Initial Soil Conditions Research Articles

Evaluating the risk of non-point source pollution from biosolids: integrated modelling of nutrient losses at field and catchment scales

Abstract. A semi-distributed model, INCA, has been developed to determine the fate and distribution of nutrients in terrestrial and aquatic systems. The model simulates nitrogen and phosphorus processes in soils, groundwaters and river systems and can be applied in a semi-distributed manner at a range of scales. In this study, the model has been applied at field to sub-catchment to whole catchment scale to evaluate the behaviour of biosolid-derived losses of P in agricultural systems. It is shown that process-based models such as INCA, applied at a wide range of scales, reproduce field and catchment behaviour satisfactorily. The INCA model can also be used to generate generic information for risk assessment. By adjusting three key variables: biosolid application rates, the hydrological connectivity of the catchment and the initial P-status of the soils within the model, a matrix of P loss rates can be generated to evaluate the behaviour of the model and, hence, of the catchment system. The results, which indicate the sensitivity of the catchment to flow paths, to application rates and to initial soil conditions, have been incorporated into a Nutrient Export Risk Matrix (NERM).

Animal-powered tillage erosion assessment in the southern Andes region of Ecuador

While water erosion has been the focus of past research in the Andes, former studies show that soil erosion could also be related to the methods used in cultivating the fields. The main objective of the present study was to assess (i) tillage erosion caused by the traditional animal-powered “yunta” or ard plough in the Andes and the factors controlling the process and (ii) the implications for soil conservation. Erosion rates were experimentally measured on 27 sites, having slopes from ca. 0% to 60% and soils ranging from Andosols to Cambisols, in the Andes region of Ecuador (Gima, Azuay). Different tillage methods were assessed: (i) tillage parallel to the contour lines (‘Paralelo’) and (ii) tillage at an angle with the contour lines. Statistical analysis points out that erosion caused by animal-powered tillage is gravity-driven. A strong correlation exists between slope and downslope displacement: furthermore, tillage depth and initial soil condition are important. For the ‘Paralelo’ tillage method the tillage transportation coefficient ( k) is below 100 kg m − 1 Tillage Pass − 1 , for the combined ‘Arado’–‘Cruzado’ tillage method k may exceed 300 kg m − 1 . Tillage erosion is responsible for the reduction of the slope between the contour strips over a relatively short time period of 20 years, resulting in the formation of terraces and therefore the reduction of the water erosion risk. However, at the same time it may negatively affect soil quality.

Soil organic carbon dynamics following afforestation of degraded pastures with eucalyptus in southeastern Brazil

Afforestation of degraded pastures can potentially enhance carbon sequestration, but little is known about the effects of eucalyptus plantations on soil organic matter (SOM) fractions. We used density and particle size SOM fractionations to evaluate changes in SOM for a chronosequence of eucalyptus plantations established on degraded pastures in two contrasting regions in southeastern Brazil. Declines in the content of soil C derived from the pasture (C4 photosynthetic pathway) and the accumulation of that derived from the eucalyptus (C3 photosynthetic pathway) were followed through 13C natural abundance of the SOM. The two study areas were in the Rio Doce River Valley, Minas Gerais State, namely: 1, Belo Oriente (BO, a region with lower elevation, higher mean annual temperature, lower forest productivities and dominated by clayey Typic Haplustoxs); 2, Virginópolis (VG, a region of higher elevation, higher forest productivities and dominated by clayey Rhodic Ustoxs). In the BO region the chronosequence included 0, 4.2, 13.2, 22.2, 32.0 and 34.2 years of eucalyptus cultivation and in the VG region soils were cultivated with eucalyptus for 0, 8.2, 19.2, 29.2 and 33.2 years. The accumulated cultivation time corresponds to about five rotations. In both regions the initial soil condition was represented by a site that was still under pasture. Soil samples collected at 0–10 and 10–20 cm depths were analyzed for: total organic carbon (TOC); C in the fulvic (FAF), humic (HAF) and humin (HF) fractions; C in the free (FLF) and occluded (OLF) light fractions; C associated with the heavy fractions, namely, the sand (SF), silt plus clay (S + CF) and clay (CF) fractions; and C in the microbial biomass (MB). Carbon stocks of virtually all SOM fractions were about twice as high in the VG region than in the BO region. Eucalyptus cultivation in the BO region increased C stocks in all SOM fractions, except for the MB fraction that was not altered and the SF fraction that declined with time of eucalyptus cultivation. Increases in TOC, FAF and HF organic C fractions were observed in the VG region. Mean annual accumulations across the entire cultivation period in the 0–10 cm soil layer showed virtually no difference averaging 0.22 Mg C ha −1 year −1 for the BO region and 0.23 Mg C ha −1 year −1 for the VG region. However, gains of TOC peaked by the end of the third rotation in the BO region and the second rotation in the VG region, so the TOC accumulation rate during the respective aggrading periods would correspond to 0.35 and 0.57 Mg C ha −1 year −1. There was no clear evidence that the more labile, fast cycling SOM fractions were more sensitive indicators of the impact of eucalyptus cultivation in the BO region, whereas in the VG region the FLF and OLF were more sensitive to the land use change than TOC.

Impact of a conversion from cropland to grassland on C and N storage and related soil properties: Analysis of a 60-year chronosequence

Land use change can lead to changes in a range of soil properties, including soil carbon (C) and nitrogen (N) content, bulk density and pH. Previous investigations on the effects of land use change have been biased towards the impact of forest clearing and afforestation in tropical environments. Therefore, the aim of this study is to determine the impact of a conversion from cropland to grassland on soil properties in two districts of the Lahn-Dill Highlands, Germany. We determined a land use history for the period 1945–2004 from aerial photographs and field surveys. This land use history was used to build a chronosequence of grassland sites with a different age since the conversion from cropland for both districts. Each chronosequence was sampled to determine bulk density, pH, coarse material fraction, C/N ratio and soil C and N content as a function of grassland age. Results showed that there was no clear dependency of soil properties on grassland age for both districts. It was concluded that observed differences within each district are much more related to differences in soil parent material or slope position, instead of land use. Interestingly, the reduction of the chronosequence analysis to a paired site survey led to an opposite conclusion for the Erda district because all investigated soil properties were significantly different for continuous grassland and cropland. This indicates that care is required when interpreting results from paired site surveys, especially when the equality of initial soil conditions is not (or cannot be) tested.

Differences of Soil Microbial Biomass and Nitrogen Transformation under Two Forest Types in Central Germany

In order to observe the tree species effect on soil N status, soil microbial biomass C and N (Cmic, Nmic), potential N mineralization and potential nitrification (under laboratory incubation conditions, 22 °C) in different subhorizons (LOf1, Of2, Oh and mineral soil at 0–10 cm depth) were determined at three forest sites in central Germany. At each site, two contrasting stands (Beech, Norway spruce or Scots pine) were selected, where the initial soil conditions were similar. Three sampling dates that represented different stages of tree growth were selected: growing season - August, dormant season - November, after budbreak – April. In organic layers, Cmic-to-total C (Ct) ratios under beech and under conifer were 0.72–4.74% and 0.34–2.11%, respectively. Nmic-to-total N (Nt) ratios were 2.47–11.61% and 0.71–5.77%, respectively. Both concentrations of Cmic and Nmic were significantly affected by the stand type and sampling time. Potential N mineralization rates, ranging from 3.7 to 19.7 mg N kg−1 d−1, showed no clear pattern in relation to stand type. However, potential nitrification rates were mostly significantly higher under beech than under contrasting conifer. In mineral soils, concentrations of Cmic and Nmic showed a clear temporal pattern in the order: August>November>April. The average Nmic and Nmic-to-Nt were higher in soils from beech than conifer, while Cmic and Cmic-to-Ct ratios were similar between the two forest types. In organic layers, the highest values of Cmic-to-Nmic ratio and Cmic were found in November samples, especially under beech. By contrast, in mineral soils the highest value of Cmic-to-Nmic ratios were found in April samples, and at that time the Cmic concentrations were the lowest, especially under conifer. These results revealed the differences in microbial growth form and survival strategy associated with different tree species and soil layers.

A winter rainfall simulation study of sediment and phosphorus loss in arable land in Denmark

Geografisk Tidsskrift, Danish Journal of Geography 106(1):57–69, 2006 Most of the Danish freshwater environments do not live up to the high ecological standards set by the European Water Framework Directive. This is mainly due to high phosphorus loadings originating from agricultural areas. Phosphorus pollution caused by soil erosion is generally considered to be insignificant in Denmark, even though soils are rich in phosphorus. There have only been a few erosion monitoring programs conducted in Denmark resulting in little specific data. This paper presents the results of a rainfall simulation study with a low rainfall intensity of 10 mm h−1 conducted in winter conditions in the Haraldsted catchment on Zealand. The test nozzle (Hardi 37E) proved suitable for generating kinetic energy to represent yearly recurring rainstorm events in eastern Denmark. A correlation of sediment and phosphorus loss (total phosphorus (TP), dissolved phosphorus (DP) and dissolved reactive phosphorus (DRP)) compared to different physical soil parameters showed a dependency of soil loss when compared to runoff while total phosphorus could be correlated with both the soil loss and total runoff. Carbon and initial soil conditions were important parameters in determining runoff and soil loss. The average concentration of particulate phosphorus when compared to the total P in runoff was 95.8%. To calculate the percent of DRP of TP in runoff, a multivariate linear regression model was determined. The average phosphorus enrichment ratio was found to be 2.25 but the variation of phosphorus concentration in the eroded sediment was high. In 91% of all the runoff samples, the critical load of DRP was exceeded which may indicate that eroded sediment may pose a potential risk of eutrophication of Danish water bodies.

Long‐Term Effects of Biosolids on Revegetation of Disturbed Sagebrush Steppe in Northwestern Colorado

Abstract A study was conducted to evaluate the long‐term effects of biosolids amendment on restoration of disturbed sagebrush steppe habitat in northwestern Colorado. Twenty‐four years after biosolids amendment, soil fertility and plant community development were studied in replicated plots receiving various biosolids amendments on two different substrates. The two substrates used were a subsoil, determined to have low initial fertility, and a topsoil over retorted shale substrate, determined to have relatively high initial fertility. Results suggest that biosolids amendments have long‐lasting effects on soil fertility and plant community composition, but these effects vary between the two substrates that were utilized. Within the plots established on subsoil, the long‐term effect of biosolids was a reduction in plant species diversity and dominance by perennial grasses. On the topsoil substrate, there was a decrease in perennial grasses and an increase in shrub dominance with increasing biosolids. Results demonstrate the importance of considering initial soil conditions, seed mixture, and biosolids application rate when using biosolids for restoration of disturbed sagebrush steppe habitat. The long‐term effects of the biosolids treatments at this site demonstrate the need to consider restoration treatment effects over longer and more ecologically meaningful time frames.

Cyst Distribution and Hatching Pattern ofChirocephalus ruffoi (Crustacea, Anostraca) in an Experimental Undisturbed Pool

The pattern of cyst distribution in the absence of turbation and their hatching behaviour were studied in an outdoor artificial pool, where just differentiated adults of the anostracan Chirocephalus ruffoi (sex ratio 1:3) lived until it dried up. The horizontal and vertical distributions of cysts in the pool bed were determined. The comparison between cyst bank estimate (Mura, 2004) and the actual number of cysts counted in the pool bed revealed an estimate error of 20.9%. Resting eggs occurred only in the upper 2.5 cm thick soil sections and decreased within this section as depth increased. Peripheral areas of the pool contained significantly larger numbers of cysts than the central area. Multiway analysis on the results recorded in hatching success (nested ANOVA) revealed that the differences were significantly affected by initial soil conditions, treatment and vertical distribution of cysts. Among these factors, vertical distribution (sections nested in cores) was the most influential. Hatching success was significantly inversely related to depth. Differences in the timing of hatching depending on the above considered factors were also noted. A nearly synchronous hatching pattern was observed only for cysts from initially dry sediment of the uppermost layers. In all successively deeper layers, hatching showed multiple peaks and was increasingly delayed and erratic (already mentioned). ANCOVA within each of the experimental conditions revealed significant differences in hatching frequencies (time as covariate) depending on sediment depth. Within any given layer, ANCOVA revealed a significant influence of initial sediment conditions and treatment on the timing of hatching. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Regional variation in soil carbon and δ13C in forests and pastures of northeastern Costa Rica

Recent studies suggest that the direction and magnitude of changes in soil organic carbon (soil C) pools following forest-to-pasture conversion in the tropics are dependent upon initial soil conditions and local factors (e.g. pre-conversion soil C content, soil texture, vegetation productivity, and management practices). The goal of this study was to understand how landscape-scale variation in soil-forming factors influenced the response of soil C pools to forest clearing and pasture establishment in northeastern Costa Rica. We measured soil C and its stable isotopic composition in 24 paired pasture and reference forest sites distributed over large gradients of edaphic characteristics and slope throughout a 1400 km2 region. We used the large difference in stable C isotopic signatures of C3 vegetation (rain forest) versus C4 vegetation (pasture grasses) as a tracer of soil C dynamics. Soil C pools to 30 cm depth ranged from 26% lower to 23% higher in pastures compared to paired forests. The presence of non-crystalline clays and percent slope explained between 27 and 37% of the variation in the direction and magnitude of the changes in soil C storage following pasture establishment. Stable carbon isotopes (δ13C) in the top soil (0–10 cm) showed a rapid incorporation of pasture-derived C following pasture establishment, but the vegetation in these pastures never became pure C4 communities. The amount of forest-derived soil C in pasture topsoils (0–10 cm) was negatively correlated to both pasture age and the concentrations of non-crystalline iron oxides. Together these results imply that site factors such as soil mineralogy are an important control over soil C storage and turnover in this region.

Changes in Topographical Features and Soil Properties after Exclosure Combined with Sand-Fixing Measures in Horqin Sandy Land, Northern China

In a field experiment we evaluated the effects of exclosure and sand-fixing measures on land restoration of shifting sand dunes in a semi-arid, sandy grassland located in Northern China. We fenced an area of about 2.1 ha, enclosing 2 shifting sand dunes—one large and the other small. In 1996, on each dune we applied 2 sand-fixing measures developed in China—burying wheat straw in a checkered pattern (Sc) and planting seedlings of a sand-fixing shrub, Artemisia halodendron (Ar). Changes in the topographical features and soil properties were then monitored for 6 years. The effectiveness of the sand-fixing measures was assessed by monitoring the topographical features and soil properties. Sand dune fixation took 1 year, and a biological soil crust with high contents of soil organic carbon and fine particles (clay+silt) was formed within 3 years. These results are consistent with those reported in previous studies, confirming that such crusts reduce wind erodibility. The effectiveness of exclosure alone, without the other measures, was also implied, especially for the small dune, based on the changes in the topographical features and soil properties outside the exclosure, where a “no-grazing” policy had been implemented from 2000. The rate of soil restoration was higher on the small dune than on the large dune. This was due to the differences in the initial soil conditions caused by the topographical features, suggesting that the capacity of a site to reverse desertification by itself depends on the dune size. Sc was slightly more effective than Ar, presumably because Ar inhibited the invasion of other plant species, whereas Sc allowed invasion of various species. Although the soil properties in the very thin surface crust were restored, the restoration of the soil properties in deeper layers was not significant even after 6 years. It would take a longer time (e.g., 10 to 20 years) for the restoration of the soil properties in deeper layers to sustain more intensive land use in this region.

The Effect of Raindrop Impact and Initial Soil Conditions on Surface Crust Formation

The Effect of Raindrop Impact and Initial Soil Conditions on Surface Crust Formation

Precipitation pulses and carbon fluxes in semiarid and arid ecosystems.

In the arid and semiarid regions of North America, discrete precipitation pulses are important triggers for biological activity. The timing and magnitude of these pulses may differentially affect the activity of plants and microbes, combining to influence the C balance of desert ecosystems. Here, we evaluate how a "pulse" of water influences physiological activity in plants, soils and ecosystems, and how characteristics, such as precipitation pulse size and frequency are important controllers of biological and physical processes in arid land ecosystems. We show that pulse size regulates C balance by determining the temporal duration of activity for different components of the biota. Microbial respiration responds to very small events, but the relationship between pulse size and duration of activity likely saturates at moderate event sizes. Photosynthetic activity of vascular plants generally increases following relatively larger pulses or a series of small pulses. In this case, the duration of physiological activity is an increasing function of pulse size up to events that are infrequent in these hydroclimatological regions. This differential responsiveness of photosynthesis and respiration results in arid ecosystems acting as immediate C sources to the atmosphere following rainfall, with subsequent periods of C accumulation should pulse size be sufficient to initiate vascular plant activity. Using the average pulse size distributions in the North American deserts, a simple modeling exercise shows that net ecosystem exchange of CO2 is sensitive to changes in the event size distribution representative of wet and dry years. An important regulator of the pulse response is initial soil and canopy conditions and the physical structuring of bare soil and beneath canopy patches on the landscape. Initial condition influences responses to pulses of varying magnitude, while bare soil/beneath canopy patches interact to introduce nonlinearity in the relationship between pulse size and soil water response. Building on this conceptual framework and developing a greater understanding of the complexities of these eco-hydrologic systems may enhance our ability to describe the ecology of desert ecosystems and their sensitivity to global change.

Factors controlling soil water-recharge in a mixed European beech (Fagus sylvatica L.)?Norway spruce [Picea abies (L.) Karst.] stand

The spatial dynamics of soil water-recharge in a forest stand is the product of a number of interacting processes. This study focuses on the role of tree species and antecedent soil water content upon horizontal and vertical patterns of soil water recharge in heavy clay soils of a mixed European beech–Norway spruce stand and of a pure Norway spruce stand after rewetting periods with different rain quantities and intensities. Volumetric water content (VWC) was measured at 194 locations across 0.5-ha plots in each stand using time-domain reflectometry (TDR) with fixed 30- and 60-cm vertical waveguides. This was repeated 28 times (as close as possible) before and after rewetting periods during the vegetation seasons in 2000 and 2001. In addition, the locations of all trees within the plots were recorded. Geostatistics was used to describe the spatial correlation between VWC measurements and to interpolate soil water recharge in space. Spatial patterns of soil water recharge were then evaluated according to antecedent soil water-content and tree species distribution. Open-field precipitation of 30 mm (maximum intensity 10 mm h−1) on extremely dry initial soil conditions resulted in higher subsoil (30–60 cm soil depth) recharge and erratic recharge patterns. This was presumably due to preferential flow in opening shrinkage cracks of the heavy clay soil. A comparable quantity and intensity of rainfall under moderately dry antecedent soil water conditions resulted in almost exclusively topsoil (0–30 cm soil depth) water recharge and patterns of recharge that were clearly related to tree species distribution. The higher recharge around beech trees can be attributed to the lower interception rates there. Spatial patterns of soil water recharge reflect patterns of antecedent soil water conditions.

Comparison of soil nitrogen dynamics under beech, Norway spruce and Scots pine in central Germany

To investigate the effect of tree species on soil N dynamics in temperate forest ecosystems, total N (Nt), microbial N (Nmic), net N mineralization, net nitrification, and other soil chemical properties were comparatively examined in beech (64–68 years old) and Norway spruce (53–55 years old) on sites 1 and 2, and beech and Scots pine (45 years old) on site 3. The initial soil conditions of the two corresponding stands at each site were similar; soil types were dystric Planosol (site 1), stagnic Gleysols (site 2), and Podzols (site 3). In organic layers (LOf1, Of2, Oh), Nmic and Nmic/Nt, averaged over three sampling times (Aug., Nov., Apr.), were higher under the beech stands than under the corresponding coniferous ones. However, the Nmic in the organic layers under beech had a greater temporal variation. Incubation (10 weeks, 22 °C, samples from November) results showed that the net N mineralization rates in organic layers were relatively high with values of 8.1 to 24.8 mg N kg−1 d−1. Between the two corresponding stands, the differences in net N mineralization rates in most of the organic layers were very small. In contrast, initial net nitrification rates (0.2–17.1 mg N kg−1 day−1) were considerably lower in most of the organic layers under the conifer than under the beech. In the mineral soil (0–10 cm), Nmic values ranged from 4.1–72.7 mg kg−1, following a clear sequence: August>November>April. Nmic values under the beech stands were significantly higher than those under the corresponding coniferous stands for samples from August and April, but not from November. The net N mineralization rates were very low in all the mineral soils studied (0.05–0.33 mg N kg−1 day−1), and no significant difference appeared between the two contrasting tree species.

Comparison of Estimates of First‐Year Dairy Manure Nitrogen Availability or Recovery Using Nitrogen‐15 and Other Techniques

Measurements of dairy manure nutrient availability to crops typically show great variability. Approaches that are more accurate are needed to improve manure management and reduce nutrient loss to the environment. In this study, we compared direct (15N recovery) and indirect (difference method [Diff Meth] and fertilizer equivalence [FE] approach) methods of determining first-year dairy manure N availability or recovery during three cropping seasons. A field experiment was conducted on a Plano silt loam (fine-silty, mixed, superactive, mesic Typic Argiudolls) planted to corn (Zea mays L.). Plots received either manure, fertilizer N, or no N. Microplots receiving 15N-labeled manure were also established each study year. Manure was applied to a new plot each cropping season. Whole-plant N uptake was the best crop parameter to use for FE estimates. Estimates of N availability by relative effectiveness (Rel Eff), which are derived from the Diff Meth, and FE were similar (32 and 41%, respectively) and higher than unlabeled N or 15N recovery measurements because these indices factor in N use efficiency. Measures of the Rel Eff of manure N use were highly affected by control plot N uptake. The FE approach is less influenced by control plots, but it requires the inclusion of several more treatments and use of mathematical functions to describe crop response to N. These limitations are reflected in the wide ranges obtained for N availability estimates (-60 to 148%). Although apparent N recovery by the Diff Meth (14%) or direct measurements of 15N recovery (16%) were close on average, variability tended to be much lower for the 15N method. In addition, the Diff Meth was highly dependent on initial soil conditions. Use of 15N-labeled manure, although more costly and time-consuming, provided more consistent and reliable results.

Drying, soil surface condition and interrill erosion on two Ontario soils

Interrill erodibility summarizes the resistance of soil to rainsplash, sheetwash and rainflow erosion and incorporates changes in infiltration, roughness and resistance to detachment and transportation. Resistance to interrill erosion varies during storms, reflecting the effect of wetting, sealing, crusting, and erosion on surface conditions. Significant changes in soil conditions also occur as a result of progressive drying between storms, producing considerable uncertainty in event-based erosion prediction. However, a clear link between drying, primary soil properties and soil condition, as well as interrill erosion during subsequent storms has not been established. In this study, soil condition and interrill erosion of soils from southern Ontario were examined in detail during a sequence of simulated rainstorms, and linkages between primary soil properties, drying, surface soil structure, and interrill erosion were established. Interrill erosion after drying differed depending on the persistence of the changes in soil structure caused by drying. Pronounced stabilization of aggregates on drying led to an increase of erodibility, indicating a transition from a coherent to granular structure, requiring lower stresses for particle detachment. The sensitivity of soil erodibility to drying appears to be determined by the tendency of a soil to form aggregates. The results show that a better understanding of the linkages between primary soil properties, rainfall and drying history, soil condition and interrill erosion is required for effective event-based erodibility assessment. However, the wide range of possible initial soil conditions and the complex nature of soil–climate interaction are likely to limit the effectiveness of erodibility assessment.

Mechanisms of soil vulnerability to compaction of homogenized and recompacted Ultisols

Soil compaction is a main cause of soil degradation in the world and the information of soil compaction in subtropical China is limited. Three main Ultisols (quaternary red clay, sandstone and granite) in subtropical China were homogenized to pass through 2 mm sieve and recompacted into soil cores at two bulk densities (1.25 and 1.45 g cm −3). The soil cores were equilibrated at different matric potential values (−3, −6 and −30 kPa) before subjected to multi-step compaction tests. Objectives of this study were to determine how different initial soil conditions and loading time intervals influence pre-compression stress and to evaluate an easy measure to determine soil vulnerability to compaction. It became evident that the soil strength indicator, pre-compression stress, was affected by soil texture, initial soil bulk density and matric potential. The coarser the soil texture, the lower the bulk density and the higher the matric potential, the lower was the pre-compression stress. The pre-compression stress decreased exponentially with increasing initial soil water content. Soil water content and air permeability decreased after compaction. The amount of water loss was affected not only by soil texture, bulk density and initial water content but also by loading time interval. These results indicate soil pore structure and hydraulic conductivity changed during compactions. The applied stress corresponding to the highest changes of pore water pressure during compaction had a significant linear relationship with the pre-compression stress ( R=0.88, P<0.001). The correlation was ascribed to that the changes in pore water pressure describe the dynamics of the interactive effects of soil pore characters and soil water movement during compaction. The results suggested the evaluation of soil vulnerability to compaction have to consider the initial soil condition and an easy method to measure the changes in pore water pressure can be applied to compare soil strength and soil vulnerability to compaction.

Effect of surface layer thickness on simultaneous transport of heat and water in a bare soil and its implications for land surface schemes

A physically‐based multi‐layer numerical model is developed to determine the coupled transport of heat and water in the soil and in the soil‐atmosphere boundary layer. Using inputs of standard weather data and initial soil conditions the model is capable of predicting the surface energy balance components as well as water content and temperature profiles in the soil. It is used to predict these variables for a bare silt loam soil under two tillage treatments, viz. culti‐packed and left loose after disc‐harrowing, and the predicted results are compared with measurements. Very good agreement between the model predictions and measured evaporation and heat fluxes and soil water and temperatures for a ten‐day period shows that the model is capable of simulating the coupled transport of soil heat and soil water and their transfer across the soil surface‐atmosphere interface adequately. Model predictions were compared with those of CLASS (Canadian Land Surface Scheme). It is shown that CLASS, version 2.6, provides good estimates of evaporation and hence the latent heat flux density, QE, under wetter soil conditions, but overestimates QE at moderately wet soil conditions and underestimates it under dry soil conditions. Under dry to moderately wet soil conditions the calculation of evaporation from bare soil is very sensitive to the thickness of the top layer particularly as the thickness approaches 10 cm.

A neural network framework for mechanical behavior of unsaturated soils

In this paper, a neural network approach is used to describe the mechanical behavior of unsaturated soils. A sequential architecture was chosen for the network, that is, a multilayer perceptron network with feedback capability. The input layer consisted of nine neurons, where six of them represented the initial soil conditions and the remaining three neurons were continuously updated for each increment of axial strain based on outputs from the previous increment. The output layer consisted of three neurons representing values of deviatoric stress, volumetric strain, and change in suction at the end of each increment. Next, a database was developed from triaxial test results available in the literature. The database was used to train and test the network. Neural network simulations were compared with experimental results. The comparison indicates the good performance of the proposed network for predicting the mechanical behavior of unsaturated soils. Moreover, the trained network was employed to simulate other stress paths not present in the database to model the so-called "collapse phenomena." The results were promising.Key words: unsaturated soil, neural network, stress–strain, collapse, modeling, constitutive law.

RESPONSES OF ENGELMANN SPRUCE FORESTS TO NITROGEN FERTILIZATION IN THE COLORADO ROCKY MOUNTAINS

Two old-growth coniferous forests in Colorado with differing initial soil conditions responded differently to four years of low-level fertilization with ammonium nitrate. The site (Fraser) with an average initial organic horizon soil C:N ratio of 36 and nitrogen (N) pool of 605 kg/ha showed no significant increase in net N mineralization rates. At the Fraser site, foliar and organic horizon soil percentage N increased significantly. In contrast, N mineralization rates and inorganic soil N increased significantly at the site (Loch Vale) with greater soil N (C:N of 24, N pool of 991 kg/ha), while foliar N and soil percentage N in the organic layer did not change. We predict continued fertilization at Fraser will narrow the soil C:N ratio to a point where increases in biogeochemical N cycling and fluxes will be detected. Additional N inputs to the site with already low soil C:N ratios will enhance N mineralization rates and leaching losses. The coniferous forests at Fraser and Loch Vale are similar in species composition, stand age, substrate, aspect, and climate. The differences in soil conditions strong enough to cause contrasting responses to fertilization could be due to differences in atmospheric N deposition. Regardless of the reason, the size of the organic soil N pool and C:N ratio of mature coniferous forests in Colorado controls the responsiveness of N pools and fluxes to fertilization, and even low levels of fertilization are sufficient to initiate measurable biogeochemical changes. Corresponding Editor: W. L. Silver