Enzyme activities and microbial biomass in coastal soils of India

Soil salinity is a serious problem for agriculture in coastal regions, wherein salinity is temporal in nature. We studied the effect of salinity, in summer, monsoon and winter seasons, on microbial biomass carbon (MBC) and enzyme activities (EAs) of the salt-affected soils of the coastal region of the Bay of Bengal, Sundarbans, India. The average pH of soils collected from different sites, during different seasons varied from 4.8 to 7.8. The average organic C (OC) and total N (TN) content of the soils ranged between 5.2–14.1 and 0.6–1.4 g kg−1, respectively. The electrical conductivity of the saturation extract (ECe) of soils, averaged over season, varied from 2.2 to 16.3 dSm−1. The ECe of the soils increased five fold during the summer season (13.8 dSm−1) than the monsoon season (2.7 dSm−1). The major cation and anion detected were Na+ and Cl−, respectively. Seasonality exerted considerable effects on MBC and soil EAs, with the lowest values recorded during the summer season. The activities of β-glucosidase, urease, acid phosphatase and alkaline phosphatase were similar during the winter and monsoon season. The dehydrogenase activity of soils was higher in monsoon than in winter. Average MBC, dehydrogenase, β-glucosidase, urease, acid phosphatase and alkaline phosphatase activities of the saline soils ranged from 125 to 346 mg kg−1 oven dry soil, 6–9.9 mg triphenyl formazan (TPF) kg−1 oven dry soil h−1, 18–53 mg p-nitro phenol (PNP) kg−1 oven dry soil h−1, 38–86 mg urea hydrolyzed kg−1 oven dry soil h−1, 213–584 mg PNP kg−1 oven dry soil h−1 and 176–362 mg PNP g−1 oven dry soil h−1, respectively. The same for the non-saline soils were 274–446 mg kg−1 oven dry soil, 8.8–14.4 mg TPF kg−1 oven dry soil h−1, 41–80 mg PNP kg−1 oven dry soil h−1, 89–134 mg urea hydrolyzed kg−1 oven dry soil h−1, 219–287 mg PNP kg−1 oven dry soil h−1 and 407–417 mg PNP kg−1 oven dry soil h−1, respectively. About 48%, 82%, 48%, 63%, 40% and 48% variation in MBC, dehydrogenase activity, β-glucosidase activity, urease activity, acid phosphatase activity and alkaline phosphatase activity, respectively, could be explained by the variation in ECe of saline soils. Suppression of EAs of the coastal soils during summer due to salinity rise is of immense agronomic significance and needs suitable interventions for sustainable crop production.