Climate change as a factor in export of dissolved organic matter from agricultural watersheds

Background/Question/Methods

Many general circulation models predict that precipitation and subsequent surface runoff will increase in many areas of the Midwestern United States with climate change.  Current trends in increasing precipitation and predicted climatic influences are superimposed in agricultural landscapes over a suite of land management practices that also contribute to increasing water yield.  These hydrologic trends are likely to change biogeochemical processes in ways that influence carbon cycling in agroecosystems and the waterways that drain them.  In this context, a series of field- and watershed-scale studies were conducted to investigate dissolved organic carbon (DOC) export from agroecosystems in response to changing hydrology. 

Results/Conclusions

In field scale plots (located in south central Minnesota) that have been modified with subsurface tile drainage, DOC concentrations typically increase during the periods following flow events but there is no correlation with measured discharge (in contrast to watershed-scale results). In general, plots with more intense drainage exhibit higher DOC concentrations and greater annual water yields than plots with standard drainage, resulting in greater annual DOC export from plots with more intense drainage.  When measured in a stream draining an 850 km² agricultural watershed located in west central Indiana, DOC export is primarily controlled by increased stream discharge related to rainfall and snowmelt events.  Molecular and isotopic proxies show that increasing quantities of DOC exported during elevated stream flow are characterized by increasing terrestrial contributions as well as changes in relative degradation state (as inferred from lignin monomers).  In this watershed, total carbon loads are dependent on a few key flow events and roughly 80% of the annual load was exported during events occurring less than 20% of the time.  Overall, these results suggest that the processes controlling DOC export differ depending on spatial scale but that hydrology is still dominant in determining total fluxes.