Effect of plant communities on aggregate composition and organic matter stabilisation in young soils

© 2014, Springer International Publishing Switzerland. Results: Deciduous forest soil accumulated the highest C content in the 0–5 cm layer (43 g C kg−1), whereas values in coniferous forest and arable soils were lower (30 and 12 g C kg−1, respectively). The highest portion of C in arable soil was accumulated in the mineral fraction (80 %), whereas 50–60 % of the C in forest soils were in POM. More C was associated with minerals in deciduous forest soil (16 g C kg−1 soil) than under coniferous forest and arable land (8–10 g C kg−1 soil). Conclusions: Particulate organic matter explains most of the differences in organic C accumulation in soils developed during 45 years under the three vegetation types on identical parent material. The C content of the mineral soil fraction was controlled by plant cover and contributed the most to differences in C accumulation in soils developed under similar vegetation type (forest). Objectives: Carbon (C) content in pools of very young soils that developed during 45 years from loess was analysed in relation to vegetation: deciduous and coniferous forests and cropland. We hypothesised that variations in the amount of particulate organic matter (POM) can explain the C accumulation and also affects the C bound to mineral surfaces in soil under various vegetation. Methods: Soil samples were collected under three vegetation types of a 45-year-old experiment focused on initial soil development. Aggregate and density fractionations were combined to analyse C accumulation in large and small macro- and microaggregates as well as in free and occluded POM and mineral factions

Depth rather than microrelief controls microbial biomass and kinetics of C-, N-, P- and S-cycle enzymes in peatland

© 2018 Elsevier B.V. The formation of microrelief forms in peatlands - elevated and dry hummocks, depressed wet hollows and intermediate lawns - is controlled by the interaction of water table, nutrient availability and dominant plant communities. This affects the composition and activity of various functional groups of microorganisms. With depth, the change in peat quality from less to more highly processed organic material additionally regulates microbial activity. We hypothesized that microbial biomass and enzyme activities are driven by aeration and by peat quality and therefore (i) they increase from hollows (water saturated/anaerobic) through lawns (intermediate) to hummocks (aerobic) in the top peat and ii) they decrease with depth due to increasing distance from fresh plant-derived inputs and lower oxygen availability. These hypotheses were tested for enzymes catalysing the decomposition of C-, N-, P- and S-containing organic compounds in peat of the three microform types at three depths (15, 50 and 200 cm). Microbial biomass and peat chemical characteristics were compared with enzyme kinetic parameters, i.e. maximal potential activity (Vmax) and the Michaelis constant (Km). Microbial biomass carbon (MBC) and Vmax of β-glucosidase and N-acetyl glucosaminidase increased by 30–70% from hummocks and lawns to hollows in the top 15 cm, contradicting the hypothesis. Similarly, Km and the catalytic efficiency of enzymes (Ka = Vmax/Km) were best related to MBC distribution and not to the aeration gradient. With depth, Vmax of β-glucosidase, xylosidase and leucine aminopeptidase followed the hypothesized pattern in hollows. In contrast, MBC was 1.3–4 times higher at 50 cm, followed by successively lower contents at 15 and 200 cm in all microforms. The same depth pattern characterized the Vmax distribution of 6 out of 8 enzymes. Phosphatase activity decreased from drier hummock to wetter hollows and the higher activity throughout the peat profile suggested a high microbial demand for P. Enzyme activities and catalytic efficiency in peat were closely linked to the distribution of microbial biomass with depth, which in turn was best explained by P content. From the ecological perspective, these results clearly show that peat decomposition will be accelerated when microbial activity is stimulated e.g. by increased P availability

Weaker priming and mineralisation of low molecular weight organic substances in paddy than in upland soil

© 2017 Elsevier Masson SAS Although soil organic matter (SOM) and microbial biomass pools in flooded paddy soils are generally larger than they are in upland soils, the processes (i.e., slower mineralisation, other types of C stabilization, and a negative priming effect) underlying higher SOM stocks in paddy soil are unclear. To elucidate these processes, three 13 C labelled low molecular weight organic substances ( 13 C-LMWOS) (i.e., glucose, acetic acid, and oxalic acid) were incubated in upland and paddy soils under simulated field conditions. Within 30 days of incubation, acetic acid exhibited the highest mineralisation in both soils. The amount of mineralisation of glucose in upland soil was higher than that of oxalic acid (p < 0.05), whereas the opposite was observed for paddy soil. Mineralisation of all three LMWOS was lower in paddy soil than that in upland soil (p < 0.05), illustrating that the molecular structure of the LMWOS as well as soil management determined the mineralisation rate. The priming effect evoked by oxalic acid and glucose was lower in paddy than in upland soil (p < 0.05). Therefore, the generally weaker mineralisation and priming effect of LMWOS observed in paddy soil contributed to higher carbon accumulation than they did in upland soil. Priming effect was positively correlated with fungal abundance, which was lower in paddy soil than in upland soil. Thus, slow organic C turnover in paddy soil is partly attributed to the suppression of fungal activity by flooding

Vegetation Type Dominates the Spatial Variability in CH<inf>4</inf> Emissions Across Multiple Arctic Tundra Landscapes

Methane (CH4) emissions from Arctic tundra are an important feedback to global climate. Currently, modelling and predicting CH4 fluxes at broader scales are limited by the challenge of upscaling plot-scale measurements in spatially heterogeneous landscapes, and by uncertainties regarding key controls of CH4 emissions. In this study, CH4 and CO2 fluxes were measured together with a range of environmental variables and detailed vegetation analysis at four sites spanning 300 km latitude from Barrow to Ivotuk (Alaska). We used multiple regression modelling to identify drivers of CH4 flux, and to examine relationships between gross primary productivity (GPP), dissolved organic carbon (DOC) and CH4 fluxes. We found that a highly simplified vegetation classification consisting of just three vegetation types (wet sedge, tussock sedge and other) explained 54% of the variation in CH4 fluxes across the entire transect, performing almost as well as a more complex model including water table, sedge height and soil moisture (explaining 58% of the variation in CH4 fluxes). Substantial CH4 emissions were recorded from tussock sedges in locations even when the water table was lower than 40 cm below the surface, demonstrating the importance of plant-mediated transport. We also found no relationship between instantaneous GPP and CH4 fluxes, suggesting that models should be cautious in assuming a direct relationship between primary production and CH4 emissions. Our findings demonstrate the importance of vegetation as an integrator of processes controlling CH4 emissions in Arctic ecosystems, and provide a simplified framework for upscaling plot scale CH4 flux measurements from Arctic ecosystems

Effect of plant communities on aggregate composition and organic matter stabilisation in young soils

© 2014, Springer International Publishing Switzerland. Results: Deciduous forest soil accumulated the highest C content in the 0–5 cm layer (43 g C kg−1), whereas values in coniferous forest and arable soils were lower (30 and 12 g C kg−1, respectively). The highest portion of C in arable soil was accumulated in the mineral fraction (80 %), whereas 50–60 % of the C in forest soils were in POM. More C was associated with minerals in deciduous forest soil (16 g C kg−1 soil) than under coniferous forest and arable land (8–10 g C kg−1 soil). Conclusions: Particulate organic matter explains most of the differences in organic C accumulation in soils developed during 45 years under the three vegetation types on identical parent material. The C content of the mineral soil fraction was controlled by plant cover and contributed the most to differences in C accumulation in soils developed under similar vegetation type (forest). Objectives: Carbon (C) content in pools of very young soils that developed during 45 years from loess was analysed in relation to vegetation: deciduous and coniferous forests and cropland. We hypothesised that variations in the amount of particulate organic matter (POM) can explain the C accumulation and also affects the C bound to mineral surfaces in soil under various vegetation. Methods: Soil samples were collected under three vegetation types of a 45-year-old experiment focused on initial soil development. Aggregate and density fractionations were combined to analyse C accumulation in large and small macro- and microaggregates as well as in free and occluded POM and mineral factions

Review and synthesis of the effects of elevated atmospheric CO<inf>2</inf> on soil processes: No changes in pools, but increased fluxes and accelerated cycles

© 2018 Elsevier Ltd 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

Source determination of lipids in bulk soil and soil density fractions after four years of wheat cropping

Preservation of soil organic matter (SOM) is strongly affected by occlusion within aggregates and by association of SOM with minerals. Protection of organic carbon (C) due to adsorption to mineral surfaces can be assessed by investigation of SOM in soil density fractions. Apart from the physical properties the preservation of SOM is affected by its chemical composition. While for bulk organic C this was demonstrated for numerous soils, SOM density fractions have been scarcely studied regarding their molecular composition. Lipids as a compound class that can derive from plants, microorganisms and contamination by products from incomplete combustion or fossil carbon were not investigated in density fractions so far. We hypothesized that molecular proxies deriving from lipid composition yield a large potential to elucidate the sources of organic matter entering soil, and in combination with density fractions they enable the identification of incorporation and preservation pathways of SOM. We determined distribution patterns of aliphatic hydrocarbons and fatty acids as two representative groups for total lipids in soil density fractions. The fatty acids showed a predominant input of plant-derived poly unsaturated short chain and saturated long chain fatty acids in free particulate organic matter (fPOM). The microorganism-derived compounds such as unsaturated short chain fatty acids were largely abundant in fPOM and especially in occluded particulate organic matter (oPOM 1.6). The proportion of plant-derived components like long chain fatty acids increased with increasing density of the fractions, whereas the abundance of short chain fatty acids decreased in the same direction as indicated by the ratio of long chain vs. short chain fatty acids. The main portion of soil lipids (60% of total lipids) was recovered in the mineral (Min) fraction, which denotes the strongest protection of lipids adsorbed to mineral surfaces. For the aliphatic hydrocarbons the contribution of plant- and microorganism-derived components was the largest in fPOM. Short chain alkanes as part of the aliphatic hydrocarbons showed contamination of soil by an incompletely burned plant biomass or fossil carbon. These contaminants were the most abundant in fPOM and subsequently attributed to particles with a low density, which derived probably from soot. However, a large contribution of fossil C was found in the Min fraction as well, which is thought to be attributed to degraded soot particles being adsorbed to minerals. We demonstrated at the molecular level that the incorporation of individual C sources varies with the density fractions. It was found that microorganism-derived compounds were most abundant in fPOM and oPOM 1.6 fractions, whereas plant-derived long chain biopolymers were enriched in mineral dominated fractions (oPOM 2.0 and especially Min). Thus, preservation of plant-derived lipids in soil is strongly attributed to the association with minerals. Based on lipid composition in density fractions we evaluated several molecular proxies, which help to elucidate the sources of SOM

Effect of plant communities on aggregate composition and organic matter stabilisation in young soils

© 2014, Springer International Publishing Switzerland. Results: Deciduous forest soil accumulated the highest C content in the 0–5 cm layer (43 g C kg−1), whereas values in coniferous forest and arable soils were lower (30 and 12 g C kg−1, respectively). The highest portion of C in arable soil was accumulated in the mineral fraction (80 %), whereas 50–60 % of the C in forest soils were in POM. More C was associated with minerals in deciduous forest soil (16 g C kg−1 soil) than under coniferous forest and arable land (8–10 g C kg−1 soil). Conclusions: Particulate organic matter explains most of the differences in organic C accumulation in soils developed during 45 years under the three vegetation types on identical parent material. The C content of the mineral soil fraction was controlled by plant cover and contributed the most to differences in C accumulation in soils developed under similar vegetation type (forest). Objectives: Carbon (C) content in pools of very young soils that developed during 45 years from loess was analysed in relation to vegetation: deciduous and coniferous forests and cropland. We hypothesised that variations in the amount of particulate organic matter (POM) can explain the C accumulation and also affects the C bound to mineral surfaces in soil under various vegetation. Methods: Soil samples were collected under three vegetation types of a 45-year-old experiment focused on initial soil development. Aggregate and density fractionations were combined to analyse C accumulation in large and small macro- and microaggregates as well as in free and occluded POM and mineral factions

Oxygen matters: Short- and medium-term effects of aeration on hydrolytic enzymes in a paddy soil

Rapid exposure of anoxic microbial communities to oxygen (O2) can have unpredictable effects, including strong suppression of their enzymatic activity. Nonetheless, most medium- and long-term incubation studies on soil organic matter transformations fail to consider aeration effects during sample post-processing and/or assays. Moreover, it remains unclear whether anoxic enzymatic systems are adapted to quick switch to oxic conditions. We evaluated the effects of short-term (2-h oxic (+O2) vs. anoxic (–O2) assays) and medium-term aeration (after 10-day oxic vs. anoxic pre-incubation) on the kinetic parameters (Vmax, Km) of phosphomonoesterase, β-glucosidase, and leucine aminopeptidase in top bulk, rooted, and bottom bulk paddy soil of flooded rice mesocosms. We hypothesized contrasting short- and medium-term responses of hydrolytic enzyme activities to aeration (i) a negative short-term effect caused by reactive O2 species toxicity and/or other mechanisms, and (ii) adaptation of anoxic microbial communities to medium-term aeration reducing the impact of ongoing O2 exposure. Overall, 2-h aeration suppressed Vmax values by 7–43% and catalytic efficiency Ka (Vmax/Km) by 3–22%, and extended the substrate turnover time Tt (7–33%) of three tested enzymes in all soil compartments pre-incubated without O2. In contrast, no short-term suppressive effect of O2 was observed on three tested enzymes after oxic pre-incubation. Medium-term aeration increased Vmax (by 12–253%) and Ka (by 3–78%) of the enzymes and shortened Tt (4–42%) as compared to the anoxic counterpart. These findings support our hypothesis about anoxic microbial community adaptation over the medium-term aeration. Accordingly, the sensitivity of anoxic hydrolytic enzymes to a short-term O2 exposure and the O2 adaptation mechanisms require strong consideration (i) for enzyme assays of anoxic soils and (ii) for understanding the soil organic matter dynamics in environments with O2 fluctuations. © 2021 Elsevier B.V

To shake or not to shake: Silicone tube approach for incubation studies on CH<inf>4</inf> oxidation in submerged soils

© 2018 Elsevier B.V. Incubation experiments are the most common approach to measure methane (CH4) oxidation potential in soils from various ecosystems and land-use practices. However, the commonly used headspace CH4 injection into microcosms and the shaking of the soil slurry during incubation fully removes CH4 (soil-born) and O2 (air-born) gradients common in situ, and may also induce various errors and disturbances. As an alternative, we propose CH4 input into microcosm soils via a silicone tube located within the slurry. We hypothesized that (i) poor CH4 diffusion in slurry will be compensated by direct CH4 delivery into the slurry via a silicone tube and, consequently, (ii) shaking of microcosms can be substituted with the soil silicone tube CH4 injection. During a 29-day submerged paddy soil incubation, the highest net CH4 oxidation rate was 1.6 μg C g−1 dry soil h−1, measured between the 3rd and 7th day after injecting 13CH4 into the slurry via a silicone tube without shaking. This rate was 1.5–2.5 times faster than the respective CH4 oxidation after headspace injection without shaking (1st hypothesis supported). As expected, shaking accelerated CH4 oxidation regardless of injection methods by 3.2–3.7 times (most intensively on days 3–7) compared to headspace injection without shaking. Nonetheless, the rates were similar between silicone tube injection without shaking and headspace injection with shaking. This supports the hypothesized potential of silicone tubes to substitute the common shaking method (2nd hypothesis). Furthermore, shaking increased the incorporation of 13C from CH4 into soil organic matter and microbial biomass by 1.8–2.7 times compared with CH4 injection into tubes and the static control without tubes. This reflects an overestimation of CH4 oxidation due to shaking. We conclude that direct soil CH4 injection via silicone tubes is advantageous in incubation experiments because gas concentration gradients are maintained and thereby more realistically reflect natural soil conditions