Effects of interrow grass planting on soil quality characteristics in jujube orchard in the extremely arid areas of southern Xinjiang

Abstract:【Objective】Jujube is one of the important fruits and has a long history of planting in China. The superior natural conditions with ample light and heat resources, rare precipitation, long duration of sunshine, long frost-free period, large diurnal temperature difference, few pests and diseases, no indus- trial pollution, and adequate soils are very suitable for the growth and development of jujube in southern Xinjiang. In the past decades, jujube has been introduced and cultivated in many northwest regions due to its excellent ecological adaptability, high production and popularity in domestic market. Jujube industry therefore plays an important role in rural economy, farmers’income and in eliminating poverty and achieving prosperity in the poverty areas in southern Xinjiang. In 2018, the planting area of jujube reached 4×105 hm2 with a production of 2.4 million tons. It has become an important pillar industry in local rural areas. It is well known that soil organic matters and nutrients are fundamental for sustainable  orchard production. However, poor soil quality and water shortage are the adverse factors, and tradition- al orchard soil management with clear tillage has led to a series of ecological and environmental prob- lems such as soil water decrease, soil property deterioration, surface roughness, soil erosion, pesticide use increase, environmental pollution, loss of biodiversity and weakening of system resistance in a long run, which leads to reduce agricultural productivity and decrease economic benefit, and restrict the sus- tainable development of horticulture industry. Currently, inter-row planting grass is a recommended soil management practice in orchards. It improves soil physical and chemical properties, reduces soil water evaporation, increases soil organic matter content, maintains soil nutrient balance, ameliorates orchard ecological environment, and effectively solves the potential negative effect due to long-term clean till- age in orchard. However, there has been limited investigation on the influence of artificially planted grass on soil quality in extremely arid areas of southern Xinjiang. In order to provide theoretical and practical basis for orchard soil-water management technique, the effects of inter-row planting white clo- ver (Trifolium repens), bluegrass (Poa annua) and clear tillage on soil physical-chemical properties in a jujube orchard were studied in the extreme arid area【. Methods】The experiment was conducted in a ju- jube orchard with three treatments: inter-row planting white clover (Trifolium repens) or bluegrass (Poa annua), and the clean tillage (CK). The grasses were planted in the spring of 2016. The average sowing volume was 15-22.5 kg·hm-2, with mowing twice a year. In the third year after successive grass plant- ing, soil samples were taken at different layers from 0 to 60 cm using the five-point method from March to September 2018. We tried to minimize the disturbance of soil samples during the collection and trans- port processes. The soil physical-chemical properties including soil bulk density, porosity, water-hold- ing capacity and nutrient content were measured【. Results】Interrow grass planting could reduce soil bulk density, increase soil water content and porosity, change soil particle composition and significantly improve the physical structure of soil, especially in 40 cm soil layer. Interrow grass planting enhanced the water-holding capacity of soil, and reduced the optimal lower limit of water content and irrigation quota. The soil water holding capacity, available water content and water retention function were im- proved significantly. The maximum capacity, capillary capacity, field capacity and soil saturated water content, soil absorbed water content, soil retained water content in grass treatment within 60 cm were in- creased by 4.02%, 3.54%, 3.49% and 8.42 mm, 0.96 mm, 9.42 mm compared to CK, respectively, and the optimal lower limit of water content and irrigation quota decreased by 2.37 mm, 3.84 mm, respec- tively. Interrow grass planting enhanced soil infiltration capacity. The irrigation water could rapidly in- filtrate into soil, reduce runoff and improve the utilization of soil water. The soil initial permeability rate, steady permeability rate, average permeability rate and permeability coefficient in grass treatments within 60 cm were increased by 0.73 mm·min-1, 0.29 mm·min-1, 0.32 mm·min-1, and 0.10 mm·min-1compared to CK, respectively. Interrow grass planting improved the soil quality. The soil organic matter and available N, P, K content were increased by 4.10 g·kg-1, 18.48 mg·kg-1, 49.58 mg·kg-1, 61.12 mg·kg-1compared to CK, respectively. White clover was better than bluegrass in improving soil quality【. Conclu- sion】Interrow grass planting can significantly improve soil physical-chemical properties, and improve soil quality. The effect of planting clover was much better in orchard, southern Xinjiang.