Effect Of Nitrogen Addition And Litter Treatment On Soil Respiration In A Mid-subtropical Mountain Forest

Nitrogen deposition has increased three-to fivefold over the past century due to intense human activities, and it is expected to increase by a factor of 2.5 by the end of this century. China is one of the three high nitrogen deposition areas. N deposition has a significant effect on global carbon cycle. Increased N deposition usually result in an increase of exogenous carbon input, which can cause a serious consequences on ecosystem carbon cycle and soil respiration. The influence of nitrogen deposition on soil respiration depends on forest type, soil nutrient status and the amount of nitrogen deposition including promotion, inhibition or no effect. Studies were mostly conducted in temperate and boreal forests with inconsistent results. Few studies have examined the response of soil respiration to N addition and litter treatment in subtropical forests where N deposition rates are projected to increase as a result of rapid expansion in industrial and agricultural activities. Here we conducted N addition and litter treatment experiments in a subtropical mountain forest in central China to better understand the short-term responses of soil respiration. The treatments consist of control (no N addition) and two level of N addition:low-N (30 kg N ha-1yr-1) and high-N (60 kg N ha-1yr-1) with double amount of litters and litter exclusion. The main conclusions are as follows:(1) There were no significant difference among different nitrogen treatments of soil respiration. Nitrogen addition had no significant effect on soil respiration. The maximum value appeared in July or August and the average rate of soil respiration were 1.91,1.6 and 1.52μmol CO2·m-2s-1. Although differences among N treatment were not statistically significant, the average soil respiration was lower for the high-N addition.(2) There were obvious monthly pattern in autotrophic respiration and heterotrophic respiration, while nitrogen addition had no effect on the temporal dynamics or autotrophic respiration and heterotrophic respiration.(3) Litter treatment had significant effects on respiration:litter addition increased the soil respiration by 34.5% and litter exclusion reduced the soil respiration by 23.9%. The decrement in litter exclusion was lower than the increment in the litter addition, suggesting that the effect of litter on soil respiration was not only from itself, but also from the priming effect caused by it.(4) Litter addition with low nitrogen treatment increased the average soil respiration with 11.5% increment and 42.5% increment with high nitrogen treatment, while litter exclusion reduced by 34.9% in low nitrogen treatment and increased by 3% in high nitrogen treatment. There existed significant difference between litter addition and litter exclusion treatment in the same nitrogen treatment and no significant difference with control treatment. The results suggests that nitrogen may influence the priming effect and the magnitude and direction may change with the different nitrogen concentration due to the C/N supplement.The results indicated that nitrogen addition had no significant influence while litter treatment had significant effect on soil respiration. The influence of litter treatment varied with nitrogen supply, the reason of which was likely that nitrogen could affect the priming effect of litters. Our findings were based on short-term treatment while it is urgent to conduct long-term research on the effect of nitrogen deposition and litter dynamics on soil respiration.