Refractory Fraction Research Articles

Effects of Damage to Living Plants on Leaf Litter Quality

The leaves of plants in nature are commonly subjected to damage from a wide variety of agents, including herbivory, air pollutants, and simple physical damage. Despite the attention paid to damage effects on living plants, the potential effects on the quality of litter derived from damaged leaves has not been considered. We used controlled laboratory assays of decomposition to show that both ozone (0.2 mL/m3, 4 h) and mite damage, but not ultraviolet radiation (UV‐B) exposure, to living leaves of cottonwood plants resulted in a decrease in decomposition rate of litter derived from damaged leaves. Decomposition rates were ≈ 50% slower for litter from damaged plants, and there was a twofold increase in the refractory fraction. Contrary to expectation, there was a negative relationship between rate of decomposition and litter nitrogen content. Our finding of slow decomposition of high‐nitrogen litter is explained by a general mechanism whereby cellular damage causes increases in complex phenolic material. Such materials can lead to reductions in decomposition and binding of available nitrogen. We suggest that this mechanism can translate a common occurrence, damage by a diversity of processes, into long‐term and possibly large‐scale alterations in detritus processing.

Distribution of Dissolved Organic Carbon and Bacteria at the Interface between the Rhône River and Its Alluvial Aquifer

To understand the efficiency of interstitial habitats in the elimination of organic matter as it moves from surface water to groundwater (bank filtration), we studied spatial and temporal variations of sediment organic matter concentration, biodegradable (BDOC) and refractory (RDOC) fractions of the dissolved organic carbon, bacterial abundances, and microbial enzymatic activity in the first metre of sediment of the Rhone River immediately downstream of a large city. The study area was fed most of the year by the surface water inflow (downwelling area), because of groundwater pumping wells located - 80 m from the river. Decreasing gradients from surface water to deep sediments and from the river to the shore were observed in most of the cases for the four variables. The decrease in RDOC concentrations did not vary seasonally (this decrease is probably due to physical process, such as adsorption on fine mineral particles), whereas decreases in BDOC concentrations only occurred when microbial enzymatic activities were high; BDOC is rapidly assimilated by microbial communities. Physical and biological processes together make this first metre of sediment an efficient filter for organic matter.

Interaction of detritus with abundance of cyanobacteria and microalgae

Detritus particles interact with phytoplankton growth through light attenuation and nutrient retention. A model is described for predicting abundance of cyanobacteria and microalgae in relation to the detritus dynamics in shallow lakes with varying phosphorus load. Steady-state P distribution among cyanobacterial biomass, detritus and the dissolved pool depending on mortality and detritus mineralisation rate was predicted from kinetics for P-limited growth and refractory fraction of the detritus. The depth-averaged irradiance was computed for a constant background attenuation and specific attenuation coefficients of biomass and detritus estimated from field and laboratory data. This light was compared to the light critical to net growth of microalgae based on initial slope of growth vs. irradiance relationship, maximal growth rate and constant maintenance rate. It was predicted that the probability of abundance of microalgae relative to that of cyanobacteria is high at low population loss rates, coinciding with the lowest depth-averaged irradiances at a particular P load. This prediction is consistent with observations on many shallow lakes.

Interaction of detritus with abundance of cyanobacteria and microalgae

Detritus particles interact with phytoplankton growth through fight attenuation and nutrient retention. A model is described for predicting abundance of cyanobactena and microalgae in relation to the detritus dynamics in shallow lakes with varying phosphorus load. Steady-state P distribution among cyanobactenal biomass, detritus and the dissolved pool depending on mortality and detritus mmeralisation rate was predicted from kinetics for P-limited growth and refractory fraction of the detritus. The depth-averaged uradiance was computed for a constant background attenuation and specific attenuation coefficients of biomass and detntus estimated from field and laboratory data. This light was compared to the light critical to net growth of microalgae based on initial slope of growth vs. irradiance relationship, maximal growth rate and constant maintenance rate. It was predicted that the probability of abundance of microalgae relative to that of cyanobacteria is high at low population loss rates. coinciding with the lowest depth-averaged uradiances at a particular P load. This prediction is consistent with observations on many shallow lakes.

Hydrotreatment process kinetics for bitumen and bitumen-derived liquids

Hydrodenitrogenation, hydrodesulfurization and resid conversion data for the Whiterocks bitumen and bitumen-derived liquid were analysed using a modified power rate law model. The data were obtained with a fixed-bed reactor in the upflow mode. A space velocity (WHSV α) term was included to account for deviations from plug flow behaviour. The upflow mode was preferred to the trickle-bed mode to ensure complete wetting of the catalyst and nearly isothermal operation. The apparent kinetic parameters were obtained by combined non-linear regression and ODE solver techniques for the analysis of laboratory data. A simple nth-order power rate law expression for hydrodenitrogenation and hydrodesulfurization was examined. The higher than first-order kinetics for hydrodenitrogenation and hydrodesulfurization of the bitumen and bitumen-derived liquids were explained by invoking two parallel first-order reactions for the facile and refractory fractions or lumps. Parallel and consecutive reaction schemes were used to examine the extent of conversion of the resid fraction to middle distillate, gas oil and naphtha. The apparent kinetic parameters were also determined.

Changes in carbon storage in temperate humic loamy soils after forest clearing and continuous corn cropping in France

Soil samples from forest and agricultural sites in three areas of southwest France were collected to determine the effect of forest conversion to continuous intensive corn cropping with no organic matter management on soil organic carbon (C) content. Soils were humic loamy soils and site characteristics that may affect soil C were as uniform as possible (slope, elevation, texture, soil type, vegetation). Three areas were selected, with adjacent sites of various ages of cultivation (3 to 35 yr), and paired control forest sites. The ploughed horizon (0-Dt cm) and the Dt-50 cm layer were collected at each agricultural site. In forest sites, each 10 cm layer was collected systematically down to 1 meter depth. Carbon concentrations were converted to total content to a given depth as the product of concentration, depth of sample and bulk density, and expressed in units of kg m-2. For each site and each sampled layer, the mineral mass of soil was calculated, in order to base comparisons on the same soil mass rather than the same depth. The pattern of C accumulation in forest soils showed an exponential decrease with depth. Results suggested that soil organic carbon declined rapidly during the first years of cultivation, and at a slower rate thereafter. This pattern of decrease can be fitted by a bi-exponential model assuming that initial soil organic carbon can be separated into two parts, a very labile pool reduced during the first rapid decline and more refractory fractions oxidizing at a slower rate. Sampling to shallow depths (0-Dt cm) resulted in over-estimation of the rate of carbon release in proportion to the initial amount of C, and in under-estimation of the total loss of C with age. The results for the 0–50 cm horizon indicated that losses of total carbon average about 50% in these soils, ranging in initial carbon content from 19 to 32.5 kg m-2. Carbon release to the atmosphere averaged 0.8 kg m-2 yr-1 to 50 cm depth during the first 10 years of cultivation. The results demonstrate that temperate soils may also be an important source of atmospheric carbon, when they are initially high in carbon content and then cultivated intensively with no organic matter management.

Use of alkaline fly ash as an amendment for swine manure

Alkaline fly ash was characterized for the leaching potential of some major and minor constituents, and then added to swine manure at 10% and 20% w/v ratio. The influence of fly ash on microbial production of CO 2 was studied during a 12-day experimental period. pH, ammonium- and total N, organic C, available B, available and more refractory fractions of P, K, Mg, Cu, Fe, Mn and Zn were determined on the fly ash-amended manure. On the basis of the results, it appears that the reduction in CO 2 production which occurred in the amended manure was probably due to the high pH values of the manure caused by fly ash addition, rather than to an inhibition of microbial activity and respiration. A marked mobilization of inorganic P compounds of fly ash, not correlated with the gradual acidification of the mixtures during the experimental period, occurred in the amended manure, possibly as a consequence of the microbial activity. Boron was solubilized in the 20%-amended manure at an extent that might be detrimental to crops.

The investigation of dissolved and suspended-particulate trace metal fractionation in the Black Sea

A sequential filtration-ion-exchange column scheme has been developed for the investigation of dissolved trace metal fractionation. The method is designed to separate water column metals rapidly into particulate and colloidal-sized species, anionic dissolved metal-organic and/or metal-sulfide complexes, and ‘free’ (hydrated) metal cations and/or labile major ion complexes. The suspended-particulate matter (0.4 μm filterable material) is further subjected to a sequential leaching procedure to separate particulate metals into weak-acid leachable, strong-acid leachable, and refractory fractions. These methods are evaluated with respect to laboratory experiments and field results. These methods have been applied to the study of trace metal chemical fractionation in the Black Sea, the world's largest anoxic basin. We present here the results for Co, Ni, Cu, Zn, Cd, and Pb. The dissolved Co distribution is best explained in terms of a scavenging-regeneration cycle with Mn-oxyhydroxides across the sulfide interface and coprecipitation of Co with Fe-sulfides in the deep waters. Nickel displays nearly constant dissolved metal concentrations with depth and is apparently unaffected by redox processes. The Class B metals (Cu, Zn, Cd, and Pb) have high dissolved metal concentrations in the surface waters then decrease rapidly across the sulfide interface to low deep water concentrations, consistent with metal-sulfide precipitation below the interface. The dissolved Class B metal fractionation was generally dominated in the oxic zone by ‘free’ metal species, shifting below the interface to anionic species, probably dissolved metal-sulfide complexes. The suspended matter trace metal fractionation is dominated by weak-acid soluble forms throughout most of the water column. For particulate Co, strong-acid leachable forms, probably metal-sulfide phases, are important in the anoxic deep waters. The Class B metals form relatively reactive (weak-acid leachable) metal-sulfide precipitates just below the sulfide interface.

15N and 13C natural abundance in suspended particulate organic matter from a Kuroshio warm-core ring

Vertical profiles of the natural abundances of 15N and 13C in suspended particulate organic materials (POM) were determined together with PON and POC concentrations to the depth of ca 4000 m at five stations in and out of a warm-core ring (WCR). The 15N and 13C profiles showed generally, with one exception for a 15N profile at the northern edge of the ring, the same trend: an increasing abundance of 15N and 13C in the subsurface layer, followed by a decrease with depth. At the northern station, the 15N abundance of PON in the euphotic zone was unexpectedly high (∼20.4‰). PON with high 15N abundance was also observed at 30, 75 and 150 m at the southern periphery of the ring. This was not seen for the 13C of POC. T-S analysis indicated that POM with high 15N content was transported from the northern periphery by the ‘cold streamer’ of the WCR. In the depth zone of 100–500 m, where thermostad appeared in the ring, the 15N of PON increased with depth while the 13C of POC remained constant in the ring; conversely, the 13C of POC decreased with depth while 15N of PON remained constant out of the ring. Temporal variation was noted for the 15N abundance of PON in the thermostad of the WCR, but little variation was observed for the 13C of POC. The spatial and temporal differences between the 15N and the 13C profiles were explained assuming an intensified vertical mixing of the water column in the thermostad of the WCR, and an order of magnitude difference between the abundance of refractory fractions in POC and PON enriched with both 15N and 13C. It is suggested that the POM 15N and 13C isotopie ratios provide complementary information to study particle dynamics in the ocean.

Anaerobic treatment of leachates in UASB reactors

AbstractLeachates from the solid urban wastes landfill of Asturias were characterized and treated in upflow anaerobic sludge blanket laboratory reactors. The highest percentage of COD removal was 88% for a hydraulic retention time of 2–4 days. Gas production was close to the theoretical values, with CH4 content up to 88%. HCl was needed to keep pH neutral, and phosphate was added as nutrient. There was a refractory fraction to the biodegradation of 7%.

Phanerozoic atmospheric reconstructions: a terrestrial perspective

The Paleozoic stands out in geochemical reconstructions as a time of unusually high organic carbon burial and disturbed O 2 and CO 2 regimes. The anomaly is apparently of terrestrial origin. Evidence and explanations for reconstructed trends are sought in the terrestrial record. The correspondence between reconstructed O 2 and fusinite abundance in coal, a possible proxy for charcoal formation and fire activity, is ambiguous due to problems of data quality and interpretation. Patterns in Silurian, Devonian, and Carboniferous plant evolution are consistent with stress imposed by progressive CO 2 starvation followed by leveling out, as predicted in reconstructions. I hypothesize that the Paleozoic disturbance was caused by bottlenecks in the cycling of lignin and other refractory compounds synthesized by land plants, and that these bottlenecks were eventually reduced by the evolution of more effective decomposer organisms and/or reduction of the refractory fraction in land plant biomass.

Phanerozoic atmospheric reconstructions: a terrestrial perspective

The Paleozoic stands out in geochemical reconstructions as a time of unusually high organic carbon burial and disturbed O 2 and CO 2 regimes. The anomaly is apparently of terrestrial origin. Evidence and explanations for reconstructed trends are sought in the terrestrial record. The correspondence between reconstructed O 2 and fusinite abundance in coal, a possible proxy for charcoal formation and fire activity, is ambiguous due to problems of data quality and interpretation. Patterns in Silurian, Devonian, and Carboniferous plant evolution are consistent with stress imposed by progressive CO 2 starvation followed by leveling out, as predicted in reconstructions. I hypothesize that the Paleozoic disturbance was caused by bottlenecks in the cycling of lignin and other refractory compounds synthesized by land plants, and that these bottlenecks were eventually reduced by the evolution of more effective decomposer organisms and/or reduction of the refractory fraction in land plant biomass.

Evidence for a factor required for transcriptional stimulation by the chimeric acidic activator GAL-VP16 in HeLa cell extracts.

We provide biochemical evidence for the existence of a transcriptional intermediary factor (TIF) in HeLa whole-cell extracts (WCE) that is distinct from the basic transcription factors and that is required for transcriptional stimulation by the chimeric acidic activator GAL-VP16. We have fractionated HeLa WCE by heparin-agarose chromatography. Of transcriptionally active fractions eluting in a step between 0.24 and 0.6 M KCl, the initial fractions are refractory to GAL-VP16 stimulation, whereas subsequent fractions are strongly stimulated by the activator. Aliquots of GAL-VP16-responsive fractions efficiently complement refractory fractions for transcriptional stimulation. Aliquots of responsive fractions are also far more efficient than those of refractory fractions in overcoming transcriptional inhibition that is brought about by high concentrations of GAL-VP16. Experiments performed with heat-treated WCE support the idea that HeLa cells contain a TIF that is essential for GAL-VP16 stimulation, but that is not required for basal transcription. Addition of recombinant yeast or human transcription factor TFIID (rTFIIDY and rTFIIDH, respectively) to a WCE heated at 48 degrees C for 15 min restores basal transcription, but in neither case is the reconstituted system activated by GAL-VP16. However, a 45 degrees C heat-treated WCE reconstituted with either rTFIIDH or rTFIIDY is stimulated by GAL-VP16, suggesting that a HeLa TIF can be selectively inactivated by heating at 48 degrees C, but not at 45 degrees C. Interestingly, a TFIID fraction partially purified from HeLa cell extracts, but not rTFIIDH, efficiently relieves transcriptional inhibition by GAL-VP16, suggesting that there may be an association between TIF(s) and TFIID and, moreover, that TIF(s) may be the direct target of the acidic domain of GAL-VP16. In summary, our results support the existence of a TIF that is not essential for basal transcription but that is required to mediate the stimulatory activity of the acidic activator GAL-VP16.

Dynamics of Carbon, Nitrogen, and Phosphorus Cycling in a Sawgrass Tidal Marsh with Special Reference to the Aboveground Primary Production

AbstractThe litter‐bag method was used to assess the disappearance rates of organic C, N, and P from the plant detritus of a sawgrass (Cladium jamaicense Crantz) tidal marsh in North Florida. A steady‐state kinetic model was used to simulate the effects of C, N, and P cycling dynamics on the quantity and quality of soil organic matter in the marsh. Four basic parameters were required for the model, namely, (i) net annual primary production of C, N, and P, (ii) soil organic C, N, and P in the biologically active layer, (iii) disappearance rate of dead standing crop, and (iv) disappearance rates of sawgrass C, N, and P from the litter‐bag. The decomposition of sawgrass C, N, and P can be described by a two‐component model through which a labile fraction and a refractory fraction of the primary production are defined. The rate constants of the labile fraction are one to two orders of magnitude greater than those of the refractory fraction. Simulation study indicated that the time required for soil C, N, and P to reach the steady state were 30, 60, and 70 yr, respectively. The simulation also indicated that the C/N ratio of the soil organic matter changed most significantly in the first 10 yr of the marsh development, whereas the N/P ratio remained relatively the same during the same period of time. The C/N and N/P ratios and the mean fraction remaining of soil organic matter all approach their respective steady‐state values, which are 23, 13, and 0.47, respectively. The observed C/N and N/P ratios of the soil organic matter were 21 and 13, respectively. The calculated labile fractions of the sawgrass were 0.34, 0.205, and 0.488 for organic C, N, and P, respectively. The mean retention time of C, N, and P in the labile fraction was 8.5 months. The MRTs of the refractory fractions were 10, 29, and 33 yr for C, N, and P, respectively. The bulk of the soil organic matter consists almost entirely of the refractory fraction of the primary production. According to the simulation, decomposition rate constants of plant detritus were the most significant and sensitive parameters that determine the quantity and quality of the soil organic matter. The annual N and P cycling rates between the aboveground sawgrass and the soil were estimated to be 18.8 g/m2 and 1.9 g/m2, respectively.

Comparative Analysis of the Chemical Composition of Mixed and Pure Cultures of Green Algae and Their Decomposed Residues by 13 C Nuclear Magnetic Resonance Spectroscopy

It is known that macromolecular organic matter in aquatic environments, i.e., humic substances, is highly aliphatic. These aliphatic macromolecules, predominantly paraffinic in structure, are prevalent in marine and lacustrine sediments and are believed to originate from algae or bacteria. A comparative study of mixed and pure cultures of green algae and their decomposed residues was performed by using solid-state C nuclear magnetic resonance spectroscopy as the primary analytical method. Results obtained in this study confirm the presence of components that are chemically refractory and that are defined as alghumin and hydrolyzed alghumin. These were detected in heterogeneous, homogeneous, and axenic biomasses composed of several genera of Chlorophyta. Although the chemical composition of algal biomass varied with culture conditions, the chemical structure of the alghumin and hydrolyzed alghumin, demonstrated by C nuclear magnetic resonance spectroscopy appeared to be constant for members of the Chlorophyta examined in this study. The alghumin was dominated by carbohydrate-carbon, with minor amounts of amide or carboxyl carbon and paraffinic carbon, the latter surviving strong hydrolysis by 6 N HCI (hydrolyzed alghumin). Bacterial decomposition of heterogeneous algal biomass labeled with C was conducted under both aerobic and anaerobic conditions to determine chemical structure and stability of the refractory material. The refractory fraction ranged from 33% in aerobic to 44% in anaerobic cultures. The refractory fraction recovered from either aerobic or anaerobic degradation comprised 40% alghumin, which represented an enrichment by 10% relative to the proportion of alghumin derived from whole cells of algae. The paraffinic component in the hydrolyzed alghumin of whole algal cells was found to be 1.8% and increased to 5.1 and 6.9% after aerobic and anaerobic bacterial degradation, respectively. It is concluded that members of the Chlorophyta contain a common insoluble structure composed of paraffinic carbon that is resistant to chemical and bacterial degradation under conditions used in this study. The paraffinic structure is identical to those constituting humin of aquatic origin. Thus, alga-derived macromolecular compounds deposited in aquatic environments (alghumin) probably contribute to sedimentary humic substances.

Global trends in the nature of organic matter in river suspensions

Riverine organic matter consists of a labile (metabolizable) and a residual (non-metabolizable) fraction1. The labile fraction can be oxidized or 'lost' within the rivers2, their estuaries3, and in the marine environment4. Previous estimates of the river fluxes of organic carbon have not considered such potential losses, being based on measurements of the bulk carbon and nitrogen contents5. Here I report detailed chemical analyses of organic matter associated with suspended matter from several major world rivers that have allowed me to differentiate it into labile and refractory fractions. Globally, 35% (81X 1012 g Cyr−1) of it belongs to the labile fraction and may become oxidized in estuaries and in the marine environment. The rest (150 x 1012g Cyr−1) appears to be highly degraded, with the bulk entering present-day tropical and subtropical sea areas. This degraded fraction could represent a significant source of organic carbon accumulating in marine sediments.

Inter-tumor heterogeneity and radiation dose-control curves

This communication presents a theoretical analysis of the influence of inter-tumor heterogeneity on clinical radiation dose-response curves. Any clinically defined group of tumors consists of individuals each having a unique individual dose-control relation (I-TCP/D). This I-TCP/D relation is steep and is based on Poisson statistics. The population as a whole possesses a heterogeneous dose-control relation (H-TCP/D relation) that is a resultant of the weighted summation of the I-TCP/D curves, each weighted according to its relative frequency in the population. This summation leads to an H-TCP/D curve that is considerably flatter than the steep portion of any I-TCP/D curve. For any specified radiation regimen, three subclasses of tumors exist in any population: the stochastic fraction, which consists of those tumors whose individual tumor control probability lies between 99% and 1% and individual outcome is probabilistic; the refractory fraction, which consists of those tumors each of which has no prospect for control with the specified treatment; and the sensitive fraction, which consists of tumors each of which is absolutely controllable with the specific treatment. Based on plausible parameters, it is estimated that, for tumors associated with an overall tumor control probability of approximately 50%, some 70-85% of failures come from a predictably radioresistant fraction. To optimize and improve the radiation therapy of tumors with unsatisfactory local control, it is necessary to predictably identify the location of each individual in one of the three subclasses.

Structural analysis of sea loch sedimentary humic substances by 13C 1H dipolar dephasing NMR

Nuclear magnetic resonance (NMR) techniques have been applied to the structural characterization of humic substances isolated from an organic-rich sediment in Loch Thurnaig, northwest Scotland. Both the sedimentary humic acid (SHA) and sedimentary fulvic acid (SFA) fractions from Loch Thurnaig contained substantial quantities of carboxylic and alkyl carbon with a small contribution (26 and 22% respectively) of aromatic/olefinic carbon atoms. The latter structures were shown by dipolar dephasing 13C NMR to be largely non-protonated. Differences in the alkyl structures of the two fractions were noted, with the SHA containing a larger amount of branched chain aliphatic material. As branched chain compounds are more resistant to microbial degradation than straight chain compounds, this may indicate that SHA is the older, more refractory fraction of the sedimentary organic matter, although it is possible that the results reflect different algal precursor material. The fraction of aromatic carbon was higher in the SHA than in the SFA as is generally found for humic and fulvic acids isolated from the same source. The low aromaticities and highly branched aliphatic structures show that the sedimentary humic substances from the loch resemble dissolved marine humic substances rather than their terrestrial counterparts.

Anaerobic treatment of landfill leachates: Kinetics and stoichiometry

Abstract The kinetics and stoichiometry of anaerobic digestion of landfill leachates have been studied using a series of four laboratory digesters working at 37 °C with HRT 5–35 days. Most of the COD of the leachates (18.0 ‐ 40.0 g.L‐1) comes from VFA (60–70%) followed by protein and hydroxyaromatic compounds. HCl was regularly added in order to keep pH approximately neutral. A lack of phosphate in the media was observed, and it was necessary to add nutrient phosphate for organic loading rates higher than 1.3 kg COD.m3. day‐1. The deviation of the experimental data from Chen & Hashimoto's equation fitted by means of a non‐linear algorithm was in all cases less than 5%. The values of the kinetic parameters of the model were μ = 0.275, K= 0.465 and R = 0.1; the latter indicates the presence of a refractory fraction. Methane production was also modelled using an experimental value of Y= 377 L CH4. (kg of COD removed).‐1

Decomposition and nutrient dynamics of litter from four species of freshwater emergent macrophytes

Nitrogen and phosphorus concentrations and mass remaining were followed for 30 months in decomposing litter of the perennial macrophytes Typha latifolia L., Carex lacustris Willd., Calamagrostis canadensis (Michx.) Nutt., and the annual Zizania aquatica L. in a fresh water tidal marsh in Massachusetts. Step-wise decreases in the mass remaining that corresponded to seasonal temperature changes were observed for all species. A model that assumes that initial litter inputs to the marsh surface consist of refractory and labile fractions and that the decay rate of the labile fraction is an exponential function of inverse temperature produced an acceptable description of the observed litter decomposition. The model suggests that a refractory fraction of 11% of initial litter weight of the annual Zizania and from 18 to 23% in the perennial species persists while labile organics are largely degraded within 1 yr. Zizania litter, which had the highest initial concentrations of both N and P, contained the lowest amounts of N and P after 30 months of decomposition. In the three perennial species studied, which are the dominant macrophytes in this marsh, there was a net accumulation of N and P in litter during the first 5 months of decomposition that was about 36% and 100%, respectively, of the annual N and P losses by vegetation in litterfall. This phase of nutrient accumulation was followed by nutrient release, particularly of N, after plant roots had invaded the litter. This sequence of nutrient accumulation, root invasion, and nutrient release represents a mechanism for nutrient conservation and/or accumulation in this ecosystem.