Impact of Salinity on Respiration and Organic Matter Dynamics in Soils is More Closely Related to Osmotic Potential than to Electrical Conductivity

Osmotic potential (OP) of soil solution may be a more appropriate parameter than electrical conductivity (EC) to evaluate the effect of salts on plant growth and soil biomass. However, this has not been examined in detail with respect to microbial activity and dissolved organic matter in soils with different texture. This study evaluated the effect of salinity and sodicity on respiration and dissolved organic matter dynamics in salt-affected soils with different texture. Four non-saline and non-sodic soils differing in texture (S-4, S-13, S-24 and S-40 with 4%, 13%, 24% and 40% clay, respectively) were leached using combinations of 1 mol L−1 NaCl and 1 mol L−1 CaCl2 stock solutions, resulting in EC (1:5 soil:water ratio) between 0.4 and 5.0 dS m−1 with two levels of sodicity (sodium absorption ratio (SAR) < 3 (non-sodic) and 20 (sodic), 1:5 soil:water ratio). Adjusting the water content to levels optimal for microbial activity, which differed among the soils, resulted in four ranges of OP in all the soils: from −0.06 to −0.24 (controls, without salt added), −0.55 to −0.92, −1.25 to −1.62 and −2.77 to −3.00 Mpa. Finely ground mature wheat straw (20 g kg−1) was added to stimulate microbial activity. At a given EC, cumulative soil respiration was lower in the lighter-textured soils (S-4 and S-13) than in the heavier-textured soils (S-24 and S-40). Cumulative soil respiration decreased with decreasing OP to a similar extent in all the soils, with a greater decrease on Day 40 than on Day 10. Cumulative soil respiration was greater at SAR = 20 than at SAR < 3 only at the OP levels between − 0.62 and − 1.62 MPa on Day 40. In all the soils and at both sampling times, concentrations of dissolved organic C and N were higher at the lowest OP levels (from − 2.74 to − 3.0 MPa) than in the controls (from − 0.06 to − 0.24 MPa). Thus, OP is a better parameter than EC to evaluate the effect of salinity on dissolved organic matter and microbial activity in different textured soils.