Correlation between the soil microbial biomass and yield and quality in Kuerlexiangli pear orchard with different nitrogen application levels

Abstract:【Objective】Nitrogen is one of the most active elements in soil fertility. Excessive application of nitrogen leads to decline in fruit yield and quality and reduces soil microbial activity. Soil micro-bial biomass represents the active part of soil nutrients and is considered an important indicator of soil quality and ecological function. Application of nitrogen fertilizer can affect the growth and metabolism of soil microorganisms, which invariably affects material conversion and energy cycle of the whole or-chard ecosystem. Subsequently, growth and development of the aboveground part of the plant is affect-ed. Understanding of the correlations between soil microbial biomass, yield and quality under different nitrogen application rates are important to realize efficient nitrogen utilization for the sustainable devel-opment of fruit trees. However, few in-depth studies on the correlation between soil microbial biomass, yield and quality under different nitrogen application rates are presently available. This study investigat-ed the effect of nitrogen application rate on the yield and fruit quality of Kuerlexiangli pear based on soil obtained in orchards. The correlations between nitrogen application rate and soil microbial biomass, yield and fruit quality were analyzed. The optimal nitrogen application range for improvement of Kuer-lexiangli pear yield and quality was recommended.【Methods】Soil from a 6-7-year-old Kuerlexiangli pear orchard was used as the research material. Four nitrogen application levels of 0, 150, 300 and 450 kg·hm^-2 (expressed by N0, N1, N2 and N3 respectively) were used. Soil samples were collected by soil drilling method at different growth stages of Kuerlexiangli pear (fruit setting period: June 1st; fruit swell-ing period: August 1st; and fruit ripening period: September 15th). The fruit samples were collected at the fruit ripening stage (September 15th). Soil microbial biomass carbon (SMBC) was determined with fumigation extraction volumetric analysis. Soil microbial biomass nitrogen (SMBN) was determined with fumigation extraction ninhydrin colorimetry. The yield per plant was the number of fruit per plant multiplied by average fruit weight. Yield per plant was used to calculate yield per hectare. The content of soluble solids was determined with a hand-held refractometer. The content of vitamin C (Vc) was de-termined using 2, 6-dichloroindophenol method. Total acid content was determined with acid-base indi-cator titration according to the national standard (GB/T 12456—2021). The content of stone cells was determined using frozen H2SO4 treatment.【Results】With the increase in nitrogen application rate,yield, soluble solids, Vc and sugar/acid ratio increased initially and then decreased. The best effect was observed in N2. The total acid content decreased with the increase in nitrogen application (N0＞N1＞N2＞N3). Yield, soluble solids, Vc and sugar/acid ratio were positively correlated with SMBC and SMBN in the three stages, and negatively correlated with SMBC/SMBN. Stone cells and total acids were negatively correlated with SMBC and SMBN, and positively correlated with SMBC/SMBN. Re-gression analysis of nitrogen application rate and SMBC/SMBN with yield and quality showed that the interaction between nitrogen application rate and SMBC/SMBN was strong in effect on the yield, solu-ble solids, Vc, total acid content and sugar/acid ratio, especially at the fruit swelling period.【Conclusion】The yield and fruit quality of fragrant pear increased with the increase in concentration of nitro-gen within certain range. N2 treatment was the best level. Excessive nitrogen application reduced the yield and fruit quality of fragrant pear. According to the results, application rate of 300-380 kg·hm-2 and an SMBC/SMBN range of 9-15 were recommended for 6-7-year-old Kuerlexiangli pear orchard.