Nitrogen mineralization rates and kinetics in soils freshly amended with green manures

Long term incubation studies to determine the nitrogen (N) mineralization rates and kinetics in soils freshly amended with some commonly used green manures such as Sesbania rostrata, Gliricidia maculata, Leucaena leucocephala and Azolla pinnata are scarce. A long term aerobic study was, therefore, conducted by incubating soils freshly amended with the above-mentioned green manures in PVC columns at 35 +/- 1 degrees C and with 0.01 Mpa moisture content. The soils were then leached at periodic intervals for up to 36 weeks. The N-mineralization rates were greatest during the first week and decreased with time in all soils. The green manure amended soils leached 247 mg kg(-1) more NO3 + NO2 - N than the unamended control. In general, the total N mineralized (mean 61%) was almost twice that of net N mineralized (mean 30%) in the amended soils. The percent N mineralized (total and net), however, varied with the nature of green manure incorporated into the soil. It was greatest in the soil amended with sesbania and lowest in the soil amended with azolla. The kinetic parameters derived using the double exponential model indicated that green manure amended soils possessed significantly higher N-mineralization potentials and rate constants compared to the unamended control. The kinetic parameters also varied with the nature of green manure incorporated into the soil. Among the various parameters lignin content, lignin to N ratio and lignin + polyphenol to N ratio of the green manures were the key factors governing the rate of decomposition and subsequent N mineralization from the amended soils.