Surface Soil Horizons Research Articles

Root systems of some Chihuahuan Desert plants

Root systems of 11 shrub or shrub-like species, 11 grass species, 19 perennial forb species and four annual forb species were excavated on the Jornada Experimental Range in the northern Chihuahuan Desert in southern New Mexico, U.S.A. Maximum radial horizontal spread of shrub root systems usually occurred above calcic and petrocalcic horizons at depths above 1 m. With one exception, all shrub species roots were traced through calcic and petrocalcic horizons to depths down to 5 m. Upward growing roots reaching very shallow depths (<10 cm) were common for most shrub species. Thus, the shrubs can readily access soil water and nutrients from both surface and deep soil horizons. Grass root systems on sandy soils extended radially up to 1·4 m, perhaps an adaptation to capture more soil water from the frequent small rainfall events. Grass roots did not extend through calcic or petrocalcic horizons and none penetrated deeper than 1·6 m. Perennial forb root systems varied in depth of branching but often penetrated into or through calcic and petrocalcic horizons and, like shrubs, have an advantage over grasses during droughts. Root systems of the annual forbs, excavated at the end of a season of above average precipitation, reached depths of 0·5–1·2 m. At each of the 18 excavation sites the roots of all the plant life forms were highly intermingled in the upper soil horizons, indicating that competition for water and soil nutrients is intense. It is believed that the plasticity and architecture of the shrub root systems, enabling them to compete with the grasses for soil water in the upper soil horizons and also access soil water at depths beyond the reach of grass roots, has been a major reason for the increase of shrubs during the historical period in this arid environment.

Comparação de métodos de extração de cobre, zinco, ferro e manganês em solos do estado do Rio de Janeiro

As conseqüências da deficiência ou toxidez de micronutrientes na produção agrícola do estado do Rio de Janeiro são pouco conhecidas. Desta forma, em 1997, foram quantificados os teores de cobre, zinco, ferro e manganês em 103 amostras de horizontes superficiais de solos representativos do estado, sendo utilizados os extratores Mehlich-1, DTPA-TEA e HCl 0,1 mol L-1. De maneira geral, o extrator HCl 0,1 mol L-1 foi o que apresentou maior poder de extração dos micronutrientes estudados, tendo o Mehlich-1 extraído as menores quantidades de cobre e ferro e o DTPA-TEA as menores quantidades de zinco e manganês. Segundo os resultados, equações de conversão entre extratores, para um mesmo elemento, e a utilização de faixas de interpretação desenvolvidas para regiões edáficas diferentes das condições do trabalho podem não estimar corretamente a disponibilidade dos micronutrientes às plantas.

Forms of organic phosphorus in zonal steppe soils of Russia assessed by 31P NMR

Different soil organic P (SOP) structures mineralize at different rates. Actual SOP composition reflects steady-state equilibrium of SOP build-up and loss. This study was conducted to investigate heterogeneity of SOP composition among particle-size fractions in zonal steppe soils. Composite samples were taken from four surface (0–10 cm) and subsurface (ca. 50–60 cm) soil horizons along a climosequence in the native steppe and forest steppe from Tula to Kalmykia Republik, 120 km south of Volgograd, Russia. The SOP composition was assessed by 31P nuclear magnetic resonance (NMR) spectroscopy on alkaline NaOH/NaF extracts obtained from bulk soils (<2 mm), clay (<2 μm), silt (2–20 μm), and sand (20–2000 μm) fractions. About 50–60% of extracted P were monoesters, the proportions being highest in the sand fractions. Diester-P structures, including teichoic and nucleic acids, amounted to 19–31% of extracted P. They were significantly enriched in the clay fractions. There was little alteration of SOP composition with increasing soil depth, i.e. we did not find indications that in the Mollisols labile P diesters were degraded in preference to the more stable monoester-P structures as in other soils. The enrichment of diester P in the clay fractions increased as the ratio of mean annual precipitation (MAP) to potential evaporation (pET) decreased; in turn, the proportion of monoester P decreased. However, also P extractability declined with increasing MAP/pET ratio. On the one hand this may indicate different P availability in the zonal soils, however, on the other hand this complicates the assessment of SOP dynamics with the help of 31P NMR. Elucidating the ecological significance of P structures accessible to liquid-state NMR spectroscopy might thus warrant further attention.

Hillslope and wetland hydrodynamics in a tussock grassland, South Island, New Zealand

AbstractPrevious research on two experimental catchments at the Glendhu Forest (Otago) showed that runoff dynamics from this landscape differ substantially from other areas in New Zealand. In particular, the recession curves from these catchments have two distinct phases: a rapid decline for a period of several hours, followed by a slow decline that may last many days to weeks. In the current study, soil water matric potential measurements indicated that perched saturated zones develop very rapidly in the surface (0–30 cm) soil horizons. Deep (50–250 cm) loess horizons remain moist throughout the year and may be near or at saturation at the bedrock contact, and respond only slowly to rain events. Soils in the middle horizons are often drier than the surface soil above or the deep soils below. Throughflow measurements showed that storm runoff occurs primarily as interflow through a retentive but pervious mat of mosses, litter, and roots at the soil surface. Despite the presence of abundant earthworm burrows, less water is transmitted through the soil horizon just below this surface mat. Only a small fraction of stormflow was transmitted through the deep, mineral horizons. However, baseflow could be sustained for long periods (up to 4 weeks) from unsaturated flow of soil moisture stored in the deep loess horizons. Water storage in bog peats and associated riparian soils (which are numerous in this area) is insufficient to sustain baseflow for more than a few days. The results from this study are consistent with previous hydrometric, soils, and isotopic observations and suggest that any land‐use practices that disrupt the surface soil horizons are likely to change the runoff characteristics of this area. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Effects of Pinus caribaea forests on the C, N, P, and S status of Brazilian savanna Oxisols

Abstract The accumulation of humus in soils under Pinus caribaea Morelet forests may increase the availability of organically bound N, P, and S to plants and thus improve soil fertility. To test this hypothesis, we compared C, N, P, and S concentrations and forms in soil solid phase and soil solution under Pinus (PI) and natural savanna vegetation ( Cerrado , CE). We sampled the 0–0.15, 0.15–0.3, 0.3–0.8, 0.8–1.2, and 1.2–2 m soil depth layers of Oxisols from three spatially separated plots under CE and PI; at each plot, soil solution at 0.15, 0.3, 0.8, 1.2, and 2 m soil depth and under PI additionally below the organic layer was collected during two rainy seasons (21 October–28 April 1997/1998 and 1998/1999). Under PI, the organic layer stored more C (24.5 t ha −1 ), N (2.6 t ha −1 ), P (140 kg ha −1 ), and S (261 kg ha −1 ) than under CE (1.2 t C ha −1 , 23 kg N ha −1 , 1.2 kg P ha −1 , 2.3 kg S ha −1 ). No significant difference in C, N, P, and S concentrations between CE and PI in the surface horizons of the mineral soil (C: 20.7–23.4 g kg −1 , N: 1.2–1.5 g kg −1 , P: 0.16–0.26 g kg −1 , S: 0.18–0.21 g kg −1 ) was observed. However, at 1.2–2 m soil depth C and organic N (N org ) concentrations were significantly higher under PI (C: 6.9 g kg −1 , N org : 0.54 g kg −1 ) than under CE (C: 6.4 g kg −1 , N org : 0.41 g kg −1 ). C/P and C/S ratios in soil solid phase decreased with increasing soil depth under CE and PI, those of C/N org only under PI, leading to narrower C/N org ratios under PI (12.9) than under CE (15.6) at 1.2–2 m soil depth. The result indicated enhanced N transport to the subsoil under PI. Total organic C concentrations (TOC) in the soil solution were similar under CE and PI and throughout the mineral soil. Under PI, TOC concentrations decreased sharply between the litter leachate (30 mg C l −1 ) and 0.15 m soil depth (5 mg C l −1 ) indicating strong TOC sorption onto the surface mineral soil. Total N concentrations in the soil solution were 11–23 times higher under PI than under CE. Organic N concentrations also decreased between the litter leachate and 0.15 m soil depth under PI but NO 3 − concentrations increased from the litter leachate to 1.2 m soil depth and decreased between 1.2 and 2 soil depth, probably due to enhanced anion sorption at 2 m soil depth. No S and P was detectable in any solution sample. Higher C, N, and S storages and higher N availability under PI indicated that PI forests improve C, N, P, and S supply for plants in subsequent cultures and may therefore be considered sustainable with respect to organically bound nutrients.

Effects of deep ripping and organic matter amendments on Ap horizons of soil reconstructed after coal strip-mining

Deep ripping is a common management technique used to shatter dense subsurface soil horizons that limit percolation of water and penetration of roots. Most studies have concentrated on the effects of deep ripping on subsoils, with few examining the effects on surface soil horizons. A study was conducted on reclaimed fields (undisturbed soil Albic Luvisol) at the Highvale coal mine approximately 80 km west of Edmonton, Alberta, Canada, with objectives to evaluate the effect of deep ripping, with and without the use of organic matter amendments, on the physical and chemical properties of Ap horizons of reconstructed minesoils. Significant treatment effects were identified for particle size distribution (all fractions), plasticity index and the October mean penetration resistance (PR), but effects on bulk density, soil water content at the time of sampling, water retention characteristics and modulus of rupture were non-significant. Soil chemical properties significantly affected included pH, soluble Ca and Mg, exchangeable Na and K, exchangeable Ca:Na ratio and loss of ignition. Addition of subsoil to the Ap horizon as a result of deep ripping likely caused the changes. They are expected to make the seedbed more difficult to cultivate and manage unless specific management practices such as organic matter additions are adopted to overcome the detrimental changes in soil properties that have occurred. Key words: Manure, peat, reclamation, soil properties, subsoiling, surface horizon, tilth

Dissolved Organic Carbon in Terrestrial Ecosystems: Synthesis and a Model

The movement of dissolved organic carbon (DOC) through soils is an important process for the transport of carbon within ecosystems and the formation of soil organic matter. In some cases, DOC fluxes may also contribute to the carbon balance of terrestrial ecosystems; in most ecosystems, they are an important source of energy, carbon, and nutrient transfers from terrestrial to aquatic ecosystems. Despite their importance for terrestrial and aquatic biogeochemistry, these fluxes are rarely represented in conceptual or numerical models of terrestrial biogeochemistry. In part, this is due to the lack of a comprehensive understanding of the suite of processes that control DOC dynamics in soils. In this article, we synthesize information on the geochemical and biological factors that control DOC fluxes through soils. We focus on conceptual issues and quantitative evaluations of key process rates to present a general numerical model of DOC dynamics. We then test the sensitivity of the model to variation in the controlling parameters to highlight both the significance of DOC fluxes to terrestrial carbon processes and the key uncertainties that require additional experiments and data. Simulation model results indicate the importance of representing both root carbon inputs and soluble carbon fluxes to predict the quantity and distribution of soil carbon in soil layers. For a test case in a temperate forest, DOC contributed 25% of the total soil profile carbon, whereas roots provided the remainder. The analysis also shows that physical factors—most notably, sorption dynamics and hydrology—play the dominant role in regulating DOC losses from terrestrial ecosystems but that interactions between hydrology and microbial–DOC relationships are important in regulating the fluxes of DOC in the litter and surface soil horizons. The model also indicates that DOC fluxes to deeper soil layers can support a large fraction (up to 30%) of microbial activity below 40 cm.

Radioactive Fallout and Accumulated Level of Soil Surface Contamination with 90Sr and 137Cs around St. Petersburg in 1954-1999

Systematic data on radioactive atmospheric fallout in 1954-1999 and accumulated levels of contamination of the soil-vegetable cover with 90Sr and 137Cs around St. Petersburg, estimated on 1998, are presented. Comparative analysis is made of these data and directly obtained data on the vertical distribution of these radionuclides in the surface horizon (0-25 cm) of soddy-podzolic sandy loam soil: the levels of surface contamination with 90Sr and 137Cs were found to be 580±90 and 6500±600 Bq m-2 on 1998. Before April 28, 1986 the 90Sr and 137Cs radioactivity in the investigated region corresponded to the global level. After Chernobyl accident the contamination levels with 90Sr and 137Cs increased, respectively, from 1200 to 1280 (by 6.3%) and from 2400 to 7050 Bq m-2 (by a factor of 2.9). The 90Sr and 137Cs vertical distributions throughout the soil profile are considerably different: 137Cs is practically totally fixed in the surface horizon, its activity being exponentially declining; whereas the stock of 90Sr in horizons 0-10 and 0-25 cm represents only 22 and 59% of the integral accumulated inventory of 90Sr in the soil (990 Bq m-2). The activity ratio of 137Cs to 90Sr in the atmospheric fallout in 1963-1985 was 1.85±0.42, and in river water in the same period, ca. 0.18, suggesting higher mobility in the soil of 90Sr as compared to 137Cs, and, therefore, higher rate of self-cleaning of the surface soil horizon with respect to 90Sr.

PHYSICAL RECONSTRUCTION OF MINE TAILINGS AFTER SURFACE MINING MINERAL SANDS FROM PRIME AGRICULTURAL LAND

Minable quantities of heavy minerals occur in prime agricultural lands of the Atlantic Coastal Plain. We evaluated the use of yard waste compost to reclaim tailings mined from these lands. A 30 × 60-m area was excavated to a depth of 5 m to establish a reclamation study site. Soils were removed, treated in a manner simulating the separation of the mineral sands, and returned in a slurry form. Undisturbed natural soils, Hapludults and Paleudults, have sand and loamy sand surface horizons. Mining these soils and redistributing the tailings as a slurry resulted in sandy loam and sandy clay loam textures. Treatments established in the tailings were: unamended tailings; tailings amended with 23 to 270 t/ha compost; and tailings capped with 45 cm of topsoil. Selected treatments were sampled and analyzed to test for differences in the physical properties related to treatment. Gravimetric water contents of tailings amended with 135 t/ha compost were significantly higher (at the 0.05 level) than unamended tailings, tailings capped with top soil, or the natural soil. Mechanical resistance values recorded in the Fall for tailings amended with 135 t/ha of compost were lower than the unamended tailings and tailings capped with topsoil. Mean hydraulic conductivity (Ksat) and porosity values for tailings amended with 135 t/ha compost were higher than the natural soil surface horizons. Tailings amended with as little as 45 t/ha compost had higher porosity (48%) and lower bulk density (1.38 g/cm3) values than the natural soil surface (45% and 1.43 g/cm3). Tailings capped with topsoil had the highest bulk density (1.64 g/cm3) and lowest porosity (38%) and Ksat (0.13 cm/hr), suggesting that this reclamation approach may not be the best way to try to reclaim the tailings. Average corn yields for tailings amended with compost were higher than those for tailings capped with topsoil or unamended tailings. Results suggest that mine soils can be constructed with excellent physical properties with regard to plant growth by amending tailings with yardwaste compost.

Land-Use Management Using a Soil Survey Geographic Database for Finney County, Kansas

The determination of best management practices for land resources is often complicated by the lack of a means for evaluation and lack of quality data. Soil surveys are an important source of data that can be used to improve farm and ranch planning and environmental protection. In this study, we examined the use of a soil survey geographic (SSURGO) database within a geographic information system (GIS) coupled with remote sensing data for land-use management in Finney County, Kansas. The objectives were (i) to identify land-use change; (ii) to evaluate the influence of soil, groundwater, and physiography on land use; and (iii) to assess land-use potential and present management alternatives. Land-use/land-cover (LULC) maps for 1987, 1989, and 1992 were derived from Landsat Thematic Mapper data. These LULC layers were manipulated with layers: organic matter content, thickness, and texture of the surface soil horizon; land capability class; aquifer thickness (AT); and physiography. The acreage of fallow land decreased and the acreage of grassland increased from 1987 to 1992 because of an increase in the acreage of land used in the Conservation Reserve Program (CRP). Broad cropping patterns (irrigated vs. nonirrigated) did not change significantly between 1987 and 1992 and were related to AT. Some currently cropped areas had high erosion potential, whereas some grasslands had relatively low erosion hazards. These grasslands could be used as alternatives for cropping. The study demonstrates the potential of using SSURGO within a GIS coupled with remote sensing information in planning and management for natural resources.

Changes in the surface charge characteristics of degraded soils in the wet tropics through the addition of beneficiated bentonite

In their pristine state, soils of the wet tropics maintain highly productive climax rainforests that have an intrinsically tight nutrient cycling capacity. When these ecosystems are disturbed and placed under agronomic production, soil organic matter is rapidly lost due to continuous stirring of surface soils, and consequently, there is a rapid decline in fertility. In this study a methodology is presented that quantifies the degree of degradation that an agronomic system has undergone since land conversion. In an effort to reverse this degradation, a glasshouse study was undertaken to evaluate the effect of applying beneficiated bentonite clays on the surface charge characteristics of 2 degraded soils and their influence on the growth of sorghum. The properties of an Oxisol cleared of climax rain forest 53 years previously and currently under tea production were compared with an adjacent undisturbed forest. Soil pH declined by approximately 0.6 unit. Organic carbon levels decreased dramatically under the disturbed site, along with exchangeable basic cations. The degree of degradation associated with changed land use was estimated to be 85% for the surface soil horizon. In an effort to remediate the aforementioned degraded Oxisol and a similarly degraded light-textured Ultisol currently under sugarcane, varying rates (0–40 t/ha) of beneficiated (Ca 2+ , Mg 2+ , and K + saturated in a ratio of 8 : 4 : 1) bentonite were applied. Charge fingerprints were produced for each treatment prior to and after the growing of a sorghum crop. The basic cation exchange capacity at soil pH was increased from 1.15 to 3.00 cmol c /kg on a light-textured Ultisol and from 0.8 to 2.00 cmol c /kg on the Oxisol through the addition of beneficiated bentonites. This increase in surface charge was found to be permanent. Concomitant with the improved charge characteristics was a significant and sustained increase in forage sorghum biomass production with increasing additions of bentonites on both soil types. The cumulative increase in yield between the control and 40 t/ha bentonite application was a 7.7- and 3.1-fold increase for the Ultisol and Oxisol soil types, respectively.

Phosphorus forms and related soil chemistry of Podzolic soils on northern Vancouver Island. II. The effects of clear-cutting and burning

When cedar-hemlock (CH) forests of northern Vancouver Island are clear-cut and replanted, growth of replanted trees is often poor. This growth check can be overcome with nitrogen (N) and phosphorus (P) fertilization, suggesting that it may be because of deficiencies of these elements. A widely used site-preparation tool in these forests is slash burning. Because fire is known to alter nutrient cycling in forests, this burning may be contributing to the problem of poor seedling growth. Thus, the objective of this study was to compare P in forest floor and soils from clear-cut CH stands 10 years, 5 years, and immediately after burning to P concentrations and forms in undisturbed old growth CH stands. Analytical methods included extraction and digestion procedures, fractionation and31P nuclear magnetic resonance spectroscopy. Soon after burning, an "ashbed effect" was noted, with increased pH and higher concentrations of available P in surface soil horizons. Available P concentrations and pH returned to preburn levels within 10 years. However, destruction of organic matter appeared to disrupt illuviation processes throughout the soil profile, producing long-term changes in organic matter, organic P, and organically complexed Fe and Al in lower mineral horizons. Total P concentrations were unchanged, but there was a shift from organic to inorganic P forms and changes in P forms with time at depth in the profile. These changes in P distribution and movement in the soil may contribute to the growth check observed in these forests.

Changes in soil organic carbon storage under grassland as evidenced by changes in sulphur input–output budgets

Information about temporal changes in soil organic carbon (C) pools may be obtained indirectly from changes in input–output budgets of organically combined nutrients such as sulphur (S). Sulphur budgets were therefore evaluated for Northern Ireland (NI) for the period 1940–1990, inclusive. These budgets indicated that the land or soil had acted first as a sink but then as a source for S, and that reserves of soil S built up between 1940 and 1965 were totally depleted by the mid-1980s. Pooled data from six long-term soil-monitoring sites on undisturbed grassland suggested that negative S budgets from the late-1970s onwards had been due to the net mineralization of soil organic matter and thus were indicative of net losses of organic C from surface soil horizons. There was some evidence that the decline in rainfall and fertiliser S inputs from the mid-1960s may have precipitated the breakdown of soil organic matter.

Effects of surface and sub-surface soil horizons on the seasonal performance of Larrea tridentata (creosotebush)

1. In warm arid and semiarid environments, the accumulation of clay minerals produces increasingly well developed soil horizons with the passage of time. Differences in the strength of development of two prominent soil horizons, silt- and clay-rich surface vesicular (Av), and clay-enriched subsurface argillic (Bt), may strongly influence the amount and seasonal continuity of plant-available water and the physiological activity of long-lived desert shrubs. Three sites were selected on an alluvial piedmont (bajada) in the Mojave Desert that varied in surface and subsurface horizon development. The first site, a deep deposit of stabilized dune sand, entirely lacked soil horizon development. The second site had a well developed surface stone pavement and underlying Av horizon, but lacked an argillic horizon in the sandy subsoil. The third had a well developed surface pavement and Av horizon, and a deeper, well developed clay-rich argillic horizon. Seasonal water potential and gas-exchange responses of the evergreen desert shrub Larrea tridentata[DC.] Cov., and volumetric soil water content (θ), were measured monthly in 1996 on these three soils in order to test the hypothesis that desert pavements, Av and subsurface Bt horizons differentially affect the effectiveness and utilization of seasonal precipitation. 2. Predawn and midday water potentials (ψpd and ψmid), net photosynthetic rates (Anet), and stomatal conductances (gs) in L. tridentata were highest in the deep, sandy dune soils lacking horizons that could restrict surface and subsurface infiltration. Plants growing in these soils also showed no physiological response to summer precipitation events. Following a single large precipitation event during the growing season (3·8 cm), the water potentials, Anet and gs in L. tridentata were similar in the first (sand dune) and second (pavement and Av horizon) sites. However, plant performance on these soil surfaces showed marked seasonal declines, and did not respond to a small pulse of summer rainfall. Plants growing at the third site (older soils with strongly developed pavement, Av and Bt horizons) had very low gas-exchange rates and water potentials. However, following convectional summer thunderstorms L. tridentata showed improved water relations and gas exchange in these soils. 3. Midday water potentials were frequently anomalously higher than predawn water potentials, up to +6 MPa late in the growing season, especially on soil surfaces with well developed soil horizons. This anomaly was due to seasonal decreases in ψpd accompanied by invariant midday ψ. In general, ψpd was correlated with θ across the depths measured. 4. Correlations between ψmid with θ at 35 cm increased dramatically with increasing Bt horizon development, suggesting that seasonal ψmid may have been due to the vertical translocation of water (‘hydraulic lift’). 5. Our results show that subsurface and surficial soil horizon development differentially affects the seasonal availability of water for desert plants. During wetter parts of the season, subsurface horizons limit the degree of water availability, while surface soil characteristics have the greatest influence on the effectiveness of summer precipitation. These findings suggest that the effectiveness of climatic precipitation and attendant plant utilization of water resources in warm desert systems may depend on the physical soil condition.

Effects of surface and sub-surface soil horizons on the seasonal performance of Larrea tridentata (creosotebush)

1. In warm arid and semiarid environments, the accumulation of clay minerals produces increasingly well developed soil horizons with the passage of time. Differences in the strength of development of two prominent soil horizons, silt- and clay-rich surface vesicular (Av), and clay-enriched subsurface argillic (Bt), may strongly influence the amount and seasonal continuity of plant-available water and the physiological activity of long-lived desert shrubs. Three sites were selected on an alluvial piedmont (bajada) in the Mojave Desert that varied in surface and subsurface horizon development. The first site, a deep deposit of stabilized dune sand, entirely lacked soil horizon development. The second site had a well developed surface stone pavement and underlying Av horizon, but lacked an argillic horizon in the sandy subsoil. The third had a well developed surface pavement and Av horizon, and a deeper, well developed clay-rich argillic horizon. Seasonal water potential and gas-exchange responses of the evergreen desert shrub Larrea tridentata[DC.] Cov., and volumetric soil water content (θ), were measured monthly in 1996 on these three soils in order to test the hypothesis that desert pavements, Av and subsurface Bt horizons differentially affect the effectiveness and utilization of seasonal precipitation. 2. Predawn and midday water potentials (ψpd and ψmid), net photosynthetic rates (Anet), and stomatal conductances (gs) in L. tridentata were highest in the deep, sandy dune soils lacking horizons that could restrict surface and subsurface infiltration. Plants growing in these soils also showed no physiological response to summer precipitation events. Following a single large precipitation event during the growing season (3·8 cm), the water potentials, Anet and gs in L. tridentata were similar in the first (sand dune) and second (pavement and Av horizon) sites. However, plant performance on these soil surfaces showed marked seasonal declines, and did not respond to a small pulse of summer rainfall. Plants growing at the third site (older soils with strongly developed pavement, Av and Bt horizons) had very low gas-exchange rates and water potentials. However, following convectional summer thunderstorms L. tridentata showed improved water relations and gas exchange in these soils. 3. Midday water potentials were frequently anomalously higher than predawn water potentials, up to +6 MPa late in the growing season, especially on soil surfaces with well developed soil horizons. This anomaly was due to seasonal decreases in ψpd accompanied by invariant midday ψ. In general, ψpd was correlated with θ across the depths measured. 4. Correlations between ψmid with θ at 35 cm increased dramatically with increasing Bt horizon development, suggesting that seasonal ψmid may have been due to the vertical translocation of water (‘hydraulic lift’). 5. Our results show that subsurface and surficial soil horizon development differentially affects the seasonal availability of water for desert plants. During wetter parts of the season, subsurface horizons limit the degree of water availability, while surface soil characteristics have the greatest influence on the effectiveness of summer precipitation. These findings suggest that the effectiveness of climatic precipitation and attendant plant utilization of water resources in warm desert systems may depend on the physical soil condition.

Nitrogen‐15 and Bromide Tracers of Nitrogen Fertilizer Movement in Irrigated Wheat Production

AbstractIn irrigated agricultural systems, NO3 leaching is believed to result from high fertilizer rates combined with the need to periodically leach salts from surface soil horizons. The purpose of this research is to estimate N fertilizer movement in the soil of commercial fields of flood‐irrigated wheat (Triticum spp.) while documenting best management practices (BMPs). Potassium bromide and 15N labeled ammonium sulfate were applied as tracers of N fertilizer movement to 1‐ × 1‐m microplots replicated 9 or 10 times in three commercial fields of flood‐irrigated wheat. The soil was sampled at harvest to a depth of 2.4 to 4.0 m. More fertilizer was applied at two out of three sites and more irrigation water was applied at all sites than recommended by BMPs. Bromide recovery in the soil and plant tissue at harvest was 29, 68, and 61% of that applied at the three sites. Most of the 15N measured in the soil profile was contained in the 0‐ to 0.3‐m increment of soil. Recovery on 15N in the soil and plant tissue at harvest was 54, 54, and 69% of that applied. The Br recovery data suggests that 32 ± 28% (standard deviation) of the 15N applied may have leached. The highest leaching potential was measured at the site that adhered to BMPs for N fertilizer management but had the most permeable soil. Nitrate leaching in flood‐irrigated wheat production seems inevitable even if BMPs for N fertilizer management are followed.

Simple phosphorus saturation index to estimate risk of dissolved P in runoff from arable soils

Abstract. There is increasing evidence that phosphorus has been accumulating in the surface horizons of agricultural soils to the extent that some soils represent a potential diffuse source of pollution to surface waters. The relationships between equilibrium phosphorus concentration at zero sorption (EPC 0) of soil and a number of soil physicochemical variables were investigated in the surface layers of arable and grassland agricultural soils sampled from the Thame catchment, England. Soil EPC0 could be predicted from an equation including soil test (Olsen) P, soil phosphate sorption index (PSI) and organic matter content (OM) (R2=0.88; P&lt;0.001) across a range of soil types and land use. The simple index Olsen P/PSI was found to be a good predictor of EPC0 (R2=0.77; P&lt;0.001) and readily desorbable (0.02 m KCl extractable) P (R2=0.73; P&lt;0.001) across a range of soil types under arable having soil organic matter contents of &lt;10%.

Efficacy of single application ammonium sulfate in suppressing potato common scab

The objective of this study was to suppress potato common scab by lowering the soil pH and increasing the concentration of water-soluble aluminum (Al) in soil with a single application of ammonium sulfate into each row. Superphosphate (P) and potassium sulfate (K) were applied to the surface soil horizon and ammonium sulfate (N) was applied only into the rows along which potato plants were to be planted. By this application method, the soil pH was lowered and the concentration of water-soluble Al was increased in the soil of the rows where potato tubers were grown. Potato common scab was suppressed in the soil containing water-soluble Al in concentrations of 0.2 to 0.3 mg L−1 or higher. The pH of the soil fertilized as indicated above remained lower than that of the control soil to which the mixture of N, P, and K was uniformly applied. In soil types such as Haplic Andosols containing allophane at high concentrations of 71 g kg−1 in Memanbetsu, the suppression of potato common scab by this single application of ammonium sulfate was less effective due to the low soluble Al concentration. In other soil types, the soil pH was easily controlled and common scab was suppressed by this method. The advantages of this method are that (a) it minimizes the use of fertilizers, thus reducing the adverse effects of unnecessary fertilizers on the soil; and (b) it lowers the cost by eliminating additional agricultural chemicals and extra fertilizers.

SOIL CHARACTERISTICS OF FOUR SOUTHERN APPALACHIAN FENS IN NORTH CAROLINA, USA

Abstract Mountain fens are uncommon and unique wetlands in the southern Appalachian Highlands. We selected four mountain fens in North Carolina to compare soil particle-size distribution, organic carbon, pH, cation exchange capacity (CEC), and exchangeable Mg+2, Ca+2, and K+. Three of the sites are depressional areas on alluvial floodplains at elevations between 700 and 1130 m above sea level, while the fourth site is located on a slope intersecting ground-water seepage at an elevation of 950 m. Peat accumulation amounted to a shallow surface Sphagnum layer (usually <5 cm) at three sites, and the organic carbon content of the surface soil horizon ranged from 4 to 21%. Three of the soils would be classified as Cumulic Humaquepts, and the other is a Terric Haplohemist. The pH of the surface horizon of the four soils ranged from 4.3 to 4.9. The alluvial fens had higher silt concentrations than the seepage fen, and two of the alluvial fens had a subsurface, fine-textured deposit. The seepage fen showed little...

A cation exchange index for assessing degradation of acid soil by further acidification under permanent agriculture in the tropics

SummarySoil acidification and related land degradation are becoming more important worldwide and are challenging the concept of sustainability of current land management systems. We have studied the impact on soil properties of two management systems, one in a tropical savanna, the other in a humid subtropical environment, particularly in relation to their cation chemistry. In Northeast Thailand the properties of an Acrisol cleared of dipterocarp forest some 37 years previously and used for continuous crop production are compared with adjacent undisturbed forest. In southern Queensland, Australia, the properties of a Luvisol used for intensive pasture production for 36 years are compared with those prior to introduction of the management system. Under both regimes, the soil pH fell by about 1 unit, with an increase in aluminium saturation of the cation exchange complex. Organic carbon in the Acrisol greatly decreased, but in the Luvisol it increased significantly. Consequently, the effects on surface charge characteristics were strikingly different, in that the Acrisol had been almost irreversibly degraded, whereas acidification and base diminution in the Luvisol can be easily reversed through conventional management. The amount of degradation caused by each management system has been quantified in terms of a Saturation Index, Su: S u = 100 × (Cu5.5 − Σ)/Cu5.5,where Cu5.5 refers to the cation exchange capacity on the undisturbed (forested) site and Σ is the sum of the base cations at field pH. This index encompasses an assessment of the degree of degradation taking into account the previous land use, in this case forest, and that due to anthropogenic disturbance. For the Queensland Luvisol Su does not change, because the pasture management has not appreciably altered the CEC at pH 5.5. However, for the Thailand site Su increased from 0 to 86%. Consequently in its present state, the surface horizon of this cultivated soil can retain only about 15% of the basic cations it could ideally hold under undisturbed conditions.