Accumulation Of Particulate Organic Matter Research Articles

Can models adequately reflect how long-term nitrogen enrichment alters the forest soil carbon cycle?

Abstract. Changes in the nitrogen (N) status of forest ecosystems can directly and indirectly influence their carbon (C) sequestration potential by altering soil organic matter (SOM) decomposition, soil enzyme activity, and plant–soil interactions. However, model representations of linked C–N cycles and SOM decay are not well validated against experimental data. Here, we use extensive data from the Fernow Experimental Forest long-term whole-watershed N fertilization study to compare the response to N perturbations of two soil models that represent decomposition dynamics differently (first-order decay versus microbially explicit reverse Michaelis–Menten kinetics). These two soil models were coupled to a common vegetation model which provided identical input data. Key responses to N additions measured at the study site included a shift in plant allocation to favor woody biomass over belowground carbon inputs, reductions in soil respiration, accumulation of particulate organic matter (POM), and an increase in soil C:N ratios. The vegetation model did not capture the often-observed shift in plant C allocation with N additions, which resulted in poor predictions of the soil responses. We modified the parameterization of the plant C allocation scheme to favor wood production over fine-root production with N additions, which significantly improved the vegetation and soil respiration responses. Additionally, to elicit an increase in the soil C stocks and C:N ratios with N additions, as observed, we modified the decay rates of the POM in the soil models. With these modifications, both models captured negative soil respiration and positive soil C stock responses in line with observations, but only the microbially explicit model captured an increase in soil C:N. Our results highlight the need for further model development to accurately represent plant–soil interactions, such as rhizosphere priming, and their responses to environmental change.

Climate and mineral accretion as drivers of mineral‐associated and particulate organic matter accumulation in tidal wetland soils

AbstractTidal wetlands sequester vast amounts of organic carbon (OC) and enhance soil accretion. The conservation and restoration of these ecosystems is becoming increasingly geared toward “blue” carbon sequestration while obtaining additional benefits, such as buffering sea‐level rise and enhancing biodiversity. However, the assessments of blue carbon sequestration focus primarily on bulk SOC inventories and often neglect OC fractions and their drivers; this limits our understanding of the mechanisms controlling OC storage and opportunities to enhance blue carbon sinks. Here, we determined mineral‐associated and particulate organic matter (MAOM and POM, respectively) in 99 surface soils and 40 soil cores collected from Chinese mangrove and saltmarsh habitats across a broad range of climates and accretion rates and showed how previously unrecognized mechanisms of climate and mineral accretion regulated MAOM and POM accumulation in tidal wetlands. MAOM concentrations (8.0 ± 5.7 g C kg−1) (±standard deviation) were significantly higher than POM concentrations (4.2 ± 5.7 g C kg−1) across the different soil depths and habitats. MAOM contributed over 51.6 ± 24.9% and 78.9 ± 19.0% to OC in mangrove and saltmarsh soils, respectively; both exhibited lower autochthonous contributions but higher contributions from terrestrial or marine sources than POM, which was derived primarily from autochthonous sources. Increased input of plant‐derived organic matter along the increased temperature and precipitation gradients significantly enriched the POM concentrations. In contrast, the MAOM concentrations depended on climate, which controlled the mineral reactivity and mineral–OC interactions, and on regional sedimentary processes that could redistribute the reactive minerals. Mineral accretion diluted the POM concentrations and potentially enhanced the MAOM concentrations depending on mineral composition and whether the mineral accretion benefited plant productivity. Therefore, management strategies should comprehensively consider regional climate while regulating sediment supply and mineral abundance with engineering solutions to tap the OC sink potential of tidal wetlands.

The expansion of evergreen and deciduous shrubs changed the chemical characteristics and biological community of alpine meadows soil

Global warming drives shrubs expansion. However, it is unclear how shrubs expansion affect the nutrients cycling, decomposer diversity and activity of alpine meadows. We selected two dominant shrub species in an alpine forest-Tundra ecotone on the eastern Tibetan Plateau: Rhododendron lapponicum (evergreen shrubs), and Sorbus rufopilosa (deciduous shrubs), the shrub-free tundra site as the control site. The fauna community diversity, microbial biomass, extracellular enzyme activity and enzyme stoichiometry of organic soil and mineral soil layer were determined to explore the effects of shrubs expansion. The soil total nitrogen, microbial biomass carbon and nitrogen, and some extracellular enzyme activity of the two shrubs were higher than those in CK. In addition, deciduous shrub expansion had a stronger effect on meadows ecosystem process than evergreen shrub expansion. However, both of them may alleviate the limitation of nitrogen and phosphorus in alpine meadows ecosystem, and the expansion of evergreen shrubs may also contribute to the accumulation of particulate organic matter. It is of great significance to probe the biogeochemical cycle of alpine meadows ecosystems under climate change.

Organic amendments effects on soil aggregation and carbon sequestration in saline‐alkaline croplands in China

AbstractSaline‐alkaline soils are wide‐spread and approximately 6.5% of the world's land area is reported as being saline and sodic. Increasing soil organic carbon (SOC) concentration and aggregate stability in saline‐alkaline soils has been challenging due to the deleterious effects of soil salinity and sodicity. To address this issue, a 2‐year filed experiment was conducted to evaluate the effects of biochar (BC), farmyard manure (FM), corn straw (CS), and sheep manure (SM) in combination of chemical fertilizer (CK) on soil aggregation and C sequestration in mildly and moderately alkaline soils in Hetao Irrigation District, China. For mildly alkaline soil, the proportion of macroaggregates (>0.25 mm) was significantly higher under FM, CS, and SM (34.6%–51.2%) compared with CK (13.2%). The mean weight diameter (MWD) and geometric mean diameter (GMD) of water‐stable aggregates significantly increased under FM, CS, and SM by an average of 172.7% and 58.7% over CK. In addition, FM, CS, and SM enhanced the distribution of organic carbon (OC) in macroaggregates (>0.25 mm) and accelerated the accumulation of particulate organic matter within macroaggregates. Treatment BC enhanced OC distribution in microaggregates (0.053–0.25 mm) and exhibited a 7.40 g kg−1 SOC content, followed by SM and FM which exhibited SOC contents of 7.10 and 6.76 g kg−1. For moderately alkaline soil, data obtained in this study showed a slight influence from organic amendments on soil aggregates and C sequestration, suggesting that the moderately alkaline soil was not as sensitive to organic amendments as the mildly alkaline soil. Overall, the results of this study demonstrated that FM and SM combined with chemical fertilizer were the top organic amendments to improve soil aggregation and C sequestration in the mildly alkaline soil but not the moderately alkaline soil over the short term.

Long‐term fertilization and plastic film mulching modify temporal incorporation of 13C/15N‐labelled particulate organic matter

AbstractPlastic film mulching (PFM) is critical for agricultural planting and maximizing production in semiarid and arid areas. Particulate organic matter (POM) is assumed to be a sensitive indicator for evaluating the effects of different agricultural practices on soil fertility and the soil organic carbon (SOC) pool. Soil aggregates have the function of ‘wrap’ and protect the POM stored in them. However, there is limited information regarding how PFM and fertilization jointly influence the dynamic changes of newly added stalk‐derived POM in brown earth. Consequently, an in‐depth study of the fate of carbon (C) and nitrogen (N) derived from maize stalk residues within the particulate organic carbon (POC) and particulate organic nitrogen (PON) fractions in soil aggregates was undertaken. Its outcome would contribute to better predictions on the active organic matter components sequestered in the soil. The dynamics and accumulation of newly added maize stalk C and N as POC and PON in different soil aggregates (using the dry sieving method divided into >2, 1–2, 0.25–1 and <0.25 mm) were analysed by an in situ 13C/15N‐tracing technique under PFM and different fertilization treatments. Over 360 days of cultivation, the POC and PON contents were significantly (p < 0.05) larger in the nitrogen (N) and organic manure (M) treatments than in the MN (manure combined with nitrogen) and Control treatments. The PFM treatment accelerated the decomposition of maize stalk C in the N fertilizer treatment, with an increase of 64% in stalk‐derived POC after the 1‐day cultivation period. Stalk‐derived POM tended to accumulate in <0.25 mm microaggregates in the early cultivation period and then decreased rapidly with the extension of the cultivation period affected by PFM coupled with fertilization. However, stalk‐derived POM accumulation in macroaggregates (>0.25 mm) fluctuated over the 360‐day cultivation period. Accordingly, PFM application and fertilization practices had important effects on the accumulation of newly added stalk‐derived POM in soil aggregates. We conclude that the accumulation of maize residue POM was primarily affected by soil fertilization type, rather than by the presence or absence of PFM. These results provide new insights into agricultural management strategies for improving soil carbon sequestration capacity.

The path less taken: Long-term N additions slow leaf litter decomposition and favor the physical transfer pathway of soil organic matter formation

Understanding soil organic matter (SOM) formation as a balance between soil microbial access to organic plant inputs and protection by chemical recalcitrance and mineral associations can greatly improve our projections of this important terrestrial carbon pool. However, gaps remain in our understanding of the processes controlling the formation and destabilization of SOM and how these processes are affected by persistent global changes, such as nitrogen (N) deposition. To assess how elevated N deposition influences decomposition dynamics and the fate of plant inputs in a temperate deciduous forest, we coupled a reciprocal transplant leaf litter decomposition study with an analysis of the distribution of SOM in mineral associated and particulate organic matter fractions at a long-term, whole-watershed, N fertilization experiment. Nearly 30 years of N additions slowed leaf litter decomposition rates by about 11% in the fertilized watershed, regardless of the watershed from which the initial litter was collected. An apparent consequence of the altered rates of decomposition was that the proportion of SOM in light particulate organic matter in soil from the fertilized watershed was about 40% greater than that of the reference watershed, and was positively correlated with the bulk soil carbon to nitrogen ratio. Collectively, our results suggest that N saturation in a temperate forest alters SOM formation by slowing decomposition and favoring the accumulation of particulate organic matter as opposed to microbially processed mineral associated organic matter.

Modeling the Origin of the Particulate Organic Matter Flux to the Hypoxic Zone of Chesapeake Bay in Early Summer

Although hypoxia has been well-studied in Chesapeake Bay, little attention has been given to the origin of the particulate organic matter (POM) that potentially contributes to early summer hypoxia. A combination of a high-resolution baroclinic physical model and a Lagrangian particle tracking model was used to study the sources of POM to the deep channel of the mesohaline Chesapeake Bay. The circulation model reasonably reproduced the salinity structure and circulation compared with observations. The particle tracking model was improved with a fast search algorithm, and a predict-correct method was developed in the backward tracking model to ensure a similar trajectory with the forward tracking. Backward tracking results revealed that the pathway of the POM has a strong dependence on the tracking initiation time and location of the particles, as well as the magnitude of the sinking speed. In general, the behavior of the POM trajectories can be explained by the three-dimensional residual circulation of the mesohaline mainstem Chesapeake Bay. The particles that accumulated in the deep channel of the upper mesohaline Chesapeake Bay where the onset of hypoxia occurs mainly come from downstream when particles stop tracking when they backtracked to the surface layer. Downstream sources accounted for roughly 83.5% of particles for a sinking speed of 1 m/day, and the proportion from the downstream decreased to 60.5% as the sinking speed increased to 25 m/day. In addition, the source from the eastern shore was larger than that from the western shore for particle sinking speed less than 8 m/day. Moreover, particles from the Potomac estuary with a sinking speed of 1 m/day can contribute 7.4% of the bottom POM accumulation in the upper mesohaline Chesapeake Bay. When particles were allowed to stay on the surface for a period before they sink, they backtracked to the upper Chesapeake Bay suggesting that more long-lived refractory organic particles originating from the upper Bay can contribute to organic matter accumulation in the deep channel. These results help to explain why hypoxia in the deep channel of the upper mesohaline Bay occurs to the north of the primary production maximum and also why hypoxia is earlier and more severe in this region compared to the rest of hypoxic zone. Overall, the model results suggest that more local sources of POM with relatively high sinking rates are most important in driving oxygen depletion. This study improves understanding of the origin and pathways of the POM that can potentially contribute to the development of hypoxia in the deep channel of the mesohaline Chesapeake Bay. Future studies need to be undertaken to better understand the relative importance of remote versus more local sources of organic matter in driving oxygen drawdown.

Stabilization of soil aggregate and organic matter under the application of three organic resources and biochar-based compound fertilizer

Biochar mediation of soil structure and carbon (C) sequestration has been tested, but the underlying mechanisms in this process are poorly understood. We investigated the effects of different organic matter applications on soil aggregation and associated C in Northeast China. Non-disrupted soil samples (0–20 cm in depth) were collected from the field improvement experiment (established in 2009) with four addition treatments: (1) corn straw (CS), (2) pig manure compost (PMC), (3) biochar from corncob (BIO), and (4) biochar-based compound fertilizer (BF), of which CS, PMC, and BIO were combined with extra mineral fertilizers including nitrogen (N), phosphorus (P), and potassium (K). Soil aggregate distribution and associated C concentrations were estimated using physical fractionation techniques. The CS and PMC additions increased mean weight diameter (MWD), large macroaggregate fractions (>2 mm), and small macroaggregate fractions (0.25–2 mm). In contrast, BIO and BF significantly increased silt + clay fractions (< 0.053 mm). Compared with CS and PMC, applying BIO and BF had limited benefits for soil aggregation. Nevertheless, BIO promoted fine and coarse particulate organic matter (POM) accumulation significantly, and BF promoted fine POM accumulation significantly (within small macroaggregates). The concentrations of mMPOM (POM occluded in microaggregates within small macroaggregates) in BF and mPOM (POM within microaggregates) in BIO were the highest (P < 0.05). Biochar addition did not significantly improve soil structure compared to corn straw and pig manure compost treatments, but it was beneficial to SOC accumulation. Furthermore, C sequestration in the organic addition treatments was driven by the POM dynamics, and biochar debris was preferentially incorporated into the < 0.25 mm fractions.

Effects of reduced organic matter loading through membrane filtration on the microbial community dynamics in recirculating aquaculture systems (RAS) with Atlantic salmon parr (Salmo salar)

A key challenge in recirculating aquaculture systems (RAS) is the accumulation of particulate organic matter, especially the fine and colloidal fraction due to low removal efficiency of today's technology. The supply of organic matter is typically the limiting resource determining the carrying capacity (CC) of heterotrophic bacteria in the system. An appropriate and stable CC is proposed as a strategy for an optimal microbial environment in RAS with less blooms of opportunistic bacteria and more stable community dynamics. In this study, we investigated the effects of including a membrane for ultrafiltration in the RAS water treatment loop (treating 10–15% of the total water flow) to reduce the amount of fine and colloidal organic matter. Atlantic salmon parr (Salmo salar) were reared in two pilot-scale RAS (mRAS: membrane, cRAS: conventional). To evaluate the bacterial dynamics with and without membrane filtration at different organic loadings, the water exchange rates of the systems were manipulated equally to create periods with high and low loading of organic matter. The results showed that in the mRAS water, the level of organic matter was more stable throughout the experiment for the changing organic matter loadings. As a consequence, water in mRAS had higher microbial diversity, lower and shorter bacterial blooms and generally lower bacterial densities than in cRAS. All variables indicate a better microbial environment in the water of the system with membrane filtration. Also, the physicochemical water quality was better in mRAS in terms of lower turbidity and particulate organic matter (POC), and slightly lower concentrations of total ammonia nitrogen (TAN). The composition of the microbial communities was significantly different between the two systems, and temporal variations in the community dynamics were observed in both systems during the periods with different organic loadings. At high organic loading, the genus Mycobacterium had high relative abundance in the cRAS water (up to 0.25) compared to mRAS (0.01–0.03). The fish in mRAS were significantly bigger (14%) than fish in cRAS at the end of the experiment, however it is hard to conclude whether the better growth in mRAS was due to higher temperatures (caused by membrane operation) or better water quality, as it was probably a combination of both. We can conclude that membrane filtration gave more stable and better physicochemical and microbial water quality, which will reduce the probability for microbially related accidents in RAS.

Performance of full-scale woodchip bioreactors treating effluents from commercial RAS

The aquaculture industry needs new cost-effective solutions for removing effluent nitrogen to reduce their environmental impact. Recent studies have shown that laboratory and pilot-scale woodchip bioreactors can remove nitrate from aquaculture effluent. Encouraged by these results, several Danish fish farmers recently constructed commercial scale woodchip bioreactors prior to- or integrated into existing free water surface constructed wetlands without knowing the effects of design and upscaling on removal performance. The objective of the current study was to monitor initial performance (first 28–52 weeks of operation) of three such custom-designed denitrifying woodchip bioreactors installed in full-scale (350, 650, and 1250 m3 woodchips) for effluent treatment at three commercial recirculated rainbow trout (Oncorhynchus mykiss) farms.All three woodchip bioreactors removed nitrogen (essentially nitrate-nitrogen) continuously from the start and at relatively stable rates (5.3–8.5 g N/m3/d; 4.5–7.8 g NO3-N/m3/d) during the investigation period. Leaching of dissolved organic matter continued for up to half a year whereas there was no similar leaching of dissolved phosphorus. A net production of dissolved organic matter was observed under nitrate-limited conditions in one of the bioreactors characterized by a loading of 5.5 compared to 16.4–16.8 g NO3-N/m3/d at the other sites, and an empty bed contact time (EBCT) of 26 h compared to 15–16 h at the other sites. An increase in water level difference between inlet and outlet indicated that accumulation of particulate organic matter and associated bacterial growth may lead to head loss potentially reducing the long-term performance of the bioreactors. In conclusion, the study confirms that woodchip bioreactors can be applied at full-scale for removing NO3-N from fish farm effluent. Woodchip bioreactors should be designed to minimize head loss and the risk of clogging, and loading and retention time should ensure that denitrifiers do not become nitrate-limited.

Production of fluorescent dissolved organic matter in Arctic Ocean sediments.

Little is known about the production of fluorescent dissolved organic matter (FDOM) in the anoxic oceanic sediments. In this study, sediment pore waters were sampled from four different sites in the Chukchi-East Siberian Seas area to examine the bulk dissolved organic carbon (DOC) and their optical properties. The production of FDOM, coupled with the increase of nutrients, was observed above the sulfate-methane-transition-zone (SMTZ). The presence of FDOM was concurrent with sulfate reduction and increased alkalinity (R2 > 0.96, p < 0.0001), suggesting a link to organic matter degradation. This inference was supported by the positive correlation (R2 > 0.95, p < 0.0001) between the net production of FDOM and the modeled degradation rates of particulate organic carbon sulfate reduction. The production of FDOM was more pronounced in a shallow shelf site S1 with a total net production ranging from 17.9 to 62.3 RU for different FDOM components above the SMTZ depth of ca. 4.1 mbsf, which presumably underwent more accumulation of particulate organic matter than the other three deeper sites. The sediments were generally found to be the sources of CDOM and FDOM to the overlying water column, unearthing a channel of generally bio-refractory and pre-aged DOM to the oceans.

Effects of physical constraints on the lability of POM during summer in the Ross Sea

The 0–200m surface layer of the Ross Sea was studied during summer 2014 to investigate the lability of the particulate organic matter (POM) in response to physical parameters. With the use of satellite information, we selected three zones, characterised by different physical setting: a northern offshore area, crossing the summer-polynya area of the Ross Sea (hereafter called ROME 1), a more coastal area next to the Terra Nova Bay polynya (ROME 2); a southern offshore area, towards the Ross Ice Shelf (ROME 3). Ice-maps showed that the seasonal ice retreat had already occurred in early December for most of the stations. Statistical analysis of the quantitative and qualitative characteristics of the POM pointed to significant differences between the stations, especially in the upper mixed layer (UML). A comparison with previous studies showed that the localised pulses of POM accumulation in the UML were similar to those recorded at the highly productive marginal ice zones, providing notable trophic support to the ecosystem. The UML, although rather thin and easily subjected to alterations, confirmed its pivotal role in the ecosystem dynamics. A POM quality favourable to consumers was highlighted at several stations in ROME 1 and ROME 3. Reduced trophic support was, instead, found in ROME 2. Limited POM consumption where deep-water formation takes place would increase the POM role in the transfer of C to the depths.

Water mass characteristics and their temporal changes in a biological hotspot in the southern Chukchi Sea

Abstract. We analysed mooring and ship-based hydrographic and biogeochemical data obtained from a Hope Valley biological hotspot in the southern Chukchi Sea. The moorings were deployed from 16 July 2012 to 19 July 2014, and data were captured during spring and autumn blooms with high chlorophyll a concentrations. Turbidity increased and dissolved oxygen decreased in the bottom water at the mooring site before the autumn bloom, suggesting an accumulation of particulate organic matter and its decomposition (nutrient regeneration) at the bottom. This event may have been a trigger for the autumn bloom at this site. The bloom was maintained for 1 month in 2012 and for 2 months in 2013. The maintenance mechanism for the autumn bloom was also studied by hydrographic and biogeochemical surveys in late summer to autumn 2012 and 2013. Nutrient-rich water from the Bering Sea supplied nutrients to Hope Valley, although a reduction in nutrients occurred in 2012 by the influence of lower-nutrient water that would have remained on the Chukchi Sea shelf. In addition, nutrient regeneration at the bottom of Hope Valley could have increased nutrient concentrations and explained 60 % of its nutrient content in the bottom water in the autumn of 2012. The high nutrient content with the dome-like structure of the bottom water may have maintained the high primary productivity via the vertical nutrient supply from the bottom water, which was likely caused by wind-induced mixing during the autumn bloom. Primary productivity was 0.3 g C m−2 d−1 in September 2012 and 1.6 g C m−2 d−1 in September 2013. The lower productivity in 2012 was related to strong stratification caused by the high fraction of surface sea ice meltwater.

Pulsing hydrology determines top‐down control of basal resources in a tropical river–floodplain ecosystem

Variable hydrology of rivers strongly affects biophysical factors that influence primary production and population densities, thereby affecting the relative influence of bottom‐up and top‐down processes in trophic networks. Many tropical floodplain rivers have sustained seasonal flood pulses driven by precipitation patterns of the Intertropical Convergence Zone. These changes in flow alter concentrations of dissolved nutrients, aquatic primary productivity, and per‐unit‐area densities of aquatic organisms. Therefore, one would predict that the strength of top‐down effects of animals on basal resources should shift as the annual flood pulse progresses. We conducted a series of field experiments in a Neotropical lowland river to test for effects of hydrologic phase, habitat (in‐channel vs. floodplain aquatic habitat), and benthic‐feeding fish and meiofauna on particulate organic matter, chlorophyll, and benthic microalgae. Net ecosystem productivity of this oligotrophic river is higher during the low phase of the annual flood cycle, which is also when resident fishes are at highest densities and there is a seasonal influx of migratory benthic‐feeding fish. We therefore hypothesized that top‐down effects of benthic‐feeding fish would fluctuate temporally, with strongest effects during low water levels. We found that fish controlled the abundance of particulate organic matter and algae on solid substrates, but not on sand, during falling‐ and low‐water phases within both channel and floodplain habitats. Except for diatom assemblages, which responded to fish exclusion, the taxonomic structure of algal and meiofauna assemblages was not significantly influenced by fish‐exclusion treatments, but varied in relation to habitat type and hydrologic phase. Meiofauna densities were highest during the low‐water period; experimental exclusion of meiofauna during this period had a significant effect on accumulation of particulate organic matter in sand. By controlling abundance of important basal resources, fishes and meiofauna have a large potential to influence other components of this tropical ecosystem. Our findings emphasize the predictable, gradual, changes in consumer–resource interactions associated with the seasonal flood pulse in tropical river systems.

Microbial carbon and nitrogen production under experimental conditions combining warming with increased ultraviolet-B radiation in Mediterranean coastal waters

The effects of warming and increased ultraviolet-B radiation (UVBR, 280–320nm) have been rarely studied at food web scale and very few studies have considered the effect of combining these two climatic stressors. Microbial carbon and nitrogen dynamics were studied under the single and combined conditions of +3°C warming and +20% UVBR above the natural levels (control) during a 10-day mesocosm experiment in coastal Mediterranean waters. The effect of increased UVBR on primary production (PP) and bacterial production (BP) rates was rarely significant during the experiment. Warming alone or combined with increased UVBR significantly reduced BP by about 30% but also significantly increased PP by an average of 90%. No accumulation of particulate organic matter was observed during the experiment but, in the warmed mesocosms, the cumulative carbon and nitrogen losses were greater (ca. +40%). The main short-term consequence of warming was, therefore, a shift of the food web dynamics leading to higher C and N losses. This suggests a more efficient transfer of the newly produced microbial production to the upper trophic levels and a greater exportation into deeper waters through settlement under warmer conditions in Mediterranean coastal waters in the future.

Foam fractionation efficiency of a vacuum airlift—Application to particulate matter removal in recirculating systems

The accumulation of particulate organic matter (POM) in recirculating aquaculture systems (RAS) has become an important issue with the intensification of finfish production. The objective of this study was to assess the foam fractionation efficiency of a vacuum airlift in different conditions (POM concentrations, airflow rates, bubble sizes, water renewal rates and feed addition). In sea water, the vacuum airlift allowed removing 20% of the initial POM concentration per hour (foam fractionation efficiency), corresponding to a 20.7-fold concentration factor between the tank and the foam. In rearing conditions, efficiency increased with decreasing water renewal rate or increasing POM concentration. An increase in airflow rate from 10 to 80Lmin−1 in the vacuum airlift significantly decreased foam fractionation efficiency when feed was added to the water. The impact of feeding was only observed with high airflow rates where bubble coalescence occurred. Calculated POM production by fish ranged between 15.9 and 23.5gh−1 and was equivalent to estimations based on feed conversion ratio (FCR). This indicated that all the POM produced was extracted by the vacuum airlift.

Year-round accumulation of particulate organic matter in the estuarine turbidity maximum: comparative observations in three macrotidal estuaries (Chikugo, Midori, and Kuma Rivers), southwestern Japan

The inner part of the Ariake Sea is one of the most productive estuarine systems in Japan. To examine potential food items for estuarine organisms, we conducted monthly observations of the dynamics of particulate organic matter along the macrotidal Chikugo River estuary in 2005 and 2006. In the neighboring macrotidal Midori and Kuma River estuaries, comparative observations were made. High turbidity and strong vertical mixing were observed only at low salinities (<10) in the Chikugo River estuary. In contrast, the Midori and Kuma River estuaries were characterized by less turbid and less mixed waters. Concentrations of particulate organic carbon often exceeded 5 mg l−1 in or close to the estuarine turbidity maximum (ETM) of the Chikugo River estuary. However, such high concentrations were rarely observed in the other two estuaries. The observed differences could be attributable to different hydrodynamic processes related to the different lengths of tidal reaches: 23, 8, and 6 km in the Chikugo, Midori, and Kuma Rivers, respectively. In the Chikugo River estuary, spatiotemporal changes of chlorophyll a suggested that phytoplankton occurred abundantly up- and/or downstream from the ETM especially during the warm season. In contrast, pheophytin (i.e., plant detritus) always accumulated in or close to the ETM. Carbon stable isotope ratios and carbon to nitrogen ratios indicated that the plant detritus was derived from phytoplankton and terrestrial plants. The Chikugo River estuary has a high potential to support the production of estuarine organisms through abundant plant detritus in the well-developed ETM all the year round.

Distribution of particulate organic matter (POM) in the Southern Ocean during BROKE-West (30°E - 80°E)

Abstract The distribution of particulate organic carbon (POC), nitrogen (PON) and biogenic silica (BSi) in the south Indian sector of the Southern Ocean was investigated during summer 2006 as part of the BROKE-West survey. Differences in the distribution of particulate organic matter (POM) were attributed to spatial pattern related to the influence of regional hydrography and sea-ice retreat pattern. Offshore to the west, low NCP and POM accumulation were observed in a region influenced by the Weddell gyre, whereas to the east higher NCP and POM were found in a region influenced by the southern Boundary of the ACC and the southern Antarctic Circumpolar Front circulation (SB-sACCf). The highest NCP (5.4 molC m −2 ), POC (1.6 molC m −2 ) and BSi stocks (0.5 molSi m −2 ) were observed in the coastal/Marginal Ice Zone (MIZ). Spatial differences were also found in particulate elemental composition, with lower POC:PON found in the coastal zone, illustrating the presence of an active autotrophic community. Analysis of elemental ratios also showed a strong decoupling between silica and nitrogen cycling over the whole survey region, particularly in association with the circulation of SB-sACCf. Estimates of silica and nitrogen export production illustrate this decoupling with BSi exported production. BSi export ranged between 54 and 94% of BSi production whereas PON ranged between 4 and 70%, suggesting that organic matter was actively recycled in the surface water, hence illustrating the efficiency of the silicate pump in this region.

English

Particulate organic matter (POM) and aggregate recovery following mining disturbances are important for soil biogeochemical properties and ecosystem function. The objective of this study was to track POM accumulation and aggregation in reclaimed soils following coal mining in southwestern Virginia. A chronosequence of sites was selected based on shifts in vegetation communities with succession, typically occurring between 0-2, 5-7, 16-20, and 38-42 years since reclamation. Undisturbed adjacent forested sites were also sampled. The 0-2 yr old sites were covered with grasses and forbs, the 5-7 yr sites by thick stands of Lespedeza cuneata, the 16-20 yr sites predominately with Festuca arundinaceae and patches of deciduous trees (Acer rubrum, Oydendrum arborea, etc.) and the 38-42 yr old sites with a mix of Pinus taeda and deciduous forest with a grass understory. Undisturbed sites predominantly supported mixed Appalachian deciduous forest. Available POM (inter-aggregate) and physically protected (intra-aggregate) forms were determined using a density flotation technique and aggregate size distribution with wet sieving. Inter-aggregate POM did not change across site ages; however, intra-aggregate POM increased significantly between the 5-7 and 16-20 yr old sites and remained unchanged through the 38-42 yr old reclaimed site. Inter-aggregate POM reached levels similar to undisturbed sites, while intra-aggregate POM weights were almost threefold that of undisturbed sites after 16-20 years. By observing just the available POM, we would conclude that reclaimed systems recover to an protected POM, reclaimed system POM storage greatly exceeded undisturbed soil conditions. A positive relationship was observed between small macroaggregates (250-2000 μm) and intra-aggregate POM, suggesting that protection of POM by small macroaggregates in reclaimed systems is extremely important for POM accumulation and subsequently ecosystem function within a period of 20 years. We can also argue the greatest rates of POM accumulation and aggregate formation occur under early succession communities with grasses and forbs rather than under late succession forested communities.

Spreading Dead Zones and Consequences for Marine Ecosystems

Dead zones in the coastal oceans have spread exponentially since the 1960s and have serious consequences for ecosystem functioning. The formation of dead zones has been exacerbated by the increase in primary production and consequent worldwide coastal eutrophication fueled by riverine runoff of fertilizers and the burning of fossil fuels. Enhanced primary production results in an accumulation of particulate organic matter, which encourages microbial activity and the consumption of dissolved oxygen in bottom waters. Dead zones have now been reported from more than 400 systems, affecting a total area of more than 245,000 square kilometers, and are probably a key stressor on marine ecosystems.