We sought to examine the distribution of carbon (C) decomposition within the framework of the soil pore system. Soils were sampled from a transect having a natural gradient in pore-size distribution. After the addition of labelled wheat straw ( 13 C) the repacked soil columns were incubated (25 C) at soil water matric potentials of either À75 kPa or À5 kPa and for either 4 or 90 days. Pore-size distribution was determined for each soil column after incubation and soils were then analysed for soluble C, label-derived residual C, label-derived and native biomass C, nematode abundance, and ergosterol concentration as an indicator of fungal biomass. Overall, the data suggested that pore-size distribution and its interaction with soil water give rise to a highly stratified biogeography of organisms through the pore system. This results in different rates of decomposition in pores of different size. Added plant material seemed to decompose most rapidly in soils with a relatively large volume of pores with neck diameters c. 15-60 m and most slowly in soils with large volumes of pores with neck diameters < 4 m. Regression analysis suggested that at matric potentials of both À75 kPa and À5 kPa the fastest decomposition of organic substrate occurred close to the gas-water interface. This analysis also implied that slower rates of decomposition occur in the pore class 60-300 m. Correlations between the mass of soil biota and the pore volume of each pore class point to the importance of fungi and possibly nematodes in the rapid decomposition of C in the pores c. 15-60 m during the early stages of decomposition.

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Small undisturbed soil volumes (c. 1·7 cm3) were collected from the surface of a small field plot. Soil volumes were treated with clover-derived substrate, dried and rewetted, or retained continuously moist from the field. These soil volumes were then incubated for 20 days at a matric water potential of either –10 or –30 kPa. At the end of the incubation the soil was analysed for volumetric water content (θv), NO-3 -N, NH+4 -N, total N (%N), and percentages of sand, silt, and clay. The texture terms were included in linear regression models, together with %N and θv as predictors of N mineralisation and nitrification. Clay and sand were often observed to have a significant influence on N mineralisation and nitrification, but silt rarely appeared to influence these processes. In soils retained continuously moist, %clay had a negative relationship with N mineralisation and nitrification, but this relationship was positive in soils that had been dried and rewetted. The results suggest that during periods of relatively high moisture content, soils that are higher in clay are able to protect organic N more effectively from microbial attack. However, on drying and rewetting, the protective mechanisms of clay are undermined, the relatively large protected reservoirs of organic N in high clay soils become more vulnerable to microbial attack, and these soils therefore experience a greater flush of N mineralisation than soils with lower clay levels. The negative influence of clay in the continuously moist soils was not as clearly observed in the soils incubated at –10 kPa as in soils incubated at –30 kPa, suggesting that the decomposition of organic N resident in larger pores (10–30 µm neck diameter) may not be as strongly regulated by clay as that resident in smaller pores. When soils were treated with clover-derived substrate, clay had a positive relationship with N mineralisation and nitrification rates. This may have been because clay limited the diffusion of partially decomposed organics away from the decomposing microbial population, thereby helping to facilitate more complete decomposition of the organic material. Texture had very little influence on the nitrification of urea-derived ammonium.

The existence of microsites of low pH around active colonies of nitrifying soil bacteria has previously been suggested but has been difficult to verify. A study was undertaken to examine whether observed decreases in bulk soil pH that occur during nitrification are in accordance with the theory of acidified nitrification microsites. A red earth soil (sieved <2 mm) was retained at a pH of 5·3 or amended with KHCO3 to achieve a pH of 6·3. Ammonium [(NH4)2SO4] was added to the soils and they were incubated for 35 days. In both soils the pH dropped rapidly and severely limited further nitrification. The soil with the higher initial pH experienced limitations to nitrification at a pH which was 0·2 units higher than that of the soil with the lower initial pH. The explanation for this result is in terms of acidified nitrification microsites. It is suggested that an active nitrifying colony may lower the pH within its immediate vicinity to a critical pH at which nitrification almost ceases. This critical pH achieved at the nitrification microsite is probably unrelated to the initial pH of the soil, but the pH of the soil matrix which is distant from the immediate influence of the nitrification microsite would remain at a pH closer to that of the soil initially. This less acidified region of the soil matrix would have an overriding influence on the measured pH of the bulk soil and account for the discrepancy between the measured pH of the two soils at the end of the incubation. These data provide further evidence that acidified nitrification microsites exist in soil, and that the measured soil pH is a poor estimate of the pH experienced by the microbial biomass.