Particulate Organic Matter Pools Research Articles

A stoichiometric approach to estimate sources of mineral‐associated soil organic matter

AbstractMineral‐associated soil organic matter (MAOM) is the largest, slowest cycling pool of carbon (C) in the terrestrial biosphere. MAOM is primarily derived from plant and microbial sources, yet the relative contributions of these two sources to MAOM remain unresolved. Resolving this issue is essential for managing and modeling soil carbon responses to environmental change. Microbial biomarkers, particularly amino sugars, are the primary method used to estimate microbial versus plant contributions to MAOM, despite systematic biases associated with these estimates. There is a clear need for independent lines of evidence to help determine the relative importance of plant versus microbial contributions to MAOM. Here, we synthesized 288 datasets of C/N ratios for MAOM, particulate organic matter (POM), and microbial biomass across the soils of forests, grasslands, and croplands. Microbial biomass is the source of microbial residues that form MAOM, whereas the POM pool is the direct precursor of plant residues that form MAOM. We then used a stoichiometric approach—based on two‐pool, isotope‐mixing models—to estimate the proportional contribution of plant residue (POM) versus microbial sources to the MAOM pool. Depending on the assumptions underlying our approach, microbial inputs accounted for between 34% and 47% of the MAOM pool, whereas plant residues contributed 53%–66%. Our results therefore challenge the existing hypothesis that microbial contributions are the dominant constituents of MAOM. We conclude that biogeochemical theory and models should account for multiple pathways of MAOM formation, and that multiple independent lines of evidence are required to resolve where and when plant versus microbial contributions are dominant in MAOM formation.

Seasonal riverine inputs may affect diet and mercury bioaccumulation in Arctic coastal zooplankton

Climate change driven increases in permafrost thaw and terrestrial runoff are expected to facilitate the mobilization and transport of mercury (Hg) from catchment soils to coastal areas in the Arctic, potentially increasing Hg exposure of marine food webs. The main aim of this study was to determine the impacts of seasonal riverine inputs on land-ocean Hg transport, zooplankton diet and Hg bioaccumulation in an Arctic estuary (Adventfjorden, Svalbard). The Adventelva River was a source of dissolved and particulate Hg to Adventfjorden, especially in June and July during the river's main discharge period. Stable isotope and fatty acid analyses suggest that zooplankton diet varied seasonally with diatoms dominating during the spring phytoplankton bloom in May and with increasing contributions of dinoflagellates in the summer months. In addition, there was evidence of increased terrestrial carbon utilization by zooplankton in June and July, when terrestrial particles contributed substantially to the particulate organic matter pool. Total (TotHg) and methyl Hg (MeHg) concentrations in zooplankton increased from April to August related to increased exposure to riverine inputs, and to shifts in zooplankton diet and community structure. Longer and warmer summer seasons will probably increase riverine runoff and thus Hg exposure to Arctic zooplankton.

Changes in soil organic matter after conversion from irrigated to dryland cropping systems

Global water resources are under increasing pressure, and some regions face the need to retire irrigation due to groundwater depletion or to meet governmental regulations. In arid and semiarid climates, irrigated lands tend to have more soil organic carbon (SOC) than non-irrigated croplands. However, little is known about how SOC might change following irrigation retirement. Our objective was to quantify changes in SOC and nitrogen stocks after irrigation retirement in semiarid agroecosystems of the High Plains. We sampled fields that stopped using irrigation and transitioned into either dryland crops or ungrazed perennial grasslands and compared SOC and nitrogen stocks in these fields with still irrigated and long-term dryland situations. Currently irrigated fields had more SOC (83.0 vs 73.3 Mg ha-1) and nitrogen (10.2 vs 9.4 Mg ha-1) at 0–80 cm depth than their dryland counterparts, confirming the reported positive impact of irrigation in semiarid agroecosystems. However, there was no legacy effect of irrigation on SOC and nitrogen topsoil stocks 7–10 years after transition to dryland crops as retired fields had lower stocks than still irrigated fields and did not differ from long-term dryland cropping zones of the same fields. This lack of a legacy effect was related to the preferential accumulation, during irrigation, of carbon and nitrogen in the less stable particulate organic matter pool at 0–10 cm soil depth. The transition from irrigated agriculture to perennial grasses resulted in intermediate SOC stocks, that did not differ from the still irrigated nor from the sites retired to dryland crops. Our results suggest that perennial systems are a viable option to mitigate, at least partially, the negative impact of irrigation retirement on SOC. However, the advantage of perennial grasses was explained by the accumulation of carbon and nitrogen in the particulate organic matter fraction, which poses some questions about the stability of SOC in these systems after changes in management.

Biogeochemical and oceanographic conditions provide insights about current status of an Antarctic fjord affected by relatively slow glacial retreat.

Understand the origin, transport, and character of organic matter entering Antarctic fjords is essential as they are major components of the global carbon cycle and budget. Macromolecular pools of particulate organic matter, bulk organic geochemistry, major and trace elements in surface sediments from Collins Bay were analysed as source indicators. Oceanographic conditions, bathymetry (multibeam) and grain size were considered as environmental controlling factors. Sediment samples were taken with a van Veen grab, during the ANTAR XXV Peruvian expedition (February 2018), onboard the R/V "BAP Carrasco" from the Peruvian Navy. Biopolymeric composition revealed the predominance of fresh marine protein-rich organic matter in the seafloor of Collins Bay, denoting high quality food resource for marine benthic heterotrophs. Based on Igeo values (between 0 and 1) Collins Bay can be considered unpolluted with natural levels of As, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn. Distribution of most of these elements with a gradient of decrease from the shallow inner fjord towards the outer deepest fjord, suggest their association with the deposition of detrital material and lithogenic particles supplied by Collins Glacier frontal ablation and runoff. This first comprehensive baseline information would assist in interpreting downcore sedimentary reconstructions and future climate-induce changes.

Temporal dynamics of total microbial biomass and particulate detritus at Station ALOHA

Particulate adenosine-5′-triphosphate (P-ATP) and particulate carbon (PC) concentrations were measured on approximately monthly intervals throughout the upper water column (0–1000 m) over a 30-yr (1989–2018) period at Station ALOHA to track the seasonal-to-decadal variability in total microbial biomass and the dynamics of living-to-nonliving particulate organic matter pools. On selected cruises, samples were also collected to a depth of ∼4800 m. P-ATP concentrations were relatively uniform (27–34 ng l−1) throughout the upper euphotic zone (0–100 m) with a distinct peak at 45 m. P-ATP concentrations were significantly higher (p < 0.001) in summer (Jun-Aug) than in winter (Dec-Feb), especially between 45 and 100 m where the seasonal differences averaged 28%. Below 100 m, P-ATP concentrations decreased rapidly with depth to a 30-yr mean value of 3.5 ng ATP l−1 at 250 m, and then decreased more gradually to a 30-yr mean value of 0.9 ng ATP l−1 at 1000 m. Between 125 and 175 m, the seasonal peak in P-ATP shifted to spring (Mar-May), with minima in fall (Sep-Nov) and winter (Dec-Feb). No consistent seasonal variations in P-ATP were detected at depths >175 m, suggesting a temporally stable habitat. Assuming a PC:P-ATP ratio of 250:1 (g g−1), the 0–100 m, 100–250 m, and 250–1000 m depth-integrated microbial biomass estimates were 775, 425, and 350 mg C m−2, respectively. Bathypelagic zone (>1000 m) P-ATP concentrations, based on a more limited data set than the upper portions of the water column, were low (0.4–0.7 ng ATP l−1). However, when integrated over the entire deep water habitat (1000–4800 m), bathypelagic zone microbial biomass was substantial (425 mg C m−2). Expressed as a percentage of total PC, microbial biomass ranged from ∼30% in the upper euphotic zone to ∼3% at depths >3000 m, emphasizing the preponderance of detrital PC throughout the entire water column. The total water column inventory of microbial biomass at Station ALOHA was 2 g C m−2 compared to ∼18 g C m−2 for total suspended PC; approximately 50% of the total microbial biomass is resident in the aphotic zone (>175 m). Although daily gross primary production at Station ALOHA is on par with the total euphotic zone (0–175 m)-integrated microbial biomass (∼1 g C m−2 d−1 and ∼1 g C m−2, respectively), the sources of C and energy fueling the substantial aphotic zone microbial biomass (∼1 g m−2) are not well understood at the present time.

What can we learn from amino acids about oceanic organic matter cycling and degradation?

Abstract. Amino acids (AAs) mainly bound in proteins are major constituents of living biomass and non-living organic material in the oceanic particulate and dissolved organic matter pool. Uptake and cycling by heterotrophic organisms lead to characteristic changes in AA composition so that AA-based biogeochemical indicators are often used to elucidate processes of organic matter cycling and degradation. We analyzed particulate AA in a large sample set collected in various oceanic regions covering sinking and suspended particles in the water column, sediment samples, and dissolved AA from water column and pore water samples. The aim of this study was to test and improve the use of AA-derived biogeochemical indicators as proxies for organic matter sources and degradation and to better understand particle dynamics and interaction between the dissolved and particulate organic matter pools. A principal component analysis (PCA) of all data delineates diverging AA compositions of sinking and suspended particles with increasing water depth. A new sinking particle and sediment degradation indicator (SDI) allows a fine-tuned classification of sinking particles and sediments with respect to the intensity of degradation, which is associated with changes of stable isotopic ratios of nitrogen (δ15N). This new indicator is furthermore sensitive to sedimentary redox conditions and can be used to detect past anoxic early diagenesis. A second indicator emerges from the AA spectra of suspended particulate matter (SPM) in the epipelagic and that of the meso- and bathypelagic ocean and is a residence time indicator (RTI). The characteristic changes in AA patterns from shallow to deep SPM are recapitulated in the AA spectra of the dissolved organic matter (DOM) pool, so that deep SPM is more similar to DOM than to any of the other organic matter pools. This implies that there is equilibration between finely dispersed SPM and DOM in the deep sea, which may be driven by microbial activity combined with annealing and fragmentation of gels. As these processes strongly depend on physico-chemical conditions in the deep ocean, changes in quality and degradability of DOM may strongly affect the relatively large pool of suspended and dissolved AA in the ocean that amounts to 15 Pg amino acid carbon (AAC) and 89 ± 29 Pg AAC, respectively.

Functional Feeding Groups of Macrofauna and Detritus Decomposition along a Gradient of Glacial Meltwater Influence in Tropical High-Andean Streams

Tropical Andean glaciers are retreating rapidly, with possible consequences for trophic structure and ecosystem processes in high Andean meltwater streams. Here, we measured the environmental characteristics, quantified pools of particulate organic matter (POM) and periphyton (Chl. a), sampled benthic macroinvertebrates, determined functional feeding groups (FFG), and performed mesh bag decomposition experiments with Calamagrostis grass detritus at 17 stream sites along a gradient of glacial influence (GI) with 0–23% glacier cover in the catchment at 4050–4200 m a.s.l. in the Andes of Ecuador. POM was unrelated to GI while Chl. a. showed a weak (non-significant) negative relationship to GI. The macrofauna abundance decreased while taxon richness and the number of FFGs per site showed a hump-shaped relationship with increasing GI. Taxa with an opportunistic and generalist feeding mode generally dominated benthic assemblages and were related to high GI levels and low Chl. a. Only shredders were negatively related to GI, but unrelated to POM. Decomposition rates were comparable to those found in temperate alpine streams, and for both fine (0.0010–0.0065; median 0.0028 d−1) and coarse (0.0019–0.0088; median 0.0048 d−1) mesh bags, peaked at intermediate GI values, while the difference between bag types was small and almost constant along the GI gradient. This indicates an overall minor effect of macroinvertebrate shredders compared to that of microbes, in particular at high GI. It also suggests that the relatively high average temperature of these high-altitude equatorial streams (7–10 °C) does not produce higher decomposition rates than those in comparable but colder streams at temperate latitudes. The results suggest that, at the lower end of glacier cover, tropical glacier loss will not change the dominant microbial role in detritus decomposition, but that part of the physical abrasion could be partially replaced by biological shredding.

Biochemical Composition of Seston Reflecting the Physiological Status and Community Composition of Phytoplankton in a Temperate Coastal Embayment of Korea

The biochemical composition of seston along a salinity gradient were examined in the low-turbidity, temperate, estuarine embayment, Gwangyang Bay in Korea. Seasonal variations in sestonic protein (PRT), carbohydrate (CHO), and lipid (LIP) concentrations were analyzed to assess the effects of physiological status and taxonomic composition of phytoplankton. The concentrations of biochemical compounds displayed a close relationship with chlorophyll a (Chla). PRT:CHO ratios were high (&gt;1.0) in the estuarine channel in warmer months and in whole bay in February, indicating a N-replete condition for phytoplankton growth. High CHO:LIP ratios (&gt;2.5) in the saline deep-bay area during the warmer months (&gt;2.0) emphasized the importance of temperature and photoperiod over nutritional conditions. The low POC:Chla (&lt;200), molar C:N (~7) ratios, and biopolymeric carbon concentrations coupled with high primary productivity indicated a low detrital contribution to the particulate organic matter pool. Diatom dominance throughout the year contributed to consistently high carbohydrate concentrations. Furthermore, generalized additive models highlighted that phytoplankton community (i.e., size) structure may serve as an important descriptor of sestonic biochemical composition. Collectively, our results suggest that physiological and taxonomic features of phytoplankton play prominent roles in determining the biochemical composition of seston, supporting the fact that the ecosystem processes in Gwangyang Bay are largely based on phytoplankton dynamics.

Traces of sunlight in the organic matter biogeochemistry of two shallow subarctic lakes

Global environmental change alters the production, terrestrial export, and photodegradation of organic carbon in northern lakes. Sedimentary biogeochemical records can provide a unique means to understand the nature of these changes over long time scales, where observational data fall short. We deployed in situ experiments on two shallow subarctic lakes with contrasting light regimes; a clear tundra lake and a dark woodland lake, to first investigate the photochemical transformation of carbon and nitrogen elemental (C/N ratio) and isotope (δ13C, δ15N) composition in lake water particulate organic matter (POM) for downcore inferences. We then explored elemental, isotopic, and spectral (inferred lake water total organic carbon [TOC] and sediment chlorophyll a [CHLa]) fingerprints in the lake sediments to trace changes in aquatic production, terrestrial inputs and photodegradation before and after profound human impacts on the global carbon cycle prompted by industrialization. POM pool in both lakes displayed tentative evidence of UV photoreactivity, reflected as increasing δ13C and decreasing C/N values. Through time, the tundra lake sediments traced subtle shifts in primary production, while the woodland lake carried signals of changing terrestrial contributions, indicating shifts in terrestrial carbon export but possibly also photodegradation rates. Under global human impact, both lakes irrespective of their distinct carbon regimes displayed evidence of increased productivity but no conspicuous signs of increased terrestrial influence. Overall, sediment biogeochemistry can integrate a wealth of information on carbon regulation in northern lakes, while our results also point to the importance of considering the entire spectrum of photobiogeochemical fingerprints in sedimentary studies.

Oceanographic transport along frontal zones forms carbon, nitrogen, and oxygen isoscapes on the east coast of New Zealand: Implications for ecological studies

Characterizing the spatial distribution of δ18O in seawater and δ15N and δ13C in marine particulate organic matter pools can be useful in identifying isotopic discriminators of major water masses and serve as a baseline for food web and other ecological studies. Nevertheless, we do not know how isoscapes of δ18O, δ15N and δ13C are distributed and interact with mesoscale (10–100 km) physical oceanographic features for large parts of the global ocean. Here samples and measurements were collected during research cruises in the austral summer of 2017/18 and 2018/19 along the east coast of the South Island of New Zealand. The physical structure of the water column, δ18O of seawater, and δ15N and δ13C of suspended particulate organic matter were analyzed. Interannual differences in inferred circulation, oceanographic features, and the interaction of Subtropical and Subantarctic water masses in the study area were described. The relationships between nutrient concentrations, physical oceanographic variables and δ18O, δ15N and δ13C isoscapes were investigated by running linear models with all combination of variables, then averaging the most parsimonious ones that best fit the data. Results demonstrate that different environments along the east coast are connected via northward transport of Subtropical Water near the surface and Subantarctic Mode Water at depth (220–500 m). The northward transport of modified STW was associated with nutrient inputs and deepening of the maximum fluorescence depth. The spatial distribution of isotopic values had minimal temporal variability and was primarily linked to patterns of nutrient input (δ15N), coastal productivity (δ13C) and water mass type (δ18O). δ15N values demonstrated significant spatial variation, while δ18O displayed distinct values among depths. The present study was the first to resolve the distribution of δ15N and δ13C isotopes at a regional scale along the east coast of the South Island of New Zealand, δ18O in New Zealand waters, and to investigate the extent to which these distributions were influenced by patterns in physical oceanography. Average values of δ18O, δ15N and δ13C for different fishery management areas and depths are presented to support future ecological studies along Southeast New Zealand.

Elevated CO2 decreases soil carbon stability in Tibetan Plateau

The lack of ecosystem-scale CO2 enrichment experiments in alpine regions considerably restricts our ability to predict the feedback of the global carbon (C) cycle to climate change. Here we investigate soil C response in an experiment with 5-year CO2 enrichment and nitrogen (N) fertilization in a Tibetan meadow (4585 m above the sea level). We found that despite non-significant increase in bulk soil C pool, elevated CO2 dramatically altered the allocation of C in different soil fractions and soil mineralization potentials. By changing soil microbial composition and enhancing enzyme activities, elevated CO2 significantly accelerated soil organic matter (SOM) mineralization rates and stimulated the microbial utilization of ‘old C’ relative to that of ‘new C’. Furthermore, N fertilization under elevated CO2 altered the decomposition process, increased the fungi to bacteria ratio, and decreased the coarse particulate organic matter pool and enzyme activities, indicating that N fertilization counters the CO2 fertilization effect. Overall, our findings suggest a growing threat of elevated CO2 in reducing SOM stability, and highlight the key role of N availability in driving soil C turnover under elevated CO2.

Volatile organic compounds from leaf litter decomposition alter soil microbial communities and carbon dynamics.

Investigations into the transfer of carbon from plant litter to underlying soil horizons have primarily focused on the leaching of soluble carbon from litter belowground or the mixing of litter directly into soil. However, previous work has largely ignored the role of volatile organic compounds (VOCs) released during litter decomposition. Unlike most leaf carbon, these litter-derived VOCs are able to diffuse directly into the soil matrix. Here, we used a 99-d microcosm experiment to track VOCs produced during microbial decomposition of 13 C-labeled leaf litter into soil carbon fractions where the decomposing litters were only sharing headspace with the soil samples, thus preventing direct contact and aqueous movement of litter carbon. We also determined the effects of these litter-derived VOCs on soil microbial community structure. We demonstrated that the litter VOCs contributed to all measured soil carbon pools. Specifically, VOC-derived carbon accounted for 2.0, 0.61, 0.18, and 0.08% of carbon in the microbial biomass, dissolved organic matter, mineral-associated organic matter, and particulate organic matter pools, respectively. We also show that litter-derived VOCs can affect soil bacterial and fungal community diversity and composition. These findings highlight the importance of an underappreciated pathway where VOCs alter soil microbial communities and carbon dynamics.

Soil fertility status controls the decomposition of litter mixture residues

AbstractIncreasing agroecosystem biodiversity with cover crops can restore many ecosystem functions that are lost with simplified crop rotations. Mixtures of species with complementary plant traits, such as legumes and grasses, may increase multiple functions at once, including soil nutrient supply and retention, which depend on microbial decomposition dynamics. Litter mixtures can stimulate decomposition compared to individual species, and decomposition also varies with soil properties. However, the interactive effect of cover crop functional type and soil fertility status on decomposition are not known. Here, we tested for mixture effects with a legume–grass litter in soils with lower and higher levels of fertility. We also identified specific soil properties that are associated with those effects. We incubated hairy vetch, cereal rye, and vetch‐rye litter treatments for 360 d in two soils from fields with contrasting management histories and fertility levels, as defined by biological indicators such as particulate organic matter (POM) pools. We measured decomposition dynamics through respired CO2, microbial biomass, microbial extracellular enzyme activity, and inorganic nitrogen (N) mineralization. With no litter addition, the soil with larger POM pools had twofold greater microbial biomass C, seven times more net N mineralization, and respired 58% more CO2. Across both soils, after 30 d microbial biomass C increased by 58–208% following litter addition, and litter addition significantly increased CO2 production compared to the no‐litter control. However, there was no difference in the magnitude of CO2 production among cover crop treatments and soils after litter addition. The lower fertility soil had a greater response to the litter C input for CO2 production and enzyme activities in soil. Furthermore, the size and N content of free and intra‐aggregate POM pools were associated with differences in the microbial response to litter addition. Our results demonstrate that cover crop litter affects microbial decomposition dynamics differently in soils with distinct soil fertility levels and suggests that new C inputs have larger effects in lower fertility soils. Understanding how soils with different fertility levels respond to diverse cover crops, including mixtures, will inform management of agroecosystems for sustainability.

Nutrients and particulate organic matter dynamics in the outer-shelf of the South Brazil Bight: Two distinct scenarios during summer 2013

The pool of particulate organic matter driven by primary production in western boundary continental margins is known to be smaller than the highly eutrophic upwelling systems of eastern margins. Its production and export, however, can be high compared to productive ocean environments due to inherent specific physical dynamics such as mesoscale instabilities driven by the boundary current. To investigate how such physical process influence nutrients, chlorophyll-a and particulate matter on the outer shelf of the South Brazil Bight, two oceanographic cruises were conducted three months apart (November 2012 and January 2013), both under wind conditions that favor intrusions of South Atlantic Central Water (SACW). Our results showed two distinct hydrographic scenarios derived by the relative positions of oceanographic fronts in the outer shelf and shelf break region, associated with the Brazil Current (BC). In November 2012, the oligotrophic Tropical Water occupied a larger area of the continental shelf. Low nutrients and chlorophyll-a concentrations were found, typical of a regenerated production system. In contrast, maximum concentration of particulate matter and particulate inorganic carbon occurred at deeper layers in this period, together with low POC:PON ratio — probably from sediment resuspension. In January 2013, a strong shelf break upwelling was detected that enhanced SACW intrusion, notably increasing nutrient availability in the euphotic zone and the chlorophyll-a concentration at deeper layers. These deep chlorophyll maximum layers contribute to the production and accumulation of particulate organic carbon and nitrogen under these conditions. BC mesoscale instabilities appear to act as a particulate matter transport and export agent to the deep waters throughout the South Brazil Bight outer shelf.

Soil carbon storage informed by particulate and mineral-associated organic matter

Effective land-based solutions to climate change mitigation require actions that maximize soil carbon storage without generating surplus nitrogen. Land management for carbon sequestration is most often informed by bulk soil carbon inventories, without considering the form in which carbon is stored, its capacity, persistency and nitrogen demand. Here, we present coupling of European-wide databases with soil organic matter physical fractionation to determine continental-scale forest and grassland topsoil carbon and nitrogen stocks and their distribution between mineral-associated and particulate organic matter pools. Grasslands and arbuscular mycorrhizal forests store more soil carbon in mineral-associated organic carbon, which is more persistent but has a higher nitrogen demand and saturates. Ectomycorrhizal forests store more carbon in particulate organic matter, which is more vulnerable to disturbance but has a lower nitrogen demand and can potentially accumulate indefinitely. The share of carbon between mineral-associated and particulate organic matter and the ratio between carbon and nitrogen affect soil carbon stocks and mediate the effects of other variables on soil carbon stocks. Understanding the physical distribution of organic matter in pools of mineral-associated versus particulate organic matter can inform land management for nitrogen-efficient carbon sequestration, which should be driven by the inherent soil carbon capacity and nitrogen availability in ecosystems. Land management strategies for enhancing soil carbon sequestration need to be tailored to different soil types, depending on how much organic matter is stored in pools of mineral-associated and particulate organic matter, suggests an analysis of soil organic matter across Europe.

Eukaryotic influence on the oceanic biological carbon pump in the Scotia Sea as revealed by 18S rRNA gene sequencing of suspended and sinking particles

AbstractSuspended marine particles constitute most of the particulate organic matter pool in the oceans, thereby providing substantial substrates for heterotrophs, especially in the mesopelagic. Conversely, sinking particles are major contributors to carbon fluxes defining the strength of the biological carbon pump (BCP). This study is the first to investigate the differential influence of eukaryotic communities to suspended and sinking particles, using 18S rRNA gene sequencing on particles collected with a marine snow catcher in the mixed layer and upper mesopelagic of the Scotia Sea, Southern Ocean. In the upper mesopelagic, most eukaryotic phytoplankton sequences belonged to chain‐forming diatoms in sinking particles and to prymnesiophytes in suspended particles. This suggests that diatom‐enriched particles are more efficient in carbon transfer to the upper mesopelagic than those enriched in prymnesiophytes in the Scotia Sea, the latter more easily disintegrating into suspended particles. In the upper mesopelagic, copepods appeared most influential on sinking particles whereas soft‐tissue metazoan sequences contributed more to suspended particles. Heterotrophic protists and fungi communities were distinct between mixed layer and upper mesopelagic, implying that few protists ride along sinking particles. Furthermore, differences between predatory flagellates and radiolarians between suspended and sinking particles implied different ecological conditions between the two particles pools, and roles in the BCP. Molecular analyses of sinking and suspended particles constitute powerful diagnostic tools to study the eukaryotic influence on the BCP in a more holistic manner compared to classic carbon export studies focusing on sinking particles.

Impact of soil use on aggregate stability and its relationship with soil organic carbon at two different altitudes in the Colombian Andes

The stability of soil aggregates depends on the organic matter, and the soil use and management can affect the soil organicmatter (SOM) content. Therefore, it is necessary to know therelationship between aggregate stability and the content of SOMin different types of soil use at two different altitudes of theColombian Andes. This study examined the conditions of soilaggregate stability expressed as a distribution of the size classes of stable aggregates (SA) and of the mean weighted diameter of the stable aggregates (MWD). To correlate these characteristics with the soil organic carbon (OC), we measured the particulate organic matter pool (POC), the OC associated with the mineral organic matter pool (HOC), the total organic carbon content (TOC), and the humification rate (HR). Soils were sampled at two altitudes: 1) Humic Dystrudepts in a cold tropical climate (CC) with three plots: tropical mountain rainforest, pastures, and crops; 2) Fluvaquentic Dystrudepts in a warm tropical climate (WC) with three plots: tropical rainforest, an association of oil palm and pastures, and irrigated rice. Soils were sampled at three depths: 0-5, 5-10 and 10-20 cm. The physical properties, mineral particle size distribution, and bulk density were measured. The content of SA with size&gt;2.36 mm was higher in the CC soil (51.48%) than in the WC soil (9.23%). The SA with size 1.18-2.36 mm was also higher in the CC soil (7.78%) than in the WC soil (0.62%). The SA with size 0.60-1.18 mm resulted indifferent. The SA with size between 0.30 and 0.60 mm were higher in the WC soil (13.95%) than in the CC soil (4.67%). The SA&lt;0.30 mm was higher in the WC soil (72.56%) than in the CC soil (32.15%). It was observed that MWD and the SA&gt;2.36 mm increased linearly with a higher POC, but decreased linearly with a higher HR. For the SA&lt;0.30 mm, a linear decrease was observed at a higher POC, while it increased at a higher HR.

Effects of intercropping and soil properties on root functional traits of cover crops

Cover crop trait diversity may enhance multiple ecosystem functions and environmental sustainability in agroecosystems, providing a mechanistic link between crop diversity and changes in ecosystem processes. In agroecosystems, root traits play a key role in functions related to soil nutrient cycling processes, but belowground traits of cover crops are poorly understood. In this study, we analyzed root functional traits of hairy vetch (V. villosa, a legume) and cereal rye (S. cereale, a grass) across five organically-managed agroecosystems in southeastern Michigan selected to span diverse soil fertility levels. Specifically, in this study we ask: (1) what is the extent of cover crop root functional trait variation and is this modified in mixture? (2) Does the expression of cover crop root traits co-vary in predictable patterns? And (3) Which soil properties best explain functional trait variability of cover crops? We found some support for root trait trade-offs; for instance, root C:N and root diameter were positively correlated for rye in mixture (p = 0.049), as were root %N and root length for rye in both mixture (p = 0.003) and monoculture (p < 0.0001). Acquisitive root traits (higher root length and specific root length) increased by 43–66% for vetch grown in mixture, and conservative traits (larger root diameter and higher C:N) were favored in monoculture. As expected, nitrogen in soil particulate organic matter pools, and plant-available phosphorus, were correlated with root trait expression across the farm gradient. These findings contribute to predictive understanding of root trait variation of cover crops across different environmental and management conditions, and inform the design of agroecosystems for delivery of belowground ecosystem services with cover crops.

Post-agricultural restoration: Implications for dynamics of soil organic matter pools

In order to bridge the gap of knowledge in respect of soil organic matter (SOM) dynamics of post-agricultural Luvisols during restoration, the study focused on alteration processes of functionally different SOM pools, 14C ages, and C turnover rates in a chronosequential approach. The study comprises three Luvisol chronosequences of 37 years (North), 120 years (Middle), and 42 years (South) across a climate gradient of the temperate broad-leaved forests of European Russia. Restoration led to SOM accumulation within both active (free particulate organic matter (POM) and occluded POM and passive (clay fraction) pools, and as a result, in gains of total organic carbon (C). Accumulation of new C was confirmed by increasing ∆14C signatures of occluded POM and clay fractions. After 22 years of restoration, ∆14C signatures progressed from negative to positive values for the occluded POM fraction, whereas they showed a similar development but remained negative for the clay faction. This highlights the fact that the incorporation of new C into the passive pool takes place, but that it occurs at a slower rate compared to that of the active pool. The 14C age of occluded POM fraction declined from 600 to 1200 years before present (BP) in arable sites to about 200 years BP after 22 years of restoration and thereafter. The 14C age of the clay fraction decreased from 1670 to 2660 years BP in arable soils to 750 years BP in 22 year abandoned site, 450–310 years BP in 32–66 year abandoned sites, and 200–370 years BP in near-natural forests. This decline of the 14C age reveals a change of the proportions between old and young C within the passive pool caused by management and the development of an ecosystem towards a steady state. Recovery of C was accompanied by initially increasing and then decreasing turnover rates of SOM, both again, much more pronounced in the active pool. These dynamics trace an initial ecosystem disturbance after abandonment, which slows down with time. Although C recovery and adjusted turnover rates of SOM progressed towards a new steady state of ecosystem; full restoration was not achieved within the chronosequential time scale of 120 years.

Particulate Organic Matter in Noncultivated and Arable Soils

Data on the contents of particulate organic matter (POM) in the humus horizons of different soil types were obtained. The influence of the land use system and fertilizers on the carbon of particulate organic matter (CPOM) in gray forest soil and typical chernozem was shown. The mineralization rate of POM under constant conditions of temperature and moisture was estimated, and the ratios between the pools of POM and biologically active organic matter in the soils were calculated. In the soils of natural cenoses, the percentage of POM together with the sand particle-size fraction varied within 12–36% of the soil mass, and the CPOM content varied from 1.66 to 8.03% of the fraction mass. The portion of CPOM in the total Corg ranged from 20 to 48%. Cultivated gray forest soil and typical chernozem were depleted of CPOM by 3.0–3.2 and 2.0–2.8 times, respectively, in comparison with the samples of noncultivated soils. The mineralization rate of POM in the gray forest soil and typical chernozem was about the same (7.9–12.1 and 7.2–8.2% of CPOM, respectively), which was about 1.2–2.4 times higher than the mineralization rate of the total soil organic matter. It was noted that POM could be a sensitive indicator of the Corg dynamics in the soil, a good predictor of changes in the content and quality of soil organic matter, and a significant source of potentially mineralizable organic matter in the soil.