Temporal evolution of the impact of a woody biochar on soil nitrogen processes - A 15N tracing study

Biochar addition to soils has been proposed as a means to increase soil fertility and carbon sequestration. However, its effect on soil nitrogen (N) cycling and N availability is poorly understood. Furthermore, there is a need for biochar N studies that aim at investigating long-term analogues. In order to investigate the temporal evolution of biochar effect on the gross rates of simultaneously occurring N transformations, we conducted two 15N tracing studies in combination with numerical data analysis using sandy loam soil from a field trial amended with a woody biochar. For the first experiment we sampled immediately after biochar application; for the second experiment one year later.
Altogether, our results show that application of a woody biochar type to a C-poor sandy loam soil accelerated N cycling just after biochar addition through stimulating soil microbial activity, thereby increasing N bio-availability through increased mineralization and nitrification rates. However, available N was quickly biotically immobilized. One year after biochar addition, results show that these effects are temporary, probably due to the transient effects of biochar pH and labile C fraction.