Nitrogen Deposition Effects on Soil Carbon Dynamics in Temperate Forests

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandlingForskning

Shimon Ginzburg Ozeri

Soils contain the largest fraction of terrestrial carbon (C). Understanding the factors regulating the decomposition and storage of soil organic matter (SOM) is essential for predictions of the C sink strength of the terrestrial environment in the light of global change. Elevated long-term nitrogen (N) deposition into forest ecosystems has been increasing globally and was hypothesized to raise soil organic C (SOC) stocks by increasing forest productivity and by reducing SOM decomposition. Yet, these effects of N deposition on forest SOC stocks are uncertain and largely based on observations from high dose-N fertilization experiments. Here, I present a synthesis of results from three studies aiming at elucidating the effects of long-term N deposition on SOC stocks and fluxes in temperate conifer forests. In two of the studies, short N deposition gradients occurring naturally in forest edges were used to study the effects of varying N deposition load on SOC stocks and fluxes as well as on the temperature sensitivity of SOM respiration. In a third study, the effects of 20 years of continuous experimental N addition (35 kg N ha-1 year-1) on soil C budget were investigated. Our general hypotheses were that elevated N deposition will: i) increase SOC stocks owing to positive effect of N on litterfall C inputs combined with negative effect on SOM decomposition regardless of negative effects on belowground C inputs by roots and associated mycorrhiza; ii) reduce the temperature sensitivity of SOM heterotrophic respiration. The five forest edges investigated exhibited N deposition gradients decreasing from the edges toward the interior of the stands. Thus, increasing distance from the edge was used as proxy for decreasing N deposition. At the two edges receiving intermediate-N deposition rates (≤ 23 kg N ha-1 year-1), forest floor SOC stocks tended to decrease with distance from the edge while a decade of experimental N additions insignificantly increased the size of the humus (H) layer providing weak evidence for some positive effect of N deposition on SOC stocks. The three edges receiving higher N deposition rates (> 29 kg N ha-1 year-1) were N-saturated where SOC stocks in the top soil were either unchanged or negatively affected by elevated N deposition. Similarly, forest floor SOC stocks were unchanged following two decades of experimental-N additions. Aboveground C inputs by needle litterfall were generally not significantly affected by N deposition at the edge sites but tended to increase with increasing distance from the edge in two of the N-saturated sites. The experimental N additions resulted in reduced C inputs by foliar litter relative to control concomitant with reduced tree and moss growth. At three edges, fine-root biomass tended to increase with distance from the edge while over a decade of N additions reduced the means of fine-root biomass, ectomycorrihizal (EM) mycelial production and species composition insignificantly compared with control suggesting reduced belowground C inputs under elevated N deposition. At two edge sites, forest floor C outputs by respiration tended to decrease with decreased forest floor C/N and distance from the edge indicating positive effect of elevated N deposition on SOC sequestration. Correspondingly, N-enriched litter incubated in litterbags had significantly lower late-stage decomposition rates compared with control litter. However, potential respiration of forest floor and mineral soil was overall unaffected by the experimental N-additions. A temperature treatment of forest floor samples taken from one edge site revealed decreasing respiration rates and temperature sensitivity (Q10) of labile and recalcitrant SOM with increasing forest floor C/N ratio. I conclude that N deposition at rates typical to agriculturally intensified regions in north-western Europe may reduce the decomposition rate of SOM thereby positively affecting SOC sequestration but at the same time above- and belowground C inputs also decrease resulting in no change in SOM stocks. Moreover, elevated N deposition may reduce the potential positive feedback of SOM decomposition on global atmospheric CO2 concentrations. These results have implications for modelling the carbon sink-strength of temperate forests under global change.
ForlagDepartment of Geosciences and Natural Resource Management, Faculty of Science, University of Copenhagen
StatusUdgivet - 2014

ID: 124495404