Nutrient Release Dynamics in Argentinean Pampean Soils Amended with Composts under Laboratory Conditions

Abstract

Composted manure is the greatest nitrogen (N) source in organic agriculture, where the use of chemical fertilizers, except the use of rock phosphate as P source, is not allowed. The aim of this study was to evaluate the effect of the soil type and compost characteristics on the potential rates of mineralization and on the kinetics of the nitrogen (N), phosphorus (P), and carbon (C) release patterns. For this, an aerobic incubation study was conducted over the course of 119 days at 25 °C using two different composts as amendments: a compost from biosolids and wood shavings (CRUB), and a compost from poultry manure with wheat straw (CSP). The composts were incorporated into two different soils of contrasting texture, sandy loam (S1) and silty clay loam (S2), at the dose of 160 kg N ha−1. Samples were collected eight times during the incubation period. The N and C dynamics were studied separately according to the different soil type. In both soils, a predominance of the nitrification process immediately after compost application was observed, showing a higher mineralization in the sandy loam soil, this trend being followed by a decrease and even an immobilization process, associated with stable organic materials such as compost. Moreover, both composts presented high contents of available P, but without exceeding 50 mg P kg−1, due to the slow release from the stable organic P fractions present in compost. The results obtained showed that despite the initial mineralization, both composts showed a significant period of N immobilization after their application into the pampean soils, which makes it necessary to anticipate their incorporation to avoid this lack period for crops. Furthermore, the application of these composts to the pampean soils implied an incorporation of available P, but with a slow release that avoided overfertilization, but also reducing N:P ratio and producing a relative deficit of N. Therefore, the results obtained suggested that it is necessary to study nutrient mineralization processes of the amendments prior to their use, according to the edaphoclimatic condition of each crop, in order to predict the release dynamics on soils and the fertilization potential to minimize potential environmental impacts.