An Improved Framework for Estimating Organic Carbon Content of Mangrove Soils Using loss-on-ignition and Coastal Environmental Setting

The use of loss on ignition (LOI) measurements of soil organic matter (SOM) to estimate soil organic carbon (OC) content is a decades-old practice. While there are limitations and uncertainties to this approach, it continues to be necessary for many coastal wetlands researchers and conservation practitioners without access to an elemental analyzer. Multiple measurement, reporting, and verification (MRV) standards recognize the need (and uncertainty) for using this method. However, no framework exists to explain the substantial differences among equations that relate SOM to OC; consequently, equation selection can be a haphazard process leading to widely divergent and inaccurate estimates. To address this lack of clarity, we used a dataset of 1,246 soil samples from 17 mangrove regions in North, Central, and South America, and calculated SOM to OC conversion equations for six unique types of coastal environmental setting. A framework is provided for understanding differences and selecting an equation based on a study region's SOM content and whether mineral sediments are primarily terrigenous or carbonate in origin. This approach identifies the positive dependence of conversion equation slopes on regional mean SOM content and indicates a distinction between carbonate settings with mean (± 1 S.E.) OC:SOM of 0.47 (0.002) and terrigenous settings with mean OC:SOM of 0.32 (0.018). This framework, focusing on unique coastal environmental settings, is a reminder of the global variability in mangrove soil OC content and encourages continued investigation of broadscale factors that contribute to soil formation and change in blue carbon settings.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13157-023-01698-z.

Emergence of the Reactivity Continuum of Organic Matter from Kinetics of a Multitude of Individual Molecular Constituents

The reactivity continuum (RC) model is a powerful statistical approach for describing the apparent kinetics of bulk organic matter (OM) decomposition. Here, we used ultrahigh resolution mass spectrometry data to evaluate the main premise of the RC model, namely that there is a continuous spectrum of reactivity within bulk OM, where each individual reactive type undergoes exponential decay. We performed a 120 day OM decomposition experiment on lake water, with an untreated control and a treatment preexposed to UV light, and described the loss of bulk dissolved organic carbon with RC modeling. The behavior of individual molecular formulas was described by fitting the single exponential model to the change in peak intensities over time. The range of the empirically derived apparent exponential decay coefficients (k_exp) was indeed continuous. The character of the corresponding distribution, however, differed from the conceptual expectations, due to the effects of intrinsic averaging, overlaps in formula-specific loss and formation rates, and the limitation of the RC model to include apparently accumulating compounds in the analysis. Despite these limitations, both the RC model-simulated and empirical (mass spectrometry-derived) distributions of k_exp captured the effects of preexposure to UV light. Overall, we present experimental evidence that the reactivity continuum within bulk OM emerges from a range of reactivity of numerous individual components. This constitutes direct empirical support for the major assumption behind the RC model of the natural OM decomposition.

Study on SOC Forecast Model in Regions of Hilly Purple Soil by Water Erosion

The soil productive forces are mainly affected by soil erosion as soil nutrients and organic carbon are taken away. On the premise of analysis of the last 17 years’ data of rainfalls and vegetation coverage in Xiejiawan small watershed, the grey model was established to forecast and evaluate the losses of organic carbon caused by water erosion in the hilly area of purple soils. Moreover, according to comparison of four-dimensional, five-dimensional and six-dimensional model groups, we found that, six-dimensional GM (1,1) model was the best one, posterior-variance-test accuracy of which amounted to the first class, and maximum simulated relative error ,average simulated relative error and predicted relative error of which were respectively 43.26 % ,16.85% and -17.09%, besides, its simulation and prediction accuracy was also high. Furthermore, the accuracy of simulated forecast value was all satisfactory. So the SOC loss by water erosion in purple areas of Xiejiawan small watershed was simulated and predicted well by grey model, also the unique evolution patterns of changing organic carbon in purple soil by water erosion could be well described and explained by the grey model.

Thermophilic anaerobes in Arctic marine sediments induced to mineralize complex organic matter at high temperature

Marine sediments harbour diverse populations of dormant thermophilic bacterial spores that become active in sediment incubation experiments at much higher than in situ temperature. This response was investigated in the presence of natural complex organic matter in sediments of two Arctic fjords, as well as with the addition of freeze-dried Spirulina or individual high-molecular-weight polysaccharides. During 50 degrees C incubation experiments, Arctic thermophiles catalysed extensive mineralization of the organic matter via extracellular enzymatic hydrolysis, fermentation and sulfate reduction. This high temperature-induced food chain mirrors sediment microbial processes occurring at cold in situ temperatures (near 0 degrees C), yet it is catalysed by a completely different set of microorganisms. Using sulfate reduction rates (SRR) as a proxy for organic matter mineralization showed that differences in organic matter reactivity determined the extent of the thermophilic response. Fjord sediments with higher in situ SRR also supported higher SRR at 50 degrees C. Amendment with Spirulina significantly increased volatile fatty acids production and SRR relative to unamended sediment in 50 degrees C incubations. Spirulina amendment also revealed temporally distinct sulfate reduction phases, consistent with 16S rRNA clone library detection of multiple thermophilic Desulfotomaculum spp. enriched at 50 degrees C. Incubations with four different fluorescently labelled polysaccharides at 4 degrees C and 50 degrees C showed that the thermophilic population in Arctic sediments produce a different suite of polymer-hydrolysing enzymes than those used in situ by the cold-adapted microbial community. Over time, dormant marine microorganisms like these are buried in marine sediments and might eventually encounter warmer conditions that favour their activation. Distinct enzymatic capacities for organic polymer degradation could allow specific heterotrophic populations like these to play a role in sustaining microbial metabolism in the deep, warm, marine biosphere.