Carbon and nitrogen mineralisation in sand, silt, and clay fractions of soils under maize and pasture

Although several studies have quantified either C mineralisation or net N mineralisation in particle-size fractions, no work has examined simultaneous C and net N mineralisation. Therefore, we conducted an 18-week laboratory incubation to compare simultaneous mineralisation in sand, silt, and clay fractions. The soils (silt loams) were collected from fields of long-term pasture and maize. Sand, silt, and clay were separated by mild dispersion in water followed by centrifugation. Samples were incubated at 25 degrees C in the dark in a quartz matrix, and were leached every 2 weeks with 0.004 M CaCl2 at a suction of 20 kPa to remove soluble products. C mineralisation was determined from CO2-C evolved each 2 weeks, and mineral N was measured in the leachate. C mineralisation, on a C basis, followed the order sand > clay > silt, and was related (r(2) = 0.88) to the proportion of O-alkyl C (carbohydrate C) estimated from C-13 NMR. The low mineralisation in the silt may also have been a result of the physical protection of substrates in small pores in this fraction. The rates of N release were initially rapid from the maize soil fractions, and were consistent with the high initial mineral-N contents; subsequently, the rates were slower, and probably related to C mineralisation. For the pasture soil, N mineralisation followed the order clay>silt>sand, and was inversely related to the C:N ratios. Immobilisation appeared to take place in the sand fraction, whereas a large part of the net N mineralisation occurred in the clay fraction. There is now good evidence that rates of C and net N mineralisation differ within discrete particle size fractions, and data on such fractions could be useful for constructing soil organic matter models.