Soil organic matter in the Netherlands: Quantification of stocks and flows in the top soil

Soil organic matter (SOM) and especially decreasing SOM are since many decades on the agenda of different stakeholders due to the importance of SOM for various issues ranging from local crop profitability to global climate change. Globally large amounts of organic carbon are stored in the soil and changes in the amount of SOM may sequester or release CO2 from/into the atmosphere. The global stock of soil organic carbon (SOC) in the upper 100 cm equals roughly two times the amount of carbon in the atmosphere and soil respiration equals circa ten times the release of carbon by burning fossil fuels. Other functions of SOM with a (more) local dimension relate to e.g. soil fertility, soil structure, soil erosion, regulation of soil water flows, plant productivity and maintenance of soil biodiversity. Declining SOM is considered as one of the most serious processes of soil degradation and has been identified as one of the main soil threats. Next to positive effects, decomposition of SOM may also have adverse effects by enhancing N2O and CH4 emissions, and releasing nutrients of which part is leached to surface and ground waters. In the Netherlands, the “Technische Commissie Bodem” (TCB) gives advice to the government on soil related issues and has recently developed an advice for the Dutch government on the effects of future trends (such as the biobased economy, climate change, safeguarding food productivity, water management) on soil functioning. As part of the information gathering underlying this advice, the TCB asked Plant Research International and Alterra to conduct a literature research of (a) SOM stocks, flows and recent trends, (b) variation and uncertainty in the data and (c) determination of areas of having/reaching low SOM levels in the Netherlands. In this study we have focussed on the top soil of 0-30 cm and mainly on soils under agricultural use. SOM in deeper soil layers may be important (e.g. globally the layer 30-100 cm contains approximately an equal amount of SOC as compared to the 0-30 cm layer), but due to lack of data this fell outside the scope of this study. The findings of this study have been presented to the working group “Koolstofstromen” of the TCB in three separate sessions in 2013-2014.