Spatial location of carbon decomposition in the soil pore system

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English. 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 (C-13) the repacked soil columns were incubated (25°C) at soil water matric potentials of either -75 kPa or -5 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 mum and most slowly in soils with large volumes of pores with neck diameters < 4 mum. Regression analysis suggested that at matric potentials of both -75 kPa and -5 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 mum. 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 mum during the early stages of decomposition.