Shading gradients shape soil carbon stabilization mechanisms in a horticultural agri-photovoltaic field in Germany

Agri-photovoltaic (Agri-PV) systems combine crop production with renewable energy generation, but their effects on soil carbon (C) stabilization remain poorly understood. We investigated how shading in a Agri-PV field influences C pools in western Germany. Soils were sampled (0–30 cm) in 2025 in transects (six sampling points, six field replicates) across under-panel (UP) and inter-panel row (GAP) zones of a horticultural Agri-PV system established in 2021. An adjacent open field with same crop history (broad bean and green rye 2023, calendula and peat/oat 2024, spinach 2025) was sand ampled as control. Measurements included soil water content, microbial biomass C (Cmic), total organic C (TOC), and C in organic matter fractions (free-light, FLF; occluded-light, OLF; mineral-associated, MAOC). Relative to the control, soil moisture was 9–25% lower in UP zones and 11–21% higher in GAP zones in Agri-PV, reflecting rainfall redistribution by panels. Also, Cmic decreased by 39–48% in UP and 18–26% in GAP zones. Compared to the control, the stocks of TOC, FLF, OLF, and MAOC in Agri-PV soil declined by 16–29%, 31–72% (mainly in UP), 45–53% (mainly in GAP), and by 20–29% (mainly in UP). Within the Agri-PV field, UP zones were drier, had lower Cmic, TOC, FLF, and MAOC stocks, whereas OLF was more significantly reduced in GAP zones. These gradients were absent in the control. Overall, fixed-tilt south-facing Agri-PV systems can markedly disrupt soil C stabilization by reducing biomass inputs and destabilizing soil structure, with implications for long-term SOC storage, soil resilience and health.