Abstract: Increasing soil organic carbon (SOC) mineralization rate is an important ecological process that most terrestrial ecosystems are undergoing under the context of global UVB enhancement. Iron (Fe) oxides could affect the stability of SOC through adsorption or coprecipitation with carbon, and play an important role in regulating the process of soil carbon cycling. With an incubation experiment, we examined the effects of Fe addition on the photodegradation of SOC in surface soils (0-5 cm depth) from the southeast and northwest slopes of Shunwangping subalpine meadow in Lishan National Nature Reserve, Shanxi, northwest China. There were four treatments: (1) ambient light intensity under natural room light exposure (AM), (2) AM+0.1 g Fe(II) as FeCl₂/0.02 kg soil (AM+Fe), (3) minus 280-320 nm ultraviolet-B radiation with Mylar tape under no-Fe addition (-UVB), and (4) UVB + Fe addition (-UVB+Fe). The UVB radiation was 60 μW·cm^-2 at noon of 12:00 pm and no UVB radiation was detected under the -UVB treatment. Fresh soil was incubated at 25±1 oC under 50% field water-holding capacity for 1, 3, 5, 10, 15, 20, 30, and 50 days. CO₂ released from incubated soil was absorbed with 0.1 mol·L^-1 NaOH and then analyzed firstly with 1 mol·L^-1 BaCl₂ and then 0.05 mol·L^-1 HCl. Results showed that the cumulative mineralization and mineralization rate of SOC were significantly greater in the northwest slope than that in the southeast slope, mainly because the lower concentration of amorphous iron (Feo-p) in soil from the northwest slope led to a weaker SOC protection. Moreover, photodegradation of SOC occurred under UVB radiation, which promoted the mineralization of soil organic carbon. The increasing extent of mineralization amount and mineralization rate under UVB radiation, namely photo-priming, was significantly higher in the northwest slope than that in the southeast slope. The addition of exogenous Fe resulted in a significant decrease of both cumulative mineralization and mineralization efficiency of SOC, as well as a significant decrease of SOC photodegradation in both slopes. Our results demonstrated that SOC in the northwest slope has strong photodegradation potential with the continuous enhancement of UVB radiation in the future, and that exogenous Fe addition could alleviate the photodegradation of surface SOC. This study can provide theoretical basis for understanding of SOC sequestration mechanism in subalpine meadows under the background of climate change.

Key words: UVB radiation, soil organic carbon mineralization, iron oxide, photochemical degradation