Gravel-associated organic material is important to quantify soil carbon and nitrogen stocks to depth in an agricultural cropping soil

Abstract

Context: Gravel is a common constituent in soil and is routinely excluded when estimating soil carbon (C) and nitrogen (N) stocks. Aims: We investigated the contribution that the gravel fraction (>2 mm) makes to C and N stocks in an agricultural soil. Methods: The amount of gravel and the C and N content of gravel-associated organic matter (OM) was assessed to 180 cm in a long-term cropping soil with differing nutrient treatments. Key results: Gravel-associated C and N accounted for ∼5% of the total C and N stocks in the upper layers (0-30 cm) of soil and up to 40% below 100 cm. The C:N ratio of the gravel-associated OM was similar to that in fine earth fraction (FEF) soil, with C:N ratio of ∼13 in surface layers to ∼8 at depth. Conclusions: We estimated that 19% and 23% of the total stock of C and N, respectively, were associated with gravel over the whole soil profile. In the two nutrient treatments, with differing C and N stocks in the FEF, gravel-associated OM accounted for 9.3-10.6 t C ha-1 and 1.1-1.3 t N ha-1. Implications: Our work highlights the significance of gravel in contributing to soil OM and the importance of sampling to depth to estimate soil C and N stocks. Importantly, disregard of the gravel fraction results in an underestimation of total soil C and N, which has implications for the accounting of C in agricultural soils and for the development of strategies to sequester soil C.