Meta analysis offers a better solution, for example. Nitrogen fixation is known to occur but is difficult to measure, especially in the low amounts that are thought to occur with non-symbiotic N fixation (Barkman and Schwintzer, 1998, Rosén and Lindberg, 1980, Roskoski, 1980 and Son, 2001). Perhaps because of difficulties with measurement, non-symbiotic N fixation is often invoked as Ferroptosis activation an explanation for so-called “occult N inputs”: that is, changes in
ecosystem N content that cannot be explained by other known and better-measured inputs such at atmospheric deposition, fertilization, manuring, etc. Early studies, such as Richards (1964), reported increased N availability and quantities in Queensland conifer stands but could not explain the process. It is not the intention of this paper to review processes and changes as a result of symbiotic N fixation,
but a few comments are in order. Symbiotic N fixation by forest species has been the focus of a number of studies and reviews (e.g. Sprent, 2005 and Dommergues and Ganry, 1986) especially considering the rates of fixation, N turnover and accumulation in the biological component (vegetation and forest floor) but little on net changes in soil quantities (e.g. Miller 1982). In the review by Miller (1982) estimates of annual N fixation ranged from 8 to 323 kg N ha−1 yr−1. Similar reviews have been undertaken for other genera indicating potential high rates of N fixation, especially in relation to short term plantations see more (e.g. Adams et al., 2010 and Binkley et al., 2003). There is evidence that presence of N fixing species as a component of mixed stands many fixes significant quantities of nitrogen but there is no information on limiting factors for this N accumulation (e.g. Turner et al., 2011). However there are few studies reconciling short term estimates of N fixation (from process studies)
with actual longer term net accumulation within the system (changes in pool size), whether the N in soil pools increases significantly above those of non-N fixing species on comparable soils or whether N saturation is attained. Over the last decade, evidence has emerged showing that in some ecosystems we may have missed a substantial part of soil N content by analyzing only the <2 mm fraction. Whitney and Zabowski (2004) found that rocks contained from 0.3% to 34% of total N in soils from a variety of sites in Alaska, Oregon, Washington, and Puerto Rico. Dahlgren (1994) documented a very interesting case in northern California where N release from N-containing rocks and subsequent nitrification and nitrate-base cation leaching caused sufficient soil acidification on a local scale to kill all vegetation. In the surrounding forest, N released from the rocks is taken up by forest vegetation and nitrate leaching rates are low.