converted to DON by the process described in the ferrous wheel hypothesis (see Section 8.4.2). Although nitrogen was lost, it was not as much as was expected: 85% was retained over the first 6 years. The cause is still being explored.
Active fungal biomass (i.e. biomass that was functional) was reduced in both stands but more so in the pine stands: biomass was 27-62% (low - high N plots, respectively) lower than the control in the hardwood stand and 42-69% lower than the control in the pine stand (Frey et al., 2004).
Further background information can be obtained from Magill et al. (2004). This and 11 other papers make up a special section in Forest Ecology and Management (2004,196, 1, pp. 1-186) entitled: 'The Harvard Forest (USA) nitrogen saturation experiment: results from the first 15 years.'
An earlier series of 29 papers on nitrogen enrichment in Europe can be found in Foresst Ecology and Management (1998, 101, 1-3, pp. 1-363) entitled: 'The whole ecosystem experiments in the NITREX and EXMAN projects', including Boxman et al. (1998). (NITREX = Nitrogen saturation experiments; EXMAN = Experimental manipulation of forest ecosystems in Europe.)
There is also a series of 5 papers published in Bioscience (2003,53,4, pp. 341-420) on nitrogen pollution problems in the USA (including Galloway et al., 2003).
self-sufficient in energy would require prohibitive amounts of space but using energy from biomass could reduce carbon emissions in EU countries by 2-30% (Schwaiger and Schlamadinger, 1998).
About two-thirds of the global pool of forest carbon is underground in roots and soils, amounting to 787 Gt (Table 11.1) and thus much research has been aimed at understanding the dynamics of soil carbon, in particular the carbon found in the passive carbon pool (Section 7.6.1) with its very slow turnover. Tropical forests form 42% of the area of all forests but hold only 27% of the soil carbon (Table 11.1). Rather it is the northern forests making up 33% of all forests that hold 60% of the soil carbon. But they are not necessarily sponges ready to soak up extra carbon. In fact both tropical and northern soils are vulnerable to carbon loss because northern forests contain so much carbon (and warming is predicted to be greatest nearer the poles) and in tropical forests small changes in temperature may be enough to degrade greatly the forest and lead to carbon loss (Attiwill and Weston, 2001). Moreover, climate warming is thought to reduce soil carbon by increasing decomposition in the soil. Experiments using heating cables in a northern hardwood forest indicate that decomposition is increased exponentially with increasing soil temperature (McHale et al, 1998). However, it is unclear whether this would continue once the more readily decomposed carbon has gone.
Recent evidence from nitrogen fertilization studies in temperate forests indicate that soils rather than plants are the dominant long-term sink of applied nitrogen. But sequestration of nitrogen in soils promotes less carbon storage than would plant uptake and so nitrogen pollution may diminish the ability of forest soils to sequester and hold carbon (Nadelhoffer et al., 1999).
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