Opposing patterns of carbon and nitrogen stability in soil organic matter fractions compared to whole soil

Nitrogen (N) released from soil organic matter (SOM) is quantitatively important for crop uptake, even when adequate fertiliser N is supplied. Understanding of SOM has shifted to recognise distinct fractions that correlate with properties such as turnover time, carbon (C) and N content, and chemical composition. Yet, how these fractions relate to N supply from SOM is poorly understood. This study aimed to link N mobilisation and C stability in coarse (>= 50 mu m) and fine (<= 50 mu m) fractions, and evaluate the roles of these fractions in supplying N in cropping soil. Soils from long-term continuous cotton, cotton-vetch, and cotton-wheat rotations and a nearby uncleared site (NV) were separated into coarse and fine fractions, left as whole soil, or dispersed and freeze-dried as a fractionation control. Initial C chemistry in fractions and whole soils was measured by solid state C-13 NMR spectroscopy. N mobilisation and CO2 loss were measured over a 14-day incubation experiment. In the cropping soils, net immobilisation of N was measured in the separate fractions, while net mobilisation was found in the whole soils. In the NV soil, N mobilisation was greater in the fine fraction. C mineralisation followed the order fine fraction > fractionation control = coarse fraction > whole soil. C stability was best explained by physical protection within whole soil structure rather than chemical recalcitrance or mineral stabilisation. The results revealed an unexpected contrast between C and N mineralisation from SOM fractions and demonstrated the importance of soil aggregates for SOM stability. We show a cautionary impact of fractionation on C and N dynamic, highlighting the need for further research to understand the synergistic behaviour of SOM fractions in whole soils.