Crust Cover Research Articles

Management to reduce nitrogen losses in animal production.

Reduction of nitrogen loss in animal production requires whole-farm management. Reduced loss from one farm component is easily negated in another if all components are not equally well managed. Animal excretion of manure N can be decreased by improving the balance of protein or amino acids fed to that required by individual animals or animal groups or by improving production efficiency. Management to increase milk, meat, or egg production normally improves efficiency by reducing the maintenance protein required per unit of production. Large losses of manure nitrogen occur through the ammonia and nitrous oxide that are emitted into the atmosphere and the nitrate leached into groundwater. Up to half of the excreted nitrogen is lost from the housing facility, but this loss can be decreased through frequent manure removal and by avoiding deep litter systems and feedlots. Techniques such as acid treatment of manure, scrubbing of ventilation air, and floor designs for separating feces and urine substantially reduce ammonia emissions, but these practices are often impractical or uneconomical for general use. Manure storage units improve nutrient utilization by allowing better timing of nutrient application with crop needs. At least 70% of the nitrogen entering anaerobic lagoons is typically lost, but a less than 10% loss can be maintained using slurry storage with a natural crust or other cover, or by drying poultry manure to at least 50% dry matter. Irrigation and surface spreading of manure without soil incorporation often ensures the loss of all remaining nonorganic nitrogen (typically, 20 to 40% of remaining nitrogen). Rapid incorporation and shallow injection methods decrease this loss by at least 50%, and deep injection into the soil essentially eliminates this loss. For grazing animals, excessive loss can be avoided by not overstocking pastures and avoiding late fall and winter grazing. Reducing emissions between the animal and the soil can lead to greater leaching and denitrification losses from the soil if this additional nitrogen is not used properly. The use of a crop rotation that efficiently absorbs these nutrients and applying nitrogen near the time it is needed by crops reduce the potential for further loss. Maintaining the proper number of animals per unit of land available for manure application is always crucial for efficient recycling of nitrogen. Our understanding of nitrogen loss processes is improved through modeling, and computer models assist in the development of integrated systems for efficient and economical nitrogen use in animal production.

Effects of Historic Livestock Grazing on Vegetation at Chaco Culture National Historic Park, New Mexico

Abstract: Livestock grazing is the most ubiquitous land use in western North America, yet it rarely has been studied in a controlled manner because of the lack of large areas free of grazing. We compared the ecological effects of three grazing treatments—long‐term protection, short‐term protection, and currently grazed—at Chaco Culture National Historic Park in northern New Mexico. Chaco has a long history of human habitation and is now one of the largest grazing exclosures in the American West. We studied the effects of livestock grazing on the cover of plants, soil crusts, and plant species richness at six sites with different potential natural vegetation. Species richness was higher under long‐term protection than under current grazing at all six sites. Trends in shrub and grass response varied significantly across the six sites. Shrub cover increased with long‐term protection at four upland sites, and grass cover increased with protection at four sites. The response of Chaco vegetation to release from grazing varied significantly according to each site's ecological potential, determined in part by edaphic and topographic characteristics. These nuances in vegetation response represent natural ecological variation and contrast with the notions of widespread shrub “invasion” often inferred in the past.

Wind tunnel simulation of aeolian sandy soil erodibility under human disturbance

Inappropriate anthropogenic activities such as overcultivation and overgrazing of steppe and excessive collection of fuelwood are largely responsible for current desertification in China. However, quantitative information concerning the impacts of human disturbance on soil erosion remains sparse. This study investigated aeolian sandy soil erodibility under human disturbance by wind tunnel simulation. The fixed aeolian sandy soils were taken from an artificial vegetation protective system on mobile dunes at the southeastern edge of the Tengger Desert. The results indicate that: (1) human disturbance such as cultivation can accelerate the erodibility of the fixed aeolian sandy soil. The ratio between total soil loss from the undisturbed soil and the cultivated soil is about 0.004; (2) surface vegetation and microbiotic crust are the main factors responsible for the natural wind erodibility of the fixed sandy soil. Wind erosion rate increases with decreasing percent of the vegetation and crust cover; (3) the grain size distribution shows a higher percentage of particles in the range >1.0 mm and a lower percentage in the range <0.05 mm for the cultivated sandy soil than for the undisturbed fixed sandy soil.

Comparison of wind erosion measurements in Germany with simulated soil losses by WEPS

The Wind Erosion Prediction System (WEPS) is a process-based model that simulates daily weather and field conditions along with hourly wind speeds and erosion. Its physical basis should allow model application to regions outside the US for which it was originally developed. The objective of this study was to compare results from measured wind erosion with simulated soil losses as a first example of WEPS use in Germany. Another objective was to introduce methods and techniques for quantifying wind erosion in preparation for model comparison studies to be undertaken within the Global Change and Terrestrial Ecosystems Soil Erosion Network. Comparisons between the measured and simulated erosion were based on single erosion events and outputs from the erosion submodel of WEPS. Multiple runs of the model were performed with varying roughness parameters to set the simulated threshold wind speed equal to that measured during the first erosion event after a tillage operation. This initial data set was used, depending on the rainfall or erosion, as the basis for the gradually changing roughness, crust cover and the fraction of erodible material on crust for all the following erosion events. Thus, the accuracy of the simulation depends much more on the relationship between erosion events, than on a good agreement for one single event. The results showed excellent agreement between measured and simulated erosion ( R 2>0.9). This is mainly attributed to the good agreement with the four largest erosion events in which transport exceeded 100 kg/m width. Excluding these events, R 2 was reduced to about 0.6 for all other erosion events. Spatial and temporal variability of the soil transport were also reasonably simulated.

Exploring some relationships between biological soil crusts, soil aggregation and wind erosion

A portable wind tunnel was used to test the contribution of biological and physical elements to overall soil aggregation on a soil dominated by biological soil crusts in south-eastern Australia. After moderate disturbance and simulated wind erosion, 90% of surface aggregates on the loamy soil and 76% on the sandy soil were dominated by biological elements (cryptogams). Lower levels of biological bonding were observed on the severely disturbed treatment. Linear regression indicated a significant positive relationship ( r 2=0·72) between biological soil crust cover and dry aggregation levels greater than 0·85mm. To maintain sediment transport below an erosion control target of 5gm −1s −1 for a 65kmh −1 wind at 10m height, a crust cover of approximately 20% is required. When a multiple regression model which sequentially fitted biological crust cover and dry aggregation greater than 0·85mm was applied to the data, dry aggregation accounted for more of the variation in sediment transport rate than biological crust cover. These data were used to develop a conceptual model which integrates crust cover and dry aggregation, and provides a useful framework within which to predict the likely impacts of changes in soil crust cover and aggregation.

Impacts of off-road vehicles on nitrogen cycles in biological soil crusts: resistance in different U.S. deserts

Biological soil crusts are an important component of desert ecosystems, as they influence soil stability and fertility. This study examined and compared the short-term vehicular impacts on lichen cover and nitrogenase activity (NA) of biological soil crusts. Experimental disturbance was applied to different types of soil in regions throughout the western U.S. (Great Basin, Colorado Plateau, Sonoran, Chihuahuan, and Mojave deserts). Results show that pre-disturbance cover of soil lichens is significantly correlated with the silt content of soils, and negatively correlated with sand and clay. While disturbance appeared to reduce NA at all sites, differences were statistically significant at only 12 of the 26 sites. Cool desert sites showed a greater decline than hot desert sites, which may indicate non-heterocystic cyanobacterial species are more susceptible to disturbance than non-heterocystic species. Sandy soils showed greater reduction of NA as sand content increased, while fine-textured soils showed a greater decline as sand content increased. At all sites, higher NA before the disturbance resulted in less impact to NA post-disturbance. These results may be useful in predicting the impacts of off-road vehicles in different regions and different soils.

Community Characteristics of Old-Growth Western Juniper Woodlands

While considerable attention has been given to the areal expansion of juniper (Juniperus sp.) in the western United States, the presence and ecological significance of old-growth juniper communities has gone largely unnoted. Increased recognition of these communities has prompted questions about how to recognize old-growth, community structure, ecological importance, and appropriate management. As an initial analysis of old-growth western juniper woodlands (Juniperus occidentalis Hook var. occidentalis Vasek) in central Oregon, this study investigated old-growth community structure on eolian-sand derived soils. These woodlands represent the most extensive old-growth western juniper woodlands throughout its range. Nine study plots were established at 7 sites. Within each plot, densities and physical attributes of all live trees and large standing and fallen woody detritus were recorded. Additional measurements for live trees included canopy cover, apparent age class (pre- or postsettlement), and a sampling of tree ages. Aging of trees older than 250 years was complicated by extensive heartwood rot. Shrub density and cover were measured by species. Understory cover was measured by species and functional type. Bare ground, rock, juniper litter, other litter, moss, and cryptogamic crust cover were also measured. Plant cover ranged from 11 to 33% for trees, 0 to 10% for shrubs, 3 to 12% for perennial grasses, 1 to 2% for forbs, and from 0 to 0.1% for annual grasses. The woodlands contained at least 80 trees ha(-1) aged over 200 years. Correlations between tree parameters and understory structure and composition were generally poor. Differences in plant composition among these stands was primarily attributed to elevation, slope, and percent sand content. Structural characteristics that distinguished old-growth stands from younger stands included tree growth form, presence of standing and dead large woody debris, lichen on dead branches, and a relatively open canopy. Results provide a preliminary basis for identifying old-growth Juniperus occidentalis stands, as a prelude to the development of management plans and further research into the functional characteristics of the systems. A definition of old-growth juniper woodlands is presented. DOI:10.2458/azu_jrm_v54i5_waichler

Biotic Soil Crusts of Oregon's Shrub Steppe: Community Composition in Relation to Soil Chemistry, Climate, and Livestock Activity

Abstract We examined biotic soil crust cover and composition at nine shrub-steppe sites in central and eastern Oregon, U.S.A. One pair of livestock-grazed and excluded transects was established at each site. Data were collected on the cover of biotic soil crust and vascular plant species, soil surface pH and electrical conductivity, and other environmental variables. Using gradient analysis, we found that differences in community composition among sites were most strongly related to soil pH, electrical conductivity (EC), and Calcareous Index Value (CIV; a scale representing the relative calcium carbonate content of soils). Other important variables included precipitation, elevation, aspect, and temperature. We found total crust cover to be highest at sites with lower pH, EC, and CIV. Dominant species differed markedly between the more calcareous sites with higher pH, and the less calcareous, lower pH sites. Livestock exclusion was not an important gradient in the ordination of these data, being overshadow...

Patterns of plant invasions: a case example in native species hotspots and rare habitats

Land managers require landscape-scale information on where exotic plant species have successfully established, to better guide research, control, and restoration efforts. We evaluated the vulnerability of various habitats to invasion by exotic plant species in a 100,000 ha area in the southeast corner of Grand Staircase-Escalante National Monument, Utah. For the 97 0.1-ha plots in 11 vegetation types, exotic species richness (log10) was strongly negatively correlated to the cover of cryptobiotic soil crusts (r =− 0.47, P< 0.001), and positively correlated to native species richness (r = 0.22, P< 0.03), native species cover (r = 0.23, P< 0.05), and total nitrogen in the soil (r = 0.40, P< 0.001). Exotic species cover was strongly positively correlated to exotic species richness (r = 0.68, P< 0.001). Only 6 of 97 plots did not contain at least one exotic species. Exotic species richness was particularly high in locally rare, mesic vegetation types and nitrogen rich soils. Dry, upland plots (n = 51) had less than half of the exotic species richness and cover compared to plots (n = 45) in washes and lowland depressions that collect water intermittently. Plots dominated by trees had significantly greater native and exotic species richness compared to plots dominated by shrubs. For the 97 plots combined, 33% of the variance in exotic species richness could be explained by a positive relationship with total plant cover, and negative relationships with the cover of cryptobiotic crusts and bare ground. There are several reasons for concern: (1) Exotic plant species are invading hot spots of native plant diversity and rare/unique habitats. (2) The foliar cover of exotic species was greatest in habitats that had been invaded by several exotic species. (3) Continued disturbance of fragile cryptobiotic crusts by livestock, people, and vehicles may facilitate the further invasion of exotic plant species.

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.

Community characteristics of old-growth western juniper woodlands

While considerable attention has been given to the areal expansion of juniper (Juniperus sp.) in the western United States, the presence and ecological significance of old-growth juniper communities has gone largely unnoted. Increased recognition of these communities has prompted questions about how to recognize old-growth, community structure, ecological importance, and appropriate management. As an initial analysis of old-growth western juniper woodlands (Juniperus occidentalis Hook var. occidentalis Vasek) in central Oregon, this study investigated old-growth community structure on eolian-sand derived soils. These woodlands represent the most extensive old-growth western juniper woodlands throughout its range. Nine study plots were established at 7 sites. Within each plot, densities and physical attributes of all live trees and large standing and fallen woody detritus were recorded. Additional measurements for live trees included canopy cover, apparent age class (pre- or postsettlement), and a sampling of tree ages. Aging of trees older than 250 years was complicated by extensive heartwood rot. Shrub density and cover were measured by species. Understory cover was measured by species and functional type. Bare ground, rock, juniper litter, other litter, moss, and cryptogamic crust cover were also measured. Plant cover ranged from 11 to 33% for trees, 0 to 10% for shrubs, 3 to 12% for perennial grasses, 1 to 2% for forbs, and from 0 to 0.1% for annual grasses. The woodlands contained at least 80 trees ha(-1) aged over 200 years. Correlations between tree parameters and understory structure and composition were generally poor. Differences in plant composition among these stands was primarily attributed to elevation, slope, and percent sand content. Structural characteristics that distinguished old-growth stands from younger stands included tree growth form, presence of standing and dead large woody debris, lichen on dead branches, and a relatively open canopy. Results provide a preliminary basis for identifying old-growth Juniperus occidentalis stands, as a prelude to the development of management plans and further research into the functional characteristics of the systems. A definition of old-growth juniper woodlands is presented.

Stream biomonitoring at an agricultural test site using benthic algae

The percent cover and composition of benthic macroalgal growths and the structure of epilithic diatom communities were analyzed in 1995 at four sites in a stream flowing through a cultivated field near Kintore, Ontario, Canada. The dominant epilithic diatom taxa did not differ greatly among sites. However, a correspondence analysis of the diatoms sampled monthly between July and September separated an upstream site from those receiving agricultural runoff. Sites downstream of agricultural inputs showed an increase in the cover of Cladophora glomerata (L.) Kütz and other green algae, and in general a reduction in the cover of diatoms and crusts dominated by Oscillatoriaceae (aff. Oscillatoria sp.) and the red alga Audouinella violacea (Kütz.) Hamel. In particular, there was a positive correlation between the cover of Cladophora and nitrate concentration.Key words: biomonitoring, diatoms, periphyton, agriculture, correspondence analysis (CA).

Dynamics of cryptogamic soil crusts in a derived grassland in south‐eastern Australia

AbstractWe examined the dynamics of cryptogamic soil crusts in a derived (disclimax) grassland near Orange in south‐eastern Australia. Changes in the cover of cryptogamic crusts and floristics and abundance of the constituent species were measured on four treatments with two levels each of grazing and cultivation. Twenty‐two lichens, mosses and liverworts were found at the study site and, of these, 13 were collected in the quadrats. Three moss species (Barbula calycina, Eccremidium arcuatum and Bryum pachytheca) and one lichen species (Cladonia tessalata) accounted for 67% of total cover‐abundance scores. Generally, cover‐abundance was significantly higher in the unvegetated microsites than in the vegetated microsites. Species richness was not significantly different between the four grazing‐cultivation treatments but, on average, there were significantly more species in the unvegetated microsites (mean = 3.2 species) than in the vegetated microsites (0.54 species). Grazing and cultivation resulted in significantly greater cover of bare ground and consequently significantly greater crust cover. Averaged across all treatments, approximately half of the area of unvegetated soil was occupied by cryptogams. Overall, the results indicate that lichens and bryophytes are important components of derived temperate grasslands, surviving in even densely vegetated swards. This study suggests that strategies which disturb the soil surface (e.g. grazing and cultivation) will stimulate the abundance and cover of soil crust organisms by increasing the availability of unvegetated microsites.

Stream biomonitoring at an agricultural test site using benthic algae

The percent cover and composition of benthic macroalgal growths and the structure of epilithic diatom communities were analyzed in 1995 at four sites in a stream flowing through a cultivated field near Kintore, Ontario, Canada. The dominant epilithic diatom taxa did not differ greatly among sites. However, a correspondence analysis of the diatoms sampled monthly between July and September separated an upstream site from those receiving agricultural runoff. Sites downstream of agricultural inputs showed an increase in the cover of Cladophora glomerata (L.) Kutz and other green algae, and in general a reduction in the cover of diatoms and crusts dominated by Oscillatoriaceae (aff. Oscillatoria sp.) and the red alga Audouinella violacea (Kutz.) Hamel. In particular, there was a positive correlation between the cover of Cladophora and nitrate concentration.Key words: biomonitoring, diatoms, periphyton, agriculture, correspondence analysis (CA).

Differential water distribution over dune slopes as affected by slope position and microbiotic crust, Negev Desert, Israel

Runoff is one of the main water sources responsible for water redistribution within a given ecosystem. Water redistribution is especially important in arid regions, and may be of great importance on sandy dunes, where the likelihood of runoff is low owing to the high infiltration rates of sand. Redistribution of water may significantly affect plant and animal distribution, and may explain vegetation patterns within an ecosystem. Runoff yield over sandy dune slopes in the western Negev Desert was measured under natural conditions during 1990–1994. The magnitude of runoff yield on different slope sections and on north and south exposures was established. The results demonstrate that while slope position controlled the microbiotic crust cover, crust cover and crust biomass controlled the amounts of runoff obtained. Whereas no runoff was measured on the upper dune sections devoid of crust, only meagre quantities were measured on the midslope sections, characterized by discontinuous crust cover. Substantially larger amounts were, however, obtained at the bottoms of the slopes, characterized by continuous crust cover. North-facing slopes, usually characterized by a chlorophyll a content of 29–41 mg m−2, yielded on average 3·2 times more runoff than south-facing footslopes, characterized by a 17 mg m−2 chlorophyll a content. Whereas microbiotic crust was found to be responsible for runoff generation, additional water supply owing to runoff may also explain the occurrence of a high biomass crust and the dense vegetation belt at the dune–interdune interface of the northern exposure, where runoff tends to collect. Thus, whereas crust may reduce infiltration in certain habitats, runoff generated by crust may also be responsible for the promotion of crust growth in other habitats. Runoff may also be used to promote vegetation growth at the dune footslopes. The possibility of using runoff to facilitate agroforestry is discussed. Copyright © 1999 John Wiley & Sons, Ltd.

Distribution and floristics of moss- and lichen-dominated soil crusts in a patterned Callitris glaucophylla woodland in eastern Australia

The distribution and abundance of soil crust lichens and bryophytes was examined in a patterned Callitris glaucophylla woodland in eastern Australia. Twenty-one lichen species and 26 bryophyte species were collected within thirty quadrats along a sequence of runoff, interception and runoff zones. Crust cover was significantly greatest in the interception zones (79.0 %), followed by the runoff zones (24.0 %), and lowest in the groved, runon zones (6.6 %). Lichens and bryophytes were distributed across all geomorphic zones, and, although there were significantly more moss species in the interception zones (mean = 9.1) compared with either the runoff (4.2) or runon (3.2) zones, the number of lichen species did not vary between zones. Ordination of a reduced data set of 32 species revealed a separation of taxa into distinct groups corresponding to the three geomorphic zones. Canonical correspondence analysis (CCA) of the 32 species and thirteen environmental variables revealed that the most important factors associated with the distribution of species were sheet and scarp erosion, soil stability and coherence, litter cover and crust cover. Surface cracking, microtopography and plant cover were of intermediate importance. The CCA biplot revealed that the timbered runon zones (groves) were dominated by `shade-tolerant' mosses Fissidens vittatus and Barbula hornschuchiana, whilst the heavily eroded runoff zones supported sparse populations of `erosion tolerant' lichens (Endocarpon rogersii) and mosses (Bryum argenteum and Didymodon torquatus). Interception zones supported a rich suite of `crust forming' mosses and lichens capable of tolerating moderate inundation by overland flow. Two other groups of taxa were identified by this analysis: the `pioneer' group, comprising mainly nitrogen-fixing lichens which occupy the zone of active erosion at the lower edge of the groves, and the `opportunists' dominated by liverworts, occupying the shallow depressions or bays at the margins of the groves and the interception zones. This study confirms that the non-vascular lichens and bryophytes in these arid soil crusts, are, like the vascular plants, strongly patterned according to geomorphic zone, being most strongly associated with soil surface and erosional features.

Short-Term Influence of Tank Tracks on Vegetation and Microphytic Crusts in Shrubsteppe Habitat

/ I examined vegetation and microphytic crust cover on two sites in burned and two sites in unburned big sagebrush (Artemisia tridentata Nutt.) habitat within the Idaho Army National Guard Orchard Training Area in southwestern Idaho. The purpose of this study was to determine the short-term (1-2 years) influence of tank tracks on vegetation and microphytic crusts in shrubsteppe habitat. The two types of tank tracks studied were divots (area where one track has been stopped or slowed to make a sharp turn) and straight-line tracks. Divots generally had a stronger influence on vegetation and microphytic crusts than did straight-line tracks. Tank tracks increased cover of bare ground, litter, and exotic annuals, and reduced cover of vegetation, perennial native grasses, sagebrush, and microphytic crusts. Increased bare ground and reduced cover of vegetation and microphytic crusts caused by tank tracks increase the potential for soil erosion and may reduce ecosystem productivity. Reduced sagebrush cover caused by tank tracks may reduce habitat quality for rodents. Tank tracks may also facilitate the invasion of exotic annuals into sagebrush habitat, increasing the potential for wildfire and subsequent habitat degradation. Thus, creation of divots and movement through sagebrush habitat by tanks should be minimized.KEY WORDS: Divots; Fire history; Idaho; Military; Sagebrush; Straight-line tracks

Cover and floristics of microphytic soil crusts in relation to indices of landscape health

An extensive field survey of 282 sites over 500 000 km2 of rangeland in eastern Australia was carried out to investigate the relationships between cover and floristics of microphytic soil crusts and indices of landscape health. Empirical quadrat-based data were used to calculate three indices of landscape health (soil stability, infiltration capacity and nutrient status) according to the method of Tongway (1994). Analyses were based on a subset of 104 sites from landscapes dominated by red earths and calcareous earths, where the loss of microphytic crust cover is indicative of a loss in soil productive potential. Crust cover and floristics were further compared with an objective but widely used method of assessing rangeland condition based on the use of photostandards. Crust cover was a significant though weak predictor of stability of calcareous earth soils only (P<0.001, R2=0.106). On red earths, cover was a significant descriptor of rangeland condition (P<0.001, R2=0.171). However, overall, the percentage of the soil occupied by soil crusts was regarded a poor predictor of landscape health. Generally, increases in the number of lichen and bryophyte taxa at a site were associated with increasing rangeland condition (P<0.001, R2=0.178), landscape stability (P=0.047, R2=0.027) and nutrient status (P<0.001, R2=0.115), though these relationships varied according to soil type. Canonical Correspondence Analyses indicated that some lichens and bryophytes were consistently associated with either stable, good condition sites (Xanthoparmelia spp., Catapyrenium lacinulatum, Buellia subcoronata, Aloina bifrons, Riccia lamellosa, Fossombronia spp.) or unstable, poor condition sites (Heterodea beaugleholei, Cladonia spp., Barbula calycina, Bryum spp., Desmatodon convolutus, Eccremidium arcuatum). The majority of taxa however had no strong affinity with the four indices of landscape health. When easily recognisable morphological groups of lichens were related to landscape health indices, the group comprising yellow-green foliose lichens was the most strongly associated with stable, healthy sites. The results suggest that disturbances influencing soil stability and soil surface condition such as trampling and fire are likely to lead to changes in the composition of soil crust communities.

Effects of spent mushroom substrate on soil physical conditions and plant growth in an intensive horticultural system

A 2-year field trial determined the influence of applying spent mushroom substrate (SMS) on soil physical properties and the growth of 4 consecutive vegetable crops (sweetcorn, cabbage, potato, cabbage). Treatments comprised 0, 20, 40, and 80 t/ha of moist SMS, both with and without inorganic fertiliser, applied to each crop, giving a range of SMS rates up to 320 t/ha. SMS improved the environment for plant root growth by decreasing soil bulk density (by 0· 05-0·25 g/cm 3 at 100 mm depth), increasing aggregate stability (by 13-16%), reducing clod and surface crust formation (by 16-31 and 18-94%, respectively), increasing the infiltration rate (by 130-207 mm/h), increasing the water content of the soil (by 0-7% w/w), and reducing diurnal temperature changes. Some of these changes were not evident until repeated applications of 80 t/ha SMS had been made. Soil physical properties were related to crop yield, and soil physical properties’ principal components were related to crop principal components using regression analysis (r2 of 0·20-0·60 and 0·16-0·54, respectively). The soil physical properties that had the most influence on plant growth were specific to each crop and included bulk density, water content, surface crust cover, infiltration rate, and aggregate size distribution. Soil physical properties had a large influence on the potato yield irrespective of fertiliser use and on both cabbage crop yields when fertiliser was not used, but not on the sweetcorn yield (the first crop to be grown). The effect of changing soil physical properties on plant growth was most apparent when fertiliser was not used. This was because the improved physical properties increased plant yield (at least in part) because of increased plant nutrient uptake.

Hillslope dynamics of on-farm generation of surface water flows: The case of rain-fed cultivation of pearl millet on sandy soil in the Sahel

Results from on-farm measurements of surface overland flow of both runon ( R on) and runoff ( R off) are presented for a continuously cultivated pearl millet ( Pennisetum glaucum (L.) R.Br.) field, along a characteristic catena, in the Sahel (Niger). Despite the deep sandy soil along the studied hillslope, classified as a Typic Haplustult, non-negligible depths of R off were measured during two rainy seasons: 12–13% of total rainfall for the upslope position and 6–8% for the downslope position. This was explained by the presence of surface crusts with low infiltrability. Upslope, more than 30% of the soil surface was covered with erosion crusts (runoff coefficient (K r) = 60 – 70% for moist soil conditions) during 40% of the rainy season. The mean coverage of erosion crust for two rainy seasons was 26% upslope, compared with 11% and 15% midslope and downslope, respectively. Weeding, carried out manually with hand hoes, had a very positive effect on soil infiltrability. The destruction of surface crusts resulted in zero Roff (Kr = 0) for rainstorms directly following weeding operations. However, surface crusts were quickly re-established as a result of the erosive effect of the following storms. Large volumes of runon (Ron) water were measured at the upslope limit of the field (14.4 m 3 m −1 in 1995, corresponding to a rainfall depth of 47.5 mm if redistributed over the 8.5 ha millet field). Small volumes were registered downslope (R on = 0.75 m 3 m −1), indicating redistribution of R on along the hillslope. The Ron originated from the upstream degraded fallow zone and the adjacent sparsely vegetated plateau, which in effect functioned as water-harvesting zones. R off was modelled based on field observations of surface coverage of soil crusts, data on hydraulic characteristics for the observed crusts, rainfall data and estimates of antecedent soil moisture. Despite high variance in the observed R off, the model provides a reasonable estimate of R off, indicating the possibility of predicting R off on cultivated fields where crust coverage changes quickly over time. The large volumes of measured overland flow suggest that R on and R off cannot be excluded from on-farm water-balance studies. The findings also indicate the potential of developing water-harvesting systems for protective irrigation during the frequent periods of water stress in the rain-fed Sahelian agriculture.