Soil Resource Heterogeneity Research Articles

English

  A study was conducted to determine the effect of different land use systems on soil physical and chemical properties in Sodo Zuria Woreda of Wolaita zone Southern Ethiopia. Differences in soil properties in different land use types at two depths (0-15 and 15-30 cm) were observed on many soil properties important to crop growth. Enset (Ensete Ventricosum) fields had higher pH, electrical conductivity (EC), available P and Zn, exchangeable Ca and K which is attributed due to the addition of manure, whereas maize fields had lowest average K and Mg, cation exchange capacity (CEC), percentage of base saturation (PBS), total N and OC. These results suggest that land use has persistent, multi-decadal effects on the spatial heterogeneity of soil resources and also the need of land use and landscape research to determine ecologically sound and sustainable land use and management strategies.   Key words: Land use, Enset, grass land, manure, continuous cultivation, soil properties

Broadening the ecological context of ungulate–ecosystem interactions: the importance of space, seasonality, and nitrogen

Spatial heterogeneity of soil resources, particularly nitrogen availability, affects herbaceous-layer cover and diversity in temperate forest ecosystems. Current hypotheses predict that ungulate herbivores influence nitrogen availability at the stand scale, but how ungulates affect nitrogen availability at finer spatial scales that are relevant to the herb layer is less understood. We tested the hypothesis that ungulate exclusion reduces the spatial complexity of nitrogen availability at neighborhood scales (1-26 m) apart from mean stand scale effects. This outcome was expected due to a lack of ungulate nitrogenous waste deposition within exclosures and seasonally variable ungulate habitat use. To test this hypothesis we examined spatial patterning of ammonium and nitrate availability, herb-layer cover and diversity, and under-canopy solar radiation using geostatistical models. Our study sites included six stands of eastern hemlock (Tsuga canadensis) forest: three where white-tailed deer (Odocoileus virginianus) were excluded and three that were accessible to deer. Where deer were present, patch sizes of ammonium availability, cover, and diversity were smaller compared to deer exclosures, whereas mean site-level effects were not significant. Within deer exclosures cover and solar radiation were more similar in patch size than were cover and nitrogen availability. Our results suggest that browsing ungulates affect spatial patterns of herb-layer cover and diversity through the excretion of nitrogenous wastes in small, discrete patches. Ungulate-excreted nitrogen deposition and herbivory were concentrated in the dormant season, allowing herb-layer plants a greater opportunity to benefit from nitrogen additions. Therefore, the impact of ungulates on nitrogen cycling in forest ecosystems varies with spatial scale and the seasonal timing of ungulate impacts. In this way, ungulates may function as a seasonally dependent link between fine-scale and landscape-level ecological processes.

Fine root response to soil resource heterogeneity differs between grassland and forest

Soil resource heterogeneity has clear effects on plant root development and overall plant performance. Here we test whether contrasting vegetation types have similar or different responses to soil patches of differing resource availability. We examined the fine root responses of grassland and forest vegetation at the northern edge of the Great Plains to transplanted patches of resource-poor and resource-rich soils, using rhizotron imaging. Every aspect of measured root behavior, including root length, production, mortality, turnover, variability and size distribution, varied significantly between patch types, and most aspects also varied between vegetation types. Most importantly, differential responses to patches between grassland and forest were shown by significant interactions between patch type and vegetation for two response variables. First, root length variability was significantly lower in resource-rich compared to resource-poor patches in forest but not grassland. Second, the proportion of very fine roots was significantly greater in resource-rich than resource-poor patches in forests but not grassland. Thus, compared to grassland, forest more fully occupied resource-rich patches relative to resource-poor patches by allocating more growth to very fine roots. We report the first example of significant differences between vegetation types (grassland and forest) in root responses to soil resource heterogeneity measured in a field experiment. The relatively high ability of forest roots to more fully occupy resource-rich patches is consistent with the global expansion of woody vegetation and associated increases in soil heterogeneity.

A negative heterogeneity–diversity relationship found in experimental grassland communities

Recent meta-analyses and simulation studies have suggested that the relationship between soil resource heterogeneity and plant diversity (heterogeneity-diversity relationship; HDR) may be negative when heterogeneity occurs at small spatial scales. To explore different mechanisms that can explain a negative HDR, we conducted a mesocosm experiment combining a gradient of soil nutrient availability (low, medium, high) and scale of heterogeneity (homogeneous, large-scale heterogeneous, small-scale heterogeneous). The two heterogeneous treatments were created using chessboard combinations of low and high fertility patches, and had the same overall fertility as the homogeneous medium treatment. Soil patches were designed to be relatively larger (156 cm(2)) and smaller (39 cm(2)) than plant root extent. We found plant diversity was significantly lower in the small-scale heterogeneous treatment compared to the homogeneous treatment of the same fertility. Additionally, low fertility patches in the small-scale heterogeneous treatment had lower diversity than patches of the same size in the low fertility treatment. Shoot and root biomass were larger in the small-scale heterogeneous treatment than in the homogeneous treatment of the same fertility. Further, we found that soil resource heterogeneity may reduce diversity indirectly by increasing shoot biomass, thereby enhancing asymmetric competition for light resources. When soil resource heterogeneity occurs at small spatial scales it can lower plant diversity by increasing asymmetric competition belowground, since plants with large root systems can forage among patches and exploit soil resources. Additionally, small-scale soil heterogeneity may lower diversity indirectly, through increasing light competition, when nutrient uptake by competitive species increases shoot biomass production.

Root system size determines plant performance following short-term soil nutrient pulses

Heterogeneity in soil resources is common in natural communities, in both space and time. In contrast to substantial research into plant responses to spatial nutrient heterogeneity, little is known about the factors that influence plants responses to temporally heterogeneous nutrient supplies. We examine (1) the effects of nutrient pulse length on plant biomass allocation patterns, (2) whether a plant’s root system prior to a nutrient pulse influences the benefit a plant can gain from that pulse, and (3) how competition and nutrient pulses interact to influence plant biomass. To address these issues, we used a pot experiment with varying initial conditions (nutrient levels and competition) designed to produce a range of root system sizes in the grass Poa pratensis. We then subjected the plants to short-term (3 and 10 days long) nutrient pulses where the same total nutrients were delivered at varying rates per unit time (intensity). Plant biomass and tissue nitrogen concentration were lowest when pulses were short relative to either long pulses or continuous supply. Plants with larger root systems at the start of the nutrient pulse gained more benefit than smaller plants because final plant size relative to initial root system size decreased as pulse intensity (nutrients delivered per unit time) increased. This study has two important implications. First, since larger plants benefit disproportionately more from a nutrient pulse than smaller plants, pre-pulse conditions such as the competitive environment and resource supply are likely key determinants of a plant’s ability to capture nutrients from a pulse. Second, the importance of pre-pulse size provides a belowground mechanism to enhance the size variability among plants in a community.

Application of distance–decay models for inferences about termite mound-induced patterns in dryland ecosystems

Although termites have been widely reported to induce heterogeneity in soil resources, vegetation and patch use by herbivores, little effort has been made to examine spatial patterns around mounds. In this mini review we re-examine published studies on the variation in soil properties, herbage biomass and herbivore foraging with distance from termite mounds with reference to distance–decay models. The analyses revealed a significant decline in soil clay and silt, soil nutrients, herbage biomass, grazing and browsing with distance from termite mounds consistently with distance–decay models. This has both theoretical and practical significance to our understanding of processes and resource degradation in dryland ecosystems where termite mounds are common. From a sampling perspective, our analyses imply that measured variables around termite mounds are spatially structured. Therefore, we advocate geostatistical model-based sampling as a framework in future studies on termite mounds.

Coupling vegetation organization patterns to soil resource heterogeneity in a central Kenyan dryland using geophysical imagery

In dryland ecosystems, understanding the effects of heterogeneity in soil moisture and geophysical properties on vegetation structure and dynamics poses a suite of challenging research questions. Heterogeneity in soil depth can affect resource availability and the subsequent organization of woody vegetation, while spatiotemporal variation in soil moisture can reveal important ecohydrological feedbacks that govern the outcome of anthropogenic activities on the organization of dryland vegetation. In this research we investigate two cases of soil resource heterogeneity that affect the organization of dryland vegetation patterns by expanding previous electromagnetic induction (EMI) imaging techniques. In the first case we examine the influence of soil depth as a control on soil resource availability on hillslopes in tree‐grass systems in central Kenya. Our results indicate that woody vegetation clumping occurs where soil depth changes, and the deeper rooted Acacia tortilis occurs on deep soils while the drought tolerant Acacia etbaica occurs on shallow soils. In the second case we examine daily patch–interpatch scale moisture dynamics following two different‐sized rain events in a degraded landscape. With the aid of a numerical subsurface flow model, EMI, and soil moisture data, we have identified a possible positive feedback mechanism (“soil moisture halo effect”) that we believe may have contributed to the proliferation and two‐phase pattern formation of a native succulent Sansevieria volkensii in degraded ecosystems of Kenya. By determining how different plants respond to, and modify, the soil environment, we can better understand resource capture and dynamics, which in the longterm will help to develop management strategies.

Resource Heterogeneity, Soil Fertility, and Species Diversity: Effects of Clonal Species on Plant Communities

Spatial heterogeneity in soil resources is widely thought to promote plant species coexistence, and this mechanism figures prominently in resource-ratio models of competition. However, most experimental studies have found that nutrient enhancements depress diversity regardless of whether nutrients are uniformly or heterogeneously applied. This mismatch between theory and empirical pattern is potentially due to an interaction between plant size and the scale of resource heterogeneity. Clonal plants that spread vegetatively via rhizomes or stolons can grow large and may integrate across resource patches, thus reducing the positive effect of small-scale resource heterogeneity on plant species richness. Many rhizomatous clonal species respond strongly to increased soil fertility, and they have been hypothesized to drive the descending arm of the hump-shaped productivity-diversity relationship in grasslands. We tested whether clonals reduce species richness in a grassland community by manipulating nutrient heterogeneity, soil fertility, and the presence of rhizomatous clonal species in a 6-year field experiment. We found strong and consistent negative effects of clonals on species richness. These effects were greatest at high fertility and when soil resources were applied at a scale at which rhizomatous clonals could integrate across resource patches. Thus, we find support for the hypothesis that plant size and resource heterogeneity interact to determine species diversity.

Resource heterogeneity does not explain the diversity-productivity relationship across a boreal island fertility gradient

Many studies at the regional scale have found either negative or hump-shaped relationships between productivity and diversity, and some theories propose that these occur because soil resource heterogeneity is either lower or less important in more productive environments. However, there have been few explicit tests of these theories in natural ecosystems. We evaluated the relationship between soil resource heterogeneity and plant richness within a well characterized system of 30 islands in northern Sweden across which soil fertility and productivity declines, and species richness increases, as a consequence of ecosystem retrogression. On each island we created a spatially explicit grid consisting of 49 sampling points in a 9.5 m quadrat, which we used to quantify spatial heterogeneity of five soil variables (NH4 -N, amino N, PO4 # -P, microbial biomass, and decomposition), and plant community composition. Using a hierarchical Bayesian approach, we estimated mean semivariograms of each variable for each island size class to compare three components of spatial heterogeneity: total variability, spatial grain, and patchiness. This analysis showed that variability within islands was usually lowest on small islands, where species richness was highest and productivity lowest; however, NH4 -N and amino N had greater patchiness and spatial grain on small islands. We did not detect any significant across-island correlations between whole-plot plant species richness and either whole-plot standard deviation or coefficient of variation of any soil variable. Using partial Mantel tests, we found that mean correlation coefficients between within-plot plant community composition and the soil variables were never significant for any island size class, and did not differ between island size classes. Our findings do not provide any evidence that soil resource heterogeneity controls the productivity! diversity relationship in this system, and suggests other mechanisms are primarily responsible.

Determining Soil–Tree–Grass Relationships in a California Oak Savanna Using Eco‐Geophysics

Savanna ecosystems have long fascinated ecologists due to the codominance of trees and grasses. This codominance is often explained by the vertical juxtaposition of rooting depths leading to niche separation for water uptake. The tree–grass spatial patterns observed are often investigated within the competition, disturbance, and resource heterogeneity framework. Ecohydrologic studies are developing models to describe patterns based on this framework and the complex interplay between climate, soil moisture, and resource heterogeneity. A common impediment to unraveling the relative importance of each of these factors is a lack of detailed spatial and temporal data describing resource heterogeneity. In this research, we investigated the use of electromagnetic‐induction (EMI) imaging as a way of determining soil spatial resource heterogeneity. The aim was to determine if EMI signal response, reflecting soil texture and moisture, can provide a rapid method for identifying soil spatial resource heterogeneity, thus providing a novel method to help differentiate between the importance of competition, disturbance, and resource heterogeneity in controlling tree–grass pattern development. We found a significant difference (t = 10.18, P = 1.57 × 10−21) between signal response under grass and trees. Higher bulk soil electrical conductivity (ECa) values occurred under grass (∼32 mS m−1), indicating more clay, while lower ECa values (∼21 mS m−1) occurred under trees where clay content was less. Significant ECa differences were also observed between the oak (Quercus spp.) and buckeye [Aesculus californica (Spach) Nutt.] tree communities, but not between the evergreen and deciduous oak communities. Soil ECa provides a rapid measurement method to identify soil resource heterogeneity in savanna tree–grass systems and could provide a vital tool for woodland restoration planning.

Spatial heterogeneity of soil nutrients and plant species in herb-dominated communities of contrasting land use

Recent interest in spatial pattern in terrestrial ecosystems has come from an awareness of the intimate relationship between spatial heterogeneity of soil resources and maintenance of plant species diversity. Soil and vegetation can vary spatially in response to several state factors of the system. In this study, we examined fine-scale spatial variability of soil nutrients and vascular plant species in contrasting herb-dominated communities (a pasture and an old field) to determine degree of spatial dependence among soil variables and plant community characteristics within these communities by sampling at 1-m intervals. Each site was divided into 25 1-m2 plots. Mineral soil was sampled (2-cm diameter, 5-cm depth) from each of four 0.25-m2 quarters and combined into a single composite sample per plot. Soil organic matter was measured as loss-on-ignition. Extractable NH4 and NO3 were determined before and after laboratory incubation (28 days at 27°C) to determine potential net N mineralization and nitrification. Cations were analyzed using inductively coupled plasma emission spectrometry. Vegetation was assessed using estimated percent cover. Most soil and plant variables exhibited sharp contrasts between pasture and old-field sites, with the old field having significantly higher net N mineralization/nitrification, pH, Ca, Mg, Al, plant cover, and species diversity, richness, and evenness. Multiple regressions revealed that all plant variables (species diversity, richness, evenness, and cover) were significantly related to soil characteristics (available nitrogen, organic matter, moisture, pH, Ca, and Mg) in the pasture; in the old field only cover was significantly related to soil characteristics (organic matter and moisture). Both sites contrasted sharply with respect to spatial pattern of soil variables, with the old field exhibiting a higher degree of spatial dependence. These results demonstrate that land-use practices can exert profound influence on spatial heterogeneity of both soil properties and vegetation in herb-dominated communities.

Vascular plant species richness on wetland remnants is determined by both area and habitat heterogeneity

There is an ongoing ecological debate on whether area per se or habitat heterogeneity is the main driver for species richness. The wetland remnants in the Sanjiang Plain, NE China harbor a high biodiversity and play an important role for local ecosystems. Fifty-one wetland remnants were sampled to examine the effect of area and habitat heterogeneity on vascular plant species richness. Number of community types, elevation, water heterogeneity and soil resource heterogeneity were employed as habitat heterogeneity variables, but only water heterogeneity was identified as the proper surrogate for habitat heterogeneity. Compared with the classic species-area model, the choros model achieved better fitness when water heterogeneity and elevation were employed as habitat heterogeneity variables. Nevertheless, elevation was poorly correlated with species richness. It suggests, without a comprehensive analysis of habitat heterogeneity variables, the choros model might result in a misleading result. In this study, species richness was significantly influenced by water heterogeneity, area and number of community types. Water heterogeneity and area both controlled the number of community types, and they were the two main determinants of species richness. As area was significantly and positively correlated with water heterogeneity, the variance in species richness was mainly related to the mutual effect of area and water heterogeneity. The results of this study confirmed that the relationship between the area per se hypothesis and the habitat heterogeneity hypothesis was conjunct rather than mutually exclusive. In addition, it is critical that both area and water heterogeneity should be taken into account for biodiversity conservation and management in wetland remnants.

Decomposing stumps influence carbon and nitrogen pools and fine-root distribution in soils

Decomposing stumps could significantly increase soil resource heterogeneity in forest ecosystems. However, the impact of these microsites on nutrient retention and cycling is relatively unknown. Stump soil was defined as the soil fraction directly altered by the decomposition of the primary rooting system (e.g. taproots) and aboveground stumps. Study sites were located in mature hardwood stands within the Jefferson National Forest in the Ridge and Valley Physiographic region of southwest Virginia. The objectives of this study were to: (i) determine the total soil volume altered by the decomposition of stumps and underlying root system, (ii) compare and contrast total C and N, extractable ammonium (NH 4 +) and nitrate (NO 3 −), potentially mineralizable N, microbial biomass C (MBC), root length and root surface area between the bulk soil (i.e. O, A, B and C horizons) and stump soil and (iii) evaluate how nutrient concentrations and fine-root dynamics change as stumps decompose over time using a categorical decay class system for stumps. Potentially mineralizable N was 2.5 times greater in stump soil than the A horizon (103 mg kg −1 vs. 39 mg kg −1), 2.7 times greater for extractable NH 4 + (16 mg kg −1 vs. 6 mg kg −1) and almost 4 times greater for MBC (1528 mg kg −1 vs. 397 mg kg −1). Approximately 19% of the total fine-root length and 14% of fine-root surface area occurred in the stump soil. Significant differences occurred in C and N concentrations between all four decay classes and the mineral soil. This validated the use of this system and the need to calculate weighted averages based on the frequency and soil volume influenced by each decay class. In this forest ecosystem, approximately 1.2% of the total soil volume was classified as stump soil and contained 10% and 4% of soil C and N. This study illustrates that including stump soil in soil nutrient budgets by decay class will increase the accuracy of ecosystem nutrient budgets.

Soil resource heterogeneity in the form of aggregated litter alters maize productivity

Soils are spatially heterogeneous. To better understand the effects of sub-plant scale soil resource heterogeneity on primary productivity we aggregated plant litter (Trifolium pratense L. shoots and Secale cereale L. leaves) into 8, 24, 32, and 72 patches and uniformly distributed in 50-L containers (0.15 m2) of soil and grew Zea mays L. throughout two seasons. On average aggregated T. pratense litter enhanced final aboveground maize biomass by 14% relative to uniformly distributed T. pratense litter. This effect may be related to the reduction in root carbon allocation observed as lower apparent root respiration rates compared to uniform litter distribution. In contrast, the spatial distribution of S. cereale litter did not affect productivity. The common experimental approach of uniformly distributing resources to understand their influence on soil-plant processes likely oversimplifies field conditions because our results indicate that the spatial distribution of plant litter alone can influence productivity and plant carbon allocation.

Can biological invasions induce desertification?

Can biological invasions induce desertification?

Sediment deposition and soil nutrient heterogeneity in two desert grassland ecosystems, southern New Mexico

The role of wind in changing the spatial heterogeneity of soil resources in erosion-dominated semiarid ecosystems is well known. Yet the effect of windblown sediment deposition on soil nutrient distribution and ecosystem dynamics at local and landscape scales has received little attention. We examined the effects of enhanced sediment deposition on the spatial distribution of soil nutrients at the Jornada Experimental Range, southern New Mexico. Enhanced sediment deposition was obtained as a result of grass cover reduction in the upwind portion of the experiment in two sites co-dominated by mesquite and one of two grass species with different morphologies. The spatial characteristics of soil available nitrogen (including ammonium, nitrite, and nitrate), phosphate, potassium, and calcium were quantified using a variety of traditional and geostatistical analyses. Our results showed that enhanced deposition led to considerable reduction in both mean soil nutrient concentrations and coefficients of variation over a two-year period (2004–2006). Given the observed increase in the scale of spatial dependence for available nitrogen, but not for potassium, phosphate, and calcium following enhanced sediment deposition, we suggest that soil available nitrogen may be particularly responsive to increased aeolian activities due to livestock grazing and other anthropogenic activities that remove vegetation. Our study further suggests that soil particles deposited in the downwind area may be “nutrient-imbalanced.” Specifically, the lower-than-normal available nitrogen concentrations in the wind-deposited soils may inhibit the growth of grasses and the germination of seeds. For wind-erodible ecosystems found in southern New Mexico, structures of Bouteloua-dominated communities may be particularly susceptible to change under enhanced soil erosion conditions.

Microsite and herbaceous vegetation heterogeneity after burning Artemisia tridentata steppe

Woody vegetation can create distinct subcanopy and interspace microsites, which often result in resource islands in subcanopies compared to interspaces. This heterogeneity in soil resources contributes to herbaceous vegetation heterogeneity in plant communities. However, information detailing the impact of disturbance, such as fire, that removes the woody vegetation on microsites and herbaceous vegetation heterogeneity is limited. The purpose of this study was to determine the influence of burning on microsites and herbaceous vegetation in subcanopies and interspaces. Six study sites (blocks) were located at the Northern Great Basin Experimental Range in shrub (Artemisia tridentata ssp. wyomingensis (Beetle & A. Young) S.L. Welsh)-bunchgrass plant communities and one half of each block was burned to remove A. tridentata. Herbaceous vegetation and microsite characteristics were measured 2 years post-fire in intact and burned subcanopies and interspaces. Burning resulted in microsite and herbaceous vegetation differences between intact and burned subcanopies and intact and burned interspaces. However, burned subcanopies and burned interspaces appeared to be relatively similar. The similarity in microsite characteristics probably explains the lack of differences in herbaceous vegetation cover and biomass production between burned subcanopies and burned interspaces (P > 0.05). However, some microsite and herbaceous vegetation characteristics differed between burned subcanopies and burned interspaces. Our results suggest that disturbances that remove woody vegetation reduced microsite and herbaceous vegetation heterogeneity within plant communities, but do not completely remove the resource island effect. This suggests soil resource heterogeneity may influence post-fire community assembly and contribute to diversity maintenance.

Soil resource heterogeneity in terms of litter aggregation promotes nitrous oxide fluxes and slows decomposition

To better understand the role of resource heterogeneity in decomposition and nitrous oxide (N 2O) flux we systematically altered the degree of plant litter aggregation in soil, from uniformly distributed to highly aggregated. In laboratory incubations, we distributed 4.5 g of dried clover shoots ( Trifolium pratense L.) in two particle sizes (1 or >5 mm) into 1, 3, or 9 patches versus uniformly distributed. Soil moisture content was also varied to manipulate soil oxygen (O 2) concentrations. In moist soil (50% water-filled pore space, WFPS), litter aggregation delayed the peak litter decomposition rate by 3–5 days compared to uniformly distributed litter regardless of the litter particle size. In contrast, under near-saturated soil conditions (80% WFPS) litter aggregation suppressed decomposition throughout the 26-day incubation period. This significant interaction between litter aggregation and soil moisture treatments suggests that diffusion of soil resources (likely O 2) plays an important role in the influence of litter aggregation on decomposition. Specifically, O 2 diffusion may more adequately meet O 2 consumption rates when litter is distributed than when aggregated. In contrast to the temporary influence of aggregation on litter decomposition, N 2O fluxes under 50% WFPS conditions were consistently greater and on average 7.9, 7.2, and 4.7-fold greater than fine aggregated litter (1, 3, and 9 patches, respectively) than when uniformly distributed. Coarse litter aggregation also stimulated N 2O emissions, but not as much as fine litter. Under field conditions with growing maize ( Zea mays L.), litter aggregation also stimulated N 2O emissions. The results suggest that litter aggregation plays a role in N 2O flux from agricultural soils and it might be manipulated to provide an additional N 2O mitigation strategy.

Spatial heterogeneity and plant species richness at different spatial scales under rabbit grazing

Herbivores influence spatial heterogeneity in soil resources and vegetation in ecosystems. Despite increasing recognition that spatial heterogeneity can drive species richness at different spatial scales, few studies have quantified the effect of grazing on spatial heterogeneity and species richness simultaneously. Here we document both these variables in a rabbit-grazed grassland. We measured mean values and spatial patterns of grazing intensity, rabbit droppings, plant height, plant biomass, soil water content, ammonia and nitrate in sites grazed by rabbits and in matched, ungrazed exclosures in a grassland in southern England. Plant species richness was recorded at spatial scales ranging between 0.0001 and 150 m(2). Grazing reduced plant height and plant biomass but increased levels of ammonia and nitrate in the soil. Spatial statistics revealed that rabbit-grazed sites consisted of a mixture of heavily grazed patches with low vegetation and nutrient-rich soils (lawns) surrounded by patches of high vegetation with nutrient-poor soils (tussocks). The mean patch size (range) in the grazed controls was 2.1 +/- 0.3 m for vegetation height, 3.8 +/- 1.8 m for soil water content and 2.8 +/- 0.9 m for ammonia. This is in line with the patch sizes of grazing (2.4 +/- 0.5 m) and dropping deposition (3.7 +/- 0.6 m) by rabbits. In contrast, patchiness in the ungrazed exclosures had a larger patch size and was not present for all variables. Rabbit grazing increased plant species richness at all spatial scales. Species richness was negatively correlated with plant height, but positively correlated to the coefficient of variation of plant height at all plot sizes. Species richness in large plots (<25 m(2)) was also correlated to patch size. This study indicates that the abundance of strong competitors and the nutrient availability in the soil, as well as the heterogeneity and spatial pattern of these factors may influence species richness, but the importance of these factors can differ across spatial scales.

Effects of wind erosion on the spatial heterogeneity of soil nutrients in two desert grassland communities

Wind is known to affect the spatial heterogeneity of soil resources in arid and semiarid systems, but multi-year, quantified observations are largely absent. We studied the effects of wind erosion on the spatial distribution of soil organic carbon (SOC) and other soil nutrients at the Jornada Experimental Range, in southern New Mexico. Enhanced wind erosion was encouraged by grass cover reduction in a Sporobolus-mesquite dominated site (SM) and a Bouteloua-mesquite dominated site (BM). The scale and magnitude of spatial dependence for the soil analytes were quantified using geostatistical analyses. Results of this study show that soil organic matter related analytes such as SOC, TN, Navail, and SO4 2­ are among the first to be eroded and redistributed; cations such as Ca2+ and Mg2+ may not be removed and redistributed significantly; and other ions such as K+, Na+ and Cl− showed no discernible pattern of change. Geostatistics show that wind appeared to increase the scale of spatial autocorrelation, but decrease the scale of spatial dependence of most soil analytes over 2–3 windy seasons. In the wind enhanced plot of the SM site, up to 99% of the spatial dependence of SOC was autocorrelated at the distance of 1.45 m before the initiation of wind erosion, but the spatial dependence dropped significantly to only 60% at a larger autocorrelation distance of 2.76 m after three windy seasons. Similar but less significant changes were observed for SOC in the BM site. Despite the differential effects of wind on the soil analytes, we conclude that the overall results of wind on the grass cover reduction plots are the disappearance of small, strong fertile islands, which may be related to grasses; and the reinforcement of large fertile islands, which are likely related to mesquite shrubs. In addition, the change of the spatial patterns of SOC and other soil nutrients induced by enhanced wind erosion may persist and reinforce soil islands associated with shrubs, thus allowing a positive feedback for further desertification in this arid grassland.