Nutrient availability of composted and noncomposted residues in a Patagonian Xeric Mollisol

The main objectives of this study were to determine: (1) the agronomic valu e of composted and noncomposted residues originated in the Andean-Patagonia n region (APR), and (2) whether aerobic incubations provide a reliable inde x of nutrient availability to estimate application rates. A Xeric Mollisol of the transitional zone between the APR and the Patagonian steppe, amended with composted and noncomposted residues, was employed in laboratory incub ations without plants and in a 5-month greenhouse trial with ryegrass. Nonc omposted residues were biosolids and fish wastes (FW) collected under farmi ng cages. Composts were obtained from fish offal and biosolids mixed with s awdust, woodshavings and yard trimmings. A commercial compost was also incl uded. In laboratory incubations at similar rates of application (10 g kg(-1 )), net N mineralization (N-min) was about two-fold higher with the noncomp osted (17-23%) than with the composted residues (0-12%) and P release was v ery high with the FW. Ryegrass yields were closely related to the total Kje ldahl N and mineralized N of the organic residues and weakly related to the ir P contents (total or extractable). Yields were increased even in the cas e of composts which exhibited very low values of N-min land slight N immobi lization) during laboratory incubation, suggesting that the presence of pla nts enhanced N-min at rates of 10-20 g kg(-1). Soil residual extractable P after ryegrass removal was high for fish-derived amendments and the commerc ial product, suggesting a potential risk of P pollution when these are appl ied according to N requirements. Aerobic incubations provided a relatively good index with which to assess adequate rates of application.