Soil Organic Carbon Distribution, Enzyme Activities, and the Temperature Sensitivity of a Tropical Rainforest in Wuzhishan, Hainan Island

Soil carbon (C) distribution, which is deeply correlated with soil extracellular enzyme activity and its temperature sensitivity (Q(10)), are important for predicting the soil organic carbon (SOC) pools under climate warming. However, the high spatial heterogeneity in tropical forest soils makes the predicted results vary significantly. Thus, a total of 87 soil samples of three forest types, eight slope orientations, and four vegetation types were selected from Wuzhishan in Hainan Island, China. SOC distribution, the activities of six soil enzymes, and Q(10) at 10, 20, 30, and 40 degrees C were investigated to supplement the tropical data. The results showed that: (1) SOC ranged from 24.82 to 87.72 g/kg. SOC in the primary forest was significantly higher than that of the secondary forest, and SOC of the cloud forests and hilltop scrub at higher elevations was significantly higher than that of the lowland rainforests and montane rainforests at lower elevations. However, the amount of recalcitrant carbon in the primary forest was the lowest. (2) Under lab temperature conditions, the acid phosphatase and beta-1,4-glucosidase in the plantation forest were significantly higher than that of the secondary forest, and the polyphenol oxidase and catalase in the plantation forest were significantly higher than that of the primary forest. Enzyme stoichiometry analysis indicated that microbials were limited by nitrogen in the study area. The six soil enzyme activities were strongly correlated with SOC and total nitrogen. (3) The Q(10) of soil enzymes ranged from 0.61 to 1.92 under three temperature gradients. Most hydrolases enzymes (Q(10) > 1) showed a positive response with temperatures from 10 to 30 degrees C, and showed a negative response (Q(10) < 1) with temperatures from 30 to 40 degrees C. We concluded that the negative response of enzyme Q(10) with global warming would slow down the SOC decomposition. Primary tropical forests could still sequester SOC; however, their ability to do so may be vulnerable to climate change, as the amount of soil C quality index was low.