Evidence of increasing nitrate (NO3−) leaching losses from soils under various land-use systems has elevated the interest and need to find better land-management practices. An essential step is to find and understand the common parameters that closely relate to leaching losses across a wide range of land-use systems. The overall objective of this study was to relate NO3− leaching losses to various properties across soil types and land-use systems. We chose two locations in southeastern Oklahoma with different soil types. The land-use systems were old-field (abandoned grassland), forest and cultivated arable crop. Parameters determined were soil biomass C, biomass N, total C, total N, extractable mineral N, pH, moisture, CO2 emission and lysimeter-drained water NO3− concentration. Firstly, stepwise multiple linear-regression analyses were performed between biannual NO3− leaching losses and monthly measured independent variables. Secondly, annual and biannual timescale-independent variables and respective NO3− leaching losses were used to perform a regression model. Monthly measured parameters explained over 75% of the biannual NO3− leaching losses, but often with different explanatory variables. Land use was the dominant parameter on monthly models, while soil total C, total C/N ratio, extractable NH4+ and NO3− concentrations and water flux were the most frequently included parameters. When annual- and biannual-mean parameter values were used in regression, respective NO3− leaching losses were explained by somewhat similar parameters to monthly models. Together, these parameters explained only 63, 53 and 44% of the total variation in NO3− leaching loss for the first, second and biannual period, respectively. These models explained a consistently lower amount of variation than the monthly models. Additionally, more than 70% of the total variation could be explained by NO3− concentration and the tested soil-type related parameters. Results from this study revealed that annual and biannual trends of NO3− leaching losses across land-use systems can be explained by a number of parameters collectively rather than a single parameter. Thus, there is no single parameter that can be used to fully predict NO3− leaching losses across different land-use systems on a monthly, annual and biannual basis. Additionally, there is no specific month or season for sampling to better predict NO3− leaching losses across different soil and land-use systems.

Nitrogen (N) enrichment of terrestrial ecosystems dramatically changes ecosystem diversity and structure of plant communities. Research designed to elucidate effects of nitrogen addition on mammalian assemblages is rare. We investigated nitrogen requirements of hispid cotton rats (Sigmodon hispidus) and fulvous harvest mice (Reithrodontomys fulvescens), small mammals native to the tallgrass prairie of the southern Great Plains, USA, to better understand population responses of these species to nitrogen enrichment. We studied reproductive requirements by measuring growth of offspring under varying levels of dietary nitrogen. We predicted that dietary niche would dictate nitrogen requirements, such that the larger herbivore (S. hispidus) would have a lower dietary need for nitrogen per unit mass than the small omnivore/granivore (R. fulvescens). Reproductive output (measured as mass gain of litters and offspring) was responsive to varying nitrogen in cotton rats but not in harvest mice. Nitrogen intake that supported 50% survival of juvenile harvest mice (1.34% dietary nitrogen) also was adequate for maximum growth (1.29%). Cotton rats potentially drew on maternal nutrient stores to support litter growth at low levels of dietary nitrogen (as low as 1.08%). Overall, nitrogen requirements for maximum reproduction were greater (2.31% dietary nitrogen) for cotton rats. We conclude that life history characteristics and body size constraints rather than dietary niche explain the differential species response to variation in dietary nitrogen. Our results imply that nitrogen enrichment in old‐field succession in the southern Great Plains may lead to dominance by cotton rats and a reduction in diversity of the small‐mammal assemblage. Consumers with similar abilities to take advantage of increased environmental nitrogen may likewise dominate other ecosystems.

Loss of nitrate (NO3−) from grazing land is a major cause of surface and groundwater contamination. These losses increase when N sources such as fertilizer are applied to grazing land. The objectives of this work were to (1) study the impact of dairy effluent (DE) or urea on N losses and plant uptake when DE or urea was applied with and without cattle urine and; (2) determine the effect of organic C rich DE on the fate of urine N. The experiment was conducted using lysimeters that contained Templeton sandy loam soil extracted from a pasture in New Zealand. Application of DE resulted in significantly less (p < 0.05) NO3− leaching compared with urea in the first year, but not in the second year. Differences between years could be attributed to the comparatively lower C:N ratio of applied DE in the second year, causing relatively greater N mineralization and greater NO3− leaching. Differences could also be due to cumulative effects of DE (first year applied) on second year NO3− leaching. Total annual pasture N uptake was similar for DE and urea treatments. During the first year, the average NO3− concentration was lower when DE was combined with urine compared to urine alone, but not in the second year. The combination of DE with urine resulted in significantly greater (p < 0.01) annual pasture N uptake compared with the urine alone treatment in both years. Urine plus urea resulted in the greatest leaching losses in both years, but its impact on pasture N uptake was mixed. The total leaching loss of N from urine plus DE (90 kg N ha−1) was similar to urine alone (77 kg N ha−1) in the second year. Likewise, the annual percentage of 15N recovered in the leachate from urine plus DE (9%) was not significantly different from urine alone (6%). However, 15N recoveries revealed that the contribution of urine N to NO3− leaching was greater when urine was combined with DE (98.8%) compared to urine alone (83%). The greater NO3− leaching from urine when combined with DE could be a result of greater nitrification due to the low C:N ratio of DE. Additionally, the annual percentage of urine N uptake by the pasture from urine plus DE (29%) was significantly less than from urine alone (39%) (p < 0.01). The application of organic C rich DE had no significant effect on soil N retention or denitrification when combined with urine.

We conducted a mark-recapture experiment to examine population dynamics of the fulvous harvest mouse (Reithrodontomys fulvescens) and plains harvest mouse (R. montanus) in response to low-level nitrogen amendments (16.4 kg N/ha/y) in an old-field grassland. The experimental design consisted of 16, 0.16-ha plots with four replicates of each treatment combination (fenced, nitrogen amendment; unfenced, nitrogen amendment; fenced, control; unfenced, control). We predicted that densities, survival, and transition probabilities would be greater for both species on nitrogen-amended plots because of greater aboveground biomass (i.e., enhanced concealment from predators). We observed no distinct patterns in survival or transition probabilities of R fulvescens or R. montanus with regard to treatments. Although population densities of R. fulvescens did not exhibit any distinct patterns with regard to treatments, densities of R. montanus tended to be highest on nitrogen plots, but lowest on nitrogen-fenced plots during winter 1999-2000. As low-level nitrogen amendments continue to be applied, we predict survival and densities of R. montanus and R. fulvescens on control plots, especially fenced plots with no nitrogen amendment, will eventually exceed those on nitrogen-amended plots as a result of higher plant species diversity, food availability and better quality cover.

The size and quality of the soil organic matter (SOM) pool can vary between ecosystems and can affect many soil properties. The objectives of this study were to examine the relationship between gross N transformation rates and microbial populations and to investigate the role that SOM plays in these factors. In our study, culturable microbial and actinomycete populations were positively correlated with gross mineralization and ammonium (NH4+) consumption rates over time in both ecosystems. These correlations provide evidence that microbial plate counts could be a good representation of all microbes responsible for gross mineralization and gross NH4+ consumption. Rates of gross mineralization, nitrification and NH4+ consumption were significantly greater in forest soil than old-field soil. These greater rates in forest soil could be due to the presence of higher levels of readily transferable substrates in SOM. Gross nitrification rates were considerably lower than gross mineralization and NH4+ consumption rates over the experimental period, indicating heterotrophic uptake of NH4+ rather than use by autotrophic nitrifiers under soil and environmental conditions in this study. Additionally, microbial populations were significantly (p<0.01) greater in forest soil than in old-field soil, which could also be related to the higher level of SOM in the forest soil. Net mineralization and nitrification rates were similar between ecosystems. Results also showed that net rates were highly correlated to each other, but were not correlated with culturable microbes or gross N transformation rates, indicating the isolation of net rates in relation to fundamental controlling factors.

The results of nitrogen (N) fertilization experiments have shown inconsistent rates of plant litter decomposition, a phenomenon that may be explained by dispropotionate influence of animal detritivores (macro-detritivores) on litter mass loss versus that of microbial decomposers, whose activity may be dependent on inorganic N. In turn, macrodetritivores may be influenced by plant species composition via their selection of optimal food resources and habitats. In our experiment, fertilizer had no apparent effect on litter decomposition, suggesting that microbial decomposers did not use the additional inorganic N and/or that macrodetritivores had a greater influence on decomposition. Manipulation of macrodetritivores suggested that plant species composition-dominated in this study by Festuca arundinacea, an exotic, invasive grass, and Aster ericoides, a native forb-caused shifts in detrivore communities and/or feeding patterns that tended to increase litter mass loss. Canopy cover of F. arundinacea and A. ericoides ranged from 0% to 11%, suggesting that low-intensity invasion may produce significant changes in ecosystem function, such as decomposition.

Abstract Question: Does increasing Festuca canopy cover reduce plant species richness and, therefore, alter plant community composition and the relationship of litter to species richness in old-field grassland? Location: Southeastern Oklahoma, USA. Methods: Canopy cover by species, species richness, and litter mass were collected within an old-field grassland site on 16, 40 m × 40 m plots. Our study was conducted during the first three years of a long-term study that investigated the effects of low-level nitrogen enrichment and small mammal herbivory manipulations. Results: Succession was altered by an increase in abundance of Festuca over the 3-yr study period. Species richness did not decline with litter accumulation. Instead, Festuca increased most on species-poor plots, and Festuca abundance remained low on species-rich plots. Conclusions: Festuca may act as an invasive transformer-species in warm-season dominated old-field grasslands, a phenomenon associated more with invasions of cool-season grasses at higher latitudes in North America. Nomenclature: Anon. (1986).

Small mammal microhabitat research has greatly influenced vertebrate community ecologists. There exists a "microhabitat paradigm" that states that sympatry among small mammal species is enabled by differential use of microhabitat (i.e., microhabitat partitioning). However, several studies have failed to detect microhabitat partitioning, and research has consistently indicated that microhabitat phenomena do not explain larger spatial scale (i.e., macrohabitat) variation. Possible reasons for these difficult to reconcile observations are explored by reviewing and tabulating data from 70 studies. The meaning of the term microhabitat has changed subtly since 1969. This review demonstrates that the existing knowledge of small mammal microhabitat partitioning is highly concentrated among small-scale studies, conducted with modest intensity, that measure microhabitat at inappropriate spatial scales. This concentration of knowledge appears to be an insufficient foundation on which to accept microhabitat partitioning as a widely generalizable phenomenon. The observation that microhabitat phenomena do not explain larger spatial scale variation suggests the importance of underappreciated adaptive mechanisms that relate to the ability of species to coexist, use habitat, and ultimately persist.

We examined the reliability of using nitrogen concentration of stomach contents from hispid cotton rats (Sigmodon hispidus) as an index of dietary nitrogen. Stomach contents of hispid cotton rats fed pelleted diets varying in nitrogen concentration were analyzed for stomach nitrogen. Regression analysis revealed a positive linear relationship between stomach and dietary nitrogen, but the relationship was not 1 : 1. Thus, inverse estimation of the regression equation can be used to adjust for a lack of a 1 : 1 ratio to obtain more reliable and accurate estimates of diet quality. Although we expected this relationship to be robust in its application to field studies, the pelleted diet model consistently underestimated dietary nitrogen during model evaluation experiments with natural forages. We conclude that the applicability of using nitrogen concentration of the stomach contents of cotton rats as an index to dietary nitrogen is dependent on the level of accuracy and precision required in estimating nitrogen concentration of foods consumed.

We conducted a mark–recapture experiment to examine the population dynamics of hispid cotton rats (Sigmodon hispidus) in response to low-level nitrogen amendments (16.4 kg nitrogen/ha per year) and exclosure fencing in an old-field grassland. The experimental design consisted of sixteen 0.16-ha plots with 4 replicates of each treatment combination. We predicted that densities, reproductive success, movement probabilities, and survival rates of cotton rats would be greater on nitrogen-amended plots because of greater aboveground biomass and canopy cover. Population densities of cotton rats tended to be highest on fenced nitrogen plots, but densities on unfenced nitrogen plots were similar to those on control and fenced plots. We observed no distinct patterns in survival rates, reproductive success, or movement probabilities with regard to nitrogen treatments. However, survival rates and reproductive success tended to be higher for cotton rats on fenced plots than for those on unfenced plots and this was likely attributable to decreased predation on fenced plots. As low-level nitrogen amendments continue to be applied, we predict that survival, reproduction, and population-growth rates of cotton rats on control plots, especially fenced plots with no nitrogen amendment, will eventually exceed those on nitrogen-amended plots as a result of higher plant-species diversity, greater food availability, and better quality cover.

Along tire northern periphery of their range, populations of the hispid cotton rat, Sigmodon hispidus, are vulnerable to major reductions in density and occasional local extinctions as a result of severe winter weather. Between our sampling periods on 3 December 2000 and 14 January 2001, 3 independent winter weather events, in conjunction with the coldest month in the state since 1983, affected central and eastern Oklahoma. We recorded a drastic decline in the population of hispid cotton rats at the Center for Subsurface and Ecological Assessment Research in central Oklahoma following these winter weather events. Densities dropped from 58.6 cotton rats/ha on 3 December 2000 to 1.2 cotton rats/ha on 14 January 2001. Although hispid cotton rat densities were declining before these winter weather events occurred, we attributed the dramatic decrease to severe winter weather and below-normal temperatures. As of 19 November 2001, the population of hispid cotton rats at the site had not recovered. Abundances between January and November 2001 ranged from 0.6 to 2.6 cotton rats/ha compared with a range of 30.1 to 112.5 during the same period in 2000. Additionally, we present evidence of populations of hispid cotton rats being affected at a statewide scale as a result of the weather in December 2000. We suspect that severe winters, such as the events described, might slow the northward advance of hispid cotton rats and serve to indirectly regulate populations along the intermediate and northern fringes of its range.

Small mammals locate buried wet seeds more efficiently than buried dry seeds. This may be attributable to emission of volatile compounds by the seeds. To test this hypothesis I measured emission of volatile compounds from seeds of three plant species (Pinus contorta, Purshia tridentata and Achnatherum hymenoides) under wet and dry conditions using solid phase micro-extraction, gas chromatography/mass spectrometry headspace analysis. Seeds responded in two ways: (1) wet seeds released different, generally greater, amounts of volatile gas than dry seeds and (2) wet seeds and dry seeds released different compounds. Pinus contorta seeds release greater amounts of three compounds when wet; Purshia tridentata seeds release two compounds when dry that are not released when wet, and release increased amounts of two compounds when wet, and increased amounts of two compounds when dry; Achnatherum hymenoides releases at least 22 compounds, one of which is released in large concentrations when wet. These data suggested two mechanisms by which small mammals locate buried seeds. First, small mammals may be sensitive to release of differing concentrations of volatile compounds by seeds. Second, small mammals may be sensitive to compounds released by wet seeds that are not released by dry seeds. Ability of seeds to survive depredation by granivores may be an adaptive trait influenced by natural selection.

Quadrats are widely used for measuring characteristics of vascular plant communities. it is well recognized that quadrat size affects measurements of frequency and cover. The ability of quadrats of varying sizes to adequately measure diversity has not been established. An exhaustive search of sixteen 40 by 40 m old field plots documented presence or absence of 88 vascular plant species (mean = 58.8 per plot). Against these benchmarks, measurements of species richness were obtained from 50 samples in each plot and compared among three different sized quadrats (0.2 by 0.5 m, 0.5 by 0.5 m and 0.5 by 1.0 m). These quadrats differed in the number of species measured, with an average of 33.6, 38.7 and 41.4 species respectively (P <0.001; Least Significant Difference test critical difference = 0.7 species). Quadrats are relatively ineffective for documenting the actual number of vascular plant species present on old field plots (at least 30% of species undetected). However, under the intensity of sampling used in this study (n=50) a difference of less than one species per plot was detected at P < 0.05. Thus, quadrat sampling regardless of quadrat size can provide a precise index of vascular plant species richness.

Habitat associations and diversity of rodents were studied in four vegetation types associated with a Chihuahuan Desert/desert plains grassland ecotone in southern New Mexico during spring 1993 and 1994. Vegetation types included two types of draw (i.e., wide bottomed vegetated drainage) (Rhus and grass) and two types of upland (Acacia and mesa relic) characterized by occurrence of unique shrub and/or grass species. In 14,400 trap nights 1,314 individuals of 16 species ware collected. Of these, 13 demonstrated associations with vegetation types that remained constant even as rodent populations increased 99% from 1993 to 1994. Only a population increase of 1,309% for Perognathus flavus in grass draws was sufficient to cause a vegetation type x year interaction. Diversity measurements also remained consistent among vegetation types. This suggests that within the ecotone studied, vegetation types support consistent rodent populations when compared to other nearby vegetation types. Ecotones are probably important reservoirs of biodiversity within landscapes. whereas the prevailing view of ecotones is that they comprise edges or regions of intergradation between habitats, data from this suggest that (for rodents) ecotones such as those investigated may be more appropriately understood to comprise a patchwork of adjacent discrete habitats. Under this view, habitats within ecotonas can be identified and classified on their own merit without specific reference to the biomas that border the ecotone.

Line intercept transects are commonly used to measure vegetation although their length and number are variable among studies. Although line intercepts are used to compare other sampling techniques, few studies have investigated the effect of changing the number and length of transects on measurements. This study compared results from line intercept sampling with conventional 45 m transects to 4 m transects at 10 study sites in south-central New Mexico. In a plant community dominated by creosotebush (Larrea tridentata), tarbush (Florensia cernua), and bush muhly (Muhlenbergia porteri) no differences were detected in measurements made with three 45 m transects compared to thirty 4 m transects or for six 45 m transects compared to one hundred 4 m transects (P > 0.05). However, sampling with the 4 m transects can be easily conducted by a single technician whereas the 45 m transects typically require a two person team. This study suggests that a substantial amount of sampling efficiency can be gained through use of short line transects with no loss in data quality regarding estimates of mean and variance.

Many insights into community ecology over the past 3 decades were derived from investigations of associations of rodent species with microhabitats. Nonetheless, studies of microhabitat use of rodents are inconsistent, suggesting spatially dependent interacting factors. We investigated the relative ability of microhabitat and macrohabitat to predict rodent captures in traps placed in 48 trapping grids of 90 traps each during spring and autumn of 1993 and 1994 (17,280 data points). Trapping grids represented eight replications of six discrete macrohabitats. We used discriminant function analysis and random null models to compare the ability of microhabitat and macrohabitat to predict use of individual traps by 13 rodent species. Classification rates for presence at a trap by dummy variables of macrohabitats exceeded those obtained with principle components of microhabitats for nine of 13 species. In seven of those cases, classification rate exceeded that expected from a random distribution of dummy variables. Of the four cases where principle components of microhabitats out-classified dummy variables of macrohabitats, only two exceeded rates expected from a random distribution of dummy variables. Thus, microhabitat partitioning for many species is constrained by local macrohabitat conditions.