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The impact of nitrogen on the fate of recently assimilated carbon in forest soils

Final Report Summary - NITFOR (The impact of nitrogen on the fate of recently assimilated carbon in forest soils)

Global climate change is strongly linked to the accumulation of greenhouse gases in the atmosphere. In particular, CO2 contributes almost 45-60% to the observed anthropogenic global warming yet has the potential to be captured by trees and stored either in woody biomass or in soils over long time periods. Consequently, the Kyoto protocol recommends afforestation as one viable option to help reduce atmospheric CO2 concentrations. While the above-ground carbon cycle is well constrained, there remain great uncertainties in below-ground carbon cycling. For example, it is currently not known what proportion of carbon, fixed by trees in photosynthesis as CO2, is stored in soil, released back to the atmosphere as CO2 or CH4. We proposed to tackle specific key questions about the fate of carbon in forests by taking advantage of existing afforestation experiments in England (main phase) and in Russian Siberia (return phase), by combining new stable isotopes methods and innovations in 'in-growth' core approaches.
It is known that adding N to most forests soils alters soil surface CO2 flux. One of the current hypotheses to explain this is that the added N results in a decline in transfer of freshly assimilated C to mycorrhizal hyphae. We tested this hypothesis using in situ application of N and P to an experimental Ponderosa pine plantation forest soil located at the Wheldrake Forest, Yorkshire, UK, separating the biological sources of CO2 using mesh collars. We used shallow (including all components of the soil community) and deep 'exclusion' collars to separate autotrophic and heterotrophic components of CO2 efflux. The deep collars were fitted with windows covered with two types of nylon mesh allowing, or excluding, mycorrhizal in-growth.
Nitrogen addition increased CO2 efflux in this system. and the effect was only observed in shallow collars, suggesting that the growth of tree roots (autotrophic component) was responsible for the observed N addition effect.

See attached Figure 1

Parallel laboratory incubations involving N and 13C-glucose additions showed no N effect on heterotrophic microorganisms under lab conditions, confirming that at the present site it was root-derived CO2 flux which was sensitive to N fertilization. Addition of P had no effect on total CO2 flux or any of the potential sources. This is probably due to the nature of the site; P fertilization was routine practice during tree planting at the site 30 years ago. Contrary to our expectations, there was not a higher CO2 flux in collars where the growth of mycorrhiza was excluded, in contrast to those with collars. It appears that mycorrhiza inhibited the activity of heterotrophs at this site.

Overall, the project 1) considerably advanced our knowledge of below-ground C cycling in forest ecosystems and 2) established a new collaborative research link between scientists at the Institute of Forest in Krasnoyarsk and the University of York.

As a direct result of the current work, and the research period spent at York, four peer-reviewed papers have been published in the peer reviewed journals Global Change Biology, Soil Biology and Biochemistry, Eurasian Journal of Soil Science, and the Russia Germany Humboldt Journal; at least two further papers are planned for the high-impact journals Global Change Biology and New Phytologist.