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

Abstract

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. © 2011 Elsevier Masson SAS.