Forested lands have lower soil carbon priming effects than croplands in hedgerow agroforestry systems

The priming effect induced by exogenous organic substrate addition influences soil carbon (C) and nutrient cycling. Agroforestry systems offer a promising land-use approach to increase soil organic C (SOC) sequestration while sustaining agricultural productivity; however, the influence of these systems and their interaction with nitrogen (N) fertilizer application on the soil priming effect remain poorly understood. We conducted a lab incubation experiment with additions of ¹³C-labeled glucose and N to assess C loss via the priming effect and the net balance of SOC in top- and subsoils across two common agroforestry systems (hedgerows and shelterbelts) and their component land uses: forested lands and adjacent annual croplands, in central Alberta, Canada. Glucose addition caused a positive priming effect, which was more pronounced in the subsoil than in the topsoil. Nitrogen addition reduced the priming effect in subsoils by 32 %, suggesting that N limitation was a key driver of priming-induced SOC loss. In addition, agroforestry systems and their component land uses interactively affect the priming effect. The priming effect was 34 % lower in the forested land than in the adjacent cropland in the hedgerow system with a more diverse plant community, likely due to greater labile C and nutrient availability in forested lands, reducing the vulnerability of SOC to the priming effect. However, the priming effect was not different between the two land uses in the shelterbelt system, likely due to the smaller differences in SOC and N availability between the two land uses, reducing the contrast in microbial responses to labile C input. Our findings underscore the risk of priming effect-enhanced SOC loss in croplands, and the potential for agroforestry systems to reduce SOC loss through damping the priming effect and mitigate climate change.