Environmental changes in biodiversity hotspot ecosystems of
South Ecuador: RESPonse and feedback effECTs (FOR2730)

Abstract:

In nutrient-poor tropical forests litter decomposition is essential for the supply of nutrients to plants. This process is in turn controlled by nutrient availability. Beside nitrogen (N) and phosphorus (P) other elements may co-limitate litter decomposition. Studies revealed a relationship between sodium (Na) shortage and reduced litter decomposition which can decrease the mineralization of stored carbon (C). Investigations of element fluxes of a Na-poor tropical montane rainforest in southern Ecuador have shown low atmospheric deposition rates since 1998. Additionally, Na was retained in different parts of the ecosystem. A study found that the Na-retention in the canopy was related to the Na demand of the microorganisms in the phyllosphere. Because the Na budget of the organic layer was also positive, it was assumed that decomposers may experience a lack of Na due to low atmospheric deposition. Thus, one main goal was to investigate whether Na fertilization accelerates litter decomposition. Based on the assumption of a higher Na demand of soil fauna than of microorganisms, it was also tested whether the stimulation of soil organisms by Na is limited to the soil fauna. To test these hypotheses, a combined litter decomposition and Na fertilization experiment was conducted in an evergreen premontane rainforest and a montane rainforest in south Ecuador. Fresh litter was incubated in the field and fertilized weekly with different Na concentrations which were similar to the natural deposition rates. Litterbags with different mesh sizes allowed to study the effects of Na fertilization on microbes and mesofauna separately. Additionally, a bait lamina experiment was installed in the organic layer to study the uptake of Na fertilized baits by soil organisms. Differences of the bait loss between the fertilized and the control group should indicate a potential stimulation of the soil organisms by a Na-addition. The statistical analysis confirmed the stimulation of litter decomposition by Na additions. The comparison of both sites revealed a slightly stronger effect of Na additions in the premontane forest than in the montane forest. The accelerated litter decomposition was related to an increased activity of the soil fauna. However, a significant stimulation of microorganisms by Na additions could not be detected. The bait lamina experiment confirmed the results of the litter decomposition experiment and showed a marginally significant increase of the activity of soil organisms by Na fertilization. Sodium can stimulate the decomposition of the organic layer in the studied premontane and montane rainforest, thus affecting ecosystem functioning. Na supply is of higher importance in the ecosystem than usually assumed and its relevance for the C turnover in the organic layer may increase if the atmospheric Na deposition remains on the present low level.

Abstract:

Recent studies raise the hypothesis that Na shortage restricts decomposition and affects the carbon cycle in tropical forests. When Na concentrations in soils are low and the stands are far off-coast, they do not receive substantial Na inputs from the atmosphere. Since terrestrial plants have low concentrations of Na, which is not considered as an essential element, the demand of soil fauna may not be covered. Yet, in contrast to animals, little is known on Na demands of phyllosphere microorganisms. This thesis presents results from a study on Na limitation in a montane forest ecosystem in South Ecuador, which is located on the Eastern cordillera of the Andes, in a microcatchment under an undisturbed lower montane rainforest. The study area is characterized by low Na concentrations because of low deposition rates with incident precipitation and by low Na stocks in in the soils and in the organic layer. Sodium fluxes in rainfall, throughfall, stemflow, litter leachate, litterfall and organic layer have been monitored since 1998. Results reveal overall low Na concentrations in the ecosystem fluxes. Higher Na fluxes with incident rainfall than with throughfall suggest that Na is retained in the canopy. Therefore, this study aims at testing the hypothesis that Na is retained in the canopy because of Na limitation of microorganisms in phyllosphere. To explore the role of the phyllosphere in Na retention, I sampled leaves covered by phyllosphere microorganisms and leaves without phyllosphere cover from 12 tree species belonging to 7 plant families frequently occurring in the study area. The fresh leaves were sprayed with a NaCl solution containing 1 mg L-1 Na, corresponding to the Na concentration in incident rainfall in the study area during La Niña events. Comparison with a control treatment excluded effects by abiotic Na fixation on the surface of the leaves. The results show that increasing phyllosphere cover leads to a significantly enhanced Na retention, which is much higher on understory tree leaves than on leaves of the upper canopy. Leaching of K, Ca and Mg was higher with increasing degree of phyllosphere cover, which can be attributed to increasing element exchange between foliage and phyllosphere with leaf age. These results suggest that Na availability possibly plays a regulating role in the study ecosystem which might even grow in importance if Na deposition from the atmosphere continues to decrease or stabilizes at the current low level.