Woody perennial polycultures and their influence on soil organic carbon

Woody perennial polycultures (WPPs) can contribute to sequestration of soil organic carbon (SOC). We studied how WPP diversity and density, and landscape features influence SOC change. We quantified SOC, particulate organic matter‐carbon and ‐nitrogen (POM‐C and POM‐N), potentially mineralizable nitrogen (PMN), and inherent properties in soil collected in 2015 and 2022 using plot‐ and grid‐based sampling approaches. The SOC stocks increased in the top 60 cm in 3‐ and 4‐species (3‐Sp and 4‐Sp) treatments, double density 3‐Sp (3‐SpX2), and corn ( Zea mays L.)‐soybean ( Glycine max (L.) Merr.) control (CS). Gains ranked 3‐Sp > CS > 3‐SpX2 > 4‐Sp and ranged from 0.6 to 3.5 Mg C ha −1  year −1 . Only differences between 3‐Sp and 4‐Sp were statistically significant. Gains in the CS are attributable to their relatively small plot size (one‐fifth that of WPP plots) that reduced erosion and increased deposition. Gains in POM found in the pastured inter‐rows drove SOC stock change in the 0‐ to 30‐cm depth. Comparatively, small POM gains in the WPP rows suggest WPP diversity and density were not important drivers of SOC change. Multiple linear regression of grid‐based data revealed that SOC gains in the surface were positively related to PMN and negatively related to elevation, and SOC change was less than that seen at the 30–60 cm that was positively related to available P. Net SOC change is the result of plant productivity in the surface and of erosion, deposition, and leaching at depth. The use of both treatment‐ and spatial‐based sampling was needed to understand where and how vegetative inputs and landscape features determined SOC change.