Litter decomposition dynamics following land-use change are driven by land-use legacies

Background/Question/Methods

Land-use change - especially conversion to agricultural production - is typically accompanied by dramatic shifts in plant communities and soil microbial resource availability.  Given that microbial communities are highly sensitive to their environment, these changes may influence microbial-mediated processes such as decomposition.  However, land-use history may also exert strong controls on current soil microbial functions, ultimately influencing the trajectory of decomposition and soil organic matter dynamics.  This may occur if soil microbes are capable of maintaining their original functional capacities through adaptive mechanisms or if abiotic conditions, which persist following conversion, are the primary regulators shaping the original community. We initiated a litter decomposition experiment in spring 2010 at the W.K. Kellogg Biological Station in southwest Michigan to assess how decomposition dynamics of various plant litter inputs are mediated by land-use history, initial litter chemistry, and current land management. Switchgrass, mixed prairie and corn were each planted in fields that were previously under either continuous corn cultivation or unmanaged grassland for over 20 years.  Litterbags of corn stover, switchgrass, and prairie-grass were deployed at each field site for serial destructive sampling over the course of a 2-year period. Over two growing seasons, soil and litterbags were analyzed for extracellular enzyme activity, microbial substrate utilization, microbial biomass and growth efficiency, mass of litter remaining, and litter chemistry.  

Results/Conclusions

Land-use history had a stronger effect on decomposition dynamics than either litter type or current land management. However, current land management and litter quality did influence litter and soil microbial biomass and litter mass loss (P<0.05). An initial assessment of enzyme activities shows that historical agricultural sites had lower enzyme activity in both litter and soil compared to historical grassland sites. Catabolic response profiling indicated greater diversity of substrate utilization and higher microbial growth efficiency under historic grassland soils compared to historic agricultural soils. Corn litter had significantly higher mass loss rates and microbial biomass concentrations relative to switchgrass and prairie litter, but no differences in enzyme activities were detected across different litter types.  There was an interaction between land-use history and current land management in litter mass loss rates, where mass loss was highest under historical agriculture sites, but only in the switchgrass fields.  Our results suggest that historical land-use practices can strongly influence current decomposition dynamics and should be considered when assessing soil carbon cycling following land-use conversions.