Integrated Impacts of Soil Salinity and Drought Stresses on the Decomposition of Plant Residues

Soil salinity and drought are major environmental challenges that significantly affect soil functioning and soil organic matter (SOM) decomposition. Despite their importance, the combined effects of drought and salinity on residue decomposition are not well understood. This study addresses this gap by evaluating the decomposition of maize residue under salinity and drought stresses over a 75-day incubation period at 20 °C under controlled conditions. The experiment included two moisture levels: optimum moisture at 80% water-holding capacity (WHC) and drought conditions at 30% WHC, in both normal (ECe = 1.48 dS m −1 ) and saline (ECe = 8 dS m −1 ) soils, with 5 g DM kg −1 soil maize residues mixed in. A control treatment without maize residue addition was also included. The results indicated that salinity stress reduced maize residue decomposition, as evidenced by lower soil respiration, decay constant, metabolic quotient (qCO 2 ), and soil extracellular enzyme activities. While drought did not affect total soil respiration in the presence of maize residue, it significantly decreased soil extracellular enzyme activities and decay constant rates. Combined drought and salinity stress further diminished maize residue decomposition, marked by reduced soil respiration, decay constant, microbial biomass carbon, and soil extracellular enzyme activities, while dissolved organic carbon (DOC) and qCO 2 increased significantly. Similarly, extracellular enzyme activities were significantly reduced under abiotic stresses and further diminished under combined stress conditions. In conclusion, the simultaneous occurrence of drought and salinity can have compounded detrimental effects on microbial functioning, particularly in the presence of fresh plant residues.