Ten-year effects of perennial cropping systems on soil organic carbon stock and stability in sandy soils: Mechanisms and biochemical drivers

Perennial crops can be a sustainable alternative to annual crops due to plant traits and management practices that improve productivity and may contribute to soil organic carbon (SOC) sequestration. However, our understanding of the mechanisms behind the potential differences in SOC pools between perennials and annuals is incomplete, particularly in the sandy soils that dominate Danish croplands. Based on a 10-year field experiment on a temperate sandy soil with perennials (tall fescue, grass/legume mixture) and annuals (triticale monoculture, triticale in a rotation), we investigated SOC pools at depths of 0–20 cm (topsoil) and 20–50 cm (subsoil) through (i) physical fractionation into pools of particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) and (ii) chemical analysis of aliphatic-to-aromatic/carboxylic ratios via Diffuse Reflectance Infrared Fourier Transform Mid-Infrared Spectroscopy. These analyses were complemented with measurements of extracellular enzyme activities, microbial biomass, root biomass, and aboveground biomass C/N ratio. We found that tall fescue had significantly higher activities of nutrient-releasing enzymes (e.g., N-acetylglucosaminidase and acid phosphatase), and lower oxidase activities (peroxidase and phenol oxidase) in the topsoil compared to annuals. The grass/legume mixture had higher activities of C-, N-, and P-acquiring enzymes than annuals at both soil depths. Soil fractionation analyses showed no significant differences between tall fescue and annuals in POC and MAOC stocks in the topsoil. However, tall fescue exhibited a lower aliphatic to aromatic/carboxylic ratio in the topsoil compared to annuals, which correlated negatively with root biomass and phenol oxidase activity. In the subsoil, the MAOC stock in the tall fescue system tended to accumulate at a rate of 0.35 Mg ha⁻¹ yr⁻¹ compared to annual triticale, and was positively correlated to microbial biomass carbon. In contrast, the grass/legume mixture, without N fertilization, had limited potential for SOC stock increases at both soil depths. Altogether, the results emphasize the role of microbial processes in SOC dynamics and the importance of perennial cropping systems, such as tall fescue, in enhancing SOC stability during the transition from annual to perennial crops for biorefining on sandy soils.