Biogeochemistry最新文献

Nitrogen (N) cycling in organic tundra soil is characterised by pronounced seasonal dynamics and strong influence of the dominant plant functional types. Such patterns in soil N-cycling have mostly been investigated by the analysis of soil N-pools and net N mineralisation rates, which, however, yield little information on soil N-fluxes. In this study we investigated microbial gross N-transformations, as well as concentrations of plant available N-forms in soils under two dominant plant functional types in tundra heath, dwarf shrubs and mosses, in subarctic Northern Sweden. We collected organic soil under three dwarf shrub species of distinct growth form and three moss species in early and late growing season. Our results showed that moss sites were characterised by significantly higher microbial N-cycling rates and soil N-availability than shrub sites. Protein depolymerisation, the greatest soil N-flux, as well as gross nitrification rates generally did not vary significantly between early and late growing season, whereas gross N mineralisation rates and inorganic N availability markedly dropped in late summer at most sites. The magnitude of the seasonal changes in N-cycling, however, clearly differed among plant functional types, indicating interactive effects of seasonality and plant species on soil N-cycling. Our study highlights that the spatial variation and seasonal dynamics of microbial N transformations and soil N availability in tundra heath are intimately linked with the distinct influence of plant functional types on soil microbial activity and the plant species-specific patterns of nutrient uptake and carbon assimilation. This suggests potential strong impacts of future global change-induced shifts in plant community composition on soil N-cycling in tundra ecosystems.

As important carbon sinks, alpine wetlands on the Tibetan Plateau are undergoing severe degradation. To reveal warming-induced ecological shifts in alpine environments, this study determined soil nutrient contents, enzyme activities, absorption and fluorescence spectra and quadrupole time-of-flight mass spectra (metabolomes) of dissolved organic matter (DOM) and metagenomes based on short-term incubation (0 °C, 10 °C and 20 °C) of topsoil from alpine wetlands and meadows (degraded wetlands). Compared with meadows, wetlands had higher contents of soil DOM (dissolved organic carbon, dissolved organic nitrogen and dissolved phosphorous) and greater activities of hydrolases (β-glucosidase, cellobiohydrolase, β-N-acetylglucosaminidase and acid phosphatase), with those parameters all being highest at 20 °C in meadows and showing various dynamics in wetlands. Soil DOM in wetlands presented the lowest values of specific ultraviolet absorbances (SUVA₂₅₄ and SUVA₂₆₀) at 0 °C and the highest values at 10 °C, whereas the opposite was true in the meadows. Wetland soils had greater diversities of DOM molecular compositions and microbial communities, with warming gradually increasing the number of identified DOM compounds in meadows and decreasing the number of microbial species in both soils. Wetland soils had more Proteobacteria (44.2%) and Acidobacteria (21.1%) and fewer Actinobacteria (18.0%) than meadow soils and contained many temperature-sensitive archaea (which were abundant at 0 °C). Distance-based redundancy analysis and Procrustes analysis indicated the greater complexity of ecological responses in alpine wetlands, which may be attributed to the higher adaptive capacity of soil microbial communities. Our results suggest that both degradation and warming decrease soil DOM content and microbial activities in alpine wetlands, providing important references for alpine wetland conservation under current climate change.

Fire exclusion is a key factor driving conifer expansion into temperate semi-arid grasslands. However, it remains unclear how reintroducing fire affects the aboveground storage of carbon (C) and nitrogen (N) in the expanding tree species and belowground in soils. To assess the impacts of fire reintroduction C and N pools and fluxes in areas of conifer expansion we targeted a region of the Northern Great Plains that has experienced extensive woody plant expansion (WPE) of two species: ponderosa pine (Pinus ponderosa) and juniper (Juniperus spp). We quantified tree mortality of both species to estimate the amount of dead biomass C and N produced by a recent prescribed fire, in addition to changes in soil C, pyrogenic C (PyC), and N concentrations across a woody-cover gradient using a before/after/control experimental design. Post-fire soil chemical analysis revealed a 2 year increase in mineral soil C, PyC and N, suggesting the return of fire led to the transfer of partially combusted plant organic matter back to the soil. Further, we found that functional trait differences between the two species influenced the distribution of living conifer biomass-N prior to fire. Despite junipers having 41% less total aboveground biomass than ponderosa, they contained two times more aboveground N. Prescribed fire resulted in 88% mortality of all mature juniper stems and increased fire severity correlated with greater pre-fire juniper cover. Ponderosa mortality varied by size class, with > 40 cm stem diameter class having only 28% mortality. High mortality and greater aboveground N storage in juniper biomass, compared to ponderosa, led to 77% of the total conifer biomass N lost. Consequently, the functional attributes of expanding trees differentially contribute to fluxes of C and N after the return of fire, with junipers acting as conduits for N movement due to their relatively higher N content in less fire-resistant tissues and ponderosa serving as important and more stable storage pools for C. Together, these findings highlight the importance of considering species-specific traits when planning WPE management strrategies at landscape-scales, particularly when goals include C storage or soil nutrient status.

Graphical abstract

Riverine export of carbon (C) is an important part of the global C cycle; however, most riverine C budgets focus on individual forms of C and fail to comprehensively measure both organic and inorganic C species in concert. To address this knowledge gap, we conducted high frequency sampling of multiple C forms, including dissolved organic C (DOC), inorganic carbon (as alkalinity), particulate organic C (POC), coarse particulate organic C (CPOC), and invertebrate biomass C across the main run-off season in a predominantly rain-fed watershed in Southeast Alaska. Streamwater concentrations were used to model daily watershed C export from May through October. Concentration and modeled yield data indicated that DOC was the primary form of riverine C export (8708 kg C/km²), except during low flow periods when alkalinity (3125 kg C/km²) was the dominant form of C export. Relative to DOC and alkalinity, export of particulate organic C (POC: 992 kg C/km²; CPOC: 313 kg C/km²) and invertebrates (40 kg C/km²) was small, but these forms of organic matter could disproportionately impact downstream food webs because of their higher quality, assessed via C to nitrogen ratios. These seasonal and flow driven changes to C form and export likely provide subsidies to downstream and nearshore ecosystems such that predicted shifts in regional hydroclimate could substantially impact C transfer and incorporation into aquatic food webs.

In flooded paddy fields, peak greenhouse gas nitrous oxide (N₂O) emission after rewetting the dry soils is widely recognised. However, the relative contribution of biotic and abiotic factors to this emission remains uncertain. In this study, we used the isotope technique (δ¹⁸O and δ¹⁵N^SP) and molecular-based microbial analysis in an anoxic incubation experiment to evaluate the contributions of bacterial, fungal, and chemical denitrification to N₂O emissions. We collected eight representative paddy soils across southern China for an incubation experiment. Results show that during the 10-day incubation period, the net N₂O emissions were mainly produced by fungal denitrification, which accounted for 58–77% in six of the eight investigated flooded paddy soils. In contrast, bacterial denitrification contributed 6–15% of the net N₂O emissions. Moreover, around 11–35% of the total N₂O emissions were derived from chemical denitrification in all soil types. Variation partitioning analysis (VPA) and principal component analysis (PCA) demonstrated that initial soil organic carbon (OC) concentrations were the primary regulator of N₂O source patterns. Soils with relatively lower OC concentration (7–15 mg g⁻¹) tend to be dominated by fungal denitrification, which accounted for the net N₂O production at the end of the incubation period. Overall, these findings highlight the dominance of the fungal denitrification pathway for N₂O production in flooded paddy soils, which predominates in soils with relatively lower OC content. This suggests that fungal contribution should be considered when optimizing agricultural management system timing to control N₂O emissions in flooded paddy soil ecosystems, and for the relevant establishment of predictive numerical models in the future.

Inland waters are important sources of the greenhouse gases carbon dioxide (CO₂) and methane (CH₄). Ponds have amongst the highest CO₂ and CH₄ fluxes of all aquatic ecosystems, yet seasonal variation in fluxes remain poorly characterized, creating challenges for accurately estimating annual emissions. Further, ponds can exhibit a range of mixing regimes, yet the impact of mixing regimes on gas emissions remains unclear. Here, we assessed annual dynamics of CO₂ and CH₄ in four temperate ponds (Minnesota, USA) that varied in mixing regimes. The ponds ranged from annual sinks to sources of CO₂ (−1 to 15 mol m⁻² yr⁻¹) and were all significant sources of CH₄ (4.3–8.2 mol m⁻² yr⁻¹), with annual fluxes in CO₂ equivalents of 1.8–4.1 kg CO₂-eq. m⁻² yr⁻¹. Mixing regimes impacted CO₂ and CH₄ dynamics, as stratified periods were associated with more anoxia, greater accumulation of gases in the bottom waters, higher emissions of CH₄, and lower fluxes of CO₂. Ponds with stronger summer stratification also had increased CO₂ and CH₄ fluxes associated with fall turnover. Overall, the two ponds with the strongest stratification had higher annual fluxes (2.6, 4.1 kg CO₂-eq. m⁻² yr⁻¹) compared to the two ponds that more frequently mixed (1.8, 2.2 kg CO₂-eq. m⁻² yr⁻¹).

Despite the rising interest in understanding how climate change could affect the emissions of greenhouse gases (GHGs) from river systems, including floodplains, we still lack a mechanistic understanding of how changing environmental conditions, such as moisture and nutrient availability, limit the temperature responses of GHG production in floodplain sediments. To examine the environmental co-limitations on the temperature responses of three major GHGs (CO₂, CH₄, and N₂O) produced in floodplain sediments, sediments from a constructed wetland on the floodplain of the lower Han River were incubated for 24 d at four temperatures spanning 4–28 ℃, under three conditions (closed, open/wetting, and open/drying). The net production of all three GHGs exhibited nonlinear temperature responses with gas-specific patterns and magnitudes of response varying over the incubation period. During the later incubation phase, positive temperature responses were weakened for the net production of CO₂ and CH₄ in the dried treatments, whereas a similar weakening occurred for N₂O production in the wet treatments. This, combined with incubation-induced changes in dissolved organic carbon and its fluorescence components, indicated the lack or excess of moisture and associated changes in O₂ and organic carbon availability as critical co-limiting factors for the temperature responses of GHG production. Warming decreased δ¹³C in the CH₄ emitted from wet and hypoxic sediments, implying a stronger warming effect on CH₄ production over oxidation. Unlike many studies assuming a consistent relationship between temperature and GHG production in sediments irrespective of other environmental conditions, our results suggest that warming effects on the GHG emissions from floodplain sediments would depend on the balance between gas production and consumption under the prevailing constraints of moisture, O₂, and labile carbon availability.

Black carbon (BC) is a significant component of the global carbon cycle both in terrestrial and aquatic systems. Dissolved black carbon (DBC) is a significant portion of the total dissolved organic carbon (DOC) pool and represents a major flux of recalcitrant carbon to the coastal and deep oceans. Dissolved black carbon can originate from multiple sources related to its relative biogeochemical reactivity with the dynamics of highly recalcitrant DBC integral to long-term sequestration. Thus, understanding how the more recalcitrant fractions of DBC varies in diverse catchments is critical and currently underexplored. We used hydrogen pyrolysis to isolate the fraction of DBC with aromatic clusters above 7 rings, representing the more stable components. Here we report the dynamics of DBC_HyPy over a hydrological year in a temperate catchment, with a long history of coal mining extraction. Quarterly measurements of DBC were undertaken from two main channel and four tributary sites. Hydrogen pyrolysis derived DBC comprised a significant percentage of the total DOC flux (3.2% to 28.3%) and included significant spatial variability. Unlike other studies examining more reactive DBC fractions, bulk DOC concentrations and DBC_HyPy were poorly correlated when considered over an annual scale. Rather, DBC_HyPy was correlated with indicators of groundwater such as dissolved inorganic carbon and conductivity. Data suggest a consistent source of DBC_HyPy not subject to the same mobilisation drivers as DOC, which shows substantial seasonality. Rather, our data shows a potentially consistent supply of stable DBC originating from the coal mining-influenced groundwater. Petrogenic sources of DBC have been poorly constrained to date, the data presented here suggests in some catchments it may be significant and yield catchment scale DOC-DBC decoupling. The dynamics of DBC have implications for carbon fluxes, pollution transport and water quality/treatment requirements. These preliminary findings suggest potentially complex drivers in spatially heterogeneous catchments, contrasting with previous work finding tight DOC-DBC mobilisation dynamics.

Direct and indirect anthropopressure on water ecosystems is the serious problem throughout the world.. In the Northern Hemisphere, an increase in average air temperatures is observed, which implies the occurrence of a shorter period of snow and ice cover during the winter season. The winter 2019/2020 was unusual, because that was the first time in the record, that a complete lack of permanent ice cover was observed on numerous lakes in Poland. Such unusual conditions could influence lake functioning. Hence we analyzed the chemistry of the water–sediment interface (near-bottom and interstitial water and sediment) in the shallow, eutrophic Mielenko Lake (area 7.9 ha, max depth 1.9 m) in 2013 and 2019–2022 period to assess the influence of prolonged water circulation on the bottom zone. Mielenko Lake was subjected to a phosphorus inactivation procedure using Al and Fe salts (PAX 18, PIX 111) in 2020 and 2021. Our research revealed that unusually prolonged winter circulation caused a significant decrease in organic matter content in bottom sediment in 2020, as well as a decrease in NaOH-nrP fraction and TP amounts. That effect was short-term and it did not significantly influence the NaOH-rP fraction amounts. The released P was probably built in macrophytes biomass during vegetation season, because P inactivation has been limiting phytoplankton proliferation, and it favored shifting to a clearwater state with macrophytes domination. This was confirmed by decreasing in phytoplankton biomass, and a massive expansion of the macrophytes range noted in the second year of restoration. Our study shows, that P inactivation could mitigate the negative effects of warm winters in shallow lakes.

Deadwood represents a dynamic carbon pool in forest ecosystems where microbial decomposition causes fluxes of CO₂ to the atmosphere through respiration and organic carbon to the soil through leakage and fragmentation. This study characterises different stages of deadwood of Norway spruce (Picea abies). 35 Norway spruce trees were sampled and categorized on a 0–5 decay scale. For the 14 trees in classes 0–3, two stem discs were collected from two heights. For the 21 trees in classes 4 and 5, a single sample per tree was taken, because decay was relatively uniform throughout the stem. The relative amount of hemicellulose and cellulose declined moderately from decay class 1 to 3 and substantially from decay class 3 to class 4 but small amounts were still present in decay class 5. The relative lignin proportion increased substantially from decay class 3 to 4 and dominated in decay class 5. Relative carbon content increased from 50 to 56% during the decomposition process due to the increasing accumulation of lignin residuals being a typical signature of brown rot decay. A laboratory experiment including three species of brown rot fungi verified decomposition close to 70% of Norway spruce biomass and resulted in 55% carbon content. This was similar to the carbon content in decay class 4 and 5. A novel approach is presented to quantify the carbon flux from deadwood to the soil. First, we calculated the residual proportion of carbon in decayed wood compared to the initial carbon content of live trees. Subsequently, we extended the calculation to determine the amount of remaining carbon from non-decayed wood that was transferred to the soil during each decay class. The approach showed that Norway spruce wood decomposition under field conditions transfers at least 39–47% of the initial wood carbon to the soil carbon pool, depending on soil type. This strengthens the previously under-communicated fact that the carbon flux from deadwood to soil is higher from brown rot decomposition in boreal forests than the corresponding carbon flux in temperate and tropical forests where deadwood is more influenced by white rot fungi.