Examining the impacts of plant species diversity and rooting characteristics on the accumulation of soil organic matter in temporary grasslands

Global soils are under increasing pressure from a range of drivers including intensive agriculture, deforestation, urbanisation and climate change. These are leading to dangerous losses of soil organic matter (SOM) which is vital for effective soil function. Soils provide a range of essential ecosystem services including food provision, flood prevention and carbon sequestration. Soil degradation through the loss of SOM could reduce the range and quality of ecosystem services that global societies receive from soil. It has been proposed that SOM in grasslands may be enhanced by increasing the plant species diversity of swards. However, research has previously not investigated this process within the context of temporary agricultural grasslands, nor have such SOM gains been quantified down to the composite fraction level to identify how responses vary in labile and persistent SOM fractions respectively. Two projects were conducted; firstly, a sampling exercise of an agricultural grassland which involved extracting deep soil cores for subsequent SOM fractionation and analysis; and secondly a glasshouse experiment which involved six herbaceous grassland species (selected from three functional groups determined by rooting characteristics) grown both in monoculture and 2-species diversity treatments. A subsidiary experiment was conducted concurrently allowing a detailed assessment of rooting characteristics of all six herbaceous species grown in the main glasshouse experiment. The combined study found that SOM was not higher in the higher diversity treatment, although some individual SOM fractions were. Additionally, SOM was not consistently correlated with a specific rooting characteristic or with total root biomass. While some trends were observed linking higher SOM with more extensive root structures, these trends were inconsistent and SOM values were not proportional to root biomass. Finally, an inconsistent relationship was observed between SOM and soil depth, which may have resulted from variable water content and or bulk density at different depths between the two projects.