Key role of symbiotic dinitrogen fixation in tropical forest secondary succession

Forests contribute a significant portion of the land carbon sink, but their ability to sequester CO2 may be constrained by nitrogen1, 2, 3, 4, 5, 6, a major plant-limiting nutrient. Many tropical forests possess tree species capable of fixing atmospheric dinitrogen (N2)7, but it is unclear whether this functional group can supply the nitrogen needed as forests recover from disturbance or previous land use1, or expand in response to rising CO2 (refs 6, 8). Here we identify a powerful feedback mechanism in which N2 fixation can overcome ecosystem-scale deficiencies in nitrogen that emerge during periods of rapid biomass accumulation in tropical forests. Over a 300-year chronosequence in Panama, N2-fixing tree species accumulated carbon up to nine times faster per individual than their non-fixing neighbours (greatest difference in youngest forests), and showed species-specific differences in the amount and timing of fixation. As a result of fast growth and high fixation, fixers provided a large fraction of the nitrogen needed to support net forest growth (50,000¿kg carbon per hectare) in the first 12¿years. A key element of ecosystem functional diversity was ensured by the presence of different N2-fixing tree species across the entire forest age sequence. These findings show that symbiotic N2 fixation can have a central role in nitrogen cycling during tropical forest stand development, with potentially important implications for the ability of tropical forests to sequester CO2

Phosphatase activity and nitrogen fixation reflect species differences, not nutrient trading or nutrient balance, across tropical rainforest trees

10.1111/ele.13129Ecology Letters21101486-149

Global pattern of leaf litter nitrogen and phosphorus in woody plants

Forest ecosystems exert an important influence on global biogeochemical cycles. A global dataset of nitrogen (N) and phosphorus (P) concentrations in leaf- litter of woody plants was compiled from the literature. Among the 677 data sets, 482 included P concentrations and the N:P ratio. At a global scale, the mean leaf-litter N and P and N:P ratio were 10.9 mg g-1, 0.85 mg g-1 and 18.3, respectively. Leaf-litter N and P were significantly correlated. When the data was grouped by continents, the highest mean N was found in Africa (19.5 mg g-1), and the lowest in North America (8.18 mg g-1). P was significantly smaller in the Asian Islands (Japan and Malaysia, 0.44 mg g-1) than on the Asian mainland. For the global dataset, leaf-litter N increased linearly with mean annual temperature and annual precipitation and decreased with latitude. Although leaf- litter P showed no significant relationship with temperature, it declined linearly with precipitation and there was a convex quadratic relationship with latitude. For the global dataset and also for different functional groups (e.g. shrubs, evergreen broadleaf, deciduous broadleaf, and conifers) the leaf-litter N:P ratio generally followed a positive linear relationship with temperature and precipitation, and showed a concave quadratic response with latitude. The differences in leaf-litter N:P ratio among functional groups and among continents should be taken into account when modeling biogeochemical cycles in different regions as well as on a global scale

Seasonal variations of dissolved nitrogen and DOC:DON ratios in an intermittent Mediterranean stream

22 páginas, 6 figuras, 4 tablas.Seasonal variations of dissolved inorganic nitrogen (DIN) (NO3–N and NH4–N) and dissolved organic nitrogen (DON) were determined in Fuirosos, an intermittent stream draining an unpolluted Mediterranean forested catchment (10.5 km2) in Catalonia (Spain). The influence of flow on streamwater concentrations and seasonal differences in quality and origin of dissolved organic matter, inferred from dissolved organic carbon to nitrogen ratios (DOC:DON ratios), were examined. During baseflow conditions, nitrate and ammonium had opposite behaviour, probably controlled by biological processes such as vegetation uptake and mineralization activity. DON concentrations did not have a seasonal trend. During storms, nitrate and DON increased by several times but discharge was not a good predictor of nutrient concentrations. DOC:DON ratios in streamwater were around 26, except during the months following drought when DOC:DON ratios ranged between 42 and 20 during baseflow and stormflow conditions, respectively. Annual N export during 2000–2001 was 70 kg km 1 year 1, of which 75% was delivered during stormflow. The relative contribution of nitrogen forms to the total annual export was 57, 35 and 8% as NO3–N, DON and NH4–N, respectively.This study was supported by a Formacio´n de Personal Investigador (FPI) grant and funds provided by the Comisio´ n Interministerial de Ciencia y Tecnologı´a (CICT, reference REN2001-3327).Peer reviewe