Journal
WATER AIR AND SOIL POLLUTION
Volume 177, Issue 1-4, Pages 335-347Publisher
SPRINGER INTERNATIONAL PUBLISHING AG
DOI: 10.1007/s11270-006-9177-z
Keywords
deposition; eutrophication; forestry; grassland; heathland; pollution; retention
Funding
- Natural Environment Research Council [ceh010023] Funding Source: researchfish
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Nitrate leaching occurs when the soil's nitrogen immobilisation and plant uptake capacity has been saturated. Several widely-used models of nitrogen saturation incorporate a breakthrough function in which N begins to be leached at C/N values below an upper threshold, and is completely leached at C/N values below a lower threshold. In a survey of deciduous and coniferous woodland, acid grassland and heathland sites for which both C/N and nitrate flux measurements were available, deciduous woodland and acid grassland typically had lower C/N ratios, and began leaching nitrate at a lower C/N ratio, than coniferous woodland and heathland. Least-square fits of nitrate breakthrough functions gave upper thresholds (no nitrate leaching) of 27 mol C mol(-1) N for deciduous woodland and acid grassland and 50 mol C mol(-1) N for coniferous woodland and heathland. Upper thresholds were similar, at 24 and 51 mol C mol(-1) N, respectively, for total inorganic N (NH4 + NO3) leaching flux as a proportion of total inorganic N influx. In conifer plantations, stand maturity had a large effect, suggesting that a breakthrough function is unsuitable for modelling systems that are in disequilibrium. However, there was sufficient evidence to suggest that using different breakthrough C/N thresholds for different groups of vegetation would improve predictions of N saturation and leaching at both plot and catchment scales. The difference may be related to the reactivity of soil carbon; soils with a large proportion of recalcitrant carbon are likely to begin leaching nitrate at a higher C/N value than soils with more labile carbon.
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