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 farming 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 and lentil rotation) in an attempt to improve nutrient availability and plant growth. Plant available pools (N and P) and N-mineralization rates 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 straw + fertilizer, straw and control plots. N-mineralization rate ranged from 2.7 to 24.2 mug g-1 month-1. The increase in nitrogen mineralization in straw + fertilizer treatment was 97% over the control. The rate of N-mineralization was greatest during the wet period of the year. It increased exponentially with increase in soil moisture content from 5-23%, indicating that the N-mineralization process is moisture limited in these drylands. At similar moisture content, straw + fertilizer treatment resulted in greater rate of N-mineralization compared to the control. Available P ranged from 7.9 to 17.7 mug P g-1 dry soil. From 26 to 69% increase in available P was found in straw + fertilizer treated plots. Both available P and mineralization rates were maximum in straw + fertilizer treated plots and minimum in control plots. In conclusion, straw + fertilizer application substantially increased available nutrient pools, and also the rate of N-mineralization, particularly in the wet period.