Rhizosphere activity and atmospheric methane concentrations drive variations of methane fluxes in a temperate forest soil

Aerated soils represent an important sink for atmospheric methane (CH⁠4), due to the effect of methanotrophic bacteria, thus mitigating current atmospheric CH⁠4 increases. Whilst rates of CH⁠4 oxidation have been linked to types of vegetation cover, there has been no systematic investigation of the interaction between plants and soil in relation to the strength of the soil CH⁠4 sink. We used quasi-continuous automated chamber measurements of soil CH⁠4 and CO⁠2 flux from soil collar treatments that selectively include root and ectomycorrhizal (ECM) mycelium to investigate the role of rhizosphere activity as well as the effects of other environmental drivers on CH⁠4 uptake in a temperate coniferous forest soil. We also assessed the potential impact of measurement bias from sporadic chamber measurements in altering estimates of soil CO⁠2 efflux and CH⁠4 uptake. Results show a clear effect of the presence of live roots and ECM mycelium on soil CO⁠2 efflux and CH⁠4 uptake. The presence of ECM hyphae alone (without plant roots) showed intermediate fluxes of both CO⁠2 and CH⁠4 relative to soils that either contained roots and ECM mycelium, or soil lacking root- and ECM mycelium. Regression analysis confirmed a significant influence of soil moisture as well as temperature on flux dynamics of both CH⁠4 and CO⁠2 flux. We further found a surprising increase in soil CH⁠4 uptake during the night, and discuss diurnal fluctuations in atmospheric CH⁠4 (with higher concentrations during stable atmospheric conditions at night) as a potential driver of CH⁠4 oxidation rates. Using the high temporal resolution of our data set, we show that low-frequency sampling results in systematic bias of up-scaled flux estimates, resulting in under-estimates of up to 20% at our study site, due to fluctuations in flux dynamics on diurnal as well as longer time scales

Chars produced by slow pyrolysis and hydrothermal carbonization vary in carbon sequestration potential and greenhouse gases emissions

Bio-char, biomass that has been deliberately charred to slow its rate of decomposition, has been proposed as an amendment with the potential to sequester carbon and improve certain soil properties. Slow pyrolysis (temperature 500 C) and hydrothermal carbonization (low temperature, high pressure) are two efficient methods to produce bio-char with high yield and are applicable to a broad range of feedstocks. Chars made using slow pyrolysis (PC) and hydrothermal carbonization (HTC) of the same feedstock material (corn, C4) differed in physical appearance, chemical properties and decomposition behavior. We added these HTC and PC chars as amendments to three soils with C3-derived organic matter that differed in clay content, pH, and land use (managed spruce forest, unmanaged deciduous forest and agriculture), and compared their impacts on carbon sequestration and net greenhouse gas (CO2, 13CO2, N2O and CH4) emissions. HTC addition (1% w/w) significantly increased CO2 emissions in all three soils (p < 0.001), with much of the extra C derived from HTC decomposition. In contrast, PC addition (1% w/w) had almost no impact on deciduous forest soil and actually decreased CO2 emission from the agricultural soil. HTC treatment resulted in increased CH4 emission from all soils but reduced N2O fluxes in the agricultural and spruce forest soils. PC amendment had no significant effect on CH4 emission, and resulted in intermediate levels of N2O emission (between control and HTC treatments). Although both HTC and PC chars were produced from the same feedstock, PC had markedly higher potential for carbon sequestration than HTC

Model structure and parameter identification of soil organic matter models

Soil organic matter models with complex ecological mechanisms usually include a large number of parameters than simpler models that omit detailed processes. Finding parameter values for these complex models is challenging given the poor availability of comprehensive datasets that describe different processes. Depending on the type of data available, the estimation of parameters in complex models may lead to identifiability problems, i.e. obtaining different combinations of parameters that give equally good predictions in comparison with the observed data. In this manuscript, we explore the problem of identifiability in soil organic matter models, pointing out combinations of empirical data and model structure that can minimize identifiability issues. We found that only sets of up to 3 or 4 parameters may be uniquely identifiable, depending on the number of data constrains used for parameter identification. When only using data on soil respiration fluxes from soil incubations or mass loss from litter decay studies, up to 2 parameters can be uniquely identifiable independently on the model structure. For nonlinear microbial models, all parameters cannot be identified simultaneously with mass loss or respiration data, combined with additional constraints from isotopes. Parameter identifiability possess series challenges for proposing complex model structures in global soil carbon models given the limitation of comprehensive datasets at a global scale

Interactions among temperature, moisture, and oxygen concentrations in controlling decomposition rates in a boreal forest soil

Determining environmental controls on soil organic matter decomposition is of importance for developing models that predict the effects of environmental change on global soil carbon stocks. There is uncertainty about the environmental controls on decomposition rates at temperature and moisture extremes, particularly at high water content levels and high temperatures. It is uncertain whether observed declines of decomposition rates at high temperatures are due to declines in the heat capacity of extracellular enzymes as predicted by thermodynamic theory, or due to simultaneous declines in soil moisture. It is also uncertain whether oxygen limits decomposition rates at high water contents. Here we present results from a full factorial experiment using organic arctic soils incubated at high temperatures (25 and 35 degrees C), a wide range of water-filled pore space WFPS (15, 30, 60, 90 %), and contrasting oxygen concentrations (1 and 20 %). We found support for the hypothesis that decomposition rates increase at high temperatures provided enough moisture and oxygen is available for decomposition. Furthermore, we found that decomposition rate is mostly limited by oxygen concentrations at high moisture levels; even at 90 % WFPS, decomposition proceeded at high rates in the presence of oxygen. Our results suggest an important degree of interactions among temperature, moisture, and oxygen in determining decomposition rates at the soil-core scale

Chars produced by slow pyrolysis and hydrothermal carbonization vary in carbon sequestration potential and greenhouse gases emissions

Bio-char, biomass that has been deliberately charred to slow its rate of decomposition, has been proposed as an amendment with the potential to sequester carbon and improve certain soil properties. Slow pyrolysis (temperature ≤500°C) and hydrothermal carbonization (low temperature, high pressure) are two efficient methods to produce bio-char with high yield and are applicable to a broad range of feedstocks. Chars made using slow pyrolysis (PC) and hydrothermal carbonization (HTC) of the same feedstock material (corn, C4) differed in physical appearance, chemical properties and decomposition behavior. We added these HTC and PC chars as amendments to three soils with C3-derived organic matter that differed in clay content, pH, and land use (managed spruce forest, unmanaged deciduous forest and agriculture), and compared their impacts on carbon sequestration and net greenhouse gas (CO2, 13CO2, N2O and CH4) emissions. HTC addition (1% w/w) significantly increased CO2 emissions in all three soils (p&lt;0.001), with much of the extra C derived from HTC decomposition. In contrast, PC addition (1% w/w) had almost no impact on deciduous forest soil and actually decreased CO2 emission from the agricultural soil. HTC treatment resulted in increased CH4 emission from all soils but reduced N2O fluxes in the agricultural and spruce forest soils. PC amendment had no significant effect on CH4 emission, and resulted in intermediate levels of N2O emission (between control and HTC treatments). Although both HTC and PC chars were produced from the same feedstock, PC had markedly higher potential for carbon sequestration than HTC. © 2013 Elsevier Ltd

Soil methanotroph abundance and community composition are not influenced by substrate availability in laboratory incubations

Variations in the rates of atmospheric CH4 uptake in upland soils can arise from both abiotic and biotic factors. Among the less-studied biotic factors is the degree to which methanotroph activity and community composition interact with supply of CH4 to the soil. Here, we investigated whether the abundance of high affinity methanotrophs in a range of soils representing different land use types is substrate (CH4) dependent. Field replicates of three soils sampled from deciduous forest, spruce forest and agricultural sites were incubated in columns flushed continuously for 24 days with air at one of four CH4 concentrations: <1 ppm (starvation), 1.8 (ambient), 30 (low elevated) and 60 (high elevated) ppm. In all soils, CH4 oxidation rates increased linearly with CH4 supply. For all levels of CH4 supply, CH4 oxidation rates were the highest in deciduous forest soil followed by spruce forest and agricultural soils. Terminal restriction fragment length polymorphism (T-RFLP) analysis indicated that the agricultural soil had a distinct methanotrophic community compared to the two forest soils. In particular, the T-RFs (Terminal restriction fragments) associated with USCα and Type II methanotrophs (Methylocystis sp, Methylosinus sp.) were the most abundant in forest soils while Type 1a associated T-RFs dominated in agricultural soil. The agricultural and forest soils also differed in their fractionation of stable isotopes, 13C and 2H, during CH4 oxidation. Altering CH4 concentration in the inlet air did not change methanotroph abundance, as evidenced by three different assays, two qPCR and T-RFLP, that recorded no changes in the number of pmoA genes and/or the relative abundance of T-RFs. Altogether, it is proposed that intrinsic differences in CH4 oxidation rates between soils, particularly between temperate agricultural and forest soils, are driven by methanotroph community structure. The population size of methanotrophs in upland soils did not respond to CH4 availability and is most probably regulated by other factors, such as the availability of nitrogen, cross-feeding or other carbon sources