Effects of self-sown grass on soil nitrogen and apple fruit quality in the Weibei dry platean

Abstract: Weibei dry plateau of western China is an important production base of high-quality apples in the world. But a large amount of nitrogenous chemical fertilizer is normally applied to promote the growth of fruit trees and yield in orchards, due mainly to the low content of organic matter in soil in recent years. An accumulation layer of nitrate nitrogen in soil inevitably incurred to break the supply balance of soil nutrients, leading to the physiological diseases of fruit trees and soil environmental pollution. Alternatively, the self-sown grass can be widely expected to enhance the soil organic matter, nutrient, and moisture in the local ecological environment, suitable for a better-balanced supply of soil nutrients, fruit quality and yield. This study aims to systematically investigate the effects of self-sown grass on soil nitrogen and apple quality in the Weibei dry plateau. The self-sown grass (Stellaria media and Malachium aquaticum community) of 2, 4, 6, and 8 years was taken as the research object, with the clean tillage as control. Some parameters were continuously monitored, including the soil organic matter, total nitrogen, nitrate nitrogen, ammonium nitrogen, and available calcium content, as well as the contents of nitrogen, phosphorus, potassium, and calcium in apple fruit, together with the fruit yields and quality. The results showed that the self-sown grass contributed to the increase of the organic matter, total nitrogen and available calcium content in the soil layer of 0-40 cm in the apple orchard. The soil available calcium content also slightly increased in the soil layer of 40-60 cm. There were much higher organic matter, total nitrogen and available calcium content in the soil, as the years of self-sown grass increased. But there was no effect on the organic matter and total nitrogen content in the soil layer below 40 cm and the available calcium in the soil layer below 60 cm. In addition, the ammonium nitrogen contents of self-sown grass of 2 and 4 years in the soil layer of 0-60 cm were slightly lower than that of clean tillage, while those of 6 and 8 years were slightly higher, but those of different years under 60 cm soil layer were basically the same as that of clean tillage. Compared with the clean tillage, the nitrate nitrogen contents of self-sown grass of 2, 4, 6 and 8 years decreased by 16.28%, 31.31%, 40.13% and 47.41% in the soil layer of 0-80cm, respectively, while those in the soil layer of 80-240 cm decreased by 4.38%, 12.41%, 16.90%, and 19.39%, respectively, but there was no significant effect on the nitrate nitrogen in the soil below 240 cm soil layer. There were all the same contents of total carbon, nitrogen, phosphorus, potassium, calcium, and biomass of self-sown grass in different growth years. The calcium content increased significantly, while the contents of N/Ca, P/Ca and K/Ca decreased in apple fruits. It inferred that the self-sown grass contributed to reducing the incidence of calcium deficiency symptoms, while improving the coloring area, firmness, and soluble solid content in the fruit. Therefore, the self-sown grass can be expected to extensively promote the organic matter content, total nitrogen, and available calcium, while effectively reduce the accumulation of nitrate nitrogen in an apple orchard for a high-quality apple fruit free of calcium deficiency in the Weibei dry plateau of China.