Is CO2 evolution in saline soils affected by an osmotic effect and calcium carbonate?

Abstract

Salt-affected soils are widespread, particularly in arid climates, but information on nutrient dynamics and carbon dioxide (CO2) efflux from salt-affected soils is scarce. Four laboratory incubation experiments were conducted with three soils. To determine the influence of calcium carbonate (CaCO3) on respiration in saline and non-saline soils, a loamy sand (6.3% clay) was left unamended or amended with NaCl to obtain an electrical conductivity (EC) of 1.0 dS m⁻¹ in a 1:5 soil/water extract. Powdered CaCO3 at rates of 0%, 0.5%, 1.0%, 2.5%, 5.0% and 10.0% (w/w) and 0.25-2 mm mature wheat residue at 0% and 2% (w/w) were then added. Cumulative CO2-C emission from the salt amended and unamended soils was not affected by CaCO3 addition. To investigate the effect of EC on microbial activity, soil respiration was measured after amending a sandy loam (18.8% clay) and a silt loam (22.5% clay) with varying amount of NaCl to obtain an EC1:5 of 1.0–8.0 dS m⁻¹ and 2.5 g glucose C kg⁻¹ soil. Soil respiration was reduced by more than 50% at EC1:5 ≥ 5.0 dS m−1. In a further experiment, salinity up to an EC1:5 of 5.0 dS m⁻¹ was developed in the silt loam with NaCl or CaCl2. No differences in respiration at a given EC were obtained between the two salts, indicating that Na and Ca did not differ in toxicity to microbial activity. The effect of different addition rates (0.25–2.0%) of mature wheat residue on the response of respiration to salinity was investigated by adding NaCl to the silt loam to obtain an EC1:5 of 2.0 and 4.0 dS m⁻¹. The clearest difference between salinity levels was with 2% residue rate. At a given salinity level, the modelled decomposition constant ‘k’ increased with increasing residue addition rate up to 1% and then remained constant. Particulate organic carbon left after decomposition from the added wheat residues was negatively correlated with cumulative respiration but positively correlated with EC. Inorganic N (NH4⁺-N and NO3⁻-N) and resin P significantly decreased with increasing salinity. Resin P was significantly decreased by addition of CaCl2 and CaCO3.