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Both herbivorous and bacterivorous microfauna have been shown to influence root development, soil nitrogen (N) mineralization, and plant productivity. However, our knowledge of these effects is limited as multitrophic interactions remain largely unexplored. We investigated whether and how herbivorous nematodes (Pratylenchus zeae) and bacterivorous nematodes (Poikilolaimus oxycercus), alone and in combination, affect plant biomass (Lolium multiflorum) through root traits and/or soil N mineralization. Bacterivorous nematodes increased, whereas herbivorous nematodes decreased, plant productivity. We found that root trait coordination in response to soil microfauna was consistent with the concept of root economics space. The negative interaction between herbivorous and bacterivorous nematodes on plant productivity at high herbivorous nematode infestation could be explained by reduced N mineralization and variation in the root nitrogen concentration-root tissue density (RNC-RTD) axis aligned with increased herbivory intensity. This study revealed that herbivorous and bacterivorous nematodes moderated each other's effect on plant productivity via root trait coordination and N mineralization, and suggests, for the first time, the value of the root economics space concept for interpreting phenotypic root plasticity and functioning in response to local biotic factors.

Soil microbial communities (bacteria, fungi, archea, protozoa) directly and indirectly influence all other trophic interactions in soil. Partial soil sterilization by gamma irradiation is emerging as a promising defanauting technique to remove specific target group of organisms with minimal soil disturbance. While considering the selective elimination of a target organism, it is crucial that microbial community has recovered to its original state. The microbial community should return back to its original state shortly after partial sterilization, but to date there are no studies on the dynamics of microbial recovery following partial sterilization.

Soil organic matter (SOM) quality and iits decomposibility is influenced by cropping pattern. However, little is known on this matter, particularly so far rice monoculture or comination of vegetables with rice culture. Therefore, four pairs of fields having vegetable-rice and rice-rice cropping pattern were selected from four different locations in Bangladesh, covering floodplain and terrace soils. Soils were first physically fractionated into particulate organic matter (POM) and silt and clay sized OM. The silt and clay sized OM was further chemically fractionated by oxidation with 6% NaOCl to isolate an oxidation-resistant OM fraction, followed by extraction of mineral bound OM with 10% HF (HF-res OM). The results show that there is a small increase in POM in vegetable compared with rice soils. Among the chemical treatments, NaOCl oxidized the largest amount of OC and N from the silt and clay fractions of vegetable soils. The silt and clay N in vegetable soil was found to be more susceptible to NaOCl treatment compared with OC. Both the HF-extracted (HF-ex) and HF-res OM fractions were found to be lower in vegetable soils compared with rice soils. Results from both physical and chemical fraction of SOM suggest that SOM accumulated in vegetable-rice cropping pattern are more labile than the solely rice based cropping pattern and prone to decompose quickly in any change of land use.

Elevated depositions of non-marine sulphate (SO42-) and inorganic nitrogen (N), as a consequence of air pollution, resulted in a progressive acidification and eutrophication of Flemish forests. Since the 1980s emission abatement reduced the acidifying and eutrophying emissions and depositions in Flanders. This thesis aimed to evaluate the impact of the evolution in depositions on soil solution chemistry, using long-term data collected in 5 plots of the ICP Forests monitoring (Level II) network in Flanders. The sharp decrease of SO42- and ammonium (NH4+) depositions made that abiotic N status started to improve and acidification slowed down during the past two decades. However, N depositions are still far above the critical loads for ectomycorrhiza and epiphytic lichens. Given the still very low soil pH (3.5‒4.5) unfavourable for microbial life, the generally observed tendency of increased dissolved organic carbon (DOC) and nitrogen (DON) mobility is likely a direct result of lowered ionic strength and partly rise in pH. Abiotic recovery is delayed by a simultaneous decrease in the deposition of base cations (Ca2+, K+ and Mg2+) and SO42- desorption. Biotic recovery is lagging behind on the changes in soil solution chemistry, as indicated by the stable but unbalanced tree mineral nutrition. Acidification and eutrophication will likely continue to produce after-effects for many decades. The results from this thesis indicate that the Programmatic Approach to Nitrogen (PAS) is partly missing its target for oxidized N compounds and that extra measures will be necessary to bring NOx emissions at an acceptable level.