Drying and wetting in saline and saline-sodic soils - effects on microbial activity, biomass and dissolved organic carbon

Aims: There are few studies on the interactive effect of salinity and sodicity in soils exposed to drying and wetting cycles. We conducted a study to assess the impact of multiple drying and wetting on microbial respiration, dissolved organic carbon and microbial biomass in saline and saline-sodic soils. Methods: Different levels of salinity (EC 1:5 1.0 or 2.5) and sodicity (SAR < 3 or 20) were induced by adding NaCl and CaCl 2 to a non-saline/non-sodic soil. Finely ground wheat straw residue was added at 20 g kg -1 as substrate to stimulate microbial activity. The constant moist (CM) treatment was kept at optimum moisture content for the length of the experiment. The drying and rewetting (DW) treatments consisted of 1 to 3 DW cycles; each DW cycle consisted of 1 week drying after which they were rewet to optimum moisture and then maintained moist for 1 week. Results: Drying reduced respiration more strongly at EC2.5 than with EC1.0. Rewetting of dry soils produced a flush in respiration which was greatest in the soils without salt addition and smallest at high salinity (EC2.5) suggesting better substrate utilisation by microbes in soils without added salts. After three DW events, cumulative respiration was significantly increased by DW compared to CM, being 24% higher at EC1.0 and 16% higher at EC2.5 indicating that high respiration rates after rewetting may compensate for the low respiration rates during the dry phase. The respiration rate per unit MBC was lower at EC2.5 than at EC1.0. Further, the size of the flush in respiration upon rewetting decreased with each ensuing DW cycle being 50-70% lower in the third DW cycle than the first. Conclusions: Both salinity and sodicity alter the effect of drying and rewetting on soil carbon dynamics compared to non-saline soils. © 2011 Springer Science+Business Media B.V.Manpreet S. Mavi, Petra Marschne