Effect of Agroforestry Systems on Soil NPK and C Improvements in Karst Graben Basin of Southwest China

Abstract

Agroforestry systems can fully exploit the ecological benefits of an ecosystem’s component plants, and improve the ecological environment, soil quality, and land use efficiency, all of which have recently attracted the attention of many scholars. Southwest China’s karst graben basins have barren soil that needs immediate improvement. The karst graben basin in southwest China was used for this study to examine the impacts of several forest–grass composite systems of Bingtang orange–alfalfa–ryegrass on soil improvement, which is located in Jianshui County, Honghe Prefecture, Yunnan Province. The experiment had four treatments, Bingtang orange–alfalfa sowing (B2), Bingtang orange–ryegrass × alfalfa mixed sowing (A), Bingtang orange–ryegrass × alfalfa intercropping (R), and Bingtang orange monocropping (CK). The results showed that different forest–grass composite patterns had noticeable effects on improving the soil’s organic carbon (SOC), total nutrients, and available nutrients, especially in the rainy season when plants grew vigorously. Forage grass intercropping under forest exerted the best effect on soil improvement in the surface layer, and the effect decreased with the increase in soil depth. Alfalfa intercropping under Bingtang orange forest had the strongest effect on improving SOC, total N, ammonia nitrogen, and nitrate nitrogen, whose content increased by 30.7%, 27.3%, 35%, and 36.3%, respectively, in the dry season and 38%, 46.7%, 48.7%, and 55.3%, in the rainy season. However, the effect of alfalfa–ryegrass intercropping under the Bingtang orange forest on soil total P, total K, and available P was better than that of the Bingtang orange–alfalfa intercropping system. The C:N ratio is more suitable in the Bingtang orange–ryegrass-alfalfa composite system. Forage grass intercropping under the Bingtang orange forest effectively improved the soil NPK and C status, and the results of this study provided a basis for the selection of forest–grass composite patterns for soil improvement.