Natural Grass Cultivation Management Improves Apple Fruit Quality by Regulating Soil Mineral Nitrogen Content and Carbon–Nitrogen Metabolism

Orchard grass cultivation management is an effective measure to safeguard the sustainable development of the fruit industry in China. However, details of the influence of natural sod culture management on carbon (C)–nitrogen (N) nutrition of trees and fruit quality in Hanfu apple orchards are lacking. Therefore, a field experiment was conducted, which consisted of two treatments: clean tillage (CT) and natural grass cultivation (NG). Results revealed that NG treatment contributed to the increases in soil organic matter (SOM), total N, and soil NH4+-N at depths of 0–20 cm and 20–40 cm, while the soil NO3−-N concentration under NG treatment was significantly decreased at the same depth, within the range of 0–200 cm of the soil profile, compared with CT. NG treatment also significantly promoted leaf photosynthesis and enhanced leaf N and fruit sugar metabolism. The results of isotope labeling showed that NG treatment obviously elevated the 13C accumulation and distribution rate in fruits, as well as the 15N accumulation in the whole tree, whereas the 15N accumulation in fruits decreased. Furthermore, NG treatment significantly increased the fruit anthocyanin content. These results provide theoretical references for the feasibility of natural sod culture management to improve fruit quality in Hanfu apple orchards.