Win-win: Application of sawdust-derived hydrochar in low fertility soil improves rice yield and reduces greenhouse gas emissions from agricultural ecosystems.

As a good soil synergist, biochar has a wide prospect in improving soil fertility and crop production. Although hydrochar, produced by hydrothermal carbonization process has attracted attention due to production advantages, hydrochar application in low fertility soils as well as its impact to the associated greenhouse gas (GHG) emissions in farmlands is rarely reported. To advance our understanding on the effect of hydrochar addition on grain yield from low fertility soils and the corresponding CH4 and N2O emissions, a soil-column experiment, with two hydrochar types (sawdust-derived hydrochar (SDH), microbial-aged hydrochar (A-SDH)) at two application rates (5‰, 15‰; (w/w)), was conducted. The results showed that hydrochar addition evidently increased rice yield. The N2O emissions were mainly related to the substrate supply of the hydrochar itself and less affected by the denitrifiers (functional genes) present. Hydrochar amendment at low application rate (5‰; SDH05, A-SDH05) significantly decreased the cumulative N2O emissions by 26.32% ~ 36.84%. Additionally, hydrochar amendment could not increase the CH4 emissions due to the substrate limitation; the cumulative emissions were similar with those from the control, ranging between 11.1-12.8 g m-2. Regarding grain yield and global warming potential, greenhouse gas intensity from the soils subjected to hydrochar (SDH05, A-SDH05, A-SDH15) were significantly lower than that of the control, observation attributed to the high yield and low N2O emissions. Overall, hydrochar addition is an effective strategy to ensure grain yield in low fertility soils with relatively low/controlled GHG emissions, especially when the amendment is applied at low application rate.