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Changes in mineral and mineralizable N of soil incubated at varying salinity, moisture and temperature regimes

ASMA Lodhi^1*, M. Arshad², F. Azam³ AND M.H. Sajjad¹

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 EC_e 7, 9, and 18 dS m^-1 (original EC_e 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 40^oC 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 40^oC. 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.

¹Nuclear Institute for Agriculture and Biology (NIAB), Faisalabad, Pakistan

2Department of Botany, University of Agriculture, Faisalabad, Pakistan

³ Nuclear Institute for Food and Agriculture (NIFA), Peshawar, Pakistan