Effects of nitrogen input on soil microbial biomass in Kuerlexiangli pear orchard

Abstract:【Objective】Nitrogen is one of the most active elements in soil fertility. However, excessive application of nitrogen will threaten the ecological environment of the soil and reduce the activity of microorganisms. Soil microbial biomass represents the active part of soil nutrients and is often regarded as an important indicator of soil quality and ecological functions. The application of nitrogen fertilizer affects the soil microbial biomass and soil quality. Therefore, it is very important to study the effect of different nitrogen application rates on soil microbial biomass. Nitrogen fertilizer management has become one of the important means to adjust the soil ecological balance. Recently, with the rapid development of the cultivation technology of Kuerlexiangli pears, excessive nitrogen fertilizers have been applied to obtain high yields. It has an adverse impact on productivity, increases production costs, and poses a potential threat to the soil ecological environment. However, there are few studies on the effect of nitrogen application rate on soil biological characteristics in Kuerlexiangli pear orchards. This study takes the soil in a Kuerlexiangli pear orchard as the research object. The changing pattern of soil microbial biomass carbon (SMBC), soil microbial biomass nitrogen (SMBN) and other biochemical indicators under different nitrogen application rates were examined. Relationships between SMBC, SMBN and different concentrations of soil nitrogen addition were analyzed. The study provides a scientific basis for establishing a scientific and reasonable nitrogen fertilization for maintaining the soil ecological balance in fragrant pear orchards.【Methods】The soil in a 6-7 year-old Kuerlexiangli pear orchard was used as the material to study the effects of different nitrogen application levels on the soil microbial biomass. Four levels of nitrogen application including 0, 150, 300 and 450 kg·hm^-2 (represented by N0, N1, N2 and N3), were set. The soil was sampled by the soil drilling method at different growth periods of the trees (the early bud period (April 1st), the fruit setting period (June 1st), the fruit swelling period (August 1st), the fruit ripening period (September 15th). Soil organic carbon was measured by the external heating method of potassium dichromate. Total nitrogen in the soil was determined by the semi-micro Kjeldahl method. The determination of SMBC was carried out with the fumigation extraction-volume analysis method. The determination of SMBN was done with the fumigation extraction-ninhydrin colorimetric method. SMBC was calculated as Ec/kEC, where Ec is the difference between fumigated and unfumigated soil and kEC the conversion coefficient, with a value of 0.38. SMBN was calculated as mEmin-N, where Emin-N is the difference between fumigated and unfumigated soil and m is the conversion factor with a value of 5.0【. Results】With the continuous changes in the growth period of fruit trees, SMBC and SMBN under different treatments generally increased first and then decreased in each soil layer (the fruit swelling period ＞ the fruit setting period ＞ the fruit ripening period ＞ the early bud period). From the spatial distribution point of view, in different fertilization treatments in each growth period, the SMBC and SMBN decreased with the increase of soil layer. The effect of fertilization on the SMBC and SMBN of the 0-60 cm soil layer in each growing season showed the same change trend, an increase first and then a decrease (N2 ＞ N3 ＞ N1 ＞ N0). The treatment effect with N2 was the best. SMBC and SMBN were extremely significantly positively correlated with the nitrogen concentration and total nitrogen applied (p ＜0.01), and extremely significantly negatively correlated with the soil depth (p ＜0.01). The change in SMBC/SMBN was also directly affected by the concentration of nitrogen applied, total nitrogen, and organic carbon. The result suggested that the concentration of nitrogen application, soil depth, total nitrogen, organic carbon and soil microbial biomass are closely related.【Conclusion】Soil microbial biomass tended to increase as result of nitrogen addition at a suitable concentration, indicating that nitrogen addition can promote soil microbial activity to a certain extent. The application of nitrogen fertilizer can significantly improve SMBC and SMBN, and the improvement effect of SMBC and SMBN was the best under N2 treatment. However, excessive nitrogen application will inhibit the accumulation of soil microbial biomass and significantly reduce the utilization efficiency of soil total nitrogen by microorganisms. Therefore, in the cultivation of Kuerlexiangli pears, the supply of nitrogen should be appropriately controlled. In this study, combining the effects of nitrogen application rate on soil microbial biomass in Kuerlexiangli pear orchard under different nitrogen application rates, it is recommended that the nitrogen application rate of 300kg·hm^-2 was the best for 6-7-year-old Kuerlexiangli pears.