Arid Land Soils Research Articles

Linking water and nutrients through the vadose zone: a fungal interface between the soil and plant systems

Plant water availability, use, and management have largely focused on physical processes of infiltration and the role of roots in uptake and transpiration. However, roots and mycorrhizal fungi redistribute water in complex patterns. Here I describe some of our observations and experiments showing that mycorrhizal fungi play key roles in moving water for both transpiration and to facilitate nutrient acquisition under dry conditions. Mycorrhizal fungal hyphae grow from both surface and deep roots even into bedrock to help extract water under dry conditions. In both deep and surface roots, mycorrhizal fungi acquire water from pores too small for roots and root hairs to access, and at distances from roots and root hairs. Mycorrhizal fungi are also able to utilize hydraulic-lifted water from plants to obtain nutrients in extremely dry surface soils. The importance of these root symbionts in water and nutrient dy- namics, and as integrators of surface and deeper water dynamics need further investigation. Predicting water use by irrigated crops in mesic lands has become relatively accurate, facilitating manipula- tion of water application and for predicting crop growth and production. However, these models are largely based on the relatively even distribution of soil structural elements through the root zone, coupled with an understanding in the vertical spatial distribu- tion of water inputs. Even then, however, water movement within the soil profile is chaotic, resulting in complex patterns of water movement under unsatu- rated conditions. Much of the research on water movement and use has been undertaken as a physical process, but here I discuss the role of mycorrhizal fungi in redistributing soil moisture for plant use and for plant re-allocation to mycorrhizal fungi to acquire surface nutrients. Water and nutrients are tightly coupled in regulating ecosystem processes including productivity and de- composition. However, in arid land soils, water and nutrients are spatially segregated. Most of the nutrients are applied (as fertilizers) or continually cycled in the upper soil layers and the highest nutrient availability is generally in a thin organic layer between the litter and the mineral soils, and in the upper mineral soil. In

Woodland Hypothesis for Devonian Tetrapod Evolution

The rarity of Devonian tetrapods and the absence of tetrapods during the first 14 million years of the Mississippian (Romer’s Gap) have inspired hypotheses of fish-tetrapod evolutionary transition as an escape from difficult habitats such as deserts or stagnant waters. These hypotheses and Romer’s Gap are tested here using depth to calcic horizon in paleosols of the northern Appalachians as a proxy for precipitation and vegetation changes during the Devonian and Mississippian. All tetrapod bones and trackways in this region, as well as fossil tree remains, were found at times of high precipitation (indicated by deep calcic paleosols), when subhumid woodlands expanded at the expense of semiarid shrublands in alluvial lowlands. No tetrapods or tree fossils were found in abundant paleosols with the shallow calcic horizons of aridland soils. The mean annual precipitation requirement of Devonian-Mississippian tetrapods and trees was at least mm. Global distribution of prototetrapods and aquatic tetrapods also shows that they lived earliest (Eifelian) in coastal lagoon or estuary margin soils of humid regions rather than in aridlands or intertidal flats. Tetrapods later spread to a variety of habitats, but continued preference for noncalcareous soils of woodlands and forests may explain Romer’s Gap as a preservational artifact. A woodland hypothesis of tetrapod evolution is presented here: limbs and necks were selected for by scavenging and hunting in shallow-flooded woodlands and oxbow lakes during a unique period in Earth history, after evolution of flood-ponding trees and before effective terrestrial predator resistance.

Evaluation of four grasses for use in phytoremediation of Cs-contaminated arid land soil

We used greenhouse experiments to evaluate cesium (Cs) uptake by four grasses, Agropyron spicatum (Pursh) Scribn & Smith, Leymus cinereus Scribn & Merr., Agropyron cristatum (L.) Gaertn. and Bromus tectorum L., to determine their potential as phytoremediation agents for Cs-contaminated soils. These four species grow well in the Intermountain region of western North America primarily on moderately coarse to coarsely textured soils where annual precipitation ranges from 20 to 50 cm. Whereas A. cristatum and B. tectorum are introduced species from Asia and the Mediterranean region of Europe, respectively, A. spicatum and L. cinereus are native to the Great Plains and Intermountain regions of North America. Plants were grown under two treatments each of: (1) soil Cs (ambient and 50 mg kg−1 + ambient), (2) soil fertility (ambient and ambient + 40 mg N kg−1 and 60 mg P kg−1), and 3) soil moisture (35% and 70% water holding capacity of the potted soils). Shoot Cs concentration in high soil Cs treatments was approximately ten times greater than in low soil Cs treatments. Even though shoot Cs concentrations tended to be lower in high soil Cs-high fertility-high soil moisture treatments than in high soil Cs-ambient fertility-low soil moisture treatments, total Cs uptake was greater because shoot biomass was approximately ten-fold greater. Shoots did not show signs of Cs toxicity in the high soil Cs treatments. We concluded, however, based on the low transfer factors (∼1.0) for all these grasses, that none were strong candidates as phytoremediation agents for Cs-contaminated soils.

Growth promotion of the freshwater microalga Chlorella vulgaris by the nitrogen-fixing, plant growth-promoting bacterium Bacillus pumilus from arid zone soils

Immobilization of Bacillus pumilus ES4 from arid land soils, a plant growth-promoting bacterium and the freshwater, green microalga Chlorella vulgaris enhanced microalgal growth only in the absence of combined nitrogen in synthetic growth medium (SGM), but not in medium with combined nitrogen. B. pumilus was able to fix nitrogen in N-free SGM and its growth yielded an accumulation of ammonium in the medium. On its own, B. pumilus is a poor agent for removing nitrogen and phosphorus from wastewater, while C. vulgaris is a capable microorganism. By jointly immobilizing the two microorganisms, the capacity to remove nitrogen and phosphorus from the medium by the microalgae culture was not enhanced, but, at the cell level, removal of these nutrients was significantly enhanced. It appears that growth promotion induced by B. pumilus on C. vulgaris is related to nitrogen fixation.

Pulse dynamics and microbial processes in aridland ecosystems

Summary Aridland ecosystems cover about one‐third of terrestrial environments globally, yet the extent to which models of carbon (C) and nitrogen (N) cycling, developed largely from studies of mesic ecosystems, apply to aridland systems remains unclear. Within aridland ecosystems, C and N dynamics are often described by a pulse‐reserve model in which episodic precipitation events stimulate biological activity that generate reserves of biomass, propagules and organic matter that prime the ecosystem to respond rapidly to subsequent precipitation events. The role of microbial C and N processing within the pulse‐reserve paradigm has not received much study. We present evidence suggesting that fungi play a critical and underappreciated role in aridland soils, including efficient decomposition of recalcitrant C compounds, N‐transformations such as nitrification, and nutrient storage and translocation of C and N between plants and biotic soil crusts. While fungi may perform some of these functions in other ecosystems, this ‘fungal loop’ assumes particular importance in the N cycle in aridlands because water availability imposes even greater restrictions on bacterial activity and physicochemical processes limit accumulation of soil organic matter (SOM). We incorporate these findings into a Threshold‐Delay Nutrient Dynamics (TDND) model for aridland ecosystems in which plant responses to pulsed precipitation events are mediated by a fungal loop that links C and N cycling, net primary production (NPP) and decomposition in aridland soils. Synthesis. Arid ecosystems are highly sensitive to global environmental change including N deposition and altered precipitation patterns; yet, models from mesic ecosystems do not adequately apply to aridland environments. Our ‘fungal loop’ N cycle model integrates spatial structure with pulse dynamics and extends the pulse‐reserve paradigm to include the key role of microbial processes in aridland ecosystem dynamics.

Effects of Altered Temperature and Precipitation on Desert Protozoa Associated with Biological Soil Crusts

Biological soil crusts are diverse assemblages of bacteria, cyanobacteria, algae, fungi, lichens, and mosses that cover much of arid land soils. The objective of this study was to quantify protozoa associated with biological soil crusts and test the response of protozoa to increased temperature and precipitation as is predicted by some global climate models. Protozoa were more abundant when associated with cyanobacteria/lichen crusts than with cyanobacteria crusts alone. Amoebae, flagellates, and ciliates originating from the Colorado Plateau desert (cool desert, primarily winter precipitation) declined 50-, 10-, and 100-fold, respectively, when moved in field mesocosms to the Chihuahuan Desert (hot desert, primarily summer rain). However, this was not observed in protozoa collected from the Chihuahuan Desert and moved to the Sonoran desert (hot desert, also summer rain, but warmer than Chihuahuan Desert). Protozoa in culture began to encyst at 37 degrees C. Cysts survived the upper end of daily temperatures (37-55 degrees C), and could be stimulated to excyst if temperatures were reduced to 15 degrees C or lower. Results from this study suggest that cool desert protozoa are influenced negatively by increased summer precipitation during excessive summer temperatures, and that desert protozoa may be adapted to a specific desert's temperature and precipitation regime.

Soil nematode communities are ecologically more mature beneath late- than early-successional stage biological soil crusts

Biological soil crusts are key mediators of carbon and nitrogen inputs for arid land soils and often represent a dominant portion of the soil surface cover in arid lands. Free-living soil nematode communities reflect their environment and have been used as biological indicators of soil condition. In this study, we test the hypothesis that nematode communities are successionally more mature beneath well-developed, late-successional stage crusts than immature, early-successional stage crusts. We identified and enumerated nematodes by genus from beneath early- and late-stage crusts from both the Colorado Plateau, Utah (cool, winter rain desert) and Chihuahuan Desert, New Mexico (hot, summer rain desert) at 0–10 and 10–30 cm depths. As hypothesized, nematode abundance, richness, diversity, and successional maturity were greater beneath well-developed crusts than immature crusts. The mechanism of this aboveground–belowground link between biological soil crusts and nematode community composition is likely the increased food, habitat, nutrient inputs, moisture retention, and/or environmental stability provided by late-successional crusts. Canonical correspondence analysis of nematode genera demonstrated that nematode community composition differed greatly between geographic locations that contrast in temperature, precipitation, and soil texture. We found unique assemblages of genera among combinations of location and crust type that reveal a gap in scientific knowledge regarding empirically derived characterization of dominant nematode genera in deserts soils and their functional role in a crust-associated food web.

Attaining Sustainable Farm Management Systems in Semi-Arid Areas in Kenya: Few Technical Options, Many Policy Challenges

Soils in arid and semi-arid lands (ASAL) are fragile, low in fertility and susceptible to erosion and leaching. To address these problems, activities were implemented in 1998–2003 to identify current problems, and design, test, implement, demonstrate and disseminate improved, integrated soil fertility management techniques. Current soil fertility management results in slightly negative nutrient balances, especially for phosphorus and potassium. Recycling of nutrients through crop residues and animal manure is inefficient, with evidently high losses. Due to the relatively high price of fertilisers and the high risks of crop failure, use of mineral fertilisers is restricted to market-oriented farms with access to irrigation facilities. Of the total farm household population, 35–85% lives below the poverty line. Applying higher rates of Farm Yard Manure (FYM) and/or fertilisers is unattractive and risky. Combinations of FYM and fertilisers give better financial returns than either of the two alone. Where irrigation is available, farmers adopt innovative farming systems that include higher application of mineral and organic fertilisers, and result in higher and more stable yields and higher financial returns. A set of specific policy measures for the semi-arid areas were identified to arrive at necessary changes in the economic environment, leading to a wider range of financially attractive technology options for smallholders.

Estimates of global cyanobacterial biomass and its distribution

We estimated global cyanobacterial biomass in the main reservoirs of cyanobacteria on Earth: marine and freshwater plankton, arid land soil crusts, and endoliths. Estimates were based on typical population density values as measured during our research, or as obtained from literature surveys, which were then coupled with data on global geographical area coverage. Among the marine plankton, the global biomass of Prochlorococcus reaches 120 x 10 12 grams of carbon (g C). and that of Synechoccus some 43 x 10 12 g C. This makes Prochlorococcus and Synechococcus, in that order, the most abundant cyanobacteria on Earth. Tropical marine blooms of Trichodesmium account for an additional 10 x 10 12 g C worldwide. In terrestrial environments, the mass of cyanobacteria in arid land soil crusts is estimated to reach 54 x 10 12 g C and that of arid land endolithic communities an additional 14 x 10 12 g C. The global biomass of planktic cyanobacteria in lakes is estimated to be around 3 x 10 12 g C. Our conservative estimates, which did not include some potentially significant biomass reservoirs such as polar and subarctic areas, topsoils in subhumid climates, and shallow marine and freshwater benthos, indicate that the total global cyanobacterial biomass is in the order of 3 x 10 14 g C, surpassing a thousand million metric tons (10 15 g) of wet biomass.

Trace N gas losses and N mineralization in Mojave desert soils exposed to elevated CO 2

We examined N cycling processes in desert soils exposed to elevated CO 2 to better understand how some features of aridland soil N and C cycling may respond to an altered atmospheric composition. We measured rates of denitrification, potential denitrification, N 2O fluxes, NH 3 volatilization, and net mineralization in an intact Mojave Desert ecosystem with elevated CO 2 (Free Air Carbon Enrichment technology) over 2 y. All measurements were performed on soil under four different cover types: Larrea tridentata; Lycium spp.; Pleuraphis rigida; and plant interspaces. The mean rate of denitrification was 161±96 μg N m −2 d −1. Field fluxes of N 2O occurred sporadically, with a mean of 30±20 μg N m −2 d −1. Rates of NH 3 volatilization experienced less variability than did N 2O fluxes, with a mean of 120±45 μg N m −2 d −1. Mean potential denitrification enzyme activity (DEA) was 146±8 mg N m −2 d −1. Rates of net mineralization were highest in soil under L. tridentata and Lycium spp. (398±108 mg N m −2 d −1) and lowest in the plant interspaces (129±28 mg N m −2 d −1). There was no effect of elevated CO 2 on N 2O fluxes or mineralization rates. There was a 39% increase in NH 3 volatilization with elevated CO 2 during March 2000. Potential DEA increased by 193% with elevated CO 2 in October 1999 and decreased by 45% in March 2001. These results suggest that NH 3 volatilization may be a more important component of aridland gaseous N emissions than previously thought, and that the potential for high DEA does not necessarily induce large fluxes of N 2O under natural conditions, especially in aridlands where rainfall primarily occurs in winter when soil temperatures can limit microbial activity. This study also suggests that the effects of elevated CO 2 on soil microbial activity may not be consistent for all seasons.

Physiological variation among native and exotic winter annual plants associated with microbiotic crusts in the Mojave Desert

Microbiotic crusts are important components of many aridland soils. Research on crusts typically focuses on the increase in soil fertility due to N-fixing micro-organisms, the stabilization of soils against water and wind erosion and the impact of disturbance on N-cycling. The effect of microbiotic crusts on the associated plant community has received little attention. We quantified the influence of crusts on the production, species diversity, nutrient content and water relations of winter annual plant species associated with microbiotic soil crusts in the northeast Mojave Desert. Shoot biomass of winter annuals was 37% greater and plant density was 77% greater on crusts than were biomass and density on soils lacking crust cover (=bare soils). This greater production of annuals on crusts was likely due to enhanced soil conditions including an almost two-fold increase in soil organic matter and inorganic N compared to bare soils. Crusted soils also had 53% greater volumetric water content than bare soils during November and December, the time when winter annuals become established. As plant development progressed into spring, however, soil water availability decreased: More negative plant xylem water potentials were associated with greater plant biomass on crusted soils. Plants associated with microbiotic soil crusts had lower concentrations of N in shoots (mg N g−1 dry mass). However, total shoot N (mg N m−2) was the same in plants growing on the different soil types when biomass production peaked in April. Shoots had similar patterns in their concentration and content of P. Species diversity of annuals was not statistically different between the two soil types. Yet, while native annuals comprised the greatest proportion of shoot biomass on bare soils, exotic forbs and grasses produced more biomass on crusts. Total shoot nutrient content (biomass×concentration) of the two exotic annual species examined was dramatically greater on crusts than bare soils; only one native species had greater shoot content of N and P when growing on crusts than bare soils. Microbiotic crusts appear to increase site fertility in the northeast Mojave Desert, but nutrients and water distributed within a greater biomass of annual plants growing on microbiotic crusts likely resulted in lower concentrations of nutrients in plant tissue and lower xylem pressure potentials than plants growing on bare soils. Exotic annuals growing on crusts appear to respond to the higher N availability by growing faster, potentially outcompeting native annual species.

Carbon Sequestration in Soils

The production, transport, and application of N fertilizer incur COemissions to the atmosphere, which must be subtracted from any apparent increases in soil organic matter before calculating the net carbon sequestration in agricultural soils. Similarly, net carbon sequestration in arid-land soils is reduced by COis emissions when fossil fuels are used to pump irrigation water and when pedogenic carbonate precipitates in soils. Application of manure, important to maintaining agricultural tilth, does not offer a mechanism for the net sequestration of carbon in soils that is relevant to the Kyoto Protocol.

Vulnerability of desert biological soil crusts to wind erosion: the influences of crust development, soil texture, and disturbance

Biological soil crusts, consisting of cyanobacteria, green algae, lichens, and mosses, are important in stabilizing soils in semi-arid and arid lands. Integrity of these crusts is compromised by compressional disturbances such as foot, vehicle, or livestock traffic. Using a portable wind tunnel, we found threshold friction velocities (TFVs) of undisturbed crusts well above wind forces experienced at these sites; consequently, these soils are not vulnerable to wind erosion. However, recently disturbed soils or soils with less well-developed crusts frequently experience wind speeds that exceed the stability thresholds of the crusts. Crustal biomass is concentrated in the top 3mm of soils. Sandblasting by wind can quickly remove this material, thereby reducing N and C inputs from these organisms. This loss can result in reduced site productivity, as well as exposure of unprotected subsurface sediments to wind and water erosion. Actions to reduce impacts to these crusts can include adjustments in type, intensity, and timing of use.

Simulation of daily energy budget and mean soil temperatures at an arid site

Soil temperature is often inadequately based upon relatively few measurements at widely dispersed locations. Within arid regions, such as the desert southwestern United States, soils, microclimates, and thus soil temperature may be markedly heterogeneous. Because extensive measurement of soil temperature is often not feasible, models are needed that simulate soil temperature based on readily available soil survey and “above-ground” weather information. This paper describes a simple energy-budget based model for simulating daily mean temperatures within a bare arid land soil. The model requires basic information on soil physical properties, and daily weather data including air temperature, windspeed, rainfall, and solar radiation to calculate daily surface energy budget components and surface temperature. One of two alternative numerical methods is then used to calculated subsurface temperatures. Tests of the model using 1987 daily temperature data from an arid site at Yuma, Arizona resulted in root mean square deviations within 1.4°C between daily modeled and measured temperatures at both 0.05 and 0.10 m depths. Sensitivity analysis showed modeled temperatures at 0.05 m depth to be most sensitive to parameters affecting the surface energy balance such as air temperature and solar radiation. Modeled temperatures at 1.0m depth were relatively more sensitive to initial temperature conditions and to parameters affecting distribution of energy within the profile such as thermal conductivity.

The effect of sodium acetate buffer pretreatment on the clay minerals of some arid land soils and reference clays

Abstract The pretreatment of soils with pH5 sodium acetate (NaOAc) buffer is widely used to remove carbonates from soils prior to dispersion and separation of clays for mineralogical analysis. The effect of this pretreatment on other minerals such as palygorskite and zeolite (analcime) was evaluated. Soil samples from Arizona and Egypt as well as reference samples of palygorskite and analcime spiked with CaCO3were used. The pretreatment of the soils and reference samples had no observable change in the mineralogy as indicated by X‐ray diffraction and SEM analysis. The study further suggests that the somewhat tedious and time consuming step can be avoided for the mineralogical analysis of many arid land soils. The common procedure for clay separation of calcareous soils for mineralogical analysis and study by X‐ray diffraction is to remove the carbonates by a pH5 sodium acetate pretreatment(1,2). For soils low in carbonates, the pretreatment may be feasible, but for highly calcareous soils the procedure is...

Introduction: Potential of biological processes in desertification control

Abstract The economic importance of arid land soils was neglected for a long time. Increased population pressure, associated with concurrent droughts, resulted in lowered agricultural yields and in degradation of the fragile arid environment. A number of recommendations of the UN Conference of Desertification 1977, based on known grazing and agricultural practices, have been implemented but have shown only partial success. Soil biota has an instrumental role in regulating arid soil fertility. It is the sole agent controlling soil organic matter cycling, providing for N input by N2 fixation, and maintaining nutrient turnover in soils. Use of agronomic methods involving soil biological properties has been mostly neglected in lands of arid zones. The following actions involving soil biology are deemed as initial priorities for implementation to achieve increased arid soil fertility and for soil rehabilitation: (1) utilization and management of available organic matter resources, including organic wastes, (2)...

The Effect of CaCO3on the Uptake of Phosphorous by Two Desert Shrub Species, Larrea tridentata (DC.) Cov. and Parthenium incanum H. B. K.

In a phytotron growth study, the effects of soil CaCO3 on the uptake of P by two desert shrub species were species specific. Larrea tridentata seedlings responded to soil CaCO3 with increased root: shoot ratios, increased N:P ratios in tissues, and decreased specific absorption rates of P, indicating that CaCO3 directly inhibited the uptake or the availability of P to roots. Parthenium incanum seedlings responded negatively to CaCO3, but this response was not specific to P uptake and did not appear to be caused by a reduction in P availability. Pedogenic carbonates, present in many arid-land soils of the world, might greatly affect both P availability and the biogeochemical cycle of P in desert ecosystems.

Clay Dispersion and Hydraulic Conductivity of Some Salt‐Affected Arid Land Soils

AbstractCumulative clay dispersion from four arid land soils are compared with their relative hydraulic conductivities as affected by electrolyte concentration and sodium adsorption ratio (SAR). As electrolyte concentration decreased and SAR increased, clay dispersion increased and hydraulic conductivity decreased correspondingly. Threshold relations between electrolyte concentration and SAR associated with clay dispersion of 5 g of clay per kilogram of soil corresponded well with hydraulic conductivity threshold relations. This correspondence between clay dispersion and sodicity‐related reductions in soil hydraulic conductivity can be utilized to provide an index based on clay dispersion for screening soils for their sodicity hazard sensitivity and for predicting reductions in soil hydraulic conductivity related to soil and water sodicity.

Dispersion of Clay From Some Salt‐Affected, Arid Land Soil Aggregates

AbstractThe dispersion characteristics of five arid land soils were determined upon reaction with solutions having various levels of electrolyte concentration, C (C = 0, 1, 4, 10, 30, 50, and 100 mmolcL−1), and sodium adsorption ratios, SAR (0, 5, 10, 20, and 40) with either Ca or Mg as the divalent counter‐ion. With either divalent cation, clay dispersion increased as SAR increased and C decreased. Greater dispersion was observed with Mg as the divalent counter‐ion due to the higher SAR and lower C levels, which resulted in the Mg systems compared to the Ca systems when the soils were reacted with comparable SAR and C treatment solutions. Dispersion threshold relations are concluded to be a useful criterion for screening soils for their sodicity hazard potentials.

Research Priorities and Science Policy Objectives for the Management of Soils in Arid Lands : Research Policy

Research Priorities and Science Policy Objectives for the Management of Soils in Arid Lands : Research Policy