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Previous research suggests that soil organic C pools may be a feature of semiarid regions that are particularly sensitive to climatic changes. We instituted an 18-mo experiment along an elevation gradient in northern Arizona to evaluate the influence of temperature, moisture, and soil C pool size on soil respiration. Soils, from underneath different tree canopy types and interspaces of three semiarid ecosystems, were moved upslope and/or downslope to modify soil climate. Soils moved downslope experienced increased temperature and decreased precipitation, resulting in decreased soil moisture and soil respiration (as much as 23 and 20%, respectively). Soils moved upslope to more mesic, cooler sites had greater soil...
Bare-soil evaporation is an important component of the water balance in semiarid systems. However, little is known quantitatively about the influence of soil texture on bare-soil evaporation. We hypothesized that soil texture would have a great influence on both the temporal dynamics of bare-soil evaporation as well as on the depth ?to which evaporation-influenced soil water content throughout a 51-d simulated drought period. We measured soil water in lysimeters filled with three different soils. We measured daily evaporation gravi metrically and estimated evaporation rates using the energy balance method of Ben-Asher et al. We estimated soil water content at depths of 3.8 cm, 11.4 cm, 19.0 cm, 26.6 cm, and 34.2...
Previous research suggests that soil organic C pools may be a feature of semiarid regions that are particularly sensitive to climatic changes. We instituted an 18-mo experiment along an elevation gradient in northern Arizona to evaluate the influence of temperature, moisture, and soil C pool size on soil respiration. Soils, from underneath different tree canopy types and interspaces of three semiarid ecosystems, were moved upslope and/or downslope to modify soil climate. Soils moved downslope experienced increased temperature and decreased precipitation, resulting in decreased soil moisture and soil respiration (as much as 23 and 20%, respectively). Soils moved upslope to more mesic, cooler sites had greater soil...
In many semiarid regions, runoff and erosion differ according tovegetation patch type. These differences, although hypothesized tofundamentally affect ecological processes, have been poorly quanti-fied. In a semiarid pinon-juniper woodland [Pinus edulis Engelm.and Juniperus monosperma (Engelm.) Sarg.] in northern New Mexico,we measured runoff and erosion from the three patch types thatcompose these woodlands: Canopy patches (those beneath woodyplants), vegetated patches in intercanopy areas, and bare patches inintercanopy areas. The bare intercanopy patches exhibited the highestrates, followed by vegetated intercanopy patches and then by canopypatches. Large convective summer storms, though relatively infre-quent,...
Nutrient availability is an important constraint on sustainable forest productivity, and it is crucial to understand the long-term effects of management practices, including soil scarification, on soil microbial communities because they store and cycle nutrients. In addition, because forests are subject to wildfires, it would be useful to understand potential interactive effects of wildfire and management practice on forest soil ecosystems. We studied the individual and combined effects of soil scarification and a subsequent wildfire on microbial community structure of a ponderosa pine (Pinus ponderosa C. Lawson) forest soil in the central Rocky Mountains. Experimental plots were scarified by rototilling in 1981,...
Following a 200-kg urea-N/ha fertilization in a loblolly pine stand (Pinus taeda), soil mineral N levels (almost entirely NH4+) declined from 200 ppm 20 days after fertilization to < 10 ppm within 161 days. Similar patterns had been previously observed following urea fertilization in a Douglas-fir stand. After the decline in soil mineral N, 20% (40 ppm) of fertilizer N was mineralized within 4 weeks of aerobic incubation in the laboratory at 25 deg C. Nitrogen mineralization in control soils did not occur after 7 weeks incubation. In contrast to field results, urea additions to control soils in the laboratory resulted in rapid net N mineralization. The difference between laboratory and field fertilization results...
The physiological mechanisms utilized by soil bacteria for acclimation to sudden increases in soil water potential are poorly understood. In this study, we examined the physiological responses of soil isolates of Pseudomonas chlororaphis, P. fluorescens, Bacillus pumulis, and Streptomyces griseus to a sudden increase in solution water potential (dilution). Bacterial isolates were cultured at a low solute water potential (-3.0 MPa) and subjected to rapid water potential increases of 0.5 to 2.0 MPa. The small amount of protein and DNA released by a 2.0 MPa dilution suggests that water potential increases up to 2.0 MPa did not cause significant cell lysis. In response to dilution, intracellular solutes were released...
Soil properties under stands of vegetation dominated by mountain big sagebrush (Artemisia tridentata ssp. vaseyana) and grass were examined 14 yr after spraying with 2,4-dichlorophenoxy acetic acid (2,4-D) to control sagebrush. Changes in only a few soil chemical properties were found after conversion to grassland. Phosphorus and K were apparently redistributed from depth to the surface 5 cm of soil by grass-dominated vegetation. Conversely, surface concentrations of N were lower under grass vegetation than under undisturbed vegetation. No changes attributable to vegetation conversion were found for total C, Na, Mg, cation exchange capacity, base saturation, pH, bulk density, or potential net N mineralization rates...
Organic acids can form stable complexes with metals and therefore can affect metal solubility and speciation. The low-molecular-weight aliphatic organic acids extracted in water and present in soil solution from O, A, Bh and Bt horizons from a group of forested Utlisols, Entisols and Spodosols were identified by high performance liquid chromatography (HPLC). Oxalic acid was found in all samples and was present generally in the highest concentrations. Oxalate concentrations in soil solution ranged from 25 to 1000 microM and were greater in the Bh and Bt horizon soils than in the A horizon soil. High concentrations of formic acid were also identified in most soils, ranging from 5 to 174 microM in soil solution. Trace...
Aggregate disintegration and crust formation at soil surfaces exposed to rain are caused by rapid wetting of the dry aggregates and by raindrop impact. The relative importance of these two mechanisms was evaluated by studying the effects of the wetting rate (WR) and raindrop impact on crust strength and crust micromorphology. Two soils, a loess (Typic Haplustalf) and a black soil (Pachic Udic Argiboroll), varying in their organic matter content (15.2 and 42.6 g kg-1, respectively) and aggregate stabilities, were packed in splash cups, prewetted at WRs of 2 or 50 mm h-1, and exposed to 5 to 60 mm of simulated rainfall. For the unstable loess soil, crust strength increased and crust microfabric deteriorated with increases...
A stepwise multiple regression procedure was developed to predict oven-dried bulk density from soil properties using the 1997 USDA-NRCS National Soil Survey Characterization Data. The database includes both subsoil and topsoil samples. An overall regression equation for predicting oven-dried bulk density from soil properties (R2 = 0.45, P < 0.001) was developed using almost 47000 soil samples. Partitioning the database by soil suborders improved regression relationships (R2 = 0.62, P < 0.001). Of the soil properties considered, the stepwise multiple regression analysis indicated that organic C content was the strongest contributor to bulk density prediction. Other significant variables included clay content, water...
Encroachment of pinyon–juniper woodland into rangeland ecosystems is prevalent across the western United States. Mechanisms associated with this successful encroachment are speculative, but probably, in part, involve the effective use of water resources. We explored the ecohydrological characteristics of a two-needle pinyon pine (Pinus edulis Engelm.)–Utah juniper [Juniperus osteosperma (Torr.) Little] woodland on the Colorado Plateau in Utah. We have discovered that a high level of natural soil water repellency or hydrophobicity exists under the canopies of both pinyon and juniper species. We found, following summer precipitation events, that soil water repellency under trees concentrated the soil water below...
Mapping salt-affected soils in remote rangelands is challenging. We used Landsat 7 ETM data to facilitate digital mapping of gypsic and natric soil areas in the upper Colorado River drainage. Optimum index factor band combinations were used to explore the scene. Normalized difference ratio models and threshold values were developed by comparing spectral signatures with gypsic and natric soil areas verifi ed in the fi eld. Gypsic soil areas were mapped using the normalized difference ratio of Bands 5 and 7 with a threshold >0.11, probably related to the spectral refl ectance of gypsum within a few centimeters of the surface. All sites predicted to be gypsic soil areas were determined to be gypsic by fi eld assessment,...
In ponderosa pine (Pinus ponderosa Dougl. ex Laws.) forests of the western USA, fire exclusion by Euro-American settlers facilitated pine invasion of grass openings, increased forest floor detritus, and shifted the disturbance regime toward stand-replacing fires. We evaluated the impacts of two replicated ecological restoration treatments involving tree thinning alone (thinning restoration) and a combination of tree thinning, forest floor reduction, and prescribed burning (composite restoration) on soil microbial activity, biomass, and function approximately 8 yr after initial treatments. Microbial-N levels in the two restoration treatments were not significantly different from the control during either the dry...
The suitability of the foliar N/P ratio was evaluated as a predictor of nutrient limitation in an alpine ecosystem of the Colorado Front Range. We hypothesized that foliar N/P ratios are directly correlated with the alpine soil nutrient status. We used a long-term fertilization experiment conducted in three alpine plant communities, where 48 plots were established consisting of four replicates of control, N, P, and N + P additions. We characterized four extractable P fractions, maximum P sorption capacity, and extractable NO3?N and NH4?N, measured N and P adsorption fl uxes, determined the soil moisture content, and measured the total N and P concentrations in soils and below- and aboveground plant material. The...
Invasive annual cheatgrass (Bromus tectorum L.) and medusahead wildrye [Taeniatherum caput-medusae (L.) Nevski ssp. asperum (Simonk.) Melderis] have decreased livestock productivity and biological diversity and increased the frequency of wildfire on rangelands in the western United States. On disturbed sites, squirreltail (Elymus sp.), a short-lived North American perennial, appears to compete against invasive exotic annuals when available soil N and nitrification are reduced. We tested the hypothesis that differences in N uptake activity could account for this phenomenon. North American seedlings of perennial bluebunch wheatgrass [Pseudoroegneria spicata (Pursh) �. Löve], four populations of squirreltail, and...
The rapid rewetting of a dry soil often yields a pulse in soil CO2 production that persists for 2 to 6 d. This phenomenon is a common occurrence in surface soils, yet the mechanism responsible for producing the CO2 pulse has not been positively identified, We studied the effects of a single drying and rewetting event on soil C pools, to identify which specific C substrates are mineralized to produce the observed pulse in respiration rates. We labeled two soils with C-glucose and measured the enrichment and pool sizes of the released CO2, extractable biomass C, and extractable soil organic matter (SOM-C) throughout a drying and rewetting cycle. After rewetting, respiration rates were 475 to 370% higher than the rates...
Encroachment of pinyon?juniper woodland into rangeland ecosystems is prevalent across the western United States. Mechanisms associated with this successful encroachment are speculative, but probably, in part, involve the effective use of water resources. We explored the ecohydrological characteristics of a two-needle pinyon pine (Pinus edulis Engelm.)?Utah juniper [Juniperus osteosperma (Torr.) Little] woodland on the Colorado Plateau in Utah. We have discovered that a high level of natural soil water repellency or hydrophobicity exists under the canopies of both pinyon and juniper species. We found, following summer precipitation events, that soil water repellency under trees concentrated the soil water below the...
In the marginally productive rangelands of the semiarid, southwestern USA, the maintenance of organic C (OC) is essential to the stability of the ecosystem. This study was conducted to identify landscape factors responsible for the distribution of OC in watershed soils, its loss from upland areas and subsequent transport within the stream system of a large semiarid watershed (Walnut Gulch Experimental Watershed [WGEW], Tombstone, AZ). Samples were collected along transects from the surface 5 cm of each major soil mapping unit in six subwatersheds (SW). Data were recorded for slope class, landscape position, and aspect at each of the 435 sampling points. Soil analyses consisted of: total C and OC, particle-size distribution,...
In many semiarid regions, runoff and erosion differ according tovegetation patch type. These differences, although hypothesized tofundamentally affect ecological processes, have been poorly quanti-fied. In a semiarid pinon-juniper woodland [Pinus edulis Engelm.and Juniperus monosperma (Engelm.) Sarg.] in northern New Mexico,we measured runoff and erosion from the three patch types thatcompose these woodlands: Canopy patches (those beneath woodyplants), vegetated patches in intercanopy areas, and bare patches inintercanopy areas. The bare intercanopy patches exhibited the highestrates, followed by vegetated intercanopy patches and then by canopypatches. Large convective summer storms, though relatively infre-quent,...