Soil-plant nitrogen isotope composition and nitrogen cycling after biochar applications

Biochar has strong potential to improve nitrogen (N) use efficiency in both agricultural and horticultural systems. Biochar is usually co-applied with full rates of fertiliser. However, the extent to which N cycling can be affected after biochar application to meet plant N requirement remains uncertain. This study aimed to explore N cycling up to 2 years after biochar application. We applied pine woodchip biochar at 0, 10 and 30 t ha⁻¹ (B0, B10, B30, respectively) in a macadamia orchard and evaluated the N isotope composition (δ¹⁵N) of soil, microbial biomass and macadamia leaves. Soil total N (TN) and inorganic N pools were also measured up to 2 years after biochar application. Biochar did not alter soil TN but soil NO₃⁻-N increased at months 12 and 24 after biochar application. Soil NO₃⁻-N concentrations were always over ideal levels of 15 μg g⁻¹ in B30 throughout the study. Stepwise regression indicated that foliar δ¹⁵N decreases after biochar application were explained by increased NO₃⁻-N concentrations in B30. Foliar TN and photosynthesis were not affected by biochar application. The soil in the high rate biochar plots had excess NO₃⁻-N concentrations (over 30 μg g⁻¹) from month 20 onwards. Therefore, N fertiliser applications could be adjusted to prevent excessive N inputs and increase farm profitability.