Bernard Ludwig

Dynamic of soil chemical parameters in shifting agriculture in the Eastern Amazon

Abstract In Northeast Para, Brazil, shifting agriculture by land settlers has been practiced for around 100 years. After a common cropping period of 2 years the fields are left fallow for 4 to 8 years. Changes in extractable cations and C, N, and P of soils were studied on six fields which were under different phases of the rotational cycle. The chemical composition of soil solutions was also monitored for a period of 19 months. In topsoils, the differences in C and N contents and extractable Ca and Al could be related to the landuse history expressed as time elapsed since last burn. Repeated sampling on two ‘slash and burn’ plots, showed significant increases in pH, CEC, extractable K, Ca and Mg, but decreases in extractable Na and Al, C and N content in the plots from 7-year old fallow to the first-year cropping field. Soil solution chemistry at 105 cm depth did not change with increasing fallow age. On the ‘slash and burn’ field, big variations in solute composition at 10 cm depth were observed during the initial months of the cropping period. In the soil solutions after burning, Ca concentrations ranged from 4.8 to 30.1 mg l −1 , but decreased to values between 1.2 and 2.7 mg l −1 within 2 months. Slash burning and subsequent cropping lead to increases of Cl − and NO − 3 in the soil solution at 105 cm depth. A small dose of NPK fertilizer (11 kg N ha −1 , 13 kg P ha −1 and 17 kg K ha −1 ) did not increase NO − 3 concentration and chloride concentrations at this depth increased to a small extent. After the harvest of cassava, high nitrate concentrations of 11 mg N l −1 were observed. NO − 3 and Cl − concentrations on the cropping fields were significantly correlated ( n = 350) with the sum of the concentration of the M b cations (Na, K, Ca, Mg), (NO − 3 M b , r = 0.82; Cl − M b , r = 0.81). Nitrate also showed significant correlations with protons ( r = 0.59) and Al ( r = 0.47) concentrations. Ion concentrations in the soil solution can be reduced by agricultural management. During harvest, the aboveground vegetation should not be removed completely in one single step. If clear cutting cannot be avoided, the timing should be chosen as to keep some vegetation with high uptake potential at the beginning of the rainy season. A small dose of NPK-fertilizer stimulated the nutrient uptake by the vegetation and did not enhance nutrient concentrations in the seepage water.

Effects of fertilization and soil management on crop yields and carbon stabilization in soils. A review

The study of sustainable land use is complex and long-term experiments are required for a better understanding of the processes of carbon stabilization. Objectives were (i) to describe for four long-term experiments the effects of fertilization and soil management on crop yields and the dynamics of soil organic carbon (SOC) and total N, and (ii) to discuss the usefulness of models for a better understanding of the underlying processes. Data of soil organic carbon and total N of four long-term experiments in Germany and China which studied the effect of fertilization (Bad Lauchstädt, Darmstadt) and tillage (Göttingen, Quzhou) were evaluated and soil organic carbon fractionation was carried out. The Rothamsted Carbon Model was used for a description and prediction of soil organic carbon dynamics as affected by fertilization and tillage in Bad Lauchstädt and Quzhou. The type of fertilizer added at common rates — either mineral N or farmyard manure — affected the crop yields only slightly, with slightly lower yields after manure application compared with mineral N fertilization. For both fertilization trials, manure applications at common rates had beneficial effects on soil organic carbon stocks in the labile pool (turnover time estimated as <10 years) and to a greater extent in the intermediate pool (turnover time estimated to be in the range of 10 to 100 years). A comparison of the effects of conventional tillage, reduced tillage and no-tillage carried out in Göttingen and Quzhou indicated only small differences in crop yields. Reduced tillage in Göttingen resulted in an increased C storage in the surface soil and C was mainly located in the mineral-associated organic matter fraction and in water-stable macro-aggregates (>0.25 mm). For Quzhou, no-tillage and conventional tillage had similar effects on total C stocks, with a greater spatial variability in soil organic carbon stocks in the no-tillage plots. Modeling required site-specific calibrations for the stock of inert organic matter for each of the sites, indicating that not all carbon stabilization processes are included in the model and that application of a model to a new site may also need site-specific adjustments before it can be used for predictions. After site-specific calibration, however, model predictions for the remaining treatments were generally accurate for the fertilization and tillage trials, which emphasizes the importance of temperature, moisture, soil cover and clay content on the decomposition dynamics of soil organic carbon and the significance of amounts and quality of carbon inputs in the soil for maintaining or increasing soil organic carbon stocks in arable soils.

Use of near infrared spectroscopy to determine biological and chemical characteristics of organic layers under spruce and beech stands

The chemical composition of organic layers of forest soils shows a high spatial variability and fast methods may be required for its study at a landscape level. The objective was to assess the applicability of near infrared spectroscopy (NIRS) to measure several chemical and biological properties of organic layers in spruce, beech, and mixed spruce-beech stands. Spectra in the VIS-NIR region (400—2500 nm) were recorded for 406 samples representing Oi, Oe, and Oa layers of forest soils from Solling (Germany), 195 of them were used for calibration and 211 for validation. The calibration equations for each constituent were developed using the whole spectrum (0th to 3rd derivative). Humus samples were analyzed for contents of C and N and contents of P, S, Na, K, Ca, Mg, Mn, Fe, and Al after pressure digestion in HNO3. Additionally, basal respiration and microbial C (Cmic) were measured. NIRS predicted well the contents of C, N, P, S, Ca, Na, K, Fe, and Al and C/N and C/P ratios: the regression coefficients (a) of a linear regression (measured against predicted values) ranged from 0.9 to 1.1, and the correlation coefficients (r) were greater or equal 0.9. Cmic (a = 0.87, r = 0.83) was predicted satisfactorily, whereas the prediction of the basal respiration (a = 0.74, r = 0.87) was less satisfactory. Due to liming of some of the plots NIRS failed to predict contents of Mg (a = 1.27, r = 0.68). For all chemical and biological characteristics the best prediction performances were achieved using the whole sample population. Splitting the samples into smaller groups according to a dominant tree species or an organic layer did not improve the predictions. Anwendung von Nahinfrarotspektroskopie zur Bestimmung biologischer und chemischer Charakteristika organischer Auflagen unter Fichten- und Buchenbestanden Die chemische Zusammensetzung organischer Auflagen von Waldboden weist eine hohe raumliche Variabilitat auf und schnelle Methoden sind erforderlich fur Untersuchungen auf der Landschaftsebene. Ziel dieser Studie war es, die Anwendbarkeit von Nahinfrarotspektroskopie (NIRS) zur Bestimmung chemischer und biologischer Eigenschaften organischer Horizonte unter Fichten-, Buchen- und Mischbestanden zu untersuchen. Fur 406 Proben (L-, F- und H-Lagen organischer Auflagen) wurden die Spektren im VIS-NIR-Bereich (400—2500 nm) gemessen. 195 Proben wurden fur die Kalibrierung und 211 fur die Validierung eingesetzt. Die Kalibrierungsgleichungen wurden fur jedes Merkmal bei Berucksichtigung des gesamten Spektrums (0. bis 3. Ableitung) berechnet. Die Humusproben wurden bezuglich der C- und N-Gehalte und der Gehalte an P, S, Na, K, Ca, Mg, Mn, Fe und Al nach HNO3-Druckaufschluss untersucht. Zusatzlich wurden Basalatmung und mikrobielle Biomasse (Cmic) bestimmt. NIRS war gut geeignet zur Abschatzung der Gehalte an C, N, P, S, Ca, Na, K, Fe und Al und der C/N- und C/P-Verhaltnisse: die Regressionskoeffizienten (a) einer linearen Regression (gemessene gegen abgeschatzte Werte) lagen zwischen 0.9 und 1.1, und die Korrelationskoeffizienten waren groser oder gleich 0.9. Cmic (a = 0.87; r = 0.83) wurde befriedigend abgeschatzt, wahrend die Abschatzung der Basalatmung (a = 0.74, r = 0.87) weniger befriedigend war. Aufgrund von Kalkungen einiger Teilflachen war die Abschatzung der Mg-Gehalte unbefriedigend (a = 1.27, r = 0.68). Fur samtliche chemische und biologische Charakteristika wurde die groste Abschatzungsgenauigkeit bei Verwendung des gesamten Probenkollektivs erreicht. Eine Auftrennung des Kollektivs in kleinere Gruppen nach Baumart oder Lagen (L, F, bzw. H-Lage) der organischen Auflage verbesserte die Genauigkeit der Abschatzungen nicht.

Effects of Lime and Wood Ash on Soil-solution Chemistry, Soil Chemistry and Nutritional Status of a Pine Stand in Northern Germany

Lime and wood ash may be useful to improve acidic forest soils. A field experiment was conducted in a pine stand on a sandy podzol at Fuhrberg, Germany, which involved an application of dolomitic lime (3 t ha-1) with three replications or wood ash (4.8 t ha-1) without replications on the forest floor. During the 2 yr study period, lime affected the soil solution composition only slightly. Ash had a marked effect on solution chemistry of the mineral soil at 10 cm and the pH values dropped temporarily from 3.7 to 3.1. Nineteen months after the treatments, exchangeable calcium in the organic layer and mineral soil increased by 222 (lime addition) or 411 kg ha-1 (ash addition) and exchangeable magnesium increased by 101 (lime addition) or 39 kg ha-1 (ash addition). After ash addition, no marked change in heavy metal content was found below 4 cm of the organic layer. In the ash treatment, the potassium concentration of the 1-yr-old pine needles increased from 5.6 to 5.9 g kg-1. This study suggests that ash from untreated wood may be recommended for amelioration of forest soils.

Modelling the dynamics of organic carbon in fertilization and tillage experiments in the North China Plain using the Rothamsted Carbon Model—initialization and calculation of C inputs

Modelling of the carbon dynamics in arable soils is complex and the accuracy of the predictions is unknown before the model is applied to each specific site. Objectives were (i) to test the accuracy of predictions of the carbon dynamics using the Rothamsted Carbon (RothC) Model in a field trial in Quzhou, North China Plain, using different methods for initialization and estimation of carbon input into the soil and (ii) to test the applicability of the RothC model for plots with either conventional tillage (CT) or no-tillage (NT) systems. A field trial was conducted with applications of differing amounts of N (0, 112 or 187 kg N ha−1 year−1), P (0, 75 or 150 kg P2O5 ha−1 year−1) and wheat straw (0, 2.25 or 4.5 t DM ha−1 year−1) in differing combinations with either CT or NT for 18 years. CT and NT affected stocks of soil organic carbon (SOC) similarly. Carbon inputs from crops were either estimated from published regression functions that relate C inputs to crop yield including rhizodeposition (models 1 and 2) or published root:aboveground biomass ratios (model 3). Model 1, which was not calibrated to the site conditions, was successful in predicting the carbon dynamics in seven out of nine treatments (model efficiencies EF ranged from 0.28 to 0.87), whereas for two treatments, EF (−0.35 and−2.3) indicated an unsuccessful prediction. The prediction of the C dynamics in NT experiments using model 1 was generally successful, but this may have been due to the fact that NT did not have a specific effect on SOC stocks for this trial. Model 2, which was the same as model 1 except for an optimization of the stock of inert organic matter using one treatment, predicted SOC stocks in the remaining eight treatments overall better than model 1. Model 3 was less successful than models 1 and 2 in all treatments (−19 ≤ EF ≤ 0.56). The results indicate that the RothC model may successfully predict C dynamics—for the site studied even without prior calibration as in model 1—, but care should be taken in choosing an appropriate approach for estimating C inputs into the soil.

Modelling cation composition of soil extracts under ashbeds following an intense slashfire in a eucalypt forest

Following an intense slashfire in clearfelled mixed-species Eucalyptus forest, soils under ashbeds were collected 1 day, 2 yrs and 4 yrs afterwards. The amount of salt-extractable cations was measured, as well as the chemical composition of equilibrium soil extracts obtained from sequential batch experiments. A model of coupled equilibria which included solubility of sparingly soluble salts, multiple cation exchange and inorganic complexation was used to describe the observed soil chemical changes. Fire caused large increases in the salt-extractable and soluble cations in the surface (0–2.5 cm) soil in ashbeds, but this decreased with time after fire. The model adequately described the temporal changes in the sorption of Na, K, Mg and Ca from the batch experiments using surface soil. The concentration of Ca measured in the equilibrium extracts indicated that sparingly soluble Ca compounds in the ash were controlling the solubility of Ca until at least 4 yrs after slash fire. Changes in the concentration of Ca and Mg could be described by including solubility product principles in the model. The modelling results suggested that the nature of sparingly soluble salts in the ash changed from a relatively soluble magnesian calcite 1 day after the fire to an insoluble magnesian calcite (or calcite) after 4 yrs. Soil organic C increased after the fire, but this was not reflected in any change in the cation exchange capacity of the soil. It is suggested that the measured increase in organic C was mainly due to the progressive inclusion of more decomposing fine root fragments in the sieved (<5 mm) soil with time after intense heating of soil under the ashbeds. This fraction of organic matter although increasing total soil C content, added little to active exchange sites.

Effect of temperature on the mineralization of C and N of fresh and mature compost in sandy material

Information about the mineralization rate of compost at various temperatures is a precondition to optimize mineral N fertilization and to minimize N losses in compost-amended soils. Objectives were to quantify the influence of the temperature on the mineralization rate and leaching of dissolved organic carbon (DOC) and nitrogen (DON), NO3—, and NH4+ from a fresh (C : N = 15.4) and a mature (C : N = 9.2) organic household waste compost. Compost samples were mixed with quartz sand to ensure aerobic conditions, incubated at 5, 10, 15, 20, and 25°C and irrigated weekly for 112 days. For the fresh compost, cumulative CO2 evolution after 112 days ranged from 36% of the initial C content at 5°C to 54% at 25°C. The CO2 evolution was only small in the experiments with mature compost (1 to 6% of the initial C content). The data were described satisfactorily by a combined first-order (fresh compost) or a first-order kinetic model (mature compost). For the fresh compost, cumulative DOC production was negatively related to the temperature, probably due to leaching of some of the partly metabolized easily degradable fractions at lower temperatures. The production ratios of DOC : CO2-C decreased with increasing temperature from 0.094 at 5°C to 0.038 at 25°C for the fresh and from 1.55 at 5°C to 0.26 at 25°C for the mature compost. In the experiments with fresh compost, net release of NO3— occurred after a time lag which depended on the temperature. Cumulative net release of NO3— after 112 days ranged from 1.8% of the initial N content at 5°C to 14.3% at 25°C. Approximately 10% of the initial N content of the mature compost was released as NO3— after 14 days at all temperatures. The DOC : DON ratios in the experiments using fresh compost ranged from 11.5 to 15.7 and no temperature dependency was observed. For the mature compost, DOC : DON ratios were slightly smaller (7.4 to 8.9). The DON : (NH4+ + NO3—) ratio decreased with increasing temperature from 0.91 at 5°C to 0.19 at 25°C for the fresh compost and from 0.21 at 5°C to 0.12 at 25°C for the mature compost. The results of the dynamics of C and N mineralization of fresh and mature compost can be used to assess the appropriate application (timing and amount) of compost to soils. Einfluss der Temperatur auf die C- und N-Mineralisierung von frischem und reifem Kompost in sandigem Material Information uber die Mineralisierungsrate von Kompost bei ver-schiedenen Temperaturen ist eine Voraussetzung zur Optimierung des Verhaltnisses zwischen zusatzlichen Mineral-N-Dungergaben und vermeidbaren N-Verlusten. Ziel dieser Studie war es, den Einfluss der Temperatur auf die Mineralisierungsrate und die Auswaschung von gelostem organischen Kohlenstoff (DOC) und Stickstoff (DON), NO3— und NH4+ eines frischen (C : N = 15,4) und eines reifen (C : N = 9,2) Kompostes aus der Biotonne zu quantifizieren. Kompostproben wurden mit Quarzsand vermischt, um aerobe Bedingungen zu gewahrleisten. Die Proben wurden fur 112 Tage bei 5, 10, 15, 20 und 25°C inkubiert und wochentlich beregnet. Die kumulative CO2-Freisetzung in den Experimenten mit frischem Kompost reichte nach 112 Tagen von 36% des initialen C-Gehalts bei 5°C bis zu 54% bei 25°C. Die CO2-Entwicklung in den Experimenten mit reifem Kompost war nur gering (1 bis 6%). Die Daten wurden befriedigend mit kinetischen Modellen beschrieben, die zwei Terme (frischer Kompost) bzw. einen Term (reifer Kompost) erster Ordnung beinhalteten. Fur den frischen Kompost stieg die kumulative DOC-Produktion mit fallender Temperatur an, vermutlich aufgrund von Auswaschung von einigen der teilweise metabolisierten leicht abbaubaren Fraktionen bei niedrigeren Temperaturen. Die Quotienten DOC-Produktion/CO2-C-Produktion verminderten sich mit steigender Temperatur von 0,094 bei 5°C auf 0,038 bei 25°C fur den frischen Kompost und von 1,55 bei 5°C auf 0,26 bei 25°C fur den reifen Kompost. In den Experimenten mit frischem Kompost trat eine Netto-NO3—-Freisetzung nach einer Verzogerung auf, die temperaturabhangig war. Die kumulative Netto-Freisetzung nach 112 Tagen reichte von 1,8% des initialen N-Gehalts bei 5°C bis zu 14,3% bei 25°C. Ungefahr 10% des initialen N-Gehalts des reifen Komposts wurde bei allen Temperaturen nach 14 Tagen als NO3— ausgewaschen. Die DOC : DON-Verhaltnisse in den Experimenten mit frischem Kompost reichten von 11,5 bis 15,7, unabhangig von der Temperatur. Fur den reifen Kompost waren die DOC : DON-Verhaltnisse enger (7,4 bis 8,9). Das DON : (NH4+ + NO3—)-Verhaltnis verminderte sich mit stei-gender Temperatur von 0,91 bei 5°C auf 0,19 bei 25°C fur den frischen Kompost und von 0,21 bei 5°C auf 0,12 bei 25°C fur den reifen Kompost. Die Ergebnisse der Dynamik der C- und N-Mineralisierung von frischem und reifem Kompost konnen zur Festlegung geeigneter Applikationen (Zeitpunkt und Menge) von Kompost zu Boden verwendet werden.

Determination of chemical and biological properties of composts using near infrared spectroscopy

Successful use of compost to maintain plant health and soil fertility requires consistent monitoring of compost quality. For this purpose, near infrared (NIR) spectroscopy might be a useful alternative to standard procedures which are often time-consuming and laborious. Ninety-eight yard-waste compost samples were analysed by conventional methods and NIR spectroscopy. Reference analysis included the determination of age, organic C (Corg) and total N (Nt) contents, C/N ratio, microbial biomass (Cmic), the ratio of Cmic to organic C (Cmic/Corg), basal respiration, metabolic quotient (qCO2), hydrolysis of fluorescein diacetate (FDA–HR), specific enzyme activity, i.e. FDA–HR related to Cmic, and suppression of pathogens. All samples were scanned in the visible light and near infrared regions (400–2500 nm). Cross-validation equations were developed using the whole spectrum (first and second derivative) and a modified partial least-square regression method. NIR predicted basal respiration and age successfully [ratio of standard deviation and standard error of cross-validation (RPD) was 4.3 or 2.9, respectively]. All other properties, i.e. Corg and Nt contents, C/N ratio, Cmic, Cmic/Corg, qCO2, FDA–HR, specific enzyme activity and suppression of pathogens at an inoculation level of 5‰ related to rating or fresh weight, respectively, were predicted with moderate success (1.4 ≤ RPD ≤ 2.0). However, the coefficients of determination for specific enzyme activity and suppression of pathogens related to fresh weight were rather low (r2 = 0.49 and 0.47, respectively). The results presented indicate that NIR spectroscopy is able to determine important compost quality parameters. However, further research is needed concerning the basis of and limitations for the determination of specific enzyme activity and suppressiveness by NIR spectroscopy.

Assessment of cation and anion exchange and pH buffering in an Amazonian Ultisol

Abstract Knowledge about cation and anion exchange and pH buffering in soils is essential for developing an efficient nutrient management system, especially in tropical soils where nutrient retention capacity may be low. Sorption characteristics and pH buffering of an Amazonian Ultisol were studied in sequential batch experiments. The objective was to gain additional information on the cation and anion exchange and pH buffer properties of this soil by using a coupled equilibrium model. The CEC values in the surface soil and subsoil were below 30 mmolc kg−1, indicating that the soil is highly limited in retaining nutrient cations against leaching. The anion exchange capacity (AEC) of the soils for both depths was small (1–4 mmolc kg−1), indicating insignificant retention of NO3 and its enhanced losses in seepage water following fertilization. The exchange coefficients (Gaines–Thomas formulation) of the surface soil followed the order Al>Ca>K>Mg>Na, whereas the order for the subsoil was Al>K>Na>Mg=Ca, indicating that K transferred by leaching from the surface soil to deeper depths may be retained preferentially in the subsoil. More protons could be buffered in the surface soil than in the subsoil. Addition of protons to the surface soil released equivalent amounts of Mb (Ca, Mg, K, Na) cations because of CEC reduction, and an insignificant amount of Al. However, input of protons to the subsoil released large amounts of Al. The model satisfactorily predicted the sorption values for various elements in both depths, indicating that the main processes are understood, but some deviations between modeled and experimental values were noted for the pH in the surface soil. The results indicated a very poor capacity of the soil for nutrient retention and pH buffering and a high risk of Al toxicity. Management options should substitute ‘slash and burn’ practices (which result in further nutrient losses) with mulch producing technologies and strive for only small additions of fertilizer (to reduce nutrient losses and avoid further soil acidification).

Dissolved Organic Carbon in Seepage Water – Production and Transformation during Soil Passage

Dissolved organic carbon (DOC) in seepage water can combine with organic pollutants, with Al and heavy metal ions and transport them through the soil profile with a potential to contaminate groundwater. We studied the production of DOC in aerobic decomposition experiments at 8 °C and moisture close to field capacity in soils from two sites with different microbial activities (spodic dystric Cambisols with moder (SLB) and mor-moder (SLS) layers) using 13C-depleted plants of differing decomposability (Epilobium angustifolium and Calamagrostis epigeios). Additionally, we investigated the DOC transformation during soil passage in decomposition experiments and in the field for the sites SLB and SLS. For SLS, decomposition of Epilobium resulted in a cumulative CO2 production of 14% of the added C within 128 days. Priming effects were negligible. CO2 production for the experiments using Calamagrostis was less with 11% for SLB and 10% for SLS. Cumulative DOC production was markedly high in the Epilobium decomposition experiment, being 25 g m–2, out of which 11 g m–2 were Epilobium-derived (2% of the added C). For the Calamagrostis experiments, cumulative productions of DOC and Calamagrostis-derived DOC (0.1% of the added C for SLS and SLB) were much less. During the soil passage, much of the DOC was removed by sorption or decomposition processes. Field studies at SLS and SLB using 13C natural abundance showed that 13C distribution of soil organic matter increased with depth, probably mainly due to a discrimination of C isotopes by decomposing microorganisms. DOC, however, showed a depletion of 13C from –28γ PDB to –29γ (SLB at 40 cm) or –28 to –30γ (SLS at 20 cm) with depth, owing to preferential decomposition of 13C-enriched substances or preferential adsorption. This study indicates that DOC production is strongly affected by litter composition and that significant changes in DOC composition may occur during its passage through a soil depth of 40 cm. Geloster organisch gebundener Kohlenstoff im Sickerwasser – Produktion und Transformation wahrend der Bodenpassage Geloster organisch gebundener Kohlenstoff (DOC) im Sickerwasser kann mit organischen Schadstoffen, Al- und Schwermetallionen Bindungen eingehen und durch das Bodenprofil transportiert werden mit dem Potential, Grundwasser zu kontaminieren. Wir untersuchten die DOC-Produktion in aeroben Abbauexperimenten bei 8 °C und Feuchtegehalten nahe der Feldkapazitat in Boden zweier Standorte mit unterschiedlichen mikrobiellen Aktivitaten (spodic dystric Cambisols mit Moder- (SLB) und rohhumusartiger Moder-Auflage (SLS)) bei Verwendung 13C-abgereicherter Pflanzen unterschiedlicher Abbaubarkeit (Epilobium angustifolium und Calamagrostis epigeios). Zusatzlich untersuchten wir die DOC-Transformation wahrend der Bodenpassage in Abbauexperimenten und im Freiland fur die Standorte SLB und SLS. Fur SLS wurde beim Abbau von Epilobium eine kumulative CO2-Produktion von 14% des zugegebenen Kohlenstoffs innerhalb von 128 Tagen gefunden. Priming-Effekte waren vernachlassigbar. Die CO2-Produktion beim Abbau von Calamagrostis war geringer: 11% fur SLB und 10% fur SLS. Die kumulative DOC-Produktion war betrachtlich in dem Abbauexperiment mit Epilobium: 25 g m–2, wobei 11 g m–2Epilobium-burtig waren (2% des zugegebenen C). Fur die Abbauexperimente mit Calamagrostis waren die kumulativen Produktionen an DOC und an Calamagrostis-burtigem DOC (0.1% des zugegebenen C bei SLS und SLB) erheblich geringer. Wahrend der Bodenpassage wurde ein Grosteil des DOC durch Sorption oder Abbauprozesse aus dem Sickerwasser entfernt. Freilandstudien bei SLS und SLB unter Verwendung der naturlichen 13C-Haufigkeit zeigten, dass die 13C-Verteilung der organischen Bodensubstanz mit zunehmender Tiefe anstieg, vermutlich hauptsachlich aufgrund einer Diskriminierung der C-Isotope durch zersetzende Mikroorganismen. DOC wies jedoch mit zunehmender Tiefe eine 13C-Abreicherung von –28γ PDB bis zu –29γ (SLB bei 40 cm) oder von –28 bis zu –30γ (SLS bei 20 cm) auf, aufgrund eines bevorzugten Abbaus 13C-angereicherter Substanzen oder bevorzugter Adsorption. Diese Studie zeigt, dass die DOC-Produktion erheblich von der Qualitat der Streu abhangt und dass signifikante nderungen in der DOC-Zusammensetzung wahrend der Bodenpassage bis zu einer Tiefe von 40 cm auftreten konnen.A