Nitrate-nitrogen, land use/land cover, and soil drainage associations at multiple spatial scales.

Managing non-point-source pollution of water requires knowledge of land use/land cover (LULC) influences at altering watershed scales. To gain improved understanding of relationships among LULC, soil drainage, and dissolved nitrate-N dynamics within the Calapooia River Basin in western Oregon, we selected 44 watersheds ranging in size between 3 and 33 km(2) for monthly synoptic sampling of surface water quality between October 2003 and September 2004. Seasonal associations were examined between dissolved nitrate-N and proportion of woody vegetation or poorly drained soils at differing scales (10, 20, 30, 60, 90, 150, 300 m, and entire watershed), which we defined as influence zones (IZs), surrounding stream networks. Correlations between nitrate-N and proportion woody vegetation or poorly drained soil at each IZ were analyzed for differences using the Hotelling-Williams test. We observed negative correlations (r = -0.81 to -0.94) between nitrate-N and proportion of woody vegetation during winter and spring. Poorly drained soils had positive correlations (r = 0.63-0.87) with nitrate-N. Altering the scale of analysis significantly changed correlations between nitrate-N and woody vegetation, with IZs <150 m being stronger than the watershed scale during winter. However, absolute differences in correlation values were small, indicating minimal ecological consequence for significant differences among scales. In contrast, nitrate-N correlations with poorly drained soil were stronger at the watershed scale than the 10- through 90-m IZs during winter and spring, and absolute differences were sufficient to suggest that scale is ecologically important when determining associations between dissolved nitrate-N and poorly drained soils.

Mitigation of shallow groundwater nitrate in a poorly drained riparian area and adjacent cropland.

Riparian ecosystems, through their unique position in the agricultural landscape and ability to influence nutrient cycles, can potentially reduce NO3 loading to surface and ground waters. The purpose of this study was to determine the fate of NO3 in shallow groundwater moving along a lateral flowpath from a grass seed cropping system through an undisturbed mixed-species herbaceous riparian area. Soil A (30-45 cm) and C horizon (135-150 cm) NO3, dissolved oxygen, and nitrous oxide concentrations were significantly higher in the cropping system than the adjacent riparian area. Nitrate concentrations in both horizons of the riparian soil were consistently at or below 0.05 mg N L(-1) while cropping system concentrations ranged from 1 to 12 mg N L(-1). Chloride data suggested that NO3 dilution occurred from recharge by precipitation. However, a sharp decrease in NO3/Cl ratios as water moved into the riparian area indicated that additional dilution of NO3 concentrations was unlikely. Riparian area A horizon soil water had higher dissolved organic carbon than the cropping system and when the riparian soil became saturated, available electron acceptors (O2, NO3) were rapidly reduced. Dissolved inorganic carbon was significantly higher in the riparian area than the cropping system for both horizons indicating high biological activity. Carbon limitation in the cropping system may have led to microbial respiration using primarily O2 and to a lesser degree NO3. Within 6 m of the riparian/cropping system transition, NO3 was virtually undetectable.

Molecular phylogenetics of the clover genus (Trifolium--Leguminosae).

Trifolium, the clover genus, is one of the largest genera of the legume family. We conducted parsimony and Bayesian phylogenetic analyses based on nuclear ribosomal DNA internal transcribed spacer and chloroplast trnL intron sequences obtained from 218 of the ca. 255 species of Trifolium, representatives from 11 genera of the vicioid clade, and an outgroup Lotus. We confirm the monophyly of Trifolium, and propose a new infrageneric classification of the genus based on the phylogenetic results. Incongruence between the nrDNA and cpDNA results suggests five to six cases of apparent hybrid speciation, and identifies the putative progenitors of the allopolyploids T. dubium, a widespread weed, and T. repens, the most commonly cultivated clover species. Character state reconstructions confirm 2n=16 as the ancestral chromosome number in Trifolium, and infer a minimum of 19 instances of aneuploidy and 22 of polyploidy in the genus. The ancestral life history is hypothesized to be annual in subgenus Chronosemium and equivocal in subgenus Trifolium. Transitions between the annual and perennial habit are common. Our results are consistent with a Mediterranean origin of the genus, probably in the Early Miocene. A single origin of all North and South American species is hypothesized, while the species of sub-Saharan Africa may originate from three separate dispersal events.

Fatty acid methyl ester analysis to identify sources of soil in surface water.

Efforts to improve land-use practices to prevent contamination of surface waters with soil are limited by an inability to identify the primary sources of soil present in these waters. We evaluated the utility of fatty acid methyl ester (FAME) profiles of dry reference soils for multivariate statistical classification of soils collected from surface waters adjacent to agricultural production fields and a wooded riparian zone. Trials that compared approaches to concentrate soil from surface water showed that aluminum sulfate precipitation provided comparable yields to that obtained by vacuum filtration and was more suitable for handling large numbers of samples. Fatty acid methyl ester profiles were developed from reference soils collected from contrasting land uses in different seasons to determine whether specific fatty acids would consistently serve as variables in multivariate statistical analyses to permit reliable classification of soils. We used a Bayesian method and an independent iterative process to select appropriate fatty acids and found that variable selection was strongly impacted by the season during which soil was collected. The apparent seasonal variation in the occurrence of marker fatty acids in FAME profiles from reference soils prevented preparation of a standardized set of variables. Nevertheless, accurate classification of soil in surface water was achieved utilizing fatty acid variables identified in seasonally matched reference soils. Correlation analysis of entire chromatograms and subsequent discriminant analyses utilizing a restricted number of fatty acid variables showed that FAME profiles of soils exposed to the aquatic environment still had utility for classification at least 1 wk after submersion.