Patch level plant-soil feedback heterogeneity: A matter of scale?
Plant-soil feedback (PSF) studies have become a central component of our understanding of many terrestrial plant community processes. Recent work has shown that spatial interactions in PSF can strongly influence the plant performance. However, while theoretical work has included the spatial scale of PSF interaction, emperical data on the role of the spatial scale (grain) of PSF heterogeneity is lacking.
In a greenhouse experiment we explicitly manipulated the spatial grain of PSF heterogeneity within experimental units, with coarse- (12x12cm patches) and finegrained (6x6cm patches) treatments, as well as homogeneous controls. Soils with different PSF were placed in patches using a customized grid that was removed when the treatments were setup. Monocultures of six common grassland species were planted full factorially with the soil treatments. The homogeneous soil treatments were used as a baseline for the performance in heterogeneous PSF conditions. Our hypothesis was that 1) plant monocultures would be affected by spatial PSF heterogeneity because roots would encounter different soil communities in the different patches and 2) that the effect would be most pronounced in the fine-grained heterogeneity treatments as roots on average would encounter shifts in microbial community more frequently and earlier in their life-cycle.
Spatial PSF heterogeneity has pronounced effects on plant performance, which in general was reduced in the heterogeneous PSF treatments (~10% reduction in biomass). The effects were particularly strong belowground. Although the strenght of the effect of PSF heterogeneity varied strongly across test species and soil conditioning, most were negative, in fact we found only one case where a species performed better on heterogeneous PSF soil than on homogeneous soils. Surprisingly, the scale of PSF heterogeneity, however, did not affect plant performance.
We conclude that spatial PSF heterogeneity reduces plant performance. At the scale of our experiment the grain of heterogeneity played a minor role. We think the heterogeneity effect is driven by more diverse set of PSF histories encountered in the heterogeneous treatment. Since plant roots grew throughout the experimental containers and soil biota can use these root systems to disperse, the effect of spatial grain at this scale dissapates quickily without the conditioning host. Our results suggest that in the field it may be more difficult for any plant to gain dominance than would be expected from spatially homogeneous PSF studies, as it would have to colonize patches with a diverse set of PSF histories.