Journal of Geophysical Research: Biogeosciences最新文献

Understanding factors influencing carbon effluxes from soils to the atmosphere is important in a world experiencing climatic change. Two important uncertainties related to soil organic carbon (SOC) stock responses to a changing climate are (a) whether soil microbial communities acclimate or adapt to changes in soil temperature and (b) how to represent this process in SOC models. To further explore these issues, we included thermal adaptation of enzyme-mediated processes in a mechanistic SOC model (ReSOM) using the macromolecular rate theory. Thermal adaptation is defined here to encompass all potential responses of soil microbes and microbial communities following a change in temperature. To assess the effects of thermal adaptation of enzyme-mediated processes on simulated SOC losses, ReSOM was applied to data collected from a 13-year soil warming experiment. Results show that a model omitting thermal adaptation of enzyme-mediated processes substantially overestimates observed CO₂ effluxes during the initial years of soil warming. The bias against observed CO₂ effluxes was lower for models including thermal adaptation of enzyme-mediated processes. In addition, for a simulated linear 3°C soil warming over 100 years, models including thermal adaptation of enzyme-mediated processes simulated SOC losses of a factor of three smaller than models omitting this process. As thermal adaptation of microbial community characteristics is generally not included in models simulating feedback between the soil, biosphere and atmosphere, we encourage future studies to assess the potential impact that microbial adaptation has on soil carbon – climate feedback representations in models.

Spectral induced polarization (SIP) exhibits potential to be a nonintrusive approach to monitor bacterial activity in biological hotspots associated with the critical zone of the earth. The polarization of bacteria in a low-frequency electrical field is related to the polarization of their electrical double layer coating their surface. However, few studies have quantified the induced polarization responses on both gram-negative (GN) and gram-positive (GP) bacteria in soil column experiments. To address this gap, 17 experiments using two strains, Pseudomonas aeruginosa O1 (PAO1, GN) and Brevibacillus centrosporus (L3, GP) are conducted. Complex conductivity spectra are collected in the frequency range 10 mHz–10 kHz during bacterial growth and decay phases in soils. The complex conductivity spectra are fitted using a double Cole-Cole model to remove the effect of Maxwell-Wagner polarization. The change in the magnitude of the polarization (quadrature conductivity or normalized chargeability of the low-frequency contribution) is linearly related to the bacterial density, regardless of the type of bacteria. The changes in the normalized chargeability and Cole-Cole relaxation time are directly proportional to the density of bacteria. Furthermore, it is inferred that the thickness of microcolonies plays a critical role in the relaxation time rather than the diameter of individual bacteria. This study expands the potential of SIP for in situ monitoring of microbial activity in soils.

Understanding controls on primary productivity is essential for describing ecosystems and their responses to environmental change. In lakes, pelagic gross primary productivity (GPP) is strongly controlled by inputs of nutrients and dissolved organic matter. Although past studies have developed process models of this nutrient-color paradigm (NCP), broad empirical tests of these models are scarce. We used data from 58 globally distributed, mostly temperate lakes to test such a model and improve understanding and prediction of the controls on lake primary production. The model includes three state variables–dissolved phosphorus, terrestrial dissolved organic carbon (DOC), and phytoplankton biomass–and generates realistic predictions for equilibrium rates of pelagic GPP. We calibrated our model using a Bayesian data assimilation technique on a subset of lakes where DOC and total phosphorus (TP) loads were known. We then asked how well the calibrated model performed with a larger set of lakes. Revised parameter estimates from the updated model aligned well with existing literature values. Observed GPP varied nonlinearly with both inflow DOC and TP concentrations in a manner consistent with increasing light limitation as DOC inputs increased and decreasing nutrient limitation as TP inputs increased. Furthermore, across these diverse lake ecosystems, model predictions of GPP were highly correlated with observed values derived from high-frequency sensor data. The GPP predictions using the updated parameters improved upon previous estimates, expanding the utility of a process model with simplified assumptions for water column mixing. Our analysis provides a model structure that may be broadly useful for understanding current and future patterns in lake primary production.

Droughts typically exert negative effects on vegetation growth, which largely depend on the timing of drought onset. However, huge inconsistencies exist in the seasonal vegetation response to drought among diverse regions across the globe. Here, using the leaf area index (LAI) and solar-induced chlorophyll fluorescence (SIF), we quantified the vegetation susceptibility by calculating the coincidence rate between vegetation suppression extremes and soil droughts, and further investigated the spatiotemporal changes of vegetation susceptibility during different seasons from 2001 to 2021. We found the vegetation during summer and dry seasons were most susceptible to soil droughts in the extra-tropics and tropics, respectively. Temporally, the autumn vegetation susceptibility was strengthening in drought-susceptible regions of extra-tropics, albeit with insignificant change during spring, summer and the entire growing season. Both the dry and wet seasons showed evidently increasing vegetation susceptibility on the dry tropical ecosystems, which dominated the enhanced vegetation susceptibility of global drought-susceptible regions. Our findings determined the spatial pattern of most susceptible seasons to soil droughts across the globe and highlighted the enhanced risk to soil droughts, especially in the dry tropics.

The variability in stocks and accumulation rates of organic carbon (C_org), nitrogen (N), and carbonate (CaCO₃) was studied in fifteen Posidonia oceanica meadows spread throughout the South Aegean Sea (Greece). In addition, the abiotic and biotic drivers determining the pattern of variability in the accumulation rates were assessed by exploring the influence of sediment characteristics, seagrass traits, and environmental settings. The meadows supported on average (±STDEV) 14.6 ± 5.0 kg C_org m⁻², 0.47 ± 0.17 kg N m⁻², and 249 ± 210 kg CaCO₃ m⁻² in the top meter of their sediments, with mean accumulation rates over the last 500 years of 33.6 ± 23.6 g C_org m⁻² yr⁻¹, 1.00 ± 0.62 g N m⁻² yr⁻¹, and 405 ± 336 g CaCO₃ m⁻² yr⁻¹ across sites. A redundancy analysis (RDA) explained 70% of the variation in C_org, N, and CaCO₃ accumulation rates, with three sediment characteristics (i.e., sediment C_org:N and C_org:C_inorg ratios and P. oceanica contribution to the sediment C_org pool) emerging as the primary set of factors shaping the accumulation of matter, followed by seagrass traits (i.e., leaf biomass and rhizome elongation) and environmental variables (i.e., suspended organic matter). The high degree of variability within the region emphasizes the need for fine-scale assessments to understand the local conditions influencing sequestration. Our findings underscored the critical role of seagrass meadows in carbon and nitrogen sequestration in the region, urging conservation efforts to protect these ecosystems and prevent potential losses of stored carbon and nitrogen following seagrass degradation.

Understanding on relationships between seasonality of vegetation phenology and photosynthesis is lacking for desert ecosystems. We used digital camera (i.e., PhenoCam) to monitor the phenology of forest (i.e., 2 sites with one being closer to a lake) and grassland (i.e., 1 site) ecosystems in the Badain Jaran Desert, China. The vegetation phenology was quantified using vegetation indices calculated from the red, green, and blue digital numbers in images obtained by the PhenoCams. Additionally, various meteorological variables were continuously measured, and gross primary production (GPP) was obtained using the eddy covariance technique at the grassland site. The difference between the phenological periods extracted from the PhenoCam images and the artificial visual method was small (≤6 days), indicating that the digital camera can effectively obtain desert vegetation phenology. The key meteorological factors affecting changes in the vegetation indices were identified, with temperature being the most important factor (i.e., correlation coefficients = 0.4–0.8 and p-value < 0.001 for all three study sites). Although precipitation showed weak correlation with the vegetation index (correlation coefficient = 0.18–0.14, p-value < 0.01), rapid increases in the vegetation index were observed in response to precipitation events. Vegetation indices were strongly correlated with GPP variations at the grassland, and the strongest correlation was observed in the green-up stage (correlation coefficient = 0.67 to 0.85, p-value < 0.001). The highest GPP lagged about 1 month behind the peak in the vegetation indices in summer (June–August). Our results can markedly improve the knowledge of desert ecosystem processes and aid in assessing the influence of future climate changes in drylands.

Global fluctuations in vegetation productivity are intricately tied to climatic variability, but how climate change will alter climatic limitations on productivity is unclear. Here, we used shapley additive explanation (SHAP), a novel technique based on game theory, for identifing the contributions of climatic factors to vegetation productivity. We also delineated climatic limitations on productivity and traced their temporal evolution during 1982–2018 using the SHAP values. The results identified that, in temperate, boreal, and polar zones, temperature primarily limited productivity during the early growing season, and temperature and radiation jointly limited productivity during the peack and late growing season. In contrast, water and radiation predominantly limited productivity mainly in arid and equatorial zones, respectively. We also observed an alleviated temperature but an intensified water limitations on productivity across different months. The alleviated temperature limitation was particularly notable in June for the northern hemisphere (July for the southern hemisphere), with the temperature-constrained area decreasing significantly at a rate of 2.2‰/y (1.2‰/y). In contrast, the exacerbation of water limitation was most pronounced in June (September), with the water-constrained area expanding significantly at a rate of 2.8‰/y (3.3‰/y). Our findings underscore the imperative for a more explicit incorporation of the impact of water limitation in understanding regional and global carbon dynamics under a warming climate.

Isotope hydrological studies to understand groundwater-surface water interactions in tropical, high-elevation catchments are limited. These interactions are important in controlling lake water residence time, aqueous biogeochemistry, and water availability for downstream communities and ecosystems. To better comprehend the complexity of spatio-temporal variations in the aquifer-lake domain in tropical volcanic regions, a multi-tracer approach including water and inorganic carbon stable isotopes (δ²H, δ¹⁸O, δ¹³C_DIC), hydrochemistry, and ²²²Rn was applied in Lake Hule, northern Costa Rica. Seasonal isotope mass balance calculations using lake, stream, precipitation, and groundwater compositions were supplemented with local hydrometeorological information. Evaporation to inflow ratios (E/I) revealed a small variability between the dry (December–April) and wet seasons (May–November), with relatively low evaporation losses, 2.9 ± 1.0 % and 3.2 ± 1.8 %, respectively. Bayesian end-member analysis indicated that annual inputs from groundwater, precipitation, and runoff represented 61.3 ± 8.1%, 24.4 ± 8.4, and 14.3 ± 5.9% of total lake inflow, respectively. Temporal variations of δ¹³C_DIC confirmed the key role volcanic carbonate buffering plays in this lake and indicated greater CO₂ degassing from groundwater sources in the wet season. This tracer-aided assessment in a volcanic lake maar of northern Costa Rica provides evidence of previously unknown groundwater-surface water interactions and illustrates the application of isotopic tools for estimating water balances and seasonal variability of groundwater discharge into natural lakes across the volcanic front of Central America.

During the last decades, intensive forest dieback due to drought events and bark beetle infestation was globally observed leading to accumulation of deadwood. However, data on molecular composition of deadwood DOM, of its bacterial and photo-transformation, and of the interaction of these processes are scarce. Here, we investigate the fate of DOM leached from deadwood into streams. We hypothesized that (a) bacterial degradation dominates quantitatively over photodegradation in stream water, (b) bacterial degradation is further promoted by labile and easily degradable photoproducts, and (c) DOM compositional changes reflect both the bacterial and light transformation. A leachate of spruce branches and bark in pure water was used for a degradation experiment in a 2 × 2 factorial design without and with stream bacteria and light, respectively. Dissolved organic carbon concentration did not change in dark incubation without bacteria but decreased slightly (3%) in the light. The decrease with bacteria in the dark was stronger (9%), that is, photodegradation of spruce leachate was less important than bacterial degradation (a). Photodegradation and bacterial degradation added in the light plus bacteria treatment (12%), and bacterial degradation was similar in light and dark, indicating no quantitative priming by easily available photoproducts but some qualitative modifications were detected (b). Light induced the production of mostly small and polar molecules, mainly from stream water DOM, while bacteria preferentially degraded nonpolar molecules from dead-wood leachate (c). Our results indicate distinct transformation pathways and high microbial availability for deadwood-derived DOM as compared to stream water DOM that may stimulate heterotrophic processes in headwater streams.

Heterotrophic bacteria can contribute to improve stream water quality by taking up nitrate (NO₃⁻) from the water column, although microbial demand for this nutrient is usually lower than for other inorganic nitrogen (N) forms, such as ammonium. Heterotrophic NO₃⁻ uptake has been related to the availability of dissolved organic carbon (DOC) relative to nutrients (i.e., DOC: nutrients ratios). Yet, how dissolved organic matter (DOM) composition and specific microbial assemblages influence NO₃⁻ uptake remains poorly understood. We conducted laboratory incubations to investigate heterotrophic NO₃⁻ uptake kinetics in 9 Mediterranean freshwater ecosystems, primarily headwater streams, exhibiting wide variation in DOC:NO₃ ratios (from 1.5 to 750). Moreover, we characterized DOM composition using spectroscopic indexes and its degradation via a reactivity continuum model approach. Microbial community composition and functioning were assessed by analyzing extracellular enzymatic activities and the potential abundance of N-cycling genes. Our results revealed that NO₃⁻ uptake rates (k_NO3) were positively related with DOC:NO₃ ratios (r² = 0.4) and to NO₃:SRP ratios as well (r² = 0.6). Furthermore, k_NO3 was negatively correlated to the humification index (r² = 0.7), suggesting that a higher proportion of humic-like compounds slow down heterotrophic NO₃⁻ uptake. A partial least squares regression model (PLS) pinpointed that DOC and nutrient stoichiometry, DOM composition and reactivity, and microbial composition and activity collectively contributed to explain the variability in k_NO3 observed across treatments. Our findings suggest that heterotrophic NO₃⁻ uptake may show significant responsiveness to shifts toward more labile DOM sources and nutrient imbalances induced by global change.