基于通量观测数据的玉米水碳交换量及水分利用效率分析

Analysis of corn water consumption, carbon assimilation and ecosystem water use efficiency based on flux observations

揭示玉米生长期内水分、二氧化碳交换量及水分利用效率的变化规律,对于认识玉米生长规律,指导农业灌溉具有重要意义。该文采用美洲通量网(Ameri Flux)3个农田通量站的数据,计算玉米生育期内的水分消耗量ET(evapotranspiration)、总初级生产力GPP(gross primary productivity)和生态系统净交换量NEE(net ecosystem exchange)及水分利用效率,并采用统计分析方法分析饱和水汽压差和光合有效辐射对水分利用效率的影响。结果表明,该区域玉米整个生育周期约为156~180 d,ET为440~520 mm,GPP为1 320~1 640 g/m2,以C计,NEE为-560^-620 g/m2,以C计;水分利用效率,NEE/ET为1.2~1.4 g/kg,GPP/ET为3.0~3.5 g/kg。水分利用效率与饱和水汽压差(VPD)成负指数关系,存在常数k使得GPP/ET正比于VPD-k,最优k值为0.42~0.63。水分利用效率与光合有效辐射无显著相关性。

It has a great significance on understanding the growth process of corn and guiding the agricultural irrigation to reveal the patters of corn water consumption, carbon assimilation and ecosystem water use efficiency. Therefore, this research collected flux and meteorological observations from 1997 to 2005 from three flux observations sites of the AmeriFlux network;and these three sites, focusing on farmland ecosystem, were located at corn fields in the northeast of the United States of America. At these three sites, latent heat, sensible heat and carbon dioxide fluxes were measured by eddy correlation system, and at the same time, other climatic variables, such as air temperature, vapor pressure deficit, and photosynthetic active radiation were also recorded by automatic meteorological stations. Based on the observations, this research estimated the water consumption (evapotranspiration, ET, including soil evaporation and corn transpiration) according to the latent heat flux, gross primary productivity (GPP) and net ecosystem productivity (NEE), both of which were estimated by according to the carbon dioxide flux; and then estimated the water use efficiency at different time scales, namely daily, weekly and annual, and furthermore tried to explore the relationship of water use efficiency with vapor pressure deficit (VPD) and photosynthetically active radiation (PAR) by using a correlation analysis. The results showed that the growth duration of corn is 156–180 days, ET ranges from 440 to 520 mm, GPP ranges from 1320 to 1640 g/m2, and NEE ranges from-560 to-620 g/m2;the peaks of weekly average ET, NEE and GPP are 3.5–5.3 mm/d,-10.0–-12.5 g/(m2 d) and 17.5–28.0 g/(m2 d) for the three sites. It could be found that air temperature had a notable impact on corn growth duration;and specially, air temperature at the US-Ro1 site was 2℃ warmer than that at the other sites, while the corn growth duration at the US-Ro1 site was about 14 d longer than that at the other sites. Regarding the water use efficiency, WN (defined as NEE/ET) ranges from 1.2 to 1.4 g/kg, WG (defined as GPP/ET) ranges from 3.0 to 3.5 g/kg at an annual scale. Both WG and WN had a small variation at an inter-annual scale, with a variation coefficient of 0.2;however, at an intra-annual scale, they had a large variation, with a maximum of 4.5–5.7 g/kg in WG and 2.4–3.2 g/kg in WN, and the minimums close to zero, which was synchronous with GPP variation. More remarkably, WG had a negative correlation with vapor pressure deficit (VPD) at a canopy scale, which could be explained in theory according to the Farquhar’s photosynthesis model. In this research, a more accurate relationship between ET and VPD was revealed, i.e. there was a constant k that made ET and GPP·VPDk approximately keep a direct proportion, with k value ranging from 0.42 to 0.63 for the three sites, which was close to the value of 0.5 proposed in the previous research. In addition, water use efficiency had no significant correlation with photosynthetic active radiation.