Carbon and nitrogen mineralization from added organic matter in saline and alkali soils

The mineralization of N in salt-affected soils is a subject of much controversy because there are reports of salt induced non-biological ammonification. In laboratory experiments we studied the decomposition of green manure Sesbania in saline and alkali (sodic) soils. Carbon mineralization was reduced from 38.6 to 16.8% when electrical conductivity (ECe) in saline soils was 97 dS m−1. In alkali soils C mineralization was ∼38.0% irrespective of pH variation from 8.1 to 10.0. Ammoniacal-N accumulated up to ECe 70 dS m−1 beyond which it declined; nitrite+nitrate-N declined at ECe 16 dS m−1 and was not detectable at ECe≥26 dS m−1. Net N mineralized decreased from 351.2 mg kg−1 in the non-saline control soil (ECe 1.1 dS m−1) to 277 mg kg−1 at ECe 97 dS m−1 salinity and from 399.2 in control (pH 8.1) to 317 mg kg−1 at pH 10.0 in alkali soils. There was no NH3 volatilization in the saline soils and it was negligible in the alkali soils; most of the N lost was presumably due to denitrification. Protease, amidase (asparaginase, glutaminase) and deaminase (histidase) activities in soil were low but were stimulated by organic matter addition. Glutaminase activity was appreciable at high pH and deaminase at high salinity. The depression of ammonification at ECe 97 dS m−1 and the overall decrease in net N mineralization with increasing pH or salinity, and the significant enzymatic hydrolysis of organic N at high pH or salinity were all evidence of the biological nature of N mineralization in salt-affected soils. The adaptation of nitrifiers to salt stress at later stages of incubation, the nature of N losses and the reduction in losses with increase in pH or salinity also indirectly showed that N mineralization was biological and not chemical.