Mineralogical associations with soil carbon in managed wetland soils

Practical Guide to Measuring Wetland Carbon Pools and Fluxes

Wetlands cover a small portion of the world, but have disproportionate influence on global carbon (C) sequestration, carbon dioxide and methane emissions, and aquatic C fluxes. However, the underlying biogeochemical processes that affect wetland C pools and fluxes are complex and dynamic, making measurements of wetland C challenging. Over decades of research, many observational, experimental, and analytical approaches have been developed to understand and quantify pools and fluxes of wetland C. Sampling approaches range in their representation of wetland C from short to long timeframes and local to landscape spatial scales. This review summarizes common and cutting-edge methodological approaches for quantifying wetland C pools and fluxes. We first define each of the major C pools and fluxes and provide rationale for their importance to wetland C dynamics. For each approach, we clarify what component of wetland C is measured and its spatial and temporal representativeness and constraints. We describe practical considerations for each approach, such as where and when an approach is typically used, who can conduct the measurements (expertise, training requirements), and how approaches are conducted, including considerations on equipment complexity and costs. Finally, we review key covariates and ancillary measurements that enhance the interpretation of findings and facilitate model development. The protocols that we describe to measure soil, water, vegetation, and gases are also relevant for related disciplines such as ecology. Improved quality and consistency of data collection and reporting across studies will help reduce global uncertainties and develop management strategies to use wetlands as nature-based climate solutions.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13157-023-01722-2.

Organic carbon preservation in wetlands: Iron oxide protection vs. thermodynamic limitation

The sequestration of organic carbon (OC) in wetland sediments is influenced by the presence of oxygen or lack thereof. The mechanisms of OC sequestration under redox fluctuations, particularly by the co-mediation of reactive iron (Fe) protection and thermodynamic limitation by the energetics of the OC itself, remain unclear. Over the past 26 years, a combination of field surveys and remote sensing images had revealed a strong decline in both natural and constructed wetland areas in Tianjin. This decline could be attributed to anthropogenic landfill practices and agricultural reclamation efforts, which may have significant impacts on the oxidation-reduction conditions for sedimentary OC. The Fe-bound OC (CBD extraction) decreased by 2 to 10-fold (from 8.3 to 10% to 0.7-4.5%) with increasing sediment depth at three sites with varying water depths (WD). The high-resolution spectro-microscopy analysis demonstrated that Fe (oxyhydr)oxides were colocalized with sedimentary OC. Corresponding to lower redox potential, the nominal oxidation state of C (NOSC), which corresponds to the energy content in OC, became more negative (energy content increased) with increasing sediment depth. Taken together, the preservation of sedimentary OC is contingent on the prevailing redox conditions: In environments where oxygen availability is high, reactive Fe provides protection for OC, while in anoxic environments, thermodynamic constraints (i.e., energetic constraints) limit the oxidation of OC.

Carbon-sink potential of continuous alfalfa agriculture lowered by short-term nitrous oxide emission events

Alfalfa is the most widely grown forage crop worldwide and is thought to be a significant carbon sink due to high productivity, extensive root systems, and nitrogen-fixation. However, these conditions may increase nitrous oxide (N₂O) emissions thus lowering the climate change mitigation potential. We used a suite of long-term automated instrumentation and satellite imagery to quantify patterns and drivers of greenhouse gas fluxes in a continuous alfalfa agroecosystem in California. We show that this continuous alfalfa system was a large N₂O source (624 ± 28 mg N₂O m² y^-1), offsetting the ecosystem carbon (carbon dioxide (CO₂) and methane (CH₄)) sink by up to 14% annually. Short-term N₂O emissions events (i.e., hot moments) accounted for ≤1% of measurements but up to 57% of annual emissions. Seasonal and daily trends in rainfall and irrigation were the primary drivers of hot moments of N₂O emissions. Significant coherence between satellite-derived photosynthetic activity and N₂O fluxes suggested plant activity was an important driver of background emissions. Combined data show annual N₂O emissions can significantly lower the carbon-sink potential of continuous alfalfa agriculture.

Effects of exogenous Fe addition on soil respiration rate and dissolved organic carbon structure in temperate forest swamps of northeastern China

Fe as an important redox-active transition metal plays a key role in the carbon cycle of ecosystems. To date, the mechanisms by which Fe affects organic carbon (soil respiration rate [Rs] and dissolved organic carbon [DOC] structure) remain unclear, because most studies only focused on the effect of Fe on soil organic carbon content. To understand these effects, a 30-day laboratory incubation experiment was conducted using forest swamp soils from northeastern China amended with different concentrations of exogenous Fe (no exogenous Fe added [L0], add exogenous Fe at 1 time the soil background value [L1], add exogenous Fe at 2 times the soil background value [L2]). Our results showed that exogenous Fe addition reduced the soil respiration rate by 54.8% during the incubation time. The DOC concentration decreased by 40.5% with exogenous Fe addition during the incubation time. The dissolved organic matter (DOM) characteristic parameters showed apparent variations (p < 0.05), including significant increases in the fluorescence and biological index and significant decreases in the humification index, which indicate that exogenous Fe addition reduced humification, which may lead to the increased fixation of dissolved organic carbon. In addition significant increases in tryptophan-like DOM was observed when exogenous Fe addition resulted in a soil Fe concentration of twice the background value (p < 0.05). These findings indicate that exogenous Fe addition promotes the production of endogenous soil DOC by microorganisms. Overall, Our study uses three-dimensional fluorescence spectroscopy techniques combined with the parallel factor analysis (PARAFAC) to characterize the dissolved organic matter components in soil samples under exogenous Fe addition conditions, with a view to exploring the differences in the effects of Fe on the DOC concentration and structure of wetland soils, providing a theoretical basis for the mechanisms of soil carbon fixation and soil organic matter transformation in wetland soils.

Printed Potentiometric Nitrate Sensors for Use in Soil

Plant-available nitrogen, often in the form of nitrate, is an essential nutrient for plant growth. However, excessive nitrate in the environment and watershed has harmful impacts on natural ecosystems and consequently human health. A distributed network of nitrate sensors could help to quantify and monitor nitrogen in agriculture and the environment. Here, we have developed fully printed potentiometric nitrate sensors and characterized their sensitivity and selectivity to nitrate. Each sensor comprises an ion-selective electrode and a reference electrode that are functionalized with polymeric membranes. The sensitivity of the printed ion-selective electrodes was characterized by measuring their potential with respect to a commercial silver/silver chloride reference electrode in varying concentrations of nitrate solutions. The sensitivity of the printed reference electrodes to nitrate was minimized with a membrane containing polyvinyl butyral (PVB), sodium chloride, and sodium nitrate. Selectivity studies with sulphate, chloride, phosphate, nitrite, ammonium, calcium, potassium, and magnesium showed that high concentrations of calcium can influence sensor behavior. The printed ion-selective and reference electrodes were combined to form a fully printed sensor with sensitivity of −48.0 ± 3.3 mV/dec between 0.62 and 6200 ppm nitrate in solution and −47 ± 4.1 mV/dec in peat soil.

Warming and humidification mediated changes of DOM composition in an Alfisol

Dissolved organic matter (DOM) represents the most mobile and reactive pool of soil organic matter (SOM). Climate changes, such as global warming and altered precipitation exert considerable influences on the quality and quantity of soil DOM. However, rare reports have focused on the interactive effects of soil warming and increased precipitation. In the present study, we conducted a 90-day incubation experiment to investigate how the concentration, source and chemical composition of DOM from an Alfisol respond to the variations of temperatures (15, 30 and 45 °C) and moistures (40%, 60%, and 80% of saturated soil water content). Four DOM components were identified through fluorescence excitation emission matrix (EEM)-parallel factor analysis (PARAFAC). Increased temperature alone aggravated the decomposition of plant-derived aromatic components (C2 and C4) but promoted the accumulation of microbial-derived aliphatic carbon (C1) and tryptophan-like component (C3). Increased fungi/bacteria ratio with warming was responsible for the decomposition of plant-derived components. Warming-induced disassociation of Ca-bearing mineral to colloidal Ca facilitated the accrual of microbial-derived aliphatic DOM. Humidification alone and humidification + warming significantly increased the concentration of DOM and the percentage of plant-derived aromatic carbon (C2, C4), which was attributed to the release of Fe-bearing mineral-OC. Based on the above findings along with the results of two-way ANOVA and Variation partition analysis, we infer that moisture will play a dominant role in regulating the chemical composition of DOM in Alfisols under both warming and humidification which in turn impact global C cycling and the ultimate climate.

Hot moments drive extreme nitrous oxide and methane emissions from agricultural peatlands

Agricultural peatlands are estimated to emit approximately one third of global greenhouse gas (GHG) emissions from croplands, but the temporal dynamics and controls of these emissions are poorly understood, particularly for nitrous oxide (N₂ O). We used cavity ring-down spectroscopy and automated chambers in a drained agricultural peatland to measure over 70,000 individual N₂ O, methane (CH₄ ), and carbon dioxide (CO₂ ) fluxes over 3 years. Our results showed that N₂ O fluxes were high, contributing 26% (annual range: 16%-35%) of annual CO₂ e emissions. Total N₂ O fluxes averaged 26 ± 0.5 kg N₂ O-N ha^-1 y^-1 and exhibited significant inter- and intra-annual variability with a maximum annual flux of 42 ± 1.8 kg N₂ O-N ha^-1 y^-1 . Hot moments of N₂ O and CH₄ emissions represented 1.1 ± 0.2 and 1.3 ± 0.2% of measurements, respectively, but contributed to 45 ± 1% of mean annual N₂ O fluxes and to 140 ± 9% of mean annual CH₄  fluxes. Soil moisture, soil temperature, and bulk soil oxygen (O₂ ) concentrations were strongly correlated with soil N₂ O and CH₄ emissions; soil nitrate ( NO3- ) concentrations were also significantly correlated with soil N₂ O emissions. These results suggest that IPCC benchmarks underestimate N₂ O emissions from these high emitting agricultural peatlands by up to 70%. Scaling to regional agricultural peatlands with similar management suggests these ecosystems could emit up to 1.86 Tg CO₂ e y^-1 (range: 1.58-2.21 Tg CO₂ e y^-1 ). Data suggest that these agricultural peatlands are large sources of GHGs, and that short-term hot moments of N₂ O and CH₄ are a significant fraction of total greenhouse budgets.