Variation Characteristics Of Productivity Of Summer Maize Agroecosystem And Its Response To The Extreme Precipitation Event In North China Plain

Canopy carbon flux was continuously measured by using the eddy covariance system in summer maize field in Yucheng station in 2003-2013 to investigate its dynamic characteristics and evaluatethe carbon sinkduring the growth season of summer maize.The extreme precipitation threshold was determined according to the daily precipitation during the growth season of summer maize in the 11 years. According to the conditions of extreme precipitation of Yucheng station, two extreme precipitation events were selected and the effect of extreme precipitationon productivityof summer maize ecosystemwas analyzed. The results indicated that the variation of carbon flux in the summer maize ecosystem had a significant regularity during the 11 years. The summer maize agroecosystem was the carbon sink in 2003-2013. Non-extreme precipitationevent could significantly improve the NEP and GPP of summer maize ecosystem, and extreme precipitation event could significantly decrease the NEP and GPP of summer maize ecosystem. The results were as follows: 1. The dynamic characteristics of carbon flux during the growth season of summer maizeThe monthly average daily variation（except June）and the diurnal variation in different growth period（expect VE） of CO₂ fixation were a “U” shape curve in the growing season of summer maize from 2003-2013. The daily maximum value appeared at 12:00 o’clock. The CO₂ fixation transformed from carbon emission into carbon absorption after 6:00 a.m. and then transformed from carbon absorption into carbon emission after 6:00 p.m. The monthly maximum value was-1.40 mgCO₂·m-2·s-1 and appeared in August. The summer maize agroecosystemrepresented as carbon emission at VE and R5, the Fc was positive; as carbon absorption at V6 to R3, the Fc was negative. The maximumvalue of the Fcaccumulation appeared at R3, and the average of maximumvalue of the Fc accumulation was 181.49gCO₂·m-2·d-1 in the 11 years. The average of daily largest carbon absorption was 1.33 mgCO₂·m-2·s-1 of the 11 years. The annual maximum Fc was-10.07 to-17.46 gCO₂·m-2·d-1. The GPP and Re were a“n” shape curve in the growing season of summer maize agroecosystem from 2003-2013. The maximum GPP appeared in July or August due to the high temperature and appropriate lighting. The annual maximum GPP was 17.61 to 24.70 gCO₂·m-2·d-1. The maximumvalue of the GPP accumulation appeared at R3（R0 in 2005）, and the average of maximumvalue of the GPP accumulation was 373.14gCO₂·m-2·d-1 in the 11 years. The maximum Re also appeared in July or August and the annual maximum Re was 5.41 to 10.78 gCO₂·m-2·d-1. The maximumvalue of the Re accumulation appeared at V6 or R3, and the average of maximumvalue of the Re accumulation was 196.53gCO₂·m-2·d-1 in the 11 years. 2. The evaluation of carbon sinkof summer maize agroecosystemFrom 2003 to 2013, the average of GPP, NEE, Reof summer maize agroecosystem were 894.08,-238.34, 654.68 gCO₂·m-2, respectively. The date of the largest carbon absorption appeared at middle to late June or early Julyof summer maize agroecosystem in the 11 years. The summer maize agroecosystem manifested as carbon absorption was at middle July（early August in 2011） and as carbon emission was at late September or early October. The date of the maximum of GPP was consistent to the date of the largest carbon absorption during the 11 years. The NEE was largely affected by GPP at VT-R5, and the others were Re. The summer maize agroecosystem was the carbon sink in 2003-2013. 3. The extreme precipitation eventof Yucheng stationThe daily rainfall during the summer maize growth season was analyzed from 2003 to 2013. The extreme precipitation threshold was determined as the 95 th percentile of daily rainfall, which was 57.8 mm during the summer maize growth season in 2003-2013 of Yucheng station. In this article, we selected the precipitation which reached the extreme precipitation thresholdand let the SWC achieve to 120% of field moisture capacity as an extreme precipitation event. The non-extreme precipitation event could significantly improve the NEP and GPP of summer maize agroecosystem. And compared with the BR, the NEP and GPP increased 70.81% and 35.83%, respectively. 4. The extreme precipitation eventof Yucheng stationAccording to the conditions of extreme precipitation of Yucheng station, two extreme precipitation events were selected in the growing season of summer maize from 2003-2013.The two extreme precipitation events both occurred at the filling stage. The GPP had the same change trend with NEP during two extreme precipitation events. Compared with before precipitation, the NEP and GPP decreased. The NEP and GPP decreased 38.47% and 25.54% in 2007, 15.02% and 17.98% in 2013. The extreme precipitation event was divided into four stages. They were before rainfall（BR）, flooding I（F1）, flooding II（F2）, and recovering（R）. The NEP, GPP, Reof F1, F2, R both decreased than BR in the two years, and the decrease degree of 2013 was lower compared with 2007. In 2013, there had had a heavy rain event before the extreme precipitation event studied, which made the SWC increase and close to the condition of extreme precipitation event. So the extreme precipitation event studied could be as a result of that heavy rain before.The soil moisture condition of summer maize agroecosystem could affect the range of the light saturation point. Higher SRWC（130%）, which was over the optimal soil water content could reduce the light saturation point of summer maize agroecosystem, and then reduce the photosynthetic capacityunder high light intensity. During the PAR < 1400 umol m-2s-1, the NEP was lower than before precipitation. During the flooding, the apparent quantum efficiency decreased, and then recovered with the recovery of SWC. The maximum net photosynthetic rate increased when flooding, and then decreased when recovering. And the extreme precipitation could increase the dark respiration rate, decrease the RUE of summer maize agroecosystem. The air temperature and NEP had no significant correlation during the two extreme precipitation events. And the relative humidity had significant correlation with NEP at F1 in 2007（p<0.05） and R in 2007, 2013（p<0.01）.