Impacts of leaf litter diversity and root resources on microorganisms and microarthropods (Acari, Collembola) during early stages of decomposition in tropical montane rainforest ecosystems

Soil organisms influence organic matter turnover and nutrient cycling via processing of organic matter entering the soil as litter and root-derived resources. Plant species differ enormously in the quality and quantity of litter and roots that they produce, and this diversity strongly modifies decomposition of litter by decomposer organisms. Higher plant diversity is generally assumed to improve habitat conditions and availability of resources, thereby improving the abundance and activity of decomposer organisms. Tropical Andean montane rainforest ecosystems harbor an exceptional diversity of plant and animal species. However, little is known on how the huge diversity of plants and root resources affect the activity of soil communities and the overall decomposition rates, particularly during early stages of decomposition. This thesis aims to contribute to our understanding of the effects of leaf litter diversity and root resources on microorganisms and decomposer microarthropods during the early stages of litter decomposition in Andean tropical montane rainforest ecosystems. The studies were performed as field experiments at 2000 m (Chapter 2 and 4) and along an altitudinal gradient from 1000 to 2000 to 3000 m (Chapter 3) in a tropical montane rainforest in Southern Ecuador. Chapter 2 investigates the effect of leaf litter diversity and identity on microbial functions and microarthropod abundance. The results suggest that decomposition and microbial parameters in litter vary with litter diversity as well as litter identity, while microarthropods respond only to litter identity. The results show that higher levels of diversity detrimentally affect soil microbial biomass and result in a decline in litter decomposition. Further, the results indicate that the differential response of soil biota was mostly due to differences in the initial chemical composition of litter species. However, the results also highlight the importance of leaf litter physical traits, particularly on the abundance of decomposer invertebrates. Overall, the results indicate that litter species identity functions as major driver of the abundance and activity of soil organisms and thereby exerts distinct effects on ecosystem processes such as decomposition and nutrient mobilization. Chapter 3 investigates the contribution of soil microbes and decomposer microarthropods to the decomposition of leaf and root litter along an altitudinal gradient of the studied tropical rainforests. The results suggest that the decomposition of both leaf and root litter in montane rainforests is mainly due to microorganisms, whereas the effect of microarthropods is minor along the altitudinal gradient. However, at higher altitudes soil microarthropods accelerate the decomposition of low-quality litter, such as root litter. Further, the study suggests that the abundance of microorganisms as food is of minor importance in structuring decomposer microarthropod communities, underscoring the role of litter quality. Overall, our findings highlight that resource quality or local interspecific variation in litter quality has stronger effects on decomposer organisms regardless climatic variations associated to altitude, at least during early stages of decomposition. Chapter 4 investigates the response of arbuscular mycorrhizal (AM) fungi, microorganisms and microarthropods to the rotation of hyphal-ingrowth cores, defaunation and nitrogen addition. The results suggest that in the study site AM fungi are closely associated with living roots and do not form extensive extraradical hyphae that can be cut by rotation of the cores. Nonetheless, the results suggest that on top of the litter layer, AM fungi likely compete with saprotrophic microorganisms for litter-derived resources, with mycorrhizal fungi suppressing the activity of saprotrophic microorganisms. While in the soil layer interactions of mycorrhizal fungi with other soil biota are restricted to the close vicinity of roots. Nitrogen addition increased the quality of litter material produced by plants and beneficially affected microbial activity, highlighting that decomposition processes in the studied montane rainforests are strongly limited by nutrient availability and microorganisms in these forests even respond to moderate increase in nitrogen. The results also document a restricted recovery of microorganisms and microarthropods after defaunation of the rotated cores, highlighting the importance of root-derived resources for fueling soil food webs. Chapter 5 presents a discussion and conclusions on the contribution of the research chapters to the overall state of knowledge. Generally, the results of this thesis suggest that during early stages of decomposition the abundance, diversity and activity of soil organisms are strongly associated with the quality and availability of the litter resources. Overall, the results suggest that decomposition processes in montane rainforests at early stages are mainly driven by microorganisms, whereas the contribution of microarthropods is of minor importance. Further, the results also highlight the importance of root-derived resources for fueling soil microarthropod abundance during early stages of decomposition. In addition, the results point to AM fungi as an important player for determining the abundance and activity of microbial communities during early stages of decomposition in tropical montane rainforests.