The impact of nitrogen deposition on carbon sequestration by European forests

An estimate of net carbon pool changes was made at more than 600 Intensively Monitoring plots. Carbon pool changes in trees were based on repeated forest growth surveys. Carbon pool changes in the soil were based on calculated nitrogen retention (N deposition minus N leaching) rates in soils minus N uptake and multiplied by the C/N ratio of the forest soils. Results were scale up to Europe based on data for more than 6000 plots in a systematic 16km x 16 km grid. The carbon pool changes in the tree are generally 5-10 times as high as the estimated carbon pool changes in the soil. The changes in the carbon pool in tree due to forest growth increased from Northern to Central Europe, whereas the changes in the carbon pool in soil are high in Central Europe and low in Northern and Southern Europe. Net increases in the carbon pool by forests in Europe (both trees and soil) are in the range of 0.1-0.15 Gton.yr, being an important part (about 50%) of the terrestrial carbon sink in Europe, derived from atmospheric inversion models. The contribution of N deposition to the increase in carbon in standing biomass is approximately estimated to be less than 10%. INTRODUCTION It is of importance to arrive at reliable estimates of carbon sequestration in forests since this may delay the rise in the atmospheric CO2 concentration with implications for the speed of climate change. An overview of various estimates of the carbon sequestration in Europe, focusing on different ecosystem compartments and using different methods is given in Table 1. Apart from a distinction in the type of flux and the forest compartment, a differentiation has been made in the quality of the upscaling methods, going from individual sites to the European scale. A direct comparison of these studies on carbon sequestration in forests (in Europe), is hampered because of the measurement of different carbon sink terms. First of all, there is a difference in the assessment of the so-called net ecosystem production (NEP) or net ecosystem exchange (NEE), and the net biome production (NBP). The NEP or NEE stands for the total uptake of CO2 by photosynthesis, corrected for plant and soil respiration, whereas the NBP equals the NEP corrected for CO2 emissions due to harvest and forest fires. The latter term is critical with respect to long-term carbon storage, since an aggrading forest may temporarily sequester large carbon amounts, but most of it is re-emitted to the atmosphere after logging. Secondly, a distinction can be made in sequestration in the trees and in the soil. Over the long term, the soil is the ultimate sink or source of CO2 for these ecosystems. A systematic discussion related to the various approaches and results is given in De Vries et al. (2003) [1]. Table 1 Overview of different estimates of carbon sequestration on a European wide scale. Type of C flux Compartment Method Estimated sink Gton. yr Upscaling method Reference NBP landscape NBP Landscape Inversion modelling 0.30 Good [2] NEE/NEP Whole forest/trees NEE Whole forest CO2 net flux measurements 0.47 0.25 Neural networks Forest maps [3] [4] NEP Total above-ground biomass Tree growth measurements 0.39-0.53 Multiply with forested area [5] NBP whole forest/trees NBP Trees (stem wood) Repeated forest Inventories 0.10 Country inventory data [6] [7] NBP Trees (stem wood) Modelling forest growth 0.08-0.12 Country inventory data [8] NEP contribution Trees (above-ground biomass) N retention 0.025 World average values [9] NBP forest soil NBP Forest soil (below-ground biomass) Carbon soil input minus carbon mineralisation 0.13 Multiply with forested area [5] NBP Forest soil (below-ground biomass) Modelling forest growth and decomposition 0.0380.061 Country inventory data [8] NBP Forest soil (below-ground biomass) N retention 0.022 World average values [9] 1 The first estimates was derived by [5] based on a forested area in Europe of approximately 150 million ha, whereas the second estimate is based on an area of 200 million ha, used in this study 2 These estimates were originally limited to the EU + Norway and Switzerland (approximately 138 million ha) but results were scaled to the European forested area, excluding most of Russia (approximately 200 million ha) 3 These estimates were originally global but were scaled to the European N deposition and forest area. Actually, [9] also estimated a NEP of 0.0.25 for carbon sequestration in trees but this was presented as the contribution of N deposition to NEP in trees and not the total growth Important questions with respect to carbon sequestration are related to its cause and the timeperiod in which the terrestrial sink will be saturated. Apart from changes in standing growing stock (influenced by forest management), changes in net primary productivity may also play a role due to increases in atmospheric CO2 concentrations, nitrogen deposition and temperature. In this context, N deposition is claimed to be most important [10]. Nitrogen is often the limiting nutrient in terrestrial ecosystems. The increase in nitrogen deposition on forests may thus cause a significant interaction increase in carbon sequestration by increased wood production and accumulation of soil organic matter. Insight in the latter effect is crucial since the soil is the ultimate sink or source of CO2 for forest ecosystems over the long term. Current hypotheses suggest that increased N deposition causes an increased rate of soil organic matter accumulation at least in two ways due to an increased leaf/needle biomass and litter production [e.g. 5] and a reduced decomposition of organic matter [11]. Earlier estimates suggested that this mechanism could take up one third of the global CO2 emission from fossil fuel (or 2 x 10 g.yr) if most of the deposition nitrogen was taken up by trees and used to form new woody biomass [12]. Recent data on the distribution of deposition nitrogen between trees and soil [9], however, suggest that a large part of the nitrogen is accumulated in the soil at low carbon to nitrogen ratio (10-40) and not in the trees at carbon to nitrogen ratio (200500). Nadelhoffer et al. (1999) [9] calculated additional C sequestration on a global scale from additional N uptake by trees and N immobilisation in soils in response to N deposition. From their estimate, the authors conclude that C sequestration in forest trees and forest soils over the world is of equal magnitude. The upscaling of the results to a European scale by these authors was very simple, thus hampering an adequate estimate on this large scale. This paper presents an estimate of carbon sequestration in trees and soil in Europe and the likely impact of N deposition on the sequestration rates in the period 1960-2000. Use is made of measured and estimated data on N retention (input minus output), N uptake and soil C/N ratios at approximately 120 so-called Intensive Monitoring Plots with data on atmospheric N deposition and forest ecosystems responses and of approximately 6000 so-called Level 1 plots with data on forest vitality and soil chemistry.

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