Carbon dynamics of young experimental afforestations in Thuringia

Nach Artikel 3.3. des Kyoto-Protokolls können Aufforstungen als Kohlenstoff (C) Senken angerechnet werden. Junge Aufforstungen können jedoch signifikante C-Quellen darstellen, wenn der C-Austrag durch Bodenatmung die C-Speicherung durch den Biomassezuwachs der Bäume übersteigt. Das Ziel dieser Arbeit war, i) die Faktoren zu untersuchen, die den Anwuchserfolg der Bäume bei Freiflächenaufforstungen bestimmen, ii) den Einfluss der Flächenvorbereitungsmaßnahmen und Veränderungen im Flächenmanagement auf die C-Bilanz des Systems zu quantifizieren, iii) den Einfluss historischer Landnutzungswechsel und der Bodenfauna (Regenwürmer) auf die C-Dynamik zu untersuchen und iv) die Heterogenität der C-Vorräte im Boden auf zwei Aufforstungsflächen zu analysieren, um ein optimiertes Beprobungsdesign für zukünftige Untersuchungen auf diesen Flächen zu entwickeln. Die Ergebnisse dieser Studien sind in sechs Publikationen zusammengefasst. Sie bilden die Grundlage für das Langzeitexperiment BIOTREE, in dessen Rahmen diese Arbeit angefertigt wurde. Das Experiment umfasst drei Flächen von insgesamt 70 ha in Thüringen. Das Ziel dieses Experimentes ist es in Zukunft den Einfluss der Baumartenvielfalt auf ökosystemare Prozesse zu untersuchen. Dazu wurden 300 000 Setzlinge 19 verschiedener Baumarten gepflanzt. Der Anwuchserfolg der 19 verschiedenen Baumarten, sowie die Einflussfaktoren, die den Anwuchserfolg bestimmen, wurde untersucht. Ausfälle von bis zu 79% pro Baumart verlängern die Zeit bis die Aufforstungsfläche zu einer Netto-C-Senke wird. Untersuchungsflächen mit hoher Baumartenvielfalt waren resistenter gegenüber Verbissschäden durch Schermäuse und Hasen als Flächen mit geringer Baumartenvielfalt. Die Flächen Mehrstedt und Kaltenborn wurden vor 23 und 29 Jahren teilweise von Acker zu Grünland umgewandelt. Diese vergangenen Landnutzungsänderungen ergaben keine signifikanten Veränderungen der C-Vorräte im Boden aber eine veränderte vertikale C-Verteilung. Hohe C-Vorräte wurden unterhalb der Pflugsohle auf den tonreichen Böden der Ackerfläche Mehrstedt gefunden. Die Quell- und Schrumpfdynamik der Tonminerale führte zu einem beschleunigten C-Transport in den Unterboden. 14C Altersbestimmungen des organischen Kohlenstoffs bestätigten diese Hypothese. Nur im obersten Bodenhorizont auf der Grünlandfläche Kaltenborn sind die mineralischen Oberflächen C-gesättigt und können deshalb keinen zusätzlichen Kohlenstoff physikalisch stabilisieren. Die großen ungesättigten mineralischen Oberflächen der Unterböden stellen ein ungenutztes Potenzial zur Stabilisierung und Speicherung von zusätzlichem Kohlenstoff dar. Der Netto-C-Fluss zwischen der Landoberfläche und der Atmosphäre wurde auf der Aufforstungsfläche Mehrstedt und einem angrenzendem Grünland mit zwei Eddy- Kovarianz-Türmen gemessen. Die Bruttoprimärproduktion der Aufforstungsfläche war um 41% (erstes Jahr) bis 14% (drittes Jahr) geringer als die der benachbarten Grünlandfläche. Die Flächenvorbereitung der Aufforstung mit Tieffräsen der Pflanzreihen zerstörte 30% der nicht-verholzten Vegetation, die die C-Flüsse der Fläche bestimmten. Eine beschleunigte Mineralisierung von Bodenkohlenstoff auf der Aufforstungsfläche führte im ersten Jahr zu einem Netto-C-Verlust von 1.2 t ha-1. Dahingegen war die saisonale C-Dynamik durch klimatische Faktoren bestimmt und durch Störungen durch das Flächenmanagement. Die Detektierbarkeit von Veränderungen der C-Vorräte im Boden wird durch deren räumliche und vertikale Heterogenität bestimmt. Die Variabilität der Bodenkohlenstoff-konzentration war ein bis zwei Größenordnungen größer als die der Feinbodendichte. Aus diesen beiden Parametern werden die C-Vorräte im Boden errechnet. Mit einem Simulationsmodel konnte gezeigt werden wie diese Information genutzt werden kann, um das Beprobungsdesign zu optimieren mit 12 - 19% weniger Proben aber unveränderter statistischer Genauigkeit. Der Einfluss von Regenwürmern auf den C-Transport und die C-Stabilisierung wurde untersucht, um den Effekt von verringerter Regenwurmabundanz in Wäldern auf die C Dynamik im Boden abschätzen zu können. Tiefgrabende Regenwürmer haben frischen Detritus schnell und effektive in den Unterboden transportiert und dort an den Gangwänden abgelagert. Entgegen der Hypothese, dass Regenwürmer zur C Stabilisierung beitragen wurde ein schneller C-Abbau in den Regenwurmgängen gemessen mit Umsatzzeiten von 3 bis 5 Jahren. Ein NMR (nuclear magnetic resonance) Relexationszeit-experiment und Messungen zur Enzymaktivität in den Regenwurmgängen ergaben keine Hinweise auf eine C-Stabilisierung durch Regenwürmer. Die C-Dynamik der untersuchten Aufforstungsflächen wird durch verschiedene Faktoren bestimmt, von denen sich einige kontinuierlich mit Heranwachsen des Waldes ändern werden, wie z.B. die Regenwurmabundanz oder die Bodenfeuchtedynamik. Dies wird zu Rückkopplungen auf den C-Kreislauf und auf die C-Speicherfunktion der Aufforstung führen.Afforestations are acknowledged as C sinks under the Kyoto protocol article 3.3. However, young afforestations may be considerable C sources. Losses of soil C may offset the C sink of the tree biomass. The aim of this thesis was to i) investigate the factors that affect the establishment success of the new forests, ii) quantify the impact of site preparation and management changes along with the afforestation on the C balance of the system, iii) understand how soil C dynamics are influenced by historical land use changes and activity of the soil fauna (earthworms), and iv) to explore soil C variability to set up an optimized sampling scheme for future soil C studies at the two afforestation sites. The essence of this research is presented in the form of six manuscripts. This thesis sets the basis for the long-term experiment BIOTREE which was started at three sites in Thuringia with a total of 70 ha. The future aim of this experiment is to investigate the influence of tree diversity on ecosystem processes. Therefore, 300 000 seedlings from 19 different tree species were planted. The design of the experiment is outlined in manuscript 1 together with a description of the three study sites. Manuscript 2 explores the differences between the establishment success of the tree species and the influencing factors. Establishment failure of the species up to 79% extends the time before afforestations become net C sinks. Experimental plots with higher tree diversity were found to be more resistant against damages by voles and rabbits than plots with less tree species. Parts of the sites Kaltenborn and Mehrstedt were converted from cropland to grassland, 23 and 29 years ago, respectively. The impact of this historical land use change on soil C stocks and C fractions was investigated (manuscript 3). Surprisingly, there was no significant difference in soil C stocks between both land use types but a different vertical C distribution was observed. High C stocks at the clay rich Mehrstedt site were found below the ploughing horizon. The swelling and shrinking dynamic of the clayey soil was expected to enhance the C transport into the subsoil. Measurements of the 14C age of this subsoil C confirmed this hypothesis. In the uppermost horizon of the sandy soil in the Kaltenborn grassland mineral surfaces were found to be C-saturated, thus, this horizon cannot physically stabilise additional C. The large area of unsaturated mineral surfaces in the subsoil provides an unused capacity to stabilise and store additional C at of both sites. Net C exchange fluxes between land surface and atmosphere were measured with two eddy covariance towers at the afforestation site Mehrstedt and an adjacent grassland site (manuscript 4). Gross primary productivity of the afforestation was reduced by 41% (first two years) to 14% (third year) compared to the grassland. Site preparation of the afforestation with deep ploughing damaged parts of the herbaceous vegetation that dominated the C fluxes. Enhanced C mineralisation was detected at the afforestation only during the first year, causing a net C loss of 1.2 t ha-1. Seasonal C dynamics were determined by climatic factors (mainly precipitation during summer) and disturbances by site management (grazing on grassland site, mowing on the afforestation site). The probability to detect expected soil C stock changes depends on the vertical and spatial heterogeneity of the C stocks. The variability of the soil C concentration was found to be one to two magnitudes higher than the variability of the bulk density. Both parameters directly affect the calculated soil C stocks. A simulation model revealed the possibility to improve the sampling design for soil C stocks with sample numbers reduced by 12-19% but unchanged statistical power. This is of major importance because high sample numbers are usually needed to make soil C stock changes detectable. The effect of earthworms on soil C translocation and stabilisation was investigated to understand how afforestations may influence the C cycling indirectly by reducing the earthworm abundance (manuscript 6). Deep burrowing earthworms were found to be effective in translocating recently assimilated C into the subsoil by depositing it along the burrow walls. Contrary to the original hypothesis of C stabilisation due to earthworm gut passage, organic C in earthworm burrows was lost rapidly with half life times of only 3 5 years. Nuclear magnetic resonance (NMR) relaxation experiments and enzyme activity measurements showed no enhanced C stabilisation by earthworms. The C dynamics of the investigated afforestation sites were found to be influenced by different factors. Some of them, such as earthworm abundance and seasonal soil moisture pattern, change along with the forest development feeding back on the C cycle and the C sequestration

Does conversion to conservation tillage really increase soil organic carbon stocks in organic arable farming?

Aggravation of weather extremes increases awareness of climate change consequences. Mitigation options are in demand which aim to reduce the atmospheric concentration of greenhouse gases. Amongst others, conversion from ploughing to conservation tillage is argued to increase soil organic carbon (SOC) stocks. Yet, main findings of reviews and meta-analyses comparing SOC stocks between tillage systems show different results: from a significant increase of SOC stocks to the question if there is any effect at all. Reasons are a sampling bias as in many campaigns only topsoil layers are assessed and horizons thickness is not considered adequately, different methods for SOC and bulk density determination, and the comparison of SOC stocks based on equivalent soil masses instead of equal sampling depths. In order to address these limitations, we initiated the SOCORT consortium (Soil Organic Carbon in Organic Reduced Tillage) – an international network of nine agronomical long-term trials. All trials represent common mixed organic farming systems of the respective region with organic fertilisation and crop rotations including leys. Climatic conditions are similar, but age and soil texture vary (7 to 21 years and sandy to clayey soils). A common sampling campaign was consequently elaborated to answer the question if the combination of conservation tillage and organic farming can really increase SOC stocks. Undisturbed soil cores were taken with driving hammer probes (8 cm in diameter) to a maximum depth of 100 cm. Each core was divided in the increments 0-30, 30-50, 50-70, 70-100 cm. The topsoil layer (0-30 cm) was further divided into the different tillage depths of the respective trial. All samples were analysed in the same laboratory for bulk density, organic carbon content, pH and texture. We compiled the yields for each trial to assess carbon inputs. The SOCORT consortium in combination with the common sampling campaign will entangle the driving factors of carbon sequestration through reduced tillage and add important knowledge on carbon dynamics in agro-ecosystems

Substrate quality of drained organic soils—Implications for carbon dioxide fluxes

Background: Peatlands only cover a minor fraction of the global terrestrial surface, but due to drainage, they are major contributors to carbon dioxide (CO2) emissions from soils. Previous studies have shown that hydrological conditions, nutrient availability and anthropogenic disturbance play an important role in the mineralisation of organic matter. Furthermore, microbial turnover depends on peat quality, which is determined by its botanical origin and degree of transformation under natural conditions. Aims: The objective of this study was to shed light on the interdependence between mineralisation rates, secondary transformation of peat and chemical composition by examining the differences between bog and fen peat and between strongly degraded topsoil and well-preserved subsoil. Methods: Bog and fen peat from ten different peatlands under grassland use in Germany were analysed for their chemical composition using standard 13C nuclear magnetic resonance (NMR) spectroscopy and wet chemical extractions for fibre analysis. The radiocarbon age was determined as well. The results were combined with CO2 fluxes from a previous incubation study. Results: Topsoils had higher shares of proteins and lipids, and lower shares of carbohydrates and aromatics than subsoils. Bog peat subsoils were characterised by higher shares of carbohydrates and lower shares of aromatics than fen peat subsoils. Topsoils were more similar to each other in their chemical composition than the subsoils. Considering all samples, aromatics and phenolics were negatively correlated with CO2 fluxes. Measured CO2 fluxes from topsoils were significantly higher than from subsoils. However, no influences of chemical composition on CO2 fluxes were detected when examining topsoils and subsoils separately. Even though aromatics and phenolics showed positive relationships with radiocarbon age, differences in age alone were unable to explain the higher amounts of these compounds in the subsoil. Conclusions: The results imply that chemical composition of topsoil peat is not the reason for higher mineralisation rates compared to subsoil peat, but rather a consequence of decomposition and transformation. Thus, peat mineralisation of drained organic soils under agriculture might not slow down over time due to gradually decreasing peat quality but could increase further

Direct nitrous oxide emissions from oilseed rape cropping – a meta-analysis

Oilseed rape is one of the leading feedstocks for biofuel production in Europe. The climate change mitigation effect of rape methyl ester (RME) is particularly challenged by the greenhouse gas (GHG) emissions during crop production, mainly as nitrous oxide (N2O) from soils. Oilseed rape requires high nitrogen fertilization and crop residues are rich in nitrogen, both potentially causing enhanced N2O emissions. However, GHG emissions of oilseed rape production are often estimated using emission factors that account for crop-type specifics only with respect to crop residues. This meta-analysis therefore aimed to assess annual N2O emissions from winter oilseed rape, to compare them to those of cereals and to explore the underlying reasons for differences. For the identification of the most important factors, linear mixed effects models were fitted with 43 N2O emission data points deriving from 12 different field sites. N2O emissions increased exponentially with N-fertilization rates, but interyear and site-specific variability were high and climate variables or soil parameters did not improve the prediction model. Annual N2O emissions from winter oilseed rape were 22% higher than those from winter cereals fertilized at the same rate. At a common fertilization rate of 200 kg N ha−1 yr−1, the mean fraction of fertilizer N that was lost as N2O-N was 1.27% for oilseed rape compared to 1.04% for cereals. The risk of high yield-scaled N2O emissions increased after a critical N surplus of about 80 kg N ha−1 yr−1. The difference in N2O emissions between oilseed rape and cereal cultivation was especially high after harvest due to the high N contents in oilseed rape's crop residues. However, annual N2O emissions of winter oilseed rape were still lower than predicted by the Stehfest and Bouwman model. Hence, the assignment of oilseed rape to the crop-type classes of cereals or other crops should be reconsidered

Fifty years after deep‐ploughing: Effects on yield, roots, nutrient stocks and soil structure

Deep‐ploughing far beyond the common depth of 30 cm was used more than 50 years ago in Northern Germany with the aim to break root‐restricting layers and thereby improve access to subsoil water and nutrient resources. We hypothesized that effects of this earlier intervention on soil properties and yields prevailed after 50 years. Hence, we sampled two sandy soils and one silty soil (Cambisols and a Luvisol) of which half of the field had been deep‐ploughed 50 years ago (soils then re‐classified as Treposols). The adjacent other half was not deep‐ploughed and thus served as the control. At all the three sites, both deep‐ploughed and control parts were then conventionally managed over the last 50 years. We assessed yields during the dry year 2019 and additionally in 2020, and rooting intensity at the year of sampling (2019), as well as changes in soil structure, carbon and nutrient stocks in that year. We found that deep‐ploughing improved yields in the dry spell of 2019 at the sandy sites, which was supported by a more general pattern of higher NDVI indices in deep‐ploughed parts for the period from 2016 to 2021 across varying weather conditions. Subsoil stocks of soil organic carbon and total plant‐available phosphorus were enhanced by 21%–199% in the different sites. Root biomass in the subsoil was reduced due to deep‐ploughing at the silty site and was increased or unaffected at the sandy sites. Overall, the effects of deep‐ploughing were site‐specific, with reduced bulk density in the buried topsoil stripes in the subsoil of the sandy sites, but with elevated subsoil density in the silty site. Hence, even 50 years after deep‐ploughing, changes in soil properties are still detectable, although effect size differed among sites.BonaRes http://dx.doi.org/10.13039/501100022576Peer Reviewe

The availability of carbon sequestration data in Europe

With growing interest in the carbon sequestration potential of soils, experimental research and mapping projects have produced a wealth of datasets in this subject area. However, the coverage, quality and scope of available data vary widely across Europe, and the extent to which these data are accessible to experimental researchers and modellers is also highly variable. This report describes the availability of soil carbon data at the global and European levels, and reviews the on-line resources for accessing these data and meta-data. The extent to which researchers in the field share findings, based on institutional links in projects and on-line resources, is investigated. Future priorities for research and data accessibility relating to carbon sequestration are discussed. Many soil data resources are available online. Global and European soil data portals draw together much information from across Europe, and include the outcomes of major soil carbon mapping exercises. However, much project and national research is not accessible through these portals, and information on datasets derived from many research initiatives is difficult or impossible to locate online. Data on carbon sequestration (carbon fluxes in soils) specifically is more limited, although some such datasets are available through the general soil data resources described. Improved clarity in the presentation of research, and work to link more national and sub-national data to European and global online resources is required, with initiatives such as GSIF (Global Soil Information Facility) active in encouraging direct reporting of soil-related data at the global level. Priorities for research on SOC stocks include measuring carbon storage below the topsoil (>30cm), improving records of SOC in peatlands, improving the number and distribution of samples available for Europe-wide soil carbon mapping, and developing recognised methodological standards to allow easier comparisons of datasets. In the field of carbon sequestration research specifically, priorities include linking long-term SOC data to historical land use, developing understanding of the movement of SOC between top-soil and sub-soil and increasing dialogue between modellers and empirical researchers to improve dynamic modelling of SOC

CO2-Zertifikate für die Festlegung atmosphärischen Kohlenstoffs in Böden: Methoden, Maßnahmen und Grenzen

Agrarböden besitzen durch den Aufbau von organsicher Bodensubstanz (Humus), die zu etwa 58% aus Kohlenstoff (Corg) besteht, ein großes Potential zur Kohlenstoffbindung. Positive Anstrengungen im Humusmanagement könnten daher einen wesentlichen Beitrag für den Klimaschutz leisten. Für Landwirtinnen und Landwirte stellen so genannte CO2-Zertifikate für den Aufbau von Corg („Humuszertifikate“) einen zusätzlichen Anreiz dar, humusfördernde Bewirtschaftungsmaßnahmen umzusetzen. Diese CO2-Zertifikate werden von privatwirtschaftlichen Initiativen und Unternehmen im Bereich des freiwilligen CO2-Markts vergeben. Insbesondere im Bereich der Landwirtschaft wächst im deutschsprachigen Raum derzeit der Zertifikatehandel für den Aufbau von Corg in Agrarböden. Um zum Klimaschutz beizutragen, müssen bei der Vergabe von Zertifikaten bestimmte Kriterien eingehalten werden. Wissenschaftliche Mindeststandards wurden dabei in der Praxis bislang jedoch wenig berücksichtigt. In dieser Studie werden Empfehlungen hinsichtlich der Erfassung von Corg (Probenahme, Analytik, Vorratsberechnung), eine Bewertung von Maßnahmen zum Corg-Aufbau, sowie Hinweise zu generellen Einschränkungen hinsichtlich des Klimaschutzes über CO2-Zertifikate gegeben. CO2-Zertifikate können einen positiven Anstoß geben, damit sich Landwirte verstärkt mit einer nachhaltigen Bewirtschaftung und Humusversorgung ihrer Böden auseinandersetzen. Da Humus die zentrale Steuergröße für viele Funktionen des Bodens und nicht zuletzt der Bodenfruchtbarkeit darstellt, ist jede Anstrengung für mehr Humus sinnvoll. Landwirtinnen und Landwirte, die sich für Humusaufbau interessieren, sollten daher hinsichtlich standort- und betriebsspezifischen Optionen zum Aufbau von Corg umfassend unterstützt und beraten werden

Fate and stability of dissolved organic carbon in topsoils and subsoils under beech forests

Dissolved organic carbon (DOC) from Oa horizons has been proposed to be an important contributor for subsoil organic carbon stocks. We investigated the fate of DOC by directly injecting a DOC solution from 13C labelled litter into three soil depths at beech forest sites. Fate of injected DOC was quantified with deep drilling soil cores down to 2 m depth, 3 and 17 months after the injection. 27 ± 26% of the injected DOC was retained after 3 months and 17 ± 22% after 17 months. Retained DOC was to 70% found in the first 10 cm below the injection depth and on average higher in the topsoil than in the subsoil. After 17 months DOC in the topsoil was largely lost (− 19%) while DOC in the subsoil did not change much (− 4.4%). Data indicated a high stabilisation of injected DOC in the subsoils with no differences between the sites. Potential mineralisation as revealed by incubation experiments however, was not different between DOC injected in topsoil or subsoils underlining the importance of environmental factors in the subsoil for DOC stabilisation compared to topsoil. We conclude that stability of DOC in subsoil is primary driven by its spatial inaccessibility for microorganisms after matrix flow while site specific properties did not significantly affect stabilisation. Instead, a more fine-textured site promotes the vertical transport of DOC due to a higher abundance of preferential flow paths. © 2020, The Author(s)

Relevance of aboveground litter for soil organic matter formation – a soil profile perspective

In contrast to mineral topsoils, in subsoils the origin and processes leading to the formation and stabilization of organic matter (OM) are still not well known. This study addresses the fate of litter-derived carbon (C) in whole soil profiles with regard to the conceptual cascade model, which proposes that OM formation in subsoils is linked to sorption–microbial processing–remobilization cycles during the downward migration of dissolved organic carbon (DOC). Our main objectives were to quantify the contribution of recent litter to subsoil C stocks via DOC translocation and to evaluate the stability of litter-derived OM in different functional OM fractions. A plot-scale stable isotope-labeling experiment was conducted in a temperate beech forest by replacing the natural litter layer with 13C enriched litter on an area of 20 m2 above a Dystric Cambisol. After 22 months of field exposure, the labeled litter was replaced again by natural litter and soil cores were drilled down to 180 cm soil depth. Water extraction and density fractionation were combined with stable isotope measurements in order to link the fluxes of recent litter-derived C to its allocation into different functional OM fractions. A second sampling was conducted 18 months later to further account for the stability of translocated young litter-derived C. Almost no litter-derived particulate OM (POM) entered the subsoil, suggesting root biomass as the major source of subsoil POM. The contribution of aboveground litter to the formation of mineral-associated OM (MAOM) in topsoils (0–10 cm) was 1.88±0.83 g C m−2 and decreased to 0.69±0.19 g C m−2 in the upper subsoil (10–50 cm) and 0.01±0.02 g C m−2 in the deep subsoil >100 cm soil depth during the 22 months. This finding suggests a subordinate importance of recent litter layer inputs via DOC translocation to subsoil C stocks, and implies that most of the OM in the subsoil is of older age. Smaller losses of litter-derived C within MAOM of about 66 % compared to POM (77 %–89 %) over 18 months indicate that recent carbon can be stabilized by interaction with mineral surfaces; although the overall stabilization in the sandy study soils is limited. Our isotope-labeling approach supports the concept of OM undergoing a sequence of cycles of sorption, microbial processing, and desorption while migrating down a soil profile, which needs to be considered in models of soil OM formation and subsoil C cycling