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.

Increasing soil organic carbon (SOC) stocks in agricultural soils can contribute to stabilizing or even lowering atmospheric greenhouse gas (GHG) concentrations. Cover crop rotation has been shown to increase SOC and provide productivity benefits for agriculture. Here we used a split field design to evaluate the short-term effect of cover crop on SOC distribution and chemistry using a combination of bulk, isotopic, and spectroscopic analyses of size-and density-separated soil aggregates. Macroaggregates (>250 µm) incorporated additional plant material with cover crop as evidenced by more negative δ¹³C values (−25.4‰ with cover crop compared to −25.1‰ without cover crop) and increased phenolic (plant-like) resonance in carbon NEXAFS spectra. Iron EXAFS data showed that the Fe pool was composed of 17–21% Fe oxide with the remainder a mix of primary and secondary minerals. Comparison of oxalate and dithionite extractions suggests that cover crop may also increase Fe oxide crystallinity, especially in the dense (>2.4 g cm⁻³) soil fraction. Cover crop δ¹³C values were more negative across density fractions of bulk soil, indicating the presence of less processed organic carbon. Although no significant difference was observed in bulk SOC on a mass per mass basis between cover and no cover crop fields after one season, isotopic and spectroscopic data reveal enhanced carbon movement between aggregates in cover crop soil.

Subsurface carbon stocks are a prime target for efforts to increase soil carbon storage for climate change mitigation and improving soil health. However, subsurface carbon (C) dynamics are not well understood, especially in soils under long term intensive agricultural management. We compared subsurface C dynamics 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 + cover crops) and ORG (composted poultry manure + cover crops). Our results showed a ~19 Mg/ha increase in SOC stocks down to 1 m under ORG systems, no significant SOC increases under CONV+WCC or CONV systems, and the accumulation of carboxyl rich C in the subsurface (60–100 cm) horizons of all systems. Systems also had greater amounts of aromatic carbon in the order ORG>CONV+WCC>CONV. We identified a potential interaction between cover crops and compost, theorizing that increased macropores from cover crop roots facilitate the transport of soluble C and nutrients into the subsurface, thereby increasing stocks. These results demonstrate the potential for subsurface carbon storage in tilled agricultural systems and highlight a potential pathway for increasing carbon transport and storage in subsurface soil layers.