Soil mineralized nutrients changes and soil conservation benefit evaluation on 'green project grain' in ecologically fragile areas in the south of Yulin city, Loess Plateau

The 'green project grain' was initiated to reduce water and wind erosion on marginal and highly erodible sloping croplands by removing them from permanent production planting and soil-conserving vegetation such as grassland/forest. We chose 53 plots of 265 sites in the ecologically fragile areas in the south of Yulin city in Loess Plateau, in order to assess the soil net mineralized nutrients changes and benefit evaluation. Field plots were established on a loamy site on some representative models of planting tree (grass) species for afforestation (such as Orientalis, Jujube, Amorpha, Apricot and Alfalfa) for over 5 years and on an adjacent site (<30 m) in the same elevation of sloping cropland (no-tillage). As a comparison, all soil samples were analyzed for total nitrogen, phosphorus, potassium (kalium) and soil organic matter. Results showed that after over 5 years of grassland/forest conversion of cropland to forest land, surface soils at the study area were significantly higher in the average total nutrients of nitrogen, phosphorus and organic matter by mass than sloping croplands, but the average total of kalium was in contrast. These results emphasize the need to evaluate benefits of soil conservation in fertilizer, so assessment on the ecological economic value of soil conservation in nitrogen, phosphorus and soil organic carbon in forest/grassland ecotone to steep cropland were 15.135, 0.142 and 2.412 billion yuan, respectively, but soil conservation benefit in kalium produced a negative value with 40.42 billion yuan. Benefits of soil conservation in fertilizer had a positive and negative relationship for exhibiting almost different values significantly. The soil mineralized nutrients changes and benefits of soil conservation with two-pronged relationships could be useful soil quality indicators in the ecologically fragile areas in soil management systems