Dry season soil conditions and soil nitrogen availability to wet season wetland rice

A pot experiment with Maahas clay soil covered three consecutive crops. After uniform growth of the first crop, the soils were subjected to different moisture conditions during the dry season. Prolonged drying before wet season flooded rice stimulated increased release of mineral nitrogen but moistening of the dry soil for a dryland crop or by occasional rain during the dry season reduced nitrogen use from the soil in the next wet season. One cycle of alternate wet and dry soil preparation for 20 days before transplanting rice improved soil nitrogen availability and plant uptake of fertilizer nitrogen.

The initial growth of rice was retarded after flooding the previously moist dryland or dried soil, but not in the continuously flooded soils.

Losses of applied nitrogen were small in continuously flooded soils and were greater in the previously moist dryland and dry treatments. Uptake of soil nitrogen, however, was much higher in the air-dried soil treatment and in the dry with alternate wet and dry preparation treatments. Total nitrogen uptake (soil+fertilizer) was also greater in those dry treatments. Uptake of soil nitrogen in the wet-season crop was roughly proportional to the amounts of ammonia measured just before transplanting.

The proportion of the uptake of immobilized fertilizer nitrogen to available soil nitrogen was constant among treatments. Release of immobilized fertilizer nitrogen was also greatly enhanced by soil drying. For 1976 wet-season crop, the availability of fertilizer nitrogen immobilized in the 1975 wet season was three times higher than that of native soil nitrogen.