中亚热带天然林土壤CH_4吸收速率对模拟N沉降的响应

Responses of CH_4 uptake rates to simulated N deposition in a nature forest in mid-subtropical China

陆地森林土壤是重要的大气甲烷(CH4)汇,大气氮(N)沉降增加对森林土壤CH4吸收速率影响突出。运用静态箱-气相色谱法对中亚热带天然林土壤CH4吸收速率对模拟N沉降的响应进行连续3a的观测;试验作3种N处理,分别为对照(CK,0 kg N·hm-2·a-1)、低氮(LN,50 kg N·hm-2·a-1)和高氮(HN,100 kg N·hm-2·a-1),每种处理重复3次,每个月采集气体1次,同时测定0—5 cm土壤温度和0—12 cm土壤含水量;分析不同N沉降水平土壤CH4吸收速率的差异、动态变化以及对土壤含水量和土壤温度响应,并探讨N沉降对土壤理化性质的影响。结果显示:天然林土壤(CK)平均CH4吸收速率为(-62.78±14.39)μg·m-2·h-1,LN和HN土壤平均CH4吸收速率分别下降了30.21%、7.24%,CK、LN和HN处理土壤CH4吸收速率季节变化趋势相似;观测期间土壤CH4吸收速率对LN响应达到显著水平(P<0.05),对HN响应则不显著(P>0.05);LN、HN处理前两年对土壤CH4吸收速率抑制作用均不显著(P>0.05),但在第3年LN极显著降低了土壤CH4吸收速率(P<0.01),HN处理对土壤CH4吸收速率的影响则在第3年表现为显著抑制作用(P<0.05),表明土壤CH4吸收速率对N沉降的响应随着N沉降时间的持续呈抑制效应加剧的趋势。相关分析表明:CK与HN土壤CH4吸收速率与土壤温度和土壤含水量均有显著相关性(P<0.05),但LN土壤CH4吸收速率仅与土壤含水量显著相关(P<0.05),表明土壤含水量是控制各N沉降处理土壤CH4吸收速率动态的主要环境因子。此外,LN、HN处理下土壤pH均极显著降低(P<0.01),但LN土壤pH极显著低于HN(P<0.01);LN处理极显著提高了土壤C/N比(P<0.01),HN处理则相反;LN和HN处理对土壤NH+4-N、NO-3-N、可溶性总N(TDN)、可溶性有机碳(DOC)、地面凋落物量、地下0—10 cm细根生物量影响均不显著(P>0.05),表明一定时期内N沉降首先引起了土壤pH和土壤C/N比的显著变化。

The anthropogenic emission of nitrogen （ N ） compounus 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-1 from 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 importantCH4 sink or source through the activity of methanotrophic and methanogenes bacteria. The diffusivity of CH4 through the soil profile is the primary limiting factor upon CH4 uptake, which is influenced by soil moisture density. Furthermore, soil available N contents （NH4 and NO3 ） can limit CH4 uptake 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 deposition- induced suppression of CH4 uptake in forest soils. In this research, we studied the responses of soil CH4 uptake 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-1 for control （CK） , low-N （LN） , and high-N （HN） treatment, respectively. From December 2009 to November 2012, monthly CH4 uptake rates were measured using a static chamber and gas chromatography technique. Average CH4 uptake rate in control was （-62.78±14.39）μg·m-2·h-1,LN Comparing with the control, average CH4 uptake 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-I ） and the control （ P 〉 0.05 ）. The average rates of CH4 uptake for the LN and HN were both significantly lower than the control in the third year （P〈0.05） , which demonstrated that, the CH4 uptake rates for all N treatments might be seriously inhibited with continuing N addition. Seasonal changes of CH4 uptake rate in all N treatments were similar. Correlation analysis showed that CH4 uptake rates were significantly correlated with soil temperature and soil moisture both in the control and HN treatments （P〈0.05）, but only significant correlation between CH4 uptake rates and soil moisture was found for LN （P〈0.05）. It suggested that the change of soil moisture was the most important factor which regulated the dynamics of CH4 uptake rates. In addition, soil pH values for the LN and HN were significantly lower than that in the control （P〈0.01）. 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 NHj-N, NO2- N, total dissolved N, dissolved organic carbon（C）, litterfall mass, fine root biomass in 0--10 cm soil （P〉0.05）. Thus, it indicated that N deposition might firstly induce the changes of soil pH and soil C/N ratio.