CHANGES IN MINERAL AND MINERALIZABLE N OF SOIL INCUBATED AT VARYING SALINITY, MOISTURE AND TEMPERATURE REGIMES

Abstract

A laboratory incubation experiment was conducted to study the effect of factors like moisture, salinity and temperature on the release of N in plant-available forms (NH4 and NO3+NO2-N) and potentially mineralizable N in soil over a period of 8 weeks following amendment with leguminous plant residues. In this experiment, soil samples salinized to ECe 7, 9, and 18 dS m-1 (original ECe was 5.0 dS m-1) were amended with 0.5% plant material of Sesbania aculeata and incubated at three moisture levels of 15, 30 and 45%, w/w and three temperature regimes of 20, 30 and 40oC for 8 weeks. Soil samples were sacrificed at 2, 4 and 8 weeks for the determination of NH4-N, NO3+NO2-N and mineralizable N. Ammonification of organic N as determined by the accumulation of NH4-N in soil was found to increase with time as salinity, moisture and temperature increased. However, the increase was more pronounced at higher moisture levels. While temperature had a positive effect on nitrification, increased salinity and moisture depressed the process. Net mineralization of N increased with time in all the treatments; the process being enhanced at higher incubation temperature with a maximum at 40oC. Salinity and high moisture had a depressing effect on the mineralization of N. The content of mineralizable N determined by NH4-N accumulation following 2 weeks of incubation under submerged conditions in soil remained higher under high moisture conditions, while high salinity and temperature had a variable and negative effect. Apparently, high moisture content conserved organic N due to reduced mineralization, while high temperature had an opposite effect. A complete loss of NO3-N was observed during incubation of soil samples for the determination of mineralizable N. This was attributable to denitrification as sufficient amount of easily oxidizable C was still present in the soil after 8 weeks of incubation under relatively aerobic conditions.