Graduate Faculty of North Carolina-Phd Thesis-Tracing Cropped and Uncropped Soil-Ak Muruki 2000

Abstract

In developed countries, low cost and easy availability of nitrogen (N) fertilizers has led to excessive use. In developing countries peasant farmers rely on biological N fixation to improve soil fertility because fertilizers can be prohibitively expensive and out of reach. Legumes are sometimes used as green manure in southeastern USA, but recovery of legume N is usually lower than from conventional fertilizers. To investigate how well applied N from an organic and inorganic N source could be accounted for in a cropped and uncropped soil, 15N-Iabeled KN03 (F) and clover residues (CI) were applied to samples of a Typic Kanhapludult soil. Nitrogen in the microbial biomass, inorganic, organic and potentially mineralizable soil N pools (by anaerobic incubation) was monitored for 26 wk. Ammonia volatilization from decomposing clover residues was monitored for 12 wk. Inorganic N (NH/, N02- and N03-) accumulated and increased in the order F>Cl>control (C). Organic N concentration did not differ among treatments. After 26 wk, applied N recovered in soil inorganic N was F: 66%, CI, 40%; in soil organic N, F: 18% and CI: 50%; and in microbial biomass N, F: 0.75% and CI: 1.5%. Clover residues mineralized rapidly, because of their low carbon: N ratio (9: 1), but K 15N0 3 addition resulted initially in immobilization. Ammonia volatilization was negligible. We assumed that applied N not found in the measured N pools was denitrified: F, 16% and CI, 10%. The anaerobic incubation-extraction procedure