DECOMPOSITION AND NITROGEN DYNAMICS OF CROP RESIDUES - RESIDUE QUALITY AND WATER EFFECTS

Understanding environmental and residue influences on decomposition an d nutrient dynamics under diverse conditions is critical for efficient resource management. Our objective was to evaluate the influence of w ater on decomposition and N dynamics for surface and buried residues. Decomposition of alfalfa (Medicago sativa L.), grain sorghum (Sorghum bicolor [L.] Moench), and winter wheat (Triticum aestivum L. emend. Th ell.) residues in fiberglass bags on the surface or buried at 120 mm i n Pullman soil (fine, mixed, thermic Torrertic Paleustoll) at Bushland , TX, was measured from May 1990 to May 1991. A line-source sprinkler provided five water regimes (336, 287, 166, 60, and 5 mm) while precip itation provided 305 mm water. Decomposition coefficients (k) were gre ater for alfalfa than for wheat or grain sorghum and were greater for buried than for surface residues. Rate coefficients increased linearly with water applied. The increase was greater for alfalfa than for sor ghum and wheat. Net N mineralization occurred from alfalfa residues th roughout the study. Net N immobilization was longer than 1 yr for surf ace wheat and sorghum and about 0.33 yr for buried residues. Both N(ma x) (grams N immobilized per kilogram of original biomass) and N(eqv) ( grams N immobilized per kilogram of biomass loss) were influenced by c rop and placement but not water regime. The N(max) value was similar f or surface wheat and sorghum residues but was 50% lower for buried whe at than for sorghum. The N(eqv) indicated the N requirement of microor ganisms was less for buried than for surface residues. Water and resid ue quality interactions affecting decomposition and N dynamics should be considered in residue management strategies for soil protection and nutrient cycling.