Agroforestry, soil health and delivery of public goods

Agroforestry is the practice of deliberately growing trees in combination with arable crops and/or pasture on the same piece of land (Figure 1). Agroforestry is seen as a sustainable land management practice, where trees and agriculture co-exist to provide multiple benefits. Therefore growth and innovation in agroforestry has the potential to improve farmland productivity, resilience and diversity while maintaining and/or improving the provision of other ecosystem services, via improving soil health (Dollinger & Jose, 2018;), sequestering carbon (De Stefano & Jacobson, 2018; Lorenz & Lal, 2014) and slowing water runoff (Marshall et al., 2009). While long-established in sub-tropical and tropical climates, uptake of agroforestry in temperate agricultural systems has been slow, particularly in the UK (Woodlands Trust, 2018). In order to realize this potential, there is urgent need for greater understanding of how planting trees in temperate agricultural systems impacts upon soil health indicators and thus helps to reduce flooding and mitigate climate change.
While literature reviews have shown that agroforestry can increase the amount of carbon stored in the soil (De Stefano & Jacobson, 2018; Lorenz & Lal, 2014) and thus help to mitigate climate change, the majority of studies (~80%) were located in tropical and sub-tropical climates, with less than 20% in temperate climates. Oelbermann et al. (2014) found that in temperate zones agroforestry had to be established for greater than 10 years in order to see an increase in soil carbon due to lower turnover rates than in the tropics. In addition, it is unclear whether planting trees in pastures has the same benefit for soil organic carbon content as planting trees in arable fields (Upson et al., 2016). Recent studies in the UK have shown that planting trees on farmland can increase soil infiltration rates (e.g. Marshall et al., 2009). However, these studies were carried out at one site. In addition, it is not clear if similar impacts would be observed in lowland agricultural systems; highlighting the need for further research. Given the long time required to study the development of agroforestry and the complex interactions between crops, animals and trees, calibrated and validated simulation models can significantly contribute to the understanding and quantification of environmental benefits and to forecast the resilience of the systems to the predicted climate change (Cardinael et al., 2018).

The major aim of this project is therefore to determine the impact of agroforestry on soil health, in particular its impact on soil carbon storage, soil structure and hydrological properties. You will work with scientists at the University of Leeds and supported by the Woodlands Trust to quantify the impact of different agroforestry systems on key soil health indicators (e.g. carbon, bulk density, permeability, water storage) in temperate climates to understand how agroforestry impacts upon wider ecosystem services such as climate change mitigation and flood control. This will be achieved through fieldwork at a newly established silvopastural site in the Vale of York, an older silvoarable site established at the University of Leeds farm in the 1980s, and other agroforestry sites across the UK, France, Spain, Italy and Germany. In particular, according to your particular research interests, the studentship could address a combination of the following objectives:
1. Determine short-term and longer-term impacts of agroforestry on soil health indicators.
2. Evaluate the impact of different agroforestry types (silvoarable, silvopasture, shelter belts, and hedges) on soil health indicators.
3. Investigate the effects of distance and depth on soil health indicators in a range of different aged agroforestry plots soil types .
4. Quantify the impact of climate change on carbon cycling in agroforestry systems via process-based modelling of soil processes.