The effect of nitrogen addition on soil organic matter dynamics: a model analysis of the Harvard Forest Chronic Nitrogen Amendment Study and soil carbon response to anthropogenic N deposition

Recent observations indicate that long-term N additions can slow decomposition, leading to C accumulation in soils, but this process has received limited consideration by models. To address this, we developed a model of soil organic matter (SOM) dynamics to be used with the PnET model and applied it to simulate N addition effects on soil organic carbon (SOC) stocks. We developed the model’s SOC turnover times and responses to experimental N additions using measurements from the Harvard Forest, Massachusetts. We compared model outcomes to SOC stocks measured during the 20th year of the Harvard Forest Chronic Nitrogen Amendment Study, which includes control, low (5 g N m^?2 yr^?1) and high (15 g N m^?2 yr^?1) N addition to hardwood and red pine stands. For unfertilized stands, simulated SOC stocks were within 10 % of measurements. Simulations that used measured changes in decomposition rates in response to N accurately captured SOC stocks in the hardwood low N and pine high N treatment, but greatly underestimated SOC stocks in the hardwood high N and the pine low N treatments. Simulated total SOC response to experimental N addition resulted in accumulation of 5.3–7.9 kg C per kg N following N addition at 5 g N m^?2 yr^?1 and 4.1–5.3 kg C per kg N following N addition at 15 g N m^?2 yr^?1. Model simulations suggested that ambient atmospheric N deposition at the Harvard Forest (currently 0.8 g N m^?2 yr^?1) has led to an increase in cumulative O, A, and B horizons C stocks of 211 g C m^?2 (3.9 kg C per kg N) and 114 g C m^?2 (2.1 kg C per kg N) for hardwood and pine stands, respectively. Simulated SOC accumulation is primarily driven by the modeled decrease in SOM decomposition in the Oa horizon.     

Will CO<Subscript>2</Subscript> Emissions from Drained Tropical Peatlands Decline Over Time? Links Between Soil Organic Matter Quality,Nutrients, and C Mineralization Rates

Conversion, drainage, and cultivation of tropical peatlands can change soil conditions, shifting the C balance of these systems, which is important for the global C cycle. We examined the effect of soil organic matter (SOM) quality and nutrients on CO₂ production from peat decomposition using laboratory incubations of Indonesian peat soils from undrained forest in Kalimantan and drained oil palm plantations in Kalimantan and Sumatra. We found that oil palm soils had higher C/N and lower SOM quality than forest soils. Higher substrate quality and nutrient availability, particularly lower ratios of aromatic/aliphatic carbon and C/N, rather than total SOM or carbon, explained the higher rate of CO₂ production by forest soils (10.80 ± 0.23 µg CO₂–C g C h^?1) compared to oil palm soils (5.34 ± 0.26 µg CO₂–C g C h^?1) from Kalimantan. These factors also explained lower rates in Sumatran oil palm (3.90 ± 0.25 µg CO₂–C g C h^?1). We amended peat with nitrogen (N), phosphorus (P), and glucose to further investigate observed substrate and nutrient constraints across the range of observed peat quality. Available N limited CO₂ production, in unamended and amended soils. P addition raised CO₂ production when substrate quality was high and initial P state was low. Glucose addition raised CO₂ production in the presence of added N and P. Our results suggest that decline in SOM quality and nutrients associated with conversion may decrease substrate-driven rates of CO₂ production from peat decomposition over time.     

Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways

Rapid increases in human population and land transformation in arid and semi-arid regions are altering water, carbon (C) and nitrogen (N) cycles, yet little is known about how urban ephemeral stream channels in these regions affect biogeochemistry and trace gas fluxes. To address these knowledge gaps, we measured carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄) before and after soil wetting in 16 ephemeral stream channels that vary in soil texture and organic matter in Tucson, AZ. Fluxes of CO₂ and N₂O immediately following wetting were among the highest ever published (up to 1,588 mg C m^?2 h^?1 and 3,121 μg N m^?2 h^?1). Mean post-wetting CO₂ and N₂O fluxes were significantly higher in the loam and sandy loam channels (286 and 194 mg C m^?2 h^?1; 168 and 187 μg N m^?2 h^?1) than in the sand channels (45 mg C m^?2 h^?1 and 7 μg N m^?2 h^?1). Factor analyses show that the effect of soil moisture, soil C and soil N on trace gas fluxes varied with soil texture. In the coarser sandy sites, trace gas fluxes were primarily controlled by soil moisture via physical displacement of soil gases and by organic soil C and N limitations on biotic processes. In the finer sandy loam sites trace gas fluxes and N-processing were primarily limited by soil moisture, soil organic C and soil N resources. In the loam sites, finer soil texture and higher soil organic C and N enhance soil moisture retention allowing for more biologically favorable antecedent conditions. Variable redox states appeared to develop in the finer textured soils resulting in wide ranging trace gas flux rates following wetting. These findings indicate that urban ephemeral channels are biogeochemical hotspots that can have a profound impact on urban C and N biogeochemical cycling pathways and subsequently alter the quality of localized water resources.     

The importance of denitrification for N2O emissions from an N-saturated forest in SW China: results from in situ 15N labeling experiments

Long-term elevated atmogenic deposition (~5 g m^?2 year^?1) of reactive nitrogen (N) causes N saturation in forests of subtropical China which may lead to high nitrous oxide (N₂O) emissions. Recently, we found high N₂O emission rates (up to 1,730 μg N₂O–N m^?2 h^?1) during summer on well-drained acidic acrisols (pH = 4.0) along a hill slope in the forested Tieshanping catchment, Chongqing, southwest China. Here, we present results from an in situ ¹⁵N–NO₃ ^? labeling experiment to assess the contribution of nitrification and denitrification to N₂O emissions in these soils. Two loads of 99 at.% K¹⁵NO₃ (equivalent to 0.2 and 1.0 g N m^?2) were applied as a single dose to replicated plots at two positions along the hill slope (at top and bottom, respectively) during monsoonal summer. During a 6-day period after label application, we found that 71–100 % of the emitted N₂O was derived from the labeled NO₃ ^? pool irrespective of slope position. Based on this, we assume that denitrification is the dominant process of N₂O formation in these forest soils. Within 6 days after label addition, the fraction of the added ¹⁵N–NO₃ ^? emitted as ¹⁵N–N₂O was highest at the low-N addition plots (0.2 g N m^?2), amounting to 1.3 % at the top position of the hill slope and to 3.2 % at the bottom position, respectively. Our data illustrate the large potential of acid forest soils in subtropical China to form N₂O from excess NO₃ ^? most likely through denitrification.     

Gaseous and fluvial carbon export from an Amazon forest watershed

The transfer of carbon (C) from Amazon forests to aquatic ecosystems as CO₂ supersaturated in groundwater that outgases to the atmosphere after it reaches small streams has been postulated to be an important component of terrestrial ecosystem C budgets. We measured C losses as soil respiration and methane (CH₄) flux, direct CO₂ and CH₄ fluxes from the stream surface and fluvial export of dissolved inorganic C (DIC), dissolved organic C (DOC), and particulate C over an annual hydrologic cycle from a 1,319-ha forested Amazon perennial first-order headwater watershed at Tanguro Ranch in the southern Amazon state of Mato Grosso. Stream pCO₂ concentrations ranged from 6,491 to 14,976 ??atm and directly-measured stream CO₂ outgassing flux was 5,994 ± 677 g C m^?2 y^?1 of stream surface. Stream pCH₄ concentrations ranged from 291 to 438 ??atm and measured stream CH₄ outgassing flux was 987 ± 221 g C m^?2 y^?1. Despite high flux rates from the stream surface, the small area of stream itself (970 m², or 0.007% of watershed area) led to small directly-measured annual fluxes of CO₂ (0.44 ± 0.05 g C m² y^?1) and CH₄ (0.07 ± 0.02 g C m² y^?1) per unit watershed land area. Measured fluvial export of DIC (0.78 ± 0.04 g C m^?2 y^?1), DOC (0.16 ± 0.03 g C m^?2 y^?1) and coarse plus fine particulate C (0.001 ± 0.001 g C m^?2 y^?1) per unit watershed land area were also small. However, stream discharge accounted for only 12% of the modeled annual watershed water output because deep groundwater flows dominated total runoff from the watershed. When C in this bypassing groundwater was included, total watershed export was 10.83 g C m^?2 y^?1 as CO₂ outgassing, 11.29 g C m^?2 y^?1 as fluvial DIC and 0.64 g C m^?2 y^?1 as fluvial DOC. Outgassing fluxes were somewhat lower than the 40?C50 g C m^?2 y^?1 reported from other Amazon watersheds and may result in part from lower annual rainfall at Tanguro. Total stream-associated gaseous C losses were two orders of magnitude less than soil respiration (696 ± 147 g C m^?2 y^?1), but total losses of C transported by water comprised up to about 20% of the ± 150 g C m^?2 (±1.5 Mg C ha^?1) that is exchanged annually across Amazon tropical forest canopies.     

Correlation between leaf litter and fine root decomposition among subtropical tree species

Sampling disturbance has been shown to rapidly increase net nitrification rates in some forest soils. To gain insight on mechanisms, we investigated both gross and net rates of ammonification and nitrification in intact cores and mixed composite samples. Using the isotope pool dilution method, we studied samples from two northeastern USA watersheds, Brush Brook and Sleepers River in Vermont, where previous work had found high net nitrification rates. Gross ammonification was usually not significantly different between intact cores and mixed samples. However, gross and net nitrification rates in mixed samples were similar (mean ～24?µmol N?kg^?1?hr^?1 or ～8 mg N kg^?1 d^?1) and significantly higher than in intact cores (7.7 and 3.4?µmol N kg^?1?h^?1 for means of gross and net respectively). Nitrate consumption was decreased somewhat by disturbance but did not account for the large differences in net rates. Because there were similar gross ammonification rates in both treatments, increased nitrification in these disturbed soils must be a result of an increase in the utilization of ammonium by the ammonia oxidizers at the expense of other ammonium consumption pathways. Different mechanisms may operate in different soils; increased nitrification appears to be the primary pathway in these soils with high N cycling rates.     

Net ecosystem carbon exchange and the greenhouse gas balance of tidal marshes along an estuarine salinity gradient

Tidal wetlands are productive ecosystems with the capacity to sequester large amounts of carbon (C), but we know relatively little about the impact of climate change on wetland C cycling in lower salinity (oligohaline and tidal freshwater) coastal marshes. In this study we assessed plant production, C cycling and sequestration, and microbial organic matter mineralization at tidal freshwater, oligohaline, and salt marsh sites along the salinity gradient in the Delaware River Estuary over four years. We measured aboveground plant biomass, carbon dioxide (CO₂) and methane (CH₄) exchange between the marsh and atmosphere, microbial sulfate reduction and methanogenesis in marsh soils, soil biogeochemistry, and C sequestration with radiodating of soils. A simple model was constructed to estimate monthly and annually integrated rates of gross ecosystem production (GEP), ecosystem respiration (ER) to carbon dioxide ( \( {\text{ER}}_{{{\text{CO}}_{2} }} \) ) or methane ( \( {\text{ER}}_{{{\text{CH}}_{4} }} \) ), net ecosystem production (NEP), the contribution of sulfate reduction and methanogenesis to ER, and the greenhouse gas (GHG) source or sink status of the wetland for 2 years (2007 and 2008). All three marsh types were highly productive but evidenced different patterns of C sequestration and GHG source/sink status. The contribution of sulfate reduction to total ER increased along the salinity gradient from tidal freshwater to salt marsh. The Spartina alterniflora dominated salt marsh was a C sink as indicated by both NEP (~140 g C m^?2 year^?1) and ²¹⁰Pb radiodating (336 g C m^?2 year^?1), a minor sink for atmospheric CH₄, and a GHG sink (~620 g CO_2-eq m^?2 year^?1). The tidal freshwater marsh was a source of CH₄ to the atmosphere (~22 g C–CH₄ m^?2 year^?1). There were large interannual differences in plant production and therefore C and GHG source/sink status at the tidal freshwater marsh, though ²¹⁰Pb radiodating indicated modest C accretion (110 g C m^?2 year^?1). The oligohaline marsh site experienced seasonal saltwater intrusion in the late summer and fall (up to 10 mS cm^?1) and the Zizania aquatica monoculture at this site responded with sharp declines in biomass and GEP in late summer. Salinity intrusion was also linked to large effluxes of CH₄ at the oligohaline site (>80 g C–CH₄ m^?2 year^?1), making this site a significant GHG source (>2,000 g CO_2-eq m^?2 year^?1). The oligohaline site did not accumulate C over the 2 year study period, though ²¹⁰Pb dating indicated long term C accumulation (250 g C m^?2 year^?1), suggesting seasonal salt-water intrusion can significantly alter C cycling and GHG exchange dynamics in tidal marsh ecosystems.     

Interactions between N application rate, CH4 oxidation and N2O production in soil

Here we report on a controlled environment experiment in which we applied ¹³C- and ¹⁵N-enrichment approaches to quantify methane oxidation rates and source partition N₂O production in a silt loam soil following application of NH₄NO₃, enabling us to look for potential interactions between methane oxidation and nitrifier-N₂O production. ¹⁵N-N₂O, ¹⁴⁺¹⁵N-N₂O and CO₂ fluxes and mineral N concentrations were measured over a 23-day period after application of NH₄NO₃ (5 at.% excess ¹⁵N) at rates of 0, 5, 10, 20, 30 and 40 g N m^?2 to a silt loam soil. Change in ^12/13C-CH₄ concentrations (as indicative of ¹³C-CH₄ oxidation rates) and production of ¹³C-CO₂ were monitored over the first 72 h after addition of 1.7 ??l ¹³C-CH₄ l^?1 (10 at.% excess ¹³C) to these N treatments. Oxidation of applied ¹³C-CH₄ was slower in the 5, 10, 20 and 30 g N m^?2 (5 at.% excess ¹⁵N) treatments (0.24?C0.32 ??g ¹³C-CH₄ l^?1 day^?1) than in the control (0.40 ??g ¹³C-CH₄ l^?1 day^?1), suggesting that these N loadings inhibited oxidation. N₂O production was raised after N addition, and in the 10, 20 and 30 g N m^?2 treatments nitrification was the predominant source of N₂O accounting for 61, 83 and 57% of the total ¹⁵N-N₂O produced, respectively. Our results point towards the possibility of methylotrophs switching function to oxidise ammonia in the presence of N, which may result in greater atmospheric loading of both CH₄ and N₂O.     

Net sediment N2 fluxes in a southern New England estuary: variations in space and time

Over the past three decades, Narragansett Bay has undergone various ecological changes, including significant decreases in water column chlorophyll a concentrations, benthic oxygen uptake, and benthic nutrient regeneration rates. To add to this portrait of change, we measured the net flux of N₂ across the sediment–water interface over an annual cycle using the N₂/Ar technique at seven sites in the bay for comparison with measurements made decades ago. Net denitrification rates ranged from about 10–90 μmol N₂–N m^?2 h^?1 over the year. Denitrification rates were not significantly different among sites and had no clear correlation with temperature. Net nitrogen fixation (?5 to ?650 μmol N₂–N m^?2 h^?1) was measured at three sites and only observed in summer (June–August). Neither denitrification nor nitrogen fixation exhibited a consistent relationship with sediment oxygen demand or with fluxes of nitrite, nitrate, ammonium, total dissolved inorganic nitrogen, or dissolved inorganic phosphate across all stations. In contrast to the mid-bay historical site where denitrification rates have declined, denitrification rates in the Providence River Estuary have not changed significantly over the past 30 years.     

Influence of hydrological connectivity of riverine wetlands on nitrogen removal via denitrification

Wetland ecosystems in agricultural areas often become progressively more isolated from main water bodies. Stagnation favors the accumulation of organic matter as the supply of electron acceptors with water renewal is limited. In this context it is expected that nitrogen recycling prevails over nitrogen dissipation. To test this hypothesis, denitrification rates, fluxes of dissolved oxygen (SOD), inorganic carbon (DIC) and nitrogen and sediment features were measured in winter and summer 2007 on 22 shallow riverine wetlands in the Po River Plain (Northern Italy). Fluxes were determined from incubations of intact cores by measurement of concentration changes or isotope pairing in the case of denitrification. Sampled sites were eutrophic to hypertrophic; 10 were connected and 12 were isolated from the adjacent rivers, resulting in large differences in nitrate concentrations in the water column (from <5 to 1,133 μM). Benthic metabolism and denitrification rates were investigated by two overarching factors: season and hydrological connectivity. SOD and DIC fluxes resulted in respiratory quotients greater than one at most sampling sites. Sediment respiration was coupled to both ammonium efflux, which increased from winter to summer, and nitrate consumption, with higher rates in river-connected wetlands. Denitrification rates measured in river-connected wetlands (35–1,888 μmol N m^?2 h^?1) were up to two orders of magnitude higher than rates measured in isolated wetlands (2–231 μmol N m^?2 h^?1), suggesting a strong regulation of the process by nitrate availability. These rates were also significantly higher in summer (9–1,888 μmol N m^?2 h^?1) than in winter (2–365 μmol N m^?2 h^?1). Denitrification supported by water column nitrate (D_W) accounted for 60–100% of total denitrification (Dtot); denitrification coupled to nitrification (D_N) was probably controlled by limited oxygen availability within sediments. Denitrification efficiency, calculated as the ratio between N removal via denitrification and N regeneration, and the relative role of denitrification for organic matter oxidation, were high in connected wetlands but not in isolated sites. This study confirms the importance of restoring hydraulic connectivity of riverine wetlands for the maintenance of important biogeochemical functions such as nitrogen removal via denitrification.     

The role of gross and net N transformation processes and NH4 + and NO3 − immobilization in controlling the mineral N pool of a temperate mixed deciduous forest soil

In many forests of Europe and north-eastern North America elevated N deposition has opened the forest N cycle, resulting in NO₃ ^? leaching. On the other hand, despite this elevated N deposition, the dominant fate of NO₃ ^? and NH₄ ⁺ in some of these forests is biotic or abiotic immobilization in the soil organic matter pool, preventing N losses. The environmental properties controlling mineral N immobilization and the variation and extent of mineral N immobilization in forest soils are not yet fully understood. In this study we investigated a temperate mixed deciduous forest, which is subjected to an average N deposition of 36.5 kg N ha^?1 yr^?1, but at the same time shows low NO₃ ^? concentrations in the groundwater. The aim of this study was to investigate whether the turnover rate of the mineral N pool could explain these low N leaching losses. A laboratory ¹⁵N pool dilution experiment was conducted to study gross and net N mineralization and nitrification and mineral N immobilization in the organic and uppermost (0–10 cm) mineral layer of the forest soil. Two locations, one at the forest edge (GE) and another one 145 m inside the forest (GF1), were selected. In the organic layers of GE and GF1, the gross N mineralization averaged 10.9 and 11.1 mg N kg^?1 d^?1, the net N mineralization averaged 6.1 and 6.8 mg N kg^?1 d^?1 and NH₄ ⁺ immobilization rates averaged 3.8 and 3.6 mg N kg^?1 d^?1. In the organic layer of GE and GF1, the average gross nitrification was 3.8 and 4.6 mg N kg^?1 d^?1, the average net nitrification was ?25.2 and ?31.3 mg N kg^?1 d^?1 and the NO₃ ^? immobilization rates averaged 29.0 and 35.9 mg N kg^?1 d^?1. For the mineral (0–10 cm) layer the same trend could be observed, but the N transformation rates were much lower for the NH₄ ⁺ pool and not significantly different from zero for the NO₃ ^? pool. Except for the turnover of the NH₄ ⁺ pool in the mineral layer, no significant differences were observed between location GE and GF1. The ratio of NH₄ ⁺ immobilization to gross N mineralization, gross N mineralization to gross nitrification, and NO₃ ^? immobilisation to gross nitrification led to the following observations. The NH₄ ⁺ pool of the forest soil was controlled by N mineralization and NO₃ ^? immobilization was importantly controlling the forest NO₃ ^? pool. Therefore it was concluded that this process is most probably responsible for the limited NO₃ ^? leaching from the forest ecosystem, despite the chronically high N deposition rates.     

Plant and soil responses of an alpine steppe on the Tibetan Plateau to multi-level nitrogen addition

Background

Although plant growth in alpine steppes on the Tibetan Plateau has been suggested to be sensitive to nitrogen (N) addition, the N limitation conditions of alpine steppes remain uncertain.

Methods

After 2 years of fertilization with NH₄NO₃ at six rates (0, 10, 20, 40, 80 and 160 kg N ha^?1 yr^?1), the responses of plant and soil parameters as well as N₂O fluxes were measured.

Results

At the vegetation level, N addition resulted in an increase in the aboveground N pool from 0.5?±?0.1 g m^?2 in the control plots to 1.9?±?0.2 g m^?2 in the plots at the highest N input rate. The aboveground C pool, biomass N concentration, foliar δ¹⁵N, soil NO₃ ^?-N and N₂O flux were also increased by N addition. However, as the N fertilization rate increased from 10 kg N ha^?1 yr^?1 to 160 kg N ha^?1 yr^?1, the N-use efficiency decreased from 12.3?±?4.6 kg C kg N^?1 to 1.6?±?0.2 kg C kg N^?1, and the N-uptake efficiency decreased from 43.2?±?9.7 % to 9.1?±?1.1 %. Biomass N:P ratios increased from 14.4?±?2.6 in the control plots to 20.5?±?0.8 in the plots with the highest N input rate. Biomass N:P ratios, N-uptake efficiency and N-use efficiency flattened out at 40 kg N ha^?1 yr^?1. Above this level, soil NO₃ ^?-N began to accumulate. The seasonal average N₂O flux of growing season nonlinearly increased with increased N fertilization rate and linearly increased with the weighted average foliar δ¹⁵N. At the species level, N uptake responses to relative N availability were species-specific. Biomass N concentration of seven out of the eight non-legume species increased significantly with N fertilization rates, while Kobresia macrantha and the one legume species (Oxytropics glacialis) remained stable. Both the non-legume and the legume species showed significant ¹⁵N enrichment with increasing N fertilization rate. All non-legume species showed significant increased N:P ratios with increased N fertilization rate, but not the legume species.

Conclusions

Our findings suggest that the Tibetan alpine steppes might be N-saturated above a critical N load of 40 kg N ha^?1 yr^?1. For the entire Tibetan Plateau (ca. 2.57 million km²), a low N deposition rate (10 kg N ha^?1 yr^?1) could enhance plant growth, and stimulate aboveground N and C storage by at least 1.1?±?0.3 Tg N yr^?1 and 31.5?±?11.8 Tg C yr^?1, respectively. The non-legume species was N-limited, but the legume species was not limited by N.     

Sediment microstructure and resuspension behavior depend on each other

The critical shear stress of resuspension and rates of erosion for cohesive and loosely structured sediments must be obtained by direct measurements since there is no theoretical calculation. An in situ experiment on sediment resuspension was performed in a shallow lake (Langer See, NE Germany; area = 1.27 km², z_max = 3.8 m) in summer 2006 using a hydrodynamically calibrated erosion chamber (Ø 20 cm). Shear velocity (u*) was incrementally increased in 11 steps (0–2.19 cm s^?1) to initiate resuspension events. Entrainment rates (E) of suspended particulate matter (E_SPM), total P (E_TP), chlorophyll a (E_Chl a), and soluble reactive P (E_SRP) were determined by mass balance. Two subsequent critical u* (0.53 cm s^?1 and 1.48 cm s^?1) support the ‘two-layered bed’ model of a fluffy surface aggregate layer (freshly deposited phytodetritus prone to resuspension) and an underlying more consolidated biostabilised layer. Patterns in E_SPM (2–106 g m^?2 h^?1), E_TP (11–532 mg m^?2 h^?1), and E_Chl a (3–24 μg m^?2 h^?1) revealed a sediment surface maximum of TP and Chl a and their theoretical vertical logarithmic decrease within 4 mm sediment depth, the maximum thickness of sediment layer entrained. The advective E_SRP flux (17 mg m^?2 h^?1) was 43 times higher than the diffusive SRP flux (0.4 mg m^?2 h^?1). The TP and Chl a micro-profiles suggest that cohesive sediment bed formation is a function of both settling (fluff) and consolidation (biostabilisation). Thus, sediment microstructure and resuspension behavior depend on each other.     

Biogeochemical regime shifts in coastal landscapes: the contrasting effects of saltwater incursion and agricultural pollution on greenhouse gas emissions from a freshwater wetland

Many coastal plain wetlands receive nutrient pollution from agricultural fields and are particularly vulnerable to saltwater incursion. Although wetlands are a major source of the greenhouse gases methane (CH₄) and nitrous oxide (N₂O), the consequences of salinization for greenhouse gas emissions from wetlands with high agricultural pollution loads is rarely considered. Here, we asked how saltwater exposure alters greenhouse gas emissions from a restored freshwater wetland that receives nutrient loading from upstream farms. During March to November 2012, we measured greenhouse gases along a ~2 km inundated portion of the wetland. Sampling locations spanned a wide chemical gradient from sites receiving seasonal fertilizer nitrogen and sulfate (SO₄ ^2?) loads to sites receiving seasonal increases in marine salts. Concentrations and fluxes of CH₄ were low (<100 µg L^?1 and <10 mg m^?2 h^?1) for all sites and sampling dates when SO₄ ^2? was high (>10 mg L^?1), regardless of whether the SO₄ ^2? source was agriculture or saltwater. Elevated CH₄ (as high as 1,500 µg L^?1 and 45 mg m^?2 h^?1) was only observed on dates when air temperatures were >27 °C and SO₄ ^2? was <10 mg L^?1. Despite elevated ammonium (NH₄ ⁺) for saltwater exposed sites, concentrations of N₂O remained low (<5 µg L^?1 and <10 µg m^?2 h^?1), except when fertilizer derived nitrate (NO₃ ^?) concentrations were high and N₂O increased as high as 156 µg L^?1. Our results suggest that although both saltwater and agriculture derived SO₄ ^2? may suppress CH₄, increases in N₂O associated with fertilizer derived NO₃ ^? may offset that reduction in wetlands exposed to both agricultural runoff and saltwater incursion.     

Nutrient Biogeochemistry During the Early Stages of Delta Development in the Mississippi River Deltaic Plain

Nutrient biogeochemistry associated with the early stages of soil development in deltaic floodplains has not been well defined. Such a model should follow classic patterns of soil nutrient pools described for alluvial ecosystems that are dominated by mineral matter high in phosphorus and low in carbon and nitrogen. A contrast with classic models of soil development is the anthropogenically enriched high nitrate conditions due to agricultural fertilization in upstream watersheds. Here we determine if short-term patterns of soil chemistry and dissolved inorganic nutrient fluxes along the emerging Wax Lake delta (WLD) chronosequence are consistent with conceptual models of long-term nutrient availability described for other ecosystems. We add a low nitrate treatment more typical of historic delta development to evaluate the role of nitrate enrichment in determining the net dinitrogen (N₂) flux. Throughout the 35-year chronosequence, soil nitrogen and organic matter content significantly increased by an order of magnitude, whereas phosphorus exhibited a less pronounced increase. Under ambient nitrate concentrations (>60 μM), mean net N₂ fluxes (157.5 μmol N m^?2 h^?1) indicated greater rates of gross denitrification than gross nitrogen fixation; however, under low nitrate concentrations (<2 μM), soils switched from net denitrification to net nitrogen fixation (?74.5 μmol N m^?2 h^?1). As soils in the WLD aged, the subsequent increase in organic matter stimulated net N₂, oxygen, nitrate, and nitrite fluxes producing greater fluxes in more mature soils. In conclusion, soil nitrogen and carbon accumulation along an emerging delta chronosequence largely coincide with classic patterns of soil development described for alluvial floodplains, and substrate age together with ambient nitrogen availability can be used to predict net N₂ fluxes during early delta evolution.     

Nitrous oxide and methane exchange in two small temperate forest catchments—effects of hydrological gradients and implications for global warming potentials of forest soils

The magnitude of greenhouse gas (GHG) flux rates may be important in wet and intermediate wet forest soils, but published estimates are scarce. We studied the surface exchange of methane (CH₄) and nitrous oxide (N₂O) from soil along toposequences in two temperate deciduous forest catchments: Strødam and Vestskoven. The soil water regime ranged from fully saturated to aerated within the catchments. At Strødam the largest mean flux rates of N₂O (15 μg N₂O-N m^?2 h^?1) were measured at volumetric soil water contents (SWC) between 40 and 60% and associated with low soil pH compared to smaller mean flux rates of 0-5 μg N₂O-N m^?2 h^?1 for drier (SWC < 40%) and wet conditions (SWC > 80%). At Vestskoven the same response of N₂O to soil water content was observed. Average CH₄ flux rates were highly variable along the toposequences (?17 to 536 μg CH₄-C m^?2 h^?1) but emissions were only observed above soil water content of 45%. Scaled flux rates of both GHGs to catchment level resulted in emission of 322 and 211 kg CO₂-equivalents ha^?1 year^?1 for Strødam and Vestskoven, respectively, with N₂O contributing the most at both sites. Although the wet and intermediate wet forest soils occupied less than half the catchment area at both sites, the global warming potential (GWP) derived from N₂O and CH₄ was more than doubled when accounting for these wet areas in the catchments. The results stress the importance of wet soils in assessments of forest soil global warming potentials, as even small proportions of wet soils contributes substantially to the emissions of N₂O and CH₄.     

Quantitative aspects of oxygen and carbon dioxide exchange through the lungs in Ocypode ceratophthalmus (Crustacea: Decapoda) during rest and exercise in water and air

Ghost crabs Ocypode ceratophthalmus were exercised in air and water to measure CO₂ and O₂ exchange rates using the method of instantaneous measurements of oxygen consumption rate (MO₂) where applicable. Average heart rate increased from 100 to nearly 400 pulses per minute after five minutes of exercise on a treadmill at a run rate of 0.133 m s^?1. It took less than a minute for oxygen taken up through the lung epithelium from the air inside the branchial cavity to reach the maximal oxygen consumption rate of 26.1 mmol O₂ kg^?1 h^?1. Resting MO₂ was 4.06 mmol O₂ kg^?1 h^?1 in air, but decreased to 3.37 mmol O₂ kg^?1 h^?1 in seawater. Radioactive CO₂ from injected l-lactate is released linearly by the lung. The percent accumulated 14-CO₂ in exhaled air, plotted against time, intersects zero time on the x -axis, indicating rapid gas exchange at the lung surface. The P ₅₀ values for native haemocyanin of 4.89 mm Hg before exercise, and 8.99 mm Hg after exercise, are typical of a high-affinity haemocyanin usually associated with terrestrial crabs. The current notion that Ocypode ceratophthalmus drown when submerged in seawater was not substantiated by our experiments. MO₂ in seawater increased from 3.37 mmol O₂ kg^?1 h^?1 for resting crabs to 5.72 mmol O₂ kg^?1 h^?1 during exercise. When submerged by wave-seawater in the natural environment and during exercise in respirometer-seawater O. ceratophthalmus do not swim but, having a specific density of 1.044, float nearly weightless with a minimum of body movements.     

Measurement of net ecosystem exchange,productivity and respiration in three spruce forests in Sweden shows unexpectedly large soil carbon losses

Measurement of net ecosystem exchange was made using the eddy covariance method above three forests along a north-south climatic gradient in Sweden: Flakaliden in the north, Knottåsen in central and Asa in south Sweden. Data were obtained for 2 years at Flakaliden and Knottåsen and for one year at Asa. The net fluxes (N_ep) were separated into their main components, total ecosystem respiration (R_t) and gross primary productivity (P_g). The maximum half-hourly net uptake during the heart of the growing season was highest in the southernmost site with ?0.787 mg CO₂ m^?2 s^?1 followed by Knottåsen with ?0.631 mg CO₂ m^?2 s^?1 and Flakaliden with ?0.429 mg CO₂ m^?2 s^?1. The maximum respiration rates during the summer were highest in Knottåsen with 0.245 mg CO₂ m^?2 s^?1 while it was similar at the two other sites with 0.183 mg CO₂ m^?2 s^?1. The annual N_ep ranged between uptake of ?304 g C m^?2 year^?1 (Asa) and emission of 84 g C m^?2 year^?1 (Knottåsen). The annual R_t and P_g ranged between 793 to 1253 g C m^?2 year^?1 and ?875 to ?1317 g C m^?2 year^?1, respectively. Biomass increment measurements in the footprint area of the towers in combination with the measured net ecosystem productivity were used to estimate the changes in soil carbon and it was found that the soils were losing on average 96–125 g C m^?2 year^?1. The most plausible explanation for these losses was that the studied years were much warmer than normal causing larger respiratory losses. The comparison of net primary productivity and P_g showed that ca 60% of P_g was utilized for autotrophic respiration.     

Heterotrophic soil respiration and soil carbon dynamics in the deciduous Hainich forest obtained by three approaches

Three different approaches were used to calculate heterotrophic soil respiration (Rh) and soil carbon dynamics in an old-growth deciduous forest in central Germany. A root and mycorrhiza exclosure experiment in the field separated auto- and heterotrophic soil respiration. It was compared to modeled heterotrophic respiration resulting from two different approaches: a modular component model of soil respiration calculated autotrophic and heterotrophic soil respiration with litter, climate and canopy photosynthesis as input variables. It was calibrated by independent soil respiration measurements in the field. A second model was calibrated by incubation of soil samples from different soil layers in the laboratory. In this case, the annual sum of Rh was calculated by an empirical model including response curves to temperature and a soil moisture. The three approaches showed good accordance during spring and summer and when the annual sums of Rh calculated by the two models were compared. Average Rh for the years 2002–2006 were 436 g C m^?2 year^?1 (field model) and 417 g C m^?2 year^?1 (lab-model), respectively. Differences between the approaches revealed specific limitations of each method. The average carbon balance of the Hainich forest soil was estimated to be between 1 and 35 g C m^?2 year^?1 depending on the model used and the averaging period. A comparison with nighttime data from eddy covariance (EC) showed that EC data were lower than modelled soil respiration in many situations. We conclude that better filter methods for EC nighttime data have to be developed.     

Complete and simultaneous removal of ammonium and m-cresol in a nitrifying sequencing batch reactor

The kinetic behavior, oxidizing ability and tolerance to m-cresol of a nitrifying sludge exposed to different initial concentrations of m-cresol (0–150 mg C L^?1) were evaluated in a sequencing batch reactor fed with 50 mg NH₄ ⁺-N L^?1 and operated during 4 months. Complete removal of ammonium and m-cresol was achieved independently of the initial concentration of aromatic compound in all the assays. Up to 25 mg m-cresol-C L^?1 (C/N ratio of 0.5), the nitrifying yield (Y-NO₃ ^?) was 0.86 ± 0.05, indicating that the nitrate was the main product of the process; no biomass growth was detected. From 50 to 150 mg m-cresol-C L^?1 (1.0 ≤ C/N ≤ 3.0), simultaneous microbial growth and partial ammonium-to-nitrate conversion were obtained, reaching a maximum microbial total protein concentration of 0.763 g L^?1 (247 % of its initial value) and the lowest Y-NO₃ ^? 0.53 ± 0.01 at 150 mg m-cresol-C L^?1. m-Cresol induced a significant decrease in the values of both specific rates of ammonium and nitrite oxidation, being the ammonium oxidation pathway the mainly inhibited. The nitrifying sludge was able to completely oxidize up to 150 mg m-cresol-C L^?1 by SBR cycle, reaching a maximum specific removal rate of 6.45 g m-cresol g^?1 microbial protein-N h^?1. The number of SBR cycles allowed a metabolic adaptation of the nitrifying consortium since nitrification inhibition decreased and faster oxidation of m-cresol took place throughout the cycles.