Changes in the soil N supply and organic matter of irrigated lowland rice soils with increased aeration: a progress report
1997
Belleza, E.B. | Olk, D.C. (International Rice Research Inst., P.O. Box 933, Manila (Philippines). Agronomy, Plant Physiology and Agroecology Div.)
Soil organic matter (SOM) plays a pivotal role in soil N cycling, but it is especially important in lowland rice soils, where 25-80 percent of the N taken up by the crop has cycled through SOM. Grain yields have declined by 35-40 percent in fields supporting continuous double- and tripple- cropped irrigated rice for 20-30 yr. It was hypothesized that improved soil aeration will reverse changes in SOM chemistry and soil N availability that may be linked to the declining grain yields. Any relationships between crop N uptake, mineralizable SOM-N, and the quantity and chemical nature of SOM as affected by soil aeration are being studied. Much of this work has occurred in a previously double-cropped irrigated rice fields on the IRRI farm, which for the last 4 yrs has supported wet season (WS) rice and dry season main plot treatments of irrigated rice and upland maize. Annual aeration during the maize crop has resulted in gradually increasing effects on soil N cycling during successive WS rice crops. Some properties of the SOM have also evolved gradually with annual soil aeration, even while the amounts of SOM and visible light absorption by extracted humic acid fractions change cyclically by cropping season. In general, the SOM appears to be losing its capacity to buffer soil N availability: by the fourth WS in the minus N fertilizer control treatment, the rice-maize system had 12 percent lower grain yield than did the rice-rice system. Future studies will monitor the continuing evolution of soil N supply with regular soil aeration and will quantify the soil N buffering capacity, changes in the structural chemistry of the SOM, and rates of incorporation into the SOM of rice and maize crop residues
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