Deep soil inventories reveal that impacts of cover crops and compost on soil carbon sequestration differ in surface and subsurface soils

Abstract

Increasing soil organic carbon (SOC) via organic inputs is a key strategy for increasing long‐term soil C storage and improving the climate change mitigation and adaptation potential of agricultural systems. A long‐term trial in California's Mediterranean climate revealed impacts of management on SOC in maize‐tomato and wheat–fallow cropping systems. SOC was measured at the initiation of the experiment and at year 19, at five depth increments down to 2 m, taking into account changes in bulk density. Across the entire 2 m profile, SOC in the wheat–fallow systems did not change with the addition of N fertilizer, winter cover crops (WCC), or irrigation alone and decreased by 5.6% with no inputs. There was some evidence of soil C gains at depth with both N fertilizer and irrigation, though high variation precluded detection of significant changes. In maize‒tomato rotations, SOC increased by 12.6% (21.8 Mg C/ha) with both WCC and composted poultry manure inputs, across the 2 m profile. The addition of WCC to a conventionally managed system increased SOC stocks by 3.5% (1.44 Mg C/ha) in the 0–30 cm layer, but decreased by 10.8% (14.86 Mg C/ha) in the 30–200 cm layer, resulting in overall losses of 13.4 Mg C/ha. If we only measured soil C in the top 30 cm, we would have assumed an increase in total soil C increased with WCC alone, whereas in reality significant losses in SOC occurred when considering the 2 m soil profile. Ignoring the subsoil carbon dynamics in deeper layers of soil fails to recognize potential opportunities for soil C sequestration, and may lead to false conclusions about the impact of management practices on C sequestration.

Dynamic Stability of Soil Carbon: Reassessing the “Permanence” of Soil Carbon Sequestration

“…Perennialization or inclusion of perennial crops in rotation 0.3-1.6 Increases above-and belowground plant C inputs to soils which supports production of microbially processed C, typically reduces soil disturbance which supports aggregation and pore network development Potential for C losses if additional C supply supports microbial decomposition that outstrips MAOM production (priming effect) Bird et al, 2011;Anderson-Teixeira et al, 2013;Bach and Hofmockel, 2016;Kantola et al, 2017 Cover cropping Increases above-and belowground plant C inputs to soils which supports production of microbially processed C Potential for C losses if additional C supply supports microbial decomposition that outstrips MAOM production Poeplau and Don, 2015;Tautges et al, 2019 Residue retention/ plant high C-input crops…”

Winter cover crops increase readily decomposable soil carbon, but compost drives total soil carbon during eight years of intensive, organic vegetable production in California

“…In California's Central Valley, after 20 years of organic corn (Zea mays L.)/tomato (Lysopersicum esculentum Mill.) production inputs from annual faba bean-oat (Avena sativa L.) cover crops and poultry litter compost increased SOC 3.95 Mg ha -1 , while in a conventionally managed cover cropped system SOC did not increase [20]. Increased SOC at depth increments below 15 cm was positively correlated with cumulative compost C inputs, but not cover crop C inputs.…”

“…Increased SOC at depth increments below 15 cm was positively correlated with cumulative compost C inputs, but not cover crop C inputs. Cover crop residues, particularly legumes, are rapidly degraded following soil incorporation [20]. Legume-rye mixtures decompose more slowly than pure legume cover crops, with rates decreasing with increasing proportions of rye in the mix [65].…”

“…It is important to note that we are not able to separate the effect of compost from increased cover crop frequency because the design of this study does not include annually cover cropped systems without compost application; this is an issue that should be evaluated in future research. Furthermore, our study only evaluated SOC stocks in the top 30 cm of the soil and may not reflect changes in subsoil SOC, which recently published research [20] suggests exerts a strong effect on overall SOC stocks throughout the soil profile. While compost application appears to have the greatest effect on total SOC stocks in our study, increasing cover crop frequency altered the composition of SOC by increasing the proportion of labile C, potentially increasing nutrient availability and thus likely having a greater impact on crop yields than compost.…”

“…Compost application increases the proportion of lignin in SOC as compost-derived cellulosic polysaccharides are preferentially degraded in soil [19]. Long-term compost application (20 years) has been positively correlated with increases in SOC, increasing by stocks by 0.198 Mg C ha -1 yr -1 [20]. In contrast, cover crop residues decompose rapidly following incorporation due to their low C:N ratio.…”