Nitrogen deposition in subtropical forest: effect on temperature sensitivity of soil organic matter

An increase of nitrogen (N) deposition is predicted in the next 10 years by 50%, compared to the values observed 20 years ago. This, together with the increase of atmospheric temperatures can change the soil organic matter (SOM) to either stronger mineralization or offset effects can be observed, depending on the ecosystem, and initial characteristics of the soils at the sites. Thus, this experiment aimed to reveal the effect of N deposition on the temperature sensitivity of SOM from the P limit subtropical evergreen forest ecosystem. Soils were collected at the Heshan (HS) National Field Research Station of Forest Ecosystem located in Guangdong province, where an experiment with a complete randomized block design of N deposition or control was established in 2018 (100 kg N ha⁻¹ y⁻¹, during 2.5 years). The soil (0-20 cm, four replicates for each treatment) was incubated at 3 temperatures of 15, 25, and 35 °C during 112 d at 60 % of WHC. During the incubation, soil CO₂ efflux was constantly monitored, and three destructive samplings were done (at 10, 69 days, and at the end of the experiment). The soil was analyzed for the dissolved organic C, microbial biomass, available N pool, substrate use efficiency (with ¹⁴C-glucose), activities of C and P hydrolytic, and C oxidative enzymes, the content of microbial biomarkers, and functional gene abundances. The maximal mineralized SOC amount was found under 35 °C under N deposition and minimum at control 15 °C. The highest differences for the total CO₂ efflux were observed between N deposited and control plots at 35 °C (1.3 times), and smallest at 15 °C (0.8 times). Q₁₀ was higher for the temperature increase 15-25 °C (1.3 and 2.3 for the control and N, respectively) than for the 25-35 °C (1.16 and 1.6), and for the soil experienced N deposition. Microbial biomass was affected by both, N treatment and temperature. SUE was the highest at 15 °C in the middle of the incubation under N deposition, whereas no differences were found at the other sampling points or treatments. Activities of acid phosphatase decreased with the incubation for all temperatures, whereas b-glucosidase and xylanase had maximum in the middle of the incubation; all hydrolytic decreased activities by the end of the incubation under N treatment. Both oxidases were higher under N deposition than in control during the entire experiment, with the maximum found under 35 °C. Polyphenol oxidase activity increased at 35 °C with the incubation in both control and N deposition, however stayed constant for the other temperature treatments. In contrast, peroxidase activity was the same between the sampling times. Thus, the effect of N deposition on the activity of microbial biomass, expressed in SOM mineralization, appeared more clearly with the incubation temperature. Despite that, the complex of soil enzymes reacted differently to the studied impacts, with oxidative enzymes being more vulnerable than oxidative ones.