Sources, distribution, stability and management of deep soil carbon in agricultural systems

Abstract Climate change is accelerated by increasing levels of greenhouse gases (GHGs) as a result of human activity, particularly the release of carbon dioxide (CO 2 ). Soil carbon (C) sequestration, or the transfer of atmospheric CO 2 to soil organic matter (SOM) with long-term stabilization within the soil, is an important process of C removal from the atmosphere. For the accounting of soil C and offset markets in most countries including Australia, the standard soil sampling depth is 0–30 cm, although deeper sampling is recommended for more accurate C stock assessments and to capture long-term sequestration potential. While 30 cm soil depth accounts for most short-term management impacts on C storage, a significant portion of soil C is stored below this depth (i.e., deep soil C), and sampling at greater depths can provide a more complete account of total C stocks and potential sequestration benefits. This paper aims to provide a comprehensive review, including a bibliometric analysis and a critical discussion of the link between deep soil C storage and sequestration potential in relation to climate change mitigation and soil health. Deep soil layers contain over 850 Pg C worldwide, which is approximately 50%–60% of the total carbon stock in the top 1 m of soil. Deep soil C has a major impact on SOM dynamics and is relatively more stable compared to surface layer C, which is attributed to greater interaction with clay minerals and low microbial activity. This review emphasizes the vulnerability of deep soil C, management approaches, and strategies to enhance this C reserve along with prospects for future research. Graphical Abstract