LEGUME N MINERALIZATION - EFFECT OF AERATION AND SIZE DISTRIBUTION OFWATER-FILLED PORES

The relative importance of air-filled porosity and the size distributi on 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 poten tial. Mineralization was measured over 70 d. The accumulation of-inorg anic N in the amended treatment was described using a two-pool model a nd in the control using a one-pool model. Biomass C was measured in bo th treatments. Mineralization was found to be influenced by both the a ir-filled porosity and the distribution of the volume fractions of por es (VFP) that were water-filled and the relative importance of each wa s assessed using stepwise variable-selection regression procedures. Th e parabolic dependence of mineralization on air-filled porosity that h as 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 poo l and negatively related to the size of the resistant pool in the amen ded treatment. The rate constant of the control was parabolically rela ted to air-filled porosity, but did not give rise to a predicted maxim um in N accumulation within the experimental range of air-filled poros ities. The VFP with dia <3 mu m was selected most often in the stepwis e regression analyses. Pore characteristics had a larger influence on the labile pool than on the resistant pool of the amended treatment. T he size of the labile pool and its rate constant decreased with the VF P <3 mu m dia. The size of the potentially mineralizable pool of the c ontrol treatment varied with the VFP in a manner similar to the labile pool of the amended treatment. Biomass C in the amended and control t reatments increased with the VFP < 1.5 mu m dia, and in the amended tr eatment decreased with the VFP 10-20 mu m dia. The role of pore size o n the space inhabited by soil flora and their predators suggests that protection of soil flora in pores <3 mu m contributed to decreased min eralization from the labile pool. (C) 1997 Elsevier Science Ltd.