Hydrobiogeochemical controls on C and N coupling in a temperate forested watershed
Climate change has increased the frequency and intensity of extreme weather globally and along the east coast of the United States. Changes in the hydrologic regime of a watershed, like increased drought or rainfall, may alter exert a complex hydrobiogeochemical control on ecosystem C and N limitation. However, little research has assessed the effects on watershed processes that such changes may have. Our objective was to determine how extreme weather altered the stoichiometric coupling of C and N exported form a forested watershed. We used an 8 year record of water chemistry from various watershed hydrologic compartments and stream water in one 1st and one 2nd order forested watershed in the piedmont ecoregion of Maryland, which receives high atmospheric N deposition, to compare variability in the ratios of organic and inorganic C and N in the watershed, and variation in the fluorescence composition of DOM. Additionally we used a laboratory bioavailability incubation to assess the effect of nitrogen and carbon limitation on utilization of different forms of C and N.
Watershed exports of C and N operated at both annual and longer cycles, the latter of which appears to be driven by the number of extreme storms. On average, At base flow in 2011, which saw significant drought and several tropical storms, the ratio of dissolved organic carbon to dissolved organic nitrogen had a higher DOC:DON ratios (~45 vs. ~30) than during prior and following years. This suggests that hydrologic inputs may play a critical role in regulating the microbial processes producing DOM in soils and mobilizing DOM from soils. Seasonally, and in response to storm characteristics and antecedent conditions, DOC:DON displayed different patterns of across storm hydrographs. Storms following drought exported large quantities of high DOC:DON organic matter. Spring and autumn storms exported DOM with lower DOC:DON ratios. The ratio of particulate organic carbon to particulate nitrogen export (POC:PN) varied with storm intensity, but typically decreased from ~15 to 10 at the peaks of storm. Protein-like DOM fluorescence was negatively correlated to DON and bioavailability incubations revealed that while protein-like DOM was bioavailable, DON was relatively recalcitrant. These results suggest that, via hydrology, climate may modify the composition and coupling of C and N exports in temperate forested watersheds.