|Decomposition processes in the high tropical Andes are complex and influenced by a variety of biotic and abiotic factors. Tropical montane rainforests of the high Andes harbour large stocks of dead organic material and little is known on the regulatory forces responsible for this accumulation of carbon. Litter quality, climate and the decomposer community are known from other systems as the main factors controlling decomposition rates. Microarthropods play a major role in regulating decomposition processes due to the impact they have on their surrounding habitat. They are known to regulate organic matter turnover and influence nutrient cycling via direct and indirect processing of organic matter entering the soil system as both litter fall and root exudates. However, there is a lack of knowledge on interrelationships between litter decomposition and the structure of soil microarthropod communities in tropical montane rainforests.
This thesis focuses on disentangling the relative importance of regulatory forces, e.g. litter quality and local biotic and abiotic conditions, in regulating long term decomposition processes and variations in the microbial and microarthropod community composition. Further, I investigated the role of root derived resources in structuring the soil fauna community and in regulating decomposition processes. The studies were performed as field experiments along an altitudinal gradient from 1000 to 2000 to 3000 m in a tropical montane rainforest in Southern Ecuador.
In the first study, presented in Chapter 2, I investigated the influence of litter quality, i.e. litter origin, and altitude on long term decomposition rates of both leaf and root litter in the tropical montane rainforests. The results suggested that the pronounced accumulation of dead organic material at higher altitudes at least in part is caused by reduced or even ceasing decomposition rates after 12 months. The study demonstrated that litter quality is far less important in controlling decomposition rates than previously assumed. Rather, site specific and altitude dependant biotic and abiotic factors are most relevant. Presumably, the observed long term decomposition patterns are caused by different forest floor types at the investigated altitudes as well as different rhizosphere interactions between plants and the belowground community. Litter materials are generally poor in nitrogen and do not allow effective decomposition by saprotrophic microorganisms and this applies in particular to condensed litter compounds such as lignin. The results suggest that poor nutrient conditions in organic litter layers at high altitudes inhibit microorganisms to decompose litter materials due to the lack of contact to nutrients in the mineral soil. Presumably, restriction of microorganisms to root derived resources in the organic layer results in a closer linkage between plants and the decomposer community exploiting litter resources and this linkage is most pronounced at higher altitudes.
Parallel to decomposition processes, in a second study I investigated changes in the abundance and diversity of the most abundant microarthropod groups in soil, i.e. Oribatida and Collembola, as well as Oribatida diversity and species composition, during litter decomposition (Chapter 3). As in the first study presented in Chapter 2, I investigated the role of litter quality (i.e. litter origin), altitude and litter type (root and leaf litter) as driving factors in order to quantify their role in regulating the composition and abundance of the microarthropod community. The parallel investigation of decomposition processes and microarthropods in one experiment allowed linking driving factors of both litter decomposition and microarthropod community composition. Similar to the results of the first study quality of litter material was of little importance as driving force for the composition of the microarthropod community. Rather, factors changing with altitude were most important, with both the abundance of microarthropods as well as the diversity of Oribatida decreasing with increasing altitude. Decomposition stage also strongly affected the soil microarthropod fauna with the changes in abundance and diversity occurring over time being closely associated with changes in the speed of litter decomposition and associated changes in microorganisms. Surprisingly however, microarthropods were unable to benefit from the flourishing microbial community at the early decomposition stage, potentially due to the presence of plant secondary compounds such as phenols, and the dominance of mycorrhizal fungi. At later stages microarthropods appear to benefit from the dominance of saprotrophic microorganisms decomposing more recalcitrant litter compounds, either by feeding on these microorganisms directly or by indirectly benefiting from the conditioning of the litter by microbial enzyme production.
The results of the first two studies suggest that in particular at higher altitude root-derived resources are major drivers of decomposition rates, microorganisms and soil microarthropods in the investigated tropical montane rainforests. The third experiment, presented in Chapter 4, therefore focused on investigating the effect of the exclusion of roots and mycorrhizal hyphae on litter decomposition and soil fauna community composition. The results from this study suggested that mycorrhizal fungi and saprotrophic microorganisms compete for litter-derived resources, with mycorrhizal fungi suppressing the activity of saprotrophic microorganisms at each of the investigated altitudes. Collembola were the only microarthropods to benefit from mycorrhiza exclusion and reduced competition between mycorrhizal fungi and saprotrophic microorganisms, indicating that they depend on litter-derived resources. Total soil microarthropod density as well as Oribatida diversity, however, were reduced if roots and mycorrhizal hyphae were excluded, highlighting the importance of root-derived resources for fuelling soil food webs. Along the altitudinal gradient variations in microarthropod abundance and decomposition rates did not fit the patterns observed in the studies presented in Chapters 2 and 3 suggesting that plant-soil fauna interactions influencing belowground ecosystem processes vary with season, e.g. with drought or drought related changes, especially at lower altitude.
By highlighting the importance of altitude and root derived resources for long term decomposition processes and the belowground community structure the results of this thesis improved the understanding of regulatory forces controlling decomposition processes as well as microarthropod abundance and diversity in tropical montane rainforests.