Microbial activity and biomass and N and P availability in a saline sandy loam amended with inorganic N and lupin residues

Plant residue can be a cost effective source of N and P fertilisers, which may enhance plant growth in saline soils. Salinity and limited availability of N may limit microbial activity and thus residue decomposition N and P availability. A laboratory experiment was conducted to investigate the effects of NH4–N or NO3–N on microbial activity and biomass and N and P availability in a saline sandy loam. Three levels of salinity (EC1:5 0.21, 0.51 and 0.85 dS m−1) were imposed in the sandy loam using solutions of Na+ and Ca2+. Soil was amended with or without 2% (w/w) lupin residues (C/N ratio 15.4) or 50 μg N g−1 soil as KNO3 or (NH4)2.SO4. With no residue or inorganic N added, the concentration of available N and P remained unchanged over 45 days. Soil respiration and microbial biomass C, N and P decreased with increasing salinity, but significantly increased with residue addition. Addition of inorganic N had no significant effect, but addition of NO3–N with residue significantly increased soil respiration and microbial biomass C, N and P. Salinity had no effect on N availability and decreased P availability. Nitrogen availability was lower with addition of NH4–N, N than with NO3–N. Available N and P increased with residue addition and increased further with addition of NO3–N than with NH4–N. The greater C availability in the lupin residue amended saline sandy loam stimulated microbial activity and biomass with greater N demand, thus promoted immobilization of NO3. Hence, N and P availability increased in the saline sandy loam.

► Salinity had no effect on N availability, but decreased P availability.
► Under saline conditions, soil respiration and microbial biomass decreased.
► Residue addition increased N and P availability and salt tolerance of microorganisms.
► Inorganic N had no effect on soil respiration, and.
► With residue, soil respiration and microbial biomass were higher with NO3–N than with NH4–N.