Gravimetric Soil Water Content Research Articles

Response of Soil Inorganic Nitrogen to Land Use and Topographic Position in the Cofre de Perote Volcano (Mexico)

This study addressed the effects of land use and slope position on soil inorganic nitrogen and was conducted in small watersheds. The study covered three land use types: tropical cloud forest, grassland, and coffee crop. To conduct this research, typical slope small watersheds were chosen in each land use type. Slopes were divided into three positions: shoulder, backslope, and footslope. At the center of each slope position, soil sampling was carried out. Soil inorganic nitrogen was measured monthly during a period of 14 months (July 2005-August 2006) with 11 observations. Significant differences in soil NH(4) (+)-N and NO(3) (-)-N content were detected for both land use and sampling date effects, as well as for interactions. A significant slope position-by-sampling date interaction was found only in coffee crop for NO(3) (-)-N content. In tropical cloud forest and grassland, high soil NH(4) (+)-N and low NO(3) (-)-N content were recorded, while soil NO(3) (-)-N content was high in coffee crop. Low NO(3) (-)-N contents could mean a substantial microbial assimilation of NO(3) (-)-N, constituting an important mechanism for nitrogen retention. Across the entire land use set, the relationship between soil temperature and soil inorganic N concentration was described by an exponential decay function (N = 33 + 2459exp(-0.23T), R (2) = 0.44, P < 0.0001). This study also showed that together, soil temperature and gravimetric soil water content explained more variation in soil inorganic N concentration than gravimetric soil water content alone.

Pedotransfer functions related to spatial variability of water retention attributes for lowland soils

The estimation of non available soil variables through the knowledge of other related measured variables can be achieved through pedotransfer functions (PTF) mainly saving time and reducing cost. Great differences among soils, however, can yield non desirable results when applying this method. This study discusses the application of developed PTFs by several authors using a variety of soils of different characteristics, to evaluate soil water contents of two Brazilian lowland soils. Comparisons are made between PTF evaluated data and field measured data, using statistical and geostatistical tools, like mean error, root mean square error, semivariogram, cross-validation, and regression coefficient. The eight tested PTFs to evaluate gravimetric soil water contents (Ug) at the tensions of 33 kPa and 1,500 kPa presented a tendency to overestimate Ug 33 kPa and underestimate Ug1,500 kPa. The PTFs were ranked according to their performance and also with respect to their potential in describing the structure of the spatial variability of the set of measured values. Although none of the PTFs have changed the distribution pattern of the data, all resulted in mean and variance statistically different from those observed for all measured values. The PTFs that presented the best predictive values of Ug33 kPa and Ug1,500 kPa were not the same that had the best performance to reproduce the structure of spatial variability of these variables.

Effects of biological soil crusts on profile distribution of soil water, organic carbon and total nitrogen in Mu Us Sandland, China

Aims Biological soil crusts (BSCs) can affect soil properties including water dynamics and cycling of soil carbon and nitrogen in dryland ecosystems. Previous research has mostly focused on effects of BSCs on soil water distribution or carbon and nitrogen fixation in the surface soil layer. Thus, little is known about effects of BSCs on properties throughout the soil profile. In the current study, we assessed the effects of BSCs on the distribution of soil water content (SW), soil organic carbon content (SOC) and soil total nitrogen content (STN) throughout the soil profile as well as the influence of water conditions on the effects of BSCs. Methods In a field investigation in Mu Us Sandland, North China, soil samples were taken from plots with and without BSCs on 13 and 28 September 2006, respectively. On the two sampling dates, average soil gravimetric water content was 3.83% (61.29%) and 5.08% (60.89%), respectively, which were regarded as low and high water conditions. Soil samples were collected every 5 cm to a depth of 60 cm, and SW, SOC and STN were measured in the laboratory. Important Findings (i) BSCs affected profile distribution of SW, SOC and STN. In addition, water conditions within the plots significantly modified BSCs’ effects on the profile distribution of SW, but marginally affected the effects on SOC and STN. (ii) Under high water conditions, SW in the surface soil layer (0–10 cm) was higher in soils with BSCs compared to those without BSCs, while the opposite was true in the deep soil layer (30–55 cm). (iii) Under low water conditions, SW was lower with BSCs compared with no BSCs in near-surface (5–20 cm) and deep (25–40 cm) soil layers. (iv) BSCs affected SOC and STN only in the surface soil layer (0–5 cm) and were modified by plot water conditions.

Antecedent soil moisture affecting surface cracking of a Vertisol in field conditions

Cracking of shrink–swell soils influences landscape hydrology. Watershed models that address soil cracking phenomena generally use a relationship between shrinkage and current soil water content to estimate the extent of cracking. Although antecedent soil moisture prior to soil shrinkage is found to affect the shrinking of expansive soils in laboratory measurements, field observations are limited. In a previous study, a series of in situ surface crack measurements over 10 years indicated the effect of soil moisture just prior to the start of cracking (antecedent soil moisture) on cracking extent, but this relationship was not specifically analyzed. The objectives of this study were (i) to estimate the antecedent soil water content prior to cracking, (ii) to analyze the effect of antecedent moisture on crack area density in microhighs and microlows, and (iii) to assess the temporal distribution of antecedent soil moisture in relation to an estimated water availability index. Soil cracking was measured on a 10-m × 10-m plot of Laewest clay (fine, smectitic, hyperthermic Typic Hapludert) covered with native tallgrass vegetation on 42 dates during 1989–1998. Gravimetric soil water content was measured on 50 dates; 18 dates corresponded to crack measurements. Gilgai microtopograhy was mapped, and surface crack area density was calculated. For days when soil water content was not measured, it was estimated from precipitation and evapotranspiration. Antecedent soil water content prior to cracking was estimated for depth at 10 cm using daily estimates of soil water content and field notes on cracking. Results indicated that the temporal variation in surface crack area density of the study area during 10 years was related to dynamics of current and antecedent soil water content on microhighs and microlows ( R 2 = 0.68 and 0.59, respectively). Prediction accuracies improved with classifying drying–wetting conditions during cracking. Dynamic temporal changes in the surface crack area density exhibited dependence on a long-term (multi-year) cycle of antecedent soil water content superimposed by short-term (within a year) cycles of current soil water content.

Viscous flow lobes in central Taylor Valley, Antarctica: Origin as remnant buried glacial ice

Viscous flow lobes are common throughout the McMurdo Dry Valleys (MDV) of Antarctica. These features have been described as rock glaciers, gelifluction lobes, solifluction lobes, talus mobilized by pore ice and/or segregation ice, and debris-covered glaciers. We investigate the origin, modification, and flow of a 2-km-long lobe (East Stocking Lobe or ESL) along the north wall of central Taylor Valley using field mapping techniques, shallow seismic surveys, time-dependent displacement surveys, and isotopic analyses of buried-ice samples. On the basis of these integrated analyses, we show that the ESL is cored with remnant glacier ice, most probably derived from an advance of nearby Stocking Glacier ∼ 130 kyr BP. Seismic data, coupled with results from ice-flow modeling assuming plastic flow of clean ice, suggest that the buried core of glacier ice is ∼ 14- to 30-m thick. Near its terminus, the ESL flows at a rate of ∼ 2.4 to 6.7 mm a − 1 . The loose drift that caps the buried ice (typically < 1 m thick) is composed of moderately stratified sand- and gravel-sized clasts; it is dry (1–3% soil gravimetric water content; GWC), except near ephemeral stream channels and the margins of melting snow banks (6–25% GWC). Stable isotopic analyses of samples from the upper 30 cm of the ice lie on a slope of ∼ 5.8 (when plotted on a δD vs. δ 18O graph), well below the local meteoric water line of 7.75, suggesting modification by freeze/thaw processes and evaporation/sublimation. Measured air and soil temperatures show that intermittent melting is most likely possible during summer months where buried ice is ≤ 35 cm below the ground surface. Morphological comparisons with ice-cored deposits in upland regions of the Dry Valleys, e.g., Mullins and Beacon Valleys (30 km inland and ∼ 500 m higher in elevation), and near the coast (40 km distant and ∼ 500 m lower) reveal marked contrasts in the style of near-surface ice degradation and cryoturbation. From these morphological comparisons, we infer that buried-ice deposits in the stable upland zone have not experienced the relatively warm climate conditions now found at the ESL and at lower elevations in the Dry Valleys region (e.g. sustained summertime temperatures of ≥−4 °C) for the last several million years.

Soil water and particle size distribution influence laboratory-generated PM 10

Management of soils to reduce the amount of PM 10 emitted during agricultural tillage operations is important for attainment of air quality standards in California's San Joaquin Valley (SJV). The purpose of this study was to improve and expand upon earlier work of predicting tillage-generated dust emissions based on soil properties. We focus on gravimetric soil water content (GWC) and soil texture. A mechanical laboratory dust generator was used to test 23 soils collected for this study. Averaged results showed PM 10 concentrations (mg m −3) increased logarithmically as GWC decreased below soil water potentials of −1500 kPa. Soils with clay contents less than about 10% by weight began to emit PM 10 at GWCs 1.5–4 times their GWC at −1500 kPa. Soils with clay contents greater than about 10% began to emit PM 10 at GWC values closer to −1500 kPa. We found no correlation between maximum PM 10 concentrations, measured at low GWC values, and the %sand, %silt, or %clay in a soil. However, there was a significant correlation between the %silt to %clay ratio and PM 10 concentrations. This not only suggests the dependence of dust emission magnitudes on the supply of particles of PM 10 size, but also the importance of clay in stabilizing aggregates and maintaining higher amounts of capillary water at lower water potentials. Based on modeled results of pooled data, PM 10 concentrations increased linearly (slope = 564) for every unit increase in the %silt to %clay ratio. However, when soils were separated into groups based on clay content, the slopes for PM 10 concentrations vs. %silt to %clay ratio were texture dependent. The slope for soils with <10% clay (slope = 727) was 3.3 times greater than for soils with >20% clay (slope = 221). Improved PM 10 emission prediction based on soil properties should improve management decisions aimed at reducing tillage-generated PM 10.

Light and Moisture Competition Effects on Biomass of Red Clover Underseeded To Winter Wheat

Red clover (Trifolium pratense L.) use as an underseeded cover crop in winter cereals has declined due to inability of growers to consistently establish uniform stands. The objective of this study was to assess the effect of light and soil moisture competition on underseeded red clover establishment and end of season dry matter production. Field trials were conducted at multiple locations in 2005 and 2006 in Ontario, Canada. Wheat (Triticum aestivum L.) N rate (67 and 135 kg N ha−1) and row thinning treatments (19‐cm rows, every third 19‐cm row removed at the 4–5 leaf stage) were used to alter light penetration and soil moisture competition. The high N rate and row thinning treatments consistently reduced light penetration, beginning as early as wheat stem elongation initiation, but had no effect on soil gravimetric moisture content. Soil moisture was primarily affected by location and year. Red clover dry weight in 2005, a relatively dry year, ranged from 688 to 1184 kg ha−1, and in 2006, a relatively wet year, ranged from 2336 to 2805 kg ha−1. Average final red clover stand count was 23 plants m−2 in 2005 and 55 plants m−2 in 2006. In 2005, plant mortality occurred before wheat anthesis. In both years, and at most locations, red clover final dry weight was positively correlated with light penetration, again beginning as early as initiation of wheat stem elongation. Final red clover dry weight in both years and red clover stand count in 2005 were correlated with soil gravimetric water content during wheat anthesis, but this was primarily due to location and year effects. Although both light penetration through the wheat canopy and soil moisture influence biomass production of underseeded red clover, soil moisture has the greater influence and is altered very little by wheat management.

Corn Response to Conventional and Slow‐Release Nitrogen Fertilizers across a Claypan Landscape

Improvement of N fertilizer recovery efficiency (NRE) is necessary to reduce excess N entering the environment and to increase economic returns. A 2‐yr field trial was established in 2005 in Northeast Missouri to determine the effects of conventional and slow‐release N fertilizer sources and landscape position (summit, sideslope, and low‐lying) on crop growth and NRE in corn (Zea mays L.) in a claypan soil. Nitrogen fertilizer treatments at 168 kg N ha−1 consisted of pre‐plant applied and incorporated polymer‐coated urea (PCU), urea, 50% PCU/50% urea mix by weight, anhydrous ammonia (AA), and a nontreated control. Gravimetric soil water content data in 2005 and 2006 indicated that the low‐lying position was often wetter than the summit and sideslope positions. Anhydrous ammonia and PCU treatments increased grain yield 1470 to 1810 kg ha−1 over urea in the low‐lying position in 2005 and 2006. Corresponding increases in plant N uptake and NRE were observed in the low‐lying position in 2005 with use of PCU and AA compared to urea, but not in 2006. Based on the grain yield results in this research and different fertilizer cost and crop prices, gross profit differences for use of PCU and preplant‐applied AA compared with urea in the low‐lying position could range from $50 to $642 ha−1. These results demonstrate that targeted use of different N fertilizer sources in claypan landscapes may increase grain yields and profitability.

Spatial heterogeneity provides organic matter refuges for soil microbial activity in the Patagonian steppe, Argentina

In arid and semi-arid ecosystems that are frequently classified as water limited, it is unclear how spatial and temporal variability of vegetation and climate could affect microbially-mediated soil processes. Our objective was to determine how aboveground spatial heterogeneity creates characteristic soil conditions that modulate microbial growth and activity in a semi-arid Patagonian steppe. In particular, we explored how micro-environmental and biogeochemical soil characteristics generated by the native vegetation could control soil β-glucosidase activity. Both life-form (shrubs, grasses, mosses and bare soil) and season exerted strong controls on all measured abiotic (soil temperature and gravimetric soil water content, inorganic nitrogen, pH and total C and N) and biotic (microbial biomass C and β-glucosidase activity) soil characteristics. Partial correlation between β-glucosidase activity and extracellular organic C (EOC) was high across seasons ( r = 0.5; P < 0.001) while soil water content did not correlate with soil enzymatic activity ( r = 0.09; P > 0.05). We postulate that labile soil carbon rather than water availability functions as a principal limitation of microbial activity in this semi-arid ecosystem, and the distribution of this carbon is, in large part, determined by the patchy distribution of vegetation.

Nitric oxides and nitrous oxide fluxes from typical vegetables cropland in China: Effects of canopy, soil properties and field management

In China, vegetable croplands are characterized by intensive fertilization and cultivation, which produce significant nitrogenous gases to the atmosphere. In this study, nitric oxides (NO X) and nitrous oxide (N 2O) emissions from the croplands cultivated with three typical vegetables had been measured in Yangtze River Delta of China from September 2 to December 16, 2006. The NO fluxes varied in the ranges of 1.6–182.4, 1.4–2901 and 0.5–487 ng Nm −2 s −1 with averages of 33.8 ± 44.2, 360 ± 590 and 76 ± 112 (mean ± SD) ngNm −2 s −1 for cabbage, garlic, and radish fields ( n = 88), respectively. N 2O fluxes from the three vegetable fields were found to occur in pulses and significantly promoted by tillage with average values of 5.8, 8.8, and 4.3 ng Nm −2 h −1 for cabbage, garlic, and radish crops, respectively. Influence of vegetables canopy on the NO emission was investigated and quantified. It was found that on cloudy days the canopy can only shield NO emission from croplands soil while on sunny days it cannot only prevent NO emission but also assimilate NO through the open leaves stomas. Multiple linear regression analysis indicated that soil temperature was the most important factor in controlling NO emission, followed by fertilizer amount and gravimetric soil water content. About 1.2%, 11.56% and 2.56% of applied fertilizers N were emitted as NO–N and N 2O–N from the cabbage, garlic and radish plots, respectively.

Soil Water Content and Soil Disaggregation by Disking Affects PM10 Emissions

Row crop agriculture in California's San Joaquin Valley is a major contributor of particulate matter <10 microm in aerodynamic diameter (PM10). The California Air Resources Board uses fixed PM10 emission values for various tillage operations to monitor and design attainment strategies. However, fixed emission values do not reflect emissions produced by a single implement operating under different soil conditions. This 2-yr study evaluated how PM10 mass concentrations (microg L(-1)) from disking change as a function of gravimetric soil water content (GWC), number of sequential diskings (D1, D2, D3), and the soil's weighted mean ped diameter (WMPD). Results showed PM10 increased logarithmically as the soil dried from a GWC of 14 to 4%. Average PM10 values at the lower GWCs were six to eight times greater than at the higher GWCs. Number of diskings also increased PM10, especially in drier soil. Below a GWC of 7%, PM10 for D3 was about twice that for D1. Despite strong correlations between more disking and lower WMPD, a lower WMPD did not always result in an increase in PM10. This underscored the role soil water plays in reducing PM10 at high GWCs despite low WMPDs from multiple diskings. Three-way interactions between GWC, disking, and PM10 showed, on average, that the magnitude of PM10 produced by D1 was 1.3 to 1.6 times lower than by D3, despite having insignificantly different GWC. Therefore, a disking operation can yield two different PM10 values under similar GWCs if the amount of soil disaggregation is different. Our results show that inclusion of soil parameters in PM10 emission estimates is essential to describing agriculture's role in air quality violations and to assess the value of proposed mitigation measures, such as conservation tillage.

Impact of Soil Water Content and Core Sampler Diameter at Sampling for Dry Soil Fragment‐Size Distributions

Soil conditions at sampling and the dimensions of the sample are critical factors when soil aggregation is indirectly characterized by determining the distribution of soil fragments. Our objective was to determine the effects of gravimetric soil water content and core sampler diameter (16, 54, and 84 mm) at sampling on the dry‐fragment‐size distribution of two soils (Typic Paleudalf and Typic Hapludalf) under undisturbed Festuca arundinacea L. sod and recently rototilled treatments. The 16‐mm core diameter sampler compressed the rototilled soil, and it was not appropriate for soil aggregation studies. The fragmentation of samples taken with core diameters greater than 54 mm decreased with decreasing soil water content. A greater probability of discriminating between undisturbed and fragmented silt loam or clay loam soils was observed when sampling with a 54‐mm‐diameter probe and when the soil had a mean soil water content of 237g kg−1 or at a potential of −0.61MPa.

High levels of microbial biomass and activity in unvegetated tropical and temperate alpine soils

Barren high-altitude soils are among the most extreme terrestrial environments on Earth. The present study was undertaken to quantify broad-scale patterns of total microbial biomass in unvegetated soils in the subnival zone of the Colorado Front Range and the high Andes of Perú. In order to better understand the limiting factors and substrates used by this community, we measured microbial biomass C, dissolved organic carbon (DOC), total organic carbon (TOC), soil gravimetric water content, and extracellular enzyme activity. To further investigate substrate limitation in these alpine soils, respiration after substrate addition was measured for samples from three sites in Colorado. In general, the abundance of microbes in these soils is positively correlated with soil water content. However, Perú talus soils had higher average microbial biomass than Colorado soils despite the Perú soils being higher in altitude and drier than the Colorado sites. Furthermore, the activity of the heterotrophic portion of the microbial community appears to be limited first by carbon and then by phosphorus as indicated both by results from extracellular enzyme assays and substrate addition experiments.

Assessment of Plant‐Available Potassium for No‐Till, Rainfed Soybean

Temporal and spatial availability of K can influence soybean [Glycine max (L.) Merr.] productivity. This study quantified the impact of initial soil K concentrations, soil‐water content, and soybean K uptake on soil K pools (water‐extractable solution‐phase K [Ksol], 1 mol L−1 NH4OAc‐extractable K, referred to as exchangeable K (Kexch), and 5‐min sodium tetraphenylboron [NaTPB] extractable K [KTPB]) and compared predictions of plant K availability using the NaTPB vs. NH4OAc tests. Soil and soybean samples were collected five times between the VE and R6 development stages in 2003 and 2004 from fallow and cropped no‐till Toronto–Millbrook silt loam Alfisols at the Throckmorton Purdue Agricultural Center. Gravimetric soil water content was measured weekly during the growing season. Low‐, medium‐, and high‐K fertility plots were replicated four times. Initial Kexch levels ranged from 60 to 290 and from 50 to 90 mg kg−1 at the 0‐ to 10‐ and 10‐ to 20‐cm depths, respectively. Medium‐ and high‐fertility soils had the highest grain yields. The 0‐ to 5‐cm soil layer had the highest Kexch levels, water availability, and soybean K uptake. Compared with Kexch, Ksol levels were less stratified and the surface layer was less dynamic. The NaTPB extraction was a better predictor of soybean K uptake in 1 yr, but across both years, NH4OAc was superior. Potassium measurements in the 0‐ to 10‐cm soil layer provided slightly better estimates of plant K uptake than those in the 0‐ to 20‐cm layer. Greater water availability and K uptake in the 0‐ to 5‐cm soil layer suggest that under evenly distributed, intermittent rainfall conditions in somewhat poorly drained soils, vertical soil K stratification might not be a concern for no‐till soybean production.

Comparing neural networks, linear and nonlinear regression techniques to model penetration resistance

Penetration resistance (PR) is an important property of soils, and can be expressed as cone index (CI). Because of high variability, there are no accurate and representative PR data in most cases. Variable PR is considerably affected by gravimetric soil water content (GWC) and bulk density (BD). In this study, artificial neural networks (ANNs) were used to simulate relationship between BD, GWC, and CI. A data set of 381 samples was collected from 2 study sites, Hamadan and Maragheh. Pedotransfer functions (PTFs) were developed using ANNs and linear and nonlinear regression models to predict CI for the combined data set and each data set separately. For the combined and Hamadan data sets, ANNs produced a greater correlation coefficient (R = 0.85) and lower root mean square error (RMSE) compared with the linear regression model (R = 0.70). For the Maragheh data set, however, the regression model yielded better results. Introducing TP and relative saturation (Qv/TP) into the models improved the prediction of CI. The results further showed that ANN models performed better than nonlinear regression models. Therefore, ANNs were recognized as powerful tools to predict CI by BD, GWC, TP, and Qv/TP as the independent variables under the very diverse conditions of the soils and treatments employed.

Correction of TDR-based soil water content measurements in conductive soils

Time Domain Reflectometry (TDR) is a widespread technique for measurement of soil water content (SWC). The main assumption behind the use of Time Domain Reflectometry (TDR) is of negligible losses, therefore assuming that only the real part determines the value of the TDR-measured apparent dielectric permittivity. This assumption does not hold for soils where surfaces are conductive (clay soils) or where high concentrations of electrolyte are present in the soil solution (saline soils) because under these conditions the contribution of the imaginary part becomes important. One of the main effects of dielectric losses on the TDR measurement is overestimation of SWC. In this study we present a methodology for separating the real and the imaginary part from the measurement of the apparent dielectric permittivity. This approach allows correction of the SWC overestimation, by using the TDR-measured electrical conductivity as indicator of dielectric losses. Oven-dry gravimetric soil water content was used as an independent method for soil water content assessment. The original SWC overestimation (in respect to the oven-dry gravimetric based measurement) reached values of up to 20% of total soil saturation, after the correction the differences were reduced to a 3–5%. The methodology can be applied based on knowledge of measured permittivity and electrical conductivity only, making it readily applicable to field experiments.

Root effects on soil water and hydraulic properties

Plants can affect soil moisture and the soil hydraulic properties both directly by root water uptake and indi- rectly by modifying the soil structure. Furthermore, water in plant roots is mostly neglected when studying soil hydraulic properties. In this contribution, we analyze effects of the moisture content inside roots as compared to bulk soil moisture contents and speculate on implications of non-capillary-bound root water for determination of soil moisture and calibration of soil hydraulic properties. In a fi eld crop of maize (Zea mays) of 75 cm row spacing, we sampled the total soil volumes of 0.7 m × 0.4 m and 0.3 m deep plots at the time of tasseling. For each of the 84 soil cubes of 10 cm edge length, root mass and length as well as moisture content and soil bulk density were determined. Roots were separated in 3 size classes for which a mean root porosity of 0.82 was obtained from the relation between root dry mass density and root bulk density using pycnometers. The spatially distributed fractions of root water contents were compared with those of the water in capillary pores of the soil matrix. Water inside roots was mostly below 2-5% of total soil water content; however, locally near the plant rows it was up to 20%. The results suggest that soil moisture in roots should be separately considered. Upon drying, the relation between the soil and root water may change towards water remaining in roots. Relations depend especially on soil water retention properties, growth stages, and root distributions. Gravimetric soil water content measurement could be misleading and TDR probes providing an integrated signal are difficult to interpret. Root effects should be more intensively studied for improved field soil water balance calculations.

Tree proximity, soil pathways and common mycorrhizal networks: their influence on the utilization of redistributed water by understory seedlings

Hydraulic redistribution (HR) is a process by which water moves through plant roots from moist to dry soils. An experiment was conducted to quantify the influence of common mycorrhizal networks (CMNs) and proximity to mature HR-source trees on the water relations of surrounding seedlings. Douglas-fir (Pseudotsuga menziesii var glauca (Mirb.) Franco) seedlings were planted at four distances (0.5, 1, 2.5, and 5 m) from six mature Douglas-fir trees, either directly into soil (soil plus CMN pathway) or inside 0.5 microm mesh bags (soil-only pathway). Deuterated water was used to irrigate soil beside mature trees in order to identify different HR water pathways to surrounding seedlings. This was followed by measurements of seedling deuterium enrichment, seedling water potential, soil water potential, gravimetric soil water content, and tree root density surrounding the seedlings. There was no significantly detectable difference in the quantity of HR water transferred to seedlings having access to soil and CMN pathways or soil-only pathways of water movement. Water from the irrigation plot contributed up to 1.4% of the water of Douglas-fir seedlings. Based on the assumption that the only pathway through which seedlings could access irrigation water was through the mature trees, we estimate that as much as 21.6% of the seedling water was supplied by the nearby tree. Seedling water potential was not significantly affected either by proximity to mature trees or pathway, suggesting HR may have compensated for increasing tree competitive effects with proximity. It is also possible that the lack of difference was due to a relatively moist summer. Our results suggest that residual mature trees are potentially important for hydraulic redistribution to regenerating seedlings in harvested dry interior Douglas-fir forests.

Quantification of hillslope runoff and erosion processes before and after wildfire in a wet Eucalyptus forest

Summary An experimental program was initiated following an intense wildfire in 2003 in NE Victoria Australia, to quantify an expected large increase in hillslope erosion by rill and interrill processes from a steep, wet, Eucalyptus forest. Water repellence, soil water content, infiltration capacity, infiltration-excess runoff generation, interrill erodibility, and rill erodibility were measured periodically for 3 years. Rill erodibility on 27° slopes increased immediately following the fire by a factor of 540 times, but decreased exponentially to very low background levels 2 years after the fire. Despite this increased erodibility, rill erosion was uncommon in the catchment, because soil saturated hydraulic conductivity (Ksat) remained very high (100–1000 mm h−1) and simulated concentrated flows of 3 l s−1 were found to be “transient”, infiltrating into areas with high Ksat a short distance downslope. Despite the very high Ksat values, runoff ratios from 100 mm h−1 simulated rainfall as high as 65% were recorded 6 months after the fire due to very high spatial variability in Ksat. The runoff sediment concentration under 100 mm h−1 simulated rainfall increased 10-fold immediately following the fire and declined exponentially to pre-fire levels within 2 years as ground cover recovered to 90–95%. Considered together, the above results suggest that most infiltration-excess overland flow reaching streams in this catchment is produced from within several metres of the stream edge, and that the transported sediment is generated by interrill processes. This conclusion contrasts strongly with common perceptions/observations of widespread overland flow dominated erosion processes following fire, and perhaps indicates substantial functional differences in erosion processes between different Eucalyptus forests. Measurements in unburnt areas, intended for comparative purposes only, provided unexpected insights in their own right. These areas showed a large natural seasonal oscillation in water repellence (non-repellent in winter to strongly repellent in summer) and runoff generation from rainfall simulation (1% in winter to 42% in summer) as the soil gravimetric water content crossed a hysteresis-variable threshold value. These large seasonal fluctuations in runoff generation indicate that proportional increases in erosion over the unburnt condition are highly temporally variable; 18-fold immediately after the fire, increasing to 2240-fold at six months, and then reducing to only 2.5-fold by 12 months after the fire, perhaps explaining some of the diversity of reported results in the literature. The above results suggest erosion risk is greatest in the first winter following the fire. Both the runoff and erosion results show that conventional erosion modelling approaches in this catchment will fail (both in burnt and unburnt forests) because the dominant driving processes and soil properties are not represented.

Time series forecast and soil characteristics-based simple and multivariate linear models for management of Diaprepes abbreviatus root weevil in citrus

Synthesis of insect and soil variable patterns in space and time via process models would be useful for reducing the cost of field monitoring and for achieving improved integrated pest management. During 2001–2003, we monitored the Diaprepes abbreviatus (L.) root weevil population in a flatwoods citrus grove containing a sandy Alfisol and a loamy Mollisol in Hendry County, South Florida. Our objectives were to examine the multi-year correlations of the Diaprepes root weevil with soil characteristics, and to develop a time series model and soil variable-based simple and multivariate linear models for predicting weevil patterns in space and time. Adult weevils were monitored weekly using 100 Tedders traps arranged in a 30×12 m grid. Gravimetric soil water content (SWC), time-domain-reflectory (TDR) volumetric SWC, soil organic matter content (SOM), clay, sand, silt, pH, and Mehlich-I extractable P, K, Ca, Mg, Fe and Mn concentrations were determined for each trap location. Adult weevil density was 0.023±0.018 weevils m −2 across the 3 years, and the weevil population grew exponentially ( R 2=0.81). Each year, the weevil was positively related to SWC, TDR, clay, SOM, and soil Mg, Ca and K concentrations (0.32< r<0.65, P<0.05). High weevil density, SWC, SOM, and P, K, Mg and Ca levels matched the Mollisol boundary. The autocorrelation ranges for the weevil and soil variables were within the soil type unit. Time series moving average forecast of Diaprepes weevil development was related to the 3-year mean weevil density monitored in the field ( R 2=0.88). The SWC, TDR and SOM-based simple and multivariate linear models explained 45% of the variance in the weevil patterns ( P<0.001). Time series and soil characteristics-based simple and multivariate linear models suggest a variable rate and less frequent spray for future management of the weevil in citrus production systems.