Residue properties influence the impact of salinity on soil respiration

Abstract

There is limited information about the impact of residue properties on response of microbial activity to soil salinity. It is well established that salinity decreases soil microbial activity. However, most studies on soil respiration in residue-amended salt-affected soil only used one or two types of plant residues. Residues differ in decomposability which could influence the impact of salinity on soil respiration. The aim of this study was to investigate the impact of salinity on respiration in soil amended with residues differing in chemical composition (lignin concentration, water-soluble organic carbon and C/N ratio). Three experiments were conducted. In the first experiment, finely ground residues differing in chemical composition (shoots of wheat, canola, saltbush and kikuyu, sawdust, eucalypt leaves) were added at 2 % w/w to loam soils differing in salinity: electrical conductivity in a 1:5 soil; water extract (EC_1:5) 0.1 (non-saline), 1, 2.5 and 3.3 dS m⁻¹. As expected, cumulative respiration decreased with increasing soil EC, but the negative impact of salinity differed among residues. Based on the regression between cumulative respiration in percentage of the non-saline soil and EC_1:5, it was calculated that cumulative respiration would be reduced by 20 % compared with the non-saline soil at EC_1:5 0.3 dS m⁻¹ in soil amended with the poorly decomposable sawdust or canola shoots (high C/N ratio, high lignin concentration), but at EC_1:5 4 dS m⁻¹ soil with easily decomposable saltbush shoots (low C/N ratio, low lignin concentration). In the second experiment, the C/N ratio of residues with high C/N but different lignin content (canola and sawdust) was adjusted to 20–80 by adding NH₄SO₄ to the residues prior to mixing them into the soils. In both residues, the decrease in cumulative respiration with increasing salinity was smaller when the C/N ratio was adjusted 20 or 40 compared to the original C/N (82 for canola, 114 for sawdust); cumulative respiration would be reduced by 20 % compared to the non-saline soil at EC_1:5 3 (low C/N) compared to 0.7 dS m⁻¹ (high C/N). In the third experiment, water-extractable organic carbon (WEOC) was partially removed by leaching from two residues with high WEOC concentrations (eucalypt leaves and saltbush shoots). Partial removal of WEOC reduced cumulative respiration in the saline soils, but increased the negative effect of salinity on respiration only with saltbush shoots. Compared to the non-saline soil, cumulative respiration was reduced by 20 % at EC_1:5 4.5 dS m⁻¹ with unleached saltbush shoots, compared to 1.5 dS m⁻¹ with leached residues. It is concluded that the negative impact of salinity is greater in soils amended with residues with low decomposability, e.g. high C/N and lignin content compared to easily decomposable residues and that N supply is particularly important for adaptation of microbes to salinity.