Salinity and sodicity affect soil respiration and dissolved organic matter dynamics differentially in soils varying in texture

Abstract

The individual effects of salinity and sodicity on organic matter dynamics are well known but less is known about their interactive effects. We conducted a laboratory incubation experiment to assess soil respiration and dissolved organic matter (DOM) dynamics in response to salinity and sodicity in two soils of different texture. Two non-saline non-sodic soils (a sand and a sandy clay loam) were leached 3–4 times with solutions containing different concentrations of NaCl and CaCl₂ to reach almost identical electrical conductivity (EC₁:₅) in both soils (EC₁:₅ 0.5, 1.3, 2.5 and 4.0 dS m⁻¹ in the sand and EC₁:₅ 0.7, 1.4, 2.5 and 4.0 dS m⁻¹ in the sandy clay loam) combined with two sodium absorption ratios: SAR < 3 and 20. Finely ground wheat straw residue was added (20 g kg⁻¹) as substrate to stimulate microbial activity. Cumulative respiration was more strongly affected by EC than by SAR. It decreased by 8% at EC 1.3 and by 60% at EC 4.0 in the sand, whereas EC had no effect on respiration in the sandy clay loam. The apparent differential sensitivity to EC in the two soils can be explained by their different water content and therefore, different osmotic potential at the same EC. At almost similar osmotic potential: −2.92 MPa in sand (at EC 1.3) and −2.76 MPa in the sandy clay loam (at EC 4.0) the relative decrease in respiration was similar (8–9%). Sodicity had little effect on cumulative respiration in the soils, but DOC, DON and specific ultra-violet absorbance (SUVA) were significantly higher at SAR 20 than at SAR < 3 in combination with low EC in both soils (EC 0.5 in the sand and EC 0.7 and 1.4 in the sandy clay loam). Therefore, high SAR in combination with low EC is likely to increase the risk of DOC and DON leaching in the salt-affected soils, which may lead to further soil degradation.