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Phosphorous deficiency limits productivity of rice (Oryza sativa L.)-wheat (Triticum aestivum L.) systems in the Indo-Gangetic Plains. Deterioration of soil and air quality due to straw burning is also a concern. Field experiments were conducted to determine the effects of straw and P management strategies on yield, P balance, and P transformations in soil in a rice-wheat system. Four treatments composed of different combinations of rice and wheat straw removal, burning, and incorporation were the main plots. Subplot treatments were P fertilization to wheat or to both rice and wheat and a no-P control. Wheat yield was similar where straw was burned in situ or removed. Incorporation of straw increased the wheat yield in Year 4. Significant straw x P management interactions, observed after 4 yr, suggested that residues can enhance yield under limited P supply situations. Application of 26 kg P ha-1 to wheat increased grain yield by 6 to 15% compared with no P. Rice yield did not respond to incorporation of residues or P fertilization. The P balance was negative with removal or burning of rice straw, but when both wheat and rice straw were incorporated, the balance was positive at the recommended P level (26 kg P ha-1 to wheat only). Changes in total soil P suggested that the two crops remove significant P from below 15 cm. Incorporation of residues increased soil Olsen, inorganic, and organic P; reduced P sorption; and increased P release. Data show that continuous incorporation of residues substituted for 13 kg inorganic P ha-1 yr-1 and improved system yield.

A fundamental shift has taken place in agricultural research and world food production. In the past, the principal driving force was to increase the yield potential of food crops and to maximize productivity. Today, the drive for productivity is increasingly combined with a desire for sustainability. For farming systems to remain productive, and to be sustainable in the long-term, it will be necessary to replenish the reserves of nutrients which are removed or lost from the soil. In the case of nitrogen (N), inputs into agricultural systems may be in the form of N-fertilizer, or be derived from atmospheric N sub(2) via biological N sub(2) fixation (BNF). Although BNF has long been a component of many farming systems throughout the world, its importance as a primary source of N for agriculture has diminished in recent decades as increasing amounts of fertilizer-N are used for the production of food and cash crops. However, international emphasis on environmentally sustainable development with the use of renewable resources is likely to focus attention on the potential role of BNF in supplying N for agriculture. This paper documents inputs of N via symbiotic N sub(2) fixation measured in experimental plots and in farmers' fields in tropical and temperate regions. It considers contributions of fixed N from legumes (crop, pasture, green manures and trees), Casuarina, and Azolla, and compares the relative utilization of N derived from these sources with fertilizer N.

Rice−wheat (Oryza sativa L.−Triticum aestivum L.) rotation is the major production system in Asia, covering about 18 million ha. Conventional practice of growing rice (puddled transplanting) and wheat (conventional till, CT) deteriorate soil physical properties, and are input- and energy-intensive. Zero-tillage (ZT) along with drill-seeding have been promoted to overcome these problems. A 7-yr permanent plot study evaluated various tillage and crop establishment (CE) methods on soil physical properties with an aim to improve soil health and resource-use efficiency. Treatments included transplanting and direct-seeding of rice on flat and raised beds with or without tillage followed by wheat in CT and ZT soil. Bulk density (Db) of the 10- to 20-cm soil layer was highest under puddled treatments (1.74-1.77 Mg m-3) and lowest under ZT treatments (1.66-1.71 Mg m-3). Likewise, soil penetration resistance (SPR) was highest at the 20-cm depth in puddled treatments (3.46-3.72 MPa) and lowest in ZT treatments (2.51-2.82 MPa). Compared with conventional practice, on average, water-stable aggregates (WSAs) > 0.25 mm were 28% higher in ZT direct-seeding with positive time trend of 4.02% yr-1. Infiltration was higher (0.29-0.40 cm h-1) in ZT treatments than puddled treatments (0.18 cm h-1). The least-limiting water range was about double in ZT direct-seeding than that of conventional practice. Gradual improvement in soil physical parameters in ZT system resulted in improvement in wheat yield and is expected to be superior in long-run on system (rice+wheat) basis. Further research is needed to understand mechanisms and requirements of two cereals with contrasting edaphic requirements in their new environment of ZT direct-seeding.

Rice (Oryza sativa L.)–wheat (Triticum aestivum L.) is the major cropping system occupying 13.5 million ha in the Indo-Gangetic Plains of South Asia. Conventional-tillage practices are resource and cost intensive. A 7-yr study evaluated six treatments (T) involving three tillage methods and two rice establishment methods on crop yield, water productivity, and economic profitability in a rice–wheat rotation. Average rice yields in the conventional practice of puddling and transplanting without (T1) and with (T2) mid-season alternate wetting-drying were highest (7.81–8.10 Mg ha−1) and increased with time (0.26 Mg ha−1 yr−1) in T2. Compared to T1, rice yields in direct drill-seeding with zero-tillage averaged 16% lower on flat (T5) and 43% lower in raised beds (T3). Rice yield in raised beds (T3 and T4) decreased with time (0.14–0.45 Mg ha−1 yr−1). Conversely, wheat yielded 18% higher after zero compared to conventional-tillage. Treatment 2, despite low soil matric potential during vegetative development, had higher water productivity with 25% less water use compared with T1 and 19% less compared with other treatments. Conventional-tillage and crop establishment practices had higher net cash return in rice but in wheat it was higher with zero-tillage. Overall, T2 and T5 had the highest net returns (∼1225US$) and T3 and T4 had the lowest (747–846 US$) in the rice–wheat system. Zero-tillage on flat beds (T5), however, would conceivably be more sustainable than the conventional T2 in the long-run. Yields of zero-tillage with direct-seeding of rice on flat beds (T5) must improve before adoption occurs.

For over 7 centuries, production of rice (Oryza sativa L.) in Egypt has benefited from rotation with Egyptian berseem clover (Trifolium alexandrinum). The nitrogen supplied by this rotation replaces 25-33% of the recommended rate of fertilizer-N application for rice production. This benefit to the rice cannot be explained solely by an increased availability of fixed N through mineralization of N- rich clover crop residues. Since rice normally supports a diverse microbial community of internal root colonists, we have examined the possibility that the clover symbiont, Rhizobium leguminosarum bv. trifolii colonizes rice roots endophytically in fields where these crops are rotated, and if so, whether this novel plant-microbe association benefits rice growth. MPN plant infection studies were performed on macerates of surface-sterilized rice roots inoculated on T. alexandrinum as the legume trap host. The results indicated that the root interior of rice grown in fields rotated with clover in the Nile Delta contained ~10⁶ clover-nodulating rhizobial endophytes g⁻¹ fresh weight of root. Plant tests plus microscopical, cultural, biochemical, and molecular structure studies indicated that the numerically dominant isolates of clover-nodulating rice endophytes represent 3 - 4 authentic strains of R. leguminosarum bv. trifolii that were Nod⁺ Fix⁺ on berseem clover. Pure cultures of selected strains were able to colonize the interior of rice roots grown under gnotobiotic conditions. These rice endophytes were reisolated from surface-sterilized roots and shown by molecular methods to be the same as the original inoculant strains, thus verifying Koch's postulates. Two endophytic strains of R. leguminosarum bv. trifolii significantly increased shoot and root growth of rice in growth chamber experiments, and grain yield plus agronomic fertilizer N-use efficiency of Giza-175 hybrid rice in a field inoculation experiment conducted in the Nile Delta. Thus, fields where rice has been grown in rotation with clover since antiquity contain Fix⁺ strains of R. leguminosarum bv. trifolii that naturally colonize the rice root interior, and these true rhizobial endophytes have the potential to promote rice growth and productivity under laboratory and field conditions.

Endophytic nitrogen-fixing bacteria are believed to contribute substantial amounts of N to certain gramineous crops. We have been interested to find (a) a diazotroph(s) in rice which can aggressively and stably persist and fix nitrogen in interior tissues and (b) unique rice-diazotrophic endophyte combinations. To achieve these objectives, it has been essential to find an efficient method to surface sterilize rice tissues. The method described here consists of exposing tissues to 1% Chloramine T for 15 min followed by shaking with glass beads. It has proven very efficient since (a) surface bacterial populations on the root and culm were found to be reduced by more than 90%, (b) the number of the internal colonizers was found to be significantly higher than the number of surface bacteria, and (c) colonization of root but not subepidermal tissue by gusA-marked Herbaspirillum seropedicae Z67 bacteria was found to be virtually eliminated. Nitrogen-fixing putative endophytic populations (MPN g dry wt⁻¹) in the root (7.94 × 10⁷) and culm (2.57 × 10⁶) on field-grown IR72 plants grown in the absence of N fertilizer was found to be significantly higher near heading stage. The corresponding total putative endophyte populations in the tissues of 25 highly diverse genotypes of rice and their relatives was found to range from 10⁵-10⁸ and 10⁴-10⁹, in the roots and culms, respectively. Generally, the resident bacteria were found to be non-diazotrophic, although in isolated cases diazotrophs were found, for example in the roots and culm of IR72 rice plants, or the culm of Zizaniopsis villanensis plants. The size of populations of diazotrophic bacteria in different rice genotypes was found to be 10³-10⁷ for the roots and 10⁴-10⁶ for the culms, respectively. The rice genera-related plants Potamophila pariffora and Rhynchoryza subulata showed the highest levels.

Soil C oxidized by neutral KMnO4, or permanganate-oxidizable C (POC), has been used as an index of labile C by several workers, although the nature of organic C (OC) oxidized has not been well elucidated. This study aimed to determine the reactivity of diverse organic compounds found in the soil with KMnO4 to judge the reliability of POC as an index of labile C. Sugars, amino acids, and other organic acids reacted slowly with 33 mM KMnO4 (2-45% C oxidized in 1 h), while compounds containing glycol groups (e.g., ascorbic acid and pyrogallol) were oxidized quickly by KMnO4 (25% C oxidized in 1 min). Permanganate did not oxidize cellulose, which is decomposed by soil microbial enzymes. The POC of organic manures and plant residues was positively correlated with lignin content. The rates of oxidation of SOM with KMnO4 varied among different rice (Oryza sativa L.) soils and were highly correlated with total soil C. The clay + silt/OC ratio negatively affected POC rendering physical protection for oxidizable C groups. In the soil, KMnO4 more rapidly oxidized less readily available organic compounds than the water-soluble carbohydrates, indicating that it did not discriminate the nonlabile from labile C. Soil POC was better correlated with total C (P < 0.01) than with water-soluble C (WSC) (P < 0.05) and was not correlated with microbial biomass C (MBC). Carbon oxidized by KMnO4 is not a reliable measure of labile C and should be referred to as POC when used as a parameter for characterizing soil C.

Conventional tillage and crop establishment methods such as puddled transplanting in the rice-wheat (Oryza sativa L.-Triticum aestivum L.) system in the Indo-Gangetic Plains (IGP) require a large amount of water and labor, both of which are increasingly becoming scarce and expensive. We attempted to evaluate alternatives that would require smaller amounts of these two inputs. A field experiment was conducted in the IGP for 2 yr to evaluate various tillage and crop establishment systems for their efficiency in labor, water, and energy use and economic profitability. The yields of rice in the conventional puddled transplanting and direct-seeding on puddled or nonpuddled (no-tillage) flat bed systems were equal. Yields of wheat following either the puddled-transplanted or no-tillage direct-seeded rice were also equal. Normally, puddled transplanting required 35 to 40% more irrigation water than no-tillage direct-seeded rice. Compared with conventional puddled transplanting, direct seeding of rice on raised beds had a 13 to 23% savings of irrigation water, but with an associated yield loss of 14 to 25%. Nevertheless, water use efficiency (WUE) in the rice-wheat system was higher with direct-seeded rice (0.45 g L-1) than with transplanted rice (0.37-0.43 g L-1). In Year 1, no-tillage rice-wheat had a higher net return than the conventional system, whereas in Year 2 the net returns were equal. The study showed that the conventional practice of puddled transplanting could be replaced with no-tillage-based crop establishment methods to save water and labor. However, the occurrence and distribution of rainfall during the cropping season had considerable influence on the savings in irrigation water.

The sustainability of the rice (Oryza sativa L.)-wheat (Triticum aestivum L.) rotation is important to Asia's food security. Intensive cropping with no return of crop residues and other organic inputs result in the loss of soil organic matter (SOM) and nutrient supply, and is assumed to be nonsustainable. We evaluated seven treatments comprised of various combinations of green manure (GM; Sesbania cannabina L.); wheat straw (WS), farmyard manure (FYM), and urea on yields and yield trends; P and K balance; and soil fertility in a rice-wheat experiment (1988-2000) on a loamy sand in Punjab, India. Rice yields were comparable with GM + urea, WS + GM + urea, and urea alone, but yields were reduced when FYM was supplemented with N. Except during 1 yr, integrated use of FYM and GM produced equal or higher rice yields than other GM based treatments. Wheat straw incorporation reduced average rice yields by 7% compared with WS removal. After 5 yr of continuous application, FYM and WS were at par in increasing rice yields. Organic materials applied to rice had no residual effect on wheat yields except FYM, which increased yield by about 6% compared with urea alone. Rice yield declined by 0.02 to 0.13 Mg ha(-1) yr(-1) but wheat yields remained unchanged. Soil C increased with the application of WS and FYM. Potassium balance was highly negative. Although the causes of yield decline are unknown, inadequate K applications and changes in the climatic parameters are possible reasons.

The growing concern about the sustainability of tropical agricultural systems stands in striking contrast to a worldwide decline in the use of soil-improving legumes. It is timely to assess the future role that soil-improving legumes may play in agricultural systems. This paper reviews recent progress, potential, and limitations of green manure technology, using lowland rice cropping systems as the example. Only a few legume species are currently used as green manures in lowland rice. Sesbania cannabina is the most widely used pre-rice green manure for rice in the humid tropics of Africa and Asia. Astragalus sinicus is the prototype post-rice green manure species for the cool tropics. Stem-nodulating S. rostrata has been most prominent in recent research. Many green manure legumes show a high N accumulation (80-100 kg N ha⁻¹ in 45-60 days of growth) of which the major portion (about 80%) is derived from biological N₂ fixation. The average amounts of N accumulated by green manures can entirely substitute for mineral fertilizer N at current average application rates. With similar N use efficiencies, green manure N is less prone to loss mechanisms than mineral N fertilizers and may therefore contribute to long-term residual effects on soil productivity. Despite a high N₂-fixing potential and positive effects on soil physical and chemical parameters, the use of green manure legumes for lowland rice production has declined dramatically world-wide over the last 30 years. Land scarcity due to increasing demographic pressure and a relatively low price of urea N are probably the main determining factors for the long-term reduction in pre-rice green manure use. Post-rice green manures were largely substituted for by high-yielding early-maturing grain legumes. Unreliability of green manure performance, nonavailability of seeds, and labor intensive operations are the major agronomic constraints. The recognition and extrapolation of niches where green manures have a comparative advantage may improve an often unfavorable economic comparison of green manure with cash crop or fertilizer N. Socio-economic factors like the cost of land, labor, and mineral N fertilizer are seen to determine the cost-effectiveness and thereby farmers' adoption of sustainable pre-rice green manure technology. Hydrology and soil texture determine the agronomic competitiveness of a green manure with N fertilizers and with alternative cash crops. In general, the niches for pre-rice green manure are characterized by a relatively short time span available for green manure growth and a soil moisture regime that is unfavorable for cash crops (flood-prone rainfed lowlands with coarse-textured soils). Given the numerous agronomic and socio-economic constraints, green manure use is not seen to become a relevant feature of favourable rice-growing environments in the foreseeable future. However, in environments where soil properties and hydrology are marginal for food crop production, but which farmers may be compelled to cultivate in order to meet their subsistence food requirements, green manures may have a realistic and applicable potential.