Interplay between aboveground plant biodiversity, soil organic matter, and soil microbial functional diversity in a grazed Danish island ecosystem

Animal grazing is known to affect both, soil carbon storage and above and belowground biodiversity. However, we lack a more detailed understanding how specific soil properties might determine biodiversity and soil carbon storage as affected by grazing. Thus, in the present work we aim to connect the geo- and biodiversity with organic matter decomposition functionalities of the soil community, and thus identify how this regulates soil carbon and nitrogen storage. Therefore, we analysed aboveground plant biodiversity, soil microbial functional diversity, and soil organic matter (SOM) characteristics on a summer grazed island (Eskilsø) in a Danish fjord. Specifically, plant biodiversity analyses were conducted and topsoils sampled at thirty plots (4 replicated soil samples per plot) on the ca. 140 ha island. The plots cover the island’s main habitats: salt meadows, meadows, and developing dry grasslands. Soils were analysed for organic carbon (OC), total nitrogen (TN), organic phosphorus (OP), inorganic phosphorus (IP), and pH. Additionally, community level physiological profiles (CLPP) were analysed using the Microresp technique to make inferences about soil microbial functional diversity and activities. 
We are able to demonstrate that plant biodiverse plots contained greater contents and stocks of SOM. This also correlates with an increased soil microbial functional diversity. The findings are in line with the often observed positive interaction between aboveground diversity and belowground functionality of the soil biome in grassland ecosystems, partly due to increased amounts and diversity of rhizodeposits. As microbial activity is important for mediating the turnover of plant derived organic matter into more stable soil OM pools, this reflects the correlation with higher soil OC stocks and thus links to soil carbon persistence. For the studied island ecosystem we are able to demonstrate how the fate of soil organic matter is functionally linked to the interactions between above- and below-ground components of the ecosystem.