Legume N mineralization: Effect of aeration and size distribution of water-filled pores

The relative importance of air-filled porosity and the size distribution of water-filled pores to the mineralization of legume N and soil N were assessed. Seven soils, with or without added legume, were packed to two relative bulk densities and incubated at a constant water potential. Mineralization was measured over 70 d. The accumulation of-inorganic N in the amended treatment was described using a two-pool model and in the control using a one-pool model. Biomass C was measured in both treatments. Mineralization was found to be influenced by both the air-filled porosity and the distribution of the volume fractions of pores (VFP) that were water-filled and the relative importance of each was assessed using stepwise variable-selection regression procedures. The parabolic dependence of mineralization on air-filled porosity that has been observed when soils are incubated at different potentials was not obvious in soils equilibrated at a constant potential. Air-filled porosity was positively related to the rate constant of the labile pool and negatively related to the size of the resistant pool in the amended treatment. The rate constant of the control was parabolically related to air-filled porosity, but did not give rise to a predicted maximum in N accumulation within the experimental range of air-filled porosities. The VFP with dia <3 mu m was selected most often in the stepwise regression analyses. Pore characteristics had a larger influence on the labile pool than on the resistant pool of the amended treatment. The size of the labile pool and its rate constant decreased with the VFP <3 mu m dia. The size of the potentially mineralizable pool of the control treatment varied with the VFP in a manner similar to the labile pool of the amended treatment. Biomass C in the amended and control treatments increased with the VFP < 1.5 mu m dia, and in the amended treatment decreased with the VFP 10-20 mu m dia. The role of pore size on the space inhabited by soil flora and their predators suggests that protection of soil flora in pores <3 mu m contributed to decreased mineralization from the labile pool. (C) 1997 Elsevier Science Ltd.