Trace gas fluxes and belowground carbon allocation in tropical montane forest soils of Southern Ecuador

Abstract

Tropical forest soils are important sources of the greenhouse gases carbon dioxide (CO2) and nitrous oxide (N2O), and of nitric oxide (NO), a precursor of ozone production. They are also sinks for the greenhouse gas methane (CH4). Estimates on soil trace gas fluxes and on carbon cycling from tropical forest soils are heavily biased towards lowland forests. Limited data is available on trace gas exchange from tropical montane forest (TMF) soils and their carbon cycle has only been marginally explored, although TMFs cover approximately 9% of the tropical forest area. This dissertation presents the results of a comprehensive study on soil CO2, N2O, NO, and CH4 exchange and possible biogeochemical regulators, with a special focus on the influence of nutrient availability in TMFs of southern Ecuador. Soil CO2, N2O and CH4 fluxes were determined using static closed chambers and gas chromatographic analysis at three sites along an elevation gradient from 1000 m to 3000 m (1000 m, 2000 m, 3000 m) and along topographic gradients (lower slope, midslope, ridge),. NO fluxes were measured in the field using open dynamic chambers and a LMA-3D NO2 Analyzer with chemiluminescence detection. To determine the potential atmospheric CH4 uptake of different soil substrates, a laboratory incubation experiment using organic layer and mineral soil samples from different soil depths was conducted. Nutrient availability was determined by applying various extraction methods and by evaluating several indices (C:N, C:K, C:P ratios, d 15N signature of litterfall). Finally, total belowground carbon allocation (TBCA) was estimated from year-round soil respiration and aboveground litterfall measurements. Soil organic layer thickness increased while nutrient availability decreased with both increasing elevation and along the topographic gradient from the lower slope to the ridges. Aboveground litter production appeared to be limited by nitrogen (N), phosphorus (P) and to a certain degree by potassium (K). It was positively related mainly to available nutrient stocks of the organic layer, although stocks of K and P were larger in the mineral soil. Tree basal area increment and aboveground litter production showed close positive correlations with indices of N availability (C:N ratio and d 15N signature of litterfall), while TBCA was not correlated to nutrient availability. TBCA decreased with elevation and from the lower slope position toward the ridges and was negatively correlated to soil moisture and organic layer thickness. Soil trace gas fluxes from the three TMF sites were lower than reported for tropical lowland sites but were generally within the range reported in other montane forest sites. Soil respiration was positively correlated with both parameters, whereas no correlations of N-oxides and CH4 with soil moisture and organic layer thickness were found. N-oxide fluxes showed close positive correlations with long-term indices of N availability (e.g. C:N ratio, the d 15N signature of litterfall) and with indices of forest productivity, such as aboveground litter production and tree basal area increment. Soil respiration was positively correlated with litter quality indices. CH4 uptake rates increased with soil mineral N content, total P content of the mineral soil and with increasing CO2 emissions. Incubated samples from the deepest organic layers at the 2000 m and 3000 m sites revealed high potential CH4 oxidation rates. The close correlations of nutrient availability with aboveground litterfall and tree basal area increment is an evidence that forest productivity is nutrient limited in the investigated TMFs. Dense rooting of the organic layer and the close relationships between nutrient availability in the organic layer and nutrient concentrations in aboveground litterfall, suggest that nutrient cycling is concentrated in the organic layer and is largely decoupled from the mineral soil. In contrast to aboveground forest productivity, TBCA was not related to nutrient availability. The decline in TBCA with increasing elevation at our study site corresponded with an increase in fine root biomass which can be explained with higher root longevity. Together with slow decomposition rates in the organic layers, this may contribute to the large carbon storage in organic layers of TMF soils. Soil trace gas fluxes were correlated positively with different indices of nutrient availability. We found evidence that increasingly adverse soil conditions (low nutrient availability, high soil moisture) and lower litter quality limit microbial activity and consequently reduce soil respiration, N-oxide fluxes and CH4 uptake capacity of the soils. The close linear correlations of N-availability with forest productivity and N-oxide fluxes show that N availability links N-oxide fluxes and forest productivity and opens the possibility to include forest productivity, especially tree basal area increment, as co-variable to predict N-oxide fluxes in nitrogen limited TMFs. The largest CH4 uptake corresponded with high concentrations of soil mineral N indicating that atmospheric CH4 uptake primarily nutrient-limited, and that ammonia inhibition may be neglected as regulating factor for soil CH4 oxidation in these soils. Contrary to findings in temperate forests the organic layer did not only act as gas diffusion barriers, but showed substantial potential to oxidize atmospheric CH4.