杨树人工林生态系统降水再分配及主要离子特征

大气降水是森林生态系统养分输入的主要途径之一,对养分的生物地球化学循环有着重要的意义。对13年生杨树人工林林外雨、树干流、林内雨和地表径流等水文过程中的养分特征进行了调查分析,旨在了解该生态系统的养分输入与输出规律,为杨树人工林可持续经营提供依据。结果表明,从2013年11月至2014年10月,杨树人工林生态系统林外雨量为1154.1 mm,树干流量仅占大气降水量的2.3%,15.4%的大气降水被杨树人工林的冠层截留;林内雨、树干流与大气降水量(林外雨)的动态变化规律相似。各类降水年加权平均pH值表现为林内雨林外雨树干流;各类降水的离子浓度动态变化规律基本一致,即在降水量较小的11月至次年1月份,各阴阳离子的浓度普遍较高,在降水量较大的2—9月份,阴阳离子浓度普遍较低。SO_4~(2-)-S和Ca~(2+)分别是各类降水中的主要阴离子和阳离子;整体上,树干流的离子浓度林内雨大气降水;林内雨是养分输入的主要形式,通过林内雨输入林地较多的养分离子是Ca~(2+)和K~+,分别为70.83 kg hm~(-2)a~(-1)和63.31 kg hm~(-2)a~(-1);地表径流和土壤渗漏是养分输出的主要形式,输出林地较多的离子是Cl~-和Ca~(2+),分别为196.47 kg hm~(-2)a~(-1)和123.09 kg hm~(-2)a~(-1),其次为SO_4~(2-)-S、Mg~(2+)、Na~+、K~+;NH_4~+-N和NO_3~--N的输出量不足输出离子总量的1%。所以,从水文过程看,杨树人工林生态系统无机氮(NH_4~+-N和NO_3~--N)和K~+表现为净积累,净积累量分别为10.9 kg hm~(-2)a~(-1)和56.4 kg hm~(-2)a~(-1),其他离子表现为净损失,其中Cl~-的净损失量达179.8 kg hm~(-2)a~(-1)左右,其他离子损失量50 kg hm~(-2)a~(-1)。

Reallocation and chemical characteristics of atmospheric precipitation in a poplar plantation

Atmospheric precipitation is an important vehicle for nutrient input and nutrient return in forest ecosystems; it plays a significant role in nutrient biogeochemical cycling. In the present study, we investigated the amount of precipitation and nutrient ion concentrations of atmospheric precipitation, throughfall, stemflow, and surface runoff in a 13-year-old poplar plantation during the period of November 2013 to October 2014. The results showed that:The annual bulk precipitation from November 2013 to October 2014 was 1154.1 mm. The temporal fluctuations in precipitation amounts in throughfall and stemflow were similar to those with bulk precipitation. Only 2.3% of the annual bulk precipitation was partitioned into stemflow. After subtracting throughfall and stemflow, approximately 15.4% of the annual precipitation was intercepted by the forest canopy. The highest annual average pH value was in throughfall, followed by bulk precipitation and stemflow. The temporal fluctuations in different ion concentrations in bulk precipitation, throughfall, and stemflow were similar. The ion concentrations in different kinds of precipitation were higher during November to January, which had less bulk precipitation; whereas the ion concentrations were lower during February to September, which experienced greater bulk precipitation. The dominant ions were SO₄^2--S and Ca²⁺ in bulk precipitation, throughfall, and stemflow. The order of ion concentrations generally was stemflow > throughfall > bulk precipitation. The annual weighted average concentrations of SO₄^2--S, Ca²⁺, Cl^-, K⁺, and Mg²⁺ in throughfall were 1.9, 1.3, 1.4, 5.6, and 2.0 times that of the corresponding ions in bulk precipitation, respectively. However, the annual weighted average concentration of K⁺ in stemflow was 20 times that in bulk precipitation; the concentrations of the other nutrient ions in stemflow were 2-6 times that of those in bulk precipitation. The primary form of nutrient input was throughfall, and the majority of the nutrient ions input into the plantation ecosystem through throughfall were Ca²⁺ and K⁺ with 70.83 and 63.31 kg hm^-2 a^-1, respectively; and nutrient output during hydrological fluxes occurred mainly through surface runoff and deep percolation, the most nutrient ions, which were output of the plantation ecosystem, were Cl^- and Ca²⁺ with 196.47 and 123.09 kg hm^-2 a^-1, respectively, followed by SO₄^2--S, Mg²⁺, Na⁺, and K⁺, whereas the NO₃^--N and NH₄⁺-N only accounted for < 1% of total nutrient ions that were output from the ecosystem. For the annual nutrient element fluxes, inorganic N (NO₃^--N and NH₄⁺-N) and K⁺ had a positive balance, and the other nutrient ions showed a negative balance. The net accumulations of inorganic N and K⁺ were 10.9 and 56.4 kg hm^-2 a^-1, respectively; the net losses of Cl^-, and Mg²⁺, Ca²⁺, SO₄^2--S, and Na⁺ through surface runoff and deep percolation were 179.8 and < 50 kg hm^-2 a^-1, respectively.