2.1 Site descriptions
This study was conducted in the three counties of Guyuan (41°39ʹ N, 115°46ʹ E), Kangbao (41°84ˊ N, 114°33ˊ E), and Huade (41°55ʹN, 113°58ʹE), which encompass the majority of the distribution of the agro-pastoral ecotone in Northeast China. The conversion of steppe to agricultural land started in the early 20th century due to population growth. These native grasslands have been used for grazing in history and one adjacent sites that have been cultivate about 30-60 year. The land use history was the same for both grassland and cropland sites at the same region. Before the grassland conversion, grazing (at an intensity of 3-5 sheep ha-1) had been implemented for more than 10 years. Native grasslands received no fertilizer, croplands received approximately 25 kg N ha−1year-1 as urea once a year in the middle of May. The cropland was tilled for conventional tillage practices and no irrigation every year. Dominant plants in grassland ecosystems were L. chinensis , A. cristatum and S. krylovii . Over the past several decades, a large area of grassland has been converted to croplands with annual crop rotation between naked oat (Avena nuda ), corn (Zea mays ), potato (Solanum tuberosum ), buckwheat (Fagopyrum sagittatum ), or flax (Linum usitatissimum ). The cultivated management regime has been lasted for 30-60 years. The information of study sites are described in detail in (Figure S1, Table 1).