Nutrient release from coniferous woodland stimulated by changes in forest management: A new nitrate time bomb?

Despite large UK reductions in nitrogen (N) emissions since 1970, recent total N deposition was unchanged. Nutrient models indicate that N loads in most broadleaf and conifer systems in the UK exceed critical limits. Forest ecosystems are efficient atmospheric nitrogen scavengers and can accumulate more nitrogen than other land uses, particularly near woodland edges and where habitat fragmentation has occurred. Data collected by Forest Research suggest that large amounts of nitrogen and carbon (C) have accumulated in forest soils and in particular in the upper organic soil horizons. Increased leaching of N and C species has been detected at a number of sites.

Nitrogen stored in forest soils presents a risk to water quality if N stores are mineralized and released as a result of changes in forest management. Nitrogen losses to aquatic systems cost billions of pounds per year globally in water treatment and environmental damage from eutrophication. Despite this risk, the potential impact of changes in forest management on N leaching from forests to aquatic systems is poorly understood. Current government policy proposes more afforestation, but also changes to tree species which are more suitable for future climates, and converting some conifer systems to native broadleaves. In addition, productive forests are associated with intensive forest management processes which all can disturb the soils before tree planting, during forest development and harvest. Changes in forest due to pests and diseases could also have profound impact on nitrate inputs and leaching in forest ecosystems. In summary, we are expecting significant change of forest cover which is likely to alter established N leaching regimes which we know little about, thus creating a major research need.

Approach

This PhD will quantify the impacts of changes in forest management practices on nitrogen fate and transport beneath forested areas in the UK. The PhD will use monitoring and modelling approaches to quantify N fluxes to aquatic systems at a range of scales. The PhD will achieve this through the following tasks:

1. Detailed quantitative analysis of long term (>15 years) monitoring data of soil and soil solution nitrogen (total, nitrate, ammonium, dissolved organic nitrogen) measured at 10 Forest Research monitoring sites across the UK. Additional monitoring data collection and lab analysis will be carried out (soil N leaching, groundwater nitrate concentrations, pore water profiles).
2. Development of conceptual and process-based models of N leaching to aquatic systems parameterised at each site, and a number of generalised scenarios describing N leaching from forests, to be used in large scale modelling.
3. Upscaling of site-specific models to national scale by exploiting national scale datasets from Forest Research and CEH, e.g. BioSoil (220 sites) and soil survey and hydrogeological data.
4. Application of developed models (site and national scales) to predict impacts of changes in forest management on N fluxes from forests, and resultant impacts on aquatic system N concentrations.