http://www.gesaq.org/P2Sanggou/documents/estimating%20N%20and%20P%20loads%20from%20export%20coefficients.pdf

Evaluation and management of the impact of land use change on the nitrogen and phosphorus load delivered to surface waters: the export coefficient modelling approach

Export coefficient modelling was used to model the impact of agriculture on nitrogen and phosphorus loading on the surface waters of two contrasting agricultural catchments. The model was originally developed for the Windrush catchment where the highly reactive Jurassic limestone aquifer underlying the catchment is well connected to the surface drainage network, allowing the system to be modelled using uniform export coefficients for each nutrient source in the catchment, regardless of proximity to the surface drainage network. In the Slapton catchment, the hydrological pathways are dominated by surface and lateral shallow subsurface flow, requiring modification of the export coefficient model to incorporate a distance–decay component in the export coefficients. The modified model was calibrated against observed total nitrogen and total phosphorus loads delivered to Slapton Ley from inflowing streams in its catchment. Sensitivity analysis was conducted to isolate the key controls on nutrient export in the modified model. The model was validated against long-term records of water quality, and was found to be accurate in its predictions and sensitive to both temporal and spatial changes in agricultural practice in the catchment. The model was then used to forecast the potential reduction in nutrient loading on Slapton Ley associated with a range of catchment management strategies. The best practicable environmental option (BPEO) was found to be spatial redistribution of high nutrient export risk sources to areas of the catchment with the greatest intrinsic nutrient retention capacity.

Modelling the impact of land use change on water quality in agricultural catchments

Chapter 2 – The Decomposition Process: Mineralization, Immobilization, Humus Formation, and Degradation

Because of public and political concern for the quality of surface and ground water, leaching of nitrate is of special concern in many countries. To evaluate the effects of tillage and growth of a catch crop on nitrate leaching, two field trials were conducted in spring barley ( Hordeum vulgare L.) under temperate coastal climate conditions. On a coarse sand (1987–1992), ploughing in autumn or in spring in combination with perennial ryegrass ( Lolium perenne L.) as a catch crop was evaluated. Furthermore, rotovating and direct drilling were included. The experiment was conducted on a 19-year-old field trial with continuous production of spring barley. On a sandy loam (1988–1992), ploughing in autumn or in spring in combination with stubble cultivation and perennial ryegrass, in addition to minimum tillage, was evaluated in a newly established field trial. For calculation of nitrate leaching, soil water isolates from depths of 0.8 or 1.0 m were taken using ceramic cups. No significant effect of tillage was found on the coarse sand; however, a significant effect of tillage was found on the sandy loam, where leaching from autumn ploughed plots without stubble cultivation was 16 kg N ha −1 year −1 higher than leaching from spring ploughed plots. Leaching was significantly less when stubble cultivation in autumn was omitted. Leaching on both soil types was significantly reduced by the growth of a catch crop which was ploughed under in autumn or in spring. It was concluded that soil cultivation increased leaching on the sandy loam but not on the coarse sand, and that the growth of perennial ryegrass as a catch crop reduced leaching on both soil types, particularly when ryegrass was ploughed under in spring.

Nitrate leaching as influenced by soil tillage and catch crop

Decomposing oak litter was incubated in laboratory microcosms and the effects of adding a variety of soil animals upon nitrogen and cation release were monitored. Various groups of macrofauna caused a marked increase in ammonium release with smaller increases in calcium, potassium and sodium leaching. Microfauna also had significant but much less marked effects upon nitrogen and cation release. The effects of different grazing intensities of the millipede Glomeris marginata on a variety of forest organic substrates show that the animals amplified existing patterns of nutrient release.

Nitrogen and cation mobilization by soil fauna feeding on leaf litter and soil organic matter from deciduous woodlands

Following recent reviews on nutrient cycling in temperate and British forests, two aspects of N cycling are selected for examination because of their fundamental importance and their relevance to management-ecosystem research and mobilization of N during decomposition. Despite the early work by J. D. Ovington and P. J. Rennie, only two major forest ecosystem studies have been developed in Britain. Summaries from these ecosystems show that they span the range of temperate forests in the N cycling characteristics. The Pinus nigra plantation on Culbin Sands is functioning on a low capital with low rates of transfer and shows N deficiency after 40 years. In contrast, the mixed deciduous Meathop Wood has a much larger capital of N and higher rates of transfer. The Pinus ecosystem retained a high proportion of added fertilizer N in the biomass and forest floor, and increased growth rates can be explained through N recycling within the trees. Basic concepts of N mineralization are reviewed and it is shown that they need to be modified for application to forests. Uptake by saprophytic and mycorrhizal fungi from the pool of soluble organic N can short-circuit the ammonification pathway and N can be released from substrates with a high C:N ratio by lysis of microbial tissues and faunal feeding. Some examples of evidence of these processes are given, stressing the spatial and temporal mosaic of substrates acting as sinks and sources of N. Management practices can have a marked effect, not only on the quantity of N returned in residues, but on its availability as a result of variation in the associated carbon. However, growth experiments have shown that substrates with C:N ratios above 35 can release significant amounts of N to tree seedlings. A final examination of the recycling processes in the four nutritional stages of a forest emphasizes the importance of the effects of management on the short- and long-term balance of mobilization processes in the forest soil.

Jan Mary Anderson

Papers

Nitrogen cycling in United Kingdom forests: the relevance of basic ecological research