Stoichiometric analyses can be used to investigate the linkages between N and C cycles and how these linkages influence biogeochemistry at many scales, from components of individual ecosystems up to the biosphere. N-specific NH4+ uptake rates were measured in eight streams using short-term 15N tracer additions, and C to N ratios (C:N) were determined from living and non-living organic matter collected from ten streams. These data were also compared to previously published data compiled from studies of lakes, ponds, wetlands, forests, and tundra. There was a significant negative relationship between C:N and N-specific uptake rate; C:N could account for 41% of the variance in N-specific uptake rate across all streams, and the relationship held in five of eight streams. Most of the variation in N-specific uptake rate was contributed by detrital and primary producer compartments with large values of C:N and small values for N-specific uptake rate. In streams, particulate materials are not as likely to move downstream as dissolved N, so if N is cycling in a particulate compartment, N retention is likely to be greater. Together, these data suggest that N retention may depend in part on C:N of living and non-living organic matter in streams. Factors that alter C:N of stream ecosystem compartments, such as removal of riparian vegetation or N fertilization, may influence the amount of retention attributed to these ecosystem compartments by causing shifts in stoichiometry. Our analysis suggests that C:N of ecosystem compartments can be used to link N-cycling models across streams.

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Black-tailed prairie dogs have declined by 98% in the past century. Due to continued declines, the U. S. Fish and Wildlife Service designated the species as a candidate for listing in 2000. Prairie dogs foster both plant and animal diversity, and their continued presence is a concern in the Great Plains. We compared vegetation structure and composition of black-tailed prairie dog (Cynomys ludovicianus Ord) colonies in southwest Kansas and southeast Colorado to non-colonized grassland in 1996, and 1997. Dominant species on prairie dog colonies were Bouteloua gracilis (H. B. K.) Lag. ex Griffiths (14% cover in 1996, 15% in 1997), Buchloe dactyloides (Nutt.) Engelm. (7% cover in 1996, 17% in 1997) and Aristida purpurea Nutt. (9% cover in 1996,16 % in 1997). Dominant vegetation at randomly selected non-colonized sites were Bouteloua curtipendula (Michx.) Torr. (16% cover in 1996,18% in 1997), Bouteloua gracilis (13% cover in 1996,17 % in 1997) and Buchloe dactyloides (5% cover in 1996, 5% in 1997). Non-colonized shortgrass sites were dominated by Bouteloua gracilis (23% cover in 1996, 41 % in 1997) and Buchloe dactyloides (8% cover in 1996,12% in 1997). Cover of grass was higher (P < 0.01) on random sites (44%) than on prairie dog colonies (31%) or shortgrass sites (33%) in 1996, whereas cover of forbs was higher (P < 0.01) on prairie dog colonies (18%) than on random sites (7%) or shortgrass sites (8%) that year. Vegetation height was greater (Q = 3.66) and visual obstruction was greater (Q = 3.39) on random sites (33.6 and 6.4 cm, respectively) than on prairie dog colonies (9.5 and 2.5 cm, respectively) in 1997, the only year these variables were measured. Percent bare ground did not differ (P > 0.05) among treatments either year. While components of the vegetation on prairie dog colonies differed from that found on non-colonized sites, the vegetation of prairie dog colonies was, nonetheless, characteristic of a shortgrass region. Prairie dogs undoubtably alter vegetation structure and composition in shortgrass prairie, and likely have a great influence on landscape heterogeneity, but our results suggest that shortgrass prairie is well adapted to the herbivory and soil disturbing activities of prairie dogs.

Several studies have reported that corn (Zea mays L.) hybrids differ in leaf chlorophyll concentration (LCC) to the extent that Minolta SPAD 502 meter data have to be normalized (values obtained from a general field divided by those from a fully-fertilized area in the same field). However, situations exist where the normalization method can add needlessly to production expenses and even lead to nitrogen deficiencies and reduced yield. Little information was found to indicate how much potential variability exists among modern hybrids, so we evaluated 45 entries in a fully-fertilized and irrigated standard performance test over a 2-year period in northwestern Kansas to determine: (i) how much variability was present in a large sampling of commercially available, fully-fertilized corn hybrids and (ii) if results 1 Contribution No. 97-331-J, Kansas Agricultural Experiment Station. 1794 SUNDERMAN, PONTIUS, AND LAWLESS suggested alternatives to the normalization method. We used the SPAD meter to measure LCC twice during the vegetative phase and twice during the reproductive phase. Results indicated that genetic potential for maximum LCC was achieved. Quadratic regressions of LCC on days from planting yielded highly significant R 2 values of 0.88 when adjusted to common growth stages and 0.91 when data were not adjusted. A cubic model did not add significant improvements, but we prefer this model because it provided a better fit for early-season data. For quick checks, we propose that the mean values (±standard deviation) obtained at growth stages R1 (57.9±4.5) and R6 (60.0±4.6), and to a lesser degree V6 (40.0±3.2) and V10 (48.6±3.8), be used as first approximations for evaluation and refinement. As first approximations, they are subject to error and should not be used as the sole basis for deciding if or how much supplemental fertilizer nitrogen (N) needs to be applied. Although these standards are useful, their ranges were judged to be too broad for precise estimates, so we see little alternative to the normalization method.

Intensive monitoring of bird nests to measure reproductive success is time-consuming and may influence the fate of nests. Reproductive indices that do not require searching for and visiting nests may be reasonable alternatives to nest monitoring if they provide results similar to nest-searching efforts. We evaluated the reproductive index of Vickery et al. (1992) for estimating reproductive success of the dickcissel (Spiza americana) in northeast Kansas, USA. We used nest searching and Vickery et al.'s (1992) reproductive index to compare reproductive success on 20 plots (200 x 200 m). Daily nest survival (DNS) rates averaged 0.911 (SE = 0.011, n = 72 nests), and brownheaded cowbirds (Molothrus ater) accounted for 21% of all nest failures. Surveyors underestimated reproductive index ranks when compared to nest-searching efforts and were inaccurate in their assignment of reproductive success. In particular, surveyors reported successful nests on 3 study plots that fledged no young, probably because young dickcissels moved onto plots after fledging from their natal territories. Our results indicate that the reproductive index of Vickery et al. (1992) may be inappropriate for wary species or those heavily parasitized by brood parasites. We suggest that before relying on the index alone, investigators should use pilot trials to determine whether results from this index are concordant with results from intensive nest-searching efforts for species of interest.