Nitrogen Flow Characteristics In The Agricultural System And Green Development Assessment Of The Yellow River Basin

In order to study the spatial migration law of nitrogen（N）in the agricultural system in the Yellow River Basin and its green development constraints.The Yellow River Basin was taken as a case study,through the collection of research,statistical data and literature data,combining（NUtrient flows in Food chains,Environment and Resources use）NUFER model and its extended NUFER-AGD model,to clarify the temporal and spatial variation characteristics of nutrient input and environmental loss in the Yellow River Basin,and to classify and analyze indicators according to society,economy,resources,productivity and ecological environment,and clarify the status quo of agricultural green development in the Yellow River Basin.In addition,the N flow characteristics and green development level of the Yellow River Basin under farmland measures such as converting farmland to apple orchards,returning farmland to forests,and combining planting and breeding were analyzed.The main findings are as follows:1.The total N input of the area increased from 2.10×10⁷ t in 1999 to 3.66×10⁷ t in2019,mainly originated from fertilizer（53%,2019）and fodder（39%,2019）.The total N output increased from 8.27×10⁶ t in 1999 to 13.77×10⁶ t in 2019,and the main pathway of N loss was ammonia volatilization,accounting for 53.1%in 2019 of the total losses.Based on the N input and loss per unit area in 2019,the upper reaches of the Yellow River Basin were low-input and high-emission;the middle reaches were high-input and high-emission;the lower reaches were high-input and low-emission.Fifty variables of five dimensions,society development,economy growth,agricultural production,resources input and ecological environment were selected to assess whole the Yellow River Basin for the green development index.The results show that 36%variables reached the green level,and the parameters of the agricultural production and ecological environment among all variables were the key factors of limiting green development of the the Yellow River Basin area.2.Weibei Dryland is considered as an example to evaluate changes in N flows through the food chain following conversion of croplands to apple orchards,and we also modelled effects of 20%reduction in N fertilizers on apple yields.The result showed that the apple orchards area increased from 9.6×10⁴ha in 1978 to 72×10⁴ha in 2013,while the area under cereals decreased from 186×10⁴ha in 1978 to 152×10⁴ha in 2013.Consequently,N use efficiency of crop production（NUEc）and the combination of crop and animal production（NUEc+a）decreased,but N use efficiency of animal production（NUEa）and N losses increased between 1978 and 2013.The modelled scenario of 20%reduction in N fertilization amounts will still keep optimum apple yield but will significantly decrease N losses from apple orchards.3.Ningxia,in Northwestern China,began to implement returning sloping farmland to forest/grassland in 2000.Here,we used the NUFER model to calculate the input and loss of nitrogen and phosphorus（P）,utilization efficiency,production and consumption at food-chain scale in Ningxia for the period 1985–2015.Results showed that forest coverage rate increased from 7.74%to 33.2%,while cropping area decreased by 6.6%.NH₃ loss increased from 53,000 to 83,000 tons（56%）,N₂O increased from 1200 to 2300 tons（92%）and leaching loss from 18,000 to 62,000 tons（240%）.The environmental cost of food nitrogen per unit of production decreased from 8 to 6.7 kg/kg（19.4%）.N and P use efficiencies decreased by 4.8%and 58%in crops-animal systems,respectively,but increased by 652%and 430%in animal husbandry systems.The proportion of animal protein production increased from 18.3%to 39.0%（113%）.The major source of organic waste was livestock and poultry manure and urine（70%）.Indicators for achieving green development level increased from14 to 18（14.7%）.4.This study took Shaanxi Province in Northwest China as an example,and selected cropping system（CS）and animal farming system（AFS）as the research objects.The results showed that:（a）In CS,plant nitrogen foods were all low-yield and high-consumption,while animal N foods were the opposite.Animal and plant N foods in AFS were low-yield and high-consumption.（b）The amount of straw used as feed in AFS increased year by year,from2.8×10⁴ t in 1995 to 6.1×10⁴ t in 2014,an increase of 117%,of which the sharp change appeared in 2006-2007,from 3.4×10⁵ t to 4.6×10⁵t.（c）Through the classification and statistics of 50 agricultural green development indicators,AFS green indicators account for26%,CS green indicators account for 17%,and AFS is more suitable for green development.In conclusion,the N flow characteristics in agro-pastoral systems in the Yellow River Basin have changed greatly.The N flow in the food chain under different production systems is mainly due to the large amount of nitrogen input and low nutrient use efficiency.The nitrogen use efficiency and material recycling capacity of the system should be further improved,the effect of chemical fertilizer reduction and its application on crop yield and nitrogen fertilizer use efficiency should be studied,and the combination of agriculture and animal husbandry should be promoted,and to optimized on the basis of local agricultural development models,and emphasizing the closeness of the combination of planting and breeding,which further enhanced the green development of agriculture in the Yellow River Basin.