Why does fertilization reduce plant species diversity? Testing three competition-based hypotheses

1 Plant species diversity drops when fertilizer is added or productivity increases. To explain this, the total competition hypothesis predicts that competition above ground and below ground both become more important, leading to more competitive exclusion, whereas the light competition hypothesis predicts that a shift from below-ground to above-ground competition has a similar effect. The density hypothesis predicts that more above-ground competition leads to mortality of small individuals of all species, and thus a random loss of species from plots. 2 Fertilizer was added to old field plots to manipulate both below-ground and above-ground resources, while shadecloth was used to manipulate above-ground resources alone in tests of these hypotheses. 3 Fertilizer decreased both ramet density and species diversity, and the effect remained significant when density was added as a covariate. Density effects explained only a small part of the drop in diversity with fertilizer. 4 Shadecloth and fertilizer reduced light by the same amount, but only fertilizer reduced diversity. Light alone did not control diversity, as the light competition hypothesis would have predicted, but the combination of above-ground and below-ground competition caused competitive exclusion, consistent with the total competition hypothesis.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75695/1/j.1365-2745.2001.00662.x.pd

Interactions between soil properties, soil microbes and plants in remnant-grassland and old-field areas: a reciprocal transplant approach

Background and aims: The importance of plant-soil feedback is becoming widely acknowledged; however, how different soil conditions influence these interactions is still relatively unknown. Using soil from a degraded old-field and a remnant grassland, we aimed to explore home-field advantages in plant-soil feedbacks and plant responses to the abiotic and biotic soil conditions. We quantified the soil bacterial and fungal community from these sites and their responses to soil conditions and plant species. Methods: Sterilized old-field and remnant-grassland soil was inoculated with home or away soil in a reciprocal transplant experiment using a native grass, Rytidosperma auriculatum, and an invasive grass, Avena barbata, as test species. The soil fungal and bacterial communities were characterised using high throughput sequencing. Results: Plants had a greater growth response to microbes when an inoculant was added to its home soil. However, this relationship is complex, with microbial communities changing in response to the plant species and soil type. Conclusion: The apparent home-field advantage of the soil microbes shown in this study may restrict the utility of inoculants as a management tool. However, since we inoculated sterile soil, future work should focus on understanding how the inoculated microbial community interacts and competes with resident communities.Monique E. Smith, José M. Facelli, Timothy R. Cavagnar

Biological soil crust and vascular plant interactions in Western Myall (Acacia papyrocarpa) open woodland in South Australia

Aim: Biological soil crust (BSC) is a key component of arid environments and plays a major role in determining ecological structure and function. Our research aim was to examine several mechanisms that contribute to BSC–plant interactions at species and community levels, to increase our understanding of plant community dynamics. Location: Yellabinna Regional Reserve, South Australia (30°50′17.99″ S, 132°12′10.37″ E). Methods: We used seed extraction techniques to examine soil seed banks beneath patches with and without BSC, and field and glasshouse experiments to study the effects of crust presence and disturbance on seedling emergence and survival. We also explored the influence of chemical leachates from BSC on seed germination. Results: Biological soil crust plays a pivotal role in influencing spatial patterns in soil seed bank composition. Our results showed less propagules accumulate in soils beneath crust when compared with patches without crust, and that propagule size is a contributing factor to determining their distribution between patch types. We found that late‐stage BSC physically inhibited seedling emergence, which increased when the BSC was disturbed in field experiments. Low seedling survivorship in both patch types suggests that although BSC may suppress recruitment in favourable years, it is low precipitation levels that have the over‐riding impact on recruitment success. Finally, leachates from late‐stage BSC were found to inhibit germination in three annual plant species, whilst early‐stage BSC accelerated germination in one annual plant. This clearly shows that leachate effects on seed germination vary according to plant species and crust successional stage. Conclusion: Overall, we identified several mechanisms through which BSC has complex effects on the annual and short‐lived perennial plant guilds of arid lands. These mechanisms contribute to species diversity through the creation of spatial heterogeneity in soil seed bank structure and emergence opportunities.Emma K. Steggles, José M. Facelli, Phillip J. Ainsley, Leanne M. Poun