The Response Mechanism Of Soil Carbon To Nitrogen And Phosphorus Fertilization In Sub-alpine Meadow On The Qinghai-Tibetan Plateau

Soil carbon pool plays an important role in material energy cycle in terrestrial ecosystem and carbon dioxide concentration in the atmosphere. The soil carbon is also related to environmental issues like global warming. Some studies have reported that N and P fertilization can influence soil carbon concentration through enhancing plant biomass. Furthermore, it can influence soil’s bio-physicochemical properties, activities of microbes and soil enzymes. However, there is no concensus on the mechanism how soil carbon response to nitrogen and phosphorus fertilization, due to the complexities of different soil characteristics among different sites and all the effects induced by N and P fertilization. In this study we chose urea and disodium hydrogen phosphateas as the source of N and P fertilization, respectively. We selected three fertilizer types:N alone; P alone; and N+P together. Each fertilizer type had three gradient:5g·m-2yr-1, 10·gm-2yr-1, and 15g·m-2yr-1. A control treatment (CK) without any N or P fertilization was also included. As a result, there were ten treatments:N5, N10, N15, P5, P10, P15, N5P5, N10P10, N15P15, and CK. We divided plant species into three functional groups (grasses, legumes and forbs). Soils were sampled in two layers:0-20cm and 20-40cm depth.The results showed that:1. Fertilization increased aboveground biomass, but simultaneously decreased plant C:N at the community level. At functional groups level, fertilization increased grass biomass and decreased legumes and forb biomass. N or P fertilization reduced the C:N ratio but N+P fertilization increased C:N ratio of grass and forb. And all fertilization treatments decreased C:N ratio of legume group. All fertilization treatments decreased underground biomass and the C:N ratio.2. Soil microbial biomass (expressed as microbial biomass carbon) and microbial activity (expressed as cumulative C mineralization rate) increased in all fertilization treatments.3. Fertilization decreased phenol oxidase activities in all treatments and decreased peroxidase activity except N5 andN10 in 20-40cm. Sucrase activities in all treatments were higher than that in the control.To sum up, N and P fertilization increased biomass but decreased C:N of plant biomass, resulting in the increased percentage of liable carbon components. Fertilization treatments also increased soil microbial biomass and activity, increased sucrase activity but decreased phenol oxidase activity and peroxidase activity. The interaction between all these factors led to a decline in soil organic carbon. We concluded that fertilization can be used to increase plant biomass, meet the demands of agriculture and animal husbandry in short-term. However, in long-term, fertilization may accelerate carbon emission and inhibit carbon sequestration. Fertilization might not be a good way to maintain the health and sustainability of alpine meadow ecosystems.