A warmer climate reduces the bioreactivity of isolated boreal forest soil horizons without increasing the temperature sensitivity of respiratory CO2 loss

Abstract. The large amount of soil carbon in boreal forest ecosystems has the potential to influence the climate system if released in large quantities in response to warming. Thus, there is a need to better understand and represent the environmental sensitivity of soil carbon decomposition. Most soil carbon decomposition models rely on empirical relationships omitting key biogeochemical mechanisms and their response to climate change is highly uncertain. In this study, we developed a multi-layer microbial explicit soil decomposition model framework for boreal forest ecosystems. A thorough sensitivity analysis was conducted to identify dominating biogeochemical processes and to highlight structural limitations. Our results indicate that substrate availability (limited by soil water diffusion and substrate quality) is likely to be a major constraint on soil decomposition in the fibrous horizon (40–60% of soil organic carbon (SOC) pool size variation), while energy limited microbial activity in the amorphous horizon exerts a predominant control on soil decomposition (>70% of SOC pool size variation). Elevated temperature alleviated the energy constraint of microbial activity most notably in amorphous soils, whereas moisture only exhibited a marginal effect on dissolved substrate supply and microbial activity. Our study highlights the different decomposition properties and underlying mechanisms of soil dynamics between fibrous and amorphous soil horizons. Soil decomposition models should consider explicitly representing different boreal soil horizons and soil–microbial interactions to better characterize biogeochemical processes in boreal forest ecosystems. A more comprehensive representation of critical biogeochemical mechanisms of soil moisture effects may be required to improve the performance of the soil model we analyzed in this study.

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Response of N2O and N2 Emissions in Forest Soils to Temperature Change across China

10.5194/egusphere-egu2020-12706 ◽

2020 ◽

Author(s):

Haoming Yu ◽

Yunting Fang ◽

Ronghua Kang

Keyword(s):

Global Warming ◽

Soil Temperature ◽

Forest Soil ◽

Temperature Change ◽

Temperature Sensitivity ◽

Forest Soils ◽

Soil Samples ◽

Mean Annual Precipitation ◽

Mean Annual Temperature ◽

N Cycle

N2O and N2 Emissions from soil in terrestrial ecosystems is a crucial component of the global nitrogen (N) cycle. The response of these two gases emissions from forest soil to temperature change and its underlying mechanisms are essential for predicting N cycle to global warming. Despite the warming-induced effects on soil N cycle is considered to be positive in general, our understanding of temperature sensitivity (Q10) of N2O and N2 emissions is rather limited. We quantified the Q10 of N2O and N2 emissions in forest soils and explored their major driving factors by conducting an incubation experiment using 15N tracer (Na15NO3) with soil samples from nineteen forest sites from temperate to tropical zones. The environmental conditions largely varied: mean annual temperature (MAT) ranging from -5.4 to 21.5oC and mean annual precipitation (MAP) ranging from 300 to 2449 mm. The soil pH varied between 3.62 to 6.38. We incubated soil samples under an anaerobic condition with temperature from 5 to 35oC with an interval of 5oC for 12 or 24 hours, respectively. Soil temperature strongly affected the production of N2O and N2.&#160;N2O and N2 production rates showed a positive exponential relation with incubate time and temperature for all forest soils. Our results showed that the Q10 values ranged from 1.31 to 2.98 for N2O emission and 1.69 to 3.83 for N2 emission, indicating a generally positive feedback of N2O and N2 production to warming. Higher Q10 values for N2 than N2O implies that N2 emission is more sensitive to temperature increase. The N2O/(N2O+N2) decreased with increasing temperature in fifteen of nineteen forest soils, suggesting that warming accelerates N2 emission. Strong spatial variation in Q10 were also observed, with tropical forest soils exhibiting high Q10 values and relatively low Q10 in temperate forest soils. This variation is attributed to the inherent differences in N biogeochemical cycling behavior between the microbial communities among sites. Despite soil temperature primarily controls the N2O and N2 emissions,&#160;we &#160;explored the effects of other factors such as pH, C/N, DOC and related functional genes. In addition, we partitioned N2O and N2 emissions to different microbial processes (e.g., denitrification, co-denitrification and anammox).&#160;The results indicated that denitrification was the main pathway of N2O and N2 production under anaerobic environment and the contribution increased as temperature rise.Key words: Temperature sensitivity, N2O, N2, Forest soil, Nitrogen cycle, Global warming, Denitrification

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Effect of forest soil warming on the rate and temperature sensitivity of microbial C and N processes in a temperate mountain forest

10.5194/egusphere-egu2020-11631 ◽

2020 ◽

Author(s):

Carolina Urbina Malo ◽

Ye Tian ◽

Chupei Shi ◽

Shasha Zhang ◽

Marilena Heitger ◽

...

Keyword(s):

Forest Soil ◽

Temperature Sensitivity ◽

Enzyme Activities ◽

Thermal Acclimation ◽

Soil Warming ◽

Soil C ◽

Soil Microbial ◽

C And N ◽

N Acquisition

Despite the intensified efforts to understand the impacts of climate change on forest soil C dynamics, few studies have addressed the long term effects of warming on microbially mediated soil C and nutrient processes. In the few long-term soil warming experiments the initial stimulation of soil C cycling diminished with time, due to thermal acclimation of the microbial community or due to depletion of labile soil C as the major substrate for heterotrophic soil microbes. Thermal acclimation can arise as a consequence of prolonged warming and is defined as the direct organism response to elevated temperature across annual to decadal time-scales which manifest as a physiological change of the soil microbial community. This mechanism is clearly different from apparent thermal acclimation, where the attenuated response of soil microbial processes to warming is due to the exhaustion of the labile soil C pool.The Achenkirch experiment, situated in the Northern Limestone Alps, Austria (47&#176;34&#8217; 50&#8217;&#8217; N; 11&#176;38&#8217; 21&#8217;&#8217; E; 910 m a.s.l.) is a long term (>15 yrs) soil warming experiment that has provided key insights into the effects of global warming on the forest soil C cycle. At the Achenkirch site, we have observed a sustained positive response of heterotrophic soil respiration and of soil CO2 efflux to warming after nine years (2013), making it an appropriate setting for testing hypotheses about continued or decreasing warming effects at decadal scales. We collected soil from six warmed and six control plots in October 2019, from 0-10 cm and 10-20 cm depth, and incubated them at three different temperatures: ambient, +4, and +10 &#176;C. We measured potential soil enzyme activities with fluorimetric assays, gross rates of protein depolymerization, N mineralization, and nitrification with 15N isotope pool dilution approaches, and microbial growth, respiration, and C use efficiency (CUE) based on the 18O incorporation in DNA and gas analysis.&#160; Our preliminary results show that potential enzyme activities of aminopeptidase, N-acetylglucosaminidase, b-glucosidase, and acid phosphatase were stimulated by decadal soil warming by 1.7- to 3.5-fold, measured at the same i.e. ambient temperature. In contrast, the temperature sensitivity (Q10) remained unaltered between warmed and control soils for all enzyme activities (Q10=1.63-2.28), except for aminopeptidase where we observed a decrease in Q10 by 25% in warmed topsoils (0-10 cm). Aminopeptidase also had the highest temperature-sensitivity (Q10=2.39), causing a decrease of the enzymatic C: N acquisition ratio with warming. These results indicate an increasing investment in microbial N acquisition with warming. We will follow these trends based on results on gross rates of soil C and N processes, allowing to delineate decadal soil warming effects on soil microbial biogeochemistry and to understand their effect on the cross-talk between organic C and N cycling in calcareous forest soils.

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