Decomposition of 14C-labelled plant material in a salt-affected soil

Sodicity and salinity are each known to influence the decomposition of plant residues, but the effects of their interactions are not well understood. In this study, a loamy topsoil was adjusted to 3 values of exchangeable Na (0, 3 and 25%), for each of 3 values of salinity (0.1, 0.5 and 1.5 dS m-1 in a 1:5 soil-to-water extract), by equilibration with solutions containing appropriate amounts of NaCl and CaCl2. 14C-labelled Trifolium subterraneum shoots, either finely ground or unground, were added to the soils at a rate of 2.22 g kg-1 soil. Samples were kept for 82 days at -50 kPa water potential, or 97 days with drying-rewetting cycles. Mineralisation of ground plant C increased with increasing sodicity, and decreased with increasing salinity. The influence of both sodicity and salinity were less with unground than ground plant material because of reduced interaction of substrates with the soil matrix. The effects of the treatments on mineralisation of plant C were established within the first 20 days of incubation. Mineralisation of native soil C was increased by sodicity throughout the incubation at low salinity, but was unaffected by sodicity at high salinity. At the end of the incubation, neither plant- nor soil-derived microbial biomass C were greatly affected by sodicity or salinity. Total microbial biomass C was significantly higher in the soils kept at constant water content than in those submitted to drying-rewetting, in which it was approximately the same as in the soils before incubation. Of the total microbial biomass C, 50-93% was derived from the plant material. Drying-rewetting cycles decreased mineralisation and microbial biomass C (both plant- and soil-derived), but did not influence the effects of sodicity and salinity. At each value of salinity, mineralised plant C was positively correlated with water-extractable organic C derived from plant material. This was also true for soil-derived C at low, but not at high values of salinity. Mineralisation generally resulted in a reduction in the amount of carbohydrate C relative to other forms of organic C, as determined by 13C CP-MAS nuclear magnetic resonance.