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

Abstract

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.