Cailin H. Orr

Effects of restoration and reflooding on soil denitrification in a leveed Midwestern floodplain.

River floodplains have the potential to remove nitrate from water through denitrification, the anaerobic microbial conversion of nitrate to nitrogen gas. An important factor in this process is the interaction of river water with floodplain soil; however, many rivers have been disconnected from their historic floodplains by levees. To test the effect of reflooding a degraded floodplain on nitrate removal, we studied changes in soil denitrification rates on the Baraboo River floodplain in Wisconsin, USA, as it underwent restoration. Prior to this study, the site had been leveed, drained, and farmed for more than 50 years. In late fall 2002, the field drainage system was removed, and a gate structure was installed to allow controlled flooding of this site with river water. Soil moisture was extremely variable among zones and months and reflected local weather. Soil organic matter was stable over the study period with differences occurring along the elevation gradient. High soil nitrate concentrations occurred in dry, relatively organic-poor soil samples and, conversely, all samples with high moisture soils characterized by low nitrate. We measured denitrification in static cores and potential denitrification in bulk samples amended with carbon and nitrogen, one year before and two years following the manipulation. Denitrification rates showed high temporal and spatial variability. Static core rates of individual sites ranged widely (from 0.00 to 16.7 microg N2O-N x [kg soil](-1) x h(-1), mean +/- SD = 1.10 +/- 3.02), and denitrification enzyme activity (DEA) rates were similar with a slightly higher mean (from 0.00 to 15.0 microg N2O-N x [kg soil](-1) x h(-1), 1.41 +/- 1.98). Denitrification was not well-correlated with soil nitrate, organic matter content, or moisture levels, the three parameters typically thought to control denitrification. Static core denitrification rates were not significantly different across years, and DEA rates decreased slightly the second year after restoration. These results demonstrate that restored agricultural soil has the potential for denitrification, but that floodplain restoration did not immediately improve this potential. Future floodplain restorations should be designed to test alternative methods of increasing denitrification.

Channel morphology and P uptake following removal of a small dam

Abstract Dam removal is becoming an increasingly common management solution for aging dams, and evaluation of the impact of dam removal on basic attributes of streams such as nutrient uptake and transport is essential. The removal of 2 small dams from a forested, 2nd-order stream in Wisconsin allowed us to study how nutrient dynamics were influenced by changes in channel geometry and bed material. We calculated P uptake and transient storage metrics from a series of 19 short-term injections, measured cross-sectional profiles, and determined benthic sediment size at regular intervals for 2 mo before and after the removals. We also repeated these measurements over the same time period the following year. Nutrient uptake was highly variable, and the stream changed from being a soluble reactive P (SRP) sink to being a source several times over the study period. Uptake lengths increased immediately after dam removal but differences between measurements made before and after removal were not significant. We found no significant relationship between uptake length and benthic sediment composition, channel geometry, or water residence time over the course of the dam removals. Our results indicate that changes in physical channel attributes did not play an important role in controlling SRP retention, probably because the impact of dam removal was small compared with the natural variability of this system.

Planting and vegetation recovery on exposed mud flats following two dam removals in Wisconsin.

Previous studies of vegetation establishment in dam removal sites have shown that natural vegetation community establishment is highly variable and frequently includes species often considered undesirable in restorations. In this article, we examined two case studies where dam removal sites were planted with native species following dam removal in an effort to promote native species establishment and exclude invasive species. Some planted species established soon after the dam removals, but surveys four years later showed a decline in planted species and an increase in non-native species. In both cases, reed canarygrass (Phalaris arundinacea) became well established in the interval between surveys. A seedbank analysis of sediments from four intact reservoirs with similar physical characteristics to the removal sites showed that reed canarygrass is not active in the seedbank. This suggests that there may be an opportunity to exclude it from dam removal sites if management actions are taken after removal. However, in these two cases, planting following removal did not exclude reed canarygrass and few of the planted species persisted four years after planting.

Agricultural influences on the magnitude of stream metabolism in humid tropical headwater streams

The production and respiration of organic carbon in streams (stream metabolism) is a fundamental ecosystem process. The extent to which the magnitude of stream metabolism changes with forest conversion to agriculture in humid tropical headwaters is poorly understood. We measured whole-stream metabolism in headwaters draining forest-agricultural boundaries to investigate metabolic rates in areas with abrupt land-use transitions and the role of remnant riparian vegetation. We used linear mixed models to test the hypotheses that gross primary production (GPP) and ecosystem respiration (ER) would be higher in agricultural areas due to higher light availability and nutrient concentrations, respectively. We found a 257% increase in GPP and 30% increase in ER in agricultural stream reaches. GPP was driven by light and ER was mainly controlled by GPP. These results highlight the overriding influence of light in agricultural streams with large fractions of upstream forest cover. Our findings suggest that high riparian canopy cover (~90%) is necessary to support stream metabolic rates similar to forests in agricultural areas. This study adds to our understanding of the within-biome variation of metabolism resulting from agriculture, and the potential similarities between forested biomes and the role of tropical streams in the global carbon cycle.