higher in Experiment II (Andean foothill) than in Experiment I (interior dryland). Grain yields after legumes ranged 60-110% and 72-103% of yields in the fertilized cereal rotation (oat-wheat) in Experiments I and II, respectively. There was high correlation between soil N availability vs. grain yield (R2 = 0.84 and 0.64) and N uptake vs. grain yield (R2 = 0.55 and 0.64) for Experiments I and II, respectively. Rotations with lupine (Lupinus spp.) and field peas (Pisum sativum L.) showed high NUE and NUpE in Experiment I, and Experiment II in pea. The lowest NUE was after oats with N fertilizers. It is concluded that wheat cultivated in both agro-ecological areas after grain legumes or green manure was more efficient in N uptake and use than cereal, especially when the latter was fertilized.

Site-specific fertilizer nitrogen management in irrigated transplanted rice (Oryza sativa) using an optical sensor

Blanket fertilizer nitrogen (N) recommendations for large irrigated transplanted rice tracts lead to low N use-efficiency (NUE) due to field-to-field variability in soil N supply and seasonal variability in yield. To achieve high NUE, a fertilizer N management strategy based on visible and near-infrared spectral response from plant canopies using a GreenSeeker™ optical sensor was evaluated. Seven field experiments were conducted during 2005–2007 at two locations in the Indo-Gangetic plains of South Asia to define relationships between in-season sensor measurements at panicle initiation (PI) stage and up to 2 weeks later, and yield of rice. During 2006–2010, seven field experiments were conducted to assess the sensor-based N management strategy and to work out the prescriptive N management to be followed prior to applying sensor-guided fertilizer dose. During 2010 and 2011, the sensor- based N management strategy was evaluated versus farmers’ fertilizer practice at 19 on-farm locations. Relationships with R2 values 0.51 (n = 131), 0.45 (n = 74) and 0.49 (n = 131), respectively, were observed between in-season sensor-based estimates of yield at 42 (PI stage), 49 and 56 days after transplanting of rice and actual grain yield of rice. Applications of 30 kg N ha−1 at transplanting and 45 kg N ha−1 at active tillering stage were found to be the appropriate prescriptive strategy before applying the GreenSeeker-guided dose at PI stage. Sensor-guided N management resulted in similar grain yields as the blanket rate farmer practice, but with reduced N rates, i.e. greater recovery efficiency (by 5.5–21.7 %) and agronomic efficiency [by 4.7–11.7 kg grain (kg N applied)−1]. This study revealed that high yields coupled with high NUE in transplanted rice can be achieved by replacing blanket fertilizer recommendation by an optical sensor-based N management strategy consisting of applying a moderate amount of fertilizer N at transplanting and enough fertilizer N to meet the high N demand during the period between active tillering and PI before applying a sensor-guided fertilizer N dose at PI stage of rice.

Dairy cow responses to graded levels of rapeseed and soya bean expeller supplementation on a red clover/grass silage-based diet

The effects of rapeseed and soya bean expeller (SBE) supplementation on digestion and milk production responses in dairy cows were investigated in an incomplete Latin square design using five cows and four 3-week periods. The experimental diets consisted of five concentrate treatments fed at a rate of 9 kg/day: a mixture of barley and oats, which was replaced with rapeseed or SBE at two levels (CP concentration (g/kg dry matter (DM)) of 130 for the control concentrate and 180 and 230 for the two protein supplemented levels). A mixture of grass and red clover silage (1:1) was fed ad libitum and it had a CP concentration of 157 g/kg DM. Supply of nutrients to the lower tract was measured using the omasal canal sampling technique, and total digestion from total faecal collection. Protein supplementation increased omasal canal amino acid (AA) flows and plasma concentrations of AA, and was also reflected as increased milk production. However, N use efficiency (NUE) decreased with increased protein supplementation. Rapeseed expeller (RSE) tended to increase silage DM intake and elicited higher milk production responses compared with SBE and also resulted in a higher NUE. The differences between the protein supplements in nitrogen metabolism were relatively small, for example, there were no differences in the efficiency of microbial protein synthesis or omasal canal flows of nitrogenous components between them, but plasma methionine concentration was lower for soya bean-fed cows at the high CP level in particular. The lower milk protein production responses to SBE than to RSE supplementation were at least partly caused by increased silage DM and by the lower methionine supply, which may further have been amplified by the use of red clover in the basal diet. Although feed intake, diet digestion, AA supply and milk production were all consistently improved by protein supplementation, there was a simultaneous decrease in NUE. In the current study, the milk protein production increased only 9% and energy-corrected milk production by 7% when high level of protein supplementation (on average 2.9 kg DM/day) was compared with the control diet without protein supplementation showing that dairy production could be sustained at a high level even without external protein supplements, at least in the short term. The economic and environmental aspects need to be carefully evaluated when decisions about protein supplementation for dairy cows are taken.

Site-Specific Nitrogen Management in Dry Direct-Seeded Rice Using Chlorophyll Meter and Leaf Colour Chart

The need to maintain high rice yields and improve fertilizer nitrogen (N)-use efficiency has fueled the use of tools such as leaf colour chart (LCC) and chlorophyll meter (SPAD meter) in managing fertilizer N based on colour of the leaf. Field experiments were conducted during 2011 to 2013 at Ludhiana, India to assess the need for basal N application and to establish critical threshold values of leaf greenness as measured by LCC and SPAD meter for formulating strategies for in-season management of fertilizer N in dry direct-seeded rice (DDSR). Avoiding application of N at sowing did not adversely affect rice grain yield, indicating that basal N application in DDSR was not necessary and might lead to reduced N-use efficiency. Monitoring N uptake rate during the growing season of DDSR suggested that N uptake rate peaked at the two growth stages: maximum tillering (42 to 56 days after sowing (DAS)) and panicle initiation stages (70 to 84 DAS). Using the Cate-Nelson procedure, critical LCC and SPAD meter values for fertilizer N application worked out to be 4 and 37, respectively. Real-time fertilizer N management strategy based on applying 30 kg N ha−1 whenever SPAD meter or LCC readings fell below the critical values maintained optimum rice yields along with higher N-use efficiency than that observed by following blanket recommendation for fertilizer N in the region. The fixed-time variable-dose strategy consisted of applying prescriptive doses of 20 kg N ha−1 at 14 DAS and 30 kg N ha−1 at 28 DAS and corrective doses of 30, 40 or 50 kg N ha−1 at 49 and 70 DAS depending upon LCC shade to be ≥ 4, 4–3.5, or < 3.5 and SPAD meter readings to be ≥ 40, 40–35, or < 35, respectively. This strategy also resulted in optimal rice yield along with higher N-use efficiency as compared to the blanket recommendation. This study revealed that in DDSR, fertilizer N could be managed more efficiently using the tools of LCC and SPAD meter than the current blanket recommendation.

The effects of cultivar and nitrogen management on wheat yield and nitrogen use efficiency in the North China Plain

Understanding the inherent characteristics of cultivars with different spike types based on biomass and biomass partitioning with optimal nitrogen (N) management is important to achieve both high wheat yield and N use efficiency (NUE). A field experiment was conducted over 3 years using five N rates and two winter wheat cultivars (a larger spike cultivar (TN) and a multiple spike cultivar (LX)) in the North China Plain (NCP) to evaluate whether larger spike cultivar can accumulate more post-anthesis biomass and achieve higher yields and to determine the optimal N management strategies for the two cultivars. In both cultivars over 3 years, the optimal N rate (ONR) based on in-season root zone N management (INM) reduced the N rate by 140kgNha−1 from 300 to 160kgNha−1 without any yield losses, while it increased the N uptake efficiency from 32% to 64% compared with typical farmer's N practice (FNP). Furthermore, under ONR treatment, the grain yield and N physiological efficiency of TN significantly was higher 6 and 10% over 3 years than that of the multiple spike cultivar LX, respectively. The yield increase of TN was attributed mainly to the increased harvest index (7%) and similar total biomass. The higher 11% leaf N concentration at anthesis and 10% grain number m−2 of TN significantly increased the post-anthesis biomass by 24% under ONR, which played an important role in increasing the harvest index (HI). In addition, the grain yield of 70% ONR significantly decreased by 12% and 8% for TN and LX over 3 years, respectively, while no significant increase in grain yield was observed for 130% ONR, suggesting that the ONR was optimal in practice. Changes in N management may not be required for the two cultivars, because the ONR according to INM is close to the economically optimal N rate (EONR) based on grain yield response curves. In conclusion, selecting a larger spike cultivar with optimal N management could be a useful strategy to achieve high yield and high NUE for wheat production in the NCP, especially using optimal crop management practices.

V-ATPase and V-PPase at the Tonoplast Affect NO3 − Content in Brassica napus by Controlling Distribution of NO3 − Between the Cytoplasm and Vacuole

Nitrate, once taken up by plants, can either be stored in vacuoles or reduced by nitrate reductase in the cytoplasm. High accumulation of NO3− in the vacuole occurs when assimilation into the cytoplasm is saturated. This study elucidates how proton pumps at the tonoplast (V-ATPase and V-PPase) affect the NO3− content of Brassica napus by controlling the distribution of NO3− between the cytoplasm and vacuole. Pot experiments were conducted in a greenhouse under normal N (15.0 mM nitrate) conditions using B. napus genotypes that demonstrated either high (Xiangyou15) or low (814) nitrogen use efficiency (NUE). The NO3− content of the high NUE genotype was significantly lower than that of the low NUE genotype, whereas the total N per plant of the two genotypes was almost the same, suggesting that the different NUE between the two genotypes is not due to the difference of NO3− uptake. The relative expression levels of V-ATPase (vha-a3) and V-PPase (avp1) genes in the high NUE genotype were significantly lower than in the low NUE genotype, resulting in lower V-ATPase and V-PPase activities in the high NUE genotype. The transport of NO3− and protons from the cytoplasm to the vacuole is powered by V-ATPase and V-PPase, so their lower activities increase H+ efflux from and reduce NO3− influx into the vacuoles of the high NUE genotype. We conclude that the lower activity of proton pumps at the tonoplast is the main reason the high NUE genotype possesses lower NO3− content and higher N-use efficiency.

Accumulation and distribution of nitrogen in triticale varieties with different nitrogen utilization efficiencies under different nitrogen application levels

采用土培盆栽试验, 以小黑麦氮高效利用品种'Clxt82'、'PI429186'和氮低效利用品种'Clxt74'为材料, 研究0 (不施氮)、0.033 g(N)·kg^-1(低氮)和0.066 g(N)·kg^-1(正常氮)3个不同施氮水平下, 各生长时期氮素在器官间和器官内不同功能性氮素分配的特性。结果表明: 氮高效利用品种在氮素不足的条件下优势更明显, 抽穗期高效利用品种和低效利用品种间生物量的差异随施氮量的增加而减小, 在不施氮、低氮和正常供氮时'Clxt82'、'PI429186'地上部生物量分别为'Clxt74'的1.55倍、1.19倍、1.06倍和1.79倍、1.35倍、1.30倍。不同生育时期, 小黑麦氮积累量均随施氮量的增加而显著增加, 低氮和正常供氮处理, 在分蘖期、拔节期氮高效利用品种氮积累量均显著高于低效利用品种, 而在抽穗期差异则不大。随施氮量的增加, 氮素在叶片和穗部的分配比例减小, 在茎的分配比例增大; 分蘖期和拔节期, 氮高效利用品种茎中氮素分配比例小于低效利用品种, 叶片氮素分配比例则大于低效利用品种。抽穗期氮高效利用品种穗部氮素分配比例大于低效品种, 而叶部则相反。各生育时期各器官不同形态氮素含量总体上随施氮量的增加而增加。不施氮和低氮处理, 拔节期氮高效品种'Clxt82'、'PI429186'叶片营养性氮含量是低效品种'Clxt74'的1.31倍、1.76倍和1.12倍、1.35倍, 而结构性氮含量则是低效品种的86.12%、64.01%和80.82%、71.51%; 抽穗期氮高效品种'Clxt82'、'PI429186'叶片营养性氮含量是低效品种'Clxt74'的1.01倍、1.11倍和1.04倍、1.13倍, 结构性氮含量为低效品种'Clxt74'的74.99%、63.08%和75.78%、62.84%; 各时期品种间功能性氮素含量差异不大。低氮条件下氮高效利用品种通过降低结构性氮素含量、增加营养性氮素含量来满足氮素的利用和体内循环。

Physiological and Growth Responses of Sugarcane Genotypes to Nitrogen Rate on a Sand Soil

AbstractYields of sugarcane (a complex hybrid of Saccharum spp.) in FL, USA, are lower on sand soils than on organic (muck) soils. Nitrogen (N) supply may limit sugarcane growth and yields on these sand soils. A 2‐year pot study was conducted to determine sugarcane genotypic variation in response to N rate on a sand soil. Treatments included four N rates (0, 75, 150 and 225 kg ha−1) and three sugarcane genotypes (CP 80‐1743, CP 01‐2390 and TCP 87‐3388). Nitrogen fertilizer was equally split and applied at about 55 and 125 days after planting (DAP) for each treatment. During the experiment, the number of nodes and length of the primary stalks and tillers were recorded. Leaf relative chlorophyll (soil plant analysis development (SPAD)) and net photosynthetic rate (Pn) were measured biweekly. All plants were harvested at 183 DAP to measure green leaf area (GLA), shoot biomass accumulation and partitioning, and fertilizer N use efficiency (NUE). Genotypes differed significantly in leaf SPAD, Pn, GLA, and shoot biomass accumulation and partitioning. CP 01‐2390 had the highest leaf Pn and shoot biomass, and CP 80‐1743 had the lowest GLA, shoot biomass and NUE among genotypes. Nitrogen rate affected leaf SPAD, GLA, shoot biomass and NUE, but had much less effect on leaf Pn. Green leaf area and biomass increased with increasing N rates. Our results suggest that a two‐pronged approach, selection of genotypes with high NUE while working to optimize N rates and delivery can improve sugarcane yields on sand soils.

Leaf senescence and nitrogen remobilization efficiency in oilseed rape (Brassica napus L.).

Despite its worldwide economic importance for food (oil, meal) and non-food (green energy and chemistry) uses, oilseed rape has a low nitrogen (N) use efficiency (NUE), mainly due to the low N remobilization efficiency (NRE) observed during the vegetative phase when sequential leaf senescence occurs. Assuming that improvement of NRE is the main lever for NUE optimization, unravelling the cellular mechanisms responsible for the recycling of proteins (the main N source in leaf) during sequential senescence is a prerequisite for identifying the physiological and molecular determinants that are associated with high NRE. The development of a relevant molecular indicator (SAG12/Cab) of leaf senescence progression in combination with a (15)N-labelling method were used to decipher the N remobilization associated with sequential senescence and to determine modulation of this process by abiotic factors especially N deficiency. Interestingly, in young leaves, N starvation delayed senescence and induced BnD22, a water-soluble chlorophyll-binding protein that acts against oxidative alterations of chlorophylls and exhibits a protease inhibitor activity. Through its dual function, BnD22 may help to sustain sink growth of stressed plants and contribute to a better utilization of N recycled from senescent leaves, a physiological trait that could improve NUE. Proteomics approaches have revealed that proteolysis involves chloroplastic FtsH protease in the early stages of senescence, aspartic protease during the course of leaf senescence, and the proteasome β1 subunit, mitochondria processing protease and SAG12 (cysteine protease) during the later senescence phases. Overall, the results constitute interesting pathways for screening genotypes with high NRE and NUE.

Carrot Yield and Quality as Influenced by Nitrogen Application in Cut-and-Peel Carrots

Root bulking, quality, and uniformity in cut-and-peel carrots (Daucus carota) are paramount for optimizing marketable yield and quality. Root bulking is an ecophysiological manifestation in response to inputs such as fertilizers. Understanding this ecophysioloical interaction will help to optimize yield, quality, and amount of inputs used. Three years of field trials were conducted in Kings County, Nova Scotia, to investigate the effects of varying levels of nitrogen (N) fertilizer on yield, recovery, and root and tissue N of two cut-and-peel varieties, Sugarsnax and TopCut. Seven levels of ammonium nitrate (34–0–0; 0, 50, 100, 150, 200, 300, and 400 kg N h−1) were hand broadcast in a split (60% pre-emergence and 40% 8 weeks after emergence) application. No significant interactive effects of N and variety in terms of gross yield or recovery were observed, though Sugarsnax total yields were 12.7% greater than those of TopCut. Overall, optimum yields were achieved at N rates of 150 kg N h−1 and further addition did not significantly improve yield or quality. Increased N significantly increased root and tissue N, but N concentration in both tissues peaked at the 300 kg N h−1 rate. However, neither root nor leaf tissue N had any effect on marketable or total yield. These results show that root bulking is not modulated by altering N applications, and the results also suggest that carrots may have high N-use efficiency or harness N from deeper zones.

Effects of integrated fertilizer application on nitrogen use efficiency of spring maize and soil nitrogen content on black soil in Harbin

With the increase in planting area of spring maize and nitrogen (N) application in Heilongjiang province, the high potential of N pollution to the environment and resource waste should be given urgent attention. In this study, N use efficiency (NUE), grain harvest index and partial factor productivity of nitrogen were calculated to evaluate the balance between N application, grain yield and soil N content based on a 3-year plot experiment with different N application. The results showed that high N application did not result in high NUE, rather it decreased, and that low N application with high NUE led to soil N deficit. Considering N surplus and NUE in spring maize production, the primary conclusion could be drawn that 180 kg N ha−1 are acceptable both for grain yield and N risk to environment for spring maize in Harbin area.

Nitrogen use in switchgrass grown for bioenergy across the USA

The effect of nitrogen (N) fertilizer on switchgrass biomass production has been evaluated in a number of locations on a small-plot scale; however, field-scale information regarding switchgrass response to N and N use efficiency (NUE) in different regions of the USA is limited. Switchgrass was planted in South Dakota (SD), New York (NY), Oklahoma (OK), and Virginia (VA) in 2008 and in Iowa (IA) in 2009. Three N levels (0, 56, and 112 kg ha−1) were applied to 0.4–0.8 ha plots at each location beginning in spring the year after planting. Biomass production, N removal, apparent N recovery (ANR), and NUE were determined at all locations. Biomass yield response to N varied among locations and varied according to initial soil N concentration. Low initial soil N concentration increased biomass yield response to N fertilization, while high initial soil N concentration reduced this response. High amounts of initial soil N caused fertilizer N removal to be low. Fertilizer N uptake in switchgrass might be inhibited by competition from initial soil N. Seasonal temperature and precipitation were not strongly correlated with biomass yield and N-use of switchgrass at the studied locations. In this study, ANR was below 10% at all locations and years. Nitrogen-use efficiency was significantly related to initial soil N. High NUE was observed at locations where initial soil N was low. These data suggest that NUE depends on site-specific N management strategies that are responsive to soil N supply and plant N status.

Introduction: Impact of land use change for bioenergy production on greenhouse gas emissions

The European Union has committed to increase the proportion of renewable energy from 9% in 2010 to 20% of total energy consumption by 2020. Bioenergy currently accounts for almost two-thirds of the total renewable energy in Europe and much of this comes from energy crops. One element of the rationale for growing energy crops is based on the understanding that these can be used as part of the portfolio of measures for mitigating greenhouse gas (GHG) emissions. This is based on the principle that carbon emitted during combustion is balanced by carbon fixed in photosynthesis. However, this is a gross oversimplification and bioenergy production is unlikely to be carbon neutral because of GHG emissions released during crop growth, field management, feedstock processing and transport. Additional to carbon emitted as CO2 to the atmosphere, emissions of other GHGs, particularly methane and nitrous oxide, have also to be taken into account. Clearly, a better understanding of the impact of the conversion of land from its more typical use for food production to bioenergy production is required before we can argue for the widespread adoption of crops for both energy production and GHG mitigation and this was the topic of a workshop sponsored by GHG-Europe (an EU FP7 project) in Dublin in October 2010. The papers that follow all arose from presentations at the meeting. The review of Don et al. (2012) is a compilation of existing knowledge of the GHG balances of major European bioenergy crops with a particular focus on dedicated perennial crops such as Miscanthus and short rotation coppice species. Although such second-generation crops currently account for only 3% of the current European bioenergy production, their wider use in future will have a major impact on N2O emissions as they emit 40% to >99% less than conventional annual crops due to both lower fertilizer requirements and a higher N-use efficiency. These perennial energy crops also have the potential to sequester additional carbon in the soil, particularly if established on former cropland. This is largely confirmed by the study of Zimmermann et al. (2012) who have undertaken a study of Miscanthus grown on commercial farms, although they also show that the amount of carbon stored varies considerably with soil type, crop management and previous land use. In another review, Njakou Djomo and Ceulemans (2012) make a comparative analysis of 15 studies of the carbon intensity (the amount of CO2 emitted per unit of biofuel produced) of biofuels caused by direct and indirect land use change. They find that the total land use change carbon intensity of bioethanol production from biofuels ranged from −29% to 384% of that of gasoline fossil fuel. In an experimental study in NE England, Drewer et al. (2012) show that N2O emissions from Miscanthus and willow were lower than for wheat and oilseed rape on the same land, but that perennial bioenergy crops only emit less GHGs than annual crops when they receive zero or very low rates of N fertilizer. Furthermore, the review by Monti et al. (2012), which focused on switchgrass (Panicum virgatum), another C4 perennial grass, indicates that in most life cycle analysis studies there are significant reductions in CO2 emissions associated with cultivation and processing, compared with conventional crops. Interestingly, this species also performed better than most other biomass crops in terms of N2O emissions. Further studies of the impact of first-generation biofuel production on N2O emissions are reported by Carter et al. (2012) for organic farming systems in Denmark where the greatest GHG reduction was obtained from biogas production or co-production of bioethanol and biogas on either fresh grass-clover or whole crops of maize. In contrast, biofuel production based on lignocellulosic crop residues of rye and vetch produced considerably smaller net GHG reductions. Finally, Mander et al. (2012) studied the special case of GHG emissions from abandoned peat extraction areas in Estonia cultivated with reed canary grass (Phalaris arundinacea). The associated decreases in GHG fluxes turned these areas from net sources to net sinks of carbon and the almost zero CH4 emissions from Phalaris plots was attributed to the high sulphur concentration in peat, which probably inhibits methanogenesis. Clearly further studies on other peat substrates with varying sulphur contents are required to assess the generality of these findings. Whilst the papers in this special issue largely support a dual role for bioenergy crops in GHG mitigation and energy production there are still a number of unanswered questions, including their long-term impacts on the sustainability of agricultural ecosystems, their effects on dissolved N and C losses, and the extent to which their dual functions may be constrained by climatic and soil factors. All of these factors could have important consequences for the widespread introduction of bioenergy crops.

Maize Response to Fertilizer and Nitrogen Use Efficiency in Uganda

Maize (Zea mays L.) is an important smallholder crop in Uganda. Yields are low because of low soil fertility and little fertilizer use. Yield response to nutrient application and economically optimal rates (EOxR, where x = N, P, or K) and N use efficiency (NUE) were evaluated. Twenty‐two trials were conducted in four agroecological zones. Yield was consistently increased with N application. Mean maize yield with no N applied (N0) was 1.79 Mg ha−1 and increased by 120% with N application. Mean EONRs were 45 to 24 kg ha−1 N with fertilizer use cost to grain price ratios (CPs) of 10 to 30. With N applied, the mean increase in yield due to P application was 0.28 Mg ha−1 and mean EOPRs were 9 to 1 kg ha−1 P with CPs of 10 to 50. Yield was not increased with K application. Profitability was greater for N than P application. Mean aboveground biomass N with 0 and 150 kg ha−1 N applied was 46.3 and 94.3 kg ha−1, respectively. Mean N concentration and N harvest index at the EONR were 1.60 and 63.8%, respectively, and higher than for N0. Mean recovery efficiency, partial factor productivity, and agronomic efficiency declined with increasing N rate and were 66%, 86 kg kg−1, and 41 kg kg−1, respectively, at the EONR. Fertilizer N use can be very profitable, with high NUE, for smallholder maize production in Uganda, and the financial capacity of smallholders to use fertilizer will increase with reduced CP.

Mining of Candidate Maize Genes for Nitrogen Use Efficiency by Integrating Gene Expression and QTL Data

Breeding maize varieties for high nitrogen (N) use efficiency (NUE) by marker-assisted selection using NUE quantitative trait locus (QTL) or by genetic transfer of NUE-associated genes is a viable approach for increasing grain yield in N-limited production areas. In this investigation, we evaluated a set of introgression line populations under N supply and N deficiency conditions. From 42 QTLs for grain yield and yield components, 23 were identified under N supply conditions and 33 from N limited conditions. Meta-analysis of published maize NUE QTLs revealed 37 “consensus” QTLs, of which, 18 was detected under low N conditions. In addition, 258 unique ESTs associated with low N stress response, N uptake, transport, and assimilation were aligned on the maize genome by in silico mapping. Integrating the EST map with the QTL map has resulted in the identification of candidate NUE-associated genes of the following functional categories: N uptake, transport, and assimilation; carbon (C) metabolism and assimilation; and cascades of stress response and signal transduction genes. Nine candidates that have been introgressed into Ye478 significantly altered grain yield/yield components. It is suggested that the dynamics of interactions between C and N metabolism are important for maize yield. A high NUE variety should have a highly efficient C assimilation per unit N and actively express CO2 assimilation-related genes under N-limited conditions.

Long‐Term Evaluation of Manure Application on Maize Yield and Nitrogen Use Efficiency in China

Improving N use efficiency (NUE) and reducing the negative impact of N fertilization on the environment are essential for sustaining agricultural production in many intensive farming regions around the world. The objective of this research was to investigate the effect of various fertilization regimes on yield, N uptake, and NUE by maize (Zea mays L.) in long‐term (15‐yr) experiments performed at four sites (Changping, Zhengzhou, Yangling, and Qiyang) in China. Eight treatments were examined: unfertilized control (CK); inorganic fertilizer (manufactured source) N, N and P, N and K, and N, P, and K; inorganic N, P, and K with manure (NPKM); 1.5´ the rate of NPKM (1.5NPKM); and N, P, and K with maize stover returned (NPKS). Compared with inorganic fertilization, mixed inorganic and manure fertilization (NPKM and 1.5NPKM) resulted in more stable and significantly higher yield and NUE as well as increases in soil nutrient storage at all sites, with the most significant improvement in the acid soil (pH = 5.7) at Qiyang. Phosphorous was identified as one of the most important limiting factors in maize NUE because the NUE at all four sites increased from about 20 to 45% by increasing the available P in the soil. Higher NUEs (up to 70%) were observed mostly from the continuous manure applications. We conclude that manure and P applications are the most important and effective nutrient management strategies for both the acid and alkaline soils in this study for maize production in China.