树木N_2O排放速率的测定

N_2O EMISSION RATE FROM TREES

采用封闭罩法 ,对长白山阔叶红松林的几个主要树种———水曲柳 (Fraxinusmandshurica)、红松 (Pinusko raiensis)、毛赤杨 (Alnushirsuta)和椴树 (Tiliaamurensis)的连体枝叶排放N2 O的速率进行了原位测定 ,并考察了树木的N2 O排放速率与土壤含水量的关系。结果表明 :在观测期间 ,4种树木的N2 O排放速率的变化范围分别为 :水曲柳 9.46～ 15 2 .85ngN2 O·g-1DW·h-1、红松 1.37～ 6 4.5 1ngN2 O·g-1DW·h-1、赤杨 - 12 .0～ 16 .1ngN2 O·g-1DW·h-1、椴树 - 11.2～ 16 .1ngN2 O·g-1DW·h-1。它们的平均N2 O排放速率分别为 39.5 1、2 3.2 1、2 5 .6 9、0 .0 3ngN2 O·g-1DW·h-1。树木的N2

The sources and sinks of N 2O, one of the major greenhouse gases in the atmosphere, are still unclear at present, and it is therefore needed to further investigate potential and known sources and sinks of atmospheric N 2O. It was previously thought that biogenic nitrous oxide was produced only by soil microbial processes. However, in recent years, some studies have suggested that plants may play an important role in N 2O emission/uptake. Plants make up the largest fraction of biomass in the biosphere. Forest ecosystems play an important role in the biogeochemical cyclings of nitrogen and carbon. Therefore, determining the role of plants in N 2O emission/uptake under natural conditions would provide another component to our understanding of atmospheric N 2O budget imbalance. In previous studies on N 2O emission from forest ecosystems, only soil measurements of N 2O emission were conducted. The total N 2O emission from forest ecosystem should be underestimated if plants can emit N 2O under natural conditions. Objectives of this study were: 1) to measure N 2O emission rates from trees; 2) to understand the major factors influencing N 2O emission rate by trees; 3) to estimate the contribution of trees to N 2O emission in a forest ecosystem. Using the closed bag method, N 2O emission rates from branches and leaves of Fraxinus mandshurica, Pinus koraiensis, Alnus hirsuta and Tilia amurensis within coniferous-deciduous mixed forest in Changbai Mountain were firstly measured in situ from July to September, 2000 twice or three times a month, and the factors affecting N 2O emission rate from these trees were also studied. The results showed 1) the range of N 2O emission rates from Fraxinus mandshurica, Pinus koraiensis, Alnus hirsuta and Tilia amurrensis were 9.46～152.85, 1.37～64.51, -12.0～16.1 ng N 2O·g -1 DW·h -1 and -11.2～16.1 ng N 2O·g -1 DW·h -1,respectively; 2) the mean N 2O emission rates from the four species were estimated as 39.51, 23.21, 25.69 ng N 2O·g -1 DW·h -1 and 0.03 ng N 2O·g -1 DW·h -1, respectively; 3) a positive correlation between N 2O emission rates and soil water content was also observed. The results also indicated that plants in natural condition are able to emit N 2O, but their source strengths could be equal to that of the soil. The study provided the direct evidence supporting that plants per se, in addition to forest soil, were an important source of atmospheric N 2O in a forest ecosystem. Underestimating N 2O emissions from trees in forest ecosystems previously may be one possible reason for the unbalanced global N 2O budget.