Biogeochemistry最新文献

Human activity is increasing salt concentrations in freshwaters worldwide, but effects of freshwater salinity gradients on biogeochemical cycling are less understood than in saline, brackish, or marine environments. Using controlled microcosm experiments, we characterized (1) short-term (one to five days) biogeochemical responses and (2) water column metabolism along a freshwater salinity gradient of multiple salt types. After one day, microcosms were oxic (4.48–7.40 mg O₂ L⁻¹) but became hypoxic (1.20–3.31 mg L⁻¹) by day five. After one day in oxic conditions, microbial respiration in magnesium-, sodium-, and sea salt-based salinity treatments showed a subsidy-stress response, with respiration increasing by over 100% as salinity increased from 30 to 350–800 µS cm⁻¹. Conversely, respiration consistently increased along a calcium-based salinity gradient, peaking at 1500 µS cm⁻¹. By day five, an inverse subsidy-stress response was observed with elevated respiration at upper or lower ends of the gradient except for the magnesium treatment, which had the lowest respiration at the highest salinity. Calcium- and magnesium-based salinity treatments also caused considerable changes in phosphorus concentrations and C:P and N:P. In a separate experiment, microbial respiration and water column primary production also displayed subsidy-stress responses, but imbalances in effect sizes caused consistently declining net community production with increasing salinity. Collectively, our results establish that short-term exposure to different salt ion concentrations can enhance freshwater biogeochemical cycling at relatively low concentrations and alter resource stoichiometry. Furthermore, the nature of effects of freshwater salinization may also change with oxygen availability.

While seagrass meadows are perceived to be pertinent blue carbon reservoirs, they also potentially release methane (CH₄) into the atmosphere. Seasonal and diurnal variations in CH₄ emissions from a subtropical hypersaline lagoon dominated by Halodule wrightii in southern Texas, USA, on the northwest coast of the Gulf of Mexico were investigated. Dissolved CH₄ concentrations decreased in the daytime and increased overnight during the diel observation period, which could be explained by photosynthesis and respiration of seagrasses. Photosynthetic oxygen was found to significantly reduce CH₄ emissions from seagrass sediment. Diffusive transport contributed slightly to the release of CH₄ from the sediment to the water column, while plant mediation might be the primary mechanism. The diffusive CH₄ flux at the sea-air interface was 12.3–816.2 µmol/m² d, over the range of the sea-air fluxes previously reported from other seagrass meadows. This was related to relatively higher dissolved CH₄ concentrations (11.6–258.2 nmol/L) in a mostly closed lagoon with restricted water exchange. This study emphasizes seagrass meadows in the subtropical hypersaline lagoon as a source of atmospheric CH₄, providing insights into the interactions between seagrass ecosystems and methane dynamics, with potential implications for seagrass meadow management and conservation efforts.

All over the world, peatlands have been drained, often for agricultural purposes, resulting in CO₂ emissions, soil subsidence and biodiversity loss. To combat these negative effects, drained peatlands are being rewetted, but knowledge of the effects of rewetting on peat biogeochemistry is still incomplete, especially since a variety of rewetting methods and rewetting degrees exists. We conducted a mesocosm experiment in which we exposed 100 intact agricultural fen peat cores (80 cm, 20 cm Ø) to five different water levels (0, 20, 40, 60 cm and variable—surface), two nutrient application levels to mimic continued agricultural use, and two water origins. Over an eight-month period, we harvested above-ground plant biomass five times and sampled pore water at two depths each month. Samples were analysed for nutrients. Our results show increased phosphate and ammonium availability upon fully rewetting (0 cm—surface) and less so under partially rewetted circumstances (20 cm—surface). Above-ground biomass was strongly affected by nutrient application, especially in the high water level treatments. Vegetation was primarily N-limited, and N in the vegetation decreased with increasing water levels, indicating stronger nitrogen limitation upon rewetting. We conclude that nature restoration under fully rewetted conditions will likely be challenging as a result of the large release of nutrients from the system which may also affect surrounding nature areas. Furthermore, we conclude that partial rewetting combined with low-intensity agricultural use can be a solution to slow down the adverse effects of drainage, although this will lead to decreased agricultural production.

El Niño-induced drought, which is intensified by climate change, can have huge impacts on soil microbial biomass and plant productivity in tropical forests. We tested whether drought-induced turnover of soil microbial biomass can be a potential source of phosphorus (P), the limiting nutrient, for the reproduction of tropical forest trees (mast fruiting). We measured the seasonal variations in soil microbial biomass P and soil solution P concentrations including the periods before and after drought in a dipterocarp forest in Indonesia. Drought resulted in a decrease in soil microbial biomass C, N, and P, followed by a recovery after re-wetting. There was a sharp peak of soil solution P concentrations during the drought. The significant difference between soil microbial biomass P before and after drought amounted to 2.0 kg P ha⁻¹. The potential P release from microbial turnover is not negligible compared to the additional P demand for fruit production (1.0 kg P ha⁻¹) as well as the annual demand for litter production (2.5 kg P ha⁻¹ year⁻¹). In addition to the accumulation of nutrients for several non-fruiting years and their re-distribution in tree biomass, drought-induced microbial turnover can be nutrient subsidies for dipterocarp reproduction in highly-weathered soils.

The effects of nitrogen (N) deposition on forests largely depend on the ecosystem N status and the fates of deposited N. Boreal forests are typically N-limited ecosystems and are considered to be more efficient in retaining deposited N relative to temperate and tropical forests. As a primary disturbance in boreal forests, wildfires may alleviate N limitation in the burned ecosystem and increase mineralization, resulting in the altered outcomes of the N deposition. In order to explore the effects of a severe wildfire on the retention of deposited N, we investigated the fates of newly deposited N in burned and unburned boreal larch forests by applying ¹⁵NH₄NO₃ tracers to the forest floors. Results showed that total ecosystem retention for the deposited N was 60% in the forest recovering from a severe wildfire burned five years ago, significantly lower than in the unburned mature forest (89%). The difference was mainly attributed to the substantially lower retention in vegetation (8.3%) in the burned site than in the unburned forest (32.4%), as tracer recoveries in soil were similar (51.2 and 56.6%, respectively). Although most ¹⁵N tracer was immobilized in organic soil in both burned and unburned forests (33 and 47%, respectively), a noticeably higher amount of ¹⁵N was found in mineral soil in the burned forest (19%) than in the unburned forest (10%), suggesting mineral soil as a significant sink for N deposition in the burned forest. A higher total ¹⁵N retention in the unburned forest implies that more new N input may stimulate C sequestration and promote the productivity of the Eurasian boreal forest under the background of atmospheric N deposition. However, a considerable amount of deposited N may be lost from the disturbed boreal larch forest ecosystem after a severe wildfire.

The unique pectin-like carbohydrate “sphagnan” has been shown to protect organic matter from microbial decomposition in Sphagnum-dominated peatlands. However, the bioavailability of sphagnan has not been evaluated, and it is not known if it persists or continues to affect decomposition processes over the long timescales of peat formation. To address this, we assessed the connection between sphagnan content and organic matter decomposition rates in a temperate peatland near Fennville, MI, USA. We compared the effects of sphagnan over two timescales: (1) a short-term litter incubation assay using mosses from different peatland microtopographies; and (2) oxic and anoxic incubation assays of peat collected from multiple depths within the peat profile, reflecting a natural long-term decomposition continuum. On both timescales, we hypothesized that higher sphagnan content would be associated with lower decomposition rates, and that sphagnan would be selectively preserved compared to bulk C and other carbohydrates. The litter decomposition experiment supported both hypotheses, as higher sphagnan content was associated with lower mass loss, and sphagnan content increased due to selective preservation. In the peat, we observed weak but significant correlations between the relative abundance of sphagnan (as a fraction of total non-cellulosic sugars) and both aerobic and anaerobic respiration rates. This relationship was stronger in cores collected from hollow microtopographies than those from hummocks. However, there was not a significant relationship between respiration rates and the total (C-normalized) sphagnan content. Sphagnan content increased with depth in the peat profile, indicating selective preservation compared to bulk C. Additionally, we observed the accumulation of non-cellulosic glucose in the deep peat, likely derived from microbial exopolysaccharides. Together, these results indicate that sphagnan persists in the catotelm and continues to contribute to the long-term stabilization of organic matter in Sphagnum-rich peatlands, although the weak relationship with respiration indicates that its influence is relatively minor.

Organic peat soils occupy relatively little of the global land surface area but store vast amounts of soil carbon in northern latitudes where climate is warming at a rapid pace. Warming may result in strong positive feedbacks of carbon loss and global climate change driven by microbial processes if warming alters the balance between primary productivity and decomposition. To elucidate effects of warming on the microbial communities mediating peat carbon dynamics, we explored the abundance of broad microbial groups and their source of carbon (i.e. old carbon versus more recently fixed photosynthate) using microbial lipid analysis (δ¹³C PLFA) of peat samples under ambient temperatures and before/after initiation of experimental peat warming (+ 2.25, + 4.5, + 6.75, and + 9 °C). This analysis occurred over a profile to 2 m depth in an undrained, ombrotrophic peat bog in northern Minnesota. We found that the total microbial biomass and individual indicator lipid abundances were stratified by depth and strongly correlated to temperature under ambient conditions. However, under experimental warming, statistically significant effects of temperature on the microbial community were sporadic and inconsistent. For example, 3 months after experimental warming the relative abundance of Gram-negative bacterial indicators across depth combined and > 50 cm depth and Gram-positive bacterial indicators at 20–50 cm depth showed significant positive relationships to temperature. At that same timepoint, however, the relative abundance of Actinobacterial indicators across depth showed a significant negative relationship to temperature. After 10 months of experimental warming, the relative abundance of fungal biomarkers was positively related to temperature in all depths combined, and the absolute abundance of anaerobic bacteria declined with increasing temperature in the 20–50 cm depth interval. The lack of observed response in the broader microbial community may suggest that at least initially, microbial community structure with peat depth in these peatlands is driven more by bulk density and soil water content than temperature. Alternatively, the lack of broad microbial community response may simply represent a lag period, with more change to come in the future. The long-term trajectory of microbial response to warming in this ecosystem then could either be direct, after this initial lag time, or indirect through other physical or biogeochemical changes in the peat profile. These initial results provide an important baseline against which to measure long-term microbial community and carbon-cycling responses to warming and elevated CO₂.

Non-native ungulates (sheep, goats, and pigs) have significant negative impacts on ecosystem biodiversity, structure, and biogeochemical function throughout the Pacific Islands. Elevated nitrogen (N) availability associated with ungulate disturbance has been shown to promote the success of resource-exploitive invasive plants. While ungulate removal is a common restoration intervention, evaluations of its efficacy typically focus on vegetation responses, rather than underlying nutrient cycling. We used multiple chronosequences of ungulate exclusion (10–24 years duration) in three Hawaiian ecosystems (montane wet forest, dry forest, and dry shrubland) to determine N cycle recovery by characterizing gross mineralization and nitrification, soil inorganic N concentrations and leaching, N₂O emissions, and plant tissue δ¹⁵N. Ungulate removal led to a 1–2 ‰ decline in foliar δ¹⁵N in most species, consistent with a long-term decrease in N fractionation via ecosystem N losses, or a shift in the relative turnover of N forms. This interpretation was supported by significant (dry forest) or trending (wet forest) increases in mineralization and decreases in nitrification, but conflicts with lack of observed change in inorganic N pool sizes or gaseous losses, and increased leaching in the dry forest. While results could indicate that ungulate invasions do not strongly impact N cycling in the first place (no uninvaded control sites exist in Hawai’i to test this hypothesis), this would be inconsistent with observations from other sites globally. Instead, impacts may be spatially patchy across the landscape, or ungulate invasions (possibly in combination with other disturbances) may have permanently shifted biogeochemical function or decoupled elemental cycles. We conclude that eliminating ungulate disturbance alone may not achieve restoration goals related to N cycling within the timeframe examined here.

Invasive plants often alter ecosystem function and processes, especially soil N cycling. In eastern United States forests, the shrub Rosa multiflora (“rose”) is a dominant invader, yet potential effects on N cycling are poorly understood. Moreover, invasive plant management can impact soil N cycling by decreasing plant N uptake and disturbing the soil. The objectives of this study were to evaluate N cycling along a gradient of rose invasion (observational) and investigate potential changes to N cycling (manipulative) under four different management strategies: (1) do nothing (the control), (2) invasive plant removal, (3) removal followed by native seed mix addition, (4) removal, native seed mix, and chipped rose stem addition. We selected three forest sites experiencing a Low, Medium, or High amount of shrub invasion, and measured N cycling in the early (June) and late (September) growing seasons. We found N was immobilized in June and mineralized in September. One year after experimental management, removal alone had no effect on N cycling compared to control plots, but addition of native seed mix and chipped stems reduced early-season nitrification in our Medium invasion site. Our findings suggest that rose invasion may increase N cycling rates when soils are dry, which may occur more frequently with future climate change. In addition, N cycling responds differentially to management in the year following invasive plant removal, but most noticeably under moderate rose invasion.