Impacts Of Climate Change And Human Activities On Greenhouse Gas Emissions And Water Demand From Farming

The agriculture sector is one of the largest users of water and a significant source of greenhouse gas（GHG）emissions.The development of low-GHG-emission and water-conserving agriculture will inevitably be the trend in the future.Because of the physiological differences among crops and their response efficiency to external changes,changes in planting structure,climate and input of production factors will have an impact on regional agricultural water use and GHG emissions.This paper systematically analyzed the spatial-temporal evolution characteristics of crop planting structure,climate,and production factor inputs in Heilongjiang Province,the main grain-producing region of China,from 2000 to 2015,and quantified the regional agricultural water use and GHG emissions characteristics under different scenarios by using the Penman-Monteith formula and the Denitrification-Decomposition（DNDC）model.The main research conclusions are as follows:（1）From 2000 to 2015,the planting area of major crops in Heilongjiang Province changed significantly,and the planting diversity of crops decreased.The acreage of grain crops increased by 59%,and the acreage of rice and corn increased,while the acreage of soybean,wheat and potato decreased.The main planting structure is rice,corn and soybean,among which the corn planting area is expanding rapidly.The total amount of GHG emission and water demand of planting industry in Heilongjiang Province showed an obvious upward trend.The total emissions of CO₂,N₂O and CH₄ increased by 81%,40%and 117%respectively,exacerbating the greenhouse effect.The total water demand of planting industry increased by1.99 billion m³/year,an increase of 50%.The increase in crop acreage is the most important driver.Greenhouse gas emissions per unit area of crops and water demand per unit area showed a downward trend.The global warming potential（GWP）per unit area decreased by 4%from3180 kg C/ha in 2000 to 3060 kg C/ha in 2015.The water demand per unit area of crops decreased from 526mm in 2000 to 423mm in 2015,a decrease of 20%.（2）Due to the differences in greenhouse gas emission characteristics among different crop fields and the different response efficiency to external changes,the changes of climate,planting structure and nitrogen application rate significantly affected the total greenhouse gas emissions.The control variable method was used to analyze the variation characteristics of greenhouse gases in regional planting industry under only climate change scenario（2015C）.The results showed that the annual change of air temperature and the increase of precipitation reduced the N₂O emission flux and the overall warming effect.Emissions of all three types of greenhouse gases doubled when only the planting structure change scenario was considered（2015PS）.The most significant increase in CH₄ emissions per unit area was 26%,due to the large increase in the proportion of rice planted.Taking into account the combined effects of all three greenhouse gases,the GWP per unit area is 3660 kg C/ha years,an increase of 15%over2000.Under the single change scenario（2015F）,the increase of nitrogen application rate promoted the N₂O emission from farmland,and the total N₂O emission increased by2%.Although the increase in CO₂ and CH₄ was less than 1%,due to the effect of N₂O,the GWP increased to 3,340 kg C/ha,an increase of 5%from 2000.（3）The change of meteorological factors in the study area from 2000 to 2015 was helpful to reduce the water demand of planting industry in Heilongjiang Province,but the change of planting structure resulted in the increase of the water demand of regional planting industry.Under the 2015C scenario,crop water demand per unit area in the study area decreased by about 20%,mainly due to the decrease of wind speed and the increase of relative humidity,which led to the decrease of crop evapotranspiration.Under the 2015PS scenario,due to the expansion of planting scale,the total water demand of planting industry in the study area increased by 88%.The main reason for the influence of planting structure on the water demand per unit area of crops is that the water demand of different crops is different during the growth period,and the water demand of rice is much higher than that of other dry crops.