Simulation of Increased Nitrogen Deposition to a Montane Forest Ecosystem: Partitioning of the Added 15N

Abstract. Nitrogen (N) was added over two years to a spruce-dominated (Picea abies) montane forest at Alptal, central Switzerland. A solution of ammonium nitrate (NH4NO3) was frequently sprinkled on the forest floor (1500 m2) to simulate an additional input of 30 kg N ha-1 yr-1 over the ambient 12 kg bulk inorganic N deposition. The added nitrogen was labelled with 15NH415NO3 during the first year. Results are compared to a control plot. Neither the trees nor the ground vegetation showed any increase in their N content. Only 4.1 % of N in the ground vegetation came from the N addition. Current-year needles contained 11 mg N g-1 dry weight, of which only 2 % was from labelled N; older needles had approximately half as much 15N. The uptake from the treatment was therefore very small. Redistribution of N also took place in the trunks: 1 to 2-year-old wood contained 0.7 % labelled N, tree rings dating back 3 to 14 years contained 0.4%. Altogether, the above-ground vegetation took up 12 % of the labelled N. Most 15N was recovered in the soil: 13 % in litter and roots, 63 % in the sieved soil. Nitrate leaching accounted for 10%. Factors thought to be influencing N uptake are discussed in relation to plant use of N and soil conditions

Atmospheric deposition and precipitation are important predictors of inorganic nitrogen export to streams from forest and grassland watersheds: a large-scale data synthesis

Previous studies have evaluated how changes in atmospheric nitrogen (N) inputs and climate affect stream N concentrations and fluxes, but none have synthesized data from sites around the globe. We identified variables controlling stream inorganic N concentrations and fluxes, and how they have changed, by synthesizing 20 time series ranging from 5 to 51 years of data collected from forest and grassland dominated watersheds across Europe, North America, and East Asia and across four climate types (tropical, temperate, Mediterranean, and boreal) using the International Long-Term Ecological Research Network. We hypothesized that sites with greater atmospheric N deposition have greater stream N export rates, but that climate has taken a stronger role as atmospheric deposition declines in many regions of the globe. We found declining trends in bulk ammonium and nitrate deposition, especially in the longest time-series, with ammonium contributing relatively more to atmospheric N deposition over time. Among sites, there were statistically significant positive relationships between (1) annual rates of precipitation and stream ammonium and nitrate fluxes and (2) annual rates of atmospheric N inputs and stream nitrate concentrations and fluxes. There were no significant relationships between air temperature and stream N export. Our long-term data shows that although N deposition is declining over time, atmospheric N inputs and precipitation remain important predictors for inorganic N exported from forested and grassland watersheds. Overall, we also demonstrate that long-term monitoring provides understanding of ecosystems and biogeochemical cycling that would not be possible with short-term studies alone.publishedVersio

Reconstructing terrestrial nutrient cycling using stable nitrogen isotopes in wood

Although recent anthropogenic effects on the global nitrogen (N) cycle have been significant, the consequences of increased anthropogenic N on terrestrial ecosystems are unclear. Studies of the impact of increased reactive N on forest ecosystems—impacts on hydrologic and gaseous loss pathways, retention capacity, and even net primary productivity— have been particularly limited by a lack of long-term baseline biogeochemical data. Stable nitrogen isotope analysis (ratio of ¹⁵N to ¹⁴N, termed δ¹⁵N) of wood chronologies offers the potential to address changes in ecosystem N cycling on millennial timescales and across broad geographic regions. Currently, nearly 50 studies have been published utilizing wood δ¹⁵N records; however, there are significant differences in study design and data interpretation. Here, we identify four categories of wood δ¹⁵N studies, summarize the common themes and primary findings of each category, identify gaps in the spatial and temporal scope of current wood δ¹⁵N chronologies, and synthesize methodological frameworks for future research by presenting eight suggestions for common methodological approaches and enhanced integration across studies. Wood δ¹⁵N records have the potential to provide valuable information for interpreting modern biogeochemical cycling. This review serves to advance the utility of this technique for long-term biogeochemical reconstructions

Estimate of Leaf Area Index in an Old-Growth Mixed Broadleaved-Korean Pine Forest in Northeastern China

Leaf area index (LAI) is an important variable in the study of forest ecosystem processes, but very few studies are designed to monitor LAI and the seasonal variability in a mixed forest using non-destructive sampling. In this study, first, true LAI from May 1st and November 15th was estimated by making several calibrations to LAI as measured from the WinSCANOPY 2006 Plant Canopy Analyzer. These calibrations include a foliage element (shoot, that is considered to be a collection of needles) clumping index measured directly from the optical instrument, TRAC (Tracing Radiation and Architecture of Canopies); a needle-to-shoot area ratio obtained from shoot samples; and a woody-to-total area ratio. Second, by periodically combining true LAI (May 1st) with the seasonality of LAI for deciduous and coniferous species throughout the leaf-expansion season (from May to August), we estimated LAI of each investigation period in the leaf-expansion season. Third, by combining true LAI (November 15th) with litter trap data (both deciduous and coniferous species), we estimated LAI of each investigation period during the leaf-fall season (from September to mid-November). Finally, LAI for the entire canopy then was derived from the initial leaf expansion to the leaf fall. The results showed that LAI reached its peak with a value of 6.53 m2 m−2 (a corresponding value of 3.83 m2 m−2 from optical instrument) in early August, and the mean LAI was 4.97 m2 m−2 from May to November using the proposed method. The optical instrument method underestimated LAI by an average of 41.64% (SD = 6.54) throughout the whole study period compared to that estimated by the proposed method. The result of the present work implied that our method would be suitable for measuring LAI, for detecting the seasonality of LAI in a mixed forest, and for measuring LAI seasonality for each species

N-15 immobilization in forest soil: a sterilization experiment coupled with N-15 CPMAS NMR spectroscopy

35 pages, 6 figures, 38 references.-- Este trabajo es el Capítulo 6 de la tesis doctoral nº 3580 presentada el 7 de julio 2006, en la Faculté de L'Environnement Naturel, Architectural et Construit, Laboratoire du sol et physique environnementale, Section Sciences et Ingénierie de L'environnement, École Polytechnique Fédérale de Lausanne, pour l'obtention du Grade de Docteur ès Sciences.In temperate forests, soils are the main sink for atmospheric N deposition. The main processes proposed for N retention are the microbial immobilization and the abiotic fixation in soil organic matter. The relative importance of such processes as well as the kind of resulting chemical compounds are not totally resolved. In order to improve our understanding on the subject, we carried out a laboratory incubation of sterilized and unsterilized soils (organic and organo-mineral), labeled with 15NO3 - or 15NH4 +, which are the main form of N deposition. Soils were incubated during one hour, one day and one week times and then subjected to a K2SO4 extraction and to 15N CPMAS spectroscopy measurements. After one hour of incubation, immobilization was already effective within all the incubated soils. The corresponding NMR spectra were difficult to interpret since the signal-to-noise ratio was low. However, part of the immobilized 15N was already incorporated as amides. In the sterilized soils labeled with 15NH4 +, a chemical process connected with the presence of Hg, immobilized the tracer rapidly and massively (between 80% to 90% were unextractable after one hour in the organic sterilized soils against 50% in the unsterilized). However, no corresponding specific peak was observable on the NMR spectra. In the sterilized soils labeled with 15NO3 -, between one half and one third of the added tracer was immobilized during the first hour and then, in the organic layer, 10% more during the week. We suppose, on the one hand, that the incomplete sterilization in the very short term explains the one-hour immobilization; on the other hand, an abiotic process seems to be responsible for the NO3 - immobilization over the week. In the unsterilized soils, approximately 50% of 15N was immobilized in the OL-A samples (approximately 40% in the Aca layer) during the first hour and approximately 80% during the week (approximately 60% in the Aca layer). The dynamics of immobilization were very similar for 15NH4 + and 15NO3 - , mainly immobilized as amides. Within the framework of our study and because of the low signal-to-noise ratio obtained by 15N NMR measurements, the rate of 15N immobilized as amide could not be quantified. However, we showed that the amides-peptides signal was dominant whichever layer is concerned, or chemical form added or even whether the soil is sterilized or not. Consequently, we are able to confirm the importance of the proteinaceous compounds for the immobilization of N in the soil.Peer reviewe

15N immobilization in forest soil: a sterilization experiment coupled with 15CPMAS NMR spectroscopy

9 páginas, 6 figuras, 40 referencias.In temperate forests, soils are the main sink for atmospheric N deposition. The main processes proposed for N retention are microbial and abiotic immobilization in soil organic matter. The relative importance of these processes as well as the kind of resulting chemical compounds are not totally understood. We carried out a laboratory incubation of Hg-sterilized and non-sterilized organic and organo-mineral soil horizons, labelled with either 15NO3− or 15NH4+. The labelled samples were incubated for 1 hour, 1 day, or 6 days, then subjected to K2SO4 extraction and analysed with 15N CPMAS NMR spectroscopy. N immobilization was already effective in all samples and treatments after 1 hour. The corresponding NMR spectra showed that part of the immobilized 15N was already incorporated into an amide structure. In the sterilized soils labelled with 15NH4+, the tracer was rapidly and largely immobilized by an unknown process related to the presence of Hg. In the sterilized soils labelled with 15NO3−, between one-third and one-half of the added tracer was immobilized during the first hour and only 10% more over the 6 days. These results suggest that the sterilization was incomplete at first, allowing relatively great microbial immobilization during the first hour. By contrast, over a longer time, NO3− immobilization was significantly reduced to a level corresponding to an abiotic process as Hg sterilization became more effective. Even if the low signal-to-noise ratio precluded quantitative 15N NMR measurements, we showed that the amide-peptide signal, considered as a biotic signature, was dominant in all cases.Peer reviewe