Nitrogen-15 recovery in soil incubated with potassium nitrate and clover residues

In the southeastern USA, legumes are used as green manure to meet crop N requirements of a following crop, but recovery is usually lower than from conventional fertilizers. We conducted a laboratory study for 26 wk under aerobic conditions to monitor recovery of N-15-labeled KNO3 (fertilizer) and crimson clover (Trifolium incarnatum L.) residues (clover) in organic. inorganic, and microbial biomass N pools. Volatilization of NH3 from decomposing clover residues was monitored for 12 wk. Three hundred-gram samples of a Typic Kanhapludult soil were amended with N (0.051 mg N kg(-1) dry soil in fertilizer and 0.049 mg N kg(-1) dry soil in clover). A control with no N was also included. Although inorganic N (NH4, NO2, and NO3) accumulated throughout (fertilizer > clover > control), the rate of accumulation did not differ among treatments. Organic and microbial biomass N concentration did not differ among treatments, but applied N recovery in microbial biomass was greater in clover than fertilizer (P < 0.05) throughout. Ammonia volatilized was negligible. After 26 wk, applied N recovered in soil inorganic N was 66% for fertilizer and 40% for clover; in soil organic N, 18% for fertilizer and 50% for clover; and in microbial biomass N, 0.75% for fertilizer and 1.5% for clover. Applied N presumed denitrified was 16% in fertilizer and 10% in clover. We concluded that clover green manure can meet the N requirements of a following crop from the time of emergence in the southeastern USA.