Interactive priming of soil N transformations from combining biochar and urea inputs: A 15N isotope tracer study

Biochar has been found to interact with N transformations in soil but the mechanisms remain largely unknown. In this study we investigated the priming effect of combined biochar and urea inputs on soil inorganic N pools through an isotope tracer approach. Biochar was applied in combination with urea in two complementary laboratory experiments: (i) in the first one, three N-15-labeled organic amendments (wheat straw (WS)), its biochars produced at 350 degrees C (B350) and at 550 degrees C (B550) were added to soil in combination with unlabeled urea; (ii) in the second experiment the three same, but unlabeled, amendments were added to soil in combination with N-15 labeled urea. This system allowed partitioning between three N sources: native soil N, biochar-derived N and urea-derived N. In addition, CO2 fluxes were measured to follow total C mineralization in soil and N2O emissions were monitored. The proportion of N that mineralized from biochar was always below 0.5% of the added N. The co-addition of urea increased the concentration of NH4+ derived from B350, but not from B550, demonstrating the lower mineralization of N in biochars produced at 550 degrees C. Whereas the addition of WS led to a rapid immobilization of N, we found that despite their high C:N, none of the biochars, applied at a rate of 1.5%, immobilized inorganic N in soil. On the contrary, significantly higher NH4+ concentrations derived from native soil organic N (SON) and urea were found throughout the incubation when B550 was added. This effect can be attributed to an apparent priming effect since a net decrease in CO2 fluxes was recorded when biochar was added to the soil. The addition of glucose (a low molecular weight carbon source) stimulated an increase in CO2 fluxes in all treatments along with a net N immobilization in soil. However, both biochars significantly reduced C readily available to microbes, as proved by lower soil CO2 fluxes, and limited the immobilization of NH4+ induced by glucose addition. Our results suggest that biochar may partially offset the mineralization of easily available organic C, buffering the immobilization of inorganic N in soil when labile organic compounds (e.g. root exudates, fresh manure, etc.) are incorporated.