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

Sinks for nitrogen inputs in terrestrial ecosystems: a meta-analysis of15N tracer field studies

Effects of anthropogenic nitrogen (N) deposition and the ability of terrestrial ecosystems to store carbon (C) depend in part on the amount of N retained in the system and its partitioning among plant and soil pools. We conducted a meta-analysis of studies at 48 sites across four continents that used enriched 15N isotope tracers in order to synthesize information about total ecosystem N retention (i.e., total ecosystem 15N recovery in plant and soil pools) across natural systems and N partitioning among ecosystem pools. The greatest recoveries of ecosystem 15N tracer occurred in shrublands (mean, 89.5%) and wetlands (84.8%) followed by forests (74.9%) and grasslands (51.8%). In the short term (<1 week after 15N tracer application), total ecosystem 15N recovery was negatively correlated with fine-root and soil 15N natural abundance, and organic soil C and N concentration but was positively correlated with mean annual temperature and mineral soil C:N. In the longer term (3–18 months after 15N tracer application), total ecosystem 15N retention was negatively correlated with foliar natural-abundance 15N but was positively correlated with mineral soil C and N concentration and C : N, showing that plant and soil natural-abundance 15N and soil C:N are good indicators of total ecosystem N retention. Foliar N concentration was not significantly related to ecosystem 15N tracer recovery, suggesting that plant N status is not a good predictor of total ecosystem N retention. Because the largest ecosystem sinks for 15N tracer were below ground in forests, shrublands, and grasslands, we conclude that growth enhancement and potential for increased C storage in aboveground biomass from atmospheric N deposition is likely to be modest in these ecosystems. Total ecosystem 15N recovery decreased with N fertilization, with an apparent threshold fertilization rate of 46 kg N·ha−1·yr−1 above which most ecosystems showed net losses of applied 15N tracer in response to N fertilizer additio

Convergence of soil nitrogen isotopes across global climate gradients

Quantifying global patterns of terrestrial nitrogen (N) cycling is central to predicting future patterns of primary productivity, carbon sequestration, nutrient fluxes to aquatic systems, and climate forcing. With limited direct measures of soil N cycling at the global scale, syntheses of the 15N:14N ratio of soil organic matter across climate gradients provide key insights into understanding global patterns of N cycling. In synthesizing data from over 6000 soil samples, we show strong global relationships among soil N isotopes, mean annual temperature (MAT), mean annual precipitation (MAP), and the concentrations of organic carbon and clay in soil. In both hot ecosystems and dry ecosystems, soil organic matter was more enriched in 15N than in corresponding cold ecosystems or wet ecosystems. Below a MAT of 9.8°C, soil δ15N was invariant with MAT. At the global scale, soil organic C concentrations also declined with increasing MAT and decreasing MAP. After standardizing for variation among mineral soils in soil C and clay concentrations, soil δ15N showed no consistent trends across global climate and latitudinal gradients. Our analyses could place new constraints on interpretations of patterns of ecosystem N cycling and global budgets of gaseous N loss.Fil: Craine, Joseph M. Kansas State University. Division of Biology; Estados UnidosFil: Elmore, Andrew J. University of Maryland Center for Environmental Science. Appalachian Laboratory; Estados UnidosFil: Wang, Lixing. Indiana University-Purdue University Department of Earth Sciences; Estados UnidosFil: Augusto, Laurent. INRA. Bordeaux Sciences Agro; FranciaFil: Baisden, Troy. GNS Science. National Isotope Centre; Nueva ZelandaFil: Brookshire, E.N.J. Montana State University. Department of Land Resources and Environmental Sciences; Estados UnidosFil: Cramer, Michael D. University of Cape Town. Department of Biological Sciences; SudáfricaFil: Hasselquist, Niles. Swedish University of Agricultural Sciences. Forest Ecology and Management; SueciaFil: Hobbie, Erik A. University of New Hampshire. Earth Systems Research Center; Estados UnidosFil: Kahmen, Ansgar. Departement of Environmental Sciences - Botany; SuizaFil: Kaba, Keisuke. Tokyo University of Agriculture and Technology. Institute of Agriculture; JapónFil: Kranabetter, M. British Columbia (Canadá). Ministry of Forests, Lands and Natural Resource Operations; CanadáFil: Mack, M. University of Florida. Department of Biology; Estados UnidosFil: Marin-Spiotta, E. University of Wisconsin. Department of Geography; Estados UnidosFil: Mayor, J.R. Swedish University of Agricultural Sciences. Department of Forest Ecology & Management; SueciaFil: McLauchlan, K.K. Kansas State University. Department of Geography; Estados UnidosFil: Michelsen, A. University of Copenhagen. Department of Biology; DinamarcaFil: Nardoto, G.B. Universidade de Brasília. Faculdade UnB Planaltina; BrasilFil: Oliveira, R.S. Universidade Estadual de Campinas. Instituto de Biologia. Departamento de Biologia Vegetal; BrasilFil: Perakis, S.S. Forest and Rangeland Ecosystem Science Center; Estados UnidosFil: Peri, Pablo Luis. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Santa Cruz; Argentina. Universidad Nacional de la Patagonia Austral; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Quesada, C. Instituto Nacional de Pesquisas da Amazonia. Coordenação de Dinâmica Ambiental; BrasilFil: Richter, A. University of Vienna. Department of Terrestrial Ecosystem Research; AustriaFil: Schipper, L.A. University of Waikato. Environmental Research Institute; Nueva ZelandaFil: Stevenson, B.A. Landcare Research; Nueva ZelandaFil: Turner, B.L. Smithsonian Tropical Research Institute; PanamáFil: Viani, R.A.G. Universidade Federal de São Carlos. Centro de Ciências Agrárias; BrasilFil: Wanek, W. University of Vienna. Department of Terrestrial Ecosystem Research; AustriaFil: Zeller, B. INRA Nancy. Biogéochimie des Ecosystèmes Forestiers; Franci