Impact and Interaction of Nitrogen and Phytophthora infestans as Yield-limiting and Yield-reducing Factors in Organic Potato (Solanum tuberosum L.) Crops

For organic potato producers, the two main challenges are disease and nutrient management. Both are limited by regulations that on the one hand prohibit the use of chemical fertilizers, especially nitrogen, and on the other hand prohibit most synthetic pesticides. Late blight caused by Phytophthora infestans is commonly thought to be the most yield-reducing factor. However, because there is no really effective fungicide available to control late blight, there are virtually no yield loss data available for organic farming conditions. In this paper, the state of the art of organic potato tuber growth under on-farm conditions with respect to disease and nutrient management is summarized by field trials and on-farm surveys on commercial organic crops carried out in the years 1995–1998. Soil nitrogen (N) levels, plant N uptake, disease development of P. infestans and potato yield were measured. Results indicated that N availability was most important in limiting yields in organic potato crops. From on-farm data, a model including disease development, growth duration of the crops until foliage decay and different parameters related to N status of the crop could explain 73% of the observed variation in yield. Only 25% of this variation in yield could be attributed to the influence of late blight. Differences in N availability explained 48%. In conclusion, several points emerged from the results. In organic farming, yields are mainly limited by nutrient availability in spring and early summer. The effects of late blight on yields may often be overestimated and cannot be deduced from results in conventional farming because of the strong interaction with nutrient status. Depending on N availability, tubers stop growing between mid-July (70–90 kg N ha⁻¹ uptake), the end of July (110–140 kg N ha⁻¹ uptake) and mid-August (140–180 kg N ha⁻¹ uptake) due to N limitations. The higher the N status of a potato crop, the longer the growing period needed to achieve the attainable yield and the higher the probability that late blight stops further tuber growth and becomes the key tuber-yield-limiting factor. In the second part of this paper, the interactive effects of soil N availability and the impact of P. infestans on yield in the presence and absence of fungicides from 1996 to 1998 for mid-early main crops are reported. An empirical schematic model of disease impact depending on N availability was developed.     

The value of catch crops and organic manures for spring barley in organic arable farming

The effect of nitrogen (N) supply and weeds on grain yield of spring barley was investigated from 1997 to 2004 in an organic farming crop rotation experiment in Denmark on three different soil types varying from coarse sand to sandy loam. Two experimental factors were included in the experiment in a factorial design: (1) catch crop (with and without), and (2) manure (with and without). The crop rotation included grass-clover as a green manure crop. Animal manure was applied as slurry in rates corresponding to 40% of the N demand of the cereal crops.

Application of 50 kg NH₄-N ha⁻¹ in manure (slurry) increased average barley grain DM yield by 1.0–1.3 Mg DM ha⁻¹, whereas the use of catch crops (primarily perennial ryegrass) increased grain DM yield by 0.2–0.4 Mg DM ha⁻¹ with the smallest effect on the loamy sand and sandy loam soils and the greatest effect on the coarse sandy soil. Model estimations showed that the average yield reduction from weeds varied from 0.2 to 0.4 Mg DM ha⁻¹ depending on weed species and density. The yield effects of N supply were more predictable and less variable than the effects of weed infestation. The infestation level of leaf diseases was low and not a significant source of yield variation.

The apparent recovery efficiency of N in grains (N use efficiency, NUE) from NH₄-N in applied manure varied from 29 to 38%. The NUE of above-ground N in catch crops sampled in November prior to the spring barley varied from 16 to 52% with the largest value on the coarse sandy soil and the smallest value on the sandy loam soil. A comparison of grain yield levels obtained at the different locations with changes in soil organic matter indicated a NUE of 21–26% for soil N mineralisation, which is smaller than that for the mineral N applied in manure. However, this estimate is uncertain and further studies are needed to quantify differences in NUE from various sources of N.

The proportion of perennial weeds in total biomass increased during the experiment, particularly in treatments without manure application. The results show that manure application is a key factor in maintaining good crop yields in arable organic farming on sandy soils, and in securing crops that are sufficiently competitive against perennial weeds.     

Comparison of high-yield rice in tropical and subtropical environments: II. Nitrogen accumulation and utilization efficiency

Nitrogen requirements to achieve rice grain yields higher than 13 t ha⁻¹ and the associated internal N-utilization efficiency (NUE) have not been documented. The objective of this study was to compare N accumulation and NUE of irrigated rice in tropical and subtropical environments at yield-potential levels in both climates. Field experiments were conducted in 1995 and 1996 at the International Rice Research Institute, Philippines (IRRI, tropical site), and at Taoyuan Township, Yunnan, China (subtropical site). Three to five high-yielding rice cultivars were grown under optimum crop management. Plants were sampled at key growth stages to determine tissue N concentration, plant N accumulation, N harvest index (NHI), N translocation ratio and NUE. Plant N accumulation at maturity was 19 to 30% greater at Yunnan than at IRRI. Most of this difference resulted from greater N accumulation and N uptake rate during the vegetative period at Yunnan than at IRRI. During reproductive and grain-filling periods, N accumulation and N uptake rate were similar or higher at IRRI than at Yunnan. Grain N concentration at maturity was lower and N translocation ratio from straw to grains during grain filling was higher at Yunnan than at IRRI, and these traits contributed to larger NHI and NUE at Yunnan than at IRRI. Cultivars that produced grain yields over 13 t ha⁻¹ at Yunnan required the accumulation of about 250 kg N ha⁻¹ within the crop and had a NUE of 59 to 64 kg grain per kg plant N.     

Potassium cycling and losses in grassland systems: a review

Cycling of potassium in grassland systems has received relatively little attention in research and practice in recent years. Balanced nutrient systems require consideration of nutrients other than nitrogen (N). Potassium (K) is needed in large amounts and is closely related to N nutrition. In intensive dairy farming, surpluses of K arise from the input of concentrates and fertilizer and are returned to the grassland and may lead to increasing K content in the soil. Organic farming, on the other hand, is characterized by limitations in input of nutrient sources and quantities. Leaching of K from grassland is usually low, but high levels of available soil K, high K input from fertilizer or at urine patches lead to increasing losses. High K inputs have a negative influence on Mg and Ca uptake by plants and can cause accelerated leaching of these cations. High levels of K have been associated with inducing nutrition‐related dairy cow health problems such as milk fever (hypocalcaemia) and grass tetany (hypomagnesaemia). This review gives an overview of the cycling of potassium and related cations in grassland systems especially with regard to leaching losses and identifies limitations to knowledge.     

Nitrogen and Irrigation Management Practices to Improve Nitrogen Uptake Efficiency and Minimize Leaching Losses

Summary

Nitrogen (N) is the most important nutrient for plant growth and production. Nitrogen uptake efficiency is dependent on a number of factors. Water management influences the transformation of N sources applied to the soil and transport of the nitrate form of N in the soil. Nitrate-N is the final product of N transformations and is quite mobile in soils with the water front. Leaching of nitrate below the rootzone is an economic loss and contributes to non-point source pollution of groundwater. In this chapter we summarize the factors influencing the N uptake efficiencies for various crops and production systems, and chemical and biological processes that influence the N transformation or losses. Recent advances leading to development of N and irrigation best management practices that support sustainable crop production and net returns while minimizing the non-point source nitrate pollution of ground-water are also discussed.     

Allelopathy:Potential Role to Achieve New Milestones in Rice Cultivation

Rice fields are ecosystems with many types of plants, microbes, invertebrates, birds and animals. The rice farming protects the biodiversity of the region and maintains the ecosystem for the benefit of environment. Some rice varieties release biocidal allelochemicals which might affect major weeds, microbial and pathogenic diversity around rice plants, even soil characteristics. A large number of compounds such as phenolic acids, fatty acids, indoles and terpenes have been identified in rice root exudates and decomposing rice residues, as putative allelochemicals which can interact with surrounding environment. Since these allelopathic interactions may be positive, they can be used as effective contributor for sustainable and eco-friendly agro-production system. Genetic modification of crop plants to improve their allelopathic properties and enhancement of desirable traits has been suggested. Development of crops with enhanced allelopathic traits by genetic modification should be done cautiously, keeping in view of the ecological risk assessment(non-toxic and safe for humans and ecosystem, crop productivity, ratio of benefit and cost, etc.).     

Key Indicators for Assessing Nitrogen Use Efficiency in Cereal-Based Agroecosystems

SUMMARY

Improving nitrogen use efficiency (NUE) is an important objective of agroecosystem management. We define and demonstrate key indicators of NUE that enable a broader assessment of N management strategies. Nitrogen efficiency components and indexes were defined to assess soil and crop physiological processes, and agronomic and environmental factors related to N use. Measurements of grain yield, grain N, aboveground plant N, applied N, post-harvest root-zone soil N, and N losses via subsurface drains were used to assess N retention efficiency, available N uptake efficiency, N utilization efficiency, N harvest index, N yield efficiency, N reliance index, grain N accumulation efficiency, N balance index, N fertilizer utilization efficiency, and N loss index. Nitrogen use indicators were evaluated for two field studies: (1) hard red spring wheat with four N levels and two tillage treatments: no-tillage (NT) and conventional tillage (CT); and (2) corn in crop sequences of continuous corn (C-C), corn-soybean (C-S), two years of corn following alfalfa (ALF-C-C), and two years of corn following perennial grass (CRP-C-C). Tillage, crop rotation, and applied N had large and variable effects on different indicators of N use. N efficiency components and indexes were useful for monitoring cropping system N use, assessing N management strategies, and identifying key areas for improvements in NUE.     

Problems and Perspectives of Yam-Based Cropping Systems in Africa

SUMMARY

Yams (Dioscorea spp.) constitute an important starchy staple in sub-Saharan Africa (SSA) where food security for a growing population is a critical issue. Mixed cropping in yam based systems is the norm in the region and productivity of yams in these systems is below potential. It is concluded that there is much scope for improvement of yam based cropping systems in SSA in order to meet the needs of the region. The strategy of crop breeding to select yam varieties suitable for various cropping systems must consider a truly multidisciplinary systems approach. Further manipulation must be made to tuber dormancy to expand flexibility in field propagation in different cropping systems and improve storage and marketing. The sustainability of yam based cropping systems in SSA could improve if agronomic research was focused on strategies for improving soil fertility, weed and pest management including design of cropping systems and suitable rotations.     

Numerical Modeling to Study the Fate of Nitrogen in Cropping Systems and Best Management Case Studies

Summary

Nitrogen (N) availability for crop uptake is dependent on various factors that influence the transformation of N sources and transport of N forms in soils. The fate and transport of N is site specific. Therefore evaluation of N dynamics under each condition is neither practical nor feasible. Simulation models which are adequately calibrated and tested can be used to estimate the fate and transport of N as well as crop responses under different production systems. These evaluations provide some guidelines as how to manage N and water efficiently to maximize the N uptake efficiency and minimize the losses. Thus, they contribute to the development of N and water best management practices. In this chapter, we discuss recent information on experimentally measuring the water and nutrient transport in soils as well as performing estimations using simulation models. The development and application of different simulation models for different production systems have been summarized. Some case studies on nitrogen and water best management practices are also discussed.     

Potato nutrient management in sustainable cropping systems

Sustainable nutrient management involves a set of management practices designed to conserve soil resources, to maintain or enhance productivity, and to help reduce growers’ reliance on chemical fertilizers. Sustainable nutrient management systems will most certainly rely heavily on the use of legume rotation crops and/or organic soil amendments. To realize the full benefit to the crop ecosystem, sustainable nutrient management programs will also likely require longer crop rotations with less frequent potato production. There is considerable research evidence indicating that these systems can maintain or increase productivity while having positive impacts on nutrient supply, soil physical properties, and soil conservation. However, their successful adoption and continued use on a large scale will require resolution of uncertainties related to economic risk associated with inconsistent nutrient availability in alternative nutrient management systems, potential environmental risks associated with excessive P applications in animal manures, and the potential for increased potato pest incidence resulting from manure application.     

Genotypic variation in grain phosphorus concentration,and opportunities to improve P-use efficiency in rice

Continual removal of phosphorus (P) from fields in rice grains at harvest results in lower soil fertility in low-input farming systems and drives the need for fertiliser inputs in high-input farming systems. High-P content in rice grains (the majority as phytate) contributes little to human nutrition and agronomic practices such as growing seedlings in high-P media or seed P coating at sowing (in direct-sown crops) may overcome the reported need for high-P seed for seedling establishment. Thus, reducing the amount of P in rice grains at maturity through breeding may represent a novel means to reduce ‘mining’ of soil P. We investigated the uptake and partitioning of P in rice plants and examined the scope for breeding rice with lower grain P by assessing genotypic variation for phosphorus harvest index (PHI) and seed P concentrations among a set of 38 rice genotypes in the field. At maturity approximately 75% of total plant P was found in grains and translocation of P from stems and leaves contributed substantially to grain P. However, unlike other cereals such as wheat, rice plants continued dry matter and P accumulation until maturity with approximately 40% of total plant P taken up post anthesis. In the field study, PHI differed significantly among genotypes (from 57 to 87) but was highly correlated to HI (P ≤ 0.001), suggesting that exploiting genotypic variation for this trait may be counterproductive. Grain P concentrations varied from 1.96 to 3.18 mg P g⁻¹, and were neither associated with reductions in grain yield or seed size, nor significantly correlated to HI. Grain P concentration therefore appears to be a suitable screening criterion and the 50% variation observed among genotypes suggests that scope exists for breeding rice with lower grain P concentration to reduce the off-take of P from rice fields at harvest.     

Current status,opportunities, and challenges of cover cropping for sustainable dryland farming in the Southern Great Plains

Cropping systems that integrate cover crops into crop rotations, reduce tillage intensity and frequency, and maintain residue cover have the potential to improve agricultural sustainability in drylands. However, there is much yet to learn about the benefits of cover crops in sustainable dryland farming in the southern Great Plains (SGP). We reviewed the literature on the effects of cover crops on soil organic carbon (SOC), nitrogen, soil water conservation, and crop yields in dryland cropping systems of the US Great Plains (GP), and analyzed the opportunities and challenges for integrating cover crops into dryland crop-fallow systems of the SGP. Majority of the studies in the central Great Plains (CGP) and the northern Great Plains (NGP) of the United States suggest that cover cropping improves sustainability of cropping systems through their positive effects on SOC accumulation, nutrient cycling, soil erosion control, weed suppression, and soil health improvement. However, integrating cover crops into dryland cropping systems of the SGP faces challenges because of low quantity of soil-water availability. More research on the tradeoff between water use and other agroecosystem benefits of cover cropping is required to successfully integrate cover crops into dryland cropping systems in the SGP.     

Meeting demands for increased cereal production in China

Meeting demands for increased cereal production in China is a great challenge and this paper provides updated information on cereal production and the potential adaptation of cropping systems to climate change, as well as on progress in improving yield potential and developing molecular markers and GM cereals in China. Maize production and soybean imports are increasing significantly to meet the strong demand for feed by a rapidly growing livestock industry. Extension of the rice and maize growing seasons in northeastern China and improvement of the cropping system through delayed wheat planting have contributed to improving cereal productivity despite changing climatic conditions. Significant improvements in yield potential of rice, maize, and wheat have been achieved. Comparative genomics has been successfully used to develop and validate functional markers for processing quality traits in wheat, and also for developing new varieties. Although transgenic Bt rice and maize, and maize expressing phytase have been developed, their commercialization has not been officially permitted. International collaboration has contributed significantly to cereal production by providing germplasm and improved crop management practices. Full integration of applied molecular technologies into conventional breeding programs and promotion of lower-input technologies, will play a key role in increasing and sustaining future cereal production.     

Yield and Nitrogen Use of Irrigated Processing Potato in Response to Placement,Timing and Source of Nitrogen Fertilizer in Manitoba

Optimizing nitrogen (N) fertilizer management in irrigated potato (Solanum tuberosum L.) on coarse-textured soils is challenging. The “4R” nutrient stewardship framework of using N fertilizer at the right rate, right source, right placement and right time provides approaches to improve fertilizer use efficiency while maintaining or improving yield. This 3-years replicated field plot study evaluated effects from a series of N fertilization strategies including 10 combinations of sources, placement and timing, as well as fertigation, on irrigated processing potato (cv. Russet Burbank) grown for a total of five site-years in the Province of Manitoba, Canada. Treatments were designed to provide early to late availability of N to the potato crop. Nitrogen was applied to 80% of Provincial N recommendation to increase the likelihood of observing improved fertilizer use efficiency and effects of treatments on yields. Measurements were tuber yield, size distribution, specific gravity, hollow-heart rate, fertilizer apparent N recovery (ANR) and agronomic nitrogen use efficiency (NUE). Results showed differences in yield, quality, ANR and NUE between fertilizer treatments were generally very small or absent. Average tuber marketable yields for fertilizer treatments were significantly greater than those for the unfertilized control (P?<?0.001). Split application of urea at planting and hilling, and urea at planting with fertigation occasionally increased tuber marketable yields on sites of coarse textured soils (P?<?0.05). Use of polymer-coated urea (ESN) or stabilized urea with inhibitors (SuperU) did not affect yield, quality or N use of potato. Site-year difference (P?<?0.001) were apparent for all measures highlighting the importance of soil and climatic conditions on agronomic and environmental effects of N management practices. The results indicate current grower practice of split urea application at planting and hilling and urea at planting following by in-season fertigation are sound. Results indicate growers could shift to the more convenient practice of ESN at planting without reducing yields. Absence of treatment effects suggests N was generally not a limiting factor for the current study, indicating that the current recommendation for potato production in Manitoba over-estimate site-specific crop N needs.     

Rice yield and its relation to root growth and nutrient-use efficiency under SRI and conventional cultivation: an evaluation in Madagascar

While plant growth and productivity are known to derive from the interaction between genetic potential (G) and environmental factors (E), efforts to improve rice production have usually proceeded assuming a standard E that is created by conventional rice-growing practices. Genotypes have been assessed for their performance in continuously flooded paddy soils, with optimally dense plant populations, with reliance on inorganic fertilization to raise yields. The System of Rice Intensification (SRI) developed in Madagascar and now becoming accepted in much of Asia proposes that GxE interactions can be made more productive with different management practices: optimally sparse populations, established with very young seedlings carefully transplanted, intermittent flooding of paddies, with active soil aeration and with soil organic matter enhanced as much as possible. This article evaluates the effects of alternative SRI cultural practices on grain yield with particular attention to their impact on the growth and functioning of rice plant roots and on associated nutrient-use efficiencies that could be contributing to the observed higher grain yields. On-station experiments and on-farm surveys were conducted in Madagascar to evaluate SRI practices in comparison with standard cultural methods, considering how rice plants’ expression of their genetic potential was affected by different crop management practices. Controlling for both soil and farmer effects, rice plants cultivated with SRI methods produced average yields more than double those from standard practice (6.26 vs. 2.63 t ha⁻¹). The most evident phenotypic difference was in plant root growth, assessed by root-pulling resistance (RPR), a summary measure of root system development. On average, uprooting single SRI plants required 55.2 kg of force plant⁻¹, while pulling up clumps of three conventionally grown plants required 20.7 kg hill⁻¹, or 6.9 kg plant⁻¹. SRI plants thus offered 8 times more resistance per plant to uprooting. Direct measurements confirmed that SRI methods induced both greater and deeper root growth, which could be contributing to increased nutrient uptake throughout the crop cycle, compared with the shallower rooting and shorter duration of root functioning under continuous flooding. Rice plants grown with SRI methods took up more macronutrients than did the roots of conventionally managed plants, which was reflected in the higher SRI yields. When grain yield was regressed on nutrient uptake to assess nutrient-use efficiency, SRI plants achieved higher grain yield per unit of N taken up, compared to plants grown with conventional methods. The internal efficiency (IE) of SRI plants in utilizing macronutrients was 69.2 for N, 347.2 for P, and 69.7 for K, while the IE in plants conventionally grown was 74.9, 291.1, and 70.4 for these three macronutrients, respectively. Although no significant differences in IE were observed for N and K, the uptake of P was significantly greater, indicating more efficient use of P by SRI plants for grain production. More research needs to be done on such relationships, but this study indicates that productive changes in the structure and functioning of rice plants, particularly their roots, can be induced by alternative management methods.     

Sulfur fertilization improves nitrogen use efficiency in wheat by increasing nitrogen uptake

Nitrogen (N) fertilization plays a central role for improving yield in wheat and high N use efficiency (NUE) is desired to protect ground and surface waters. Several studies showed that sulfur (S) fertilization may increase NUE, but no attempts have been made to explain whether this increase is due to greater recovery efficiency (RE), an enhanced internal efficiency (IE) or by an improvement of both efficiencies. The aim of this study was to analyze the effects of different N and S fertilizer rates, and their interaction on N uptake, its partition at maturity, NUE and its main components. Field experiments were carried out during two consecutive growing seasons in the Argentinean Pampas using a single bread-wheat genotype grown under different combinations of N and S fertilizer rates. Additional experiments were performed in farmer fields using N and S fertilization evaluating different genotypes in order to analyze the components of NUE in other environmental conditions. Plant N uptake increased linearly in response to N addition until rates of ca. 80 kg N ha⁻¹. Sulfur addition showed no effect at the lowest N fertilizer rate, but N uptake was increased when S was applied at the highest N rate, revealing a synergism between both nutrients. At the lowest S rate RE was 42%, and increased to 70% when S fertilizer was added. No changes in IE in response to S fertilization were observed. These results were also observed in farmer field experiments, in genotypes that showed different IE. This study showed that S addition increased NUE mainly by increasing the N recovery from the soil. Thus, the concurrent management of N and S is important for reducing the potential pollution of residual soil nitrate by increasing N recovery from the soil while sustaining high nitrogen use efficiency.     

Identification of traits to improve the nitrogen-use efficiency of wheat genotypes

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 analyze the genetic variability in N-use efficiency (grain dry matter (DM) yield per unit N available from soil and fertilizer; NUE) in winter wheat and identify traits for improved NUE for application in breeding. Fourteen UK and French cultivars and two French advanced breeding lines were tested in a 2 year/four site network comprising different locations in France and in the UK. Detailed growth analysis was conducted at anthesis and harvest in experiments including DM and N partitioning. Senescence of either the flag leaf or the whole leaf canopy was assessed from a visual score every 3-4 days from anthesis to complete canopy senescence. The senescence score was fitted against thermal time using a five parameters monomolecular-logistic equation allowing the estimation of the timing of the onset and the rate of post-anthesis senescence. In each experiment, grain yield was reduced under low N (LN), with an average reduction of 2.2 t ha⁻¹ (29%). Significant N × genotype level interaction was observed for NUE. Crop N uptake at harvest on average was reduced from 227 kg N ha⁻¹ under high N (HN) to 109 kg N ha⁻¹ under LN conditions while N-utilization efficiency (grain DM yield per unit crop N uptake at harvest; NUtE) increased from 34.0 to 52.1 kg DM kg⁻¹ N. Overall genetic variability in NUE under LN related mainly to differences in NUtE rather than N-uptake efficiency (crop N uptake at harvest per unit N available from soil and fertilizer; NUpE). However, at one site there was also a positive correlation between NUpE and NUE at LN in both years. Moreover, across the 2 year/four site network, the N × genotype effect for NUpE partly explained the N × genotype effect for grain yield and NUE. Averaging across the 16 genotypes, the timing of onset of senescence explained 86% of the variation in NUtE amongst site-season-N treatment combinations. The linear regression of onset of senescence on NutE amongst genoytpes was not significant under HN, but at three of the four sites was significant under LN explaining 32-70% of the phenotypic variation amongst genotypes in NutE. Onset of senescence amongst genotypes was negatively correlated with the efficiency with which above-ground N at anthesis was remobilized to the grain under LN. It is concluded that delaying the onset of post-anthesis senescence may be an important trait for increasing grain yield of wheat grown under low N supply.     

小麦氮素高效育种研究进展

氮素是植物生长的必需元素,也是粮食作物产量的一个重要限制因素。培育氮素高效利用的农作物新品种不仅可以降低农业生产成本,还可以从根本上解决氮素施用不当造成的各种问题,是新一代绿色高效农业的发展方向。本文综述了当前小麦氮素高效利用育种研究进展,包括小麦氮素利用效率的评价、小麦氮素响应的基因型差异、影响小麦氮素利用率的形态及理化性状、小麦氮素利用相关基因的挖掘及分子育种与传统育种相结合提高氮素利用率等。     

A comprehensive study of plant density consequences on nitrogen uptake dynamics of maize plants from vegetative to reproductive stages

Nitrogen (N) use efficiency (NUE), defined as grain produced per unit of fertilizer N applied, is difficult to predict for specific maize (Zea mays L.) genotypes and environments because of possible significant interactions between different management practices (e.g., plant density and N fertilization rate or timing). The main research objective of this study was to utilize a quantitative framework to better understand the physiological mechanisms that govern N dynamics in maize plants at varying plant densities and N rates. Paired near-isogenic hybrids [i.e., with/without transgenic corn rootworm (Diabrotica sp.) resistance] were grown at two locations to investigate the individual and interacting effects of plant density (low—54,000; medium—79,000; and high—104,000 pl ha⁻¹) and sidedress N fertilization rate (low—0; medium—165; and high—330 kg N ha⁻¹) on maize NUE and associated physiological responses. Total aboveground biomass (per unit area basis) was fractionated and both dry matter and N uptake were measured at four developmental stages (V14, R1, R3 and R6). Both plant density and N rate affected growth parameters and grain yield in this study, but hybrid effects were negligible. As expected, total aboveground biomass and N content were highly correlated at the V14 stage. However, biomass gain was not the only factor driving vegetative N uptake, for although N-fertilized maize exhibited higher shoot N concentrations than N-unfertilized maize, the former and latter had similar total aboveground biomass at V14. At the R1 stage, both plant density and N rate strongly impacted the ratio of total aboveground N content to green leaf area index (LAI), with the ratio declining with increases in plant density and decreases in N rate. Higher plant densities substantially increased pre-silking N uptake, but had relatively minor impact on post-silking N uptake for hybrids at both locations. Treatment differences for grain yield were more strongly associated with differences in R6 total biomass than in harvest index (HI) (for which values never exceeded 0.54). Total aboveground biomass accumulated between R1 and R6 rose with increasing plant density and N rate, a phenomenon that was positively associated with greater crop growth rate (CGR) and nitrogen uptake rate (NUR) during the critical period bracketing silking. Average NUE was similar at both locations. Higher plant densities increased NUE for both medium and high N rates, but only when plant density positively influenced both the N recovery efficiency (NRE) and N internal efficiency (NIE) of maize plants. Thus plant density-driven increases in N uptake by shoot and/or ear components were not enough, by themselves, to increase NUE.     

A review of studies on SRI effects on beneficial organisms in rice soil rhizospheres

This communication reports on separate research efforts in India and Indonesia to evaluate the effects that modifying methods of plant, soil, water and nutrient management could have on populations of soil organisms, particularly on those that can have beneficial consequences for crop growth and yield. Comparison of these parallel studies (Table 7) draws attention to the impacts that management can have on the soil biota, given that certain organisms are known to have positive implications for plants’ nutrition, health, and productivity. Data from the three studies show SRI management associated with some significant differences in soil microbial populations; higher levels of enzyme activity in SRI plant rhizospheres, indicative of increased N and P availability; and more soil microbial C and N, which would enlarge the nutrient pool for both plants and microbes. The studies reported, although more exploratory than conclusive, show enough similarity to suggest that SRI practices, which make paddy soils more aerobic and enhance soil organic matter, are supportive of enhanced populations of beneficial soil organisms. If this relationship is confirmed by further assessments, it could help researchers and practitioners to improve paddy production in resource-conserving, cost-effective ways. This review was written to encourage more studies to assess these kinds of soil biotic relationships and dynamics.