Estimating the carbon dioxide removal potential of alley-cropping agroforestry systems in Germany

Agroforestry (AF) refers to a wide range of agricultural practices that incorporate woody plants into crop- and grasslands. Agroforestry systems (AFS) can be distinguished by their share of trees and their spatial allocation, the selection of tree species, and tree management. While AFS are common in the global south to promote soil fertility, reduce heat stress and improve the water balance, they are less common in the global north. Currently, AFS are discussed as a nature-based solution for terrestrial carbon dioxide removal (CDR). Here, alley-cropping AFS are a promising system because their tree cover is sufficiently large for significant CDR rates and they are still compatible with the use of agricultural machinery that is common in modern agricultural practices. However, estimating the large-scale CDR potential of AFS is challenging because of the variety of potential systems whose performance strongly depends on environmental conditions.

We study the CDR potential of AFS by extending the process based dynamic global vegetation model Lund-Potsdam-Jena managed Land (LPJmL) to represent alley-cropping AFS on cropland. The model explicitly accounts for shading effects of tree rows depending on row and tree distance and row orientation as well as the competition for soil water and nutrients between trees and crops. As an example for potential model applications, we assessed the future CDR potential of timber alley-cropping AFS for Germany assuming a moderate linear annual increase of AF areas by 0.5% of the total cropland area until 2060 and a moderate tree cover.

With the process-based representation of AFS in LPJmL, the model can be applied to study carbon, water, and nitrogen fluxes and pools of different alley-cropping AFS and conventional cropping systems at large spatial scales, including maximum carbon sequestration rates, potential equilibrium states and reversibility.