Prediction of the vertical scaling of soil organic carbon in temperate forest soils using percolation theory

Forest soil stores a large portion of soil organic carbon (SOC), making it one of the essential components of global carbon cycling. There is apparent spatial variability of SOC in forest soils, but the mechanism that regulates the vertical pattern of SOC is still not clear. Understanding the vertical distribution as well as the transport process of SOC can be of importance in developing comprehensive SOC models in forest soils, as well as in better estimating terrestrial carbon cycling. We propose a theoretical scaling derived from percolation theory to predict the vertical scaling of SOC with soil depth in temperate forest soils, with the hypothesis that the content of SOC along soil profile is limited by the transport of solute. The powers of the vertical scaling of 5 published datasets across different regions of the world are −0.920, −1.097, −1.196, −1.062, and −1.038, comparing with the theoretical value of −1.149. Field data from Changbai Mountain region, Jilin, China, with spatial variation of SOC correlating strongly to temperature, precipitation, and sampling slope is constrained well by theoretical boundaries predicted from percolation theory, indicating that the vertical transport so as the content of SOC along soil profile is limited by solute transport, which can be described by percolation theory in both small and large scales. Prediction of SOC content in Changbai Mountain region based on an estimated SOC content at 0.15 m from available data demonstrates a good agreement with field observation, suggesting the potential of collaborating the presented model with other surface soil models to predict SOC storage and carbon cycling in temperate forest soils.

The fate of calcium in temperate forest soils: a Ca K-edge XANES study

Calcium (Ca) plays a crucial role for plant nutrition, soil aggregation, and soil organic matter (SOM) stabilization. Turnover and ecological functions of Ca in soils depend on soil Ca speciation. For the first time, we used synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy at the Ca K-edge (4038 eV) to investigate Ca speciation in soils. We present Ca K-edge XANES spectra of standard compounds with relevance in soils (e.g. calcite, dolomite, hydroxyapatite, anorthite, clay mineral-adsorbed Ca; Ca oxalate, formate, acetate, citrate, pectate, phytate). Calcium XANES spectra with good signal-to-noise ratios were acquired in fluorescence mode for Ca concentrations between 1 and 10 mg g−1. Most standard spectra differed markedly among each other, allowing the identification of different Ca species in soils and other environmental samples as well as Ca speciation by linear combination fitting. Calcium XANES spectra obtained for samples from different horizons of twelve temperate forest soils revealed a change from dominating lithogenic Ca to clay mineral-bound and/or organically bound Ca with advancing pedogenesis. O layer Ca was almost exclusively organically bound. With increasing SOM decomposition, shares of oxalate-bound Ca decreased. Oxalate-bound Ca was absent in calcareous, but not in silicate subsoil horizons, which can be explained by microbial decomposition in the former vs. stabilization by association to pedogenic minerals in the latter soils. Synchrotron-based Ca XANES spectroscopy is a promising novel tool to investigate the fate of Ca during pedogenesis and—when performed with high spatial resolution (µ-XANES), to study aggregation and SOM stabilization mechanisms produced by Ca.

Significance and driving forces of dark CO2 fixation for organic carbon inputs in temperate forest soils

<p>Soils are the largest terrestrial organic carbon pool and one of the largest terrestrial sources of CO<sub>2</sub> in the atmosphere. However, not all CO<sub>2</sub> produced in soils is released into the atmosphere, as dark CO<sub>2</sub> fixation has been shown to modulate CO<sub>2</sub> release from soils. Temperate forest soils store up to half of the soil organic carbon pool to 1m depth and are recognized as important components of the global carbon cycle, yet studies on dark CO<sub>2</sub> fixation in temperate forest soils are scarce. Using a well characterized Cambisol soil plot in the Hainich National Park (temperate forest), Germany, we explore dark CO<sub>2</sub> fixation with the aim to assess the CO<sub>2</sub> fixation rates, the influencing biogeochemical parameters, and the contribution of this process to temperate forest soil organic carbon (SOC).</p><p>Dark CO<sub>2</sub> fixation was quantified via the uptake of <sup>13</sup>C-CO<sub>2</sub> added to microcosms containing soils sampled from three depths. Under 2% CO<sub>2</sub> headspace, rates of dark CO<sub>2</sub> fixation at soil level decreased with depth from 0.86 µg C gdw<sup>-1</sup>d<sup>-1</sup> in 0 - 12 cm to 0.05 µg C gdw<sup>-1</sup>d<sup>-1</sup> in 70 -100 cm, accounting for up to 1.1% of microbial biomass and up to 0.035% of soil organic carbon. However, as differences in microbial biomass abundance and community profiles with depth were found, no significant difference in the rates across depth was observed at microbial level. This suggests that microbial biomass is an important driver of dark CO<sub>2 </sub>fixation in soils. Given a global temperate forest area of 6.9 million km<sup>2</sup> and an average soil bulk density of 1 Mg/m<sup>3 </sup>dark CO<sub>2</sub> fixation will potentially account for the gross sequestration of 0.31 - 0.48 GtC/yr to a depth of 1 m. Furthermore, an increase in headspace CO<sub>2</sub> concentration enhanced CO<sub>2</sub> fixation rates by up to 3.4-fold under 20% v:v CO<sub>2</sub> showing that dark CO<sub>2</sub> fixation can be substantial in soils with higher CO<sub>2</sub> concentrations.</p><p>To validate microbial biomass as a driver of dark CO<sub>2</sub> fixation in soils, we made comparisons with soil plots from the Schorfheide-Chorin exploratory forest, Germany, a temperate forest characterized by vegetation-specific bacterial community structure, higher sand content and acidic pH gradients. Under these conditions, CO<sub>2</sub> fixation rates at microbial level were significantly different across depth suggesting that aside microbial biomass, other abiotic factors may influence dark CO<sub>2</sub> fixation in these soils. Of all the tested abiotic variables, water content was the main explanatory factor for the variations in dark CO<sub>2</sub> fixation rates in the Schorfheide-chorin soils. Additionally, based on 16S rRNA sequencing, qPCR and PICRUSt2 analysis, only a few putative autotrophic communities were present and displayed vegetation-specific variations indicating an influence of vegetation type and input on the active community.</p><p>Our findings highlight microbial biomass, CO<sub>2</sub> and water content as the main drivers of dark CO<sub>2</sub> fixation in temperate forest soils with only a small proportion of autotrophs being present, suggesting the potential mediators of this process. We also demonstrate the significance of this process in global temperate forest SOC inputs.</p><p> </p><p> </p>

Dark microbial CO2 fixation in temperate forest soils increases with CO2 concentration

<p>Dark, that is, nonphototrophic, microbial CO<sub>2</sub> fixation occurs in a large range of soils.<br>However, it is still not known whether dark microbial CO<sub>2</sub> fixation substantially contributes<br>to the C balance of soils and what factors control this process. Therefore,<br>the objective of this study was to quantitate dark microbial CO<sub>2</sub> fixation in temperate<br>forest soils, to determine the relationship between the soil CO<sub>2</sub> concentration and<br>dark microbial CO<sub>2</sub> fixation, and to estimate the relative contribution of different<br>microbial groups to dark CO<sub>2</sub> fixation. For this purpose, we conducted a <sup>13</sup>C-CO<sub>2</sub> labeling<br>experiment. We found that the rates of dark microbial CO<sub>2</sub> fixation were positively<br>correlated with the CO<sub>2</sub> concentration in all soils. Dark microbial CO<sub>2</sub> fixation<br>amounted to up to 320 μg C kg<sup>−1</sup> soil day<sup>−1</sup> in the Ah horizon. The fixation rates were<br>2.8–8.9 times higher in the Ah horizon than in the Bw1 horizon. Although the rates of<br>dark microbial fixation were small compared to the respiration rate (1.2%–3.9% of the<br>respiration rate), our findings suggest that organic matter formed by microorganisms<br>from CO<sub>2</sub> contributes to the soil organic matter pool, especially given that microbial<br>detritus is more stable in soil than plant detritus. Phospholipid fatty acid analyses<br>indicated that CO<sub>2</sub> was mostly fixed by gram-positive bacteria, and not by fungi. In<br>conclusion, our study shows that the dark microbial CO<sub>2</sub> fixation rate in temperate<br>forest soils increases in periods of high CO<sub>2</sub> concentrations, that dark microbial CO<sub>2</sub><br>fixation is mostly accomplished by gram-positive bacteria, and that dark microbial<br>CO<sub>2</sub> fixation contributes to the formation of soil organic matter.</p><p>Reference</p><p>Spohn M, Müller K, Höschen C, Mueller CW, Marhan S. Dark microbial CO<sub>2</sub> fixation in temperate forest soils increases with CO<sub>2</sub> concentration.<br>Glob Change Biol. 2019;00:1–10. https ://doi.org/10.1111/gcb.14937</p>