Effects Of "Grain For Green" Project On Grain Production In The Loess Plateau

China’s Grain for Green project(GGP)was initially implemented in the Loess Plateau,China,in 1999,and it soon expanded to all the parts of country as a national program by the year 2000.According to the most recent statistics of China’s State Forestry Administration(2018),the GGP has converted 400 million hectares of sloping cropland to forestland and grassland in the last 20 years.The GGP has reaped remarkable benefits in land use,the variability of vegetation coverage,soil organic carbon),ecosystem services,and other social and economic achievements.Large scale and transformational changes in land use and land cover does however decrease cropland area.As the most essential material for agricultural production,cropland resource changes undoubtedly affect grain production in quantity and quality,thereby creating problems in grain supply and grain security.From a national point of view,the world’s most populous country,China,feeds20% of the global population by less than 10% of the world’s cropland;the insufficiency of cropland resources is a serious situation for the whole country.At present,China is facing agricultural products supply problems due to a growing population.In general,it is vital to protect limited croplands and strengthen supervision to avoid the rapid drain of land resources.Under the dual goals of ecological security and food security,how to determine the appropriate scale of "cultivated land" and "forest land" and coordinate the optimal boundary between "green" and "food" is particularly important.Base on the data of 239 counties in a semi-arid rain feed region on Loess Plateau from2000 to 2015,the present study revealed the characteristics of space-time evolution between grain production and GGP in the region,we study on the impact and transmission mechanism of the GGP on grain production by using a mediating effect mode,and evaluated the optimal intensity of GGP on Loess Plateau using C-D product function model and Law of diminishing marginal returns.The results showed that :(1)the implementation of the GGP in the loess plateau area is in line with the trend of decreasing cultivated land,and during 15 years,most counties have increased grain production significantly,while the increase in grain production in the early period but the decrease in total output in 2010-2015 occurred in areas with greater intensity of GGP.(2)Intensive farming plays a partial mediating effect between GGP and crop yield;the intensity of agricultural technology input is the strongest,followed by the intensity of agricultural chemical fertilizer application,while agricultural labor input is the weakest.This suggests that the implementation intensity of the GGP has a noticeable positive impact on grain yield per unit area.(3)Under the maximum marginal benefits of arable land elements to grain production,the optimal intensity of the GGP on Loess Plateau is 23%,under increasing scale returns condition,the optimal intensity is 23%.Even more than 90% counties can be chosen as potential GGP area.Combined with optimization of the GGP on the agricultural production structure and the balance of increasing grain output between other major grain-producing areas and the overall grain output in China,we considered that the Loess Plateau still has large implementing spaces in GGP.