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 NH(4)-N or NO(3)-N on microbial activity and biomass and N and P availability in a saline sandy loam. Three levels of salinity (EC(1:5) 0.21, 0.51 and 0.85 dS m(-1)) were imposed in the sandy loam using solutions of Na(+) and Ca(2+). Soil was amended with or without 2% (w/w) lupin residues (C/N ratio 15.4) or 50 mu g N g(-1) soil as KNO(3) or (NH(4))(2).SO(4). 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 NO(3)-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 NH(4)-N, N than with NO(3)-N. Available N and P increased with residue addition and increased further with addition of NO(3)-N than with NH(4)-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 NO(3). Hence, N and P availability increased in the saline sandy loam. (C) 2011 Elsevier Masson SAS. All rights reserved.