Effects of three-dimensional soil heterogeneity on the structure and functioning of plant communities in experimental mesocosms
31 August 2018
Campus Drie Eiken, O7 - Universiteitsplein 1 - 2610 Antwerpen-Wilrijk (route: UAntwerpen, Campus Drie Eiken
Organization / co-organization:
Department of Biology
Ivan Nijs & Hans De Boeck
PhD defence Yongjie LIU - Faculty of Science, Department of Biology
Soil resources such as water and nutrients are generally distributed heterogeneously, i.e. in a patchy manner. Exploring soil heterogeneity under natural conditions is complicated since many co-occurring factors make it difficult to precisely link cause and effect. Controlled experiments where other factors except soil heterogeneity are kept constant can facilitate this research. However, such experiments have been conducted in two dimensions (either horizontally or vertically) up to now, either by injecting nutrients or adding fertilizer in a clumped pattern, or by spatially redistributing different soil layers or soils from different locations. With the former method, it is difficult to achieve a stable patch size, while the latter method includes complex plant-soil feedbacks which may obfuscate the plants’ responses to heterogeneity. Moreover, natural soils are usually heterogeneous in three dimensions (i.e. both horizontally and vertically), yet, to our knowledge, no standard method exists to create soil heterogeneity in 3D. The aim of this thesis was to develop a novel technique to create soil heterogeneity in three dimensions, and then to apply it to investigate the effects of heterogeneity on root distribution and plant diversity and to explore the response of plant communities when drought and soil heterogeneity co-occur. Species such as legumes that can significantly modify soil heterogeneity were excluded from our experiments to facilitate interpretation.
To create 3D-heterogeneity, we designed a technique with which containers of flexible size could be filled layer by layer using plastic slits in different arrangements. This makes it possible to modify the two main components of soil heterogeneity, i.e. qualitative and configurational heterogeneity. The former can be altered by varying the substrate types, which can differ in nutrient content, soil water holding capacity, etc., while the latter can be adjusted by changing the dimensions of the cells that make up the ‘soil’.
We subsequently explored the effects of soil heterogeneity on the root distribution of plant communities by conducting an experiment where configurational heterogeneity was modified using a nutrient-rich and a nutrient-poor substrate. This resulted in soils looking like 3D chessboards, with the cell sizes (inversely related to heterogeneity) varying along a gradient (0, 12, 24 and 48 cm). Twenty-four species occurring in grasslands in Belgium were sown onto these ‘mesocosms’. Roots were harvested at the end of the experiment, where they were separated by substrate type in each horizontal layer. Mesocosms with relatively higher heterogeneity had more shoot biomass while root biomass was constant across the heterogeneity gradient. This is consistent with our expectation that greater proximity to nutrient-rich patches allows plants on nutrient-poor patches to invest relatively less in roots. More heterogeneous soils also yielded spatially more heterogeneous root systems, i.e. with root biomass that diverged more between nutrient-poor and nutrient-rich cells. This suggests that plants growing on nutrient-poor patches can more easily grow into adjacent nutrient-rich cells at higher soil heterogeneity.
Thirdly, we investigated the effects of soil heterogeneity on plant diversity, i.e. soil heterogeneity-diversity (SHD) relationships, in the abovementioned experiment. At the end of the experiment species richness and plant abundance were recorded in all the mesocosms and were used to calculate different diversity indices. A unimodal SHD relationship was found, with a peak at cell size 12 cm, which originated mainly from the increased diversity on nutrient-rich substrate. This is the first time such a relationship was found in experimental heterogeneity studies for plant communities. Possibly, this unimodal pattern is caused by lack of sufficient resources to support high diversity on very small nutrient-rich patches, and strong competitive exclusion within large nutrient-rich patches. This pattern carries a similarity with other unimodal responses of plant species diversity, notably in diversity-disturbance and diversity-productivity relationships. Strikingly, plant density increased monotonically with increasing soil heterogeneity, indicating that, at least up to 12 cm-sized-cells, not only more species but also more individuals were able to coexist in mesocosms with smaller cell size.
Finally, ongoing climate change significantly influences both soil heterogeneity and the growing plant ecosystem. Drought events in temperate regions are expected to increase in intensity and/or duration. To explore the responses of plant communities to the joint effects of soil heterogeneity and drought, a new experiment similar to the one described above was set up. Drought lasted three weeks in summer 2016. At mesocosm scale, soil water content (SWC) decreased more slowly at large (48 cm) than at smaller (0-12-24 cm) cell size, which coincided with a slower loss of canopy greenness. These responses mainly originated from nutrient-poor substrate, whereas the rate of decline of SWC and canopy greenness on nutrient-rich substrate was indifferent to cell size. The slower decrease of SWC and canopy greenness on large nutrient-poor patches coincides with smaller shoot and root biomass compared with smaller nutrient-poor patches. This can, in turn, be explained by more difficult access to resources on large nutrient-poor patches. After the drought, plants recovered faster on nutrient-rich than on nutrient-poor substrate, likely owing to the greater availability of resources. Overall, our results indicate that soil heterogeneity can negatively modulate the responses of plant communities to drought events, implying that heterogeneity should be considered when assessing climate change impacts in terrestrial ecosystems.
The results presented in this thesis improve our understanding of the effects of soil heterogeneity on plant communities. Higher heterogeneity allows plants on nutrient-poor patches to more easily retrieve resources from neighbouring nutrient-rich substrates, which may in turn improve species co-existence. However, such a trend might not hold for the very small cell sizes since plant diversity declined again at highest heterogeneity level. Moreover, plants growing on soils with the highest heterogeneity responded differently to drought. Thus, especially the responses at very high heterogeneity and the joint effects of heterogeneity and climate change merit further research. It would be interesting to also consider species aggregation (clumping), as this can further modify responses because clumping reduces interspecific competitive exclusion processes, potentially resulting in improved survival of competitively weak species.