Increased carbon use and extracellular enzymatic efficiency promote profile-scale soil organic carbon accumulation under a long-term no-till system integrated with winter wheat cover crop

Background/Question/Methods

Increasing the crop residue return to the soil and its processing efficiency by soil microbes are gaining traction to increase cropland soil organic carbon(SOC). Cover-cropping and no-tillage promote residue-retention, thereby accelerating SOC accumulation in croplands. Previous studies have shown a positive relationship between microbial functional traits and SOC formation and theoretically suggested that conservation agricultural practices supporting microbial functional traits such as microbial carbon use efficiency(CUE) and enzymatic efficiency(EE) can efficiently convert plant inputs into microbial biomass, improving SOC stabilization. However, field-based evidence demonstrating the dynamics of CUE and EE is limited, especially from the subsoil. We studied soil microbial functional traits affecting profile-scale SOC distribution dynamics by leveraging a 40-year long replicated field experiment in Tennessee. The management treatments examined included three cover-crops (hairy-vetch: HV; winter-wheat: WW; no cover-crop: NC) and two tillage (10-cm deep chisel-tillage:CT; no-tillage:NT). Soil samples collected from the surface (0-10 cm) and subsurface (30-40 cm) soil layers were incubated for 24-hours after adding 13C-glucose and analyzed for CUE and EE. To calculate CUE and EE, we measured the total-CO2, 13CO2, total microbial-biomass-C (MBC), MB13C, and extracellular-enzymatic activities. We also measured total SOC concentration and C-associated with particulate and mineral-associated organic matter fractions (POM-C and MAOM-C).

Results/Conclusions

Our results show increased microbial CUE under NT-topsoil (NTCUE:0.93, CTCUE: 0.9) and subsoil (NTCUE:0.94, CTCUE:0.91) compared to CT treatments. Integration of grass-type cover-crops improved microbial CUE (WWCUE:0.95, HVCUE:0.86, NCCUE: 0.95) and EE (WWEE:1.04, HVEE: 1.13, NCEE:0.47) in the subsoil. Thus, conservation practices with NT and cover-cropping improved microbial functional traits, especially in the subsoil. A positive relationship between microbial growth yield (represented by CUE) and environmental resource acquisition strategies (represented by EE) was observed (slope = 0.08, P< 0.001), with the highest values associated with no-till and cover cropping compared to tillage and winter fallow, emphasizing the hypothesis of environmental role as microbial functional trait moderators, which increases energy investment for resource acquisition and microbial growth yield under conservation management practices. Also, we found a strong relationship of microbial CUE with total SOC concentration (slopetopsoil: 21.1, slopesubsoil: 5.7, P< 0.001) and with MAOM-C concentration (slopetopsoil: 19.82, slopesubsoil: 6.3, P< 0.001), indicating that these shifts in the microbial functional traits explain the accumulation of profile-scale SOC. Our results demonstrate that integrating grass-type cover crop with no-tillage practices increases the subsoil SOC sequestration by improving soil microbial CUE and EE, leading to higher retention of C as microbial byproducts, and subsequently MAOM-C.