EFFECTS OF SOIL AERATION, LEGUME RESIDUE, AND SOIL TEXTURE ON TRANSFORMATIONS OF MACRONUTRIENTS AND MICRONUTRIENTS IN SOILS

Soil aeration is an important factor controlling microbial activity, nutrient availability, and plant root growth. We investigated the effect of hairy vetch (Vicia villosa) addition to well aerated (60% water-filled pore space (WFPS)) and poorly aerated (90% WFPS) soils of varying texture on transformations of C, N, P, K, Ca, Mg, S, Zn, Mn, Fe, and Cu for a 30-d period. Of the total residue C recovered as CO2-C, 84 and 68% evolved in 0 to 10 d and 11 and 19% from 10 to 20 d at 60 and 90% WFPS, respectively, suggesting rapid decomposition of vetch residue. Of the total nutrients added to soil in vetch residue, significant apparent mineralization of N, P, K, S, and Zn were observed after the initial 20 d of aerobic incubation. Rapidly decomposing residue enhanced anaerobiosis in poorly aerated soils resulting in chemical reduction and a severalfold increase DTPA extractable Mn, Fe, and Cu after 20 d. After 30 d, reoxidation of these nutrients resulted in decreased DTPA extractable concentrations. At 90% WFPS, coarse soil texture retarded residue composition, and texture had a controlling influence on the time required for reoxidation of reduced Fe, Mn, and Cu in the order of SIC > SiL > SL. These results imply that incorporation of easily decomposable residue may supply substantial amounts of nutrients to growing plants. Where soils remain wet for > 10 d, residues may create intensive reducing conditions and a severalfold increase in available Fe, Mn, Cu, and P. Conversely, DTPA extractable Fe, Mn, and Cu can vary greatly within a few weeks depending on soil aeration status, residue management, time of sampling, and soil texture and may lead to erroneous indices of nutrient availability to plants.