Negative Priming Effect on Organic Matter Mineralisation in NE Atlantic Slope Sediments

The priming effect (PE) is a complex phenomenon which describes a modification (acceleration or retardation) in the mineralisation rate of refractory organic matter (OM) following inputs of labile material. PEs are well-studied in terrestrial ecosystems owing to their potential importance in the evolution of soil carbon stocks but have been largely ignored in aquatic systems despite the fact that the prerequisite for their occurrence, i.e. the co-existence of labile and refractory OM, is also true for sediments. We conducted stable isotope tracer experiments in continental margin sediments from the NE Atlantic (550–950 m) to study PE occurrence and intensity in relation to labile OM input. Sediment slurries were treated with increasing quantities of the ¹³C-labelled diatom Thalassiosira rotula and PE was quantified after 7, 14 and 21 days. There was a stepwise effect of diatom quantity on its mineralisation although mineralisation efficiency dropped with increasing substrate amounts. The addition of diatomaceous OM yielded a negative PE (i.e. retardation of existing sediment OM mineralisation) at the end of the experiment regardless of diatom quantity. Negative PE is often the result of preferential utilisation of the newly deposited labile material by the microbial community (“preferential substrate utilization”, PSU) which is usually observed at excessive substrate additions. The fact that PSU and the associated negative PE occurred even at low substrate levels in this study could be attributed to limited amounts of OM subject to priming in our study area (∼0.2% organic carbon [OC]) which seems to be an exception among continental slopes (typically >0.5%OC). We postulate that PEs will normally be positive in continental slope sediments and that their intensity will be a direct function of sediment OC content. More experiments with varying supply of substrate targeting C-poor vs. C-rich sediments are needed to confirm these hypotheses.     

Variability of Organic Matter Inputs Affects Soil Moisture and Soil Biological Parameters in a European Detritus Manipulation Experiment

Over the last three decades, increased temperatures and reduced annual precipitation have resulted in significant changes in several Central European deciduous forests. These effects include changes in soil moisture content and detritus production. Within the framework of a detritus manipulation experiment carried out in an old-growth Quercetum petraea–cerris community, we examined how changes in detritus inputs affect soil moisture content and microbial activity within six treatments. CO₂ release and microbial enzyme activities are known to be highly sensitive to environmental factors such as soil moisture and detritus inputs. We applied three detritus removal (No Litter, No Roots and No Input) and two detritus addition (Double Litter and Double Wood) treatments. Although the plots received the same amount of precipitation, the various detritus inputs caused significant differences in soil moisture. Treatments excluding living roots had the highest moisture levels, while the treatment excluding only aboveground detritus inputs had the lowest. CO₂ release, arylsulphatase activity and saccharase activity showed significant seasonal differences with the highest values occurring in spring. Moisture content had a significant positive correlation with CO₂ release, and enzyme activities of the plots were affected by the quantity and quality of detritus inputs. Arylsulphatase activity showed the strongest correlation with soil moisture content (R?=?0.62 in the control plot) followed by CO₂ release (R?=?0.61) and finally saccharase activity (R?=?0.42). We observed that there was a remarkably weaker correlation between soil moisture content and the three parameters in the detritus removal treatments (R values between 0.56 and 0.13) than in the Control and detritus addition treatments (R values between 0.72 and 0.42). The correlation between the three parameters of interest and soil moisture content weakens considerably under drought conditions relative to the optimal moisture range of soil moisture content for microbial activity. If the amount of precipitation in the area continues to decrease as anticipated, then litter production and soil microbial activity may be reduced.     

Experimental Protocol for Manipulating Plant-induced Soil Heterogeneity

Coexistence theory has often treated environmental heterogeneity as being independent of the community composition; however biotic feedbacks such as plant-soil feedbacks (PSF) have large effects on plant performance, and create environmental heterogeneity that depends on the community composition. Understanding the importance of PSF for plant community assembly necessitates understanding of the role of heterogeneity in PSF, in addition to mean PSF effects. Here, we describe a protocol for manipulating plant-induced soil heterogeneity. Two example experiments are presented: (1) a field experiment with a 6-patch grid of soils to measure plant population responses and (2) a greenhouse experiment with 2-patch soils to measure individual plant responses. Soils can be collected from the zone of root influence (soils from the rhizosphere and directly adjacent to the rhizosphere) of plants in the field from conspecific and heterospecific plant species. Replicate collections are used to avoid pseudoreplicating soil samples. These soils are then placed into separate patches for heterogeneous treatments or mixed for a homogenized treatment. Care should be taken to ensure that heterogeneous and homogenized treatments experience the same degree of soil disturbance. Plants can then be placed in these soil treatments to determine the effect of plant-induced soil heterogeneity on plant performance. We demonstrate that plant-induced heterogeneity results in different outcomes than predicted by traditional coexistence models, perhaps because of the dynamic nature of these feedbacks. Theory that incorporates environmental heterogeneity influenced by the assembling community and additional empirical work is needed to determine when heterogeneity intrinsic to the assembling community will result in different assembly outcomes compared with heterogeneity extrinsic to the community composition.     

Influence of Soil Organic Matter on Copper Extraction from Contaminated Soil

Column experiments of copper extraction from four contaminated soils characterized by a content of Soil Organic Matter (SOM) ranging from 1% to 25% are presented and discussed. The extraction was performed by flushing the soil with an aqueous solution of a sodium salt of ethylene diamminotetraacetic acid (EDTA). Preliminary tests were performed on a soil containing 25% of organic matter, to investigate the influence of pH, concentration and volumes of EDTA on its chelant action and on the dissolution of SOM. Having selected the optimal conditions for the extraction process, a further series of tests was conducted on the four soils to evaluate the influence of organic content on copper extraction yields. EDTA solutions at 0.01 M, 0.05 M, 0.1 and 0.2 M were injected at 0.33 ml/s; copper and organic matter extraction yield were determined. At a pH of 5, 15 pore volume (PV) of a solution containing 0.05M EDTA, extracted about 99% of copper contained by the soil with the higher organic matter content. Under the same conditions, and for soil with > 6% SOM, extraction yields over 80% were achieved, while at lower organic content, copper extraction was dramatically reduced. This was attributed to the formation of highly stable copper-humate complexes and to their increasingly dissolution that occurred in the soils with higher organic matter level.

Experimental tests performed at different contamination levels (1200 mg/kg, 2400 mg/kg) showed that EDTA extraction effectiveness also depended upon initial soil Cu concentration.     

Xiphinema americanum as Affected by Soil Organic Matter and Porosity

The effects of four soil types, soil porosity, particle size, and organic matter were tested on survival and migration of Xiphinema americanum. Survival and migration were significantly greater in silt loam than in clay loam and silty clay soils. Nematode numbers were significantly greater in softs planted with soybeans than in fallow softs. Nematode survival was greatest at the higher of two pore space levels in four softs. Migration of X. americanum through soft particle size fractions of 75-150, 150-250, 250-500, 500-700, and 700-1,000 μ was significantly greater in the middle three fractions, with the least occurring in the smallest fraction. Additions of muck to silt loam and loamy sand soils resulted in reductions in survival and migration of the nematode. The fulvic acid fraction of muck, extracted with sodium hydroxide, had a deleterious effect on nematode activity. I conclude that soils with small amounts of air-filled pore space, extremes in pore size, or high organic matter content are deleterious to the migration and survival of X. americanum, and that a naturally occurring toxin affecting this species may be present in native soft organic matter.     

Organic Matter and Water Addition Enhance Soil Respiration in an Arid Region

Climate change is generally predicted to increase net primary production, which could lead to additional C input to soil. In arid central Asia, precipitation has increased and is predicted to increase further. To assess the combined effects of these changes on soil CO₂ efflux in arid land, a two factorial manipulation experiment in the shrubland of an arid region in northwest China was conducted. The experiment used a nested design with fresh organic matter and water as the two controlled parameters. It was found that both fresh organic matter and water enhanced soil respiration, and there was a synergistic effect of these two treatments on soil respiration increase. Water addition not only enhanced soil C emission, but also regulated soil C sequestration by fresh organic matter addition. The results indicated that the soil CO₂ flux of the shrubland is likely to increase with climate change, and precipitation played a dominant role in regulating soil C balance in the shrubland of an arid region.     

Incorporation of Petroleum Carbon Into Soil Organic Matter During Biodegradation

A procedure, based on measurement of the stable carbon isotope ¹³C, has been developed for determining the extent to which petroleum carbon is incorporated into soil organic matter (SOM) by humification of biomass produced during biodegradation of the petroleum in soil. We have shown that a crude oil having a δ¹³C of-27.4%, when biodegraded in a soil containing SOM with a δ¹³C of-15.7%, resulted in a change of the δ¹³C of the bound SOM reflecting that of petroleum carbon. Comparison of five soil biodegradation tests using different amounts and types of fertilizer to stimulate biodegradation of the oil in this soil showed that the extent of the δ¹³C change in the bound SOM varied with the extent of oil biodegradation observed. To obtain ¹³C data on the SOM, the residual petroleum was first removed by rigorous extraction with dichloromethane using a Soxhlet apparatus. The extracted soil was then combusted to release bound carbon as CO₂, which was analyzed for ¹³C. Where the SOM has a δ¹³C similar to that of petroleum, ¹⁴C measurements of SOM would give similar results. This type of data, referred to as the petroleum “footprint” in the SOM, could be useful in identifying or confirming intrinsic biodegradation of petroleum in contaminated soil.     

Permafrost Distribution Drives Soil Organic Matter Stability in a Subarctic Palsa Peatland

Palsa peatlands, permafrost-affected peatlands characteristic of the outer margin of the discontinuous permafrost zone, form unique ecosystems in northern-boreal and arctic regions, but are now degrading throughout their distributional range due to climate warming. Permafrost thaw and the degradation of palsa mounds are likely to affect the biogeochemical stability of soil organic matter (that is, SOM resistance to microbial decomposition), which may change the net C source/sink character of palsa peatland ecosystems. In this study, we have assessed both biological and chemical proxies for SOM stability, and we have investigated SOM bulk chemistry with mid-infrared spectroscopy, in surface peat of three distinct peatland features in a palsa peatland in northern Norway. Our results show that the stability of SOM in surface peat as determined by both biological and chemical proxies is consistently higher in the permafrost-associated palsa mounds than in the surrounding internal lawns and bog hummocks. Our results also suggest that differences in SOM bulk chemistry is a main factor explaining the present SOM stability in surface peat of palsa peatlands, with selective preservation of recalcitrant and highly oxidized SOM components in the active layer of palsa mounds during intense aerobic decomposition over time, whereas SOM in the wetter areas of the peatland remains stabilized mainly by anaerobic conditions. The continued degradation of palsa mounds and the expansion of wetter peat areas are likely to modify the bulk SOM chemistry of palsa peatlands, but the effect on the future net C source/sink character of palsa peatlands will largely depend on moisture conditions and oxygen availability in peat.     

Retention of Nitrate-N in Mineral Soil Organic Matter in Different Forest Age Classes

Ecosystems - Conceptual models of nutrient retention in ecosystems suggest that mature forests receiving chronically elevated atmospheric nitrogen (N) deposition should experience increased nitrate...     

Heterogeneity of Astrocytes in Grey and White Matter

Astrocytes are a diverse and heterogeneous type of glial cells. The major task of grey and white matter areas in the brain are computation of information at neuronal synapses and propagation of action potentials along axons, respectively, resulting in diverse demands for astrocytes. Adapting their function to the requirements in the local environment, astrocytes differ in morphology, gene expression, metabolism, and many other properties. Here we review the differential properties of protoplasmic astrocytes of grey matter and fibrous astrocytes located in white matter in respect to glutamate and energy metabolism, to their function at the blood–brain interface and to coupling via gap junctions. Finally, we discuss how this astrocytic heterogeneity might contribute to the different susceptibility of grey and white matter to ischemic insults.

     

Temporal Variability of Primary Production Explains Marine Ecosystem Structure and Function

Ecosystems - Understanding drivers of ecosystem structure and function is a pervasive goal in academic and applied research. We used 24 synthetic ecosystem-level indices derived from trophic...     

The Impact of Corn Residue Removal on Soil Aggregates and Particulate Organic Matter

Removal of corn (Zea mays L.) stover as a biofuel feedstock is being considered. It is important to understand the implications of this practice when establishing removal guidelines to ensure the long-term sustainability of both the biofuel industry and soil health. Aboveground and belowground plant residues are the soil’s main sources of organic materials that bind soil particles together into aggregates and increase soil carbon (C) storage. Serving to stabilize soil particles, soil organic matter (SOM) assists in supplying plant available nutrients, increases water holding capacity, and helps reduce soil erosion. Data obtained from three Corn Stover Regional Partnership sites (Brookings, SD; Morris, MN; and Ithaca, NE) were utilized to evaluate the impact of removing corn stover on soil physical properties, including dry aggregate size distribution (DASD), erodible fraction (EF), and SOM components. Each site consisted of a combination of three residue removal rates (low—removal of grain only, intermediate—approximately 50 % residue removal, and high—maximum amount of residue removal). Results showed that the distribution of soil aggregates was less favorable for all three locations when residue was removed without the addition of other sources of organic matter such as cover crops. Additionally, we found that when residue was removed and the soil surface was less protected, there was an increase in the EF at all three research sites. There was a reduction in the EF for both the Brookings, SD, and Ithaca, NE sites when cover crops were incorporated or additional nitrogen (N) was added to the system. Amounts of SOM, fine particulate organic matter (fPOM), and total particulate organic matter (tPOM) consistently decreased as greater amounts of residue were removed from the soil surface. Across these three locations, the removal of crop residue from the soil surface had a negative impact on measured soil physical properties. The addition of a cover crop or additional N helped reduce this impact as measured through aggregate size distribution and EF and SOM components.     

Tree Species Effects on Soil Organic Matter Dynamics: The Role of Soil Cation Composition

Abstract We studied the influence of tree species on soil carbon and nitrogen (N) dynamics in a common garden of replicated monocultures of fourteen angiosperm and gymnosperm, broadleaf and needleleaf species in southwestern Poland. We hypothesized that species would influence soil organic matter (SOM) decomposition primarily via effects on biogeochemical recalcitrance, with species having tissues with high lignin concentrations retarding rates of decomposition in the O and A horizons. Additionally, because prior work demonstrated substantial divergence in foliar and soil base cation concentrations and soil pH among species, we hypothesized that species would influence chemical stabilization of SOM via cation bridging to mineral surfaces in the A-horizon. Our hypotheses were only partially supported: SOM decomposition and microbial biomass were unrelated to plant tissue lignin concentrations, but in the mineral horizon, were significantly negatively related to the percentage of the cation exchange complex (CEC) occupied by polyvalent acidic (hydrolyzing) cations (Al and Fe), likely because these cations stabilize SOM via cation bridging and flocculation and/or because of inhibitory effects of Al or low pH on decomposers. Percent CEC occupied by exchangeable Al and Fe was in turn related to both soil clay content (a parent material characteristic) and root Ca concentrations (a species characteristic). In contrast, species influenced soil N dynamics largely via variation in tissue N concentration. In both laboratory and in situ assays, species having high-N roots exhibited faster rates of net N mineralization and nitrification. Nitrification:mineralization ratios were greater, though, under species with high exchangeable soil Ca²⁺. Our results indicate that tree species contribute to variation in SOM dynamics, even in the mineral soil horizons. To our knowledge the influence of tree species on SOM decomposition via cation biogeochemistry has not been demonstrated previously, but could be important in other poorly buffered systems dominated by tree species that differ in cation nutrition or that are influenced by acidic deposition.     

Uptake of Allochthonous Dissolved Organic Matter from Soil and Salmon in Coastal Temperate Rainforest Streams

Dissolved organic matter (DOM) is an important component of aquatic food webs. We compare the uptake kinetics for NH₄–N and different fractions of DOM during soil and salmon leachate additions by evaluating the uptake of organic forms of carbon (DOC) and nitrogen (DON), and proteinaceous DOM, as measured by parallel factor (PARAFAC) modeling of DOM fluorescence. Seasonal DOM slug additions were conducted in three headwater streams draining a bog, forested wetland, and upland forest using DOM collected by leaching watershed soils. We also used DOM collected from bog soil and salmon carcasses to perform additions in the upland forest stream. DOC uptake velocity ranged from 0.010 to 0.063 mm s⁻¹ and DON uptake velocity ranged from 0.015 to 0.086 mm s⁻¹, which provides evidence for the whole-stream uptake of allochthonous DOM. These findings imply that wetlands could potentially be an important source of DOM to support stream heterotrophic production. There was no significant difference in the uptake of DOC and DON across the soil leachate additions (P > 0.05), although differential uptake of DOM fractions was observed as protein-like fluorescence was removed from the water column more efficiently than bulk DOC and DON (P < 0.05). Moreover, PARAFAC analysis of DOM fluorescence showed that protein-like fluorescence decreased downstream during all DOM additions, whereas humic-like fluorescence did not change. This differential processing in added DOM suggests slow and fast turnover pools exist for aquatic DOM. Taken together, our findings argue that DON could potentially fill a larger role in satisfying biotic N demand in oligotrophic headwater streams than previously thought. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Author contributions  J.B.F. conceived of or designed study, performed research, analyzed data, contributed new methods or models, and wrote the paper. E.H. conceived of or designed study and analyzed data. R.T.E. conceived of or designed study and analyzed data. J.B.J. contributed new methods or models and analyzed data.     

Particulate Organic Matter Affects Soil Nitrogen Mineralization under Two Crop Rotation Systems

Changes in the quantity and/or quality of soil labile organic matter between and after different types of cultivation system could play a dominant role in soil nitrogen (N) mineralization. The quantity and quality of particulate organic matter (POM) and potentially mineralizable-N (PMN) contents were measured in soils from 16 paired rice-rapeseed (RR)/cotton-rapeseed (CR) rotations sites in Hubei province, central China. Then four paired soils encompassing low (10th percentile), intermediate (25th and 75th percentiles), and high (90th percentile) levels of soil PMN were selected to further study the effects of POM on soil N mineralization by quantifying the net N mineralization in original soils and soils from which POM was removed. Both soil POM carbon (POM-C) and N (POM-N) contents were 45.8% and 55.8% higher under the RR rotation compared to the CR rotation, respectively. The PMN contents were highly correlated with the POM contents. The PMN and microbial biomass N (MBN) contents concurrently and significantly decreased when POM was removed. The reduction rate of PMN was positively correlated with changes in MBN after the removal of POM. The reduction rates of PMN and MBN after POM removal are lower under RR rotations (38.0% and 16.3%, respectively) than CR rotations (45.6% and 19.5%, respectively). Furthermore, infrared spectroscopy indicated that compounds with low-bioavailability accumulated (e.g., aromatic recalcitrant materials) in the soil POM fraction under the RR rotation but not under the CR rotation. The results of the present study demonstrated that POM plays a vital role in soil N mineralization under different rotation systems. The discrepancy between POM content and composition resulting from different crop rotation systems caused differences in N mineralization in soils.     

Effects of Added Organic Matter and Water on Soil Carbon Sequestration in an Arid Region

It is generally predicted that global warming will stimulate primary production and lead to more carbon (C) inputs to soil. However, many studies have found that soil C does not necessarily increase with increased plant litter input. Precipitation has increased in arid central Asia, and is predicted to increase more, so we tested the effects of adding fresh organic matter (FOM) and water on soil C sequestration in an arid region in northwest China. The results suggested that added FOM quickly decomposed and had minor effects on the soil organic carbon (SOC) pool to a depth of 30 cm. Both FOM and water addition had significant effects on the soil microbial biomass. The soil microbial biomass increased with added FOM, reached a maximum, and then declined as the FOM decomposed. The FOM had a more significant stimulating effect on microbial biomass with water addition. Under the soil moisture ranges used in this experiment (21.0%–29.7%), FOM input was more important than water addition in the soil C mineralization process. We concluded that short-term FOM input into the belowground soil and water addition do not affect the SOC pool in shrubland in an arid region.     

A Quantitative Model of Soil Organic Matter Accumulation During Floodplain Primary Succession

Texture is an important influence on organic matter (SOM) dynamics in upland soils but little is known about its role in riverine soils. We hypothesized that texture might be especially important to SOM accumulation in young alluvial soils. We combined the soil component of the CENTURY ecosystem model, which uses sand, silt, and clay concentration as primary variables, with a simple simulation model of fluvial deposition, and forest production to predict changes in soil carbon (C) and nitrogen (N) during primary succession on floodplains and terraces of the Queets River, Washington. Simulated soil C accumulated to a plateau of about 4000 g m⁻² at 110 years, closely matching observed patterns in an empirical chronosequence. Although direct fluvial OM deposition had only a small and short-lived influence on soil C, fluvial silt and clay deposition were an important influence on equilibrium C. The model underestimated soil N by about 35%, which appears to be due to failure of the model to account for N enrichment of an OM pool after its initial formation. These results suggest that basic influences on SOM retention in these young soils are not functionally different than those that apply to upland soils, but occur within highly dynamic physical contexts. Overbank deposition of silt and clay establishes a basic capacity for SOM retention. SOM, in turn, facilitates N retention. In this way, silt and clay are instrumental in propagating N forward from N-fixing red alder (Alnus rubra) stands to mature conifer forests that are frequently N-limited.