The spectral quality of light influences the temporal development of the microbial phenotype at the arable soil surface

The uppermost zone of soil represents the primary interface between the above- and below-ground compartments of terrestrial ecosystems and is functionally important since it affects water infiltration, gaseous exchange, erosion processes and the habitat for surface and near-surface dwelling fauna. Two microcosm-scale experiments were conducted to investigate the development of microbial communities in the uppermost centimetre of an arable soil surface following a physical disturbance event, and to determine the effects of the spectral wavebands of light upon such development. Following establishment of freshly disturbed surfaces, subsequently exposed in the field, phenotypically distinct communities determined by PLFA analysis were apparent after eight weeks. Community structure subsequently diverged with respect to depth and time over a further 24 weeks, following consistent trajectories in principal component ordinations. Microbial biomass concentrations increased over time and were greater in the uppermost surface layer (c. 1 mm) compared to deeper layers (up to 11 mm), hypothesised to be driven by the development of photoautotrophs in the extreme surface, where chlorophyll a concentrations increased by an order-of-magnitude over the experimental period. The waveband of light reaching the soil surface, controlled experimentally by the use of selective filters, had a profound effect upon these microbiological properties. In the absence of either light or photosynthetically active radiation (PAR), chlorophyll a concentrations were negligible. Restricting admission of UV-A to the surface resulted in a fourfold increase in chlorophyll a concentration at the extreme surface, and significantly greater concentration in the sub-surface layer compared to the +UV-A treatment. In the surface and sub-surface layers, biomass C concentrations were greatest where UV-A was restricted, and least where PAR was restricted. Biomass C decreased significantly with depth where UV-A was restricted, but increased with depth where PAR was restricted. The waveband of light reaching the soil surface did not affect biomass in the deeper layers. The phenotypic community structure was affected by both the admission and restriction of UV-A, but only in the community present in the extreme surface layers, and principally in relation to the relative proportions of the PLFAs 16:0, 14:0 and 16:1ω7c. Otherwise, the community phenotype was relatively insensitive to the waveband of light reaching the soil surface. This research demonstrates that it is effectively the uppermost 1 mm that is the biotically distinct phase of the arable soil surface, with the presence and waveband of light being an important governing factor that influences its development and phenotypic properties.