Atmospheric change encompassing a rising carbon dioxide (CO2) concentration is one component of Global Change that affects various ecosystem processes and functions. The effects of elevated CO2 (eCO2) on belowground processes are incompletely understood due to complex interactions among various ecosystem fluxes and components such as net primary productivity, carbon (C) inputs to soil, and the living and dead soil C and nutrient pools. Here we summarize the literature on the impacts of eCO2 on 1) cycling of C and nitrogen (N), 2) microbial growth and enzyme activities, 3) turnover of soil organic matter (SOM) and induced priming effects including N mobilization/immobilization processes, and 4) associated nutrient mobilization from organic sources, 5) water budget with consequences for soil moisture, 6) formation and leaching of pedogenic carbonates, as well as 7) mobilization of nutrients and nonessential elements through accelerated weathering. We show that all effects in soil are indirect: they are mediated by plants through increased net primary production and C inputs by roots that foster intensive competition between plants and microorganisms for nutrients. Higher belowground C input from plants under eCO2 is compensated by faster C turnover due to accelerated microbial growth, metabolism and respiration, higher enzymatic activities, and priming of soil C, N and P pools. We compare the effects of eCO2 on pool size and associated fluxes in: soil C stocks vs. belowground C input, microbial biomass vs. CO2 soil efflux vs. various microbial activities and functions, dissolved organic matter content vs. its production, nutrient stocks vs. fluxes etc. Based on these comparisons, we generalize that eCO2 will have little impacts on pool size but will strongly accelerate the fluxes in biologically active and stable pools and consequently will accelerates biogeochemical cycles of C, nutrients and nonessential elements.

Review and synthesis of the effects of elevated atmospheric CO2 on soil processes: No changes in pools, but increased fluxes and accelerated cycles

Nitrous oxide (N2 O) emissions from soil contribute to global warming and are in turn substantially affected by climate change. However, climate change impacts on N2 O production across terrestrial ecosystems remain poorly understood. Here, we synthesized 46 published studies of N2 O fluxes and relevant soil functional genes (SFGs, that is, archaeal amoA, bacterial amoA, nosZ, narG, nirK and nirS) to assess their responses to increased temperature, increased or decreased precipitation amounts, and prolonged drought (no change in total precipitation but increase in precipitation intervals) in terrestrial ecosystem (i.e. grasslands, forests, shrublands, tundra and croplands). Across the data set, temperature increased N2 O emissions by 33%. However, the effects were highly variable across biomes, with strongest temperature responses in shrublands, variable responses in forests and negative responses in tundra. The warming methods employed also influenced the effects of temperature on N2 O emissions (most effectively induced by open-top chambers). Whole-day or whole-year warming treatment significantly enhanced N2 O emissions, but daytime, nighttime or short-season warming did not have significant effects. Regardless of biome, treatment method and season, increased precipitation promoted N2 O emission by an average of 55%, while decreased precipitation suppressed N2 O emission by 31%, predominantly driven by changes in soil moisture. The effect size of precipitation changes on nirS and nosZ showed a U-shape relationship with soil moisture; further insight into biotic mechanisms underlying N2 O emission response to climate change remain limited by data availability, underlying a need for studies that report SFG. Our findings indicate that climate change substantially affects N2 O emission and highlights the urgent need to incorporate this strong feedback into most climate models for convincing projection of future climate change.

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Terrestrial N2O emissions and related functional genes under climate change: A global meta-analysis

Inland waters are important sources of greenhouse gases. Measurements over eight years suggest that African inland waters are a substantial source of greenhouse gases, equivalent to a quarter of the global land and ocean carbon sink.

Globally significant greenhouse-gas emissions from African inland waters

Warming can accelerate the decomposition of soil organic matter and stimulate the release of soil greenhouse gases (GHGs), but to what extent soil release of methane (CH4 ) and nitrous oxide (N2 O) may contribute to soil C loss for driving climate change under warming remains unresolved. By synthesizing 1,845 measurements from 164 peer-reviewed publications, we show that around 1.5°C (1.16-2.01°C) of experimental warming significantly stimulates soil respiration by 12.9%, N2 O emissions by 35.2%, CH4 emissions by 23.4% from rice paddies, and by 37.5% from natural wetlands. Rising temperature increases CH4 uptake of upland soils by 13.8%. Warming-enhanced emission of soil CH4 and N2 O corresponds to an overall source strength of 1.19, 1.84, and 3.12 Pg CO2 -equivalent/year under 1°C, 1.5°C, and 2°C warming scenarios, respectively, interacting with soil C loss of 1.60 Pg CO2 /year in terms of contribution to climate change. The warming-induced rise in soil CH4 and N2 O emissions (1.84 Pg CO2 -equivalent/year) could reduce mitigation potential of terrestrial net ecosystem production by 8.3% (NEP, 22.25 Pg CO2 /year) under warming. Soil respiration and CH4 release are intensified following the mean warming threshold of 1.5°C scenario, as compared to soil CH4 uptake and N2 O release with a reduced and less positive response, respectively. Soil C loss increases to a larger extent under soil warming than under canopy air warming. Warming-raised emission of soil GHG increases with the intensity of temperature rise but decreases with the extension of experimental duration. This synthesis takes the lead to quantify the ecosystem C and N cycling in response to warming and advances our capacity to predict terrestrial feedback to climate change under projected warming scenarios.

Increased soil release of greenhouse gases shrinks terrestrial carbon uptake enhancement under warming

Ebullition was a major pathway of methane emissions from the aquaculture ponds in southeast China

The net balance of greenhouse gas (GHG) exchanges between terrestrial ecosystems and the atmosphere under elevated atmospheric carbon dioxide (CO2 ) remains poorly understood. Here, we synthesise 1655 measurements from 169 published studies to assess GHGs budget of terrestrial ecosystems under elevated CO2 . We show that elevated CO2 significantly stimulates plant C pool (NPP) by 20%, soil CO2 fluxes by 24%, and methane (CH4 ) fluxes by 34% from rice paddies and by 12% from natural wetlands, while it slightly decreases CH4 uptake of upland soils by 3.8%. Elevated CO2 causes insignificant increases in soil nitrous oxide (N2 O) fluxes (4.6%), soil organic C (4.3%) and N (3.6%) pools. The elevated CO2 -induced increase in GHG emissions may decline with CO2 enrichment levels. An elevated CO2 -induced rise in soil CH4 and N2 O emissions (2.76 Pg CO2 -equivalent year-1 ) could negate soil C enrichment (2.42 Pg CO2 year-1 ) or reduce mitigation potential of terrestrial net ecosystem production by as much as 69% (NEP, 3.99 Pg CO2 year-1 ) under elevated CO2 . Our analysis highlights that the capacity of terrestrial ecosystems to act as a sink to slow climate warming under elevated CO2 might have been largely offset by its induced increases in soil GHGs source strength.

Climatic role of terrestrial ecosystem under elevated CO2 : a bottom-up greenhouse gases budget.

Rivers are of increasing concern as sources of atmospheric methane (CH4), while estimates of global CH4 emissions from rivers are poorly constrained due to a lack of representative measurements in tropical and subtropical latitudes. Measurements of complete CH4 flux components from subtropical rivers draining agricultural watersheds are particularly important since these rivers are subject to large organic and nutrient loads. Two-year measurements of CH4 fluxes were taken to assess the magnitude of CH4 emissions from the Lixiahe River (a tributary of the Grand Canal) draining a subtropical rice paddy watershed in China. Over the two-year period, annual CH4 emissions averaged 29.52 mmol m-2 d-1, amounting to 10.78 mol m-2 yr-1, making the river a strong source of atmospheric CH4. The CH4 emissions from rivers during the rice-growing season (June-October) accounted for approximately 70% of the annual total, with flux rates at 1-2 orders of magnitude greater than those for rice paddies in this area. Ebullition contributed to 44-56% of the overall CH4 emissions from the rivers and dominated the emission pathways during the summer months. Our data highlight that rivers draining agricultural watersheds may constitute a larger component of anthropogenic CH4 emissions than is currently documented in China.

Ziheng Zou

Papers

Climatic role of terrestrial ecosystem under elevated CO2 : a bottom-up greenhouse gases budget.

High Methane Emissions Largely Attributed to Ebullitive Fluxes from a Subtropical River Draining a Rice Paddy Watershed in China.

Effects of nanopolystyrene addition on nitrogen fertilizer fate, gaseous loss of N from the soil, and soil microbial community composition.

Sulfonamide antibiotics alter gaseous nitrogen emissions in the soil-plant system: A mesocosm experiment and meta-analysis.