Effects of phosphorus addition on N2O emissions from an Acacia mangium soil in relatively aerobic condition

Effects of phosphorus (P) addition on nitrous oxide (N2O) emissions from an Acacia mangium plantation soil was examined in relatively aerobic condition with carbon (C) and nitrogen (N) addition. We hypothesized that P addition reduced N2O emissions through stimulated microbial N immobilization and subsequent decrease in inorganic N resources for producing N2O. We prepared the following four experimental sets; high C (glucose 2000 μg C g soil ) and water-filled pore space (WFPS) 40 % (H40), low C (glucose 100 μg C g soil) and WFPS 40 % (L40), high C and WFPS 60 % (H60), and low C and WFPS 60 % (L60). Nitrogen (NH4NO3, 20 μg N g soil ) was also added to all soils. We prepared P-added soils (Ca(H2PO4)2, 20 μg P g soil ) and non-added control to test the effects of P addition on N2O emissions. Contrary to our hypothesis, P addition did not reduce N2O emissions, although soil microbial N immobilization was stimulated by P addition in soils with low C addition. Stimulated total N cycling by P addition probably offset the decrease in soil inorganic N. Meanwhile P addition reduced soil microbial biomass N (MBN) content in H60, where N2O emissions increased significantly by P addition. It was possible that the microbial growth reached its peak and started dying more quickly in P-added soils in H60 due to the favorable condition for microbes (higher C and water content). Thus we concluded that (i) P addition did not necessarily stimulate soil microbial N immobilization, and (ii) N2O emissions might not decrease even if P addition stimulated soil microbial N immobilization.