Multiple additions of rapidly decomposable residue alleviate the negative impact of salinity on microbial activity

Abstract

In previous research, we found that the reduction in soil respiration with increasing salinity was smaller in soils amended with rapidly decomposable residues (low C:N ratio) compared with slowly decomposable residues (high C:N ratio). However, with a single residue addition, available organic carbon will be quickly decomposed until only recalcitrant compounds remain. A more consistent supply of residues may improve the tolerance of microbes to salinity, but this effect could depend on residue decomposability. A 56-day incubation experiment was conducted with four loam soils having electrical conductivity (EC1:5) of 0.1, 1, 2.7 and 3.7dSm–1 amended 1–4 times with finely ground slowly decomposable canola (C:N=82) and rapidly decomposable kikuyu residues (C:N=19) with a total addition of 10gCkg–1 soil. There was a greater reduction in soil respiration with increasing salinity following addition of canola than addition of kikuyu. At all salinity levels and with both residues, cumulative respiration two weeks after addition of 10gCkg–1 was higher with multiple additions than with a single addition. The reduction of cumulative respiration with increasing salinity was smaller with repeated addition of rapidly decomposable residue than with only a single addition. However, this was not the case with slowly decomposable residue. The results suggest that for amelioration of saline soils, addition of rapidly decomposable residue is more effective than adding slowly decomposable residues, particularly when rapidly decomposable residues are added repeatedly.