Clever Cover Cropping : Litter trait diversities and elemental flows
Porre, Rima J.
Cover crops incorporated in an agricultural rotation can improve ecosystem services such as nutrient cycling and carbon sequestration in soil. Cover crop mixtures, through complementarity, can have a greater effect on these ecosystem services as well as being an effective tool to improve plant biodiversity. The aim of this thesis is to mechanistically understand how cover crop mixtures can provide synergistic benefits in terms of carbon (C) and nitrogen (N) cycling. In a meta-analysis of litter decomposition studies across the world I showed that on average, mass loss of mixed litters is equal to the weighted average of the mass loss of the constituent single litters. Any interactive effects (positive or negative) between litter species are contextual, and cannot be predicted based on litter traits. In a greenhouse study synergistic effects of mixing cover crop residues on mineralisation rates and N2O emissions were also not found. Residue of the single cover crops did have divergent effects on N mineralisation and N2O emissions, highlighting that cover crop residue quality is important when considering timing of cover crop incorporation and main crop planting. In a field experiment I found that, during cover crop growth, cover crop mixtures perform equally well as the best performing monocrop in reducing nitrogen leaching. Fertilising cover crops in order to kick-start growth does not increase biomass nor N recovery. Thus fertilisation is not necessary and increases the risk of N leaching just after fertilisation. After 4 years of including cover crop (mixtures) in an agricultural rotation on a sandy soil, none of the single cover crops and 2 out of 4 cover crop mixtures increased the soil organic matter content. None of the single cover crops had any effect on the stable or labile carbon pool in the soil. This thesis shows that mixtures of cover crops perform as well as the highest performing single cover crops in terms of N cycling, and mixtures can increase C sequestration compared to single species.