Journal of Geophysical Research: Biogeosciences最新文献

Infrequent soil wetting in deserts can induce large nitrogen (N) trace gas pulses; however, how other abiotic mechanisms interactively control the timing and magnitude of these pulses are not clear. In particular, production of nitric(NO) and nitrous (N₂O) oxide may be differentially sensitive to temperature, carbon (C), and N availability. At a desert field site in Southern California, USA, we used an automated sensor system in 4 years of field campaigns to track NO and N₂O pulse responses to experimental manipulations of C and N across a range of ambient temperatures and shrub fertile islands. We observed rapid onset and shorter duration of N₂O pulses immediately after wetting compared to lagged and extended pulses of NO, suggesting preferential incorporation of N initially into N₂O in anoxic microsites and then to NO as soils dry. We identified strong nitrogen limitation and exponential temperature dependence of NO pulses, particularly for soils located under shrubs. N₂O pulses were less responsive to experimental manipulations but showed evidence of C and N colimitation as well as seasonal temperature differences. As atmospheric N deposition and temperatures continue to increase in desert systems, we can expect larger losses of N from soils as pulse-based emissions.

Restored mangroves are increasingly recognized as vital nature-based solutions for atmospheric CO₂ sequestration. We hypothesize that the seasonal dynamics of carbon fluxes and coupled regulatory mechanisms may be the key in understanding their sequestration strength, especially in the northernmost mangroves experiencing pronounced seasonality. In this study, we measured net ecosystem CO₂ exchange from 2017 through 2023 using the eddy covariance technique in the northernmost restored mangrove ecosystem in southern China. These mangroves acted as carbon sinks, with an annual net ecosystem production (NEP) of 530 g C m⁻². Throughout the study period, NEP was greater during the wet seasons than the dry seasons, primarily driven by elevated photosynthetically active radiation (PAR). Machine learning identified PAR as the most influential environmental driver of seasonal NEP differences, with its positive effect being significantly stronger in wet seasons compared to dry seasons (p < 0.01). Air temperature (TA), soil temperature (TS), and soil water content (SWC) were also key drivers of NEP. When TA and TS exceeded thresholds of 27.81°C and 27.06°C, respectively, NEP was negatively affected, although such conditions occurred in 21% of whole observation period. High SWC had a more pronounced inhibitory effect on NEP during dry seasons, potentially because of reduced soil salinity impairing photosynthetic efficiency. As mangroves evolved, NEP's sensitivity to PAR and TA increased, while its sensitivity to TS and SWC was reduced. This study enhances our understanding of seasonal carbon fluxes and their interactions with environmental drivers in the northernmost restored mangrove ecosystem.

How tropical forest leaves respond to climate change has important implications for the global carbon cycle and biodiversity. Climate change could impact the energy balance properties of tropical forest canopies through (a) long-term trait changes and (b) abrupt disruptions/damage to leaf/photosynthetic machinery. We assessed the radiative and evaporative impacts of two recently proposed impacts of climate change on tropical forest canopies: (a) long-term leaf darkening and (b) leaf death through high temperature extremes. We darkened leaves to absorb 138 Wm⁻² more energy in the upper canopy of a seasonally dry tropical moist forest in Panama. 20% of this extra energy went toward heating leaves by ∼4°C, 3% went toward warming the air, and 77% went toward evaporative cooling. This leaf warming led to the appearance of necrosis across 9 ± 5% of the leaf area on certain species. In contrast, brightening leaves decreased energy absorbed by an average of 58 Wm⁻², which mainly reduced evaporation (88%) with only 12% reducing leaf temperatures (and no change in sensible heat flux). This asymmetrical result suggests leaves may be close to hydraulic limitations to support transpirational cooling toward the end of the dry season. Similar albedo increases in a model (CLM 4.0) did not diverge between brightening and darkening leaves and generally showed sensible heat flux to dominate although there were strong geographic trends. Heat death in leaves generally heated nearby leaves (by an average of ∼1.35°C) and air temperature (by 0.5°C) but less than hypothesized because leaf albedo increased. Overall, our canopy top experiments question important potential climate feedbacks but need further study.

We introduce here the development and testing of a mechanistic model accounting for inorganic carbon dynamics in soil. We designed a microbially induced carbonate precipitation version 1 (MICPv1) reaction network to describe the transport, precipitation, and dissolution of calcium carbonate under the effect of dynamic H⁺ and HCO₃⁻ concentrations driven by microbial consumption of yeast extract and acetate as the source of C for growth and urea as a source of nitrogen and bicarbonate. MICPv1, embedded in the BRTSim general-purpose solver (BRTSim-MICPv1), was solved explicitly in space and time to replicate prior experiments in a 3.7-m soil column with three soil types, subject to two different nutrient treatments, and lasting from 17 to 26 days. We found that BRTSim-MICPv1 could capture the main experimental features in urea, NH₄⁺, microbial biomass dynamics, and calcite precipitation both spatially and over time (Nash-Sutcliffe NSE > 0.51; NRMSD < 27%). A Monte Carlo stochastic analysis on BRTSim-MICPv1 parameterization showed that the carbonate dynamics response to parameter variability was robust and more accentuated in the column with high nutrient input. Finally, we show that BRTSim-MICPv1 was mass conservative and able to correctly solve for chemical equilibrium and could be used to track the sources and sinks of Ca²⁺ and HCO₃⁻ to support our findings.

Environmental impacts related to the Ocean Alkalinity Enhancement (OAE) on marine biota remain underexplored. Ocean Alkalinity Enhancement aims to increase the ocean's total alkalinity (TA), shifting the carbonate buffer system to prompt air-sea gas exchange and CO₂ drawdown. These conditions might be favorable for calcifiers, leading to increased removal of alkalinity in CaCO₃ and reversing some of the intended benefit of the OAE. Here, we parameterize the impact of increased ocean alkalinity on two dominant end-member coccolithophore species: Gephyrocapsa huxleyi and Coccolithus braarudii. The growth rate of each species increased significantly with increased alkalinity, likely driven by increasing resupply rates of CO₂ from higher HCO₃⁻ concentrations. Both species increased growth rates relative to the control even at the lowest alkalinity treatments (∼3,000 μmol kg⁻¹), which could lead to population expansion under air-equilibrated OAE, and higher population levels of calcification. At higher TA (i.e., >3,000 μmol kg⁻¹), rates of calcification were increasingly limited by CO₂ supply to the faster growing cells which resulted in malformation suggestive that cell division is prioritized over calcification when CO₂-limited. Divergent species-specific responses may arise because large and heavily calcified C. braarudii have a far greater carbon demand, and rely on CO₂ for calcification compared to the smaller, lightly calcified rapidly-growing G. huxleyi which have the additional capacity to use HCO₃⁻ when CO₂-limited. Our study suggests constraints to ensure safe ecosystem boundaries (i.e., alkalinity 3,000 μmol kg⁻¹), and provides mechanistic insights to understand the impacts of carbonate chemistry on physiology and calcite production by major calcifiers.

Understanding how wildfires impact the biogeochemistry of dissolved organic matter (DOM) in peatland catchments is important for predicting how they may respond to climate change. However, the net effects of wildfires on the composition of DOM are not yet well understood. We investigated how fire changes the age, thermal stability, and molecular composition of stream DOM in blanket peatlands in the Flow Country and the Isle of Lewis, North of Scotland. Radiocarbon measurements showed that stream DOC was predominantly modern in both bulk and ramped thermal fractions with no apparent change observed due to wildfires. Ramped thermal oxidation revealed higher thermal stability of stream DOM in wildfire impacted areas, as demonstrated by higher activation energies, a proxy for organic C bond strength. This was prominent between 350 and 470°C and was also associated with an increase in the content of thermally stable C and a reduction in bond diversity. Using ultra high-resolution mass spectrometry, we found an increase in the molecular diversity of DOM and in the relative abundance of highly unsaturated and phenolic class. There was also a higher relative abundance of highly oxygenated N- and S-containing formula, potentially from partially combusted plant and soil material, which could explain the shift in activation energy. Together, our results demonstrate ways that wildfires can impact the reactivity and composition of DOM, with implications for its stability and residence time along the terrestrial-aquatic continuum.

Ditches are potentially important sources of methane (CH₄) in agricultural regions, but their CH₄ emissions are largely unknown due to data scarcity. Here, we investigated CH₄ concentrations and diffusive fluxes across different ditches in the North China Plain (NCP), an extensive upland agricultural region with maize-wheat rotations, and well-constructed ditch systems. Based on intensive monthly and extensive regional surveys, we found that (mean ± SD) CH₄ concentrations (11.42 ± 37.69 μmol L⁻¹) and fluxes (344.7 ± 1,198.1 μmol m⁻² h⁻¹) in the agricultural ditches (ADs) showed high variability, primarily driven by spatial and temporal heterogeneity in nutrient and carbon inputs. On average, CH₄ concentrations and fluxes were 3–12 times higher than those in the nearby agricultural-rural ditches (3.80 μmol L⁻¹, 99.8 μmol m⁻² h⁻¹) and rivers (0.92 μmol L⁻¹, 47.1 μmol m⁻² h⁻¹). Dissolved organic carbon (DOC) and ammonium (NH₄⁺–N) were primary drivers of CH₄ emissions in the ADs, highlighting the key role of nutrient and carbon inputs from surrounding fields. The annual diffusive CH₄ emission from ADs in the NCP was estimated to be 1,836.3 ± 311.6 Gg CH₄ yr⁻¹ and 68.1 ± 7.3 Gg CH₄ yr⁻¹ based on the mean and median CH₄ fluxes, respectively, acting as a significant source of CH₄ emissions, despite large uncertainty. This emission overwhelmingly offsets the CH₄ uptake by soils (i.e., −9.2 Gg CH₄ yr⁻¹) in the NCP, highlighting the necessity of including CH₄ emissions from ADs in estimating CH₄ budget from upland agricultural regions.

Southeastern U.S. longleaf pine savannas have been reduced to less than 5% of their original extent, giving space to fast-growing, high-density loblolly pine plantations. Restoring longleaf pine savannas is an alternative that has been discussed to reduce evapotranspiration and increase water yield, which is of great relevance in resource management and ecosystem restoration. Understanding the benefits of forest management and restoration requires advanced tools to better quantify the fundamental processes of energy, water, and carbon interactions at stand, landscape, and regional scales. We implement and assess the Community Land Model Version 5 (CLM5) at two long-term research sites, one representing a typical loblolly pine (Pinus taeda) plantation with a high tree density and another a typical longleaf pine (Pinus palustris) savanna with a low tree density. We also carry out numerical experiments exploring potential differences in energy, water, and carbon balances under alternate land cover scenarios at each site. Using tailored parameterizations for loblolly and longleaf pine and an adjusted C4 grass parameterization, we found that CLM5 reasonably captured the overall observed stand structure and functions at both sites, performing substantially better than with the default parameterizations. Our numerical experiments indicated 8%–17% lower evapotranspiration and 30%–125% higher water yield for longleaf pine savanna compared to loblolly pine plantation. However, net ecosystem production (NEP) and NEP-based water use efficiency were 60% lower for longleaf pine savanna than loblolly pine plantation. Our modeling framework could be implemented regionally in future studies to support forest management decisions and longleaf pine restoration initiatives.

We investigated lateral dissolved organic carbon (DOC) fluxes from lower-order streams in upland tundra underlain by permafrost to assess their contribution to carbon (C) sink or source strength. The study site, located in Healy, Alaska, is within the Panguingue Creek watershed (66 km²), where permafrost acts as a confining layer for soil pore water. We examined how seasonal hydrology affects DOC export to understand lateral C flux contributions to the site's net ecosystem C balance. Previous estimates of vertical gas exchange (net CO₂ uptake and release) indicated a loss of 52.5 g C m⁻² yr⁻¹ and methane was estimated to lead to an additional loss of 6.4 ± 0.20 CO₂-equivalent (g CO₂–C m⁻² yr⁻¹) suggesting the site is a net C source. We found that a second-order stream exported an additional 5.0 g DOC m⁻² yr⁻¹ laterally, which is approximately 10% of the vertical CO₂ loss, reinforcing the site's source status. Accounting for CO₂, CH₄, and DOC, total C losses were estimated to be 64 g C m⁻² yr⁻¹. Further, we found that shoulder seasons played a key role in C export, with the spring freshet alone accounting for 59% of total DOC flux. Seasonality also influenced DOC age, with older, permafrost-derived DOC exported primarily during spring and fall. These findings underscore the significance of lateral pathways in permafrost C budgets and highlight DOC as a critical, seasonally variable component of C loss.

Boreal forests export large amounts of terrestrial carbon into downstream waters as dissolved organic material (DOM), but how increasing wildfire frequency affects this flux remains understudied. If more DOM is exported from land into water after wildfire and/or this DOM is more readily transformed by microbes, the net loss of carbon from terrestrial ecosystems after wildfire may be greater than currently estimated. Here we investigated how wildfire changes DOM exported from boreal forests into headwater streams in northwestern Ontario, Canada over a summer growing season. We compared the concentration and molecular composition of DOM between 10 recently burned and 10 undisturbed catchments using optical spectroscopy and ultra-high-resolution mass spectrometry. We found a 29% increase, on average, in DOM concentrations in the streams of burned catchments in August only. DOM in burned catchments appeared less bioavailable, as indicated by a lower H:C and higher modified aromaticity index. As expected, because of wildfire, black carbon was 55% more abundant, on average, in burned catchments compared to controls, contributing to the greater aromaticity of DOM. However, despite the lower bioavailability of DOM, compounds in burned catchments were more thermodynamically favorable for microbial degradation and as likely to be biochemically transformed as unburned DOM during July and August, but not in June. Overall, our results suggest wildfires reduce forest carbon sequestration more than currently estimated because of fluvial DOM losses. If exported DOM is mineralized, carbon sequestration may even decrease, highlighting the need to incorporate the impacts of wildfires on receiving waters into carbon accounting.