Applying biochar to improve soil physical and chemical properties and nitrogen utilization by apple trees

Objectives The paper investigated the effects of different amounts of biochar on apple growth, soil physical and chemical properties and nitrogen utilization, which would provide a basis for rational application of biomass carbon in apple orchards.

Methods Two-year-old apple trees (Malus×domestica Borkh. ‘Red Fuji’)/Malushupehensis were used as tested materials and biomass carbon produced by pyrolysis of wood under 400℃ was used as fertilizers in a pot experiment. Biochar was applied at rates of 0, 15, 30, 45 and 60 g/kg, which represented as CK, T1, T2, T3, and T4 in turn. Plant and soil samples were collected to investigate apple growth and development, the physical and chemical properties of soil, the population of rhizosphere microorganisms and their effects on nitrogen uptake, utilization and loss using the ¹⁵N trace technique.

Results The plant height, stem diameter and total dry weight of all the treatments which were added biochar were significantly higher than those of CK. The root activities of T2, T3 and T4 were significantly higher than those of T1 and CK, but there were no significant differences between the three treatments. With the increased addition of biomass carbon, soil bulk density was decreased gradually, the soil bulk density of T3 and T4 were 1.22 and 1.20 g/cm³, respectively, and the difference between the two treatments was not significant, but the density was significantly higher than that of CK, T1 and T2. Soil organic matter, alkali hydrolyzable nitrogen, readily available phosphorus, readily available potassium and rhizosphere soil bacteria, actinomycetes and fungi were highest in the T4 treatment, followed by the T3 treatment, the difference was not significant, but were significantly higher than other treatments. Compared with CK, the addition of biochar significantly increased the uptake of fertilizer ¹⁵N, and the ¹⁵N utilization rates of the T4 and T3 treatments were 15.18% and 15.63%, respectively, which were significantly higher than those of other treatments. The residual rate of soil ¹⁵N was highest in T4 (38.2%), followed by T3, and the lowest was T1 (30.0%). The highest nitrogen loss was T1 (58.5%), the lowest was T4 treatment, and the difference between T4 and T3 was not significant. Through the analysis of plant biomass and nitrogen use efficiency and biomass carbon dosage, the maximum amount of biomass carbon was in range of 64–55 g/kg.

Conclusions Biochar application decreased soil bulk density, improved the soil nutrient contents and rhizosphere soil bacteria, actinomycetes and fungi, apple plant root and shoot growth and absorption of nitrogen fertilizer, increased the soil nitrogen fixation, reduced nitrogen loss, and improved the utilization rate of nitrogen fertilizer. Under the condition of this experiment, the suitable biomass carbon amount was 55–64 g/kg.