The broad impacts of corn stover and wheat straw removal for biofuel production on crop productivity,soil health and greenhouse gas emissions: A review

Biofuel production from crop residues is widely recognized as an essential component of developing a bioeconomy, but the removal of crop residues still raises many questions about the sustainability of the cropping system. Therefore, this study reviews the sustainability effects of crop residues removal for biofuel production in terms of crop production, soil health and greenhouse gas emissions. Most studies found little evidence that residue management had long‐term impacts on grain yield unless the available water is limited. In years when water was not limiting, corn and wheat removal rates ≥90% produced similar or greater grain yield than no removal in most studies. Conversely, when water was limiting, corn grain yield decreased up to 21% with stover removal ≥90% in some studies. Changes in soil organic fractions and nutrients depended largely on the amount of residue returned, soil depth and texture, slope and tillage. Reductions in organic fractions occurred primarily with complete stover removal, in the top 15–30 cm in fine‐textured soils. Soil erosion, water runoff and leaching of nutrients such as total nitrogen (N) and extractable soil potassium decreased when no more than 30% of crop residues were removed. Stover management effects on soil bulk density varied considerably depending on soil layer, and residue and tillage management, with removal rates of less than 50% helping to maintain the soil aggregate stability. Reductions in CO₂ and N₂O fluxes typically occurred following complete residue removal. The use of wheat straw typically increased CH₄ emissions, and above or equal to 8 Mg/ha wheat straw led to the largest CO₂ and N₂O emissions, regardless of N rates. Before using crop residues for biofuel production, it should therefore always be checked whether neutral to positive sustainability effects can be maintained under the site‐specific conditions.     

Dry season groundnut stover management practices determine nitrogen cycling efficiency and subsequent maize yields

It is generally thought that grain legume residues make a substantial net N contribution to soil fertility in crop rotation systems. However, most studies focus on effects of residues on crops immediately sown after the legume crop while in fact in many tropical countries with a prolonged dry season there is a large gap before planting the next crop with potential for nutrient losses. Thus the objectives of this study were* to improve the efficiency of groundnut (Arachis hypogaea L.) stover-N (100 kg N ha ^–1) recycling by evaluating the effect of dry season stover management, i.e. surface application and immediate incorporation after the legume crop or storage of residues until next cropping in the rainy season. N dynamics (litterbags, mineral N, microbial biomass N, N ₂O emissions) were monitored and ¹⁵N labelled residues were applied to assess the fate of residue N in the plant–soil (0–100 cm) system during two subsequent maize crops. Recycling groundnut stover improved yield of the subsequent maize (Zea mays L.) crop compared to treatment without stover. A higher N recycling efficiency was observed when residues were incorporated (i.e. 55% total ¹⁵N recovery after second maize crop) than when surface applied (43% recovery) at the beginning of the dry season. This was despite the faster nitrogen release of incorporated residues, which led to more mineral N movement to lower soil layers. It appears that a proportion of groundnut stover N released during the dry season was effectively captured by the natural weed population (54–70 kg N ha ^–1) and subsequently recycled particularly in the incorporation treatment. Despite the presence of weeds major leaching losses occurred during the onset of the rainy season while N ₂O emissions were relatively small. There was a good correlation between soil microbial biomass N and first crop maize yield. Incorporation of groundnut residues led to small increases in economic yield, i.e., 3120 versus 3528 kg ha ^–1 over two cropping cycles in the surface versus incorporation treatments respectively, with corresponding residue ¹⁵N uptakes of 4 and 8%, while ¹⁵N recovery in water stable aggregates (9–15%) was not significantly different. In contrast, when stover was removed and applied before the first crop, yield benefits were highest with cumulative maize yields of 4350 kg ha ^–1 and residue utilization of 12%. However, N recycling efficiency was not higher than in the early incorporation treatment due to an asynchrony of N release and maize N demand during the first crop.     

Soil and crop response to stover removal from rainfed and irrigated corn

Excessive corn (Zea mays L.) stover removal for biofuel and other uses may adversely impact soil and crop production. We assessed the effects of stover removal at 0, 25, 50, 75, and 100% from continuous corn on water erosion, corn yield, and related soil properties during a 3‐year study under irrigated and no‐tillage management practice on a Ulysses silt loam at Colby, irrigated and strip till management practice on a Hugoton loam at Hugoton, and rainfed and no‐tillage management practice on a Woodson silt loam at Ottawa in Kansas, USA. The slope of each soil was <1%. One year after removal, complete (100%) stover removal resulted in increased losses of sediment by 0.36–0.47 Mg ha^?1 at the irrigated sites, but, at the rainfed site, removal at rates as low as 50% resulted in increased sediment loss by 0.30 Mg ha^?1 and sediment‐associated carbon (C) by 0.29 kg ha^?1. Complete stover removal reduced wet aggregate stability of the soil at the irrigated sites in the first year after removal, but, at the rainfed site, wet aggregate stability was reduced in all years. Stover removal at rates ≥ 50% resulted in reduced soil water content, increased soil temperature in summer by 3.5–6.8 °C, and reduced temperature in winter by about 0.5 °C. Soil C pool tended to decrease and crop yields tended to increase with an increase in stover removal, but 3 years after removal, differences were not significant. Overall, stover removal at rates ≥50% may enhance grain yield but may increase risks of water erosion and negatively affect soil water and temperature regimes in this region.     

Effects of water regime on growth,yield, and nitrogen uptake of rice

Summary The effects of water regime on the performance of rice were investigated in a greenhouse experiment and two field experiments. The greenhouse experiment involved four water regimes (continuous flooding, and soil drying for 16 days — begun 2, 5, and 8 weeks after transplanting — followed by reflooding), four soils, and 0 and 100 mg N/kg. Soil drying raised the redox potentials of all soils beyond the aerobic threshold. Averaged for soils and N levels, yields from treatments in which soil drying was begun at 2 and 5 weeks after transplanting were lower than that from the continuously flooded treatment, but the simple effects of soil drying on yield and N uptake depended on the soil and the growth stage of the plant. None of the soil-drying treatments had adverse effects in the soil high in N but soil drying at 2 and 5 weeks after transplanting had adverse effects in the soil low in N. The field experiments tested the effects of three water regimes (continuous flooding, alternate drying and flooding every 2 weeks, and soil drying for 2 weeks at 6 weeks after transplanting following by reflooding), and 0, 50, 100, and 150 kg N/ha on a nearly neutral clay soil, during two seasons. None of the soil-drying treatments depressed growth, yield, or N uptake by rice at any level of N in either season. Nitrate was absent after drying, so denitrification was not possible on subsequent flooding. The adverse effects on yield of alternate flooding and drying, attributed to nitrification-denitrification, may be insignificant in wetland fields carrying an actively growing rice crop.     

Seedling vigour and the early growth of transplanted rice (Oryza sativa)

Previous studies suggest that the positive response of transplanted rice (Oryza sativa L.) to nursery fertiliser application was due to increased seedling vigour or possibly to increased nutrient content. This paper presents results of two glasshouse experiments designed to test the hypothesis that seedling vigour was responsible for the response of transplanted seedlings to nursery treatments. The aim of the present study was to explore the concept of seedling vigour of transplanted rice and to determine what plant attributes conferred vigour on the seedlings. Seedling vigour treatments were established by subjecting seedlings to short-term submergence (0, 1 and 2 days/week) in one experiment and to leaf clipping or root pruning and water stress in another to determine their effect on plant growth after transplanting. Submerging seedlings increased plant height but depressed shoot and root dry matter and root:shoot ratio of the seedling at 28 days after sowing. After transplanting these seedlings, prior submergence depressed shoot dry matter at 40 days. Nursery nutrient application increased plant height, increased root and shoot dry matter, but generally decreased root:shoot ratio. Pruning up to 60% of the roots at transplanting decreased shoot and root dry matter, P concentration in leaves at panicle initiation (PI) and straw dry matter and grain yield at maturity. By contrast, pruning 30% of leaves depressed shoot and root dry matter by 30% at PI, and root dry matter and straw and grain yield by 20% at maturity. The combined effects of leaf clipping and root pruning on shoot, root and straw dry matter were largely additive. It is concluded that the response of rice yield to nursery treatments is largely due to increased seedling vigour and can be effected by a range of nutritional as well as non-nutritional treatments of seedlings that increase seedling dry matter, nutrient content, and nutrient concentration. Impairment of leaf growth and to a lesser extent root growth in the nursery depressed seedling vigour after transplanting. However, rather than increasing stress tolerance, seedling vigour was more beneficial when post transplant growth was not limited by nutrient or water stresses.     

Establishment of phosphate-solubilizing strains of Azotobacter chroococcum in the rhizosphere and their effect on wheat cultivars under green house conditions

A pot experiment was conducted in the green house to investigate the establishment of phosphate solubilizing strains of Azotobacter chroococcum, including soil isolates and their mutants, in the rhizosphere and their effect on growth parameters and root biomass of three genetically divergent wheat cultivars (Triticum aestivum L.). Five fertilizer treatments were performed: Control, 90 kg N ha(-1), 90 kg N + 60 kg P2O5 ha(-1), 120 kg N ha(-1) and 120 kg N + 60 kg P2O5 ha(-1). Phosphate solubilizing and phytohormone producing parent soil isolates and mutant strains of A. chroococcum were isolated and selected by an enrichment method. In vitro phosphate solubilization and growth hormone production by mutant strains was increased compared with soil isolates. Seed inoculation of wheat varieties with P solubilizing and phytohormone producing A. chroococcum showed better response compared with controls. Mutant strains of A. chroococcum showed higher increase in grain (12.6%) and straw (11.4%) yield over control and their survival (12-14%) in the rhizosphere as compared to their parent soil isolate (P4). Mutant strain M37 performed better in all three varieties in terms of increase in grain yield (14.0%) and root biomass (11.4%) over control.     

Wheat as a dual crop for biorefining: Straw quality parameters and their interactions with nitrogen supply in modern elite cultivars

Agricultural residues, such as straw, offer an opportunity to produce biofuels and chemicals in biorefineries without compromising food production. The ideal “dual‐purpose cultivar” would have high yield of grain and straw. In addition, the straw should be easy to process in a biorefinery: It should have good degradability, high concentration of carbohydrates, and low concentration of ash. Nitrogen (N) is an essential nutrient important for plant growth, crop yield and grain quality. However, N production and application comes with a high cost and high environmental footprint. The N application should consequently be based on an economical optimum. Limited knowledge exists on how N application affects the potential of straw for biorefining, for example, straw yield and quality. This study, conducted over three cropping seasons, investigated the effect of N supply on the biorefining potential and included 14 wheat cultivars and one triticale cultivar. The N supply directly affected the yield of straw and grain. In addition, the protein concentration in grain and straw increased, but the composition of the straw with respect to carbohydrates and lignin was largely unaffected by N supply. The only significant change was a lower silicon concentration at increasing N application rate, which could be beneficial for lignin valorization in biorefineries. Likely due to the negligible changes in cell wall composition, the effect of N application rate on straw degradability was not significant. N application should therefore primarily be optimized with respect to grain quality and overall yield of grain and straw. Differences between cultivars were also minor with respect to their performance in a biorefinery process. From a breeding and agronomic perspective, focus should therefore be put on maximizing the biomass output from the field, that is, selecting the cultivar with highest grain and straw yield and optimizing the application of fertilizer to get optimum N use efficiency.     

Corn stover feedstock trials to support predictive modeling

To be sustainable, feedstock harvest must neither degrade soil, water, or air resources nor negatively impact productivity or subsequent crop yields. Simulation modeling will help guide the development of sustainable feedstock production practices, but not without field validation. This paper introduces field research being conducted in six states to support Sun Grant Regional Partnership modeling. Our objectives are to (1) provide a fundamental understanding of limiting factor(s) affecting corn (Zea mays L.) stover harvest, (2) develop tools (e.g., equations, models, etc.) that account for those factors, and (3) create a multivariant analysis framework to combine models for all limiting factors. Sun Grant modelers will use this information to improve regional estimates of feedstock availability. A minimum data set, including soil organic carbon (SOC), total N, pH, bulk density (BD), and soil‐test phosphorus (P), and potassium (K) concentrations, is being collected. Stover yield for three treatments (0%, 50%, and 90% removal) and concentrations of N, P, and K in the harvested stover are being quantified to assess the impact of stover harvest on soil resources. Grain yield at a moisture content of 155 g kg^?1 averaged 9.71 Mg ha^?1, matching the 2008 national average. Stover dry matter harvest rates ranged from 0 to 7 Mg ha^?1. Harvesting stover increased N–P–K removal by an average of 42, 5, and 45 kg ha^?1 compared with harvesting only grain. Replacing those three nutrients would cost $53.68 ha^?1 based on 2009 fertilizer prices. This first‐year data and that collected in subsequent years is being used to develop a residue management tool that will ultimately link multiple feedstock supplies together in a landscape vision to help develop a comprehensive carbon management plan, quantify corn stover harvest effects on soil quality, and predict regional variability in feedstock supplies.     

Effect of mulching with cereal straw and row spacing on spread of bean yellow mosaic potyvirus into narrow-leafed lupins (Lupinus angustifolius)

Two field experiments examined the effect of straw spread on the soil surface on the incidence of bean yellow mosaic potyvirus (BYMV) in plots of narrow-leafed lupin (Lupinus angustifolius) sown at narrow (17.5 cm) vs wide (35 cm) row spacing and low (25–30 kg/ha) vs medium (50–60 kg/ha) seeding rates. Virus ingress was by vector aphids flying from adjacent pastures dominated by subterranean clover. In Expt 1, in which BYMV infection was extensive, straw greatly decreased the rate and amount of virus spread regardless of row spacing or plant density, decreasing infection more than 70% by the final assessment date. This effect of straw was attributed to decreased landing rates of incoming vector alates. In the plots without added straw, narrow row spacing decreased BYMV % infection by 38% by the last assessment date. Sowing at the medium seeding rate also decreased infection. The effect of wide row spacing seemed due to delayed canopy closure between rows which is likely to have increased the landing of aphids while the effect of medium seeding rate was attributed partly to the dilution effect of greater plant numbers and partly to the effects of partial canopy development in decreasing landing rates. In Expt 2, in which the incidence of BYMV infection was low, added straw again decreased BYMV spread, but by only 25–27% at final assessment; there were no effects of row spacing or seeding rate. In both experiments, an additional “reference” treatment was included which had a high (90–100 kg/ha) seeding rate, narrow rows and no straw. The dense canopy it developed also decreased BYMV incidence but less than in the plots with added straw in Expt 1. In Expt 1, adding straw and the resulting decrease in plants killed by BYMV, were associated with an overall increase in lupin grain yield of 20%. The greater plant densities resulting from the medium seeding rate also increased grain yield but row spacing did not affect it significantly. These results indicate that retaining stubble on the soil surface at seeding will assist in management of BYMV infection in lupin crops but that wide row spacing in the absence of retained stubble is undesirable.     

Effect of rates of nitrogen and relative efficiency of sulphur-coated urea and nitrapyrin-treated urea in dry matter production and nitrogen uptake by rice

Summary A field experiment conducted for two rainy seasons (1974 and 1975) on a sandy clay loam soil at the Indian Agricultural Research Institute, New Delhi showed that at 100kg N/ha the apparent recovery of urea nitrogen by the rice crop was only 28%, which was raised to 41.7% by treating urea with Nitrapyrin and to 47.4% by coating urea withneem (Azadirachta indica Juss) cake. The recovery with sulphur-coated urea was 37.7%. Dry matter production nitrogen concentration in plant and uptake by rice were increased as the rate of nitrogen was increased from 0 to 150kg N/ha. Advantage of treating urea with Nitrapyrin or coating withneem cake was seen more in grain than straw yield.     

Potential of indigenous cyanobacteria to contribute to rice performance under different schedules of nitrogen application

Rice, IR-28, was fertilized by urea at 48, 96, 144 or 192 kg N/ha under three application schedules: (1) complete basal application before submergence, (2) split application, basal and 30 days after transplanting, or (3) 15 and 45 days after transplanting. Plant growth, yield, N accumulation, and chemical fertilizer N-use efficiency were minimum under complete basal application. N fertilization at 15 and 45 days after transplanting was superior and formed the fastest and greatest colonization by cyanobacteria. The population profile comprised 64%Nostoc, 24%Anabaena and 8%Calothrix.Aulosira, Nodularia andTolypothrix were each less than 1% of the total heterocystous cyano-bacteria.     

Effect of crop residue management on the yields of different crops and on soil properties

The effects of chopped (6–9 cm) and unchopped (long) crop residues of wheat (Triticum aestivum L.) and rice (Oryza sativa L.) in corn (Zea mays L.)— wheat and rice-wheat rotations on grain yield and soil properties were investigated in 27 field experiments during 9 years. Experiments on chopped wheat residue involved the treatments of two main plots with residue incorporation at 0 and 4 t/ha before sowing of corn and wheat and having subplots with 0, 40, 80 and 120 kg N/ha. The results obtained for 4 years showed that the incorporation of wheat residue not only improved the soil physicochemical properties but also increased the grain and stover yields of corn significantly. The yield obtained with 80 kg N in conjunction with 4 t/ha chopped wheat residue was identical to that with 120 kg N/ha alone. But the wheat yield was depressed significantly upon the incorporation of wheat residue before the sowing of wheat in all the years of investigation.The experiments on the management of unchopped wheat residue in corn-wheat rotation and of unchopped wheat (6 t/ha) and rice (12 t/ha) residues in rice-wheat rotation, involved three main treatments: physical removal, in-situ incorporation and in-situ burning of residues. Main treatments were tested at 60, 120 and 180 kg N/ha level over 5 years. Irrespective of N application, the residue management treatments had non-significant effects on the succeeding crop yield in all the years. Burning of residue improved the yield by about 0·2 t/ha, whereas residue incorporation did not affect the yield either of corn or rice. On the other hand, the wheat yield was depressed by 0·1–0·2 t/ha in both the rotations.The effect of applied N irrespective of residue management was significant in some years up to the level of 120 kg and in others, 180 kg N/ha.     

Effects of straw mulching on soil temperature, evaporation and yield of winter wheat: field experiments on the North China Plain

Straw mulching is an effective measure to conserve soil moisture. However, the existence of straw on the soil surface also affects soil temperature, which in turn influences crop growth, especially of winter crops. Five‐year field experiments (2000–2005) investigated the effects of straw mulching and straw mass on soil temperature, soil evaporation, crop growth and development, yield and water use efficiency (WUE) of winter wheat (Triticum aestivum L.) at Luancheng Station on the North China Plain. Soil is a moderately well‐drained loamy soil with a deep profile at the station. Two quantities of mulch were used: 3000 kg ha^?1 [less mulching (LM)] and 6000 kg ha^?1 [more mulching (MM)], representing half and all of the straw from the previous crop (maize). In the control (CK), the full quantity of mulch was ploughed into the top 20 cm of soil. The results showed that the existence of straw on the soil surface reduced the maximum, but increased the minimum diurnal soil temperature. When soil temperature was decreasing (from November to early February the next year), soil temperature (0–10 cm) under straw mulching was on average 0.3°C higher for LM and 0.58°C higher for MM than that without mulching (CK). During the period when soil temperature increased (from February to early April, the recovery and jointing stages of winter wheat), average daily soil temperature of 0–10 cm was 0.42°C lower for LM and 0.65°C lower for MM than that of CK. With the increase in leaf area index, the effect of mulching on soil temperature gradually disappeared. The lower soil temperature under mulch in spring delayed the development of winter wheat up to 7 days, which on average reduced the final grain yield by 5% for LM and 7% for MM compared with CK over the five seasons. Mulch reduced soil evaporation by 21% under LM and 40% under MM compared with CK, based on daily measuring of microlysimeters. However, because yield was reduced, the overall WUE was not improved by mulch.     

Stimulating effects of foliar K-fertilizer applied at the appropriate stage of development of maize: A new way to increase yield and improve quality

A series of eight experiments was conducted using large pots to (1) find the most effective date, site, concentration of K-solution and K-salt for foliar K-fertilization of maize plants (Zea mays, L.) grown with sufficient K-supply in soil, (2) explain why maize responded to the K-treatment, and (3) examine the influence of various levels of N and P supplies on the effectiveness of K-fertilizer via the leaves. A single spraying on sweet maize and field maize on any day between 50% tasselling date to 10 days after tasselling shortened maturity date, increased grain yield, stover yield, grain-stover ratio, absorption of N, P, K, Ca and Mg, sweetness of young grain (of sweet maize), and crude protein content of grain. However spraying on the third day after 50% tasselling was most effective. The second application later than 7 days after the 50% tasselling date suppressed the effects of spraying on the most effective date. In application of many repetitive sprayings covering the most effective date, a spraying program with late spraying could reduce grain yield. KNO₃, 2.5% KNO₃-solution, and applications on all aerial parts were found to be the most effective. Increases in grain yield for spraying on all aerial parts, spraying on ear leaf only, spraying on all leaves above ear leaf and applying K to soil were 74%, 51%, 41% and 23%, respectively. The foliar K-fertilization affected maize by stimulating chlorophyll synthesis and not by increasing leaf area. A balance in N and K supplies was determined to be effective for the K-fertilization.     

Residual effects of natural bush,Cajanus cajan and Tephrosia candida on the productivity of an acid soil in southeastern Nigeria

An experiment was established in 1986 to examine the contribution of Tephrosia candida and Cajanus cajan shrubs to improving the productivity of an acid soil. The main treatments were N levels (0 and 60 kg ha^-1) with subplots of maize/natural bush, maize/Tephrosia candida, maize/Cajanus cajan, maize + cassava/natural bush, maize + cassava/Tephrosia candida, and maize + cassava/Cajanus cajan. In 1988, all plots were cleared and maize uniformly planted to study the residual effects of the treatments. No residual effects of N application were observed. Tephrosia candiada and Cajanus cajan increased surface soil organic carbon and total N levels over the natural bush. However, only Tephrosia candida plots produced improved maize grain and stover yield. Highly significant correlations were found between maize grain yield and earleaf N (r=0.73^**), grain N (r=0.51^**), and stover N (r=0.54^**) contents. These results suggest that Tephrosia candida increased N availability in the soil. Therefore, the shrub has potential for improving the productivity of acid soils under traditional systems, where N is limiting due to the absence of N₂-fixing legumes in the natural bush fallow.     

Genes/QTLs affecting flood tolerance in rice

The adaptation of deepwater rice to flooding is attributed to two mechanisms, submergence tolerance and plant elongation. Using a QTL mapping study with replicated phenotyping under two contrasting (water qualities) submergence treatments and AFLP markers, we were able to identify several genes/QTLs that control plant elongation and submergence tolerance in a recombinant inbred rice population. Our results indicate that segregation of rice plants in their responses to different flooding stress conditions is largely due to the differential expression of a few key elongation and submergence tolerance genes. The most important gene was QIne1 mapped near sd-1 on chromosome 1. The Jalmagna (the deepwater parent) allele at this locus had a very large effect on internal elongation and contributed significantly to submergence tolerance under flooding. The second locus was a major gene, sub1(t), mapped to chromosome 9, which contributed to submergence tolerance only. The third one was a QTL, QIne4, mapped to chromosome 4. The IR74 (non-elongating parent) allele at this locus had a large effect for internal elongation. An additional locus that interacted strongly with both QIne1 and QIne4 appeared near RG403 on chromosome 5, suggesting a complex epistatic relationship among the three loci. Several QTLs with relatively small effects on plant elongation and submergence tolerance were also identified. The genetic aspects of these flooding tolerance QTLs with respect to patterns of differential expression of elongation and submergence tolerance genes under flooding are discussed. Received: 13 December 1999 / Accepted: 14 March 2000<@head-com-p1a.lf>Communicated by G. Wenzel     

Identification and characterization of quantitative trait loci for grain yield and its components under different nitrogen fertilization levels in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

A set of rice (Oryza sativa L.) recombinant inbred lines from a cross between Zhenshan 97 (indica) and HR5 (indica) was planted for four different growing seasons in two locations at three nitrogen (N) fertilization levels (N300, 300 kg urea/ha; N150, 150 kg urea/ha; and N0, 0 kg urea/ha). Grain yield and its components were evaluated, including grain yield per plant (GYPP), panicle number per plant (PNPP), grain number per panicle (GNPP), filled grains per panicle (FGPP), spikelet fertility percentage (SFP) and 100-grain weight (HGW). Correlation and path analysis indicated that SFP had the greatest contribution to GYPP at the N300 and N150 levels, but FGPP contributed the most to GYPP at the N0 level. Quantitative trait loci (QTL) were mapped based on a mixed linear model; genetic components (main effects, epistatic effects and QTL-by-environment interactions) were estimated separately. Six to 15 QTL with main effects were detected for each trait except SFP. Clusters of main-effect QTL associated with PNPP, GNPP, SFP and HGW were observed in regions on chromosomes 1, 2, 3, 5, 7 and 10. The main-effect QTL (qGYPP-4b and qGNPP-12) were only detected at the N0 level and explained 10.9 and 10.2% of the total phenotypic variation, respectively. A total of 33 digenic interactions among grain yield and its components were also identified. The identification of genomic regions associated with yield and its components at different nitrogen levels will be useful in marker-assisted selection for improving the nitrogen use efficiency of rice.     

Effects of shading and nitrogen on growth of corn (Zea mays L.) under field conditions

Summary Field studies were performed in the growing seasons of 1967 and 1968 to determine the differential response of vegetative corn growth to three levels of shading (27, 51, and 100% sunlight) and three levels of nitrogen (0, 222, and 444 kg/ha). Ammonium nitrate was used as the N fertilizer. Shades were removed when the unshaded plants started tasselling. Subsequent stover and grain production was determined. During the vegetative period, N increased dry matter yield per plant, and the rate of increase was enhanced at higher light levels. This was consistent in both years. The effect of shading during vegetative period on N use appeared to depend on season, or perhaps on planting date. In 1967, with a late planting date, the N-shading interaction was found in final stover and ear yields. In 1968, with a planting date in early spring shading-N interactions were noted at 8 1/2 weeks of growth but not at maturity. Part of a Ph. D. thesis submitted by the senior author in 1969 to the Graduate Faculty of Studies and Research, McGill University, Canada. Former graduate assistant and associate professor respectively.     

Nitrogen mineralization and potential nitrification at different depths in acid forest soils

Yield decline of cereals grown in monoculture may be alleviated with alternative crop management strategies. Crop rotation and optimized tillage and fertilizer management can contribute to more sustainable food and fiber production in the long-term by increasing diversity, maintaining soil organic matter (SOM), and reducing adverse effects of excessive N application on water quality. We investigated the effects of crop sequence, tillage, and N fertilization on long-term grain production on an alluvial, silty clay loam soil in southcentral Texas. Crop sequences consisted of monoculture sorghum (Sorghum bicolor (L.) Moench,) wheat (Triticum aestivum L.), and soybean (Glycine max (L.) Merr), wheat/soybean double-crop, and rotation of sorghum with wheat/soybean. Grain yields tended to be lower with no tillage (NT) than with conventional tillage (CT) early in the study and became more similar after 11 years. Nitrogen fertilizer required to produce 95% to maximum sorghum yield was similar for monoculture and rotation upon initiation of the experiment and averaged 16 and 11 mg N g^-1 grain with NT and CT, respectively. After 11 years, however, the N fertilizer requirement became similar for both tillage regimes, but was greater in monoculture (17 mg N g^-1 grain) than in rotation (12 mg N g^-1 grain). Crop sequences with double-cropping resulted in greater land use efficiency because similar or lower amounts of N fertilizer were required to produce equivalent grain than with less intensive monoculture systems. These more intensive crop sequences produced more stover with higher N quality primarily due to the inclusion of soybean in the rotation. Large quantities of stover that remained on the soil surface with NT led to greater SOM content, which increased the internal cycling of nutrients in this soil. In southcentral Texas, where rainfall averages nearly 1000 mm yr^-1, more intensive cropping of sorghum, wheat, and soybean with moderate N fertilization using reduced tillage can increase grain production and potentially decrease N losses to the environment by cycling more N into the crop-SOM system.     

Efficacy of urea spray for wheat crop under irrigated conditions in Iraq

Summary Effects of varying levels of nitrogenviz 40, 80 and 120 kg/ha and two methods of applicationviz (1) Soil application; and (2) one half applied to the soil and the other half to the plant foliage, were studied on growth and yield of wheat crop. Results showed that foliar feeding of urea in combination with soil application to wheat crop gave significant increases in their grain yields. Thus there is a great scope of utilizing this technique for increasing grain yields of wheat and other cereal crops grown in Iraq.