Changes in nutrient balance, environmental effects, and green development after returning farmland to forests: A case study in Ningxia, China.

Returning sloping farmland (>25°) to forest/grassland (RFTF) is an effective ecological measure for soil and water conservation. However, changes in nutrient cycles and green development are still unclear. Ningxia, in Northwestern China, began to implement RFTF in 2000. Here, we used the NUFER model to calculate the input and loss of nitrogen and phosphorus, utilization efficiency, production and consumption at food-chain scale in Ningxia for the period 1985-2015. Five aspects comprised the evaluation of green development: society, economy, resources, environment, and productivity. Results showed that forest coverage rate increased from 7.74% to 33.2%, while cropping area decreased by 6.6%. NH loss increased from 53,000 to 83,000 tons (56%), NO increased from 1200 to 2300 tons (92%) and leaching loss from 18,000 to 62,000 tons (240%). Phosphorus leaching loss increased from 13,000 to 35,000 tons (130%). The environmental cost of food nitrogen per unit of production decreased from 8 to 6.7 kg/kg (19.4%). Nitrogen and phosphorus 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 from 14 to 18 (14.7%). Improving the level of scientific and technological management of aquaculture system, increasing the proportion of organic matter returned to farmlands, and paying attention to the combination of planting and breeding will effectively promote future cycling efficiency of nitrogen and phosphorus in the food chain, and the sustainability of agriculture towards green development.