## 4 Discussion
### Agricultural Carbon Efficiency and the Associated Environmental Problems  
The decreasing trend of carbon intensity in output value and increasing trend of agricultural carbon efficiency in Qinghai province were mainly determined by the increasing proportion of the agricultural products with higher economic output value and the change in agricultural input structure over the study period. However, the population growth, increase in food and income demands, implementation of series of ecological and supporting agricultural policies, (especially the GFGP during 2000-2007), and the unique fragile plateau environment not only result in the increasing trend of carbon intensity in area but also cause other associated environmental problems.  
The rural population in Qinghai province rose from 3.23 million to 3.96 million between 1995 and 2016, with an annual increment of 37.60 thousand. Meantime, the grain crop production increased from 1.42 million tons to 1.46 million tons over 1995-2016, with an annual increment of 3.52 thousand tons; while the cash crop production increased from 0.55 million tons to 2.00 million tons over the study period, with an annual increment of 77.98 thousand tons. Practically, the cash crop production exceeded the grain crop production in Qinghai province after 2009. Therefore, the increasing agricultural carbon efficiency mainly depended on the increasing cash crop production. Based on National Survey Data, compared with major grain production, vegetable production requires 2.1 times input of chemical fertilizers, 5.5 times input of pesticides, and 61.9 times input of plastic film in China, respectively [42]. Thus, more agrochemical inputs have to be used to increase the production of high-valued cash crops.  
Therefore, the N fertilizer application had increased from 64.42 to 80.99 kg/ha between 1995 and 2016, with a mean of 73.54 kg/ha in Qinghai province, which was 33.91% of the mean in the whole country; while the P fertilizer application had increased from 30.51 to 45.02 kg/ha between 1995 and 2016, with a mean of 38.87 kg/ha in Qinghai province, which was 39.91% of the mean in the whole country. Based on the National Data [25], China has exceeded its safe N input limit of 170 kg/ha since 1995, and safe P\({}_{2}\)O\({}_{5}\) input limit of 80 kg/ha since 2001 [43; 44]. Although the chemical fertilizer applications were under the general safe environmental limits for N and P\({}_{2}\)O\({}_{5}\) in Qinghai province, the indisputable fact is that all agrochemical inputs increased after 2007. Thus, the agrochemical input intensity during 2005-2007, which may be comparatively reasonable to hold a balance between higher carbon efficiency and lower CA, and the unique safe environmental limits of agrochemicals and energy on the plateau need further studying.  
Currently, it is necessary to efficiently use high-quality agrochemicals and energy, and to develop more biological and eco-friendly agricultural additions on the fragile plateau in order to increase agricultural carbon efficiency and to control the associated environmental problems [45]. In addition, as the birthplace of China's major rivers, Qinghai province should impose heavy environment taxes on agrochemicals and energy and waive value-added taxes on biological and eco-friendly agricultural additions [46], which will be useful for the improvement of the local and downstream environmental conditions. New technologies and methods for carbon capture, storage and rational use (e.g., CO\({}_{2}\) injection enhanced oil recovery), and manufacture of high-quality and low-pollution agrochemicals and energy should be experimentally used to protect the fragile plateau [47; 48; 49; 50; 51].