Initial biochar effects on plant productivity derive from N fertilizations

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 (15N and 13C) 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 CO2 and N2O 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. 13C–CO2 analysis confirmed that biochar was significantly more recalcitrant than residue. 15N 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.