1. Background and aims

The International Symposium on Soil Organic Matter Management in Agriculture held 29/30 May 2018 in Braunschweig was organized by the Thünen Institute of Climate-Smart Agriculture. 88 participants from 13 different countries contributed to this symposium with 28 poster presentations and 22 oral presentations. The symposium was organized in partnership with the 4per1000 Initiative for soil organic carbon. The 4per1000 Initiative launched in 2015 reinforced the focus of agricultural management on soil organic carbon (SOC). Enhanced SOC stocks can help to mitigate climate change and make soils more resilient to climate change. Finally, a sustainable management of soil organic matter is the key for a sustainable agriculture with the aim of enhanced food security. However, SOC is variable in space due to multiple environmental drivers that influence SOC dynamics and stocks. Thus, site and farm specific options are required to enhance SOC. In addition, potential of SOC sequestration are site dependent and SOC accumulation has to be closely linked to an efficient nutrient management. The transformation pathways of carbon-input to the soils to long-term stabilised carbon and its quantification is still insufficiently understood. Moreover, systematic and holistic approaches are required to understand the multiple effects of organic matter accumulation in agricultural soils and optimise carbon fluxes beyond field scale. The aims of the conference were to

i) discuss and assess efficient management options of SOC sequestration in different
   regions and at different sites for European and global agriculture,
ii) discuss the use and development of analytical tools and models for improving reliable
    large scale monitoring and prediction of SOC stocks and dynamics,
iii) identify research gaps on these topics.

2. Results and discussion

2.1. The 4‰ initiative

4‰ initiative aims at increasing SOC in order to achieve three different aims: climate mitigation, climate adaptation and finally food security. The purpose of enhancing SOC stocks should not be reduced to only one of these aims but synergies should be elucidated. The 4‰ fraction of global soil carbon stocks equals the total annual anthropogenic CO2 emission from the use of fossile fuels. This visualises the importance of SOC in the global carbon cycle. At the same time, it is a mere number that should not be aimed for and cannot be achieved everywhere. Usefulness and realizability of such carbon sequestration is strongly context specific and should always be viewed upon by comparing it to alternative options of e.g. use of biomass.


2.2 Internal input of C to soils

Internal C input is the biomass that is produced on site and left for decomposition and buildup of soil carbon on and in the soil. The amount of C input to the soil was identified as the key to maintain SOC and for SOC sequestration. The SOC loss via mineralisation can hardly be influenced in topsoils with usual agricultural management (e.g. hardly/no significant effects of reduced tillage or no tillage on SOC), only C that is transferred to subsoils may become more stable. In contrast, C input is a component that strongly depends on the management and consequently determines SOC stocks at steady state. Steady state should not be mixed up with SOC saturation.

An increase in internal C inputs can be achieved with permeant vegetation cover, e.g. with cover crops that can be incorporated into the soil as green manure or with double cropping systems (climate specific). This is the easiest agricultural measure to enhance SOC without major efforts. Also crop rotations containing multiannual forage/grass can enhance SOC stocks if sufficiently fertilized (preferably using legumes to avoid other GHG emissions).

Roots contribute 2.3 times more to SOC than the same amount of above ground biomass. A higher C input via roots (e.g. cover crops) is more efficient than C input from above ground (e.g. straw). High SOC stocks under organic farming systems can partly be explained via higher root/shoot ratios and thus more root derived C input to the soil. However, controlling factors for root/shout rations are insufficiently understood. Rhizo-deposition of crops comprise around 30% of the below ground C input and 3% of total NPP.

High yields are essential to maintain and enhance SOC since they are directly coupled to the internal C input. Agricultural measures and options that negatively impact yields can thus threaten SOC stocks if shoot-root ration and C input quality remains the same.

Subsoil horizons have a large potential to stabilize soil organic carbon. Deep-rooting crops and crops with high root biomass have the potential to increase total SOC stocks.


2.3 External input of C to the soil

External C input comprises all kind of organic fertilisers, sewage sludge, biochar etc. If it is only a transfer of carbon from one site to another its net SOC sequestration maybe zero or negative. Compared to internal C input external C input is a minor C flux in most agricultural systems.

Biochar has little effect on yield and other ecosystem services when applied to most European soils (only climate mitigation) but can have very large effects in highly weathered tropical soils (strong synergistic effects of mitigation, adaptation, food security). Nevertheless it is among the cheapest negative emission technologies and can be useful in regions with sufficient available biomass as feedstock (e.g. from forests). Technological challenges remain with construction of pyrolysis plants.

Sewage sludge has a low potential to enhance SOC due to its low supply.

Digestate from biogas is an efficient organic amendment since its major energy content has been used (e.g. for biogas production) and not wasted (e.g. in compost) and it is relatively stable. The effect of biomass to enhance SOC that is directly left on the field or biomass that was removed, used e.g. for biogas and afterwards returned is similar. This also applies to compost.

Other sources of organic amendments derived from industry or distribution chain may not be fully utilized at the moment and should be returned to soils if possible and if free of contaminants (regional specific).

Long term field experiments indicate that there is no SOC saturation in soils but SOC sequestration is limited only by the C input. Also subsoil SOC can be influenced by management and SOC turnover in subsoils is often decreased compared to topsoils. The underlying mechanism for this is unclear.

External C inputs have to be only used in the context of efficient nutrient recycling. Any over fertilisation with N or P have strong negative environmental and climate impacts and must be avoided. Thus, some (nutrient rich) organic amendments cannot be applied in certain regions.


2.4 Monitoring SOC stocks and new analytics

Soil monitoring in many European countries indicate that cropland SOC stocks are decreasing during the last decades. Underlying reasons are still unclear.

Soil carbon monitoring on regional or national scale is costly but it is an essential basis to detect changes in SOC stocks and to assess the long-term effects of management and climate changes on SOC stocks. Costs could be decreased with stratified sampling.

Monitoring programs should always take into account that land-use changes in the past can influence recent SOC stocks and dynamics. In some European regions historical land use change (last 50 years) comprise more than 50% of the land.

New fast analytical tools (e.g. easy fractionation schemes, RockEval thermal analysis, near-infrared spectroscopy) open up new options to quantify SOC quality and dynamics beyond pure SOC amounts. Also regional high resolution dataset on crop types will become available with the developments of algorithms that use new freely available satellite data (e.g. Sentinel I and II). Analytical indicators and regional data could be used in improved regional to continental scale SOC modelling. However, additional developments and validation are required in this field.

2.5 Potential analysis of the 4‰ initiative

First European scale and national scale estimates on the potential SOC stock increases in agriculture exist but show conflicting results. Theoretically and based on the biophysical potential of soils the 4‰ target of SOC sequestration could be achieved in Europe. However, it can be achieved only if drastic changes in European agriculture are initiated. The realizable potential for SOC sequestration (including socio-economic aspects) is still completely unknown. In a Swiss case study (Keel et al.) measures to sequester SOC did merely lead to unchanged SOC, while many other ‘business as usual’-treatments in long-term experiments led to a decrease in SOC stocks. This was also the case for a country-wide Swiss model simulation. Since many measures to enhance SOC are already common practice in Switzerland (e.g. use of crop residues, FYM, cover crops) the potential to additionally increase SOC was estimated to be low. In this context, the focus should be to maintain SOC stocks.

In a Bavarian case study (Wiesmeier et al.) also more drastic measures such as conversion of cropland to grassland and 5% cropland conversion to agroforestry were considered. In this scenario, a 1‰ increase of SOC stocks in Bavarian agriculture could be achieved annually over an unknown long time period. 30% of this increase was estimated to derive from drastic measures (land use change, agroforestry) and almost 50% of the effect was estimates from an increase in the area of cover crops from 12 to 29% of all croplands. This underlines the large potential of permanent vegetation (with cover crops) for SOC sequestration.

In the discussion, larger potential for the aims of the 4per1000 initiative were identified outside the Europeans´ intensive agriculture. Highly weathered tropical soils and degraded soils with low yields have a larger potential for SOC sequestration and may benefit most from an SOC increase due effects on yields/yield stability (see biochar example).'

In European agriculture an important aim would be to stop further SOC losses. This is highly relevant in particular for organic soils (peatlands) that loose often more than 4‰ per year due to drainage with agricultural land use. However, this was not the focus of the symposium.

A synergistic view on options to enhance SOC is required. Therefore, the discussion was cautiously and participants compiling different aspects that are required to make SOC sequestration a success-story:

Measures have to be

              - Economically sound (maybe enabled via subsidies or the carbon market) in
                order to insure that they are conducted on long-term (since SOC
                sequesration is reversible),
              - should not jeopardize other limited resources (e.g. water),
              - without leakage effects (higher emissions at other places or other GHGs
                such as N2O, indirect land use changes due to yield reduction with
                extensification).
              - Tradeoffs for the use of biomass for other purposes than SOC formation
                should be small.

Since options will be region specific and adapted to each farm situation, a network of pioneer farmers was proposed that explore existing and new options for SOC sequestration and help to disseminate their experiences among farmers.

Braunschweig, June 2018

Dr. Axel Don, Dr. Christopher Poeplau, Prof. Dr. Heinz Flessa