© 2017, Springer International Publishing Switzerland. Background and aim: Biochar application to soil is widely claimed to increase plant productivity. However, the underlying mechanisms are still not conclusively described. Here, we aim to elucidate these mechanisms using stable isotope probing. Methods: We conducted two experiments with uniquely double-labelled ( 15 N and 13 C) biochar and its feedstock (residue), applied separately at 15 Mg ha −1 . Both experiments contained three treatments: biochar amendment (Biochar), unpyrolysed residue amendment (Residue) and a no addition control (Control). Experiment I was a 119 day pot experiment seeded with Lolium perenne. Experiment II was a 71 day incubation experiment without plants in which CO 2 and N 2 O fluxes were measured. Results: Both Biochar and Residue significantly increased aboveground productivity compared to Control (140% and 160%, respectively). Initial N immobilisation was stimulated in Residue, whereas not in Biochar. 13 C–CO 2 analysis confirmed that biochar was significantly more recalcitrant than residue. 15 N analysis showed that 2% and 0.3% of grass N was derived from the amended material in Residue and Biochar, respectively. Conclusions: Our results suggest that biochar-induced yield increases derive from a combination of reduced N immobilization and a moderate N fertilization effect. Although in the short term biochar might offer benefits compared to residue incorporation, it is unlikely that biochar yield gains will be sustainable for the decades to centuries that biochar C can be expected to reside in soil.
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