离子树脂法测定森林穿透雨氮素湿沉降通量--以千烟洲人工针叶林为例

Determination of nitrogen deposition in throughfall using ion-exchange resins:a field test in planted coniferous forest ecosystem at Qianyanzhou

穿透雨是大气氮素输入森林生态系统的重要途径之一,穿透雨中氮素含量的定量评估在森林生态系统氮素循环研究中的作用不可忽视。穿透雨中氮沉降通量的空间异质性很强,传统降水收集法工作量大,且容易带来测定误差。分析了国产离子树脂测定大气氮素湿沉降的可行性,并以千烟洲人工针叶林为例探讨离子树脂法测定森林穿透雨的适用性。结果表明,离子交换树脂法和传统降水收集法测定值之间的相关性显著,离子交换树脂法可以很好的反映大气氮沉降通量和季节变化特征,并且在采样周期较长时也能准确测定氮沉降组分,是适用于野外站点林内穿透雨氮沉降通量的观测方法。千烟洲人工针叶林穿透雨的氮沉降通量为9.19 kgN.hm-2.a-1,夏季的5—7月份和冬季的1—2月份出现氮沉降通量高峰。夏季穿透雨氮沉降以铵态氮为主,而冬季以硝态氮为主。千烟洲人工针叶林的氮沉降通量与附近地区针叶林穿透雨氮沉降通量近似,低于临近区域阔叶林穿透雨的氮沉降通量水平,但已可能接近森林生态系统氮输出出现强烈反应的氮沉降临界值。

Throughfall is an important pathway of atmospheric nitrogen deposition into forest ecosystems.Qualifying temporal dynamics of the atmospheric nitrogen deposition via throughfall is critical to reveal nitrogen cycle in forest ecosystems.The traditional method for collecting and analyzing throughfall nitrogen,i.e.,the throughfall-gathering method,is labor intensive,and it may induce large uncertainties due to the spatial heterogeneity of throughfall and the instability of the chemical solution.In this study,we tested the feasibility of using domestic ion-exchange resin columns,a labor-saving method,to measure throughfall nitrogen deposition at a subtropical planted coniferous forest ecosystem.A laboratory study was conducted to investigate the efficiency of using domestic anion-cation exchange resins in absorbing dissolved inorganic nitrogen and the efficiency of 0.2 mol/L KCl in releasing inorganic nitrogen ions back into solution from ion exchange resins.Results showed that domestic ion exchange resins could extract more than 99% inorganic nitrogen ions in the standard series solutes and the 0.2 mol/L KCl extraction efficiencies were 〉90% after three times of extraction.A field experiment was conducted in a planted coniferous forest at Qianyanzhou station,where both the traditional rainfall-gathering method and the ion-exchange resin columns method were deployed.Ion exchange resin columwere used to absorb nitrogen ions in rainfall and thoughfall.After a month exposure,the nitrogen ions were extracted out from the resin columns back to solute with 0.2 mol/L KCl.Results indicated that measurements of the two methods were in agreement with each other throughout the whole year,confirming that the ion-exchange resin columns could successfully detect the seasonal dynamics of wet nitrogen deposition.Further,the ion-exchange resin column method performed better in discriminating the amount of nitrate and ammonium composition,owing to its ability in capturing ions by the functional groups with different electric charges.Therefore,this ion-exchange resin columns method is practical to be used at remote sites.According to the measurements of the ion-exchange resin columns method,the total annual inorganic nitrogen deposition in throughfall in Qianyanzhou planted coniferous forest was 9.19 kgN·hm^-2·a^-1.,similar to the neighboring natural coniferous forest but lower than the broadleaf forest.Inorganic nitrogen deposition showed obvious seasonal variations within the whole year.The nitrogen deposition in throughfall at Qianyanzhou peaked in May to July and ammonium accounted for more than 80% of the total wet deposition,which was mainly due to ammonia volatilization after the rice topdressing.In addition,the decomposition of animal carcass and plant residues caused by the high temperature and humidity also contributed greatly to nitrogen deposition in summer.In January to February,due to NOx emission as a result of burning coal for heating,there was another peak of nitrogen deposition in throughfall,with nitrate being the dominant form.Nitrogen deposition in throughfall is linearly correlated with precipitation in Qianyanzhou planted coniferous forest.The annual precipitation in the study year is about 450 mm,lower than the mean annual value of this site.Therefore,the nitrogen deposition at this planted coniferous forest site is likely more than 10 kgN·hm^-2·a^-1.