The potentials for sequential reduction of inorganic electron acceptors and production of methane have been examined in sixteen rice soils obtained from China, the Philippines, and Italy. Methane, CO2, Fe(II), NO3(-), SO4(2-), pH, Eh, H2 and acetate were monitored during anaerob incubation at 30 øC for 120 days. Based on the accumulation patterns of CO2 and CH4, the reduction process was divided into three distinct phases: (1) an initial reduction phase during which most of the inorganic electron acceptors were depleted and CO2 production was at its maximum, (2) a methanogenic phase during which CH4 production was initiated and reached its highest rate, and (3) a steady state phase with constant production rates of CH4 and CO2). The reduction phases lasted for 19 to 75 days with maximum CO2 production of 2.3 to 10.9 mu mol d(exp -1) g(exp -1) dry soil. Methane production started after 2 to 87 days and became constant after about 38-68 days (one soil > 120 days). The maximum CH4 production rates ranged between 0.0 1 and 3.08 mu mol d(exp -1) g(exp -1). During steady state the constant CH4 and CO2 production rates varied from 0.07 to 0.30 mu mol d(exp -1) g(exp -1) and 0.02 and 0.28 mu mol d(exp -1) g(exp - 1) respectively. Within the 120 d of anaerobic incubation only 6-17% of the total soil organic carbon was released into the gas phase. The gaseous carbon released consisted of 61-100% CO2 <0.1-35% CH4, and <5% nonmethane hydrocarbons. Associated with the reduction of available Fe(III) most of the CO2 was produced during the reduction phase. The electron transfer was balanced between total CO2 produced and both CH4 formed and Fe(III), sulfate and nitrate reduced. Maximum CH4 production rate (r = 0. 89 1) and total CH4 produced (r = 0.775) correlated best with the ratio of soil nitrogen to electron acceptors. Total nitrogen content was a better indicator for "available" organic substrates than the total organic carbon content. The redox potential was not a good predictor of potential CH4 production. These observations indicate that the availability of degradable organic substrates mainly controls the CH4 production in the absence of inorganic electron acceptors

SUMMARYMeasurements of profiles of ferrous and ferric iron and pH in blocks of reduced soil in contact with planar layers of rice {Oryza sativa L.) roots are reported. Initially 11-d-oId plants were kept in contact with the soil for up to 12 d. Over this period, substantial quantities of iron were transferred towards the root plane, producing a welldefined zone of ferric hydroxide accumulation. The pH in this zone fell by more than two units. The profiles changed with time. The decrease in pH was in part due to protons generated in ferrous iron oxidation, and in part due to protons released from the roots to balance excess intake of cations over anions, N being taken up chiefly as NH/. But the decrease in pH was less than expected from the net acid production in these two processes, possibly because of proton consumption in CO., uptake by the roots. Because of the pH-dependence of soil acidity diffusion, the two sources of acidity greatly reinforce each other. Some implications for nutrient and toxin dynamics are discussed.

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Methane and N2O are gases that are several times more radiatively active than CO2. It is well known that flooded rice (Oryza sativa L.) soils are a globally important source of atmospheric CH4. Mitigation strategies for CH4 flux, such as mid‐season drainage, might have the opposite effect on N2O emissions. An automated chamber system at the International Rice Research Institute in the Philippines measured CH4 and N2O fluxes from flooded rice and fallow rice fields essentially 24 h a day between December 1992 and April 1994. This period included two irrigated dry rice‐growing seasons (DS) and one wet rice‐growing season (WS). Nitrous oxide fluxes were generally barely detectable during the growing seasons, but small peaks (maximum 3.5 mg N2O‐N m‐2 d‐1) appeared after N fertilizer applications. Methane fluxes, on the other hand, were evident throughout the rice‐growing seasons. Organic matter additions as straw (5.5 t ha‐1, dry) or green manure (GM; Sesbania rostrata L.; 12 t ha‐1, wet) stimulated CH4 flux severalfold. Seasonal CH4 flux with ammonium sulfate (AS) was one‐fourth to one‐third the flux with urea. During the DS, however, the seasonal N2O flux was 2.5 times higher with AS than with urea. Mid‐season drainage (2‐wk duration) at either mid‐tillering or panicle initiation was very successful in suppressing CH4 flux up to 60%. However, N2O flux increased sharply during the drainage period at mid‐tillering until reflooding, when it dropped back to near zero.

they do not cause specific symptoms on the aboveground parts of the plant, they can cause severe growth reduction, chlorosis, wilting of plants, and 20-70% yield reduction in infested fields. M. graminicola, the rice root-knot nematode, is widely distributed in rainfed upland and lowland ricefields in South and Southeast Asia, especially in lighttextured soils. It is considered to be an important pest in rainfed lowland areas in India and northeast Thailand.

Six soils used for rice (Oryza sativa L.) production were incubated using an automatic microcosm system. Production of trace gases (CO2, CH4, and N2O) and transformation of N, S, and metals (Fe and Mn) were studied in soil suspensions incubated from reducing to oxidizing conditions. Results show that soil pH variation was inversely correlated to soil redox potential (EH) change (P < 0.01). Soil CO2 production exponentially increased with soil EH increase. In contrast, soil CH4 production and DOC showed an exponential decrease with soil EH increase. Without the presence of soil oxidants, methanogenesis occurred across the entire EH range, with probable H2–supported methanogenesis at higher soil EH conditions constituting up to 200f total CH4 production. The CH4 compensation point, where CH4 concentration became constant due to equilibrium between CH4 production and consumption, exponentially decreased with soil EH increase. At pH 7, the critical EH above which soils consumed atmospheric CH4 varied among the soils, but was generally >400 mV. Significant N2O production was observed between 200 and 500 mV. Nitrification could also contribute to N2O production when EH is >500 mV, a possible critical EH for the initiation of nitrification. The critical EH for substantial immobilization of Fe and Mn was estimated to be around 50 and 250 mV, respectively. The intermediate EH range (approximately −150 to 180 mV) provided optimum conditions for minimizing cumulative global warming potential resulting from CO2, CH4, and N2O production in soils. Our results have implications in interpreting the overall benefits of soil C sequestration efforts.

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Limited information is available on the dynamics of dissolved organic C (DOC) and its relationship with CH4 emissions in flooded rice (Oryza sativa L.) soils as affected by rice cultivar. Greenhouse and laboratory experiments were conducted to determine root C release in culture solution, DOC and dissolved CH4 concentration in soil solution, and CH4 emission in a flooded soil planted with three rice cultivars. Soil solutions were sampled in the root zone (soil surrounding rice roots) and the non‐root zone (soil outside the root zone). The release of root exudates increased in the order: IR65598 (new plant type) < IR72 (modern cultivar) < Dular (a traditional cultivar). Correspondingly, DOC concentrations in the root zone and CH4 emission rates increased. The dynamics of DOC and dissolved CH4 differed greatly between the root zone and the non‐root zone. Dissolved organic C in the root zone increased with plant growth and reached maximum (13–24 mmol C L−1) between rice flowering and maturation (Week 11–13), whereas DOC in the non‐root zone remained low (1–5 mmol C L−1) throughout the growing season. Similarly, dissolved CH4 concentrations in the root zone increased sooner and were greater (mean 138 μmol CH4 L−1) than those in the non‐root zone (mean 97 μmol CH4 L−1). The seasonal patterns of CH4 emissions closely followed the dynamics of DOC concentrations in the root zone. The results suggest that (i) DOC pool in the root zone of rice plants is enriched by root‐derived C; (ii) the rates of CH4 emissions are positively correlated with the dynamics of DOC in the root zone; (iii) the intercultivar difference in root C releases is responsible for the intercultivar difference in DOC production, and consequently in CH4 flux.