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Phosphorus cycling in rainfed lowland rice ecosystems is poorly understood. Soil drying and grazing of rice straw during the
long dry season, the growth of volunteer pastures during the early wet season, and intermittent loss of soil-water saturation
while the rice crop is growing are important distinguishing characteristics of the rainfed lowlands in relation to P cycling.
We studied P cycling in an acid sandy rainfed lowland soil that covers about 30% of the rice growing area of Cambodia. Soils
with similar properties in comparable rainfed sub- ecosystems occur in Laos and northeast Thailand. We developed a general
schema of P pools and fluxes in the crop and soil for rice-based cropping systems in the rainfed lowlands of Cambodia. The
schema was derived from a number of field experiments carried out over five consecutive cropping seasons to quantify the residual
value of P fertiliser, P mass balances, soil P fractions, the effect on subsequent rice crops of crop residues and volunteer
pastures incorporated into the soils, and the dynamics of P turnover in the soil. With a single rice crop yielding 2.5–3 t
ha−1, application of 8–10 kg P ha−1 maintained yields and a small positive P balance in the soil. However, the soil P balance was sensitive to the proportion
of rice straw returned to the soil. Volunteer pastures growing during the early wet season accumulated significant amounts
of P, and increased their P uptake when soils were previously fertilised with P. These pastures recycled 3–10 kg P ha−1 for the succeeding rice crops. While inorganic soil P pools extractable with ion exchange resins and 0.1 M NaOH appeared
to be the main source of P absorbed by rice, microbial and organically-bound P pools responded dynamically to variation in
soil water regimes of the main wet, dry and early wet seasons. The schema needs to be developed further to incorporate site-specific
conditions and management factors that directly or indirectly affect P cycling, especially loss of soil-water saturation during
the rice cropping cycle. The paper discusses the application of results for acid sandy soils to other significant rice soils
in the rainfed lowlands of southeast Asia. 相似文献
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