太湖流域典型稻麦轮作农田生态系统碳交换及影响因素

利用涡度相关技术观测太湖流域典型稻麦轮作农田生态系统2a净生态系统碳交换(NEE)变化过程,分析其碳交换特征及影响机理,结果表明:太湖流域典型稻麦轮作农田年NEE为-749.49—-785.38 g C m-2a-1,考虑作物籽粒碳和秸秆还田后净吸收88.12 g C m-2a-1,为弱碳汇;稻/麦季日均NEE和白天NEE季节变化直接受作物植被生长影响;麦季夜间NEE与10 cm土壤温度呈显著指数关系,2012/2013年温度敏感系数(Q10)分别为3.03和2.67;当土壤水分低于田间持水量时,麦季夜间NEE主要受土壤温度影响,反之,夜间NEE受土壤温度和水分双重影响;降水对麦季夜间NEE有短时的激发效应;稻季淹水对土壤呼吸产生较明显的阻滞效应,降低了夜间NEE对土壤温度的敏感性,2012和2013年分别为1.88和1.39,稻季淹水与烤田交替变化对土壤呼吸产生明显的抑制或激发的短时效应。

Analysis of net ecosystem CO2 exchange (NEE) in the rice-wheat rotation agroecosystem of the Lake Taihu Basin, China

The agroecosystem plays an important role in global and regional carbon balance due to its large area and high carbon sequestration potential. Due to limitations in observation techniques and the spatial heterogeneity of the environment, net ecosystem CO₂ exchange (NEE) in the rice-wheat rotation system of the Lake Taihu Basin remains poorly understood. This paper aimed to investigate NEE in this agroecosystem based on observations of CO₂ flux with the eddy covariance technique, from December 2011 to November 2013. Annual carbon balance was estimated from observed NEE and carbon contents of grain and straw. Half-hourly flux data were first corrected by removal of anomalous data points, coordinate rotation, frequency loss correction, and WPL correction. Two neural network models for daytime and nighttime NEE gap-filling were built. The results showed that annual daytime and nighttime NEE was -749.49 gC m^-2 a^-1 and -785.38 gC m^-2 a^-1, respectively. Accounting for grain removal and return of straw to the field, total net C absorption was 88.12 gC m^-2 a^-1, characterizing the agroecosystem as a weak carbon sink. This carbon sequestration capacity is greater than those of the Huaihe River Basin, subtropical region, and north China plain. Diurnal NEE exhibited a typical "W" bimodal seasonal pattern, and both seasonal NEE and average monthly diurnal NEE showed significant annual fluctuations. Cumulative diurnal NEE ranged from -12.88 gC m^-2 d^-1 to 5.94 gC m^-2 d^-1, with a mean of -2.10 gC m^-2 d^-1. Maximum cumulative diurnal NEE occurred in the wheat season of each year, with values of -12.88 gC m^-2 d^-1 on April 26, 2012 and -11.62 gC m^-2 d^-1 on April 11, 2013, respectively. Variation in diurnal NEE and daytime NEE in the rice-wheat season was significantly correlated with crop height, suggesting that diurnal NEE and daytime NEE were both influenced by crop growth. Nighttime NEE and soil temperature at 10 cm during the wheat season exhibit a significant exponential relationship, with a temperature sensitivity coefficient (Q₁₀) of 3.03 and 2.67 in 2012-2013, respectively, larger than that of many soil respiration models (Q₁₀ = 2.0). If soil moisture is lower than the field capacity, nighttime NEE is affected mainly by soil temperature; otherwise, nighttime NEE is dually controlled by soil temperature and moisture. There exists a short excitation effect to enhance nighttime NEE after heavy rainfall. Permanent flooding significantly reduces soil respiration, simultaneously decreasing the sensitivity of nighttime NEE to soil temperature. The temperature sensitivity coefficient (Q₁₀) in the rice seasons of 2012-2013 were 1.88 and 1.39, respectively. Alternate change of water regime between the permanent flooding and soil drying in the rice season would generate significant short-term effects of inhibition or excitation on soil respiration, respectively. The effects of carbon flux observation techniques and data gap-filling methods on uncertainty are discussed. Finally, the impact of straw application on greenhouse gas emissions (CO₂, CH₄, N₂O) from the agroecosystem is suggested as an avenue of further investigation.