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Effects of soil alteration on nitrogen and carbon cycling

Final Report Summary - SLAVONIC (Effects of soil alteration on nitrogen and carbon cycling)

Human activities have been altering element cycling for several thousand years. Since the Industrial Revolution, emissions of greenhouse gases, nitrogen (N) and sulphur (S) have been increasing and their negative impacts on ecosystems led to the implementation of various international agreements to abate these emissions (United Nations Framework Convention on Climate Change, Convention on Long-range Transboundary Air Pollution, Kyoto Protocol, Paris Agreement). Besides exerting significant negative pressures on global biodiversity, it is now clear that atmospheric pollutant deposition has impacted on the wider biogeochemical cycle of terrestrial ecosystems, including the ecosystem C balance. A majority of temperate terrestrial ecosystems are N limited, thus fertilization by atmospheric N deposition is thought to have increased C storage in biomass and soils of terrestrial ecosystems and thus contributed to the terrestrial CO2 sink. Increased N availability may affect terrestrial C balances via increased net primary productivity (NPP) and/or decreased litter decomposition rates. The potential of a negative effect of increased N availability on decomposition rates was suggested by many experimental N addition studies. Stimulation of litter decomposition is expected at sites with low ambient N deposition and high-quality litter (low lignin), whereas suppression of litter decomposition is suggested at sites with higher ambient N deposition or at sites with low quality (high lignin) litter. While most recent research has focused on the impacts of N deposition on C cycling, there is now clear evidence that acidification (from either S or N deposition) may also impact on a range of key processes, including suppression of litter decomposition, as well as mobility and leaching of dissolved organic carbon. Suppression of litter decomposition at low pH may occur through reduction in the availability of C for microbial community and/or by direct effect on the microorganisms themselves. Input of N to the soil, either from deposition or fertilization, can have an acidifying effect, depending on the fate (immobilization vs. leaching, nitrification) of mineral N in the soil. Thus, it is likely that experimentally induced responses to N fertilisation might in fact be due to changes in soil acid-base status. A better, integrated understanding of soil C and N responses to changing inputs of both S and N is needed to predict whether soils will in future act as C sinks or C sources as atmospheric pollutant loadings change.
In a four-year, two-forest stand (mature Norway spruce and European beech) replicated field experiment (SLAvONIC) involving acidity manipulation, N addition and combined treatments, we tested the extent to which altered soil acidity or/and N availability affected the soil carbon cycle. We demonstrated a large and consistent suppression of soil water dissolved organic carbon concentration driven by chemical changes associated with increased hydrogen ion concentrations under acid treatments, independent of forest type. Soil respiration (Rs) was suppressed by acidity in the spruce forest, accompanied by reduced in situ decomposition of high lignin organic matter, reduced microbial biomass, increased fungal:bacterial ratios and increased C to N enzyme ratios. We did not observe equivalent effects of acid treatments on Rs in the beech forest, where microbial activity appeared to be more tightly linked to N acquisition. However the only changes in C cycling following N addition were increased C to N enzyme ratios, with no impact on C fluxes. The lack of other N effects on C cycling might be explained by forest adaptation to long-term elevated ambient N deposition. We conclude that additional N deposition is unlikely to lead to further soil C sequestration in these ecosystems, and that C accumulation previously attributed to N deposition could be partly attributable to their simultaneous acidification.
A high proportion of biogeochemical research to date has taken place within areas of Europe and North America that have, as a result of industrialisation and agricultural intensification, been exposed to multiple environmental stressors, including the simultaneous deposition of a range of atmospheric pollutants. Nevertheless, experiments carried out within these ecosystems often focus on a single driver, whilst many analyses of monitoring data have considered only a partial range of potential driving variables. Studies of forest carbon sequestration in response to atmospheric deposition have overwhelmingly focused on N deposition, yet our results suggest a potentially similar rate of soil C accumulation in response to S deposition (> 10 kg C kg S-1) as that estimated for N deposition (10 – 15 kg C kg N-1). Furthermore, S deposition to European and North American forests has changed more rapidly and more dramatically over the last 50 years than N deposition, implying that S may have had a greater influence on the soil C cycle during this period, and also raising the possibility that some observed changes may have been incorrectly attributed to N deposition. In order to correctly interpret existing records, and to accurately predict future changes in ecosystem biogeochemistry in response to continuing environmental change, we argue that a more holistic approach to the impacts of multiple environmental drivers is needed.