Exogenous iron mitigates photo-facilitation of soil organic matter

Abstract

Subalpine meadow soil carbon (C) is susceptible to ultraviolet B (UV-B) radiation, yet the mechanisms of UVB-induced soil organic C (SOC) photodegradation and the influence of iron (Fe) on this susceptibility remain largely unknown. In this study, soils from the southeastern (SE) and northwestern (NW) slopes of a subalpine meadow were exposed to three UVB treatments, elevated (ele-UVB, ∼120 μW·cm⁻²), ambient (amb-UVB, ∼60 μW·cm⁻²), or attenuated (no-UVB, 0 μW·cm⁻²), to assess the effects of UV radiation and Fe addition on SOC mineralization. A two-phase lignin incubation experiment was then conducted to elucidate the mechanism by which Fe influences the abiotic and biotic processes of lignin photodegradation. Results showed that ele-UVB increased SOC degradation by 137 % and 34 % in the SE and NW soils, respectively. Lignin phenols underwent significant photochemical degradation, which was mitigated by Fe addition. Furthermore, our findings revealed that photo-facilitation (i.e., microbial decomposition) significantly contributed to lignin photodegradation, releasing over 5 times more CO₂ than abiotic degradation did. This occurred mainly due to the depolymerization of lignin macromolecules, which increased the substrate availability for microbes, rather than shifts in microbial community composition. Fe impacted photo-facilitation by binding with lignin derivatives, reducing microbial accessibility and limiting their decomposition. These findings highlight the intricate interactions among UV-B radiation, Fe, and microbial processes in SOC turnover, offering critical insights for soil C management under global environmental change scenarios.