Ecological stoichiometry (ES) is an important index that reflects the balance of various elements in ecological processes. Therefore, it is of great significance to understand the soil nutrient cycle to clarify the environmental control of soil carbon (C), nitrogen (N), phosphorus (P), and potassium (K). In this study, we analyzed the spatial distribution of soil C, N, P, and K contents and the C:N:P:K stoichiometric characteristics of 0–20 cm and 20–40 cm of farmland and grassland in four agro-pastoral areas in Inner Mongolia. Spearman correlation was used to analyze the effects of environmental factors on the soil C:N:P:K stoichiometric relationship. The results showed that there was no fixed Redfield ratio for the soil stoichiometric relationship of farmland and grassland in Inner Mongolia, and the values were 15:2:1:9 to 145:10:1:26 and 25:1:1:29 to 228:15:1:65, respectively. The stoichiometric relationships between farmland and grassland were consistent with the law of geographical and spatial heterogeneity. The ratios of C:N, C:P, C:K, N:P, and N:K showed an N distribution from west to east, while the ratio of P:K showed a V distribution. The stoichiometric relationships in grassland soil were mainly affected by soil organic carbon and total nitrogen content, while those in farmland were mainly affected by total nitrogen and total phosphorus content. The annual mean precipitation has a significant effect on stoichiometric relationships in farmland, while the annual mean temperature has a more significant effect on grassland. In conclusion, the spatial distribution difference in the soil stoichiometric relationship in the agro-pastoral ecotone of Inner Mongolia was more significant than the difference in the land use pattern. The influences of annual mean temperature and annual mean precipitation on soil ecological stoichiometry were in accordance with the geographical spatial similarity law. Compared with grassland, the stoichiometric relationship of farmland soil was greatly affected by fertilization, and farmland in this region was mainly limited by carbon and nitrogen. Thus, field management should be carried out according to local conditions. This study is of great significance as it promotes the rational utilization of land resources and the sustainable development of agriculture.

This study explores the effect of phosphorus (P) fractions, under P addition or not, based on a common vetch-rape model cropping system in alkaline soil. A two year field experiment was conducted at Tuzuo Banner modern agricultural Park in Inner Mongolia, China. Two phosphorus levels, including P0 (no fertilizer) and P45 (45 kg·ha⁻¹ P), were performed in common vetch and rape either grown alone or intercropped. We analyzed the changes of the physicochemical properties and phosphorus fractions in the rhizosphere soil. Intercropping enhanced the common vetch and rape yield by 42.05% and 24.91%, on average, compared with corresponding sole cropping on an equivalent area basis. The average land equivalent ratio (LER) was 1.34. Intercropping had a significant AP concentration, of 65.32% and 33.99% at the P0 level, and 62.83% and 36.19% at the P45 level, respectively, compared to that of the sole common vetch and rape. With the application of P, intercropping improved the Resin-Pi and NaHCO₃-Pi fraction (61.17%, 87.03% at the P0 level and 96.50%, 41.85% at the P45 level, compared to monocropped common vetch and rape in 2019). The changes in NaOH-Pi and NaOH-Po (except for NaOH-Pi in 2019) showed no significant difference between cropping systems. Intercropping significantly accumulated concentrations of HCl-P, while depleting Residual-P, in 2020. In conclusion, common vetch/rape with the addition of P polyculture stimulated rhizosphere soil P mobilization and had a yield advantage over sole cropping.

Ecological stoichiometry is an important index to reflect ecological interaction and the balance of various elements in ecological processes. At present, the stoichiometric spatial distribution of soil carbon (C), nitrogen (N), phosphorus (P) and potassium (K) in farmland and grassland of Inner Mongolia and its influencing factors are still unclear. In this study, the spatial distribution of soil nutrient content and stoichiometry and the effects of environmental factors on soil C:N:P:K stoichiometry were analyzed under different land use patterns in four agro-pastoral areas. The results showed that there was a geographical gradient in the ecological stoichiometric relationship between farmland and grassland. RCN, RCP, RCK, RNP and RNK showed an "N" distribution from west to east, while RPK showed a "V" distribution. The difference in spatial distribution was more significant than the difference in land use pattern. The ecological stoichiometric relationship of grassland was mainly affected by the content of C and N, and was related to the similarity of soil properties. The field ecological stoichiometry was mainly affected by the content of nitrogen and phosphorus, and had a great relationship with fertilization. The effects of MAT and MAP on soil ecological stoichiometry were related to the inter-regional environmental heterogeneity. The influence of MAP on farmland and MAT on grassland was more significant. It was of great significance for field management according to local conditions, rational utilization of land resources and sustainable development of agriculture to clarify the ecological stoichiometric characteristics of farmland and grassland in agro-pastoral ecotone.