The influence of the soil matrix on nitrogen mineralisation and nitrification - V. Microporosity and manganese

Small soil cubes of dimensions 12 by 12 by 12 mm were collected from the su rface of a red earth. Treatments were addition of clover substrate or urea, drying and rewetting, or no amendment, after which soils were incubated at either -10 or -30 kPa. Each soil cube was analysed for NO3--N, NH4+-N, tot al soil N (%N), volumetric water content (theta v), microporosity (volume o f pores <0.6 mu m), and Mn2+ concentration. Multiple regression analysis wa s used to determine if microporosity and Mn2+ contributed uniquely to linea r models in which %N and theta v were also used to predict N mineralisation and nitrification. In soils incubated at -10 kPa, both microporosity and M n2+ had a strong positive influence on N mineralisation and nitrification, whereas in soils incubated at -30 kPa no such influence could be observed. These and other observations suggest that when soils with high microporosit y were incubated at -10 kPa, O-2 supply to the microbial biomass was limite d and the reduction of Mn oxides to divalent Mn was enhanced. Increased sub stitution of Mn oxides for O-2 as terminal electron acceptors in the microb ially mediated oxidation of carbon substrates considerably increases H+ con sumption. We propose that in the wetter soil (-10 kPa), this process reliev es pH stress experienced by N mineralising and nitrifying organisms, thereb y increasing their activity, but that in the drier soil (-30 kPa), O-2 diff usion is less restricted and this mechanism does not operate appreciably. T he influence of microporosity on clover-amended soils was to decrease level s of mineral N and this was attributed to greater denitrification in soils with high microporosity. Neither microporosity nor Mn2+ was an important va riable in the prediction of mineral N in the urea-treated soils. This work highlights the interaction of physical, chemical, and biological components of the soil which give rise to microbial microsites and diffusion gradient s which are important determinants of soil function.