Effects of cow dung and biochar on root growth, soil properties and nitrogen utilization of apple

Two-year-old 'Fuji' apple (Malus domestica Borkh. cv Red Fuji/Malus hupehensis) trees were used to study the effect of cow dung and biochar on soil properties, and apple root growth, nitrogen uptake and utilization using ¹⁵N isotope tracer technique. The study aimed at providing reference for rational fertilization and sustainable apple development and production. There were 6 treatments in the study-no cow dung or biochar (CK), 100% cow dung (T1), 75% cow dung + 25% biochar (T2), 50% cow dung + 50% biochar (T3), 25% cow dung + 75% biochar (T4) and 100% biochar (T5). The results showed that combined application of cow dung and biochar improved soil physical and chemical properties, and increased soil contents of organic matter, alkali-hydrolyzale nitrogen, available phosphorus and available potassium, but reduced soil bulk density. The soil contents of organic matter, alkali-hydrolyzale nitrogen, available phosphorus and available potassium decreased gradually with decreasing cow dung proportion, and they were highest under T1, which increased by respectively 97.31%, 19.01%, 24.37% and 32.73% over CK. There was no significant difference between T1 and T2 in terms of the contents of the variables. Soil bulk density decreased with gradual increase in biochar proportion. The difference between T4 and T5 treatments was not significant. Soil bulk density under T4 or T5 treatment was significantly lower than that of CK and T1. Bacteria population was highest in rhizosphere soil, followed by actinomyces, and fungus content was lowest. The addition of cow dung and biochar significantly increased the number of bacteria, actinomycetes and fungus in rhizosphere soil. The population of bacteria, actinomycetes and fungus was highest in T2 treated soil. The cow dung and biochar mixture also promoted apple root growth. Root tip, root surface area and root activity were highest in T2 treated soil, with respective increases of 47.90%, 33.47% and 44.67% over that of CK. Compared with CK, the cow dung plus biochar significantly improved the Ndff value, total nitrogen and ¹⁵N absorption of various organs of apple. It also increased ¹⁵N utilization and residual rate, while reducing ¹⁵N loss rate. The utilization rate and the residual rate of ¹⁵N were higher under treatments of mixed cow dung and biochar. Single application of cow dung or biochar was second and CK treatment the lowest. The loss rate of ¹⁵N showed the reverse trend. T2 treatment showed best with the highest ¹⁵N utilization rate and residue rate and lowest ¹⁵N loss rate. Under T2 treatment, while ¹⁵N utilization ratio increased by 5.51%, ¹⁵N loss rate decreased by 14.52% compared with CK. Comprehensive analysis showed that the 75% cow dung + 25% biochar treatment (T2) had the best effect on apple root growth, soil characteristics and nitrogen absorption and utilization.