Effects Of Simulated Nitrogen Deposition On Soil Enzyme Activity During Litter Decomposition

With the increasing of global reactive nitrogen deposition, the structure and function of ecosystem will change, which has become one of research hot topic in global change. As the main part of terrestrial ecosystem, forest plays an important role in global carbon and nitrogen cycle. The decomposition of forest litter is the major source of soil nutrient and the important process of nutrient cycle. Soil enzyme is the active participator in soil biochemical process and litter decomposition. Therefore, under the background of atmospheric nitrogen deposition, research on litter decomposition, the change of soil enzyme activity and their relationship will be benefit to explaining forest litter decomposition and soil enzymes’trends under the elevated nitrogen deposition. This research can also provide scientific theoretical basis for material circulation and energy flow in forest ecosystem and it is meanful for forest and environment management.Three kinds of typaical low subtropical species (Pinus massoniana, Cunninghamia lanceolata and Schima superba) and soil were selected as researched object in this study. Four treatments, namely Bare Soil (BS), Pinus massoniana litter+Soil (PS), Cunninghamia lanceolata litter+Soil (CS) and Schima superba litter+Soil (SS) were set respectively, and all the treatments were put in our chamber system with the constant temperature of 30℃ and the soil water content of 60% soil maximum water holding capacity. Nitrogen diposition was simulated with NH4NO3, and three nitrogen levels were set in the research, namely natural nitrogen deposition (CK,30 kg N ha-1 a-1), low nitrogen (LN,60 kg N ha-1 a-1) and medium nitrogen (MN,120 kg N ha-1 a-1). The change of soil physical and chemical properties, decomposition rate of litter, element content, the enzyme involved in C, N and P cycle, CO2 flux, soil microbial biomass and the response to nitrogen deposition were analyzed in this research. The experiment lasted 232 days and the results were as follow:1. Soil acidification was alleviated by nitrogen deposition at the early stages of the experiment (30 days). However, nitrogen deposition could cause the reduction of soil pH and aggravate soil acidification after 30 days. Compared with initial soil, the content of TN, AN, NH4+-N and NO3’-N increased significantly with nitrogen deposition, but soil TN and AN decreased slightly with time.2. During the litter decomposition, Pinus massoniana litter decomposition rate was relatively fast at the early stage, and then gradually decline. For Cunninghamia lanceolata and Schima superba litters, the decomposition rates were relatively slow at the early stage, and then gradually rose. For CK treatment, Pinus massoniana, Cunninghamia lanceolata and Schima superba litter decomposition rates were 12.57%,20.33% and 19.08%. For LN and MN treatment, the rates of litter decomposition were 13.14%,21.91%,23.60% and 13.67%,19.46% and 16.58%, respectively. This suggested that the decomposition rate of Cunninghamia lanceolata litter was the highest, with the Schima superba litter in the medimum and the lowest for Pinus massoniana litter for CK and LN treatments. The nitrogen content in litter increased with exogenous nitrogen input, causing the decrease of the C:N ratio. There was no significant difference between different nitrogen levels for TP content in litter..3. Generally, soil phenol oxidase activities increased with exogenous nitrogen input, while β-glucosidase. urease, asparaginase and acid phosphatase activities showed the downward trend. Urease activity was basically in decline with nitrogen input, and the minimum was showed at the end of experiment. (3-gIucosidase and asparaginase activities decreased at the first stage and then became stable. P-glucosidase activity became stable in 60 days, and, it was appeared in 154 days for asparaginase. Protease and cellulase activities changed little during the test, and peroxidase activity characterized by the increase druing 94 days, and then gradual decrease. Urease, asparaginase and acid phosphatase activities were inhibited by the input exogenous nitrogen (NH4NO3), which might be own to the acidification or NH4+concentration.4. There were no obvious difference between different nitrogen levels for soil enzyme activities. However, in some special time and environmental conditions, β-glucosidase, phenol oxidase, peroxidase and asparaginase activities showed significant difference between the nitrogen levels. For example, β-glucosidase activity was significantly lower in LN and MN than those in CK for CS treatment, and phenol oxidase activity was inhibited significantly by nitrogen deposition in the middle and late stage (94-232 days) for CS treatment, with the activity significant lower for LN than that for CK. In the late stage of experiment (154～232 days), peroxidase activity in MN was significantly higher than that in CK for PS treatment. Urease activity in BS and SS treatments was CK>LN>MN at the end of experiment, and significant difference between nitrogen levels gradually appeared after the 94 days for asparaginase activity, with The results of CK>LN>MN for BS. PS and SS and LN>CK for CS treatment.5. Soil enzyme activities could be affected by litter decomposition. For example, cellulase activity for BS treatment was almost greater than other treatments at the early stage of litter composition, but the soil cellulase activity for PS, CS and SS treatments was higher than that for BS treatment at the late stage of litter decomposition (154～232 days). Some difference of peroxidase, urease and protease activities among different treatments were found druing litter decomposition, which was no obvious law. At the end of experiment, there significant difference of peroxidase, urease and protease activities among different treatments disappeared. The change of β-glucosidase activity was not similar for the four treatments. with β-glucosidase activity in BS treatment being the lowest in the 232th day and showing significant difference with that in SS treatment under natural nitrogen deposition. Phenol oxidase, asparagine enzyme and acid phosphatase activities did not show regular change as litter decomposition.6. Soil pH, NH4+and NO3- content might be the main factors affecting soil enzyme activity and there were no obvious difference between CK, LN and MN, so the difference of soil enzyme activity could not be distinguished between three nitrogen levels. There are significant correlation between the litter decomposition rate and the activities of β-glucosidase, urease, asparaginase and acid phosphatase. Significant correlation between soil enzyme (urease, asparaginase and acid phosphatase) activities and CO2 flux and microbial biomass were found in this research, but just for the significant correlation between cellulase and peroxidase activities and CO2 flux, which indicated that soil microorganism and litter decomposition were factors influencing soil enzyme activity.