Co-occurrence of microplastics and hydrochar stimulated the methane emission but suppressed nitrous oxide emission from a rice paddy soil

Hydrochar (HBC) is an emerging carbon-enriched material produced from renewable resources using hydrothermal carbonization and has performed multiple functions as a soil amendment, including greenhouse gas (GHGs) emission mitigation in croplands. With more and more microplastics (MPs) entering soil, there are concerns about the impacts of the co-occurrence of HBC and MPs on GHGs emissions and the associated microbial communities, which have not been revealed. This study conducted a rice paddy soil column experiment. The co-effects of 0.5 wt% MPs (polyethylene (PE) and polypropylene (PAN)) and 1.0 wt% HBC on nitrous oxide (N2O) and methane (CH4) emissions were investigated relative to treatment of HBC alone and control check (CK). Also, the microbial mechanism regulating N2O and CH4 emissions has been clarified. Results showed that compared with CK, the addition of HBC alone reduced CH4 cumulative emission by 27.7% although the change was insignificant, but increased N2O cumulative emission by 66.3%. Overall, HBC alone showed limited impacts on global warming potential (GWP) and greenhouse gas intensity (GHGI). This effect was distinctly altered by PE and PAN addition. The co-occurrence of HBC and PE caused obviously higher methanogens gene (mcrA) in soil but lower rice grain yield, leading to the increase of CH4 emissions by 83.5% and GHGIN2O + CH4 by 36.0% compared to HBC alone. While, no apparent difference in CH4 emission occurred between HBC + PAN and HBC. By contrast, HBC + PAN obviously increased N2O reductase gene (nosZ) abundance and decreased the ratio of denitrification gene nirS and nirK to nosZ. Thus, HBC + PAN showed 3- and 4-fold lower N2O emissions than CK and HBC, respectively. Among all the treatments, HBC + PAN exhibited the strongest GWPN2O + CH4 and GHGIN2O + CH4 mitigation effect. This study firstly demonstrates that the potential of HBC in mitigating GHGs emission in the rice field would greatly and differently vary with the co-occurrence of different types of MPs.