Burial of organic matter for carbon sequestration: Potentials, processes and long-term effects

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Increased carbon sequestration in soils is acknowledged as climate mitigation measure to reduce atmospheric CO2. Until now, research and praxis have focused on reduced and no-tillage even though their effects on soil carbon stocks of soils are marginal. The option of organic matter burial to enhance carbon storage in agriculture and forestry has been overlooked so far and almost no studies exist. Moreover, the processes of long-term stabilisation and storage of carbon in subsoils is insufficiently understood.Soil management has always led to organic matter burial. Croplands have been widely managed as ridge and furrow systems since the 12th century. With mouldboard ploughing into one direction, ridges were built up and buried former topsoil underneath. With invention of the steam plough the ploughing depth was continuously increased. Deep ploughing has been used for soil melioration in Podsols and Luvisols and later on also in peatland cultivation. During the 1960s dozens of agricultural field experiments on deep ploughing were set up in Northern Germany. The aims of the project are:1) to compare the long-term effect of organic matter burial on soil carbon stocks and thus, its climate impact analysing deep ploughing in agricultural (10 sites) and forestry (5 sites) and fossil cropland (5 ridge and furrow sites). In a paired plot approach soil carbon stocks are quantified down to 150 cm depth.2) to investigate the underlying mechanisms and processes of long-term stabilisation and storage of buried carbon. In addition to analysis of the main soil specific driving factors that control carbon storage, the degree of carbon stabilisation is going to be analysed using fractionation schemes that have to be further developed to be applicable also for subsoils.3) to determine the effect of organic matter burial on carbon turnover rates via radio carbon analysis on deeply ploughed soils and comparable not deeply ploughed soils. Nuclear bomb tests during the 1960s doubled the atmospheric 14C concentration which can be used as tracer for the freshly buried carbon at the deep plough trials. Potential bioavailability of buried vs. non-buried carbon are determined in incubation experiments. Together with soil ecological parameters such as microbial biomass and the metabolic ratio they provide an insight into microbial life conditions in the subsoil.Thus, organic matter burial can be used as experimental set-up to investigate the underlying mechanisms of centennial carbon storage in subsoils and to quantify subsoil potential to store carbon via targeted soil management.