Hadas, A., Dept. of Land, Air/Water Rsrc., University of California, Davis, CA 95616, United States, Inst. of Soil, Water/Environ. Sci., A.R.O., Volcani Center, Bet Dagan 50250, Israel
Doane, T.A., Dept. of Land, Air/Water Rsrc., University of California, Davis, CA 95616, United States
Kramer, A.W., Department of Agronomy, University of California, Davis, CA 95616, United States
Van Kessel, C., Department of Agronomy, University of California, Davis, CA 95616, United States
Horwath, W.R., Dept. of Land, Air/Water Rsrc., University of California, Davis, CA 95616, United States

The rate at which available nitrogen (N) is released from organic materials in soil is often measured by applying 15N and following its recovery by the growing crop. However, the turnover of labelled N in soil modifies the ratio of labelled to unlabelled available N and thereby affects the uptake of 15N by plants. The recovery of labelled N by maize was measured in a field experiment under three management systems, with one 15N-labelled input in each: (1) conventional, with fertilizer side dressing, (2) low input, with vetch as a cover crop and fertilizer side dressing, and (3) organic, with vetch and composted manure. The NCSOIL model, which simulates C and N turnover in soil, was modified to include relevant processes related to the maize crop, and used to estimate the decomposition rate constant of vetch in the field by optimizing the simulated dynamics of labelled N uptake by maize against the measured results. A large input of C from mineralizable soil organic matter and root deposition was necessary to account for the recovery of fertilizer N by maize. Optimization of labelled N recovery in the low input system resulted in two optional rate constants for the decomposition of vetch: rapid decomposition (0.4 day-1) of a labile vetch pool (49% of total vetch N), or slow decomposition (0.008 day-1) of a single vetch pool. In the simulated organic system, where manure and vetch were incorporated at the same time, only a rapid decomposition of the labile component of vetch accounted well for the recovery of vetch N by maize. The prolonged recycling of N mineralized from the vetch, and its mixing with fertilizer side dressing in the low input system, reduced the recovery of vetch N even though it was mineralized rapidly. This demonstrates the difficulty in assessing the availability of N from organic materials.