Soil organic matter and litter chemistry response to experimental N deposition in northern temperate deciduous forest ecosystems

The effects of atmospheric nitrogen (N) deposition on organic matter decomposition vary with the biochemical characteristics of plant litter. At the ecosystem‐scale, net effects are difficult to predict because various soil organic matter (SOM) fractions may respond differentially. We investigated the relationship between SOM chemistry and microbial activity in three northern deciduous forest ecosystems that have been subjected to experimental N addition for 2 years. Extractable dissolved organic carbon (DOC), DOC aromaticity, C : N ratio, and functional group distribution, measured by Fourier transform infrared spectra (FTIR), were analyzed for litter and SOM. The largest biochemical changes were found in the sugar maple–basswood (SMBW) and black oak–white oak (BOWO) ecosystems. SMBW litter from the N addition treatment had less aromaticity, higher C : N ratios, and lower saturated carbon, lower carbonyl carbon, and higher carboxylates than controls; BOWO litter showed opposite trends, except for carbonyl and carboxylate contents. Litter from the sugar maple–red oak (SMRO) ecosystem had a lower C : N ratio, but no change in DOC aromaticity. For SOM, the C : N ratio increased with N addition in SMBW and SMRO ecosystems, but decreased in BOWO; N addition did not affect the aromaticity of DOC extracted from mineral soil. All ecosystems showed increases in extractable DOC from both litter and soil in response to N treatment. The biochemical changes are consistent with the divergent microbial responses observed in these systems. Extracellular oxidative enzyme activity has declined in the BOWO and SMRO ecosystems while activity in the SMBW ecosystem, particularly in the litter horizon, has increased. In all systems, enzyme activities associated with the hydrolysis and oxidation of polysaccharides have increased. At the ecosystem scale, the biochemical characteristics of the dominant litter appear to modulate the effects of N deposition on organic matter dynamics.     

Effects of forest conversion into grassland on soil aggregate structure and carbon storage in Panama: evidence from soil carbon fractionation and stable isotopes

Land-use and land-cover strongly influence soil properties such as the amount of soil organic carbon (SOC), aggregate structure and SOC turnover processes. We studied the effects of a vegetation shift from forest to grassland 90 years ago in soils derived from andesite material on Barro Colorado Island (BCI), Panama. We quantified the amount of carbon (C) and nitrogen (N) and determined the turnover of C in bulk soil, water stable aggregates (WSA) of different size classes (<53 μm, 53–250 μm, 250–2000 μm and 2000–8000 μm) and density fractions (free light fraction, intra-aggregate particulate organic matter and mineral associated soil organic C). Total SOC stocks (0–50 cm) under forest (84 Mg C ha⁻¹) and grassland (64 Mg C ha⁻¹) did not differ significantly. Our results revealed that vegetation type did not have an effect on aggregate structure and stability. The investigated soils at BCI did not show higher C and N concentrations in larger aggregates, indicating that organic material is not the major binding agent in these soils to form aggregates. Based on δ¹³C values and treating bulk soil as a single, homogenous C pool we estimated a mean residence time (MRT) of 69 years for the surface layer (0–5 cm). The MRT varied among the different SOC fractions and among depth. In 0–5 cm, MRT of intra-aggregate particulate organic matter (iPOM) was 29 years; whereas mineral associated soil organic C (mSOC) had a MRT of 124 years. These soils have substantial resilience to C and N losses because the >90% of C and N is associated with mSOC, which has a comparatively long MRT.     

Effects of forest management intensity on carbon and nitrogen content in different soil size fractions of a North Florida Spodosol

Pine plantations of the southeastern USA are regional carbon (C) sinks. In spite of large increases in woody biomass due to advanced growing systems, studies have shown little or even negative effects on the C content of the extremely sandy soils of this region. Hence, it is important to understand the mechanisms that determine the impact of intensive forest management on soil organic carbon (SOC) sequestration. This study was conducted to examine the C profile in a 4-year-old loblolly pine (Pinus taeda L.) plantation managed under two levels of management intensity (chemical understory control and fertilizer inputs). Soil organic C and nitrogen (N) pools were evaluated using two size fractionation methods, dry and wet sieving (2000–250 μm, 250–150 μm, 150–53 μm and <53 μm). Dry sieving was preferred over wet sieving for soil size fractionation, as it preserved more structure and water-soluble SOC components such as esters and amides and did not affect the N distribution. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) spectra were used to examine the chemical composition of the size fractions, which showed the presence of recently added organic matter in the largest sand fraction, as well as more decomposed organic matter in the <53 μm fraction. Intensive forest management reduced SOC in all three 2000–53 μm fractions, most likely due to reduced root input of understory plants that were controlled using herbicides. The 2000–250 μm fractions contained nearly half of the total SOC and showed a 23% decrease in C content due to the intensive management regime. Results from this study indicated the significance and responsiveness of sand size SOC fractions in Florida Spodosols. Results also showed that reductions in SOC due to intensive management occurred after four years and highlighted the need to understand the long-term impacts and the mechanisms responsible. Responsible Editor: Barbara Wick     

Response of Oxidative Enzyme Activities to Nitrogen Deposition Affects Soil Concentrations of Dissolved Organic Carbon

Recent evidence suggests that atmospheric nitrate (NO ₃ ⁻ ) deposition can alter soil carbon (C) storage by directly affecting the activity of lignin-degrading soil fungi. In a laboratory experiment, we studied the direct influence of increasing soil NO ₃ ⁻ concentration on microbial C cycling in three different ecosystems: black oak–white oak (BOWO), sugar maple–red oak (SMRO), and sugar maple–basswood (SMBW). These ecosystems span a broad range of litter biochemistry and recalcitrance; the BOWO ecosystem contains the highest litter lignin content, SMRO had intermediate lignin content, and SMBW leaf litter has the lowest lignin content. We hypothesized that increasing soil solution NO ₃ ⁻ would reduce lignolytic activity in the BOWO ecosystem, due to a high abundance of white-rot fungi and lignin-rich leaf litter. Due to the low lignin content of litter in the SMBW, we further reasoned that the NO ₃ ⁻ repression of lignolytic activity would be less dramatic due to a lower relative abundance of white-rot basidiomycetes; the response in the SMRO ecosystem should be intermediate. We increased soil solution NO ₃ ⁻ concentrations in a 73-day laboratory incubation and measured microbial respiration and soil solution dissolved organic carbon (DOC) and phenolics concentrations. At the end of the incubation, we measured the activity of β-glucosidase, N-acetyl-glucosaminidase, phenol oxidase, and peroxidase, which are extracellular enzymes involved with cellulose and lignin degradation. We quantified the fungal biomass, and we also used fungal ribosomal intergenic spacer analysis (RISA) to gain insight into fungal community composition. In the BOWO ecosystem, increasing NO ₃ ⁻ significantly decreased oxidative enzyme activities (−30% to −54%) and increased DOC (+32% upper limit) and phenolic (+77% upper limit) concentrations. In the SMRO ecosystem, we observed a significant decrease in phenol oxidase activity (−73% lower limit) and an increase in soluble phenolic concentrations (+57% upper limit) in response to increasing NO ₃ ⁻ in soil solution, but there was no significant change in DOC concentration. In contrast to these patterns, increasing soil solution NO ₃ ⁻ in the SMBW soil resulted in significantly greater phenol oxidase activity (+700% upper limit) and a trend toward lower DOC production (−52% lower limit). Nitrate concentration had no effect on microbial respiration or β-glucosidase or N-acetyl-glucosaminidase activities. Fungal abundance and basidiomycete diversity tended to be highest in the BOWO soil and lowest in the SMBW, but neither displayed a consistent response to NO ₃ ⁻ additions. Taken together, our results demonstrate that oxidative enzyme production by microbial communities responds directly to NO ₃ ⁻ deposition, controlling extracellular enzyme activity and DOC flux. The regulation of oxidative enzymes by different microbial communities in response to NO ₃ ⁻ deposition highlights the fact that the composition and function of soil microbial communities directly control ecosystem-level responses to environmental change.     

Decades of atmospheric deposition have not resulted in widespread phosphorus limitation or saturation of tree demand for nitrogen in southern New England

It is commonly assumed that nitrogen (N) is the primary mineral resource limiting the productivity of temperate forests. Sustained inputs of N via atmospheric deposition are altering the N status of temperate forests raising the possibility that nutrients such as phosphorus (P) are increasingly limiting productivity. The objective of this study was to determine whether P availability limits tree growth alone or in combination with N. This study was conducted in two forest types common throughout the New England landscape of the northeastern United States; in sugar maple and white ash dominated stands growing on base rich parent material characterized by rapid rates of N cycling and high N availability, and in red oak–beech–hemlock dominated stands growing on base-poor parent material characterized by slow rates of N cycling and low N availability. Starting in 2004, N and P were added to replicate plots in each forest type in factorial combination at a rate of 150 and 50 kg ha⁻¹ year⁻¹, respectively. Diameter growth rates of all trees >10 cm DBH were measured in 2005 and 2006 using dendrometer bands and converted into units of basal area increment (BAI) and wood production. Following 2 years of fertilization, basal area increment in the sugar maple–white ash forests remained strongly N limited. Fertilization with P did not significantly increase BAI alone, although both N and P fertilization tended (P < 0.10) to increase diameter growth in white ash. Wood production in the N-fertilized plots increased by 100 g C m⁻² year⁻¹, roughly doubling production in the non-fertilized plots. In the red oak–beech–hemlock stands, there was no overall effect of N or P fertilization on BAI or wood production because BAI in some species was stimulated by fertilization with N alone (e.g., black cherry, red oak), while in other species BAI was unaffected (e.g., red maple, beech) or negatively affected by fertilization with N or P (e.g., eastern hemlock). Given that BAI in several tree species responded to fertilization with N alone and that only one species responded to P fertilization once N was added, this study suggests that decades of atmospheric N deposition have not (yet) resulted in widespread P limitation or saturation of tree demand for N.     

Soil Carbon Turnover Measurement by Physical Fractionation at a Forest-to-Pasture Chronosequence in the Brazilian Amazon

The effect of conversion from forest-to-pasture upon soil carbon stocks has been intensively discussed, but few studies focus on how this land-use change affects carbon (C) distribution across soil fractions in the Amazon basin. We investigated this in the 20 cm depth along a chronosequence of sites from native forest to three successively older pastures. We performed a physicochemical fractionation of bulk soil samples to better understand the mechanisms by which soil C is stabilized and evaluate the contribution of each C fraction to total soil C. Additionally, we used a two-pool model to estimate the mean residence time (MRT) for the slow and active pool C in each fraction. Soil C increased with conversion from forest-to-pasture in the particulate organic matter (>250 μm), microaggregate (53–250 μm), and d-clay (<2 μm) fractions. The microaggregate comprised the highest soil C content after the conversion from forest-to-pasture. The C content of the d-silt fraction decreased with time since conversion to pasture. Forest-derived C remained in all fractions with the highest concentration in the finest fractions, with the largest proportion of forest-derived soil C associated with clay minerals. Results from this work indicate that microaggregate formation is sensitive to changes in management and might serve as an indicator for management-induced soil carbon changes, and the soil C changes in the fractions are dependent on soil texture.     

Extracellular Enzyme Activities and Soil Organic Matter Dynamics for Northern Hardwood Forests receiving Simulated Nitrogen Deposition

Anthropogenic nitrogen enrichment alters decomposition processes that control the flux of carbon (C) and nitrogen (N) from soil organic matter (SOM) pools. To link N-driven changes in SOM to microbial responses, we measured the potential activity of several extracellular enzymes involved in SOM degradation at nine experimental sites located in northern Michigan. Each site has three treatment plots (ambient, +30 and +80 kg N ha⁻¹ y⁻¹). Litter and soil samples were collected on five dates over the third growing season of N treatment. Phenol oxidase, peroxidase and cellobiohydrolase activities showed significant responses to N additions. In the Acer saccharum–Tilia americana ecosystem, oxidative activity was 38% higher in the litter horizon of high N treatment plots, relative to ambient plots, while oxidative activity in mineral soil showed little change. In the A. saccharum–Quercus rubra and Q. velutina–Q. alba ecosystems, oxidative activities declined in both litter (15 and 23%, respectively) and soil (29 and 38%, respectively) in response to high N treatment while cellobiohydrolase activity increased (6 and 39% for litter, 29 and 18% for soil, respectively). Over 3 years, SOM content in the high N plots has decreased in the Acer–Tilia ecosystem and increased in the two Quercus ecosystems, relative to ambient plots. For all three ecosystems, differences in SOM content in relation to N treatment were directly related (r² = 0.92) to an enzyme activity factor that included both oxidative and hydrolytic enzyme responses.     

Allochthonous litter inputs, organic matter standing stocks, and organic seston dynamics in upland Panamanian streams: potential effects of larval amphibians on organic matter dynamics

Allochthonous inputs of detritus represent an important energy source for streams in forested regions, but dynamics of these materials are not well studied in neotropical headwater streams. As part of the tropical amphibian declines in streams (TADS) project, we quantified benthic organic matter standing stocks and organic seston dynamics in four Panamanian headwater streams, two with (pre-amphibian decline) and two without (post-decline) healthy amphibian assemblages. We also measured direct litterfall and lateral litter inputs in two of these streams. Continuous litterfall and monthly benthic samples were collected for 1 year, and seston was collected 1–3 times/month for 1 year at or near baseflow. Direct litterfall was similar between the two streams examined, ranging from 934–1,137 g DM m⁻² y⁻¹. Lateral inputs were lower, ranging from 140–187 g DM m⁻¹ y⁻¹. Dead leaves (57–60%), wood (24–29%), and green leaves (8–9%) contributed most to inputs, and total inputs were generally higher during the rainy season. Annual habitat-weighted benthic organic matter standing stocks ranged from 101–171 g AFDM m⁻² across the four study reaches, with ∼4 × higher values in pools compared to erosional habitats. Total benthic organic matter (BOM) values did not change appreciably with season, but coarse particulate organic matter (CPOM, >1 mm) generally decreased and very fine particulate organic matter (VFPOM, 1.6–250 μm) generally increased during the dry season. Average annual seston concentrations ranged from 0.2–0.6 mg AFDM l⁻¹ (fine seston, <754 μm >250 μm) and 2.0–4.7 mg AFDM l⁻¹ (very fine, <250 μm >1.6 μm), with very fine particles composing 85–92% of total seston. Quality of fine seston particles in the two reaches where tadpoles were present was significantly higher (lower C/N) than the two where tadpoles had been severely reduced (P = 0.0028), suggesting that ongoing amphibian declines in this region are negatively influencing the quality of particles exported from headwaters. Compared to forested streams in other regions, these systems receive relatively high amounts of allochthonous litter inputs but have low in-stream storage. Handling editor: J. Padisak     

<Emphasis Type="Italic">Vibrio cholerae</Emphasis> in waters of the Sunderban mangrove: relationship with biogeochemical parameters and chitin in seston size fractions

Wetland dynamics are probably linked to cholera endemicity in South Asia. We focus on links between Vibrio cholerae abundance, chitin content and suspended particle load in size fractions of suspended particulate matter (SPM) along the salinity gradient of Sunderban mangrove waters. SPM decreased downstream, while salinity increased from 0.2 to 4. Particulate organic carbon (90 ± 25 μM) and nitrogen (9.1 ± 3.3 μM) highly correlated with SPM and turbidity, suggesting a significant contribution of fine particles to organic matter. Total chitin ranged 1–2 mg/l and decreased downstream. The distribution among size fractions of SPM, chitin and V. cholerae O1 (the bacterial serogroup mainly associated with cholera epidemics) was similar, with ~98% of the total in the fraction <20 μm. In comparison, the number of V. cholerae O1 attached to zooplankton and microplankton size classes >20 μm was almost negligible, in contrast to usual assumptions. Thus, microdetritus, nanoplankton and fungal cells in size classes <20 μm represent a chitinaceous substrate on which V. cholerae can grow and survive. Total bacteria, cultivable vibrios and V. cholera O1 increased 5–10 times downstream, together with salinity and nitrite concentration. Overall, nitrate and silicate concentrations were relatively constant (>22 μM N and 100 μM Si). However, nitrite increased ~9 times in the outer sector, reaching ~1.2 μM N, probably as a result of increased abundance of nitrate-reducing vibrios. A characterization of Vibrio habitats that takes account of the presence of nitrate-reducing bacteria could improve the understanding of both mangrove nitrogen cycling and cholera seasonality.     

Laccase Gene Composition and Relative Abundance in Oak Forest Soil is not Affected by Short-Term Nitrogen Fertilization

Anthropogenic nitrogen (N) deposition affects a wide range of soil processes including phenol oxidase (PO) activity and soil organic matter dynamics. Depression of phenol oxidase activity in response to N saturation is believed to be mediated by the activity of white-rot basidiomycetes, whose production of extracellular oxidative enzymes can be limited by high N availability. We examined the effect of short-term N deposition on basidiomycete laccase gene diversity and relative abundance in temperate oak forest soil in which significant decreases in phenol oxidase and increased SOM have been recorded in response to experimental N deposition. UniFrac was used to compare the composition of laccase genes between three control- and three nitrogen-fertilized (80 kg⁻¹ ha⁻¹ per year) oak forest soils. The relative abundance of laccase genes was determined from qPCR analysis of laccase and basidiomycete ITS gene abundances. Our results indicate that there was no significant shift in the composition of laccase genes between control- and N-fertilized soils, nor was there a significant change in the relative abundance of laccase genes. These data suggest that N deposition effects on mineral soil PO activity do not result from changes in laccase gene diversity of white-rot basidiomycetes but are likely the result of altered microbial abundance or expression in this ecosystem type. Furthermore, laccase gene composition may be tied to factors that structure microbial communities in general, as soil laccase gene communities are more similar to other forest soils than with the corresponding litter.     

Reversibility of Soil Productivity Decline with Organic Matter of Differing Quality Along a Degradation Gradient

In the highlands of Western Kenya, we investigated the reversibility of soil productivity decline with increasing length of continuous maize cultivation over 100 years (corresponding to decreasing soil organic carbon (SOC) and nutrient contents) using organic matter additions of differing quality and stability as a function of soil texture and inorganic nitrogen (N) additions. The ability of additions of labile organic matter (green and animal manure) to improve productivity primarily by enhanced nutrient availability was contrasted with the ability of stable organic matter (biochar and sawdust) to improve productivity by enhancing SOC. Maize productivity declined by 66% during the first 35 years of continuous cropping after forest clearing. Productivity remained at a low level of 3.0 t grain ha^-1 across the chronosequence stretching up to 105 years of continuous cultivation despite full N–phosphorus (P)–potassium (K) fertilization (120–100–100 kg ha⁻¹). Application of organic resources reversed the productivity decline by increasing yields by 57–167%, whereby responses to nutrient-rich green manure were 110% greater than those from nutrient-poor sawdust. Productivity at the most degraded sites (80–105 years since forest clearing) increased in response to green manure to a greater extent than the yields at the least degraded sites (5 years since forest clearing), both with full N–P–K fertilization. Biochar additions at the most degraded sites doubled maize yield (equaling responses to green manure additions in some instances) that were not fully explained by nutrient availability, suggesting improvement of factors other than plant nutrition. There was no detectable influence of texture (soils with either 11–14 or 45–49% clay) when low quality organic matter was applied (sawdust, biochar), whereas productivity was 8, 15, and 39% greater (P < 0.05) on sandier than heavier textured soils with high quality organic matter (green and animal manure) or only inorganic nutrient additions, respectively. Across the entire degradation range, organic matter additions decreased the need for additional inorganic fertilizer N irrespective of the quality of the organic matter. For low quality organic resources (biochar and sawdust), crop yields were increasingly responsive to inorganic N fertilization with increasing soil degradation. On the other hand, fertilizer N additions did not improve soil productivity when high quality organic inputs were applied. Even with the tested full N–P–K fertilization, adding organic matter to soil was required for restoring soil productivity and most effective in the most degraded sites through both nutrient delivery (with green manure) and improvement of SOC (with biochar).     

Soil microbial parameters and stability of soil aggregate fractions under different grassland communities on the Loess Plateau,China

Over-grazing and large-scale monocultures on the Loess plateau in China have caused serious soil erosion by water and wind. Grassland revegetation has been reported as one of the most effective counter measures. Therefore, we investigated soil aggregation, aggregate stability and soil microbial activities as key parameters for soil remediation through grassland revegetation. The results showed that soil microbial biomass carbon (Cmic) and microbial biomass nitrogen (Nmic) increased under revegetated grass communities compared to cropland and overgrazed pastures and were higher in surface layers (0–10 cm) than in the subsurface (10–20 cm). Although there are variations between the four investigated grassland communities, their values were 10 to 50 times higher in comparison to the cropland and overgrazed pastures, similar to the increase in soil enzyme activities, such as β-glucosidase and β-glucosaminidase. Soil aggregate stability (SAS) showed clear differences between the different land uses with two main soil aggregate fractions measured by ultra sound: < 63 μm and 100–250 μm, with approximately 70% and 10% of the total soil volume respectively. We also found positive correlations between SAS and soil microbial parameters, such as Cmic, Nmic, and soil enzyme activities. From this, we concluded that revegetation of eroded soils by grasses accelerates soil rehabilitation.     

Altitudinal changes in carbon storage of temperate forests on Mt Changbai, Northeast China

A number of studies have investigated regional and continental scale patterns of carbon (C) stocks in forest ecosystems; however, the altitudinal changes in C storage in different components (vegetation, detritus, and soil) of forest ecosystems remain poorly understood. In this study, we measured C stocks of vegetation, detritus, and soil of 22 forest plots along an altitudinal gradient of 700–2,000 m to quantify altitudinal changes in carbon storage of major forest ecosystems (Pinus koraiensis and broadleaf mixed forest, 700–1,100 m; Picea and Abies forest, 1,100–1,800 m; and Betula ermanii forest, 1,800–2,000 m) on Mt Changbai, Northeast China. Total ecosystem C density (carbon stock per hectare) averaged 237 t C ha⁻¹ (ranging from 112 to 338 t C ha⁻¹) across all the forest stands, of which 153 t C ha⁻¹ (52–245 t C ha⁻¹) was stored in vegetation biomass, 14 t C ha⁻¹ (2.2–48 t C ha⁻¹) in forest detritus (including standing dead trees, fallen trees, and floor material), and 70 t C ha⁻¹ (35–113 t C ha⁻¹) in soil organic matter (1-m depth). Among all the forest types, the lowest vegetation and total C density but the highest soil organic carbon (SOC) density occurred in Betula ermanii forest, whereas the highest detritus C density was observed in Picea and Abies forest. The C density of the three ecosystem components showed distinct altitudinal patterns: with increasing altitude, vegetation C density decreased significantly, detritus C density first increased and then decreased, and SOC density exhibited increasing but insignificant trends. The allocation of total ecosystem C to each component exhibited similar but more significant trends along the altitudinal gradient. Our results suggest that carbon storage and partitioning among different components in temperate forests on Mt Changbai vary greatly with forest type and altitude.     

Transport of Carbon and Nitrogen Between Litter and Soil Organic Matter in a Northern Hardwood Forest

We used sugar maple litter double-labeled with ¹³C and ¹⁵N to quantify fluxes of carbon (C) and nitrogen (N) between litter and soil in a northern hardwood forest and the retention of litter C and N in soil. Two cohorts of litter were compared, one in which the label was preferentially incorporated into non-structural tissue and the other structural tissue. Loss of ¹³C from this litter generally followed dry mass and total C loss whereas loss of ¹⁵N (20–30% in 1 year) was accompanied by large increases of total N content of this decaying litter (26–32%). Enrichment of ¹³C and ¹⁵N was detected in soil down to 10–15 cm depth. After 6 months of decay (November–May) 36–43% of the ¹³C released from the litter was recovered in the soil, with no differences between the structural and non-structural labeled litter. By October the percentage recovery of litter ¹³C in soil was much lower (16%). The C released from litter and remaining in soil organic matter (SOM) after 1 year represented over 30 g C m⁻² y⁻¹ of SOM accumulation. Recovery of litter ¹⁵N in soil was much higher than for C (over 90%) and in May ¹⁵N was mostly in organic horizons whereas by October it was mostly in 0–10 cm mineral soil. A small proportion of this N was recovered as inorganic N (2–6%). Recovery of ¹⁵N in microbial biomass was higher in May (13–15%) than in October (about 5%). The C:N ratio of the SOM and microbial biomass derived from the labeled litter was much higher for the structural than the non-structural litter and for the forest floor than mineral SOM, illustrating the interactive role of substrates and microbial activity in regulating the C:N stoichiometry of forest SOM formation. These results for a forest ecosystem long exposed to chronically high atmospheric N deposition (ca. 10 kg N ha⁻¹ y⁻¹) suggest possible mechanisms of N retention in soil: increased organic N leaching from fresh litter and reduced fungal transport of N from soil to decaying litter may promote N stabilization in mineral SOM even at a relatively low C:N ratio.     

Increase in pH stimulates mineralization of ‘native’ organic carbon and nitrogen in naturally salt-affected sandy soils

Large amounts of terrestrial organic C and N reserves lie in salt-affected environments, and their dynamics are not well understood. This study was conducted to investigate how the contents and dynamics of ‘native’ organic C and N in sandy soils under different plant species found in a salt-affected ecosystem were related to salinity and pH. Increasing soil pH was associated with significant decreases in total soil organic C and C/N ratio; particulate (0.05–2 mm) organic C, N and C/N; and the C/N ratio in mineral-associated (<0.05 mm) fraction. In addition, mineral-associated organic C and N significantly increased with an increase in clay content of sandy soils. During 90-day incubation, total CO₂-C production per unit of soil organic C was dependent on pH [CO₂-C production (g kg⁻¹ organic C) = 22.5 pH – 119, R ² = 0.79]. Similarly, increased pH was associated with increased release of mineral N from soils during 10-day incubation. Soil microbial biomass C and N were also positively related to pH. Metabolic quotient increased with an increase in soil pH, suggesting that increasing alkalinity in the salt-affected soil favoured the survival of a bacterial-dominated microbial community with low assimilation efficiency of organic C. As a result, increased CO₂-C and mineral N were produced in alkaline saline soils (pH up to 10.0). This pH-stimulated mineralization of organic C and N mainly occurred in particulate but not in mineral-associated organic matter fractions. Our findings imply that, in addition to decreased plant productivity and the litter input, pH-stimulated mineralization of organic matter would also be responsible for a decreased amount of organic matter in alkaline salt-affected sandy soils.     

Acid Sensitivity of Lakes in Nova Scotia,Canada: Assessment of Lakes at Risk

The sensitivity of surface waters to acidic deposition is governed by the interaction of catchment geology, soils, topography, land use, climate and atmospheric deposition. Accordingly at the landscape scale, catchment attributes may be used to predict lake chemistry (for example, acid neutralising capacity (ANC), pH, calcium (Ca²⁺) and dissolved organic carbon (DOC)). Empirical (multiple linear regression) models based on average measured chemistry (2000–2006) for 204 lakes in Nova Scotia (NS) Canada, and their catchment attributes, were used to predict chemistry for all lakes in NS (n = 6104). Damage to aquatic biota, such as loss of species and/or reduced biodiversity has been widely evaluated using critical chemical thresholds commonly based on pH, ANC and Ca²⁺. The proportion of sensitive lakes in NS (that is, the stock at risk) was estimated as lakes with ANC less than 20 μeq l⁻¹, pH below 6, and Ca²⁺ less than 75 μeq l⁻¹ (13, 73 and 74%, respectively). Many lakes in NS are characterized by high DOC (>7 mg l⁻¹); in these lakes organic acids contribute to total acidity, making anthropogenic influences difficult to discern. To account for the potential contribution of organic acidity, all lakes with pH below 6 (and DOC < 7 mg l⁻¹) and lakes below a threshold for ANC adjusted for organic acids were quantified; 63% of the lakes fell below either of these thresholds. Despite substantial reductions in sulphur emissions in North America since the 1980s, many lakes in NS remain at risk to acidic deposition.     

Chemical Characteristics and Origin of Dissolved Organic Matter in the Yukon River

Monthly (or bi-weekly) water samples were collected from the Yukon River, one of the largest rivers in North America, at a station near the US Geological Survey Stevens Village hydrological station, Alaska from May to September 2002, to examine the quantity and quality of dissolved organic matter (DOM) and its seasonal variations. DOM was further size fractionated into high molecular weight (HMW or colloidal, 1 kDa–0.45 μm) and low molecular weight (LMW, <1 kDa) fractions. Dissolved organic carbon (DOC), colored dissolved organic matter (C-DOM) and total dissolved carbohydrate (TCHO) species were measured in the size fractionated DOM samples. Concentrations of DOC were as high as 2830 μmol-C l⁻¹ during the spring breakup in May and decreased significantly to 508–558 μmol-C l⁻¹ during open-water season (June–September). Within the DOC pool, up to 85% was in the colloidal fraction (1 kDa–0.45 μm) in early May. As DOC concentration decreased, this colloidal portion remained high (70–85% of the bulk DOC) throughout the sampling season. Concentrations of TCHO, including monosaccharides (MCHO) and polysaccharides (PCHO), varied from 722 μmol-C l⁻¹ in May to 129 μmol-C l⁻¹ in September, which comprised a fairly constant portion of bulk DOC (24±2%). Within the TCHO pool, the MCHO/TCHO ratio consistently increased from May to September. The C-DOM/DOM ratio and the size fractionated DOM increased from May to September, indicating that DOM draining into the Yukon River contained increased amounts of humified materials, likely related to a greater soil leaching efficiency in summer. The average composition of DOM was 76% pedogenic humic matter and 24% aquagenic CHO. Characteristics of soil-derived humic substances and low chlorophyll-a concentrations support a dominance of terrestrial DOM in Yukon River waters.     

Phytoplankton productivity in a pond of brownish-colored water in a Japanese lowland marsh, Naka-ikemi

To understand the characteristics of the ecosystem in Japanese lowland marsh, we investigated chlorophyll-a (Chl. a), photosynthesis and respiration of a phytoplankton community in a brownish-colored pond in Naka-ikemi marsh, Tsuruga, Fukui Prefecture. Chl. a concentrations and volumetric gross primary production rates ranged between 1.3–57.0 μg Chl. a l⁻¹ and 148–1619 μg C l⁻¹ day⁻¹ during the study period. Higher values of Chl. a and primary production rates were clearly observed from June to September, when the dominant algae were the phytoflagellates, Peridinium (Dinophyceae) and Cryptomonas (Cryptophyceae), with swimming ability. The trophic status of the pond water of Naka-ikemi marsh was defined as being in eutrophic condition based on the biomass and productivity of phytoplankton. However, depths of Z _1% showing the productive layer in this study site were relatively narrower than those observed in the hyper-eutrophic Lake Suwa with frequent cyanobacterial water bloom. Factor-attenuating underwater light intensity in Naka-ikemi marsh was presumed to be colored dissolved organic matter. Thus, not only phytoplankton primary production, but also allochthonous organic matter supplied from the catchment area seems to be the dominant factor in the whole energy budget of the pond. In conclusion, we regarded the pond ecosystem in Naka-ikemi marsh to be in a eutrophic–dystrophic condition.     

Elevated [CO2], temperature increase and N supply effects on the accumulation of below-ground carbon in a temperate grassland ecosystem

The effects of elevated [CO₂] (700 μl l⁻¹ [CO₂]) and temperature increase (+3 °C) on carbon accumulation in a grassland soil were studied at two N-fertiliser supplies (160 and 530 kgN ha⁻¹ year⁻¹) in a long-term experiment (2.5 years) on well established ryegrass swards (Lolium perenne L.,) supplied with the same amounts of irrigation water. For all experimental treatments, the C:N ratio of the top soil organic matter fractions increased with their particle size. Elevated CO₂ concentration increased the C:N ratios of the below-ground phytomass and of the macro-organic matter. A supplemental fertiliser N or a 3 °C increase in elevated [CO₂] reduced it. At the last sampling date, elevated [CO₂] did not affect the C:N ratio of the soil organic matter fractions, but increased significantly the accumulation of roots and of macro-organic matter above 200 μm (MOM). An increased N-fertiliser supply stimulated the accumulation of the non harvested plant phytomass and of the OM between 2 and 50 μm, without positive effect on the macro-organic matter >200 μm. Elevated [CO₂2] increased C accumulation in the OM fractions above 50 μm by +2.1 tC ha⁻¹, on average, whereas increasing the fertiliser N supply led to an average supplemental accumulation of +0.8 tC ha⁻¹. There was no significant effect of a 3 °C temperature increase under elevated [CO₂] on C accumulation in the OM fractions above 50 μm. This revised version was published online in June 2006 with corrections to the Cover Date.