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  • Meyer, N.; Welp, G.; Amelung, W. (2019)
    Knowledge about spatial patterns of soil respiration (SR) and its temperature sensitivity (Q10) is of emerging relevance for assessing carbon fluxes across the landscape. Related experiments are often conducted under controlled laboratory conditions and usually rely on soil samples, which are sieved and stored. Here, we investigated the effect of sieving and storage on SR and Q10. We took 14 samples from different land use types and soil textures. Samples were sieved to 2 mm at field-moist conditions and split into four treatments: sieved/no-storage, sieved/freeze-storage (−18  °C), sieved/cold-storage (+ 4 °C), and sieved/dry-storage (+ 40 °C). The storage time was 7 weeks. Intact soil cores were used as a control. The SR was not significantly affected by sieving/no-storage, sieving/freeze-storage, and sieving/cold-storage compared with the control. Yet, sieving/dry-storage significantly increased SR but all samples were similarly affected (r = 0.81 for the correlation between SR after sieving/dry-storage and SR in the control). The Q10 of sieving/no-storage (1.94 ± 0.28), sieving/freeze-storage (1.94 ± 0.23), sieving/cold-storage (2.37 ± 0.29), and sieving/dry-storage (2.29 ± 1.35) did not differ significantly from the control (2.12 ± 0.23). All samples responded similar to sieving and storage (r = 0.68–0.73 for the correlation between Q10 in each respective treatment and Q10 in the control), with the exception of sieved/dry-storage (r = 0.09). We conclude that sieving at field-moist conditions and subsequent freeze- or cold-storage is acceptable to derive SR and Q10 for the here reported storage time. Although dry-storage may be acceptable for the comparison of SR between samples, it should be avoided for realistic estimates of SR and for the determination of Q10.
  • Group Author (2019)
    River ecosystems receive and process vast quantities of terrestrial organic carbon, the fate of which depends strongly on microbial activity. Variation in and controls of processing rates, however, are poorly characterized at the global scale. In response, we used a peer-sourced research network and a highly standardized carbon processing assay to conduct a global-scale field experiment in greater than 1000 river and riparian sites. We found that Earth's biomes have distinct carbon processing signatures. Slow processing is evident across latitudes, whereas rapid rates are restricted to lower latitudes. Both the mean rate and variability decline with latitude, suggesting temperature constraints toward the poles and greater roles for other environmental drivers (e.g., nutrient loading) toward the equator. These results and data set the stage for unprecedented "next-generation biomonitoring" by establishing baselines to help quantify environmental impacts to the functioning of ecosystems at a global scale.
  • Ribeiro-Kumara, Christine; Köster, Egle; Aaltonen, Heidi; Köster, Kajar (2020)
    Wildfires strongly regulate carbon (C) cycling and storage in boreal forests and account for almost 10% of global fire C emissions. However, the anticipated effects of climate change on fire regimes may destabilize current C-climate feedbacks and switch the systems to new stability domains. Since most of these forests are located in upland soils where permafrost is widespread, the expected climate warming and drying combined with more active fires may alter the greenhouse gas (GHG) budgets of boreal forests and trigger unprecedented changes in the global C balance. Therefore, a better understanding of the effects of fires on the various spatial and temporal patterns of GHG fluxes of different physical environments (permafrost and nonpermafrost soils) is fundamental to an understanding of the role played by fire in future climate feedbacks. While large amounts of C are released during fires, postfire GHG fluxes play an important role in boreal C budgets over the short and long term. The timescale over which the vegetation cover regenerates seems to drive the recovery of C emissions after both low- and high-severity fires, regardless of fire-induced changes in soil decomposition. In soils underlain by permafrost, fires increase the active layer depth for several years, which may alter the soil dynamics regulating soil GHG exchange. In a scenario of global warming, prolonged exposition of previously immobilized C could result in higher carbon dioxide emission during the early fire succession. However, without knowledge of the contribution of each respiration component combined with assessment of the warming and drying effects on both labile and recalcitrant soil organic matter throughout the soil profile, we cannot advance on the most relevant feedbacks involving fire and permafrost. Fires seem to have either negligible effects on methane (CH4) fluxes or a slight increase in CH4 uptake. However, permafrost thawing driven by climate or fire could turn upland boreal soils into temporary CH4 sources, depending on how fast the transition from moist to drier soils occurs. Most studies indicate a slight decrease or no significant change in postfire nitrous oxide (N2O) fluxes. However, simulations have shown that the temperature sensitivity of denitrification exceeds that of soil respiration; thus, the effects of warming on soil N2O emissions may be greater than on C emissions.
  • Ryhti, Kira; Kulmala, Liisa; Pumpanen, Jukka; Isotalo, Jarkko; Pihlatie, Mari; Helmisaari, Heljä-Sisko; Leppälammi-Kujansuu, Jaana; Kieloaho, Antti-Jussi; Bäck, Jaana; Heinonsalo, Jussi (2021)
    Changes in the climate may have unpredictable effects on belowground carbon processes and thus, the carbon balance of boreal forests. To understand the interactions of these processes in soil and to quantify the potential changes in the carbon cycle, partitioning of forest floor respiration is crucial. For this purpose, we used nine different treatments to separate the sources of forest floor carbon dioxide (CO2) emissions in a mature Scots pine (Pinus sylvestris L.) stand in southern Finland. To partition the belowground CO2 emissions, we used two different trenching methods: 1) to exclude roots and mycorrhizal fungal mycelia (mesh with 1-mu m pores) and 2) to exclude roots, but not mycorrhizal hyphae (mesh with 50-mu m pores). Additionally, we used 3) a control treatment that included roots and fungal hyphae. To partition the CO2 emissions from the forest floor vegetation, we 1) removed it, 2) left only the dwarf shrubs, or 3) left the vegetation intact. The forest floor CO2 emissions were regularly measured with a flux chamber throughout the growing seasons in 2013-2015. The total forest floor respiration was partitioned into respiration of tree roots (contributing 48%), heterotrophic soil respiration (30%) and respiration of ground vegetation other than shrubs (10%), dwarf shrubs (8%), and hyphae of mycorrhizal fungi (4%). Heterotrophic respiration increased in the trenched treatments without ground vegetation over time, due to the so-called 'Gadgil effect'. In the absence of tree mots, but when hyphal access was allowed, respiration in the dwarf shrub treatment increased throughout the experiment. This indicated that dwarf shrubs had fungal connections to outside the experimental plots via their ericoid mycorrhiza. At the same time, other ground vegetation, such as mosses, suppressed the dwarf shrub respiration in trenched treatments. Our results show that competition on the forest floor is intense between plant roots and soil microbes.
  • Meyer, Nele; Silfver, Tarja; Karhu, Kristiina; Myller, Kristiina; Sietiö, Outi-Maaria; Myrsky, Eero; Oksanen, Elina; Rousi, Matti; Mikola, Juha (2021)
    Warming will likely stimulate Arctic primary production, but also soil C and N mineralization, and it remains uncertain whether the Arctic will become a sink or a source for CO2. Increasing insect herbivory may also dampen the positive response of plant production and soil C input to warming. We conducted an open-air warming experiment with Subarctic field layer vegetation in North Finland to explore the effects of warming (+3 degrees C) and reduced insect herbivory (67% reduction in leaf damage using an insecticide) on soil C and N dynamics. We found that plant root growth, soil C and N concentrations, microbial biomass C, microbial activity, and soil NH4+ availability were increased by both warming and reduced herbivory when applied alone, but not when combined. Soil NO3- availability increased by warming only and in-situ soil respiration by reduced herbivory only. Our results suggest that increasing C input from vegetation under climate warming increases soil C concentration, but also stimulates soil C turnover. On the other hand, it appears that insect herbivores can significantly reduce plant growth. If their abundance increases with warming as predicted, they may curtail the positive effect of warming on soil C concentration. Moreover, our results suggest that temperature and herbivory effects on root growth and soil variables interact strongly, which probably arises from a combination of N demand increasing under lower herbivory and soil mineral N supply increasing under higher temperature. This may further complicate the effects of rising temperatures on Subarctic soil C dynamics.