Plants mitigate ecosystem nitrous oxide emissions primarily through reductions in soil nitrate content: Evidence from a meta-analysis.

Nitrous oxide (N2O) is a potent greenhouse gas (GHG) and an ozone-depleting substance. The presence of plants in an ecosystem can either increase or decrease N2O emissions, or play a negligible role in driving N2O emissions. Here, we conducted a meta-analysis comparing ecosystem N2O emissions from planted and unplanted systems to evaluate how plant presence influences N2O emissions and examined the mechanisms driving observed responses. Our results indicate that plant presence reduces N2O emissions while it increases dinitrogen (N2) emissions from ecosystems through decreases in soil nitrate concentration as well as increases in complete denitrification and mineral N immobilization. The response of N2O emissions to plant presence was universal across major terrestrial ecosystems - including forests, grassland and cropland - and it did not vary with N fertilization. Further, in light of the potential mechanisms of N2O formation in plant cells, we discussed how plant presence could enhance the emission of N2O from plants themselves. Improving our understanding of the mechanisms driving N2O emissions in response to plant presence could be beneficial for enhancing the robustness for predictions of our GHG sinks and sources and for developing strategies to minimize emissions at the ecosystem scale.

Microbial mechanisms regulate soil organic carbon mineralization under carbon with varying levels of nitrogen addition in the above-treeline ecosystem.

Climate change is leading to the upward migration of treelines in mountainous regions, resulting in changes to the carbon and nitrogen inputs in soils. The impact of these alterations on the microbial mineralization of the existing soil organic carbon (SOC) pool remains uncertain, making it challenging to anticipate their effects on the carbon balance. To enhance our prediction and understanding of native SOC mineralization in Himalayan regions resulting from treeline shifts, a study was conducted to quantify soil priming effects (PEs) at high elevations above the treeline ecosystem. In laboratory incubation, soils were treated with a combination of 13C-glucose and varying nitrogen rates, along with carbon-only treatments and control groups without any amendments. The addition of carbon with varying nitrogen addition rates exhibited diverse PEs on native SOC. A highly positive PE was observed under low nitrogen input due to a high carbon/nitrogen imbalance and increased L-leucine aminopeptidase (LAP) activity, coupled with low nitrogen availability and carbon use efficiency (CUE). In contrast, a positive PE declined following high nitrogen input due to a low carbon/nitrogen imbalance and LAP activity, coupled with high nitrogen availability and CUE. These findings support the concept that multiple mechanisms (i.e., microbial nitrogen mining and microbial metabolic efficiency) exist that regulate SOC mineralization under the addition of carbon with varying nitrogen rates. Thus, an increase in nitrogen availability fulfils microbial nitrogen demand, reduces the microbial carbon/nitrogen imbalance, decreases enzyme activity that requires nitrogen and enhances microbial metabolic efficiency. Consequently, this mechanism reduces the positive PE, thereby serving as a potential tool for stabilizing native SOC in above-treeline ecosystems.