Lipid profiles of decomposing plant litter: UV-B and soil deposition interactions

Background/Question/Methods

In drylands characterized by sparse vegetative cover, litter decomposition may be driven by photodegradation at certain times of the year or stages of decay.  However, low plant cover in drylands also promotes soil movement by wind and water.  As a result, soil-litter mixing is common and may counterbalance photodegradation effects via direct or indirect effects on the decomposer microbial community.  We designed a field experiment to evaluate the outcome of interactions between UV-B radiation (280-320 nm) and soil-litter mixing on microbial communities associated with decomposition of shrub (mesquite, Proposis velutina) and grass (Lehmann lovegrass, Eragrostis lehmanniana) foliage.   In a factorial experiment manipulating UV radiation levels (near-ambient and sub-ambient UV-B) and the degree of litter coverage by soil (none vs. 100%), we measured phospholipid fatty acid (PLFA) profiles to quantify how microbial and plant lipids in the decomposing material were affected by these treatments after zero, one, and three months of decay.  We hypothesized that UV effects would be strongest in the fully exposed litter and that soil coverage would negate any UV effects and promote microbial activity and decomposition.

Results/Conclusions

UV treatment effects on litter lipid profiles were subtle (averaging <5% difference in total PLFA between the treatments; ordinations showed no difference between treatments at any point in time).  When significant, exposed, near-ambient UV-B litter had greater total lipids, indicating less decomposition of plant lipids and/or more microbial biomass, than the sub-ambient UV-B treatment.  These effects were strongest at the three month harvest for 18:2n6, a fatty acid methyl ester (FAME) representative of fungi (near-ambient UV-B had 8% more 18:2n6 than sub-ambient when averaged across both plant species).  For all other measured lipids, soil coverage and plant species were the over-riding factors dictating lipid quantity and relative abundance.  Initial litter lipid profiles for the shrub and grass species differed dramatically, and lipid profiles for each were altered by soil coverage after one and three months of incubation.  Soil coverage most influenced shrub lipid profiles, primarily via stimulation of bacterial FAMEs.  These data illustrate (i) the potential complexity of lifeform x UV x soil deposition interactions that likely occur in spatially heterogeneous dryland ecosystems; and (ii) that soil-litter mixing can be a critical factor in determining lipid decomposition and microbial community structure on decomposing plant material in arid systems.