The influence of the soil matrix on nitrogen mineralisation and nitrification - III. Predictive utility of traditional variables and process locationwithin the pore system

Regression analysis was used to examine the importance of organic nitrogen (%N), soil water content (theta v), soil pH, and C : N ratio for predicting N mineralisation in a small field plot. Undisturbed soil cubes (c. 1.7 cm( 3)) were collected from the soil surface and received treatments of drying and rewetting, urea, substrate derived from clover leachate, or no amendmen t, and were incubated at either -10 or -30 kPa for 20 days. The data confirm the hypothesis that within a small field plot, theta v and %N explain most of the variation in net N mineralisation and nitrification . The pore size classes of 0.6-10 and 10-30 mu m made disproportionately sm all and large contributions to N mineralisation, respectively, apparently d ue to non-uniform distribution of organic N through the pore system. When s oluble N substrate was added to the soils, both these pore classes appeared to support mineralisation. We concluded that prior to sampling, the microb ial biomass had been more active in the pores 0.6-10 mu m, and had nearly e xhausted the organic substrates in this pore class, whereas this was not so for the 10-30 mu m pore class. Drying and rewetting increased the importan ce of %N as a predictor of N mineralisation, probably because this treatmen t disrupted physical protective mechanisms of organic N. Soil pH was genera lly not a useful predictor of N mineralisation and often seemed to be a dep endent rather than an independent variable in relation to nitrification. Ne ither was C : N ratio a useful predictor of N transformation processes, and this was probably related to physical regulatory mechanisms in the soil.