Soil Microbial Responses to Temporal Variations of Moisture and Temperature in a Chihuahuan Desert Grassland

Global climate change models indicate that storm magnitudes will increase in many areas throughout southwest North America, which could result in up to a 25% increase in seasonal precipitation in the Big Bend region of the Chihuahuan Desert over the next 50 years. Seasonal precipitation is a key limiting factor regulating primary productivity, soil microbial activity, and ecosystem dynamics in arid and semiarid regions. As decomposers, soil microbial communities mediate critical ecosystem processes that ultimately affect the success of all trophic levels, and the activity of these microbial communities is primarily regulated by moisture availability. This research is focused on elucidating soil microbial responses to seasonal and yearly changes in soil moisture, temperature, and selected soil nutrient and edaphic properties in a Sotol Grassland in the Chihuahuan Desert at Big Bend National Park. Soil samples were collected over a 3-year period in March and September (2004-2006) at 0-15 cm soil depth from 12 3 x 3 m community plots. Bacterial and fungal carbon usage (quantified using Biolog 96-well micro-plates) was related to soil moisture patterns (ranging between 3.0 and 14%). In addition to soil moisture, the seasonal and yearly variability of soil bacterial activity was most closely associated with levels of soil organic matter, extractable NH(4)-N, and soil pH. Variability in fungal activity was related to soil temperatures ranging between 13 and 26 degrees C. These findings indicate that changes in soil moisture, coupled with soil temperatures and resource availability, drive the functioning of soil-microbial dynamics in these desert grasslands. Temporal patterns in microbial activity may reflect the differences in the ability of bacteria and fungi to respond to seasonal patterns of moisture and temperature. Bacteria were more able to respond to moisture pulses regardless of temperature, while fungi only responded to moisture pulses during cooler seasons with the exception of substantial increased magnitudes in precipitation occurring during warmer months. Changes in the timing and magnitude of precipitation will alter the proportional contribution of bacteria and fungi to decomposition and nitrogen mineralization in this desert grassland.     

Soil Respiration and Organic Carbon Dynamics with Grassland Conversions to Woodlands in Temperate China

Soils are the largest terrestrial carbon store and soil respiration is the second-largest flux in ecosystem carbon cycling. Across China''s temperate region, climatic changes and human activities have frequently caused the transformation of grasslands to woodlands. However, the effect of this transition on soil respiration and soil organic carbon (SOC) dynamics remains uncertain in this area. In this study, we measured in situ soil respiration and SOC storage over a two-year period (Jan. 2007–Dec. 2008) from five characteristic vegetation types in a forest-steppe ecotone of temperate China, including grassland (GR), shrubland (SH), as well as in evergreen coniferous (EC), deciduous coniferous (DC) and deciduous broadleaved forest (DB), to evaluate the changes of soil respiration and SOC storage with grassland conversions to diverse types of woodlands. Annual soil respiration increased by 3%, 6%, 14%, and 22% after the conversion from GR to EC, SH, DC, and DB, respectively. The variation in soil respiration among different vegetation types could be well explained by SOC and soil total nitrogen content. Despite higher soil respiration in woodlands, SOC storage and residence time increased in the upper 20 cm of soil. Our results suggest that the differences in soil environmental conditions, especially soil substrate availability, influenced the level of annual soil respiration produced by different vegetation types. Moreover, shifts from grassland to woody plant dominance resulted in increased SOC storage. Given the widespread increase in woody plant abundance caused by climate change and large-scale afforestation programs, the soils are expected to accumulate and store increased amounts of organic carbon in temperate areas of China.     

Linking Microbial Community Structure and Function to Seasonal Differences in Soil Moisture and Temperature in a Chihuahuan Desert Grassland

Global and regional climate models predict higher air temperature and less frequent, but larger precipitation events in arid regions within the next century. While many studies have addressed the impact of variable climate in arid ecosystems on plant growth and physiological responses, fewer studies have addressed soil microbial community responses to seasonal shifts in precipitation and temperature in arid ecosystems. This study examined the impact of a wet (2004), average (2005), and dry (2006) year on subsequent responses of soil microbial community structure, function, and linkages, as well as soil edaphic and nutrient characteristics in a mid-elevation desert grassland in the Chihuahuan Desert. Microbial community structure was classified as bacterial (Gram-negative, Gram-positive, and actinomycetes) and fungal (saprophytic fungi and arbuscular mycorrhiza) categories using (fatty acid methyl ester) techniques. Carbon substrate use and enzymic activity was used to characterize microbial community function annually and seasonally (summer and winter). The relationship between saprophytic fungal community structure and function remained consistent across season independent of the magnitude or frequency of precipitation within any given year. Carbon utilization by fungi in the cooler winter exceeded use in the warmer summer each year suggesting that soil temperature, rather than soil moisture, strongly influenced fungal carbon use and structure and function dynamics. The structure/function relationship for AM fungi and soil bacteria notably changed across season. Moreover, the abundance of Gram-positive bacteria was lower in the winter compared to Gram-negative bacteria. Bacterial carbon use, however, was highest in the summer and lower during the winter. Enzyme activities did not respond to either annual or seasonal differences in the magnitude or timing of precipitation. Specific structural components of the soil microbiota community became uncoupled from total microbial function during different seasons. This change in the microbial structure/function relationship suggests that different components of the soil microbial community may provide similar ecosystem function, but differ in response to seasonal temperature and precipitation. As soil microbes encounter increased soil temperatures and altered precipitation amounts and timing that are predicted for this region, the ability of the soil microbial community to maintain functional resilience across the year may be reduced in this Chihuahuan Desert ecosystem.     

Impact of Environmental Factors and Biological Soil Crust Types on Soil Respiration in a Desert Ecosystem

The responses of soil respiration to environmental conditions have been studied extensively in various ecosystems. However, little is known about the impacts of temperature and moisture on soils respiration under biological soil crusts. In this study, CO₂ efflux from biologically-crusted soils was measured continuously with an automated chamber system in Ningxia, northwest China, from June to October 2012. The highest soil respiration was observed in lichen-crusted soil (0.93±0.43 µmol m⁻² s⁻¹) and the lowest values in algae-crusted soil (0.73±0.31 µmol m⁻² s⁻¹). Over the diurnal scale, soil respiration was highest in the morning whereas soil temperature was highest in the midday, which resulted in diurnal hysteresis between the two variables. In addition, the lag time between soil respiration and soil temperature was negatively correlated with the soil volumetric water content and was reduced as soil water content increased. Over the seasonal scale, daily mean nighttime soil respiration was positively correlated with soil temperature when moisture exceeded 0.075 and 0.085 m³ m⁻³ in lichen- and moss-crusted soil, respectively. However, moisture did not affect on soil respiration in algae-crusted soil during the study period. Daily mean nighttime soil respiration normalized by soil temperature increased with water content in lichen- and moss-crusted soil. Our results indicated that different types of biological soil crusts could affect response of soil respiration to environmental factors. There is a need to consider the spatial distribution of different types of biological soil crusts and their relative contributions to the total C budgets at the ecosystem or landscape level.     

Soil microarthropods from the Chihuahuan Desert of New Mexico

An account is given of the soil- and litter-dwelling microarthropods collected under three species of desert shrub in southern New Mexico. Particular attention is paid to the acarine component which represents 80% of the microarthropod fauna. Comparisons made with samples collected one hour after wetting show that only the Collembola respond rapidly to changes in the moisture content of the soil. The results of the study suggest that the depth and stability of the surface litter determine the degree to which microarthropod activity is affected by the application of water.     

Association between Plant Canopies and the Spatial Patterns of Infiltration in Shrubland and Grassland of the Chihuahuan Desert,New Mexico

Shrubs have invaded extensive areas of grassland in the southwestern United States. The zones of nutrient-rich soil found beneath plant canopies, referred to as “islands of fertility,” are more intense and spaced farther apart in shrubland than in grassland. This difference in the spatial pattern of soil nutrients may reinforce shrub invasion. Changes in water availability in the soil could also influence shrub invasion. Here we compare the spatial patterns of infiltration, defined as the total equivalent water depth entering the soil following individual rainfall events or summed over many events, at adjacent grass- and shrub-dominated sites in the Sevilleta National Wildlife Refuge. We use two infiltration data sets. First, following four rainfall events, we measured soil moisture and wetting front depth at 10-cm intervals along 24-m transects. We estimate infiltration from these data. Second, we use vertical arrays of soil moisture probes to compare infiltration between adjacent canopies and interspaces following 31 storms. In both the grassland and shrubland, infiltration is typically greater beneath plant canopies than beneath interspaces. Canopies are oases where soil moisture is higher than in the surrounding areas. However, infiltration is not greater beneath canopies when surface runoff is limited. In the shrubland, the canopy–interspace infiltration ratio increases as storm size, and therefore runoff, increases. This relationship also exists in the grassland, but it is not as strong or clear. The magnitude of spatial variability of infiltration is similar in shrubland and grassland. In addition, the distance over which infiltration is correlated is approximately 50 cm in both environments. Most of the spatial variability exists between the stem and canopy margin in the shrubland and straddling the canopy margin in the grassland. The most notable difference is that subcanopy oases are spread farther apart in the shrubland because canopies are separated by larger interspaces in this environment. Received 30 October 2001; accepted 1 August 2002.     

Soil Microbial Community Response to Drought and Precipitation Variability in the Chihuahuan Desert

Increases in the magnitude and variability of precipitation events have been predicted for the Chihuahuan Desert region of West Texas. As patterns of moisture inputs and amounts change, soil microbial communities will respond to these alterations in soil moisture windows. In this study, we examined the soil microbial community structure within three vegetation zones along the Pine Canyon Watershed, an elevation and vegetation gradient in Big Bend National Park, Chihuahuan Desert. Soil samples at each site were obtained in mid-winter (January) and in mid-summer (August) for 2 years to capture a component of the variability in soil temperature and moisture that can occur seasonally and between years along this watershed. Precipitation patterns and amounts differed substantially between years with a drought characterizing most of the second year. Soils were collected during the drought period and following a large rainfall event and compared to soil samples collected during a relatively average season. Structural changes within microbial community in response to site, season, and precipitation patterns were evaluated using fatty acid methyl ester (FAME) and polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) analyses. Fungal FAME amounts differed significantly across seasons and sites and greatly outweighed the quantity of bacterial and actinomycete FAME levels for all sites and seasons. The highest fungal FAME levels were obtained in the low desert scrub site and not from the high elevation oak–pine forests. Total bacterial and actinomycete FAME levels did not differ significantly across season and year within any of the three locations along the watershed. Total bacterial and actinomycete FAME levels in the low elevation desert-shrub and grassland sites were slightly higher in the winter than in the summer. Microbial community structure at the high elevation oak–pine forest site was strongly correlated with levels of NH₄ ⁺–N, % soil moisture, and amounts of soil organic matter irrespective of season. Microbial community structure at the low elevation desert scrub and sotol grasslands sites was most strongly related to soil pH with bacterial and actinobacterial FAME levels accounting for site differences along the gradient. DGGE band counts of amplified soil bacterial DNA were found to differ significantly across sites and season with the highest band counts found in the mid-elevation grassland site. The least number of bands was observed in the high elevation oak–pine forest following the large summer-rain event that occurred after a prolonged drought. Microbial responses to changes in precipitation frequency and amount due to climate change will differ among vegetation zones along this Chihuahuan Desert watershed gradient. Soil bacterial communities at the mid-elevation grasslands site are the most vulnerable to changes in precipitation frequency and timing, while fungal community structure is most vulnerable in the low desert scrub site. The differential susceptibility of the microbial communities to changes in precipitation amounts along the elevation gradient reflects the interactive effects of the soil moisture window duration following a precipitation event and differences in soil heat loads. Amounts and types of carbon inputs may not be as important in regulating microbial structure among vegetation zones within in an arid environment as is the seasonal pattern of soil moisture and the soil heat load profile that characterizes the location.     

Plant regulation of soil nutrient distribution in the northern Chihuahuan Desert

Vegetation throughout the southwestern United States has changed from perennial grassland to woody shrubland over the past century. Previous studies on the development of islands of fertility focused primarily on only the most limiting, plant-essential element, soil nitrogen (N). The research presented here addressed the question of whether other plant-essential elements, namely phosphorus (P) and potassium (K), showed similar concentration gradients under the desert shrub Larrea tridentata, creosotebush. It also examined whether the spatial distribution of N, P, and K differed from that of essential, but non-limiting nutrients, namely calcium (Ca), magnesium (Mg), and sulfur (S), and non-essential elements, namely sodium (Na), chloride (Cl), and fluoride (F). Within adjacent grassland and shrubland plots, surface soils were collected under and between vegetation and analyzed for a suite of soil nutrients. Soil nutrient distribution followed a uniform pattern that mirrored the spatial homogeneity of bunchgrasses in the grassland, but followed a patchy distribution that mirrored the spatial heterogeneity of individual shrubs in the shrubland. The main differences were that in the grassland, all elements were uniformly distributed, but in the shrubland the plant-essential elements, nitrogen, phosphorus, and potassium, were concentrated under the shrub canopy, and the non-limiting and non-essential elements were either concentrated in the intershrub spaces or were equally concentrated under shrubs and in the interspaces. Our results show how vegetation shifts from grassland to shrubland contribute to long-term, widespread change in the structure and function of desert ecosystems.     

Temporal Asynchrony in Soil Nutrient Dynamics and Plant Production in a Semiarid Ecosystem

A central goal of ecosystem ecology is to understand how the cycling of nutrients and the growth of organisms are linked. Ecologists have repeatedly observed that nutrient mineralization and plant production are closely coupled in time in many terrestrial ecosystems. Typically, mineralization rates of limiting nutrients, particularly of nitrogen, during the growing season determine nutrient availability while pools of mineral nutrients remain low and relatively constant. Although several previous reports suggest nitrogen mineralization has the potential to vary seasonally and out of phase with plant production, such a phenomenon has been poorly documented. Here we report results from a semiarid savanna ecosystem characterized by distinct temporal asynchrony in rates of soil nitrogen cycling and plant production. Periods of positive plant growth following the onset of rains coincide with periods of low N turnover rates, whereas higher rates occur late in the wet season following plant senescence and throughout dry seasons. Plant uptake from the substantial mineral N pool present early in the growing season is sufficient to explain most of the N allocation to aboveground plant biomass during the growing season, even in the absence of any wet-season mineralization. The mineral N pool is subsequently recharged by late wet- and dry-season mineralization, plus urine inputs at sites with high levels of ungulate activity. These findings suggest fundamental changes in the quality of substrates available to decomposers over a seasonal cycle, with significant implications for the partitioning of limiting nutrients by plant species, the seasonal pattern of nutrient limitations of aboveground production, and the effective use of N fertilizers in semiarid ecosystems.     

Biological Soil Crust Microsites Are the Main Contributor to Soil Respiration in a Semiarid Ecosystem

Biological soil crusts (BSCs) are a key biotic component of dryland ecosystems worldwide. However, most studies carried out to date on carbon (C) fluxes in these ecosystems, such as soil respiration, have neglected them. We conducted a 3.5-year field experiment to evaluate the spatio-temporal heterogeneity of soil respiration in a semiarid Stipa tenacissima steppe and to assess the contribution of BSC-dominated areas to the annual soil respiration of the whole ecosystem. We selected the six most frequent microsites in the study area: Stipa tussocks (ST), Retama sphaerocarpa shrubs (RS), and open areas with very low (<5% BSC cover, BS), low, medium and high cover of well-developed BSCs. Soil respiration rates did not differ among BSC-dominated microsites but were significantly higher and lower than those found in BS and ST microsites, respectively. A model using soil temperature and soil moisture accounted for over 85% of the temporal variation in soil respiration throughout the studied period. Using this model, we estimated a range of 240.4–322.6 g C m⁻² y⁻¹ released by soil respiration at our study area. Vegetated (ST and RS) and BSC-dominated microsites accounted for 37 and 42% of this amount, respectively. Our results indicate that accounting for the spatial heterogeneity in soil respiration induced by BSCs is crucial to provide accurate estimations of this flux at the ecosystem level. They also highlight that BSC-dominated areas are the main contributor to the total C released by soil respiration and, therefore, must be considered when estimating C budgets in drylands.     

Response of dominant grass and shrub species to water manipulation: an ecophysiological basis for shrub invasion in a Chihuahuan Desert Grassland

Increases in woody vegetation and declines in grasses in arid and semi-arid ecosystems have occurred globally since the 1800s, but the mechanisms driving this major land-cover change remain uncertain and controversial. Working in a shrub-encroached grassland in the northern Chihuahuan Desert where grasses and shrubs typically differ in leaf-level nitrogen allocation, photosynthetic pathway, and root distribution, we asked if differences in leaf-level ecophysiology could help explain shrub proliferation. We predicted that the relative performance of grasses and shrubs would vary with soil moisture due to the different morphological and physiological characteristics of the two life-forms. In a 2-year experiment with ambient, reduced, and enhanced precipitation during the monsoon season, respectively, the encroaching C(3) shrub (honey mesquite Prosopis glandulosa) consistently and substantially outperformed the historically dominant C(4) grass (black grama Bouteloua eriopoda) in terms of photosynthetic rates while also maintaining a more favorable leaf water status. These differences persisted across a wide range of soil moisture conditions, across which mesquite photosynthesis was decoupled from leaf water status and moisture in the upper 50 cm of the soil profile. Mesquite's ability to maintain physiologically active leaves for a greater fraction of the growing season than black grama potentially amplifies and extends the importance of physiological differences. These physiological and phenological differences may help account for grass displacement by shrubs in drylands. Furthermore, the greater sensitivity of the grass to low soil moisture suggests that grasslands may be increasingly susceptible to shrub encroachment in the face of the predicted increases in drought intensity and frequency in the desert of the southwestern USA.     

Above- and belowground responses to nitrogen addition in a Chihuahuan Desert grassland

Increased available soil nitrogen can increase biomass, lower species richness, alter soil chemistry and modify community structure in herbaceous ecosystems worldwide. Although increased nitrogen availability typically increases aboveground production and decreases species richness in mesic systems, the impacts of nitrogen additions on semiarid ecosystems remain unclear. To determine how a semiarid grassland responds to increased nitrogen availability, we examined plant community structure and above- and belowground net primary production in response to long-term nitrogen addition in a desert grassland in central New Mexico, USA. Plots were fertilized annually (10 g N m⁻²) since 1995 and NPP measured from 2004 to 2009. Differences in aboveground NPP between fertilized and control treatments occurred in 2004 following a prescribed fire and in 2006 when precipitation was double the long-term average during the summer monsoon. Presumably, nitrogen only became limiting once drought stress was alleviated. Belowground NPP was also related to precipitation, and greatest root growth occurred the year following the wettest summer, decreasing gradually thereafter. Belowground production was unrelated to aboveground production within years and unrelated to nitrogen enrichment. Species richness changed between years in response to seasonal precipitation variability, but was not altered by nitrogen addition. Community structure did respond to nitrogen fertilization primarily through increased abundance of two dominant perennial grasses. These results were contrary to most nitrogen addition studies that find increased biomass and decreased species richness with nitrogen fertilization. Therefore, factors other than nitrogen deposition, such as fire or drought, may play a stronger role in shaping semiarid grassland communities than soil fertility.     

Confounding Effects of Soil Moisture on the Relationship Between Ecosystem Respiration and Soil Temperature in Switchgrass

Understanding the response of ecosystem respiration (ER) to major environmental drivers is critical for estimating carbon sequestration and large-scale modeling research. Temperature effect on ER is modified by other environmental factors, mainly soil moisture, and such information is lacking for switchgrass (Panicum virgatum L.) ecosystems. The objective of this study was to examine seasonal variation in ER and its relationship with soil temperature (T _s) and moisture in a switchgrass field. ER from the nighttime net ecosystem CO₂ exchange measurements by eddy covariance system during the 2011 and 2012 growing seasons was analyzed. Nighttime ER ranged from about 2 (early growing season) to as high as 13 μmol m^?2 s^?1 (peak growing period) and showed a clear seasonality, with low rates during warm (>30 °C) and dry periods (<0.20 m³ m^?3 of soil water content). No single temperature or moisture function described variability in ER on the seasonal scale. However, an exponential temperature–respiration function explained over 50 % of seasonal variation in ER at adequate soil moisture (>0.20 m³ m^?3), indicating that soil moisture <0.20 m³ m^?3 started to limit ER. Due to the limitation of soil–atmosphere gas exchange, ER rates declined markedly in wet soil conditions (>0.35 m³ m^?3) as well. Consequently, both dry and wet conditions lowered temperature sensitivity of respiration (Q ₁₀). Stronger ER–T _s relationships were observed at higher soil moisture levels. These results demonstrate that soil moisture greatly influences the dynamics of ER and its relationship with T _s in drought prone switchgrass ecosystems.     

Vegetation Affects the Relative Abundances of Dominant Soil Bacterial Taxa and Soil Respiration Rates in an Upland Grassland Soil

Plant-derived organic matter inputs are thought to be a key driver of soil bacterial community composition and associated soil processes. We sought to investigate the role of acid grassland vegetation on soil bacterial community structure by assessing bacterial diversity in combination with other soil variables in temporally and spatially distinct samples taken from a field-based plant removal experiment. Removal of aboveground vegetation resulted in reproducible differences in soil properties, soil respiration and bacterial diversity. Vegetated soils had significantly increased carbon and nitrogen concentrations and exhibited higher rates of respiration. Molecular analyses revealed that the soils were broadly dominated by Alphaproteobacterial and Acidobacterial lineages, with increased abundances of Alphaproteobacteria in vegetated soils and more Acidobacteria in bare soils. This field-based study contributes to a growing body of evidence documenting the effect of soil nutrient status on the relative abundances of dominant soil bacterial taxa, with Proteobacterial taxa dominating over Acidobacteria in soils exhibiting higher rates of C turnover. Furthermore, we highlight the role of aboveground vegetation in mediating this effect by demonstrating that plant removal can alter the relative abundances of dominant soil taxa with concomitant changes in soil CO₂-C efflux.     

Interactive Effects of Black-Tailed Prairie Dogs and Cattle on Shrub Encroachment in a Desert Grassland Ecosystem

The widespread encroachment of woody plants throughout the semi-arid grasslands in North America has largely resulted from overgrazing by domestic livestock, fire suppression, and loss of native large and small mammalian herbivores. Burrowing-herbivorous mammals, such as prairie dogs (Cynomys spp.), help control shrub encroachment through clipping of shrubs and consumption of their seedlings, but little is known about how this important ecological role interacts with and may be influenced by co-existing large herbivores, especially domestic livestock. Here, we established a long-term manipulative experiment using a 2 × 2 factorial design to assess the independent and interactive effects of black-tailed prairie dogs (Cynomys ludovicianus) and cattle (Bos taurus) on honey mesquite (Prosopis glandulosa) abundance and structure. We found that, after five years, mesquite abundance was three to five times greater in plots where prairie dogs were removed compared to plots where they occurred together or alone, respectively. While both prairie dogs and cattle reduced mesquite cover, the effect of prairie dogs on reducing mesquite abundance, cover, and height was significantly greater than that by cattle. Surprisingly, cattle grazing enhanced prairie dog abundance, which, in turn, magnified the effects of prairie dogs on mesquite shrubs. Mesquite canopy cover per hectare was three to five times greater where prairie dogs and cattle were absent compared to where they occurred together or by themselves; whereas, cumulative mesquite height was two times lower on sites where prairie dog and cattle occurred together compared to where they occurred alone or where neither occurred. Data from our experimental study demonstrate that prairie dogs and moderate grazing by cattle can suppress mesquite growth, and, when their populations are properly managed, they may interact synergistically to significantly limit mesquite encroachment in desert grasslands.