Crop Potential Yields In Northeast China Under The Background Of Climate And Cropland Changes

Grain is an important strategic reserve resource related to national economy,people's livelihood and social stability,and food security is an important part of national security.There are many factors influencing grain yield.Cropland and climate resources are the two basic conditions determining regional crop yields.Cropland change affects the crop yields by changing the quality and quantity of cropland,while climate change changes the light,temperature and water conditions during the period of crop growth and development,thus affecting the crop yields.Northeast China is a vast region with contiguous cropland,rich climate resources and huge crop potential yields.It is China's main grain producing area and commodity grain base,and it plays an important role in national food security.Therefore,this study took Northeast China as the study area.By inputting climate,soil,topography,cropland data,the GAEZ model was used to simulate the potential yields of major food crops?maize,soybean and rice?in Northeast China from 1990 to2015,and the yield gaps were obtained by comparing with the actual crop yields.Next,the effects of climate and cropland changes on crop potential yields in Northeast China from 1990 to 2015 were further studied separately and in depth by using"Variable Control Method".Finally,by simulating the climate and cropland scenarios in Northeast China in 2050,the future crop potential yields in Northeast China were simulated.The results can provide reference for the policy making of land managers,and are of great significance to ensure the future increase of crop production and food security,increase farmers'income and maintain social stability.The main conclusions of this study are as follows:?1?By using GAEZ model to simulate the crop potential yields in Northeast China,the variation characteristics of three major crops in Northeast China were obtained.From 1990 to 2015,the potential yields of maize and soybean in nearly half of the cropland increased,with the maize potential yield in most of the cropland increased by more than 1500 kg/ha,and soybean potential yields increased by 500-1500kg/ha.Rice potential yield increased by 1500 kg/ha in most parts of Heilongjiang province,while decreased in most parts of Jilin and Liaoning provinces and eastern four cities of Inner Mongolia.After comparing the actual yield with the potential yields of three kinds of crops,it can be known that among the 40 cities in Northeast China,there were 22 cities where the rate of maize actual to potential yields were greater than80%,19 cities where the rate of soybean actual to potential yields were greater than 80%,and 30 cities where the rate of rice actual to potential yields were greater than 80%.It indicated that the utilization rate of dry land and paddy field,and human input and advanced management measures in most cities in Northeast China were higher.However,there were still large gaps in crop yields in some cities.?2?By analyzing the correlation and spatial difference between the crop potential yields estimated by GAEZ model and crop yields estimated by agricultural remote sensing method,the determination coefficient R² of maize,soybean and rice were calculated to be 0.66,0.64 and 0.72 respectively,indicating that the linear correlations between the two yield results were strong.Through spatial difference analysis,it could be found that if NPP in dryland in 2015 was assumed to be obtained from maize growth,Y_GAEZwas higher than Y_NPP in most areas.If NPP in dryland in 2015 was assumed to be obtained from soybean growth,Y_{NP P}was higher than Y_GAEZin most areas.Especially in parts of Sanjiang Plain,Songnen Plain and Liaohe Plain,Y_GAEZwas 2000-4000 kg/ha lower than Y_{NP P}.In paddy field,Y_GAEZwas less than2000 kg/ha lower than Y_NPP in most areas.?3?In the study of climate change on food potential yields in Northeast China,it could be found that the the changes of maize and soybean potential yields had obvious positice correlations with the changes of solar radiation,relative humidity,wet day frequency and precipitation.However,the correlation coefficient between the changes of maize and soybean potential yields and the changes of wind speed,mean maximum and minimum temperatures were around-0.30,showing a relatively significant negative correlation.The change of rice potential yield was also positively correlated with the changes of solar radiation,relative humidity,wet day frequency and precipitation in the first decade,but in the latter period positively correlated with the changes of mean maximum temperature and solar radiation,and negatively correlated with relative humidity and wet day frequency.?4?This study analyzed the conversion characteristics of dryland and paddy field as well as other land use types in Northeast China from 1990 to 2015.From 1990 to 2000,large-scale deforestation and grassland reclamation were more serious,and the mutual transformation between paddy field and dryland was also more intense.The net increase of dryland area was 2935100 ha,and the total increase was 4312800 ha.The conversion of woodland and grassland to drypland accounted for 77.05%of the total dryland increase.The net increase of paddy field was 678900 ha,and the total increase was1387700 ha,mainly due to the conversion of dryland,unused land and grassland.From 2000 to 2015,the phenomenon of returning farmland to woodland and grassland was obvious,but the transformation of paddy field and dryland was still very drastic.The net decrease of dryland was 1487800 ha.The total decrease of dryland was 7416200 ha.Conversion of dryland to woodland,paddy field and grassland accounted for 74.10%of the total dryland loss.The net increase of paddy field was 1043800 ha,and the total increase was 2621900 ha.Most paddy field was still converted from dryland.In the study of the impact of cropland changes on the potential yields of the three crops in Northeast China,it was found that the potential production of the three crops continued to increase in both periods.The increase of the potential production of maize and soybean in Northeast China during the first decade was mainly due to reclaim a large amount of natural forest and grassland resources,as well as the substantial increase of dryland area caused by the conversion of paddy field.The increase of the potential production of maize and soybean over the next 15 years was mainly due to the conversion of paddy field,woodland and grassland into high quality dryland.The increase of rice potential production in Northeast China during the two periods from 1990 to 2015 was mainly attributed to the massive increase in paddy area caused by the conversion of dryland and unused land to paddy field.?5?In this study,the future climate simulation data of 12 General Circulation Models?GCMs?in CMIP5were simply averaged using the multi-model ensemble method to obtain simulation results of six climate variables in the growing season of 2050 in Northeast China.Then,the CA-Markov model was used to predict the land use condition in Northeast China in 2050.Finally,GAEZ model was used to simulate the potential yields of three crops in Northeast China under climate and cropland conditions in 2050.It was found that the potential yields and production of the three crops in Northeast China were all improved,and the climate conditions of RCP4.5 scenario were more favorable to the crop growth than that of RCP6.0scenario.Therefore,in the future,we should try our best to control the emission of greenhouse gases within the scenario of RCP4.5,and pay attention to improving the crop yields,so as to ensure food security in Northeast China on the basis of building an environmentally friendly society.