Trace Gas Fluxes from Tropical Montane Forests of Southern Ecuador

Abstract

Methane (CH4), nitrous oxide (N2O) and carbon dioxide (CO2) are major greenhouse gases but their fluxes in tropical montane forests are hardly known. The distribution of known sources of CH4 (e.g. soil wetlands) fail to explain high CH4 concentrations above tropical montane forest canopies that were observed by space-borne measurements. Additionally, there are indications in recent literature that responses of soil N2O and CO2 fluxes to projected increases in nitrogen (N) deposition in tropical forest areas can be predicted from nutrient limitation of vegetation growth. However, nutrient controls on N and C cycling processes have rarely been studied in tropical montane forests. This dissertation tries to elucidate (1) the role of tank bromeliads, growing in canopy wetlands of neotropical montane forests, as a potential source of CH4 that may help to explain the high CH4 concentrations above neotropical forest canopies, (2) nutrient controls on rates of soil N cycling and soil N2O fluxes and (3) nutrient controls on soil respiration in Andean neotropical montane forests of Southern Ecuador.(1) CH4 fluxes from 167 tank bromeliads, a distinctive group of herbaceous water-impounding plants of three functional plant types with various sizes and from different strata of a moist tropical montane forest in the Ecuadorian Andes at 2100-m elevation were measured. Plant incubation chambers, 13C stable isotope probing and molecular analysis techniques were used to show that tank bromeliads can have high rates of CH4 emissions. The gas is produced in their water-filled leaf axils (the pouch-like basal section) by a diverse community of methanogenic archaea. The dissolved CH4 in bromeliad tanks appears to be absorbed by foliar hairs (trichomes), diffused into aerenchyma and emitted through stomata into the atmosphere. This thesis estimates the CH4 source from the montane tropical forest at 3.6 g ha-1 d-1, which is enough to compensate for atmospheric CH4 consumption in the soil at a rate of 3.1 g ha-1 d-1. The neotropical forest source may be in the range of 1.2 Tg yr-1.(2) Soil N2O fluxes and net rates of soil N cycling were measured in a factorial NP addition experiment (i.e. N, P, N+P, and control) (NUMEX) from January 2008 to September 2009 using vented static chambers, gas chromatographic and soil chemical analysis. The NUMEX-experiment was established in a stratified random design with three replicate plots per treatment and 20-m x 20-m area each plot at three sites along an altitudinal gradient in the Ecuadorian Andes (1000-m, 2000-m and 3000-m elevation). Fertilizers were applied at the rates of 50 kg N ha-1 yr-1 (in the form of urea) and 10 kg P ha-1 yr-1 (in the form of NaH2PO4 . 2H2O with analytical grade quality) split in two equal applications per year starting in February 2008. At the 2000-m and 3000-m elevations, where an organic layer was present and vegetation growth did not respond to nutrient additions, net rates of soil N cycling and N2O emissions started to increase following N and N+P addition after the third nutrient application in 2009 but the effects were less pronounced at the 3000-m elevation. At 1000-m elevation, where an organic layer was absent and vegetation growth did not respond to nutrient additions, net rates of soil N cycling and N2O emissions started to increase following N and N+P addition after the second nutrient application in 2008. Addition of P alone had no effect on net rates of soil N cycling and N2O emissions at any elevation.(3) Soil respiration and litterfall was measured in the NUMEX-experimental plots from January 2008 to September 2009 using vented static chambers, gas chromatographic analysis, litter traps and leaf chemical analysis. At the 2000-m and 3000-m elevations, where an organic layer was present and vegetation growth did not respond to nutrient additions, soil respiration increased following N addition which may be caused by a stimulation of microbial activity in the organic layer, leading to an increase of heterotrophic respiration. At the 1000-m elevation, where an organic layer was absent and vegetation growth did not respond to nutrient additions, soil respiration did not respond to nutrient addition.The results of this thesis show that CH4 emissions from tank bromeliads and probably other types of phytotelmata in canopy wetlands may help to explain the unidentified CH4 source strength of neotropical forests. Secondly, the response of net rates of soil N cycling, soil N2O fluxes and soil respiration to N and P additions in tropical montane forests depend mainly on the amount of nutrient added and on the soil nutrient status which may primarily be controlled by the presence or absence of an organic layer and may be independent from nutrient limitation of vegetation. Finally, projected increases in atmospheric N deposition in tropical regions may induce immediate losses of N and C through soil N2O and CO2 emissions from tropical montane forest ecosystems.