Effects of moisture contents in incorporated residues and soil on net nitrogen mineralization in a laboratory study

Abstract

In Mediterranean climates, winters are mild, and decomposition of crop residues generally continues during the rainy winter season. The objective of this study was to determine whether letting crop residues dry before incorporation slows down net nitrogen (N) mineralization and reduces the risk of nitrate losses with rain during the winter months. Incubation experiments were performed with processing tomato (Solanum lycopersicum L.) and broccoli (Brassica oleracea L. var. italica) residues. Three residue moisture treatments, ranging from fresh to air-dry residues, and three soil moisture levels were included in the study. Net N mineralization was observed during the first week in the broccoli treatments, whereas tomato residues led to net N immobilization. Increasing moisture of incorporated residues increased net N mineralization or immobilization. An exception was the fresh broccoli residue, which resulted in a lower net N mineralization rate than the drier residues, likely due to anaerobic microsites created by high microbial activity. The moisture content of the residues only had a short-term effect on N turnover. Over the 12 wk of the incubations, the incorporation of both residues resulted in net N mineralization when compared with the unamended control. Nitrogen mineralization increased as soil moisture content increased and approached field capacity. These results indicate that residue moisture at the time of incorporation will not affect N mineralization beyond the initial weeks.