Biochar application increases soil available nitrogen and plant-to-soil carbon input

Biochar has strong potential to promote soil carbon (C) sequestration and crop performance, but its effects on agricultural systems may vary over time. We applied pine wood chip biochar at 10 and 30 t ha-1 (B10, B30) in a macadamia orchard and measured soil properties, photosynthesis and leaf properties over 2 years (2012–2014). Biochar treatments initially reduced soil available nitrogen (N) (NH4+-N+NO3--N), probably due to its absorption effect, and this appeared to be responsible for the reduction of macadamia leaf photosynthetic capacity following biochar application. However, the effect of biochar on soil available N reversed in 12 months and biochar treatments (especially B30) generally displayed higher soil available N than the control during the second year. This is mainly caused by the increment of soil NO3--N in biochar treatments, probably attributable to accelerated nitrification rate and reduced NO3- leaching. The effect of biochar on soil available N matched with leaf δ15N signature: macadamia leaves were more 15N depleted in biochar treatments, suggesting better preservation of soil available N. Biochar application also led to higher soil total C and C:N ratio, and the gap between biochar treatments (especially B30) and the control increased over time. This pattern indicates that biochar increased soil C not only by instantaneous addition of recalcitrant C to soil, but also by promoting plant-to-soil C input in longer term, and this was supported by the observation of higher leaf photosynthetic capacity in B30 (than the control) by the end of the experiment. Our results suggest that the influence of biochar on soil and crop could have significant temporal effect and some benefits may exhibit with considerable time lag. This highlights the imperative need of long term studies for more complete understanding of the agronomic and climate mitigation benefits of biochar soil application