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WP 2: Carbon and nitrogen pools in pre-alpine and alpine grassland soils

Figure 1. Schematic overview of the complex effects of climatic variables, site conditions as well as management practices on SOC stock changes in Bavarian grassland soils between 1989 and 2016. The numbers 1, 2, 4, 5 and 6 in the figure are corresponding to the different scenarios

An analysis of soil organic carbon (SOC) changes at long-term monitoring sites under grassland in Bavaria between 1989 and 2016 allowed to identify factor that lead to an increase, maintenance or decrease of SOC stocks (Figure 1). Seasonal climate variables explained the highest variability of SOC stock changes, followed by organic fertilizer application. SOC increase was negatively correlated with autumn precipitation increase and winter temperature, indicating the importance of microbial decomposition conditions during winter (Figure 1, scenarios 1 and 2). Additional organic matter inputs from organic fertilizer application resulted in an increase or at least maintenance of SOC stocks (Figure 1, scenarios 1, 2 and 4). SOC stock decreases were found for sites managed conventionally with low organic fertilization and sites with high slopes (Figure 1, scenarios 5 and 6).

In the mesocosm experiment we observed different conditions for aggregate formation and SOC stabilization along the elevation gradient (Fendt, 605 m a.s.l.; Graswang, 870 m a.s.l.; Esterberg, 1300 m a.s.l.). In the soils of Esterberg and Graswang the SOC content, CaCO3 and clay content were the main gluing and cementing agents which promote soil aggregation, indicated by a higher formation of stable macroaggregates rich in organic carbon. In Fendt, the higher amount of earthworms (such as Lumbricus terrestris), the higher proportion of legumes and higher amount of aboveground inputs facilitated aggregation, but the content of stable macroaggregates was lower than in Graswang and Esterberg. Instead, in Fendt a higher amount of OC-depleted microaggregates was found.