NITROGEN AND PHOSPHORUS AVAILABILITY AND MINERALIZATION IN DRYLAND REDUCED TILLAGE CULTIVATION - EFFECTS OF RESIDUE PLACEMENT AND CHEMICAL FERTILIZER

Proper residue management may help augment soil fertility in dryland f arming systems in the developing countries. We report on the effect of residue placement and chemical fertilizer on available nutrient pools (N and P) and N-mineralization in dryland reduced tillage cultivation . Four treatments (control, fertilizer, straw, and straw + fertilizer) were initiated in June 1990 under dryland farming conditions (rice an d lentil rotation) in an attempt to improve nutrient availability and plant growth. Plant available pools (N and P) and N-mineralization rat es were measured periodically. Available N ranged from 7.0 to 25.5 mug N g-1 dry soil, maximum values were found for the fertilizer treated plots (45-66% greater than control) followed in decreasing order by st raw + fertilizer, straw and control plots. N-mineralization rate range d from 2.7 to 24.2 mug g-1 month-1. The increase in nitrogen mineraliz ation in straw + fertilizer treatment was 97% over the control. The ra te of N-mineralization was greatest during the wet period of the year. It increased exponentially with increase in soil moisture content fro m 5-23%, indicating that the N-mineralization process is moisture limi ted in these drylands. At similar moisture content, straw + fertilizer treatment resulted in greater rate of N-mineralization compared to th e control. Available P ranged from 7.9 to 17.7 mug P g-1 dry soil. Fro m 26 to 69% increase in available P was found in straw + fertilizer tr eated plots. Both available P and mineralization rates were maximum in straw + fertilizer treated plots and minimum in control plots. In con clusion, straw + fertilizer application substantially increased availa ble nutrient pools, and also the rate of N-mineralization, particularl y in the wet period.