华北低山丘陵区人工林土壤CH_4通量测定代表性时段研究

Representative Time Periods of Measuring Daily CH_4 Fluxes of Soil in Robinia psedudoacia Plantation in Low Hilly Areas of North China

在日尺度上选取合适的时间段测定土壤CH_4通量,取其均值代表日均值,可使短时间内土壤CH_4通量观测数据有效反映长时间尺度上土壤CH_4通量。为此,于2014年11月─2015年10月利用土壤CH_4自动观测系统对华北低山丘陵区刺槐(Robinia pseudoacacia L)人工林的土壤CH_4通量进行昼夜连续性观测,共取得140 d完整连续数据,其中,生长季(2015年5月─2015年10月)共82 d,非生长季(2014年11月─2015年4月)共58 d。分析各月土壤CH_4通量的日均值变化特征,计算各月的代表时段与日平均的相对差异百分率,选取相对差异百分率在±10%以内作为代表性时段。各月观测代表性时段选定后,对各月代表性时刻土壤CH_4通量的平均值与日平均值进行拟合,并对其在年尺度上的有效性进行检验评估。结果表明:(1)2014年11月─2015年10月各月份的日均值变化幅度分别为-0.74^-1.16、-0.58^-0.79、-0.48^-0.67、-0.55^-0.86、-0.59^-1.49、-0.60^-0.72、-0.70^-0.80、-0.50^-0.66、-0.54^-0.71、-0.71^-0.85和-0.84^-1.09 nmol·m^(-2)·s^(-1),日较差分别为0.42、0.21、0.19、0.31、0.90、0.12、0.10、0.16、0.17、0.15、和0.24 nmol·m-2·s-1;(2)选取8:00─9:00作为生长季(2015年5月─2015年10月)土壤CH_4通量的代表性时段;选择9:00─10:00作为非生长季(2014年11月─2015年4月)的代表性时段;(3)各月的代表性时段内土壤CH_4通量平均值与24 h日平均值的相对差异分别为5.71%(正向)、4.50%(正向)、-1.98%(负向)、3.11%(正向)、6.34%(正向)、-7.37%(负向)、-1.23%(负向)、-9.03%(负向)、-4.43%(负向)、-0.71%(负向)、4.97%(正向),均在±10%以内;(4)对各月代表性时刻土壤CH_4通量的平均值与日平均值进行拟合,发现各月代表性时段土壤CH_4通量平均值与24 h日平均值数据差异最小出现在9月,最大出现在7月;(5)将选择的代表性时段内土壤CH_4通量累加至年尺度上,表明选取的代表性时段可代表研究区的全年土壤CH_4通量(r^2=0.885,P=0.000)。

A proper period of time on a day should be chose to measure CH4 flux, and to take the measured average value to represent the daily average CH4 fluxes. The short-term observed soil CH4 flux data can also be effectively applied in the estimation of CH4 fluxes at a longer time scale. In this study, we used an automatic observation system of soil CH4 to collect the continuous day and night observational data inRobinia pseudoacaciaL. plantation in low hilly areas of north China in November 2014 to October 2015. We obtained a total of 140 days of continuous data in growing season of May 2015 to October 2015 (82 days) and non-growing season of November 2014 to April 2015 (58 days). The mean daily variation of CH4 flux was analyzed, and the relative percentage differences between the representative time and daily average in each month was calculated to select a time period which were within a relative percentage difference of ± 10% as the representative period. After the confirmation of the representative period, the mean CH4 fluxes of soil between the representative time periods and daily was fitted, and the effectiveness of its annual scale was estimated and evaluated. The results show that: (1) the change range of the mean daily variation was -0.74^-1.16, -0.58^-0.79, -0.48^-0.67, -0.55^-0.86, -0.59^-1.49, -0.60^-0.72, -0.70^-0.80, -0.50^-0.66, -0.54^-0.71, -0.71^-0.85 and -0.84^-1.09 nmol·m-2·s-1, and the diurnal range was 0.42, 0.21, 0.19, 0.31, 0.90, 0.12 , 0.10, 0.16, 0.17, 0.15 and 0.24 nmol·m-2·s-1 in each month, respectively; (2) the period of 8: 00─9: 00 was selected as the representative time in growing season; and 9: 00─10: 00 as the representative time in non-growing season; (3) The relative difference between the CH4 fluxes of the representative period and daily average were 5.71% (forward), 4.50% (forward), -1.98% (negative), 3.11% (forward), 6.34% (forward), -7.37% (negative), -1.23% (negative), -9.03% (negative), -4.43% (negative), -0.71% (negative), 4.97% (forward) they were all within ± 10% in each months, respectively; (4) the fit result between the soil CH4 flux of daily averages and the representative time showed that the minimum data of the difference of CH4 flux appeared in September while the maximum appeared in July; (5) the selected representative period can represent the annual CH4 flux of this study area (r2=0.885,P=0.000) when we accumulate soil CH4 flux of selected representative period to annual scale.