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Intensive systems with two or three rice (Oryza sativa L.) crops per year account for about 50% of the harvested area for irrigated rice in Asia. Any reduction in productivity or sustainability of these systems has serious implications for global food security. Rice yield trends in the world's longest-running long-term continuous cropping experiment (LTCCE) were evaluated to investigate consequences of intensive cropping and to draw lessons for sustaining production in Asia. Annual production was sustained at a steady level over the 50-y period in the LTCCE through continuous adjustment of management practices and regular cultivar replacement. Within each of the three annual cropping seasons (dry, early wet, and late wet), yield decline was observed during the first phase, from 1968 to 1990. Agronomic improvements in 1991 to 1995 helped to reverse this yield decline, but yield increases did not continue thereafter from 1996 to 2017. Regular genetic and agronomic improvements were sufficient to maintain yields at steady levels in dry and early wet seasons despite a reduction in the yield potential due to changing climate. Yield declines resumed in the late wet season. Slower growth in genetic gain after the first 20 y was associated with slower breeding cycle advancement as indicated by pedigree depth. Our findings demonstrate that through adjustment of management practices and regular cultivar replacement, it is possible to sustain a high level of annual production in irrigated systems under a changing climate. However, the system was unable to achieve further increases in yield required to keep pace with the growing global rice demand.

Rice (Oryza sativa L.), the main staple food in Asia, is typically produced on submerged anaerobic soils, which generally have slower decomposition of soil organic matter (SOM) than aerobic soils. We sampled four long-term experiments in the Philippines, with two or three rice crops grown each year with continuous or near-continuous soil submergence, to determine the effect of fertilizer management on long-term changes in soil C and N and on C and N balances. Soils were an Aquandic Epiaquoll, an Entic Pellustert, and a Typic Pelludert; soil pH ranged from 5.9 m 6.7. After 17 to 21 yr of continuous rice cultivation, the concentration of total soil organic C (SOC) and total soil N ([N.sub.T]) in the topsoil (0-20 cm) were greater with N-P-K fertilization than without fertilization. During 15 yr of additional continuous rice cropping, topsoil SOC and [N.sub.T] were consistently maintained or increased regardless of N-P-K fertilizer regime. Topsoil SOC increased up to 10% in an experiment with three rice crops per year and removal of all aboveground plant biomass after each crop. Subsoil SOC and [N.sub.T] (20-80 cm) were not affected by fertilization. The N balances indicated that biological [N.sub.2] fixation averaged 19 to 44 kg N [ha.sub.-1] [crop.sub.-1] across the four experiments. Anaerobic N mineralization (ANM) in the topsoil was maintained during 15 yr of continuous rice cropping with N-P-K fertilization in all four experiments. The results suggest that continuous cultivation of irrigated rice with balanced fertilization on submerged soils maintained or slightly increased SOM and maintained soil N-supplying capacity. Abbreviations: ANM, anaerobic N mineralization; BIARC, Bicol Integrated Agricultural Research Center; BNE biological dinitrogen fixation; LTCCE, long-term continuous cropping experiment; LTFE, long-term fertility experiment; [N.sub.T] total soil nitrogen; PhilRice, Philippine Rice Research Institute; POC, permanganate-oxidizable carbon; SOC, soil organic carbon; SOM, soil organic matter.

Concerns regarding the detrimental effects of burning crop residues on human health and the environment have increased interest in alternative uses of crop residues. We examined the in situ use of crop residue as a source of supplemental N for succeeding crops in rice (Oryza sativa L.)-based cropping systems at three sites during 3 yr. The experiments included a rice--wheat (Triticum aestivum L.) rotation at Yixing, Jiangsu province, China; a rice-wheat rotation at Ludhiana, Punjab, India; and double-rice cropping at Taojiang, Hunan Province, China. The supply of N from crop residues was assessed in the absence of fertilizer N as the difference in total plant N between plots with and without residue. At Yixing, incorporation of wheat residue before rice significantly increased the N supply to the rice by 14 kg N [ha.sup.-1] averaged across 3 yr. At Ludhiana, incorporation of rice residue before wheat reduced the N supply by 3 kg N [ha.sup.-1] to the wheat, but increased the N supply by 5 kg N [ha.sup.-1] to the rice crop following the wheat. In all cases, the return of crop residues had no net benefit on crop yield when fertilizer N was supplied at rates sufficient to eliminate N deficiency. The incorporation of crop residues did not increase the N supply to the succeeding crop during its vegetative growth phase, but the N supply to the crop at later growth stages was often increased. Adjustments in the timing and rate of fertilizer N are probably necessary to optimally supply N to crops receiving residues. Abbreviations: [AE.sub.N], agronomic efficiency for applied fertilizer nitrogen; DAS, days after sowing; DAT, days after transplanting; INS, indigenous nitrogen supply; LCC, leaf color chart.

Low molecular weight organic acids (OA), which are intermediates in the anaerobic decomposition of straw incorporated into submerged soils, have been implicated in causing toxicity to young rice (Oryza sativa L.) seedlings. The objective of this study was to develop a method for measuring OA in soil solution in field and greenhouse studies. Three methods of soil solution sampling were evaluated: zerotension displacement (ZTD), variable-tension centrifugation (CFG), and medium-tension suction sampling through porous tubes (PT). All solution samples were analyzed for OA by high performance liquid chromatography (HPLC) using an ion exclusion column. Use of cation exchange membrane resin strips during sample collection to sorb interfering cations improved OA recovery. Comparison between sampling methods of the OA concentration in soil solution extracted from soil amended with reagent OA was as follows: PT = CFG >> ZTD. Variables between methods that could cause artifacts in OA measurement were tested systematically: solution pH after sampling, headspace gas pressure during sampling, and sorption to sand or soil layers. No artifact effects were observed, and it was concluded that the tension methods sampled a different fraction of soil solution than the zeru-tension method. Since the tension methods extracted a higher concentration of OA, they are recommended for studies comparing treatment effects on OA production. However, it was beyond the scope of this study to determine which soil solution fraction is more relevant to OA toxicity studies with rice seedlings. Between the two tension methods, PT was more convenient for repeated sampling from the same location compared to the destructive nature of centrifugation.

The rotation of crops with planted [N.sub.2]-fixing legumes (improved fallows) is a promising agroforestry innovation for replenishing soil fertility in the tropics. We postulated that woody and herbaceous legumes with different rooting and growth patterns could be mixed in improved fallows to maximize utilization of belowground growth resources. We used a method of injecting a solution of [sup.15]N-labeled [(N[H.sub.4]).sub.2]S[O.sub.4] into soil at 0.15- and 1.0-m depths to measure soil mineral N acquisition by sesbania [Sesbania sesban (L) Merr], crotalaria [Crotalaria grahamiana Wight and Arn.], and the understory herbaceous legume siratro [Macroptilium atropurpureum (DC.) Urb.] grown in mixed stands on a Kandiudalfic Eutrudox soil in western Kenya. Crotalaria had the highest root length in the topsoil. Sesbania on the other hand had nearly half its total root length below 0.3 m at 0.3-to 1.5-m depth; sesbania took up more added [sup.15]N than crotalaria and siratro from the 1.0-m depth. Mixed sesbania and crotalaria stands, as compared with growing species in monocultures, increased root length at the 0.3- to 1.2-m depth. Sesbania mixed with siratro was more effective than sesbania mixed with crotalaria in uptake of [sup.15]N at 1.0-m depth but not at 0.15-m depth. At 2 mo after injection, the [sup.15]N was concentrated immediately below the injection point with little lateral movement. This confirmed the utility of the methodology in determining temporal N uptake for species in mixed stands. Our results suggest that opportunities exist for enhanced subsoil N retrieval through the mixing of leguminous species, which can influence root distribution and increase rooting in the subsoil.

The rotation of crops with either legumes or natural regrowth of native vegetation is known to supply N to crops in tropical land-use systems, but little is known about the effect of such rotations on P supply to crops. An experiment was conducted on two soils in Kenya, an Oxisol (very fine, isohyperthermic Kandiudalfic Eutrudox), and an Alfisol (mixed, isothermic Kandic Paleustalf), to determine the effect of unfertilized, organic-based land-use systems on fractions of soil inorganic P ([P.sub.i]) and organic P ([P.sub.o]). The land-use systems involved growth of three maize (Zea mays L.) crops (18 mo total duration) after 17 mo of either (i) Sesbania sesban (L.) Merr. tree growth (sesbania fallow), (ii) natural regrowth of vegetation without cultivation (natural fallow), (iii) three crops of unfertilized maize (maize monoculture), or (iv) bare uncultivated soil (bare fallow). Soil (0-15 cm deep) was collected before post-fallow maize and after the three post-fallow maize crops. Sesbania and natural fallows increased the amount of P in light fraction soil organic matter (OM) (>150 [[micro]meter],