Nitrogen mineralization of green manure legume residues in different soil types
Understanding nitrogen (N) mineralization patterns of green manure legume residues is crucial in the synchronization of N release from plant residue and uptake by plants. A laboratory incubation experiment was conducted over a 16-week period to determine N mineralization of three green manure legumes [(Mucuna (Mucuna pruriens), Lablab (Lablab purpureus cv. Rongai) and Sunhemp (Crotalaria juncea)] in three soils with varying clay contents (62, 20 and 12% clay) from South Africa. The amount of N mineralized from the residues was determined at 2, 4, 8 and 16 weeks from the onset of incubation. Nitrogen release pattern of the three legume residues followed a similar pattern in all the three soils, with sunhemp treated soil having the highest amount of mineral N after 16 weeks of incubation in all the three soils, followed by lablab and then mucuna. The amount of mineral N ranged from 121 to170, 96 to 134 and 92 to 108 mg kg-1 in the sunhemp, lablab and mucuna treated soils, respectively. The cumulative amounts of N from the legume residues mineralized recovered as mineral N in soil after 16 weeks of incubation ranged from 21-41% (92-121 mg kg-1), 30-68% (108-170 mg kg-1) and 26-60% (93-147 mg kg-1) of the initial added N contained in the residues in the soils with 62, 20 and 12% clay contents, respectively. Less than 50% of the initial added N was mineralized in the high clay content soil. Mineralization rate constant, k, was significantly linearly related to the residue N content, net mineralized N, C/N ratio and Lignin/N ratio. Results from this study indicate that all the three legumes can contribute significant amounts of N for uptake by plants, with sunhemp tending to release N at a faster rate, followed by lablab and then mucuna. High clay content in soil slowed down N mineralization.