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1.
The relative contributions of litter and humified organic matter as the source of dissolved organic carbon (DOC) leached from organic layers of forest soils are poorly understood. In the present investigation, 13C labelled spruce litter was used to study the role of recent litter in the leaching of DOC from a coniferous forest floor in southern Sweden, while litterbags were used to quantify the total loss of C from the labelled litter. The labelled litter applied on bare lysimeters released considerable amounts of DOC during the first weeks, but the concentration of DOC originating from labelled litter decreased gradually from 176 mg litre−1 during the first sampling period in May to 5 mg litre−1 in the last sampling period in October. Only a moderate flush of DOC from the labelled litter occurred under the Oe and Oa horizons, with concentrations of 20 and 6 mg litre−1 from labelled litter, equal to 19 and 9% of the total DOC flux, respectively, during the first sampling period. Total flux of DOC from labelled litter from May to September was 16 g m−2, whereas only 2.2 and 0.9 g m−2 were captured under the Oe and Oa horizons, respectively. The almost complete loss of new DOC implies that DOC leached from the Oe and Oa horizons consists not of recent litter‐derived carbon, but of DOC produced in these two horizons themselves. Water‐extractable organic carbon from labelled litter left in litterbags in the field for 4 months consisted of about one‐third native carbon from external sources at the experimental site and two‐thirds of the labelled litter. In contrast, the 13C content of the bulk litter from the litterbags was not changed by the incubation in the field. We suggest that the soluble native carbon in water extracts originated from throughfall DOC that had been assimilated by microorganisms in the litterbags.  相似文献   

2.
Many forest ecosystems in Germany are strongly influenced by emissions of pollutants like SO2 and alkaline dusts. To quantify and evaluate the consequences of long‐term fly ash deposition on forest soils, a study was conducted in pine stands (Pinus sylvestris) in the Dübener Heide in Northeastern Germany. This forest area has been influenced mainly by emissions from coal‐fired power plants and the chemical industry of the industrial region Bitterfeld‐Wolfen‐Zschornewitz since the early 1900. The study sites are located along a fly ash deposition gradient of 8, 16, 14, 18, and 25 km away from the main emission source in Bitterfeld (sites 1, 2, 3, 4, and 5, respectively). Samples of the organic horizons (Oi, Oe, and Oa) and mineral topsoil (0—10 cm) were taken in fall 1998 and analyzed for their ferromagnetic susceptibility and total ash content. Scanning electron microscopy (SEM) and energy dispersive X‐ray microanalysis (EDX) were performed on selected samples to differentiate between the pedogenic and atmospheric origin of the mineral components in the organic horizons. As a result of the long‐term deposition, ferromagnetic fly ash components are mainly accumulated in the Oe and Oa horizons of the forest soils studied. Ferromagnetic susceptibility was significantly higher (p ≤ 0.05) in the Oe horizon of sites 1 and 2 compared to sites 3, 4, and 5. Unusually high total ash contents for organic horizons of > 74 % were determined in the Oa at all sites. SEM revealed 3 distinct features of persistent fly ash deposits from coal‐fired power plants within the organic horizons that can be defined as ”︁stable glasses” with magnetic properties, aluminum‐silicate‐minerals, and slag fragments. SEM and EDX indicated that a great portion of the mineral particles found in the organic horizons of forests soils influenced by fly ash are from atmospheric sources. For detection of atmospheric lignite‐derived deposition into forest soils, the Oe and Oa horizons have to be considered as specific diagnostic horizons because they show indicative properties for such soils.  相似文献   

3.
Dissolved organic carbon (DOC) and nitrogen (DON) are important components of the carbon and nitrogen turnover in soils. Little is known about the controls on the release of DOC and DON from forest floors, especially about the influence of solid phase properties. We investigated the spatial variation of the release of DOC and DON from Oe and Oa forest floor samples at a regional scale. Samples were taken from 12 different Norway spruce sites with varying solid phase properties, including C/N ratio, pH, different fractions of extractable carbon and exchangeable cations. Most of these solid phase properties are available for large forested areas of Europe in high spatial resolution. The samples were incubated at water holding capacity for eight weeks at 15°C and then extracted with an artificial throughfall solution to measure DOC and DON release. The rates of soil respiration and N-mineralization were determined to estimate soil microbial activity. The release of DOC and DON from Oe samples was two- to threefold higher than from Oa samples. The amounts released differed by one order of magnitude among the sites. The DOC/DON ratios in the percolates of the Oa samples were much higher as compared to the solid phase C/N, indicating different release rates of DOC and DON. In contrast, the DOC/DON ratios of the Oe percolates were in the range of the C/N ratios of the solid phase. The release of DOC and DON from Oe samples was not statistically correlated to any of the measured solid phase parameters, but to N-mineralization. The DOC and DON release from the Oa samples was positively related only to pH and soil respiration. Overall it was not possible to explain the large spatial variation of DOC and DON release by the measured solid phase properties with satisfying accuracy.  相似文献   

4.
About the sorption of dissolved organic matter to forest soils This investigation characterizes the major forest soils of the temperate climatic zones (leptosols, vertisols, cambisols, luvisols, podzols, stagnosols, gleysols) as sorbents for dissolved organic matter (DOM). Sorption isotherms were obtained for 135 soil horizons from 36 profiles. When solutions containing no DOC were added, the release of dissolved organic carbon (DOC) was highest for horizons rich in organic C (A and Bh horizons). In subsoil horizons DOC release was much lower. Most of the investigated top soils (A and E horizons) and Bh, Bg, and C horizons showed a weak DOC sorption. This was caused by low contents of sorbents (clay and sesquioxides) and/or high contents of organic C. Organic C seems to reduce the DOC sorption by occupying binding sites. Subsoils rich in clay and sesquioxides like Bs, Bt, and Bw horizons showed a strong retention of DOC. Under the aerobic conditions of the experiments, some of the subsoils of stagnosols and gleysols also showed a strong sorption of DOC. However, in sorption experiments conducted after an anaerobic incubation, the DOC sorption decreased significantly.  相似文献   

5.
Composition and effects of additions of fibric (Oi) and hemic/sapric (Oe + Oa) layer extracts collected from a 20-year-old stand of radiata pine (Pinus radiata) on soil carbon dioxide (CO2) evolution were investigated in a 94-day aerobic incubation. The 13C nuclear magnetic resonance spectroscopy indicated that Oi layer extract contained greater concentrations of alkyl C while Oe + Oa layer extract was rich in carboxyl C. Extracts from Oi and Oe + Oa layers were added to a forest soil at two different polyphenol concentrations (43 and 85 μg g−1 soil) along with tannic acid (TA) and glucose solutions to evaluate effects on soil CO2 efflux. CO2 evolution was greater in amended soils than control (deionized water) indicating that water-soluble organic carbon (WSOC) was readily available to microbial degradation. However, addition of WSOC extracted from both Oi and Oe + Oa layers containing 85 μg polyphenols g−1 soil severely inhibited microbial activity. Soils amended with extracts containing lower concentrations of polyphenols (43 μg polyphenols g−1 soil), TA solutions, and glucose solutions released 2 to 22 times more CO2-C than added WSOC, indicating a strong positive priming effect. The differences in CO2 evolution rates were attributed to chemical composition of the forest floor extracts.  相似文献   

6.
The biodegradability of dissolved organic carbon (DOC) in different fractions from the forest floor was studied. Soil leachate (SL, the soil solution in macropores which is freely drained from forest floor after rainfall), the soil matrix solution (SMS, the soil solution in meso-/micropores of the soil matrix), and soil water extracts (SWE) from two different beech forest floors were compared. Zero-tension and tension lysimeters were used to collect SL and SMS, respectively. Loss of DOC (during 21 days) and respiration of CO2-C (during 7 days) were used as conventional measures of the availability of DOC. Bacterial production, measured using the leucine incorporation technique, and bacterial growth efficiency were also estimated. All methods were used to study differences in biodegradability between plots with and without ground flora (Deschampsia flexuosa or Anemone nemorosa) and different type of forest floor (with an organic (O) horizon or a mull (A) horizon). There were no differences in bioavailability of DOC from soil solutions extracted from plots with and without ground flora. The bioavailability of DOC in the different collected soil solutions varied, however. DOC in SWE was the most available, with a mean of 39% of DOC-loss in 21 days, and 18% of DOC being respired in 7 days. DOC in soil matrix solution was the least available of the soil solutions (7% respired), significantly less than DOC in soil leachate (11% respired). The methods measuring biodegradation of DOC, DOC-loss and CO2-C respiration gave similar results and were comparable to bacterial production and bacterial growth efficiency, with the exception of SWE from the O-horizon at the D. flexuosa site, which had low bacterial production and bacterial growth efficiency, indicating a limitation of the bacterial growth. This study is one of the first to use bacterial production and bacterial growth efficiency for measuring bioavailability in terrestrial environments, giving an extra dimension for the process of biodegradation of DOC.  相似文献   

7.
We estimated the contribution of dissolved organic matter (DOM) to cation leaching and the translocation of acidity in three acid forest soils. The analysis was based on monitored (2 years) concentrations of dissolved organic carbon (DOC) in the field, measured total acidities of DOM, and measured as well as predicted weighted mean dissociation constants of the organic acids. Although the forest floor solutions were strongly acidic (pH 3.47–4.10), a considerable proportion of the organic acids was dissociated and organic anions represented 22–40% of the total anions in the mineral soil input. The flux of DOM-associated exchangeable protons from the forest floor to the mineral soil ranged from 0.35 (Wülfersreuth) to 3.72 (Hohe Matzen) kmol ha?1 yr?1. In the subsoil, this organic acidity may be neutralized by microbial decomposition of the organic acids, but a part of the hydrogen ions may dissociate and contribute to acidification of the soil solution and to weathering processes. Due to the pronounced retention of DOM in the mineral subsoil horizons, the contribution of DOM to the output of cations and acidity from the soil is much lower than in the surface horizons but still significant.?  相似文献   

8.
Drying–rewetting cycles (D/W) occur frequently in topsoils and may mobilize phosphorus (P). We investigated the effect of repeated D/W on the release of dissolved inorganic (DIP) and organic P (DOP) from forest floors and A horizons. Samples were taken from 3 European beech sites and from 3 Norway spruce sites. Soils were desiccated up to pF 6 (–100 MPa) in three D/W cycles in the laboratory, while the controls were kept permanently at 50% water holding capacity. After each drying, P was extracted from the soils in water. D/W caused the release of DIP and DOP especially from O layers. There was no general difference in response to D/W between samples from beech and spruce. The net release of DIP after D/W was largest from the Oe horizons (average 50–60 mg P kg?1) for both beech and spruce forest soils. The net release of DIP from Oi layers was on average 7.8 mg P kg?1 and from spruce Oa layers 21.1 mg P kg?1. In the A horizons, net DIP release was similar in beech and spruce soils with 0.4 mg P kg?1. The release of DOP was less than the release of DIP except for the A horizons. Repeated cycles did not increase the release of DIP and DOP. The release of DIP and DOP was positively correlated with the microbial biomass in Oe and Oa layers but not in Oi layers. Our results suggest that D/W may significantly influence the short term availability of dissolved P in both beech and spruce forest soils.  相似文献   

9.
This paper presents aluminium (Al)-solubility data for two acid forest soils (Inceptisol and Spodosol), obtained in connection with lysimeter measurements (tension-cup and zero-tension lysimeters) and batch equilibrium experiments. The solubility of Al obtained in the batch experiments was used as a reference to test whether Al3+in soil solutions collected by the lysimeters was in equilibrium with secondary forms of solid-phase Al (Al(OH)3or organically bound Al). The relation between pH and Al3+activity found for the zero-tension lysimeter solutions collected from the Inceptisol agreed well with that obtained in the batch experiment. This suggests that Al3+in the lysimeter solutions were in, or close to, equilibrium with the solid phase, whether this was organically bound Al (A horizon) or an Al(OH)3phase (B horizon). For the tension-cup lysimeters, solutions obtained from the Inceptisol B and Spodosol Bs1 horizons were generally close to equilibrium with respect to secondary solid-phase Al (apparently Al(OH)3; average ion activity product was 109.3and 108.8, respectively), whereas the Inceptisol A and Spodosol Bh solutions were not. The Al solubility in Inceptisol A and Spodosol Bh horizons was consistently higher than that obtained in the batch equilibrium experiment, indicating that the sampled solution partly originated from the underlying horizons. Thus, tension-cup lysimeters should be used with care in soils (or in parts of soil profiles) having steep solute concentration gradients because the soil volume from which the sample is drawn with this lysimeter type seems to be poorly defined.  相似文献   

10.
The forest floor represents the major source of dissolved organic carbon (DOC) and nitrogen (DON) in forest soils. The release mechanisms of DOC and DON from forest floors and their environmental controls as well as the dynamics of concentrations and fluxes are still poorly understood. We investigated the effect of drying and rewetting on the release of DOC and DON from a Norway spruce forest floor. Undisturbed soil columns of 17 cm diameter and 15—20 cm height were taken with 7 replicates from the forest floor of a mature Norway spruce (Picea abies [L.] Karst.) site and established at 10°C in the laboratory. Columns were exposed to different periods of drying (3, 5, 10, 20 days). Each drying period was followed by a rewetting for 5 days at an irrigation rate of 10 mm d—1 with a natural throughfall solution. The percolates from the forest floor were collected daily and analyzed for DOC, total N, NH4, NO3, pH, electrical conductivity and major ions. Drying for 10 and 20 days decreased the water content of the Oi horizon from 280% dry weight to about 30%. The water content of the Oe and the Oa horizon only changed from about 300% to 200%. The fluxes of DOC from the forest floor were moderately effected by drying and rewetting with an increase after 3 and 5 days of drying, but a decrease after 10 and 20 days. On the contrary, the drying for 10 and 20 days resulted in a drastic increase of the DON fluxes and a subsequent decrease of the DOC/DON ratios in the forest floor percolates from about 50 to 3.3. These results suggest that the mechanisms for DOC release in forest floors differ from those for DON and that drying and rewetting cause temporal variations in the DOC/DON ratios in forest floor percolates.  相似文献   

11.
Atmospheric fly ash emissions from lignite-fired power plantsin the Upper Lusatian and Turówan mining districts stronglyaffected large forest areas along the German-Polish border. Afield study was conducted in old spruce stands (Piceaabies (L.) Karst.) to assess the stratigraphic distributionof fly ash in the forest floor and mineral topsoil in the St. Marienthal forest area in the eastern part of Saxony,Germany. This forest area is subjected to long-termatmospheric depositions by two German and one Polish powerplants since the early 1900s. The three study sites arelocated along a fly ash deposition gradient of 3, 6 and 15 kmfrom the power plant in Turów (Sites Ia, II and III,respectively). An additional site (Site Ib) at a distance of 3 km from Turów was chosen to study the influence of vegetationtype on fly ash deposition intensity in forest soils. Samplesof the humic layer (Oi (L), Oe (F) and Oa (H) horizons) andmineral soil (0–10 cm) were taken in Spring and Fall 1999 andanalysed for their ferromagnetic susceptibility and total ashcontent. Particle size distribution, magnetic susceptibilityof individual size fractions, scanning electron microscopy(SEM), and energy dispersive X-ray microanalysis (EDX) wereperformed on selected samples to evaluate the origin ofmineral particles found in the forest floor. High magneticsusceptibility of the Oa and Oe horizons is a result of thelong-term accumulation of lignite-derived atmospheric depositsin the forest floors of the studied area. Pure conifer stands(year-round filtration of airborne pollutants) resulted inhigher inputs of ferromagnetic fly ash particles in forestsoils. Unusually high total ash contents for humic horizons(up to 77%) were determined in the Oa and Oe horizons atSites Ia and IIb, indicating the need for a new classificationsystems for the organic layer in forest soils near coal-firedpower plants. SEM revealed 4 typical phases of persistent flyash deposits formed by combustion of Lusatian lignite: (1) iron-containing `stable glasses’, (2) aluminium-iron-silicate-minerals,(3) slag fragments and (4) lignite-derived fossilcarbon. Particle size analysis, magnetic susceptibilitymeasurements and SEM-EDX techniques indicated that a greatportion of the mineral particles found in the humic horizonsof forests soils are from atmospheric sources. Fly ashaccumulation consisting of ferromagnetic minerals contributesmainly to the 125–63 and <63 μm fractions in soils. EDXanalysis revealed that atmospheric input of lignite-derivedfly ash increases the contents of the following ecologicalrelevant elements in soils: carbon, iron, aluminium, calcium,potassium, sulphur, titanium and sodium.  相似文献   

12.
Clear‐cutting of forest provides a unique opportunity to study the response of dynamic controls on dissolved organic matter. We examined differences in concentrations, fluxes and properties of dissolved organic matter from a control and a clear‐cut stand to reveal controlling factors on its dynamics. We measured dissolved organic C and N concentrations and fluxes in the Oi, Oe and Oa horizons of a Norway spruce stand and an adjacent clear‐cutting over 3 years. Aromaticity and complexity of organic molecules were determined by UV and fluorescence spectroscopy, and we measured δ13C ratios over 1 year. Annual fluxes of dissolved organic C and N remained unchanged in the thin Oi horizon (～ 260 kg C ha?1, ～ 8.5 kg N ha?1), despite the large reduction in fresh organic matter inputs after clear‐cutting. We conclude that production of dissolved organic matter is not limited by lack of resource. Gross fluxes of dissolved organic C and N increased by about 60% in the Oe and 40% in the Oa horizon upon clear‐cutting. Increasing organic C and N concentrations and increasing water fluxes resulted in 380 kg C ha?1 year?1 and 10.5 kg N ha?1 year?1 entering the mineral soil of the clear‐cut plots. We found numerous indications that the greater microbial activity induced by an increased temperature of 1.5°C in the forest floor is the major factor controlling the enhanced production of dissolved organic matter. Increasing aromaticity and complexity of organic molecules and depletion of 13C pointed to an accelerated processing of more strongly decomposed parts of the forest floor resulting in increased release of lignin‐derived molecules after clear‐cutting. The largest net fluxes of dissolved organic C and N were in the Oi horizon, yet dissolved organic matter sampled in the Oa horizon did not originate mainly from the Oi horizon. Largest gross fluxes in the Oa horizon (control 282 kg C ha?1) and increased aromaticity and complexity of the molecules with increasing depth suggested that dissolved organic matter was derived mainly from decomposition, transformation and leaching of more decomposed material of the forest floor. Our results imply that clear‐cutting releases additional dissolved organic matter which is sequestered in the mineral soil where it has greater resistance to microbial decay.  相似文献   

13.
Temperate forest soils store large amounts of organic matter and are considered as net sinks for atmospheric carbon dioxide. Information about the sink strength and the turnover time of soil organic carbon (SOC) is required to assess the potential response of soils to climate change. Here we report on stocks, turnover times (TT) and accumulation of SOC in bulk soil and density fractions from genetic horizons of a Podzol in the Fichtelgebirge, Germany. Stocks of SOC, total nitrogen and exchangeable cations determined in nine quantitative soil pits strongly varied with stone content and thickness of horizons in both the organic layer and the mineral soil. On the basis of radiocarbon signatures, mean turnover times of 4, 9 and 133 years, respectively, were calculated for Oi, Oe and Oa horizons from three soil pits, using a non-steady-state model. The Oa horizons accumulated 4–8 g C m−2 year−1 whereas the Oi and Oe horizons were close to steady-state during the past decade. Free particulate organic matter (FPOM) was the most abundant fraction in the Oa and EA horizons with TT of 70–480 years. In the B horizons, mineral associated organic matter (MAOM) dominated with over 40% of total SOC and had TT of 390–2170 years. In contrast to other horizons, MAOM in the Bsh and Bs horizon had generally faster TT than occluded particulate organic matter (OPOM), possibly because of sorption of dissolved organic carbon by iron and aluminium oxides/hydroxides. Our results suggest that organic horizons with relatively short turnover times could be particularly vulnerable to changes in climate or other disturbances.  相似文献   

14.
《Geoderma》2005,124(1-2):157-168
Multivariate regressions on ultraviolet (UV) absorbance (210–300 nm) data were tested as a potential tool to estimate concentrations of total dissolved organic carbon (DOC), DOC in the XAD-8-adsorbable (hydrophobic) fraction (HoDOC), and dissolved nitrate (NO3-N) in forest floor leachates. Partial least square (PLS) regressions were established for 20 zero-tension lysimeter solutions sampled in plots of coniferous and deciduous forest stands from May 2002 to January 2003. These regressions were validated against data from 24 other zero-tension solutions sampled in the same stands. The root mean square error of prediction (RMSEP) was 2.0 mg/l for DOC and 1.6 mg/l for HoDOC. These errors were 39% (DOC) and 20% (HoDOC) smaller than the errors obtained with regressions using only absorbance at 280 nm. For NO3-N, RMSEP was 0.12 mg/l (estimation by single wavelength regression is not possible). DOC and HoDOC covaried strongly and could not be estimated independently based on absorbances; HoDOC varied only between 61% and 81% of the DOC. Partly for this reason, an attempt to estimate the HoDOC/DOC ratio based on specific absorbance spectra (absorbance units per mg/l of total DOC) was little successful. Partly, however, this problem resulted from the fact that both HoDOC and HiDOC fractions had a considerable UV absorptivity, which was not consistent between samples.  相似文献   

15.
Dissolved organic matter (DOM) dynamics in spruce forested sites - examinations by analytical DOM fractionation Dissolved organic matter from two spruce forested sites in the Fichtelgebirge (Germany) was divided into different chemical and functional fractions, and the budgets of the fractions obtained were calculated. For both sites hydrophobic acids (HoS), hydrophilic acids (HiS), hydrophobic neutrals (HoN), hydrophilic neutrals (HiN), and hydrophilic bases (HiB) are discriminated concerning their dynamics in the compartments. Most of the HiN and HoN are mobilized by leaching from the forest canopy. Both neutral fractions are netto retained in the forest floor as well as in the mineral soil. In contrast, HoS and HiS are mainly released in the organic layers with a total input of organic acids from the forest floor into the mineral soil of ca 100 kg C (HoS) ha?1 a?1, and 50 kg C (HiS) ha?1 a?1, respectively. HoS are selectively better retained in the mineral horizons, leading to a mineral soil output of 2.4 – 4.4 kg C (HoS) ha?1 a?1, and 2.7 – 6.5 kg C (HiS) ha?1 a?1, respectively. It is concluded that the different mobility of the DOM fractions has implications for the mobilization and transport of organic pollutants and heavy metals.  相似文献   

16.
The forest floor was shown to be an effective sink of atmospherically deposited methylmercury (MeHg) but less for total mercury (Hgtotal). We studied factors controlling the difference in dynamics of MeHg and Hgtotal in the forest floor by doubling the throughfall input and manipulating aboveground litter inputs (litter removal and doubling litter addition) in the snow‐free period in a Norway spruce forest in NE Bavaria, Germany, for 14 weeks. The MeHg concentrations in the forest‐floor percolates were not affected by any of the manipulation and ranged between 0.03 (Oa horizon) and 0.11 (Oi horizon) ng Hg L–1. The Hgtotal concentrations were largest in the Oa horizon (24 ng Hg L–1) and increased under double litterfall (statistically significant in the Oi horizon). Similarly, concentrations of dissolved organic C (DOC) increased after doubling of litterfall. The concentrations of Hgtotal and DOC correlated significantly in forest‐floor percolates from all plots. However, we did not find any effect of DOC on MeHg concentrations. The difference in the coupling of Hgtotal and MeHg to DOC might be one reason for the differences in the mobility of Hg species in forest floors with a lower mobility of MeHg not controlled by DOC.  相似文献   

17.
Dissolved organic nitrogen (DON) plays a key role in the N cycle of many ecosystems, as DON availability and biodegradation are important for plant growth, microbial metabolism and N transport in soils. However, biodegradation of DON (defined as the sum of mineralization and microbial immobilization) is only poorly understood. In laboratory incubations, biodegradation of DON and dissolved organic carbon (DOC) from Oi and Oa horizons of spruce, beech and cypress forests ranged from 6 to 72%. Biodegradation of DON and DOC was similar in most samples, and mineralization of DON was more important than microbial immobilization. Nitrate additions (0-10 mg N L−1) never influenced either DON immobilization by microorganisms or mineralization. We conclude that soil microorganisms do not necessarily prefer mineral N over DON for meeting their N demand, and that biodegradation of DON seems to be driven by the microbial demand for C rather than N. Quantifying the dynamics of DON in soils should include consideration of both C and N demands by microbes.  相似文献   

18.
Summary Potential P and C mineralization rates were determined in a 12-week laboratory incubation study on subarctic forest and agricultural soil samples with and without N fertilizer added. There was no significant difference in net inorganic P produced between N fertilized and unfertilized soils. The forest soil surface horizons had the highest net inorganic P mineralized, 32 mg P kg-1 soil for the Oie and 17 mg P kg-1 soil for the Oa. In the cropped soils net inorganic P immobilization started after 4 weeks and lasted through 12 weeks of incubation. Cumulative CO2–C evolution rates differed significantly among soils, and between fertilizer treatments, with the N-fertilized soils evolving lower rates of CO2–C than the unfertilized soils. Soils from the surface horizons in the forest evolved the highest rates of CO2–C (127.6 and 89.4 mg g-1 soil for the Oie and Oa horizons, respectively) followed by the cleared uncropped soil (42.8 mg g-1 soil C), and the cropped soils (25.4 and 29.0 mg g-1 soil C). In vitro soil respiration rates, or potential soil organic matter decomposition rates, decreased with increasing time after clearing and in accord with the degree of disturbance. Only soils with high potential C mineralization rates and high organic P to total P ratios, mineralized P by the end of the study. Mineralizable P appeared to be associated with readily mineralizable organic C.  相似文献   

19.
This study aimed to gain insight into the generation and fate of dissolved organic carbon (DOC) in organic layers. In a Free Air CO2 Enrichment Experiment at the alpine treeline, we estimated the contribution of 13C-depleted recent plant C to DOC of mor-type organic layers. In an additional laboratory soil column study with 40 leaching cycles, we traced the fate of 13C-labelled litter-DOC (22 and 45 mg l−1) in intact Oa horizons at 2 and 15 °C. Results of the field study showed that DOC in the Oa horizon at 5 cm depth contained only 20 ± 3% of less than six-year-old C, indicating minor contributions of throughfall, root exudates, and fresh litter to leached DOC. In the soil column experiment, there was a sustained DOC leaching from native soil organic matter. Less than 10% of totally added litter-DOC was leached despite a rapid breakthrough of a bromide tracer (50 ± 7% within two days). Biodegradation contributed only partly to the DOC removal with 18-30% of added litter-DOC being mineralized in the Oa horizons at 2 and 15 °C, respectively. This was substantially less than the potential 70%-biodegradability of the litter-DOC itself, which indicates a stabilization of litter-DOC in the Oa horizon. In summary, our results give evidence on an apparent ‘exchange’ of DOC in thick organic layers with litter-DOC being retained and ‘replaced’ by ‘older’ DOC leached from the large pool of indigenous soil organic matter.  相似文献   

20.
Comparison is made between the chemical composition of acid soil solutions percolated through new, acid-washed ceramic P 80 suction cups, and old, over 3 years field-equilibrated suction cups with respect to quantitative and qualitative changes of dissolved organic carbon (DOC). While new suction cups sorb DOC in significant amounts with hydrophobic constituents preferred, field-equilibrated suction cups alter DOC neither in concentration nor in composition. But at changes of DOC concentrations a percolation volume of 300 ml is necessary for reaching equilibrium. It is, therefore, concluded that field-equilibrated ceramic P 80 suction cups can be used for collecting DOC from mineral B and C horizons of acid forest soils, where DOC concentrations remain constant. In contrast, the suction cups investigated are unsuitable for collecting A horizon solutions, which show greater variations in DOC concentration.  相似文献   

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