Long-term conservation management practices on depth distribution of soil organic carbon in croplands

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Rounak Patra, Department of Biosystems Engineering & Soil Science, University of Tennessee, Knoxville, TN, Debasish Saha, Biosystems Engineering and Soil Science, University of Tennessee, Knoxville, TN and Sindhu Jagadamma, Biosystems Engineering and Soil Science, University of Tennessee, Knoxville, Knoxville, TN

Background/Question/Methods
Adopting long-term cover-cropping and no-tillage practices, two of the most popular conservation management practices, have the potential to increase soil organic carbon (SOC) accumulation in the agroecosystems to provide soil health and climate mitigation benefits. However, it is still uncertain if these conservation practices increase or redistribute the SOC throughout the soil profile. To understand this SOC dynamics better, this study was conducted by leveraging a 40-year long replicated (n=4) field experiment at the West Tennessee Research and Education Center of the University of Tennessee in Jackson, TN, in a continuous cotton (Gossypium hirsutum L.) system. The cropping system treatments examined included cover-crops [Hairy Vetch (Vicia villosa, HV), winter wheat (Triticum aestivum L., WW), and no cover-crop] and tillage (conventional-tillage and no-tillage). Soil samples were collected in June 2020 from four depths (0-5, 5-10 cm representing surface-soil and 10-30, 30-60 cm layers representing subsoil) and analyzed for total SOC and its two distinct functional pools such as particulate (POC), relatively undecomposed lightweight fraction, and microbially processed, mineral associated (MAOC) organic carbon fraction.
Results/Conclusions
Across the entire 0-60 cm soil profile, the SOC stock did not significantly differ between till and no-till practices (27.47 vs. 29.31 Mg ha-1, P: 0.34). However, the distribution of SOC under no-till plots is highly concentrated at the surface soils due to the absence of soil mixing by tillage. Increased SOC stock at the no-till surface soils could be attributed to the higher abundance of POC fraction (1.56% vs. 0.77% under tilled treatments, P: <0.0001), indicating aggregate occlusions of plant residues in particulate fraction. Lower surface layer SOC in the tilled than the no-till treatments (6.09 vs. 7.09 Mg ha-1) was compensated by SOC accrual at the tilled subsurface layers (10.435 vs. 7.31 Mg ha-1, P: 0.0035), which further indicates SOC redistribution than accrual under no-tillage to drive little net change in profile SOC, even after 40-years of conservation practices in our warm and humid climate.
Regardless of tillage, cover-crops significantly impacted SOC stock (P: 0.0091). In the surface layer, cover-cropping increased both POC and MAOC fractions. Higher SOC stock under cover cropping indicates a higher contribution of cover-crop root-derived-C to the increased MAOC fraction in the subsoil. Ignoring mechanistic differences in carbon sequestration in surface and subsurface soils would miss the opportunities to recognize potential new avenues for soil C sequestration and lead to over-simplified conclusions about the impact of conservation management practices on SOC sequestration.