The human footprint in the carbon cycle of temperate and boreal forests

Abstract

Temperate and boreal forests in the Northern Hemisphere cover an area of about 2 x 10(7) square kilometres and act as a substantial carbon sink (0.6-0.7 petagrams of carbon per year). Although forest expansion following agricultural abandonment is certainly responsible for an important fraction of this carbon sink activity, the additional effects on the carbon balance of established forests of increased atmospheric carbon dioxide, increasing temperatures, changes in management practices and nitrogen deposition are difficult to disentangle, despite an extensive network of measurement stations. The relevance of this measurement effort has also been questioned, because spot measurements fail to take into account the role of disturbances, either natural (fire, pests, windstorms) or anthropogenic (forest harvesting). Here we show that the temporal dynamics following stand-replacing disturbances do indeed account for a very large fraction of the overall variability in forest carbon sequestration. After the confounding effects of disturbance have been factored out, however, forest net carbon sequestration is found to be overwhelmingly driven by nitrogen deposition, largely the result of anthropogenic activities. The effect is always positive over the range of nitrogen deposition covered by currently available data sets, casting doubts on the risk of widespread ecosystem nitrogen saturation under natural conditions. The results demonstrate that mankind is ultimately controlling the carbon balance of temperate and boreal forests, either directly (through forest management) or indirectly (through nitrogen deposition).

Stand age and species richness dampen interannual variation of ecosystem-level photosynthetic capacity

“…In young forests, cvGPP sat might depend also on the expected trend in annual growth and GPP, as young stands are expected to rapidly increase their biomass and LAI in the first years of establishment 20 . Thus, young stands could have a higher variability of GPP sat -----but this does not necessarily reflect instability.…”

Microbial denitrification dominates nitrate losses from forest ecosystems

“…This approach takes advantage of the biological imprint of soil denitrification on the N cycle and the tendency for kinetic isotope effects to elevate 15 N/ 14 N and 18 O/ 16 O of NO 3 − systematically in forests (8). Here, we extend on this approach by developing a new way to estimate NO 3 − supply rates to denitrification, which involves a combined Δ 17 O, δ 15 N, and δ 18 O analysis.…”

“…As an alternative, natural composition of N and oxygen (O) isotopes in NO 3 − provides nonintrusive, quantitative, and integrative constraints on denitrification across a myriad of spacetime scales (8,9). This approach takes advantage of the biological imprint of soil denitrification on the N cycle and the tendency for kinetic isotope effects to elevate 15 N/ 14 N and 18 O/ 16 O of NO 3 − systematically in forests (8).…”

“…Anthropogenic emissions of reactive forms of N due to fossil fuel combustion and modern agriculture practices have drastically increased N deposition inputs to forests globally (2). Atmospherically deposited N represents a new N input to terrestrial ecosystems and may enhance carbon dioxide sequestration, potentially reducing some global climate impacts (3). On the other hand, long-term N deposition could result in N saturation and increased nitrate (NO 3 − ) losses to leaching and denitrification (4,5), pathways that have different consequences for the Earth system, including consequences on climate, biodiversity, and water and air quality for human health (6).…”

Macronutrient status of UK groundwater: Nitrogen, phosphorus and organic carbon

“…Many terrestrial ecosystems that were previously nearly-closed with respect to N and P now permit considerable export of these nutrients (Galloway et al, 2008;Smil, 2000;Withers et al, 2001), leading to eutrophication and acidification of surface waters (Bennett et al, 2001). Carbon cycling has been affected by agricultural practices (Bellamy et al, 2005) and, in semi-natural systems (Magnani et al, 2007), through changes in net primary productivity and carbon storage brought about by N deposition (Townsend et al, 1996;Waldrop et al, 2004). The groundwater pool is an important, albeit slowly changing, component of the cycles of N, P and C, and in turn impacts on receptors such as surface water (Holman et al, 2010;Holman et al, 2008b) and dependent wetlands (Krause et al, 2007).…”