An ecosystem-scale radiocarbon tracer to test use of litter carbon by ectomycorrhizal fungi

The degree to which ectomycorrhizal fungi rely on decomposing litter as a carbon source in natural ecosystems is unknown. We used a radiocarbon (¹⁴C) tracer to test for uptake of litter carbon by ectomycorrhizal fungi as part of the Enriched Background Isotope Study (EBIS) in Oak Ridge Reservation, Tennessee. In EBIS, leaf litter from a highly ¹⁴C-labeled Quercus alba (white oak) forest was reciprocally transplanted with litter from a nearby low-labeled forest that had not been as strongly exposed to ¹⁴C. These litter transplants were conducted yearly. We measured Δ¹⁴C signatures of ectomycorrhizal fungi collected from each forest four months and 2.25 years after the first litter transplant. The ectomycorrhizas were associated with white oak trees. We found no significant differences in ¹⁴C signatures of ectomycorrhizal fungi exposed to low-labeled versus high-labeled litter, indicating that less than 2% of the carbon in ectomycorrhizal biomass originated from transplanted litter. In contrast, ectomycorrhizal Δ¹⁴C signatures from the high-labeled forest were 117-140‰ higher than those from the low-labeled forest. This pattern suggests that ectomycorrhizal fungi acquired most (or all) of their carbon from their host plants, probably via direct transfer of photosynthate through the roots.     

Chemical composition of young and old carbon pools throughout Cambisol and Luvisol profiles under forests

The long-term storage of soil organic matter (SOM) in forest soils is still poorly understood. In this study, particle size fractionation in combination with accelerator mass spectroscopy (AMS) and solid state ¹³C nuclear magnetic resonance (NMR) spectroscopy was applied to investigate organic carbon (OC) stabilisation in Cambisol and Luvisol profiles under spruce (Picea abies) and beech (Fagus sylvatica L.) forests. In most samples, OC was preferentially associated with <2 μm fractions. Throughout soil profiles the contribution of OC in the clay fraction to the total OC increased from 27%-53% in A horizons to 44-86% in E, B and EB horizons. The 200-2000 μm fractions from all sites and all depths showed a percentage of modern C (pmC)>100. They were enriched in ¹⁴C owing to high inputs of recent material from leaves and roots. Clearly less active material was associated with <2 and 2-20 μm fractions. This demonstrated that the particle size fractionation procedure applied to our study was capable to isolate a young OC fraction in all samples. The pmC values were strongly decreasing with depth but the decrease was much more pronounced in the fine fractions. The <2 and 2-20 μm fractions of B, E and EB horizons revealed radiocarbon ages between 512 and 4745 years before present which indicated that the SOM in those horizons was little affected by the recent vegetation. The major components of labile and stable SOM pools in topsoils and subsoils were always O/N-alkyl C (28-53%) and alkyl C (14-48%) compounds. NMR spectra of bulk soils and particle size fractions indicated that high alkyl C and O/N-alkyl C proportions throughout the soil profile are typical of Cambisols and Luvisols which were not subjected to regular burning. A relation between radiocarbon age and chemical composition throughout soil profiles was not observed. This suggests that the long-term stabilisation of SOM is mainly controlled by the existence of various mechanisms of protection offered by the soil matrix and soil minerals but not by the chemical structure of SOM itself.     

Holocene paleosols and colluvial sediments in Northeast Tibet (Qinghai Province,China): Properties,dating and paleoenvironmental implications

Colluvial deposits consisting of silts and loams were detected in several climatologically different areas of NE Tibet (3200–3700 m a.s.l.). Layering, distinct organic content and low content of coarse matter as well as location in the relief revealed an origin from low-energy slope erosion (hillwash). Underlying and intercalated paleosols were classified as Chernozems, Phaeozems, Regosols and Fluvisols. Fifteen radiocarbon datings predominant on charcoal from both colluvial layers and paleosols yielded ages between 8988 ± 66 and 3512 ± 56 uncal BP. Natural or anthropogenic factors could have been the triggers of the erosional processes derived. It remains unclear which reason was mainly responsible, due to controversial paleoclimatic and geomorphic records as well as insufficient archaeological knowledge from this region. Determinations of charcoal and fossil wood revealed the Holocene occurrence of tree species (spruce, juniper) for areas which nowadays have no trees or only few forest islands. Thus large areas of NE Tibet which are at present steppes and alpine pastures were forested in the past.     

Soil carbon pools, plant biomarkers and mean carbon residence time after afforestation of grassland with three tree species

Afforestation of grassland has been globally identified as being an important means for creating a sink for atmospheric carbon (C). However, the impact of afforestation on soil C is still poorly understood, due to the paucity of well designed long-term experiments and the lack of investigation into the response of different soil C fractions to afforestation. In addition, little is known about the origins of soil C and soil organic matter (SOM) stability after afforestation. In a retrospective study, we measured C mass in the soil light and heavy fractions in the first 10 years after afforestation of grassland with Eucalyptus nitens, Pinus radiata and Cupressus macrocarpa. The results suggest that C mass in the soil heavy fraction remained stable, but the C mass in the light fraction decreased at year 5 under three species. Soil δ¹³C analysis showed that the decrease in the light fraction may be due to reduced C inputs from grassland species litter and low inputs from the still young trees. After the initial reduction, the recovery of soil C in the light fraction depended on tree species. At year 10, an increase of 33% in light fraction soil C was observed at the 0-30 cm depth under E. nitens, compared to that under the original grassland (year 0). Planting P. radiata restored light fraction soil C to the original level under grassland, whereas planting C. macrocarpa led to a decrease of 33%. We concluded that the increase of light fraction soil C between year 5 and 10 is most likely due to C input from tree residues. Most of the increased C was derived from root turnover under pine and from both root and leaf turnover under E. nitens, as indicated by plant C biomarkers such as lignin-derived phenols and suberin and cutin-derived compounds in the 0-5 cm soil layer. Modelling of soil ?¹⁴C‰ suggested that SOM had a greater mean residence time at year 10 than year 0 and 5 due to increased relative abundance of recalcitrant plant biopolymers.     

Soil organic carbon stocks,distribution, and composition affected by historic land use changes on adjacent sites

Historic alterations in land use from forest to grassland and cropland to forest were used to determine impacts on carbon (C) stocks and distribution and soil organic matter (SOM) characteristics on adjacent Cambisols in Eastern Germany. We investigated a continuous Norway spruce forest (F-F), a former cropland afforested in 1930 (C-F), and a grassland deforested in 1953 (F-G). For C and N stocks, we sampled the A and B horizons of nine soil pits per site. Additionally, we separated SOM fractions of A and B horizons by physical means from one central soil pit per pedon. To unravel differences of SOM composition, we analyzed SOM fractions by ¹³C-CPMAS NMR spectroscopy and radiocarbon analysis. For the mineral soils, differences in total C stocks between the sites were low (F-F = 8.3 kg m⁻²; C-F = 7.3 kg m⁻²; F-G = 8.2 kg m⁻²). Larger total C stocks (+25%) were found under continuous forest compared with grassland, due to the C stored within the organic horizons. Due to a faster turnover, the contents of free particulate organic matter (POM) were lower under grassland. High alkyl C/O/N-alkyl C ratios of free POM fractions indicated higher decomposition stages under forest (1.16) in relation to former cropland (0.48) and grassland (0.33). Historic management, such as burning of tree residues, was still identifiable in the subsoils by the composition and ¹⁴C activity of occluded POM fractions. The high potential of longer lasting C sequestration within fractions of slower turnover was indicated by the larger amounts of claybound C per square meter found under continuous forest in contrast to grassland.     

Clues to the genesis of a discontinuously distributed fragipan in the northern Apennines,Italy

At about 1000 m above sea level in the Vallombrosa Forest, northern Apennines, Central Italy, a dense pan that meets the diagnostic criteria for fragipans is discontinuously distributed. The unusual slope-parallel “digitate” distribution of the pan suggests that it could have formed on water-saturated soil material that moved as earthflows. Actually, several soils in the Vallombrosa Forest have buried or truncated horizons, indicating episodes of mass movement and deposition. However, the fragipan under study shows a homogeneous morphology throughout, hence not providing clues about the occurrence of episodes that have contributed to the emplacement of the parent material. Also, the physical, chemical, and mineralogical analyses of the fragipan do not reveal discontinuities that help to resolve distinct episodes of deposition. Nevertheless, discontinuities in features such as the degree of alteration of biotite grains and the amount of plasma, seen through the micromorphological investigation at 50–60 cm of the current depth, suggest the occurrence of at least two depositional events. Radiocarbon dating of large-size pieces of charcoal collected at three depths within the fragipan yielded three ages that may reflect different times of accumulation of the parent material during the last glacial period. Post-depositional micromorphological features, such as relatively undamaged clay coatings and a weak platy structure, indicate pedogenesis with the absence of intense pedoturbation.     

Fluvial development and the sediment regime of the lower Calder,Ribble catchment,northwest England

In the lower reaches of the Calder (Lancashire) tributary of the Ribble catchment (northwest England), five river terraces set into the surrounding glacial terrain have been mapped and their underlying sediments radiocarbon dated. The earliest terrace (T1) reflects sandur-style deposition during deglaciation and it aggraded in a reach cut into glacial diamict and glaciolacustrine muds. Incision below T1 spanned the start of the Holocene, driven by a combination of reduced sediment supply under a stabilising landscape and lower base-levels. T2 had aggraded and was being abandoned by 4000 BC; its formation latterly involving a period of comparative stability in terms of channel migration, perhaps a function of extensive forest cover and sea levels reaching above present day levels. The last 3000 years were marked by substantial changes in the sedimentary regime, with increased and varying sediment supply driving cycles of cut-and-fill and greater lateral channel migration. These changes coincided with the first widespread and substantial reductions in forest cover and a progression towards an agricultural landscape. Between the aggradation of terraces T3 and T4 there was a substantial increase in sediment supply, which is broadly in keeping with the erosion history in the surrounding uplands. This sediment slug appears to have induced a switch from highly sinuous meandering to lower sinuosity channels characterised by lateral scroll-style migration. Late Holocene vegetation changes, particularly woodland removal, driven by anthropogenic land-pressure appear to have rendered the floodplain prone to channel migration and the landscape more susceptible to erosion.     

Drying-rewetting events reduce C and N losses from a Norway spruce forest floor

Periods of prolonged summer drought are likely to be expected for this century, with possibly strong effects on carbon (C) and nitrogen (N) mineralization in soils. Drought generally reduces mineralization rates, but the possibility of excess mineralization pulses during rewetting raises the question about the net effect of drying-rewetting events. In this experiment, we measured C and N mineralization in undisturbed soil columns that were either kept under continuously moist conditions (control) or that were subjected to drying-rewetting. We had three treatments (D1-D3) with different drying intensity (increasing from D1 to D3) but uniform rewetting intensity (4 mm d⁻¹). Soil columns were taken from a Norway spruce forest in Bavaria, Germany. The CO₂ fluxes from control and treatment groups were identical before drying. Over the 80 d drought period, total CO₂ emissions from D1, D2, and D3 were only 72, 52 and 43% of that from the control, respectively. Rewetting resulted in a fast increase of CO₂ fluxes to approx. the same level as in the control. Rewetting could not restore soil moisture of the dry soil to the level of the control, presumably because of preferential flow and water repellency of soil organic matter. No significant excess C mineralization during the 40 d rewetting period was observed. Adding up total CO₂ fluxes during drought and rewetting period, the treatments D1, D2, and D3 emitted only 88, 71 and 67% of the CO₂ emitted by the control. Measurements of dissolved organic carbon (DOC) did only show minor differences between control and treatment columns, indicating that no significant accumulation of DOC took place during the drought period. Radiocarbon signature of emitted CO₂ indicated that C mineralization was reduced with decreasing water availability and no new substrate became bioavailable. Net N mineralization over the course of the whole experiment was reduced by drought to 77, 65 or 52% of the control. Net nitrification was virtually zero during drought whereas net ammonification continued at reduced levels. In summary, we found that drying-rewetting generally reduced C and N mineralization in this soil and that the total reduction increased with drought intensity.     

Tracking litter-derived dissolved organic matter along a soil chronosequence using 14C imaging: Biodegradation,physico-chemical retention or preferential flow?

The cycling of dissolved organic matter (DOM) in soils is controversial. While DOM is believed to be a C source for soil microorganisms, DOM sorption to the mineral phase is regarded as a key stabilization mechanism of soil organic matter (SOM). In this study, we added ¹⁴C-labelled DOM derived from Leucanthemopsis alpina to undisturbed soil columns of a chronosequence ranging from initial unweathered soils of a glacier forefield to alpine soils with thick organic layers. We traced the ¹⁴C label in mineralized and leached DOM and quantified the spatial distribution of DO¹⁴C retained in soils using a new autoradiographic technique. Leaching of DO¹⁴C through the 10 cm-long soil columns amounted up to 28% of the added DO¹⁴C in the initial soils, but to less than 5% in the developed soils. Biodegradation hardly contributed to the removal of litter-DO¹⁴C as only 2–9% were mineralized, with the highest rates in mature soils. In line with the mass balance of ¹⁴C fluxes, measured ¹⁴C activities in soils indicated that the major part of litter DO¹⁴C was retained in soils (>80% on average). Autoradiographic images showed an effective retention of almost all DO¹⁴C in the upper 3 cm of the soil columns. In the deeper soil, the ¹⁴C label was concentrated along soil pores and textural discontinuities with similarly high ¹⁴C activities than in the uppermost soil. These findings indicate DOM transport via preferential flow, although this was quantitatively less important than DOM retention in soils. The leaching of DO¹⁴C correlated negatively with oxalate-extractable Al, Fe, and Mn. In conjunction with the rapidity of DO¹⁴C immobilization, this strongly suggests that sorptive retention DOM was the dominating pathway of litter-derived DOM in topsoils, thereby contributing to SOM stabilization.     

Contribution of sorption,DOC transport and microbial interactions to the 14C age of a soil organic carbon profile: Insights from a calibrated process model

Profiles of soil organic carbon (SOC) are often characterized by a steep increase of ¹⁴C age with depth, often leading to subsoil ¹⁴C ages of more than 1000 years. These observations have generally been reproduced in SOC models by introducing a SOC pool that decomposes on the time-scale of millennia. The overemphasis of chemical recalcitrance as the major factor for the persistence of SOC was able to provide a mechanistic justification for these very low decomposition rates. The emerging view on SOC persistence, however, stresses that apart from molecular structure a multitude of mechanisms can lead to the long-term persistence of organic carbon in soils. These mechanisms, however, have not been incorporated into most models. Consequently, we developed the SOC profile model COMISSION which simulates vertically resolved SOC concentrations based on representations of microbial interactions, sorption to minerals, and vertical transport. We calibrated COMISSION using published concentrations of SOC, microbial biomass and mineral-associated OC (MOC), and in addition, ¹⁴C contents of SOC and MOC of a Haplic Podzol profile in North-Eastern Bavaria, Germany. In order to elucidate the contribution of the implemented processes to the ¹⁴C age in different parts of the profile, we performed model-experiments in which we switched off the limitation of SOC decomposition by microbes, sorptive stabilization on soil minerals, and dissolved OC (DOC) transport. By splitting all model pools into directly litter-derived carbon and microbe-derived organic carbon, we investigated the contribution of repeated microbial recycling to ¹⁴C ages throughout the profile. The model-experiments for this site lead to the following implications: Without rejuvenation by DOC transport, SOC in the subsoil would be on average 1700 ¹⁴C years older. Across the profile, SOC from microbial recycling is on average 1400 ¹⁴C years older than litter-derived SOC. Without microbial limitation of depolymerization, SOC in the subsoil would be on average 610 ¹⁴C years younger. Sorptive stabilization is responsible for relatively high ¹⁴C ages in the topsoil. The model-experiments further indicate that the high SOC concentrations in the Bh horizon are caused by the interplay between sorptive stabilization and microbial dynamics. Overall, the model-experiments demonstrate that the high ¹⁴C ages are not solely caused by slow turnover of a single pool, but that the increase of ¹⁴C ages along a soil profile up to ages >1000 years is the result of different mechanisms contributing to the overall persistence of SOC. The dominant reasons for the persistence of SOC are stabilization processes, followed by repeated microbial processing of SOC.