NITROFOREST

It is widely accepted that Ndep can increase forest C sequestration, however the results from our 2007 paper reported a sensitivity dC/dN (additional NEP per unit of added Ndep) which is 5-10 times higher than most earlier reported values. We contend that previous approaches have severely underestimated dC/dN and we set to test this hypothesis here.

We will:

• experimentally spray a nitrogen solution over the forest canopy of a whole sub-catchment at Griffin forest, where we have been continuously monitoring forest NEE using eddy covariance since 1996. We will install a second 'control' eddy tower, monitor both for one year and subsequently spray the N solution for four more years on one of the two sub-catchments.

We will therefore use a before-after-control-impact pair statistical design.

• install hydrological instruments (e.g., stemflow, throughfall gauges, stream flow gauging station) to determine levels of wet and dry deposition, the degree of N retention by the canopy and the degree of N retention by the two catchments in response to the increase in Ndep.
• investigate the responses of trees and soil within these two sub-catchments to added Ndep and, particularly, measure changes in nutrient concentrations in leaves, twigs and roots, measure tree growth in replicated transects and determine changes in heterotrophic respiration linked to needle litter decomposition.
• upscale our experimental results by measuring tree growth of Sitka spruce, the dominant tree species in the UK, across regional gradients in Ndep. While selecting the study sites, we will stratify for soil type, climate, management history and stand age by interfacing 1-km resolved gridded maps of Ndep obtained using the FRAME model with the data held in the proprietary sub-compartment database held by Forestry Commission and by using the extensive network of FC permanent sample plots.
• employ three different process-based models (BASFOR, Century and BGC) to compare observations against predictions and to aid in extrapolation. The models will also be extended to incorporate the putative mechanisms proposed for the high dC/dN observed by Magnani, Mencuccini et al (2007).

In addition, the tied PhD studentship will focus on determining:

1. the partitioning of Ndep within the ecosystem using pulses of isotopically labelled 15NO3 15NH4 applied to the canopy of five small plots
2. the partitioning of Ndep within the ecosystem using pulses of isotopically labelled 15NO3 15NH4 applied to the ground of five small plots.
3. quantify gaseous N losses through nitrification and denitrification.
4. as complete an N budget as possible, based on available measurements
5. use process-based models to test this detailed partitioning and the closure of the N budget

With the Knowledge Exchange plan, we aim to transfer these findings to:

1. the UK forest authorities (i.e., Forest Enterprise and Forestry Commission), to improve their predictions of regional yield and carbon sequestration under climate change (this will require a temporary secondment of the PDRA to FC) and 
2. the associations of livestock farmers, to promote the use of farm woodlands (short-rotation forestry) in sequestering the N emissions from their farms.
3. DEFRA, to improve their reporting of the so-called indirect human-induced effect of C sequestration under the rules of the Kyoto Protocol.