Decomposition and nitrogen mineralization of <em>Gliricidia sepium</em> leaf green manure under diverse nutrient management strategies in irrigated lowland rice cropping systems in Sri Lanka

Abstract

The utilization of green manure has become an essential strategy for enhancing soil fertility and substituting synthetic fertilizers. The decomposition and release of nutrients through mineralization processes are influenced by soil microbial activity, which in turn depends on the nutrient management practices employed. depending on nutrient management approaches. The study aimed to investigate the decomposition and N mineralization of Gliricidia sepium leaf green manure in an irrigated lowland rice ecosystem under different nutrient management systems (NMS), for a duration of eight weeks. Litterbags were placed at two soil depths, namely the topsoil, and subsoil, within three NMS; 100% Department of Agriculture (DOA) recommended inorganic fertilizer application (CNM), 50% of DOA recommended inorganic fertilizer combined with the organic fertilizer application (INM), and 100% organic fertilizer application (ONM). The treatments were arranged in a randomized complete block design and litterbags were retrieved weekly to determine dry mass, total N content, and total organic carbon content remaining in the leaves. The rate of decomposition and nutrient release was estimated using an exponential decay equation and analysis of variance was carried out using the repeated measures MIXED model. A consistent biphasic pattern of decomposition and mineralization was observed across all NMS treatments and soil depths, characterized by an initial rapid phase followed by a slower phase. However, significant differences (p<0.05) were observed among the treatments. The ONM treatment combined with topsoil placement exhibited a shorter time for 50% and 80% reduction in biomass and nitrogen content. Approximately 80% of the nitrogen content was released into the soil solution within 4-5 weeks of green manure incorporation, regardless of the nutrient management system employed. These findings suggest that with proper application rates and appropriate depth of placement, Gliricidia sepium green manure can effectively replace a significant portion of mineral nitrogen fertilizer, thereby enhancing nutrient use efficiency in lowland rice-based systems.