Shading gradients shape soil microbial biomass carbon and carbon stability in a horticultural agri-photovoltaic field in Germany

Agri-photovoltaic (Agri-PV) systems are increasingly implemented for combined food and renewable energy production, yet their impacts on soil carbon cycling and stabilization remain insufficiently understood. This study aimed to understand how shading in a horticultural Agri-PV field alters microbial activity, soil carbon (C) stocks and stability compared with a conventional open-field management in western Germany.

The experimental fields were established in 2021 and soils (homogeneous silt loam: 5% sand, 15% clay, 80% silt) were sampled (0–30 cm) in 2025 along replicated transects across under-panel (UP) and inter-row (GAP) zones in the Agri-PV field. This Agri-PV system consisted of south-facing, fixed-tilt PV modules inclined at 20° and mounted at a maximum height of 4.30 m. An adjacent, identically managed Control open-field was sampled using the same approach. We measured gravimetric soil water content, microbial biomass C (Cmic), aggregate size distribution (8–2 mm, <2 mm), total organic carbon (TOC) stocks, and organic carbon stocks in density organic matter fractions (free-light, FLF; occluded-light, OLF; mineral-associated, MAOC), together with δ¹³C of bulk soil and fractions.

Relative to the Control, Agri-PV soils showed degradation of biological functioning and carbon pools across UP and GAP zones. Soil moisture was 22–24% lower in UP zones and 11–21% higher in GAP zones, reflecting rainfall redistribution by the panel structures. Cmic declined by 39–48% in UP zones and by 18–26% in GAP zones. TOC stocks were 16–29% lower in Agri-PV than in the Control. FLF stocks declined by 44–72% in UP zones and by 36–45% in GAP zones, reflecting reduced plant growth and biomass addition to soil in Agri-PV field. This partially explains why OLF stocks were reduced by 45–53% in GAP zones, while MAOC declined by 20–29% in UP zones and 10–12% in GAP zones in the Agri-PV compared to respective sampling positions in Control. Carbon fractions were consistently enriched in ¹³C relative to the Control (+0.18‰ to +0.33‰ in Bulk Soil, up to +1.26‰ in FLF and +0.63‰ in OLF), indicating enhanced microbial processing and reduced fresh biomass inputs.

Within the Agri-PV system, strong spatial heterogeneity emerged due to shading. UP zones were 20–40% drier than GAP zones and contained 20–30% less Cmic. The <2 mm aggregate size percentage was 12–30% higher in UP zones than in GAP zones, indicating pronounced aggregate breakdown beneath the panels. These microenvironmental gradients drove a clear redistribution of carbon pools within the Agri-PV system: TOC, MAOC, and FLF stocks were 9–17%, 14–22%, and 13–53% higher in GAP than in UP, respectively, whereas OLF stocks accumulated preferentially in UP, where they were 36–70% higher than in GAP. No comparable spatial gradients were observed in the Control, indicating that the patterns in Agri-PV are attributable to shading.

Our results demonstrate that fixed-tilt south-facing Agri-PV systems can substantially disrupt soil C stabilization by simultaneously reducing biomass inputs and by destabilizing soil structure (in silt-rich soils), with important implications for long-term soil resilience and carbon stabilization.