C.J. Koopmans

Nitrogen deposition makes a minor contribution to carbon sequestration in temperate forests

Humans have altered global nitrogen cycling such that more atmospheric N2 is being converted (‘fixed’) into biologically reactive forms by anthropogenic activities than by all natural processes combined. In particular, nitrogen oxides emitted during fuel combustion and ammonia volatilized as a result of intensive agriculture have increased atmospheric nitrogen inputs (mostly NO3 and NH4) to temperate forests in the Northern Hemisphere. Because tree growth in northern temperate regions is typically nitrogen-limited, increased nitrogen deposition could have the effect of attenuating rising atmospheric CO2 by stimulating the accumulation of forest biomass. Forest inventories indicate that the carbon contents of northern forests have increased concurrently with nitrogen deposition since the 1950s. In addition, variations in atmospheric CO2 indicate a globally significant carbon sink in northern mid-latitude forest regions. It is unclear, however, whether elevated nitrogen deposition or other factors are the primary cause of carbon sequestration in northern forests. Here we use evidence from 15N-tracer studies in nine forests to show that elevated nitrogen deposition is unlikely to be a major contributor to the putative CO2 sink in forested northern temperature regions.

Impact of nitrogen deposition on nitrogen cycling in forests: a synthesis of NITREX data

Abstract Impact of nitrogen (N) deposition was studied by comparing N fluxes, N concentrations and N pool sizes in vegetation and soil in five coniferous forest stands at the NITREX sites: Gardsjon (GD), Sweden, Klosterhede (KH), Denmark, Aber (AB), Wales, UK, Speuld (SP), the Netherlands, and Ysselsteyn (YS), the Netherlands. The sites span a N- deposition gradient from 13 to 59 kg N ha−1 yr−1. Measurements of soil N transformation rates by laboratory and field incubations were part of the site comparison. Further, results from 4–5 yr of NH4NO3 addition (35 kg N ha−1 yr−1) at low deposition sites (GD, KH, AB) and 6 yr of N removal (roofs) at high deposition sites (SP, YS) were included in the analysis. Significant correlations were found between a range of variables including N concentrations in foliage and litter, soil N transformation rates and forest floor characteristics. Using the methods from principal component analysis (PCA) these variables were summarized to an index of site N status that assigned the lowest N status to GD and the highest to YS. Site N status increased with N deposition with the exception that AB was naturally rich in N. Nitrate leaching was significantly correlated with N status but not correlated with N deposition. Forest floor mass and root biomass decreased with increased N status. Characteristics of the mineral soil were not correlated with vegetation and forest floor variables. High C N ratios in the mineral soil at the high-N deposition sites (SP, YS) suggest that the mineral soil pool changes slowly and need not change for N saturation to occur. Nitrogen transformation rates measured in laboratory incubations did not agree well with rates measured in the field except for a good correlation between ‘gross’ mineralization in the laboratory and ‘net’ mineralization in the field. The changes in N concentrations and fluxes after manipulation of N input followed the direction expected from the site comparison: increases at N addition and decreases at N removal sites. Nitrate leaching responded within the first year of treatment at all sites, whereas responses in vegetation and soil were delayed. Changes in N status by the manipulation treatments were small compared to the differences between sites. Changes in nitrate leaching were small at the low-N status sites and substantial at the high-N status sites. Nitrogen-limited and N-saturated forest ecosystems could be characterized quantitatively.

Natural abundance of 15N in forests across a nitrogen deposition gradient

Chronic atmospheric nitrogen deposition can alter the rate of internal nitrogen cycling and increase the magnitude of N leaching losses in forested ecosystems. As fractionation of nitrogen in favour of the lighter 14N occurs during various transformations associated with N-enrichment and nitrogen loss, it has been proposed that the 15N signal of vegetation may provide a useful tool in evaluating the past and current N status of forested ecosystems. A series of coniferous forests across a European nitrogen deposition gradient within the NITREX project provided an opportunity to test the relationships between nitrogen supply from atmospheric deposition and the relative 15N-enrichment of vegetation to soil, across a large geographical area. Most δ15N values for above- and below-ground tree components, soil at four depths, bulk precipitation and/or throughfall water and soil solution or outflow water values were within those observed elsewhere except for a few notable exceptions. There was a significant positive relationship between the δ15N enrichment of the tree foliage relative to the soil horizons (or the enrichment factor), and nitrogen flux in the throughfall if Aber forest, N. Wales, was excluded from the regression analysis. An unusually high enrichment factor at the Aber site indicated that a the high rate of N cycling at the site was in excess of that predicted from current N deposition. This was attributed to the effect of ploughing and tree planting on the relatively N- and clay-rich mineral horizons at Aber compared to other sites. Highly significant relationships (P < 0.01) between enrichment factors and parameters describing internal rates of N cycling, such as litterfall N flux and nitrification rates in upper soil horizons, supported this conclusion. There appears to be a strong link between the rate of N cycling and the δ15N enrichment factor, rather than N deposition or nitrate leaching per se. These results confirm the potential use of the δ15N enrichment factor to identify sites influenced by nitrogen deposition. However, consideration should be taken of other site characteristics and land management practises which also influence soil N dynamics and N cycling.

The fate of 15N-labelled nitrogen deposition in coniferous forest ecosystems

Abstract As part of four European ecosystem manipulation experiments in coniferous forests, field-scale 15N tracer experiments have been carried out. The experiments involved a year-long addition of 15NH4+ and/or 15NO3− to throughfall at experimental plots with different N inputs. The fate of this applied 15N in the important ecosystems pools (trees, ground vegetation, forest floor and mineral soil), as well as in drainage was measured. About 10–30% of added 15N was taken up by the trees and 10–15% was retained in the mineral soil. Both retention efficiencies were found to be constant with N input. The part of 15N retained in the organic layer was relatively high (20–45% of applied) at low N inputs (0–30 kg N ha−1 yr−1) but low (10–20%) at high N inputs (30–80 kg N ha−1 yr−1). An inverse relationship between N input and the loss of 15N in drainage was found: drainage losses increased as a function of N input. These results suggest that increased N inputs exceed the capacity of the microbial population to retain throughfall-N in the organic layer, with the result that N leaching increases.

Natural 15N abundance in two nitrogen saturated forest ecosystems.

Abstract Natural 15N abundance values were measured in needles, twigs, wood, soil, bulk precipitation, throughfall and soil water in a Douglas fir (Pseudotsuga menziesii (Mirb.) and a Scots pine (Pinus sylvestris L.) stand receiving high loads of nitrogen in throughfall (>50 kg N ha−1 year−1). In the Douglas fir stand δ15N values of the vegetation ranged between −5.7 and −4.2‰ with little variation between different compartments. The vegetation of the Scots pine stand was less depleted in 15N and varied from −3.3 to −1.2‰δ15N. At both sites δ15N values increased with soil depth, from −5.7‰ and −1.2‰ in the organic layer to +4.1‰ and +4.7‰ at 70 cm soil depth in the Douglas fir and Scots pine stand, respectively. The δ15N values of inorganic nitrogen in bulk precipitation showed a seasonal variation with a mean in NH4+-N of −0.6‰ at the Douglas fir stand and +10.8‰ at the Scots pine stand. In soil water below the organic layer NH4+-N was enriched and NO3−-N depleted in 15N, which was interpreted as being caused by isotope fractionation accompanying high nitrification rates in the organic layers. Mean δ15N values of NH4+ and NO3− were very similar in the drainage water at 90 cm soil depth at both sites (−7.1 to −3.8‰). A dynamic N cycling model was used to test the sensitivity of the natural abundance values for the amount of N deposition, the 15N ratio of atmospheric N deposited and for the intrinsic isotope discrimination factors associated with N transformation processes. Simulated δ15N values for the N saturated ecosystems appeared particularly sensitive to the 15N ratio of atmospheric N inputs and discrimination factors during nitrification and mineralization. The N-saturated coniferous forest ecosystems studied were not characterized by elevated natural 15N abundance values. The results indicated that the natural 15N abundance values can only be used as indicators for the stage of nitrogen saturation of an ecosystem if the δ15N values of the deposited N and isotope fractionation factors are taken into consideration. Combining dynamic isotope models and natural 15N abundance values seems a promising technique for interpreting natural 15N abundance values found in these forest ecosystems.

Nitrogen transformations in two nitrogen saturated forest ecosystems subjected to an experimental decrease in nitrogen deposition

Nitrogen transformations were studied in the forest floor and mineral soil (0–5 cm) of a Douglas fir forest (Pseudotsuga menziesii (Mirb.) Franco.) and a Scots pine forest (Pinus sylvestris L.) in the Netherlands. Curren nitrogen depositions (40 and 56 kg N ha-1 yr-1, respectively) were reduced to natural background levels (1–2 kg N ha-1 yr-1) by a roof construction. The study concentrated on rates and dynamic properties of nitrogen transformations and their link with the leaching pattern and nitrogen uptake of the vegetation under high and reduced nitrogen deposition levels. Results of an in situ field incubation experiment and laboratory incubations were compared. No effect of the reduced N deposition on nitrogen transformations was found in the Douglas fir forest. In the Scots pine forest, however, during some periods of the year nitrogen transformations were significantly decreased under the low nitrogen deposition level. At low nitrogen inputs a net immobilization occurred during most of the year leading to a very small net mineralization for the whole year. In laboratory and in individual field plots nitrogen transformations were negatively correlated with initial inorganic nitrogen concentrations. Nitrogen budget estimates showed that nitrogen transformations were probably underestimated by the in situ incubation technique. Nevertheless less nitrogen was available for plant uptake and leaching at the low deposition plots.

Nitrate leaching in coniferous forest ecosystems: The European Field‐Scale Manipulation Experiments NITREX (Nitrogen Saturation Experiments) and EXMAN (Experimental Manipulation of Forest Ecosystems)

The results of two European field-scale manipulation projects (NITREX (nitrogen saturation experiments) and EXMAN (experimental manipulation of forest ecosystems)) were used to evaluate the effect of ecosystem disturbance on nitrate leaching in coniferous forest ecosystems. The first principle component (PC1) of a principle component analysis explained 85% of the variation in nitrate leaching between the 12 sites. This PC1 consisted of nitrogen concentrations and fluxes in the ecosystem and was interpreted as an indicator of N status. Nitrate leaching responded rapidly to manipulation of nitrogen deposition, especially in sites with ambient high nitrate leaching. This rapid response could be explained partly by an immediate hydrological response of increased drainage. However, results of field-scale 15 N tracer experiments indicated that microbial processes in the organic layer had changed after a few years of changed N deposition. In sites with already significant nitrate leaching, irrigation caused a large increase in nitrate leaching due to enhanced mineralization. Combined fertilization and irrigation had only a limited effect on nitrate leaching in nitrogen-limited sites, whereas in nitrogen-saturated sites, nitrate leaching was significantly increased. The hypothesized nitrate pulse as a result of rewetting after drought did not occur in any of the sites. We conclude that the effect of disturbance on nitrate leaching depends on the N status of the ecosystem: in sites that are nitrogen-saturated, nitrate leaching is very sensitive to disturbance.

reply: Nitrogen deposition and carbon sequestration

Nadelhoffer et al. reply — Jenkinson et al. and Sievering are justifiably concerned that our 15N additions to forest floors do not account for the potential uptake of nitrogen input by forest canopies. We agree that canopies can remove nitrogen from the atmosphere, resulting in inputs to forest floors that are less than the total nitrogen deposition. A North American study has suggested that canopies remove, on average, 16% of total (organic+inorganic) atmospheric nitrogen input to forests, and concluded that nitrogen uptake by the canopy is probably small relative to the nitrogen requirements of trees. Spraying 15N-labelled ammonium and nitrate on the crowns of five-year-old Norway spruce indicated that foliar uptake in mature forests probably constitutes only a small percentage of annual nitrogen uptake.