Biotic and Abiotic Changes in Ecosystem Structure over a Shrub-Encroachment Gradient in the Southwestern USA

In this study, we investigate changes in ecosystem structure that occur over a gradient of land-degradation in the southwestern USA, where shrubs are encroaching into native grassland. We evaluate a conceptual model which posits that the development of biotic and abiotic structural connectivity is due to ecogeomorphic feedbacks. Three hypotheses are evaluated: 1. Over the shrub-encroachment gradient, the difference in soil properties under each surface-cover type will change non-linearly, becoming increasingly different; 2. There will be a reduction in vegetation cover and an increase in vegetation-patch size that is concurrent with an increase in the spatial heterogeneity of soil properties over the shrub-encroachment gradient; and 3. Over the shrub-encroachment gradient, the range at which soil properties are autocorrelated will progressively exceed the range at which vegetation is autocorrelated. Field-based monitoring of vegetation and soil properties was carried out over a shrub-encroachment gradient at the Sevilleta National Wildlife Refuge in New Mexico, USA. Results of this study show that vegetation cover decreases over the shrub-encroachment gradient, but vegetation-patch size increases, with a concurrent increase in the spatial heterogeneity of soil properties. Typically, there are significant differences in soil properties between non-vegetated and vegetated surfaces, but for grass and shrub patches, there are only significant differences for the biotic soil properties. Results suggest that it is the development of larger, well-connected, non-vegetated patches that is most important in driving the overall behavior of shrub-dominated sites. Results of this study support the hypothesis that feedbacks of functional connectivity reinforce the development of structural connectivity, which increases the resilience of the shrub-dominated state, and thus makes it harder for grasses to re-establish and reverse the vegetation change

Erosión y desertificación.-Understanding the erosion of semi-arid landscapes subject to vegetation change: a combined approach using monitoring, isotope and c analysis.

ABSTRACT The degradation of grasslands is a common problem across semi-arid areas worldwide. Over the last 150 years much of the South-Western USA has experienced significant land degradation, with desert grasslands becoming dominated by shrubs and concurrent changes in runoff and erosion which are thought to propagate further the process of degradation. Field-based experiments were carried out to determine how runoff and erosion vary at stages over a transition from a black grama (Bouteloua eriopoda) grassland to creosotebush (Larrea tridentata) shrubland at the Sevilleta NWR LTER site in New Mexico. δ13C and δ15N analyses were carried out to investigate the age and potential provenance of eroded sedimen

Use of carbon isotope analysis to understand semi-arid erosion dynamics and long-term semi-arid land degradation

Many semi-arid areas worldwide are becoming degraded, in the form of C4 grasslands being replaced by C3 shrublands, which causes an increase in surface runoff and erosion, and altered nutrient cycling, which may affect global biogeochemical cycling. The prevention or control of vegetation transitions is hindered by a lack of understanding of their temporal and spatial dynamics, particularly in terms of interactions between biotic and abiotic processes. This research investigates (1) the effects of soil erosion on the δ13C values of soil organic matter (SOM) throughout the soil profile and its implications for reconstructing vegetation change using carbon-isotope analysis and (2) the spatial properties of erosion over a grass-shrub transition to increase understanding of biotic-abiotic interactions by using δ13C signals of eroded material as a sediment tracer. Results demonstrate that the soils over grass-shrub transitions are not in steady state. A complex interplay of factors determines the input of SOM to the surface horizon of the soil and its subsequent retention and turnover through the soil profile. A positive correlation between event runoff and δ13C signatures of eroded sediment was found in all plots. This indicates that the δ13C signatures of eroded sediment may provide a means of distinguishing between changes in erosion dynamics over runoff events of different magnitudes and over different vegetation types. The development of this technique using δ13C signatures of eroded sediment provides a new means of furthering existing understanding of erosion dynamics over vegetation transitions. This is critical in terms of understanding biotic-abiotic feedbacks and the evolution of areas subject to vegetation change in semi-arid environments

Stable carbon isotope analysis of fluvial sediment fluxes over two contrasting C4-C3 semi-arid vegetation transitions

Globally, many drylands are experiencing the encroachment of woody vegetation into grasslands. These changes in ecosystem structure and processes can result in increased sediment and nutrient fluxes due to fluvial erosion. As these changes are often accompanied by a shift from C(4) to C(3) vegetation with characteristic δ(13) C values, stable isotope analysis provides a promising mechanism for tracing these fluxes.Input vegetation, surface sediment and fluvially eroded sediment samples were collected across two contrasting C(4) -C(3) dryland vegetation transitions in New Mexico, USA. Isotope ratio mass spectrometric analyses were performed using a Carlo Erba NA2000 analyser interfaced to a SerCon 20-22 isotope ratio mass spectrometer to determine bulk δ(13) C values.Stable isotope analyses of contemporary input vegetation and surface sediments over the monitored transitions showed significant differences (p <0.05) in the bulk δ(13) C values of C(4) Bouteloua sp. (grama) grassland, C(3) Larrea tridentata (creosote) shrubland and C(3) Pinus edulis/Juniperus monosperma (piñon-juniper) woodland sites. Significantly, this distinctive δ(13) C value was maintained in the bulk δ(13) C values of fluvially eroded sediment from each of the sites, with no significant variation between surface sediment and eroded sediment values.The significant differences in bulk δ(13) C values between sites were dependent on vegetation input. Importantly, these values were robustly expressed in fluvially eroded sediments, suggesting that stable isotope analysis is suitable for tracing sediment fluxes. Due to the prevalent nature of these dryland vegetation transitions in the USA and globally, further development of stable isotope ratio mass spectrometry has provided a valuable tool for enhanced understanding of functional changes in these ecosystems

Understanding spatial variability of soil properties: a key step in establishing field- to farm-scale agro-ecosystem experiments

The spatial variability of soil properties is poorly understood, despite its importance in designing appropriate experimental sampling strategies. As preparation for a farm-scale agro-ecosystem services monitoring project, the 'North Wyke Farm Platform', there was a need to assess the spatial variability of key soil chemical and physical properties.The field-scale spatial variability of soil chemical (total N, total C, soil organic matter), soil physical properties (bulk density and particle size distribution) and stable isotope ratios (δ(13) C and δ(15) N values) was studied using geostatistical approaches in an intensively managed grassland.The scales over which stable isotopes vary (ranges: 212-258 m) were larger than those of the total nutrients, soil organic matter and bulk density (ranges: 84-170 m). Two visually and statistically distinct areas of Great Field (north and south) were identified in terms of co-occurring high/low values of several soil properties.The resulting patterns of spatial variability suggest lower spatial variability of stable isotopes than that of total nutrients, soil organic matter and bulk density. Future sampling regimes should be conducted in a grid with 5 years) on the patterns of spatial variability

Assessing multiple novel tracers to improve the understanding of the contribution of agricultural farm-waste to diffuse water pollution

A study was undertaken on drained and undrained 1 ha grassland lysimeters to assess the effectiveness of multiple novel tracing techniques in understanding how agricultural slurry waste moves from land to water. Artificial fluorescent particles designed to mimic the size and density of organic slurry particles were found to move off the grassland via inter-flow (surface + lateral through-flow) and drain-flow. Where both pathways were present the drains carried the greater number of particles. The results of the natural fluorescence and delta C-13 of water samples were inconclusive. Natural fluorescence was higher from slurry-amended lysimeters than from zero-slurry lysimeters, however, a fluorescence decay experiment suggested that no slurry signal should be present given the time between slurry application and the onset of drainage. The delta C-13 values of >0.7 mm and 0.7 mm delta C-13 in water from the drain-flow pathways was higher from the lysimeter which had received naturally enriched maize slurry compared to the lysimeter which received grass slurry indicating a contribution of slurry-derived material. Values of <0.7 mm delta C-13 from the same pathway, however, produced counter intuitive trends and may indicate that different fractions of the slurry have different delta C-13 values

A novel application of natural fluorescence to understand the sources and transport pathways of pollutants from livestock farming in small headwater catchments

This paper demonstrates the application of a low-cost and rapid natural fluorescence technique for tracing and quantifying the transport of pollutants from livestock farming through a small headwater catchment. Fluorescence intensities of Dissolved Organic Matter (DOM) present in different pollutant sources and drainage waters in the Den Brook catchment (Devon, UK) were monitored through storm events occurring between January 2007 and June 2008. Contrasting fluorescence signals from different sources confirmed the technique’s usefulness as a tracer of pollutants from livestock farming. Changes in fluorescence intensities of drainage waters throughout storm events were used to assess the dynamics of key pollutant sources. The farmyard area of the catchment studied was shown to contribute polluted runoff at the onset of storm events in response to only small amounts of rain, when flows in the Den Brook first-order channel were low. The application of slurry to a field within the catchment did not elevate the fluorescence of drainage waters during storm events suggesting that when slurry is applied to undrained fields the fluorescent DOM may become quickly adsorbed onto soil particles and/or immobilised through bacterial breakdown. Fluorescence intensities of drainage waters were successfully combined with discharge data in a two component mixing model to estimate pollutant fluxes from key sources during the January 2007 storm event. The farmyard was shown to be the dominant source of tryptophan-like material, contributing 61-81% of the total event flux at the catchment outlet. High spatial and temporal resolution measurements of fluorescence, possibly using novel in-situ fluorimeters, may thus have great potential in quickly identifying and quantifying the presence, dynamics and sources of pollutants from livestock farming in catchments