How grazing impacts mineral association and stability of organic matter in oceanic alpine soils

Soil organic matter (SOM) contributes to a large number of ecosystem services and represents an important global carbon store. In grassland ecosystems, domestic grazing by large herbivores can alter organic carbon storage in soils greatly either directly (defoliation, trampling and defecation), or indirectly (change in plant species composition, quality of organic matter, nutrient cycling, soil temperature and moisture). Due to these changes and potential interactions, grazing may not only affect the amount of SOM but also its formation pathways, which both have implications for the distribution of particulate organic matter (POM) and mineral-associated organic matter (MAOM). Most studies conducted so far concern grasslands in temperate and continental regions of low to intermediate soil organic carbon (SOC) stocks, but less is known for cold and moist regions with intrinsic high SOC levels.

Our study takes part in a unique long-term experiment in grasslands of the oceanic alpine region of Setesdal, Norway (elevation ~850-1050 m and annual precipitation 1170-1760 mm). The region has nutrient poor granitic parent material, deep, moist and acidic organic horizons and a long history of grazing. The distribution of SOC with respect to particulate - (POC) and mineral-associated organic carbon (MOC), stability mechanisms and radiocarbon dating will be analysed in grazed (44-88, sheep/km²), short term non-grazed (23 years exclusion) and long term non-grazed fields (more than 60 years exclusion). Recent studies show that grazing induced shifts in plant species composition have led to low quality litter with low decomposition rate, while long term grazing-excluded fields have more nutrient rich vegetation. We hypothesise that grazing will increase overall SOC and relative POC content compared to long-term excluded fields where there will be an overall faster turnover and higher relative MOC content.

Here we present empirical results on the stocks and fractions of SOC that can inform how grazing impacts organic matter formation and stability in a cold, oceanic climate. More than 200 soil samples have been analysed for total C, N, texture, bulk density and pH, and a selection are currently being analysed for SOC fraction distribution (POC and MOC) and their 14C age. SOC stocks vary greatly (45 to 442 tonnes/ha) due to large variations in soil depth (9-37 cm) and soil type. Initial results suggest only small differences in total SOC stocks between the grazing treatments, and a lower MOC/POC ratio in the grazed areas. Preliminary results of 14C analysis indicate that although POM is considered a labile fraction of SOM, it can be preserved for hundreds of years due to climate-induced low decomposition rate. Investigating MAOM and POM in light of historic and current grazing pressure shows how land use and vegetation directs SOM formation pathways, and how this may affect carbon preservation in the long term.