Nutrient Uptake in a Large Urban River1

Abstract: Small streams have been shown to be efficient in retaining nutrients and regulating downstream nutrient fluxes, but less is known about nutrient retention in larger rivers. We quantified nutrient uptake length and uptake velocity in a regulated urban river to determine the river’s ability to retain nutrients associated with wastewater treatment plant (WWTP) effluent. We measured net uptake of soluble reactive phosphorus (SRP), dissolved organic phosphorus, ammonium (NH₄), nitrate, and dissolved organic nitrogen in the Chattahoochee River, Atlanta, GA by following the downstream decline of nutrients and fluoride from WWTP effluent on 10 dates under low flow conditions. Uptake of all nutrients was sporadic. On many dates, there was no evidence of measurable nutrient uptake lengths within the reach; indeed, on several dates release of inorganic N and P within the sample reach led to increased nutrient export downstream. When uptake occurred, SRP uptake length was negatively correlated with total suspended solids and temperature. Uptake velocities of SRP and NH₄ in the Chattahoochee River were lower than velocities in less‐modified systems, but they were similar to those measured in other WWTP impacted systems. Lower uptake velocities indicate a diminished capacity for nutrient uptake.     

Economically optimal nitrogen rate reduces soil residual nitrate

Post-harvest residual soil NO(3)-N (RSN) is susceptible to transfer to water resources. Practices that minimize RSN levels can reduce N loss to the environment. Our objectives were (i) to determine if the RSN after corn (Zea mays L.) harvest can be reduced if N fertilizer is applied at the economically optimal N rate (EONR) as compared to current producer practices in the midwestern USA and (ii) to compare RSN levels for N fertilizer rates below, at, and above the EONR. Six experiments were conducted in producer fields in three major soil areas (Mississippi Delta alluvial, deep loess, claypan) in Missouri over 2 yr. Predominant soil great groups were Albaqualfs, Argiudolls, Haplaquolls, and Fluvaquents. At four transects in each field, six treatment N rates from 0 to 280 kg N ha(-1) were applied, the EONR was determined, and the RSN was measured to a 0.9-m depth from five treatment plots. The EONR at sampling sites varied from 49 to 228 kg N ha(-1) depending on site and year. Estimated average RSN at the EONR was 33 kg N ha(-1) in the 0.9-m profile. This was at least 12 kg N ha(-1) lower than RSN at the producers' N rates. The RSN increased with increasing Delta EONR (total N applied - EONR). This relationship was best modeled by a plateau-linear function, with a low RSN plateau at N rates well below the EONR. A linear increase in RSN began anywhere from 65 kg N ha(-1) below the EONR to 20 kg N ha(-1) above the EONR at the three sites with good data resolution near the EONR. Applying N rates in excess of the EONR produced elevated RSN values in all six experiments. Our results suggest that applying the EONR will produce environmental benefits in an economically sound manner, and that continued attempts to develop methods for accurately predicting EONR are justified.     

Convergent and Contingent Community Responses to Grass Source and Dominance During Prairie Restoration Across a Longitudinal Gradient

Restoring prairie on formerly cultivated land begins by selecting propagule seed sources and the diversity of species to reintroduce. This study examined the effects of dominant grass propagule source (cultivar vs. non-cultivar) and sown propagule diversity (grass:forb sowing ratio) on plant community structure. Two field experiments were established in Kansas and Illinois consisting of identical split plot designs. Dominant grass source was assigned as the whole-plot factor, and sown dominance of grasses (five levels of seeded grass dominance) as the subplot factor. Species density, cover, and diversity were quantified for 5 years. The effect of dominant grass source on the cover of focal grasses, sown species, and volunteer species was contingent upon location, with variation between dominant grass sources observed exclusively in Kansas. Species density and diversity showed regionally convergent patterns in response to dominant grass source. Contrary to our hypotheses, total species density and diversity were not lower in the presence of grass cultivars, the grass source we had predicted would be more competitive. Sown grass dominance effects on the cover of the focal grass species were contingent upon location resulting from establishment corresponding better to the assigned treatments in Illinois. All other cover groups showed regionally convergent patterns, with lower cover of volunteers and higher cover of sown forbs, diversity, and species density in the lowest sown grass dominance treatment in both sites. Thus, decisions regarding the diversity of propagules to reintroduce had more consequence for plant community structure than cultivar or non-cultivar source of dominant grasses.     

Cultivating hybrid collectives: research methods for enacting community food economies in Australia and the Philippines

Across the globe, groups are experimenting with initiatives to create alternatives to the dominant food system. What role might research play in helping to strengthen and multiply these initiatives? In this paper we discuss two research projects in Australia and the Philippines in which we have cultivated hybrid collectives of academic researchers, lay researchers and various non-human others with the intention of enacting community food economies. We feature three critical interactions in the “hybrid collective research method”: gathering, which brings together those who share concerns about community food economies; reassembling, in which material gathered is deliberatively rebundled to amplify particular insights; and translating, by which reassembled ideas are taken up by other collectives so they may continue to “do work”. We argue that in a climate-changing world, the hybrid collective research method fosters opportunities for a range of human and non-human participants to act in concert to build community food economies.