The influence of the soil matrix on nitrogen mineralisation and nitrification. II. The pore system as a framework for mapping the organisation of the soil matrix

Large numbers of small undisturbed soil volumes (1·7 cm3 ) were collected from the surface layer of a 2 m by 3 m field plot on a red earth near Wagga Wagga, New South Wales. The hypothesis tested was that an analysis of relationships between the volume of different pore size classes and various soil properties, measured on these soil volumes, can provide an understanding of soil organisation within the framework of the pore system. Three discrete findings were presented in confirmation of the hypothesis. (1) A non-uniform distribution of organic N through the pore system was indicated by the data analysis. Soil organic N tended to be concentrated in pores <0·6 µm and in pores 10-30 µm, but not in the intermediate pore size class (pores 0·6-10 µm). Concentrations of organic N in pores <0·6 µm are probably because of physical protection from microbial decomposition, but concentrations of organic N in pores 10-30 µm are probably because these pores are infrequently water-filled, and this limits bacterial activity more severely than in the pores 0·6-10 µm. Currently available assays for potentially mineralisable N cannot account for the effect of substrate location within the pore system, and a characterisation of the soil for the distribution of N in pores may enhance their utility. Soil disturbance is likely to alter organic matter distribution through the pore system and alter mineralisation dynamics. (2) Observations of pore size distributions before and after wetting suggested that soils which were high in organic matter and clay tended to have a greater volume of pores 0 ·6-30 µm which are unstable to drying and rewetting. It is proposed that these unstable pores 0 ·6-30 µm had been produced by the movement and alignment of clay particles during the growth of microbial colonies. (3) The volume of pores <0·6 µm had a relatively strong negative correlation with pH and a relatively strong positive correlation with Mn2+ . A mechanism based on redox chemistry principles was proposed to explain these relationships. It was suggested that the volume of pores <0·6 µm is related to the potential anaerobicity of the soil volume. In anaerobic conditions, the terminal electron acceptor for organic C oxidation may be MnO2 instead of O2, and in these circumstances, considerably more H+ would be consumed than in aerobic conditions. It is suggested that this alkaline effect extends into regions of the matrix where N mineralisation and nitrification are occurring, and stimulates these processes. The greater nitrification which may result from such a chain of events may, over time, effect greater acidification in those soil volumes which have greater microporosity.