MICROBIAL DYNAMICS IN SALT AFFECTED SOILS

Abstract

Soil salinity is a worldwide problem which not only influences the physical and chemical properties of soil but may also seriously affect the microbiological properties of soil. This project was undertaken to assess the behavior of various microbiological properties of soil in relation to salinity in a series of incubation and pot experiments during 2009-12. Initially the soil microbiological properties of thirty naturally occurring diverse salt affected soils were determined. Based on the results of preliminary experiments, further experiments were conducted to evaluate the effect of NaCl and MgCl 2 induced salinity on N dynamics and soil microbial activity in soil. Finally, a pot experiment was conducted to assess the effect of important amendments such as pressmud, gypsum and farmyard manure, which are normally used for the reclamation of salt affected soils, on growth of wheat and maize and on various microbial attributes in a highly saline-sodic soil (EC 20.3 dS m -1 ). In addition, some bacteria and fungi were isolated from a highly saline-sodic soil, and the effects of same organisms were also tested in the above experiment. For experiment 1, soil samples at 0-20 cm were collected from various salt affected soils ranged in salinity from EC < 4.0 to 32 dS m -1 in November 2009. The samples were analyzed for soil microbiological (such as microbial biomass-C, microbial biomass-N, N mineralization, nitrification, rate of soil respiration) and chemical properties (such as pH, EC, soluble Ca+Mg, soluble Na, SAR (sodium adsorption ratio), ESP (exchangeable sodium percentage), carbonates, bicarbonates, chloride). The results showed that almost all microbial indices including microbial biomass-C (MBC), microbial biomass-N (MBN), basal soil respiration, nitrification and net N mineralization were negatively correlated with increasing salinity (r = -0.89, -0.74, - 0.79, -0.64 and -0.57 respectively). The results further showed that the depressive effects of SAR and ESP on various soil microbial indices were much stronger than that of carbonates and bicarbonates. Both NaCl and MgCl 2 induced salinity significantly reduced the rate of CO 2 evolution and N mineralization during all xiiiincubation periods. The depressive effects increased with increase in salts level. The NaCl induced salinity depressed cumulative CO 2 production by about 80% and N mineralization by 50% during 40 days of incubation. Whereas MgCl 2 induced salinity decreased cumulative CO 2 production by 95% and N mineralization by 81% during 30 days of incubation. These results suggested that the impact of MgCl 2 salinity on microbial indices was stronger than that of NaCl. The amendment of saline-sodic soil with pressmud, gypsum and farmyard manure (FYM) significantly improved the rate of CO 2 evolution, N mineralization, microbial biomass-C and microbial biomass-N. The effects were more pronounced with combined application of pressmud with farmyard manure or gypsum. It was also evident that the effect of pressmud + FYM was further improved with microbial inoculation of microbial isolation from highly salt affected soil. Organic matter decomposition generally increases the production of CO 2 and liberation of H + ions. The H + ion enhance the dissolution of CaCO 3 and releases more Ca to replace Na from clay particles and thus helps in the reclamation of saline-sodic soils. Both wheat and maize seeds failed to germinate under the prevailing saline-sodic conditions despite repeated re-seeding and thus no data was generated in this respect. These results suggested that all microbial indices decreased significantly with increasing salinity levels indicating that soil microorganisms were sensitive to different types of salinity. Thus, salinity is a stress factor and can reduce microbial diversity and control microbial abundance, composition and functions. Although, amendments of saline-sodic soil with organic sources (such as FYM, pressmud) substantially improved microbial attributes they did not enable the germination and survival of wheat and maize sown in the soil. Nevertheless the evidence suggests that organic matter may play significant role in the amelioration of saline-sodic or sodic soils.