Nitrogen cycling under conifer‐to‐broadleaf forest conversion in Eastern England

In the early 20th century, political focus on timber production in Europe led to extensive afforestation and replacement of broadleaves with often non-native coniferous species. Recent interest in the reverse has risen due to recognition of the wider ecosystem services delivered by forests, such as water quality improvement. Concurrently, recognition of nitrogen saturation in forest soils driven by historically elevated atmospheric deposition has stimulated interest in management interventions aimed at mitigating deposition effects. This study investigated a pseudochronosequence of stands undergoing such conifer-to-broadleaf conversion to capture its effects on soil N cycling in Thetford Forest, UK. The pseudochronosequence covered (1) mature broadleaf stands (n = 5), (2) mature Pinus sylvestris stands (n = 5), (2) sites felled during the monitoring period (n = 3) and clearfell sites planted with broadleaves: (3) 0–2 years ago (n = 3), (4) 5–8 years ago (n = 5) and (5) 10–13 years ago (n = 5). The soil C:N ratio at 0–10 cm depth was significantly higher in mature pine sites than in all broadleaf stages. The mean total deep soil NO3-N leaching fluxes during the study period were lower in young (14.53–16.11 kg-N ha−1) and mature broadleaved stands (10.72 kg-N ha−1) than in mature conifers (23.81 kg-N ha−1 year−1). However, soil NO3-N leaching fluxes were not significantly different between forest management stages. Net nitrification rates at 10–30 cm depth were higher under low pH conditions, but soil pH and nitrification rates were not linked to soil NO3-N leaching fluxes or forest management stages. Although no significant effects of management on NO3-N leaching were found, this study suggests the need to explore the interactive effects of site characteristics, deposition and forest management impacts on soil processes. Long-term studies complementing observations such as those presented here are needed to capture the effects of conifer-to-broadleaf conversion on forest soil nitrogen dynamics.