Enzyme activities and organic matter mineralization in response to application of gypsum, manure and rice straw in saline and sodic soils

Abstract

Saline and alkaline soils are a challenge for sustainable crop production. The use of organic and inorganic amendments is a common practice to increase the fertility of salt-affected soils that can trigger faster carbon (C) and nitrogen (N) cycling. We examined the effects of gypsum (Gyps), farm manure (Manure) and rice straw (Straw) on enzyme activities, organic matter mineralization and CO₂ emissions in two salt-affected soils [Solonchak (saline); pH: 8, electrical conductivity (EC): 6.5, sodium adsorption ratio (SAR): 2.5, and Solonetz (alkaline sodic); pH: 8.9, EC: 1.6, SAR: 17]. Gypsum addition decreased soil pH up to 0.62 and 0.30 units, SAR 1.2 and 5.2 units, and EC 2.9 and 1.4 units in Solonchak and Solonetz, respectively. Dissolved organic C, microbial biomass C, dissolved organic N, mineral N (NO₃^- and NH₄⁺), enzyme activities (urease, invertase, catalase, phosphatase, phenol-oxidase), alkali extractable phenols, and available phosphorous increased with the application of all amendments in both soils. Solonetz released more CO₂ than Solonchak, whereas maximum CO₂ emissions were common after manure application (3140 mg kg^-1 in Solonchak, and 3890 mg kg^-1 in Solonetz). We conclude that high SAR and low EC increase CO₂ emissions through accelerated C and N cycling and manure decomposition in Solonetz soils.

Keywords: Enzyme activities; Farm manure; Gypsum; Land use; Mineralization; Saline and alkaline soils; Soil amelioration.