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Abstract Band 66

Steinmann, Thomas (2016): Long-term development of total organic carbon and labile organic carbon fractions in arable soils of North Rhine-Westphalia. 105 S.


Abstract BBA 66

Soil organic carbon (SOC) is an essential component of fertile soils and supports various elementary soil functions; yet, research is lagging behind with regard to the development of site specific SOC levels and their stability in time. The aim of my work was i) to elucidate the coherence of different SOC pools and their differing response to soil management, ii) to track changes in SOC stocks and related C pool composition for fertile arable soils in the Cologne-Bonn region which is characterized by low livestock densities, and iii) to evaluate the temporal development and current state of SOC contents in arable soils of different geographical regions in the federal state North Rhine-Westphalia (NRW).


To enlighten the seasonal dynamics of different SOC pools, I sampled two arable sites with contrasting humus contents in 2-month intervals over the period of one year, and fractionated the soils according to particle-size, density, and cold and hot-water extractable carbon pools. To track the temporal development of SOC, I used 268 arable sites sampled by horizon down to 60 cm in 2005 and in 2013 in the Cologne-Bonn region, Germany, and then related changes in overall SOC stocks and for 38 selected sites also changes in particle size C fractions, to soil management data. Finally I evaluated more than 32,000 SOC records from entire NRW to elucidate the temporal development of arable SOC contents from 1979 to 2015.


The seasonal dynamics of coarse-sand-sized SOM (POM1) could be well related to management while this was less clear for the dynamics of the light density fractions at given season and sampling intervals. As the equivalent diameter of soil particle-size fractions declined, the seasonal variability of their SOC contents decreased. The contents of cold and hot water extractable carbon remained nearly at the same level during the year, likely, because the temporal variations of these fractions were too dynamic to be covered by a 2-month sampling interval. Overall, there was no significant coherence between the contents and seasonal variability of the different SOC fractions, suggesting that information on SOC pool sizes and dynamics remain method-specific.


When expanding this research to the Cologne-Bonn region, I found that SOC concentrations had increased significantly in the topsoil, but had decreased significantly in the subsoil from 2005 to 2013. Intriguingly, these changes were due to changes in mineral-bound SOC and not to changes in sand-sized organic matter pools. As bulk density had declined during the observation period, the overall SOC stocks in the upper 60 cm exhibited a SOC loss of nearly 0.6 t C ha-1 a-1 after correction by the equivalent soil mass method. These SOC losses occurred even despite the overall humus balances had increased positively from about 20 kg C ha-1 a-1 in the period of 2003-2005 to about 133 kg C ha-1 a-1 for the entire period of 2005-2013 due to increased organic fertilization and intercropping.


To better understand why SOC levels declined despite good agricultural practice and positive humus balances, I evaluated SOC reports over entire NRW. The analysis showed that SOC contents had increased during the 1980s, followed by an exponential decrease from 1988 to 2015, with a new steady-state equilibrium not having been reached yet. Livestock-poor regions exhibited both a more rapid gain and also a more rapid loss of SOC than did regions with high livestock densities. The overall data correlated with changes in grassland area, which was reduced in NRW by about 360,000 ha mainly between 1970 and 1990.


In summary, there is no coherence between different labile SOC pools, but particle-size fractionation appeared suitable for monitoring early changes in SOC contents as affected by management. For the longer term, however, the development of SOC stocks and contents was not correlated with current management data. I concluded that past changes in land use are still driving the evolution of SOC levels throughout the Cologne-Bonn region and entire North Rhine-Westphalia, leaving at present little chance for changed management practices to prevent ongoing SOC loss.