中国干旱半干旱区土壤CO_(2)与CH_(4)通量的耦联解耦及其对温度的响应

Coupling-decoupling of soil CO_(2) and CH_(4) fluxes and their responses to temperature in arid and semi-arid regions of China

干旱半干旱区是一类典型的生态脆弱区,同时又对全球变暖具有重要影响。其土壤以氧化型土壤为主,被认为是重要的CH_(4)汇,然而研究发现随着土壤吸收CH_(4)速率的升高,排放CO_(2)的速率也升高。为验证该消长现象是否广泛存在以及是否发生于特定环境条件下,该研究基于中国干旱半干旱地区的土壤温室气体通量与相关环境数据整合,首次开展了多站点的土壤CO_(2)与土壤CH_(4)通量季节变化耦联与日变化耦联分析。结果显示,土壤CO_(2)与土壤CH_(4)通量间存在协同(正相关)、消长(负相关)及随机(不相关)3种模式,其中随机变化的比例更高,季节尺度与日尺度上分别占比83%与54%。相对于水分和植被状况,温度与通量间相关性的关系更强,呈现为随气温升高通量间相关性下降的二次函数关系。季节尺度上,采样期间平均气温对通量间相关关系的判别准确率为92%,通量间耦联解耦的气温阈值为12.5℃;日尺度上,日气温差对通量间相关关系的判别准确率为79%,通量间耦联解耦的温度阈值为15.2℃。此外,日尺度上土壤为吸收CH_(4)状态时,土壤CH_(4)与土壤CO_(2)通量之间并非呈现为负相关关系,而更多呈现为正相关关系,这一现象难以仅用温度进行解释,我们推测土壤呼吸和CH_(4)氧化在竞争O2过程中形成了不对等的耦联关系,即土壤呼吸可通过消耗O2抑制CH_(4)氧化,从而出现土壤CO_(2)排放增加而CH_(4)吸收降低的现象。该研究表明,土壤CO_(2)和CH_(4)通量间可能存在温度调控嵌套O2竞争调控的耦联解耦机制,气候变暖可能导致两种通量在更广的空间上以及更长的时间上发生解耦,增加区域碳循环的复杂性以及碳通量评估的不确定性。

Aims Arid and semi-arid regions are typical ecologically fragile areas,and they also have an important impact on global warming.Those regions are considered to be important CH_(4)sinks since most soils are under aerobic conditions.Studies have found that along with the increase of CH_(4)uptake velocity,the rate of CO_(2)emissions also has increased.This study was carried out to examine whether there is an offset phenomenon and under what environmental conditions it occurs.Methods Based on the integration of soil greenhouse gas fluxes and relevant environmental data in arid and semi-arid regions of China,correlations between soil CO_(2)and soil CH_(4)fluxes,on seasonal and daily scales,were analyzed.Important findings The results showed that there were three levels of soil CO_(2)and soil CH_(4)flux,i.e.,synergy(positively correlated),offset(negatively correlated),and random(not correlated).Among which,the proportion of random relationships was the highest,on seasonal and daily scales 83%and 54%,respectively.Compared to water content and vegetation conditions,air temperature correlated with the correlations between the two fluxes more strongly,showing a quadratic relationship(the absolute values of correlation coefficients between fluxes decreased with increasing temperature).On a seasonal scale,the mean air temperature during the sampling period determined the correlations between the fluxes with an accuracy of 92%,and the air temperature threshold of flux coupling-decoupling was 12.5°C.On the daily scale,the diurnal air temperature difference determined fluxes relationships with an accuracy of 79%and the temperature threshold of flux coupling-decoupling was 15.2°C.In addition,when the soil was in the state of absorbing CH_(4)on a daily scale,the relationship between soil CH_(4)flux and soil CO_(2)flux was positive in more cases.This phenomenon was difficult to explain by temperature alone.We speculate that a one-way coupling relationship between soil respiration and CH_(4)oxidation formed through O2competition,that is,soil respiration would inhibit the CH_(4)oxidation by consuming O2,resulting in an increase in soil CO_(2)emissions and a decrease in CH_(4)absorption.The study suggests that coupling-decoupling of soil CO_(2)and CH_(4)fluxes might be driven by a mechanism of temperature regulation linked with oxygen competition regulation.Climate warming may cause decoupling of the two fluxes across space and time and increase the complexity of carbon cycles,thereby increasing the uncertainty of regional carbon flux estimations.