Evaluating the potential of digestate-loaded biochar in improving soil biological health and plant nutrition with less greenhouse gas emissions

Digestate has a great potential as a carbon (C) and nitrogen (N) soil amendment. Loading digestate onto biochar can produce a C- and N- enriched biochar fertilizer (i.e., digestate-loaded biochar, DLB), and help to solve digestate-specific issues. This study aims to evaluate the potential of DLB at optimal application rates in keeping plant (annual ryegrass) nutrition level while mitigating greenhouse gas emissions and improving soil biological health compared to chemical fertilizers in an acidic soil under liming or not. Soil biological health index was assessed through quantitative PCR and amplicon sequencing. The results showed that increasing DLB addition to 150 kg N ha -1 resulted in a similar N uptake of ryegrass to that under urea despite lower mineral N provided by DLB. This application rate decreased greenhouse gas emissions relative to urea, through decreasing CO 2 emission despite increased CH 4 emission. Compared to urea, this DLB rate did not change N 2O emission, corresponding to the specifically increased abundance of nirK gene (1.3-fold) (and enriched denitrifier Dokdonella) for N 2O production and nosZ clade I and II genes (87%) for N 2O consumption. The DLB at 150 kg N ha -1 enhanced soil biological health index by 1.4-fold relative to urea through increasing microbial abundances particularly fungi, enriching beneficial microbes (plant-growth-promoting bacteria, mycorrhiza and dark-septate-endophyte), and increasing fungal diversity; this effect was less pronounced under liming. This study concludes that DLB can serve as an organic-mineral fertilizer in maintaining plant nutrition while decreasing greenhouse gas emissions and enhancing soil biological health, offering a sustainable approach to managing organic waste.