中亚热带天然林土壤CH4吸收速率对模拟N沉降的响应

Abstract

The anthropogenic emission of nitrogen( N) compounds is increasing globally. As a developing country,China has a dramatic increase in atmospheric N deposition since 1980s,and the rate of N addition in China is increasing at a speed of 0.41 kg N hm-2a-1from 1980 to 2010. China has become one of the most important N deposition zones in the world.By comparison with the national average,the rates of N deposition are higher in southeast China. Excess N deposition has aroused concerns about its negative impacts on ecosystem health and services such as loss of biodiversity,N saturation and soil acidification,and inhibition of the capability of methane( CH4) uptake in upland soils. Forest soils are an importantCH4sink or source through the activity of methanotrophic and methanogenes bacteria. The diffusivity of CH4through the soil profile is the primary limiting factor upon CH4uptake,which is influenced by soil moisture and bulk density. Furthermore,soil available N contents( NH-4and NO-3) can limit CH4uptake directly by competing with the monooxygenase enzyme of methanotrophs. Many studies including field and laboratory studies have reported that increased N availability due to N deposition may inhibit the uptake capacity of forest soils for atmospheric CH4. However,some studies in subtropical China get different results. Further studies should be undertaken to better understand the mechanisms responsible for N depositioninduced suppression of CH4uptake in forest soils.In this research,we studied the responses of soil CH4uptake rates to simulated N deposition in a natural forest in Jianou,Fujian,China based on the design and methods used in the European N ITREX project. Treatments included three N levels( three replicates for each level),viz. 0,50,and 100 kg N·hm-2·a-1for control( CK),low-N( LN),and high-N( HN) treatment,respectively. From December 2009 to November 2012,monthly CH4uptake rates were measured using a static chamber and gas chromatography technique. Average CH4uptake rate in control was(-62.78±14.39) μg·m-2·h-1.Comparing with the control,average CH4uptake rate of the LN((-43.82±4.72) μg·m-2·h-1) was decreased by 30.21%.However,there was no significant difference between HN((-58.23±5.58) μg·m-2·h-1) and the control( P0.05). The average rates of CH4uptake for the LN and HN were both significantly lower than the control in the third year( P0.05),which demonstrated that,the CH4uptake rates for all N treatments might be seriously inhibited with continuing N addition.Seasonal changes of CH4uptake rate in all N treatments were similar. Correlation analysis showed that CH4uptake rates were significantly correlated with soil temperature and soil moisture both in the control and HN treatments( P0.05),but only significant correlation between CH4uptake rates and soil moisture was found for LN( P0.05). It suggested that the change of soil moisture was the most important factor which regulated the dynamics of CH4uptake rates. In addition,soil pH values for the LN and HN were significantly lower than that in the control( P0.01). Also,the LN treatment had lower soil pH value than the HN treatment. Soil C /N ratio for the LN treatment increased significantly compared with the control( P 0. 01),while the reverse was for the HN treatment. There were no significant effects of N treatments on soil NH+4-N,NO-3-N,total dissolved N,dissolved organic carbon( C),litterfall mass,fine root biomass in 0—10 cm soil( P0.05). Thus,it indicated that N deposition might firstly induce the changes of soil pH and soil C /N ratio.