Comprehensive evaluation on soil fertility quality of jujube orchard under combined application of biochar and nitrogen fertilizer

Abstract: In this study, the study area is the main jujube producing region in China. Intensive management including excessive fertilizers input and spraying pesticides were widely performed in order to obtain higher yield and income benefits, which have shown to accelerate the deterioration of soil quality and affect the edible quality and safety of jujube to consumers. Therefore, the object of this study was to identify the optimum amendment rate under the combined application of peanut shell biochar and nitrogen (N) fertilizer at different levels in jujube plantation, which could not only provide a scientific evidence for high-efficient fertilization and enhancing soil fertility, but also decrease chemical fertilizer input to reduce environmental risks and maintain the sustainable development of jujube industry. A three-year consecutive field experiment was conducted from 2013 to 2015 in jujube orchards of the North China Plain. A widely planted jujube cultivar in Henan Province was chosen and fifteen-year-old trees were used as testing materials in this study. There were a total of 13 treatments, including CK (no fertilizer), 4 levels of biochars (0, 2.5, 5 and 10 t/hm2, namely, C0, C1, C2 and C3) and 3 levels of N fertilizers (300, 450 and 600 kg/hm2, namely, N1, N2 and N3). A total of 14 important quantitative indices (organic matter, total and available soil nutrient, soil microbial biomass, enzyme activities) related to soil chemical, physical and biological properties were selected, and the factor analysis and the minimum Euclidean distance method were employed to evaluate the overall soil fertility under biochar and N fertilizer amendments. Jujube yield was used to verify the evaluation results. The results indicated that 3 common factors extracted from the initial 14 indices could reflect 82.49% of the original information quantity without losing any original variable. The first common factor was related to soil total N, total K, available K and microbial biomass N, which were the major contributors to nutrient maintenance and supply capacity of soil. The second common factor was related to the activities of urease and invertase, and microbial biomass C, which was regarded as the microbial activity factor, followed by the alkaline phosphatase activity, which was the major contributor to the third common factor. There were significant positive correlations among soil nutrients, microbial biomass, enzyme activities and microbial population according to the correlation analysis. The highest score of soil quality was observed in the C3N1 treatment, followed by the C2N3 and C2N2 treatments, which were almost in line with the results of jujube yields. Hierarchical cluster analysis classified 13 treatments into 5 main groups on the basis of the measured parameters, which was mostly consistent with the result of soil quality scores. The optimal amounts for biochar and N fertilizer according to these results should be 10 t/hm2 and 300 kg/hm2, respectively. These findings give us a better scientific basis for high-efficient fertilization and management under the combined application of peanut shell biochar and N fertilizer amendments in orchards, suggesting that biochar-amended treatment was practical and cost-efficient measure to restore crop productivity and soil quality in this study area.