Controls on litter decomposition in Arkansas valley streams

Chapter 1 Converting forests to pasture or oil and gas pads can modify in-stream carbon cycling by altering physical and chemical properties of nearby streams. Forest loss often increases stream water temperature, sediments, nutrients, and contaminants from reduced riparian cover, soil destabilization, and nutrient leaching. These catchment-level modifications can also reduce in-stream heterogeneity and cumulatively change litter decomposition rates by altering micro and macro-detritivore activity. We predicted leaf litter decomposition would be fastest in intermediately anthropogenically altered stream catchments. Leaf litter decomposition bags were placed in nine streams across three presumed levels of catchment alteration in spring 2013. Categeories were forested (low alteration), pasture (intermediate alteration), and pasture with natural gas activity (high alteration). Microbial respiration, macroinvertebrate abundance and biomass, water-column inorganic nutrients, and other water quality parameters were measured. Stream water specific conductivity was twice as great, mean nitrate concentrations were 8 times greater, and mean daily temperature was 4ºC greater in streams draining the most versus least altered catchments. However, microbial respiration was not greater in streams with more inorganic nutrients. Contrary to our predictions, microbial respiration tended to be lowest in the most altered catchments. Relative shredder abundance tended to be greater in the least altered catchments and correlated positively in streams with more coarse and stable substrate. Leaf decomposition was significantly faster in the least altered catchments when measured by degree days and did not differ by calendar day, which suggests macroinvertebrates as primary drivers. Stream benthic substrate was altered by pasture with no detected alterations with the addition of high-density natural gas development. Macroinvertebrates density and diversity was low in pasture-dominated catchments likely from unstable substrate and limited organic matter retention. There was no measurable difference in benthic habitat with added natural gas development or additional decline in leaf litter breakdown rates.;Chapter 2 Leaf litter decomposition rates are governed directly by detrivore feeding (top-down) and detritivore feeding often increases with additional food quality from added nuturients (bottom-up). The interaction between top-down and bottom-up controls could increase leaf litter decomposition rates. Macroinvertebrate detritivore feeding, egestion, and excretion provide partially digested labile carbon and inorganic nutrients to microbial decomposers. We predicted that macroinvertebrate detritivore presence alone can stimulate microbial leaf litter decompositin by delivering labile carbon and inorganic nutrients. We placed a common macroinvertebrate detritivore, Tipula abdominalis, in mesocosms and fed them either high or low C:P leaves. To measure bottom-up controls on leaf litter decomposition rates, we placed low and high C:P leaves on the bottom of mesocosms and separated them from detritivores with a fine mesh that allowed frass and excreta to pass. Tipula abdominalis consumed 108%, and egested 84% more feeding on low versus high C:P leaf litter. However, the relatively small amounts of egesta and excreta from T. abdominalis feeding on high C:P leaf litter tended to stimulate leaf litter decomposition 28% more than when T. abdominalis was absent. We measured no indirect effects in low leaf C:P mesocosms. We also tested direct and indirect macroinvertebrate effects on decomposition in a field experiment. Sweetgum leaves in fine mesh bags measured microbial decomposition only, leaf litter in coarse mesh bags measured macro- and micro detritivore activity, and litter in fine mesh bags surrounded by litter in coarse mesh bags could measure increased microbial activity from added macro detritivore-derived nutrients. As expected, litter decomposition rates were fastest in coarse mesh bags. We did not measure any indirect macro detritivore effects on decomposition rates. We speculate that this could be from longer egesta and excreta residence time in mesocosms than streams, or micro detritivores may not have been nutrient limited. This indicates that nitrogen and phosphorus limited microbes that were preconditioned or not in low water column nutrient conditions can be stimulated by invertebrate egesta and excreta. We found that bottom-up indirect effects of macro-detritivores influence leaf decomposition less than other factors in streams. Even moderate amounts of inorganic water column nutrients obscure egesta and excreta effects, direct consumption by macroinvertebrates plays a much larger role than its indirect effects, and anthropogenic catchment alterations can alter leaf decomposition rates at the whole stream level, masking bottom-up indirect effects of macro-detritivores.