Response Characteristics and Influencing Factors of the Ecological Stoichiometry of Soil Carbon, Nitrogen and Phosphorus to Conversion of Orchards into Farmlands on the Loess Tableland Area

［Objective］ The Loess Plateau is the largest apple-producing area in China. Due to serious aging of apple trees in recent years, the single variety structure, and the instability of China's grain market, it has become a common phenomenon for orchards to be transformed into farmlands on the Loess Plateau to ensure national food security. In order to study the response characteristics of the ecological stoichiometric ratios to conversion of orchards into farmlands. ［Methods］ Farmlands of 0 year (30 years orchard), 1 year, 3 years and 5 years old was selected as the research objects in this study, with farmlands cultivated with food crops set as the control group (CK). Soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) contents in the soil of 0—100 cm depth (10 cm per layer, a total of 10 layers.) were measured, and their ecological stoichiometric ratios were calculated, and the influencing factors of the ecological stoichiometric ratios were analyzed. ［Results］ 1) With the increase of conversion years, the mean values of SOC and TN in 0—100 cm soil after orchards returning showed a trend of decreasing first and then increasing, and were higher than that in CK, but the mean value of TP in soil showed a decreasing trend. There was no significant change in C∶N in soil, but C∶P and N∶P increased gradually, while the mean values of C∶P and N∶P in farmlands after 5 years of tillage were higher than that in CK. 2) With the increase of soil depth, SOC, TN and TP in the soil of orchards with different returning years showed a downward trend as a whole. The C∶P and N∶P generally increased first and then decreased, while C∶N showed no obvious change pattern. 3) The soil particle composition was the main factor affecting soil nutrients and ecological stoichiometric ratios in the study area. ［Conclusion］ These results revealed the changed pattern of C, N and P in the soil of orchards of different ages after conversion into farmlands. The results will provide an important reference for ecological environment protection and sustainable development in the loess tableland area.