Effects of grass cover in‘Nanfeng’tangerine orchard on nutrients and microbial characteristics in soil aggregates and fruit quality

Abstract: 【Objective】‘Nanfeng’tangerine is a well-known citrus variety in China.‘Nanfeng’tanger- ine is popular in the market and has been widely cultivated in many regions. In 2018‘, Nanfeng’tanger- ine cultivation area reached 78 000 hm2 with a production of 1.35 million tons. However, due to the long-term traditional orchard cleaning practice, a series of ecological and environmental problems such as reduced soil quality, reduced fertility, increased pests and diseases, and soil erosion have resulted in serious impacts on‘Nanfeng’tangerine productivity, quality, and thus profit. Grass cover is an ad- vanced orchard management, which can significantly increase soil organic matter and nutrients, im- prove orchard soil quality, repair orchard ecological environment, improve fruit yield and quality, effec- tively solve problems caused by long-term cleaning of orchards. This experiment used traditional or- chard cultivation methods (clean cultivation) as the control to examine the effects of grass cover treat- ments on nutrient and microbial characteristics of the soil aggregates in‘Nanfeng’tangerine orchard.【Methods】Three treatments including ryegrass as the cover grass (T1), white clover as cover grass (T2), and orchard cleaning as the control (CK) were set in the experiment, which was of one-factor ran- domized block design with 3 biological replicates. In late July 2019, the five-point sampling method  was used for sample collecting. 0-20 cm soil layer sample was taken 30 to 40 cm away from the trunk of the trees. The contents of soil aggregates of different sizes.【 Results】Soil R0.25 in vari- ous treatments followed a pattern of T1> T2> CK. Grass cover treatments were favorable for the formation of large soil aggregates. Compared with CK, grass cultivation increased soil organic carbon, total nitrogen, total phosphorus, total potassium, nitrate nitrogen, available phosphorus, and available potassi- um. Their average increase was 112.37%, 29.44%, 63.00%, 80.68%, 24.39%, 21.96%, 111.09%, respec- tively in T1 and 122.57%, 55.29%, 57.62%, 54.97%, 44.54%, 23.83% and 65.17% respectively, in T2. The contents of microbial biomass carbon, nitrogen and phosphorus increased significantly in soil ag- gregates (p < 0.05), and their average increase was 69.95%, 12.73% and 26.28% respectively in T1 and 94.47%, 42.68%, and 22.06% respectively in T2. The respiration intensity of soil aggregates was en- hanced remarkably (p < 0.05) and the average increase was 27.58% and 28.02% in T1 and T2, respec- tively. Species richness index, dominance index, and evenness index of microbial communities in- creased in the soil aggregates under grass cultivation. The contents of soil carbon, nitrogen and microbi- al biomass carbon and nitrogen in T2 were significantly higher than in T1 (p < 0.05), while the contents of phosphorus and potassium in soil aggregates displayed an opposite pattern. The contents of organic carbon, nitrate nitrogen, available phosphorus, microbial biomass carbon and nitrogen in 3-5 mm aggregates were significantly higher than those in the other sized agglomerates under different treatments. The basal respiration intensity of 3-5 mm, 2-3 mm and 1-2 mm aggregates under grass cultivation was significantly higher than that of the other sized aggregates. Naturally grown grass increased soluble sug- ars, soluble solids and vitamin C and reduce titratable acid.【Conclusion】Planting white clover had a better effect in increasing carbon and nitrogen in soil aggregates but was less effective in increasing phosphorus and potassium than planting ryegrass. Grass cover had a prominent effect in increasing the carbon, nitrogen, and phosphorus in soil aggregates (3-5 mm) and in improving the microbial activity of aggregates (>1 mm). Grass cultivation promoted the formation of large aggregates in the soil in Nanfeng tangerine orchard. It could effectively increase the nutrient content and microbial biomass in soil aggregates of various sizes, enhance the carbon fixation of aggregates and microbial diversity and activi- ty, and thereby improve soil fertility and fruit quality of‘Nanfeng’tangerine.