Joren Bruggink

Many nature areas in the Netherlands are currently under high ecological pressure due to long-term excess nitrogen deposition. This increase in nitrogen availability disrupts ecosystems through many pathways. For one the higher nitrogen availability stimulates the productivity of plants and thereby facilitates the growth of fast-growing species, which causes asymmetrical competition and eventually a loss in species. Additionally, nitrogen deposition causes soil acidification, which leads to the leaching of important base cations (calcium (Ca2+), potassium (K+) and magnesium (Mg2+)) and the release of aluminium (Al3+), which is toxic for plant roots. Excess nitrogen deposition also causes ecological imbalance of nitrogen: phosphorus (N:P) ratio's in the soil, that cascades up to plants. This leads to a P limitation in plants and subsequently for the herbivores. P is a crucial element for the growth and reproduction of organisms and this limitation will affect the fitness of consumers. As a result of these effects, many forests in the Netherlands are in a degraded state. Sites on mineral poor, sandy soil are particularly vulnerable. In these forests we observe a decline in tree vitality and an accumulation of soil organic matter in the form of plant litter. Restoration of these vulnerable habitats has been an important issue for nature management. The application of silicate rock powder (SRP's) has recently emerged as a potential restoration method. SRP's are expected to assist with soil buffering by increasing the pH in the soil and with replenishing the soil with base cations. The early results of this method are promising, but its effect on the fauna community has barely been studied. My thesis will therefore focus on an important soil animal group: the soil microarthropods. Soil microarthropods consist mainly of mites (Acari) and springtails (Collembola), but also feature other animal groups within the size class of 0.1 to 2 mm. Soil microarthropods are an extremely numerous and species rich group of fauna on a local scale, which makes them an interesting target for ecological studies. Furthermore, this group of animals has an important role in decomposition processes in the soil. The exact role of various species depends on their feeding strategy, however. Therefore, I do not only distinguish between herbivores, fungivores and predators, but I will also separate species into grazer and browser strategies. Grazers can feed on both fungal cell walls and the cell content, while browsers only feed on cell content. Fungivorous grazing species in specific are important for decomposition since they can breakdown (old) fungal cell walls, which releases nutrients that stimulate the growth of other fungi. Without these fungivorous grazers there is risk that fungal matter is not decomposed well and that those nutrients stay locked in the system. Fungivorous browsers, in contrast, can only feed on living fungal cells and therefore negatively impact decomposition. Soil microarthropods are generally vulnerable to disturbance of the soil, however, and the large changes in the ecosystem caused by nitrogen deposition could therefore have detrimental effects on the species richness in these forests. Moreover, many species have poor dispersal ability, meaning that they cannot recover after species-loss, even if conditions improve. The aims of my thesis therefore are three-fold: Aim (1) to study the current status of soil microarthropod richness in Dutch forests on mineral poor, sandy soil. Aim (2) to evaluate how the current soil microarthropod community functions in relation to litter decomposition in these forests. Aim (3) to study the effectiveness of two restoration methods: SRP application and soil inoculation with microarthropods (to help with dispersal). Aim (1) I worked on in Chapters 2 and 3. In Chapter 2 I conducted a dataset study of 32 forests in the Netherlands to determine how soil age and P availability shape the soil microarthropod community. Of these 32 forests, 3 were old forest soils, 10 sites were old hedgerows and the other 19 were young forest soils The old forests and hedgerows have been forested since the Middle Ages, while young forests have been established from heathlands between 1880 and 1930. Old hedgerows are remnants of old forests, but adjacent to agricultural land and therefore have been subjected to a constant P-influx from fertilization. In my research I found a significantly higher species richness of soil microarthropods in the old forest locations compared to young forest soils, showcasing the dispersal limitation. Specifically, the fungivorous grazer species that are important for decomposition, were much lower in younger forests. While the overall species richness of old forests and old hedgerows were similar, the shape of the community was still different, which I attribute to the different P levels. Old hedgerows had a higher species richness of herbivorous mite species and a higher density of larger species. Plants are less stable in their N:P compared to fungi leading to herbivores being more vulnerable to P-limitation and furthermore the allometric effect of P-limitation means that larger species are more vulnerable as well. In Chapter 3 I researched the status of soil microarthropods in the old oak forest habitat in the Veluwe and compared it to the old forest sites studied in Chapter 2 as a reference. Since Chapter 2 established that many fungivorous species seem to be vulnerable to disturbance, I focused on this group during this study. Both the species richness and abundance of fungivorous mites was indeed low in the old oak forests compared to the reference data. From literature it is known that the species richness and biomass of fungi also decline from nitrogen deposition with a specifically strong effect on the ectomycorrhiza group. Since ectomycorrhizal fungi are likely an important food source for many fungivorous species, I hypothesize that this decline in food source is an important reason for the decline of fungivorous mites and to the slowing down of decomposition that has been noted in this habitat. In Chapter 4 I therefore studied Aim (2) with the use of a 21-month litterbag study conducted in 9 forest sites. Using 8 different sampling dates, I collected 5 litterbags in each location and studied the soil microarthropod community in relation to the decomposition of Quercus robur leaves. The first 6 months were in the early phase of decomposition, which consists of a rapid decomposition of the easily degradable compounds. During this period there was a dominance of herbivorous species that take advantage of the still relatively high nutrient levels in the leaves. After 6 months there was a shift from herbivores to a dominance of fungivorous species. The decomposition speed slows down during this period as it mostly concerns the decomposition of recalcitrant lignin-bound compounds during this point. Fungi play an important role of the decomposition of these compounds, which explains the increase in fungivores. I found an inhibition of decomposition at this point, signalling a potentially inefficient breakdown of recalcitrant material. There was a significant association of fungivorous grazer species richness and decomposition speed during this period as well, which suggests that noted decline of fungivorous grazers in Chapter 3 could also partly explain the reduction in decomposition. In Chapters 2, 3 and 4 it becomes clear that the current status of the soil microarthropod community in these forests is not sufficient for a healthy ecosystem. In Chapter 5, therefore, I focused on Aim (3) to study if two restoration methods can help the soil microarthropod community. In 5 old oak forest sites I established a full factorial experiment using either three SRP treatments (Eifelgold, Soilfeed and Control) and two inoculation treatments (inoculated or null). The old forest site Ugchelen was used as donor site for inoculation since it was in the best state from the three old forest sites. SRP was indeed able to improve the abiotic soil conditions by increasing the pH by 0.3 units and increased the Ca, K and Mg levels. This resulted in a higher biological activity with Eifelgold having higher proportion of fungivorous browsers and Soilfeed having higher proportions of predators. Fungivorous browsers are species with a generally quick life cycle and only feed on living fungi cells and are therefore a good indication of higher fungal activity. Predatory species are indication of higher prey availability. The species with the highest abundance increase, Pachylaelaps bellicosus, is a known nematode predator. Soil inoculation, however, only had limited effect on the species richness. There was a small increase in herbo-fungivorous grazers, which are important for decomposition, however this increase is likely too small to have a significant effect on the decomposition. In my synthesis (Chapter 6) I evaluated the three research aims using the results from the previous research chapters. For research Aim (1) it is clear that the soil microarthropod community is under high pressure in many Dutch forests. The limitation of P and the reduction in food source have lowered the species richness, while the dispersal limitation prevents natural restoration of the community. Particularly the decline in fungivorous species is worrying for the decomposition process. Which leads me to research Aim (2) where my results indeed show that the reduction in fungivorous grazers can be linked to a decline in decomposition speed of plant litter, which provides an explanation for the accumulation of soil organic matter in acidified forests. For research Aim (3) I found that the restoration of the soil microarthropod community is a difficult challenge as well. While SRP application helps the abiotic status, the pH and base concentration is still under critical levels, and the question is whether the increase in biological activity is successful in the long term. Especially since we have not been able to improve species richness with soil inoculation. The old forest locations in the Netherlands have declined too strongly due to nitrogen deposition as well. Potentially, forests of the same habitat in other European countries can be considered as donor instead. Regardless, my thesis gives more data on the negative impact of excess nitrogen deposition and reducing the deposition to below critical levels will be essential for any hope in restoration of these forest habitats.