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The genus Sphagnum (L.) belongs to the Bryophyte plant division and includes 150 to 400 species. As all mosses Sphagnum has no roots and can hardly regulate its water uptake. As long as enough water is available Sphagnum can grow nearly unlimited while the lower, older parts die off and may accumulate as peat. Single Sphagnum species are able to build up an acrotelm as a hydrological self-regulating mechanism of a bog, a type of intact peatland (mire) only fed by precipitation. Because Sphagnum dominates nearly half of the peatlands in the world, it is one of the globally most important peat formers.
Sphagnum biomass is an important raw material for many valuable products, but in a much larger scale Sphagnum is used in its fossil state – as Sphagnum peat. With a consumption of c. 40 million m³ per year globally, Sphagnum peat is the predominant raw material for horticultural growing media. To get Sphagnum biomass it is currently collected from wild populations, to get Sphagnum peat it is extracted from bogs.
By far, more peatlands (including bogs) are subjects to drainage for agri- and silvicultural use since centuries, which harms their ecosystem services, including their typical biodiversity, carbon storage capacity, water regulation function and palaeo-environmental archive. In Europe, c. 25 % of all peatlands are used for agriculture, in Germany more than 80 %. Globally drained peatlands cover 0.4 % of land surface but produce 5 % of all anthropogenic greenhouse gas emissions.
Sphagnum farming aims to cultivate Sphagnum biomass on rewetted degraded bogs as a new agricultural crop. Sphagnum farming is paludiculture and contributes to the protection of bogs and their peat by conserving the peat body through rewetting and by offering a climate-friendly alternative to fossil peat in horticulture. Next to climate change mitigation, Sphagnum farming has benefits for nutrient retention and biodiversity conservation.
This thesis contributes to the development of Sphagnum farming by studying the conditions under which Sphagnum may reach maximal growth. Under (semi)controlled glasshouse conditions, we tested the effects of different water regimes and fertilisation levels on the productivity of various Sphagnum species. On a 1260 m² large irrigated field on cut-over bog in Lower Saxony (Germany) we studied length increase, biomass productivity and tissue nutrient content of Sphagnum over a period of 10 years. Finally, we reviewed all scientific literature and practical experiences with respect to Sphagnum farming worldwide as a first step towards a science-based implementation manual.
The main conclusions of our studies are:
1. It is possible to cultivate Sphagnum on rewetted cut-over bog and on rewetted former bog grassland.
2. The rapid establishment of a closed, highly productive Sphagnum lawn requires the deployment of a loose, >1(–5) cm thick Sphagnum layer (80–100 m³ of Sphagnum founder material per hectare) at the start of the growing season (when long frost periods are no longer probable) and adequate water supply.
3. Water table management must be very precise until a dense, well-growing Sphagnum lawn has established. For highest yields the water table should rise with Sphagnum growth and be kept a few centimetres below the Sphagnum capitula. Water supply via open irrigation ditches seems to function better than via subsurface irrigation pipes.
4. Fertilisation does not increase Sphagnum productivity on sites with high atmospheric nitrogen deposition and irrigation with phosphate-rich surface water from the agricultural surroundings. To avoid growth reduction a balanced stoichiometry is important.
5. From all studied species, Sphagnum fallax has the highest productivity. Its fast decomposition and low water holding capacity, however, may make this species less suitable for use in horticultural substrates.
6. Vascular plant cover on Sphagnum production fields can be kept low (<50 % cover) by regular mowing. Higher covers retard Sphagnum growth and reduce its quality for growing media.
7. Pathogenic fungi occurred far more in the glasshouse than in the field and have to be controlled for highest Sphagnum yields. We found Sphagnum vitality and growth rate to be stimulated by high water levels, where Sphagnum is less vulnerable to fungal or algal infection despite high nutrient loads.
8. The rate of Sphagnum biomass accumulation may remain constant over at least 4–5 years after establishing a Sphagnum production field with sufficient water supply. At dry conditions Sphagnum biomass accumulation is lower as a result of lower biomass productivity and higher decomposition rates.
Late Quaternary evolution and carbon cycling of tropical peatlands in equatorial Southeast Asia
(2014)
Peatlands are an important component in the global carbon cycle as they act as both long-term sinks for carbon dioxide and significant sources for methane. Over the Holocene period (the past 11,700 years) continuous CO2 uptake by peat accumulation exceeded methane emissions in northern peatlands and resulted in a net-radiative cooling effect on the global climate.Although 11% of the global peatland area is located in the tropics, the role of tropical peatlands in the global carbon cycle and in influencing the Earth’s radiative budget has not been resolved. Climate-carbon cycle models have thus far not included tropical peatlands because reliable data on their past rates of carbon uptake and release are not available. In this thesis this problem has been approached by reconstructing peatland expansion and rates of carbon storage and release over the Late Quaternary (Latest Pleistocene and Holocene) for the largest tropical peatland area, which is located in equatorial SoutheastAsia (i.e. Sumatra, Borneo, Peninsular Malaysia). Peat accumulation in the tropics remains an enigmatic phenomenon, because the constantly high temperatures of 26-27°C should theoretically drive rapid soil carbon turnover and thus not enable the accumulation of peat. Therefore this thesis also explores the mechanisms that cause peat formation in the SoutheastAsian tropics as well as the drivers behind changing rates of carbon accumulation. Carbon dynamics were analyzed at the regional scale (103–105 km2) of SoutheastAsia over millennial timescales (paper, I, II) and at the local scale (101–102 m2) of a peatland site on annual to centennial timescales (paper III, IV). Paper I presents the first systematic classification of the nearly 160,000 km2 SoutheastAsian lowland peatlands (below 70 m a.s.l.) into geographic peatland types. The peatlands were divided into 1) coastal peatlands of PeninsularMalaysia, Sumatra, and Borneo (~130,000 km2) and into inland peatlands (~30,000 km2) of 2) Central Kalimantan (southern Borneo), 3) the Kutai basin (eastern Borneo), and 4) the Upper Kapuas basin (western Borneo). Coastal peatlands formed by primary mire formation directly on freshly exposed marine or mangrove soils with the lowering of the sea level during the Late Holocene. In contrast, inland peatlands formed via paludification on either terrestrial sand soils (Central Kalimantan) or by both paludification and terrestrialization (Kutai basin, Upper Kapuas basin). The sequence of peatland initiation was established by applying the common cumulative basal date frequency approach (paper I). This method revealed clear differences in the timing of peatland initiation: 1) the Upper Kapuas peatlands are the oldest postglacial peat formations and date from 20,000-13,000 cal BP (calendar years before present), 2) inland Central Kalimantan peatlands date from 14,500-9000 cal BP, 3) the Kutai peatlands date from 8300-4900 cal BP, and 4) and the coastal peatlands date from 7700-200 cal BP. Coastal peatlands have a Holocene average carbon accumulation rate of 77 g C m-2 yr-1, being recognized as the globally most effective terrestrial ecosystems in terms of long-term carbon sequestration. Except for the Kutai peatlands, the Holocene average carbon accumulation rates of inland peatlands are significantly lower (20-30 g C m-2 yr-1) and very similar to the average long-term rates of northern peatlands. Fluctuations in past rates of carbon accumulation of SoutheastAsian peatlands could for the first time be linked to paleoclimatic changes, primarily variations in moisture availability (paper I, II). Hydroclimatic influences on carbon accumulation rates were related to shifts in the mean position of the Intertropical Convergence Zone, changes in the intensity of theAustral-Asian monsoon system, and variations in the frequency of the El Niño- Southern Oscillation. In contrast, peatland initiation and expansion was driven by sea-level change (paper I, II). The deglacial rise in sea-level is identified as the primary driver for inland peatland formation in Borneo, because the rising sea-level 1) lowered the hydrological gradients in the SoutheastAsian island archipelago inducing rising ground and surface water levels on these islands, and 2) led to higher atmospheric moisture availability due to the associated expansion of marine water masses on the shelf floor. Paper II shows that inland peatland initiation and expansion was most extensive during deglacial meltwater pulses, when the rate of sea-level rise exceeded 10 mm yr-1. Only when the rate of sea-level rise had slowed down to a threshold of 2.4 mm yr-1 by ~7000 cal BP could peat accumulation along the coasts keep up with the sea-level rise and coastal peatlands could form. Hydro-isostatic adjustment of the Sunda Shelf led to a sea-level lowering by ca. 5 m over the past 4500 years. Falling sea levels exposed extensive marine areas that were rapidly colonized by peat swamp forests.Anewly 140 developed method for the reconstruction of past peatland area based on transfer functions (paper II) reveals that 70%of the peatlands of Sumatra and Kalimantan only formed during the past 4000 years.Moreover, this new transfer function approach shows that the common basal dates approach overestimated the extent of peatlands in the past. This method, in general, leads to higher rates of reconstructed cumulative peat carbon uptake for the past. By combining reconstructed peatland areas and mean rates of carbon accumulation over millennial timescales from each peatland type the carbon uptake of all peatlands from Sumatra and Kalimantan could be quantified for the past 15,000 years (paper II). Carbon uptake remained below 1 Teragram (Tg) C yr-1 from 15,000-5000 cal BP because the total area of peatlands was less than 30,000 km2. Rapid peatland expansion driven by the lowering of sea-level over the past 5000 years increased carbon uptake on Sumatra and Kalimantan to over 7 Tg C yr-1 and resulted in an exponential growth of the regional peat-carbon reservoir to a size of over 20 Pg C. SoutheastAsian peatlands therefore had no significant role in the Late Pleistocene and Early Holocene global carbon cycle. However, because of their rapid expansion after 5000 cal BP by over 100,000 km2 the peatlands of SoutheastAsia became a globally important carbon sink during the Late Holocene and likely caused an atmospheric CO2 drawdown of 1-2 ppm (paper II). This previously unrecognized biospheric carbon sink partly compensated for contemporaneous terrestrial carbon losses associated with the desertification of Northern Africa. The mechanisms that enable high rates of carbon accumulation of coastal peatlands were explored in a peat core study presented in paper III. Here the use of a new coring technique for the tropics and the application of noninvasive geophysical measurements were employed to derive a high-resolution record of carbon accumulation rates. This study provides the first description of peatland pools for SoutheastAsia, which form as tip-up pools from falling trees such as Shorea albida. Based on a pollen and macrofossil record a fossil tip-up pool could be identified in the core and an associated carbon accumulation rate of 100 to over 900 g C m-2 yr-1 determined. Thus tip-up pools function as local hot spots for carbon accumulation, fundamentally different from northern hemisphere peatland pools, which act as net-carbon sources. From a time-series of aerial photographs the rate of tree fall and thus pool formation was determined at 0.4 tree ha-1 yr-1 (paper III).Asimulation model indicates that up to 60%of the peat deposited in peat domes of Borneo is derived from filled up fossil pools – changing the paradigm that Southeast Asian peatlands mainly form from belowground biomass and providing an explanation for the rapid carbon accumulation of these ecosystems. The climate impact of peatlands is, however, not only related to their capacity to rapidly store carbon, because peatlands also release the strong greenhouse gas methane – a by-product of anaerobic decomposition.Ametaanalysis of methane emission data from SoutheastAsian peatlands (paper IV) shows that their average annual methane release of 3 g CH4 m-2 yr-1 is lower than the average annual release of ~9 g CH4 m-2 yr-1 from northern peatlands, although the higher tropical soil temperatures should lead to significantly higher emissions. The limited degree of anaerobic decay is explained by the recalcitrance of the deposited biomass, which contains high amounts of lignin and tannin, providing another explanation for rapid carbon accumulation. Low anaerobic decomposition together with high rates of carbon accumulation imply that limits to vertical peat bog growth in SoutheastAsia are not set by cumulative anaerobic decay as in northern raised bogs. Instead peat bog growth is limited by aerobic decomposition related to water-table lowering as shown by a derived linear relationship between the amount of released CO2 from aerobic peat decomposition and the mean annual depth of the peatland water-table (paper IV). The climatic effect of Southeast Asian peatlands was determined by the global warming potential (GWP) method, which compares carbon uptake with methane emissions in terms of CO2-equivalents. The low methane emissions and high carbon accumulation rates of coastal peatlands result in a net annual uptake of 1340 kg CO2- equiv. ha-1 yr-1 over a 100 year GWP time-horizon. Under natural conditions coastal Southeast peatlands exert a significant net cooling effect on the global climate in contrast to northern peatlands, which have a warming effect or act climatic neutral on this time frame. It can be concluded that the tropical peatlands of SoutheastAsia are the strongest carbon sinks among all peatlands globally with a notable influence on the Earth’s radiative budget. However, today an estimated 90,000 km2 of peatlands in SoutheastAsia is drained for agriculture (e.g. oil palm plantations) and deforestation. These drained peatlands release annually over 140 Tg C yr-1 from aerobic peat 141 decomposition. Drainage also facilitates the regular spread of peat fires in this region, which on average release around 75 Tg C yr-1. Ongoing total carbon losses (~220 Tg C yr-1) exceed the natural carbon uptake by a factor of 25 and demonstrate that the entire SoutheastAsian peatland region has recently switched from a globally important carbon sink to a globally significant source of atmospheric CO2 (paper II, IV).