Abstract
Abstract. Subsoil carbon (C) stocks are a prime target for efforts to increase soil C storage for climate change mitigation. However, subsoil C dynamics are not well understood, especially in soils under long-term intensive agricultural management. We compared subsoil C storage and soil organic matter (SOM) composition in tomato–corn rotations after 25 years of differing C and nutrient management in the California Central Valley: CONV (mineral fertilizer), CONV+WCC (mineral fertilizer and cover crops), and ORG (composted poultry manure and cover crops). The cover crop mix used in these systems is a mix of oat (Avena sativa L.), faba bean (Vicia faba L.), and hairy vetch (Vicia villosa Roth). Our results showed a ∼19 Mg ha−1 increase in soil organic C (SOC) stocks down to 1 m under ORG systems, no significant SOC increases under CONV+WCC or CONV systems, and an increased abundance of carboxyl-rich C in the subsoil (60–100 cm) horizons of ORG and CONV+WCC systems. Our results show the potential for increased subsoil C storage with compost and cover crop amendments in tilled agricultural systems and identify potential pathways for increasing C transport and storage in subsoil layers.
Highlights
Agricultural subsoils (> 60 cm) have the potential to store large amounts of C (Rumpel et al, 2012) for a longer period of time (Paul et al, 1997, 2001) relative to surface soils (< 15 cm)
An increased focus on interrogating the surface soil to answer questions about processes in the entire soil profile exacerbates the risk of subsoils being treated merely as “more dilute surface soils” (Salomé et al, 2010) and ignores decades of research into the unique role that subsoils play in increasing soil C stocks (Rapalee et al, 1998; Rumpel and Kögel-Knabner, 2011)
External N inputs to CONV + WCC systems were close to 1 Mg ha−1 higher than CONV systems over 25 years, with ∼ 40 % of the external N inputs to CONV + WCC systems coming from the decomposition of cover crop residue and the other ∼ 60 % from mineral fertilizer application, compared to 100 % of total N inputs in the CONV coming from mineral fertilizer application
Summary
Agricultural subsoils (> 60 cm) have the potential to store large amounts of C (Rumpel et al, 2012) for a longer period of time (Paul et al, 1997, 2001) relative to surface soils (< 15 cm). A focus on surface soils is problematic in agricultural studies, given how practices such as cover cropping can have drastically different effects on surface versus subsoil SOC accumulation (Bernal et al, 2016; Harrison et al, 2011; Tautges et al, 2019) depending on the cover crop used.
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