A SIMPLE-MODEL FOR ESTIMATING THE CONTRIBUTION OF NITROGEN MINERALIZATION TO THE NITROGEN SUPPLY OF CROPS FROM A STABILIZED POOL OF SOIL ORGANIC-MATTER AND RECENT ORGANIC INPUT

A simple model was developed to estimate the contribution of nitrogen (N) mineralization to the N supply of crops. In this model the soil organic matter is divided into active and passive pools. Annual soil mineralization of N is derived from the active pool. The active pool comprises stabilized and labile soil organic N. The stabilized N is built up from accumulated inputs of fresh organic N during a crop rotation but the labile N is a fraction of total N added, which mineralizes faster than the stabilized N. The passive pool is considered to have no participation in the mineralization process. Mineralization rates of labile and stabilized soil organic N from different crop residues decomposing in soil were derived from the literature and were described by the first-order rate equation dN/dt = -K*N, where N is the mineralizable organic N from crop residues and K is a constant. The data were grouped K-1 by short-term (0-1 year) and K-2 by long-term (0-10 years) incubation. Because the range of variation in K-2 was smaller than in K-1 we felt justified in using an average value to derive N mineralization from the stabilized pool. The use of a constant rate of K-1 was avoided so net N mineralization during the first year after addition is derived directly from the labile N in the crop residues. The model was applied to four Chilean agro-ecosystems, using daily averages of soil temperature and moisture. The N losses by leaching were also calculated. The N mineralization varied between 30 and 130 kg N ha(-1) yr(-1) depending on organic N inputs. Nitrogen losses by leaching in a poorly structured soil were estimated to be about 10% of total N mineralized. The model could explain the large differences in N- mineralization as measured by the potential N mineralization at the four sites studied. However, when grassland was present in the crop rotation, the model underestimated the results obtained from potential mineralization.