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RESEARCH ARTICLE (Open Access)

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

Clive A. Kirkby A , John A. Kirkegaard A and Alan E. Richardson https://orcid.org/0000-0003-0708-1299 A *
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A CSIRO Agriculture & Food, GPO Box 1700, Canberra, ACT 2601, Australia.

* Correspondence to: alan.richardson@csiro.au

Handling Editor: Brian Wilson

Soil Research 60(3) 224-233 https://doi.org/10.1071/SR21140
Submitted: 1 June 2021  Accepted: 7 September 2021   Published: 16 November 2021

© 2022 The Author(s) (or their employer(s)). Published by CSIRO Publishing. This is an open access article distributed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND)

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.

Keywords: carbon, C:N ratio, coarse fraction, gravel, sequestration, soil, SOM, stoichiometry.


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