NITROGEN AND PHOSPHORUS TRANSFORMATIONS AS AFFECTED BY CROP RESIDUE MANAGEMENT-PRACTICES AND THEIR INFLUENCE ON CROP YIELD

A 15-year field experiment investigated crop residue management practi ces, with crop residue removal, burning and incorporation as the main treatments and nitrogen levels as subtreatments. The effects of crop r esidue management practices on rice and wheat yield were measured for 11 years. Surface soil samples were taken to study nitrogen and phosph orus immobilization/adsorption and their release under laboratory cond itions. The field experiment indicated that residue burning and residu e removal resulted in greater grain yields of rice (5.57 and 5.53 t/ha , respectively) and wheat (4.12 and 4.02 t/ha, respectively) than resi due incorporation (4.51 t/ha rice and 3.72 t/ha wheat). Laboratory exp eriments indicated that by the addition of crop residues nitrogen and phosphorus were converted to unavailable forms through immobilization and adsorption, respectively. Crop residue management practices were d iscontinued after 13 years and wheat and maize crops were grown in seq uence. There were significantly greater yields of wheat (3.57 t/ha in 1992-93 and 3.6 t/ha in 1993-94) and of maize (2.1 t/ha in 1993) in pl ots where the residues had previously been incorporated than where the residues were previously either removed or burned. This is attributed to release of nitrogen and phosphorus from the incorporated residues.