WP 3: Nitrate leaching

Background

New trend analysis from the UK Acid Water Monitoring Network suggests that nitrate will be the major acid anion in many acidified systems within 5-10 years as excess sulphate continues to decline and nitrate leaching remains relatively constant (Curtis et al., submitted). Hence measures to reduce S emissions alone will be insufficient to allow recovery of acidified sites to achieve good ecological status. Furthermore, worst-case models of nitrate leaching employing long-term mass-balances for nitrogen (e.g. FAB) indicate that nitrate leaching even at constant deposition load is likely to increase to much higher levels (Curtis et al., 1998).

The DEFRA and GANE research programmes have provided insight into processes controlling the spatial and temporal development of N saturation. Results suggest that inter-regional variations in N deposition, and local variations in soil organic matter content, can to a large extent explain observed variations in N leaching and soil C:N ratio; at one extreme, deep peats typically retain most or all inorganic N, at the other extreme thin organic-poor soils may rapidly N-saturate, releasing NO3- to surface waters (Evans et al., in press). These results are largely consistent with current N process descriptions in MAGIC and VSD model, suggesting that N saturation in both terrestrial and freshwater systems may be successfully modelled. However, a key uncertainty remains to be addressed in quantifying the relative roles of physical (hydrological) versus biological processes in controlling the short- and longer-term release of inorganic N from terrestrial systems.

The timescale over which N saturation and leaching may occur and the magnitude of increase from present levels will be determined by the balance between three key processes:

  1. "hydrological" nitrate which bypasses biological retention mechanisms in the soil/vegetation system via saturation overland flow and/or pipe/macropore flow (plus direct deposition to the surface of lakes)
  2. "saturation" nitrate which is present in excess of biological demand and leaches through the soil profile unchanged
  3. nitrate produced within the soil profile by microbial nitrification, which may or may not have derived originally from anthropogenic N deposition taken up by vegetation and/or immobilised by soil microbes, and may be determined by changes in soil C:N ratio in response to elevated N deposition

Previous tracer work at four sites:

  • Allt a'Mharcaidh - Cairngorms
  • Afon Gwy - mid-Wales
  • Scoat Tarn - Lake District
  • River Etherow - southern Pennines

Potential N leaching_pathwayshas shown that the sum of pathways 1) and 2) may reach one third of deposition inputs, with the rest tightly retained in the soil-plant system (Curtis et al., submitted). This proportion is sufficient to explain observed nitrate leaching at some sites. However, linked studies have also shown that the C:N ratio could have potential as an indicator of nitrate leaching via pathways 2) and 3) (Curtis et al., 2004). High nitrification potentials in some upland soils suggest there may also be a component of microbial nitrate in leaching to surface waters (pathway 3) but this remains unproven in the UK.

The split between the three possible pathways is critical for modelling the potential future leaching of nitrate. If current surface water nitrate already contains a "saturation N" component (pathway 2) or a microbial component (pathway 3), then the function of terrestrial ecosystems as N sinks may have already changed in response to elevated N inputs, and timescales to attain the worst leaching predictions of the FAB model could be just years to decades. Furthermore, large pools of stored "anthropogenic" N in the soil-plant system could potentially sustain nitrification and leaching even with reduced deposition inputs.

On the other hand, if surface water nitrate is purely hydrological (pathway 1) then terrestrial systems have not shown a saturation response (either through reduced immobilisation or increased nitrification) to many decades of high N deposition with elevated leaching, suggesting that their capacity to retain N is very large. In the latter case there may be no measurable N saturation effects and no increase in pathways 2) and 3) for many decades to come. Responses to changes in total N deposition should therefore be very rapid.

Effectively, the routing of N through the soils determines the extent to which N impacts on terrestrial systems (i.e. soil and vegetation N enrichment) versus freshwaters (i.e. NO3- leaching). Hence the work is also of direct relevance to both the DEFRA Terrestrial Umbrella and the "Critical Loads and Dynamic Modelling" Group.

Within this work package we will assess the relative importance of the three N leaching pathways through complementary experimental approaches at a key site of interest with much existing data, the Afon Gwy. Task 3.1 will provide an independent quantification of the importance of pathways 1 and 2 through the use of hydrological tracers, building on the 15N experiments described in Curtis et al. (submitted). Task 3.2 employs a dual isotope technique (see Burns & Kendall, 2002; Campbell et al., 2002; Durka et al., 1994; S poelstra et al., 2001; Williard et al., 2001), that has not previously been used for this purpose in the UK, to determine the relative c ontributions of deposited and microbial nitrate to observed concentrations in surface waters. This work provides the split between pathway 3 and the sum of pathways 1 plus 2.

Work programme

The work programme for Work package 3 is split into five main tasks. Click on the links below for further information on an indivisual task

  • Task 3.1 - Tracer 15N studies at the Afon Gwy to determine short-term pathways for transport of N into surface waters
  • Task 3.2 - Dual isotope studies at the Afon Gwy: "new" atmospheric versus "old" microbial N in surface water nitrate (Pilot Scheme)
  • Task 3.3 - Expansion of dual isotope work to other sites
  • Task 3.4 - Moss survey of AWMN sites
  • Task 3.5 - Carbon-Nitrogen ratio (C:N) survey of AWMN sites