Soil Water Recharge Research Articles

Yield, water use, and protein content of spring wheat grown after six years of alfalfa, crested wheatgrass, or spring wheat in semiarid southwestern Saskatchewan

In the semiarid prairie of western Canada, there is renewed interest for including short durations (≤3 yr) of perennial forage in rotations with annual crops. However, there are producers who want to grow longer durations (≥4 yr) of perennial forages in rotational systems. Therefore, we assessed spring wheat (Triticum aestivum L.) yield, grain protein, and water use efficiency following 6 yr of either crested wheatgrass [Agropyron cristatum (L.) Gaertn.], or alfalfa (Medicago sativa L.), or wheat, and then 1 yr of fallow. Yield, water use, and water use efficiency were significantly lower in the first year of spring wheat production (2000) when the prior crop was crested wheatgrass or alfalfa than when it was wheat. In the second year (2001), which was a near record drought year, wheat yield and water use were significantly lower when the prior crop was alfalfa than when it was grass or wheat. From 2002 to 2005, there were no consistent differences in water use, water use efficiency, or yield of wheat due to the prior perennial crop. Wheat grain protein concentration was significantly higher following alfalfa compared with following crested wheatgrass or continuous spring wheat from 2000 to 2005. This effect was attributed to the higher N-supplying power of the soil following alfalfa. Soil water content below the rooting depth of most annual crops (≥120 cm depth) was reduced by the prior alfalfa crop, and there was no evidence from 2000 to 2005 that soil water recharge was occurring below the 150-cm depth. Key words: Semiarid prairie, alfalfa, grass, spring wheat, yield, protein, water use

Combined effect of technical, meteorological and agronomical factors on solid-set sprinkler irrigation: I. Irrigation performance and soil water recharge in alfalfa and maize

In this work, maize (Zea mays L.) and alfalfa (Medicago sativa L.) were irrigated in two adjoining plots with the same sprinkler solid-set system. Irrigation was evaluated between four sprinklers in the central position within each plot, above the canopy with pluviometers and in the soil with a FDR probe. Maize and alfalfa were simultaneously irrigated under the same operational and technical conditions during two seasons: in 2005, the solid-set irrigation system layout was rectangular, 15 m between sprinklers along the irrigation line and 15 m among lines (R15 × 15), and the seasonal irrigation applied according to the crop evapotranspiration (ETc); in 2006, the solid-set layout was R18 × 15 and the seasonal irrigation was around 30% lower than the ETc. The irrigation depth above the canopies (IDC) and the soil water recharge after irrigation (RW) were monitored using a 3 m × 3 m grid (25 points in 2005 and in 30 points in 2006). For maize, RW was assessed both in the lines of plants (CL) and between the lines (BCL). The average values of IDC were similar between crops during both seasons but the uniformity (CUC) of the IDC noticeably depended on the crop: the differences were greater between crops than between sprinklers spacings (R15 × 15 and R18 × 15). The CUC of IDC, the RW and the CUC of RW were greater for alfalfa than for maize. The CUC of IDC was greater than the CUC of RW for both crops. The RW was significantly related with the IDC throughout the irrigation season for alfalfa. The correlation was weaker for maize, with important differences between positions and between growth stages. At the beginning of the season, the RW significantly correlated with the IDC, both in the CL and BCL positions. However, the correlation weakened when the maize grew, especially in the CL, because the maize plants redistributed the water. The results show that the height and canopy architecture of the crop must be considered in the analysis of the sprinkler water distribution as factors influencing the irrigation performance.

Effect of rainfall interannual variability on the biomass and soil water distribution in a semiarid shrub community

The dynamics of biomass and soil moisture in semiarid land is driven by both the current rainfall and the ecosystem memory. Based on a meta-analysis of existing experiments, an ecosystem model was used to calculate the effect of the rainfall interannual variability on the pattern of biomass and soil moisture in a shrub community. It was found that rainfall interannual variability enabled shrubs to be more competitive than grasses, and to maintain the dominant role over a longer time. The rainfall interannual variability resulted in complex soil moisture dynamics. The soil water recharge in wet years alternated with discharge in drought years.

Soil water recharge in a semi-arid temperate climate of the Central U.S. Great Plains

Abstract The amount of soil water at the beginning of the growing season has a large impact on crop yields in rainfed agriculture, especially in semi-arid regions and in years with below-average rainfall in more humid climates. Robust algorithms are needed to estimate soil water storage before planting to aid crop management decisions. The main objectives of this paper are to investigate soil water recharge during the non-growing season (October 20 to May 1) in a semi-arid, temperate ecosystem in south-central Nebraska (USA) and to evaluate empirical models to estimate soil water content at the beginning of the summer-crop growing season. A database of soil water content measurements collected over 5 years at nine locations in south-central Nebraska was used to estimate available water-holding limits in the soil profile and to determine the change in available soil water during the non-growing season. Regression analysis was performed to analyze the relationship among soil water recharge, residual soil water (i.e., soil water content at the end of the previous growing season), total precipitation, and available water-holding capacity (AWHC) in the root zone to 1.5 m. Precipitation storage efficiency (PSE) was calculated as the quotient of soil water recharge and total non-growing season precipitation. Predictive models to estimate soil water content at the beginning of summer-crop growing season were derived from these analyses. A large portion of the variation in soil water recharge was explained by residual soil water and precipitation. PSE averaged 28% across site-years; low PSE values were associated with high residual soil water and/or low AWHC. Two predictive models (linear and linear-plateau) that used residual soil water, total precipitation, and AWHC as independent variables explained 75–80% of the variation in the measured soil water content at the beginning of the summer-crop growing season. These empirical models represent a new tool to estimate soil water content by planting date of summer crops. Site-management conditions such as residue amount and its architecture, tillage system, soil texture, and terrain slope are not currently accounted for in these models and would likely improve predictive capacity.

Soil water dynamics and deep soil recharge in a record wet year in the southern Loess Plateau of China

Terrestrial water cycles are influenced by hydrologic and textural properties of the deep loess layer in the Loess Plateau . Analyses of soil water profile distributions are needed to understand the regional water cycle processes and to guide agricultural production and sustainability . The objective of this study was to quantify the extent of deep soil water recharge and soil water profile dynamics during 1987–2003, especially in a record wet year of 2003, in common cropping systems in a semiarid-subhumid region of the southern Loess Plateau. The Chinese Ecological Research Network (CERN) site and a long-term rotation experiment site in a flat tableland were selected for this study. Soil moisture profiles were measured by a neutron probe to a depth of 6 m in 2003. The precipitation of 954 mm at the Changwu County Meteorological Station in 2003 was 63.4% higher than the long-term average (584 mm), and was a record high since 1957. Although cropping systems affected deep soil water recharge, the persistent dry soil layer formed between 2- and 3-m depths in croplands, resulting from many years of intensive cropping, was fully replenished in all cropping systems in 2003. Further frequency analysis indicated that the desiccated layer between 2- and 3-m depths would be fully recharged at least once in about 10 years for all existing cropping systems excluding continuous alfalfa. This finding should alleviate concerns about the formation of a permanent deep-soil desiccation layer as well as its potential impact on the long-term sustainability of the existing intensive cropping systems in the region.

Radial growth response of white oak to climate in eastern North America

Predicting forest responses to climate change requires an understanding of the cause–effect relationships linking climate to tree growth. Dendroecological analyses across sites that span climate gradients provide one means of characterizing such relationships. Dendroecological analyses for white oak ( Quercus alba L.) at 149 sites spanning the species range in eastern North America identified spatially replicated growth–climate associations. Early growing season site water balance variables for the year of annual ring formation had the strongest, most spatially replicated associations with growth. There was little evidence of phenological variation of these associations related to the latitudinal temperature gradient. Most spatial variation in growth–climate associations was along an east-to-west precipitation gradient. Radial growth was most strongly correlated with site water balance at sites in the northwest quadrant of the range, characterized by continental climate and high interannual variability in precipitation. There was little evidence that dormant season temperature affects white oak growth, even at the northern range limit. Correlations with dormant season precipitation were common in the northwest part of the range, where winter soil water recharge may be more variable. Spatial replication was a useful criterion for distinguishing growth–climate correlations that reflect cause–effect relationships.

Exotic plants increase and native plants decrease with loss of foundation species in sagebrush steppe

Dominant plant species, or foundation species, are recognized to have a disproportionate control over resources in ecosystems, but few studies have evaluated their relationship to exotic invasions. Loss of foundation species could increase resource availability to the benefit of exotic plants, and could thereby facilitate invasion. The success of exotic plant invasions in sagebrush steppe was hypothesized to benefit from increased available soil water following removal of sagebrush (Artemisia tridentata), a foun- dation species. We examined the effects of sagebrush removal, with and without the extra soil water made available by exclusion of sagebrush, on abundance of exotic and native plants in the shrub steppe of southern Idaho, USA. We compared plant responses in three treatments: undisturbed sagebrush steppe; sagebrush removed; and sagebrush removed plus plots covered with ''rainout'' shelters that blocked winter-spring recharge of soil water. The third treatment allowed us to examine effects of sagebrush removal alone, without the associated increase in deep-soil water that is expected to accompany removal of sagebrush. Overall, exotic herbs (the grass Bromus tectorum and four forbs) were 3-4 times more abundant in shrub- removal and 2 times more abundant in shrub- removal ? rainout-shelter treatments than in the control treatment, where sagebrush was undisturbed. Conversely, native forbs were only about half as abundant in shrub removal compared to control plots. These results indicate that removal of sagebrush facilitates invasion of exotic plants, and that increased soil water is one of the causes. Our findings suggest that sagebrush plays an important role in reducing invasions by exotic plants and maintaining native plant communities, in the cold desert we evaluated.

Temporal changes of an alfalfa succession and related soil physical properties on the Loess Plateau, China

The objective of this work was to investigate the relationship between changes in the plant community and changes in soil physical properties and water availability, during a succession from alfalfa (Medicago sativa L.) to natural vegetation on the Loess Plateau, China. Data from a succession sere spanning 32 years were collated, and vegetative indexes were compared to changes related to soil bulk density and soil water storage. The alfalfa yield increased for approximately 7 years, then it declined and the alfalfa was replaced by a natural community dominated by Stipa bungeana that began to thrive about 10 years after alfalfa seeding. Soil bulk density increased over time, but the deterioration of the alfalfa was mainly ascribed to a severe reduction in soil water storage, which was lowest around the time when degradation commenced. The results indicated that water consumption by alfalfa could be reduced by reducing plant density. The analysis of the data also suggested that soil water recharge could be facilitated by rotating the alfalfa with other crops, natural vegetation, or bare soil.

SPATIAL DECOUPLING OF FACILITATION AND COMPETITION AT THE ORIGIN OF GAPPED VEGETATION PATTERNS

Spatially periodic vegetation patterns, forming gaps, bands, labyrinths, or spots, are characteristic of arid and semiarid landscapes. Self-organization models can explain this variety of structures within a unified conceptual framework. All these models are based on the interplay of positive and negative effects of plants on soil water, but they can be divided according to whether they assume the interactions to be mediated by water redistribution through runoff/diffusion or by plants' organs. We carried out a multi-proxy approach of the processes operating in a gapped pattern in southwest Niger dominated by a shrub species. Soil moisture within the root layer was monitored in time and space over one month of the rainy season. Soil water recharge displayed no spatial variation with respect to vegetation cover, but the stock half-life under cover was twice that of bare areas. A kernel of facilitation by the aboveground parts of shrubs was parameterized, and soil water half-life was significantly correlated to the cumulated facilitative effects of shrubs. The kernel range was found to be smaller than the canopy radius (81%). This effect of plants on soil water dynamics, probably through a reduction of evaporation by shading, is shown to be a better explanatory variable than potentially relevant soil and topography parameters. The root systems of five individuals of Combretum micranthum G. Don were excavated. Root density data were used as a proxy to parameterize a kernel function of interplant competition. The range of this kernel was larger than the canopy radius (125%). The facilitation-to-competition range ratio, reflecting the above-to-belowground ratio of plant lateral extent, was smaller than 1 (0.64), a result supporting models assuming that patterning may emerge from an adaptation of plant morphology to aridity and shallow soils by means of an extended lateral root system. Moreover, observed soil water gradients had directions opposite to those assumed by alternative mathematical models based on underground water diffusion. This study contributes to the growing awareness that combined facilitative and competitive plant interactions can induce landscape-scale patterns and shape the two-way feedback loops between environment and vegetation.

Soil Water Depletion and Recharge under Ten Crop Species and Applications to the Principles of Dynamic Cropping Systems

Dynamic cropping systems principles require that farmers consider climatic, market, and ecological factors on an annual basis in making crop choices. Our objectives were to determine variability of seasonal soil water depletion (SWD) and spring soil water recharge (SWR) among crops and to apply results to dynamic cropping systems practice. A 10‐species crop sequence project was conducted under no‐tillage on silt loam Haplustoll soils in North Dakota. Mid‐May to mid‐September SWD and following April SWR were determined from 2002 to 2005 by neutron moisture meter to the 1.8‐m depth. Crops studied and average SWD amounts (cm) were: sunflower (Helianthus annuus L.), 13.5; corn (Zea mays L.), 12.6; sorghum [Sorghum bicolor (L.) Moench], 11.0; spring wheat (Triticum aestivum L.), 10.6; canola (Brassica napus L.), 10.0; millet (Panicum miliaceum L.), 9.6; buckwheat (Fagopyrum esculentum Moench), 9.4; chickpea (Cicer arietinum L.), 8.5; lentil (Lens culinaris Medik), 8.1; and dry pea (Pisum sativum L.), 5.0, with highest and lowest being 29 and 11% of average May soil water, 46 cm. Because the period of the experiment was relatively dry, recharge was less than depletion. Spring soil water was 10 cm greater following pea than following sunflower. Ranking of crops for water storage roughly followed reverse SWD rank, with several exceptions, notably wheat, which had greater water from snow capture. Lower soil water following crops such as sunflower and corn was linked to negative crop sequential effects in this project. Choosing to seed a lower water‐using crop in the spring after the occurrence of below‐average SWR on land that had a higher water‐using crop the previous season illustrates an application of information reported here along with the principles of dynamic cropping systems.

Soil Water Depletion and Recharge under Ten Crop Species and Applications to the Principles of Dynamic Cropping Systems

Soil Water Depletion and Recharge under Ten Crop Species and Applications to the Principles of Dynamic Cropping Systems

The effect of water availability on stand-level productivity, transpiration, water use efficiency and radiation use efficiency of field-grown willow clones

The effect of water availability on stand-level productivity, transpiration, water use efficiency (WUE) and radiation use efficiency (RUE) is evaluated for different willow clones at stand level. The measurements were made during the growing season 2000 in a 3-year-old plantation in Scania, southernmost Sweden. Six willow clones were included in the study: L78183, SW Rapp, SW Jorunn, SW Jorr, SW Tora and SW Loden. All clones were exposed to two water treatments: rain-fed, non-irrigated treatment and reduced water availability by reduced soil water recharge. Field measurements of stem sap-flow and biometry are up-scaled to stand transpiration and stand dry substance production and used to assess WUE. RUE is estimated from the ratio between the stand dry substance production and the accumulated absorbed photosynthetic active radiation over the growing season. The total stand transpiration rate for the 5 months lies between 100 and 325mm, which is fairly low compared to the Penman-Monteith transpiration for willow, reaching 400-450 mm for the same period. Mean WUE of all clones and treatments is 5.3 g/kg, which is high compared to earlier studies, while average RUE is 0.31 g/mol, which is slightly low compared to other results. Generally, all clones, except for Jorunn, seem to be better off concerning biomass production, WUE and RUE than the reference clone. Jorr, Jorunn and Loden also seem to be able to cope with the reduced water availability with increase in the water use efficiency. Tora performs significantly better than the other clones concerning both growth and efficiency in light and water use, but the effect of the dry treatment on stem growth shows sensitivity to water availability. The reduced stem growth could be due to a change in allocation patterns. (C) 2007 Elsevier Ltd. All rights reserved. (Less)

Impact of changing soil moisture distribution on net ecosystem productivity of a boreal aspen forest during and following drought

The interannual and seasonal variability of gross ecosystem photosynthesis (P), ecosystem respiration (R) and evapotranspiration (E), and their relationships to environmental factors were used to explain changes in net ecosystem productivity (F NEP) at the onset of, during, and following a 3-year-long (2001–2003) drought in a mature boreal aspen stand in central Saskatchewan, Canada. The forest was a moderate carbon (C) sink over its entire 11-year data record (F NEP = 153 � 99 g C m � 2 year � 1 ), including the peak drought years of 2002 and 2003. In 2001, the depletion of water near the soil surface likely reduced heterotrophic soil respiration while water remaining deep in the root zone maintained P above the pre-drought mean, resulting in above-average F NEP. In 2002 and 2003, the forest remained a C sink even though P was below average because R was also below average—a likely consequence of the influence of low soil water content in deep and shallow soil layers on both autotrophic and heterotrophic respiration. In 2004, the recharge of soil water in shallow soil layers allowed R to recover to its pre-drought values, whereas low spring temperatures, the slow recharge of soil water in deep soil layers in spring, late leaf emergence and diminished leaf area index combined to suppress P and produce the lowest annual F NEP of the 11-year record (4 g C m � 2 year � 1 ). The low F NEP and P were mirrored in the lowest stem growth and LAI values of the 11-year record. In 2005, a warm wet year, both the annual values and seasonal variations of F NEP, P and R returned to those of pre-drought years; the partial recovery of LAI to pre-drought values suggests that aspen P was able to adjust to this restriction on C assimilation. Growing season average dry surface conductance (gsv), the Priestley–Taylor coefficient (a) and light use efficiency (LUE) reached their lowest values in 2003 and became similar to predrought years in 2004–2005. Water use efficiency (WUE) was highest in 2003 and remained above average in 2004 and 2005. At the ecosystem scale, the above-average gains made in C sequestration in the first year of the drought were significantly offset by belowaverage stand F NEP in the final 2 years of the drought, and in the year following the drought. # 2006 Elsevier B.V. All rights reserved.

Analysing the space–time distribution of soil water storage of a forest ecosystem using spatio-temporal kriging

In forest the soil water balance is strongly influenced by tree species composition. For example, differences in transpiration rate lead to differences in soil water storage (SWS) and differences in canopy interception cause differences in infiltration. To analyse the influence of tree species composition on SWS at the scale of a forest stand, we compare spatio-temporal patterns in vegetation and SWS. Geostatistical space–time models provide a probabilistic framework for mapping SWS from point observations. The accuracy of these models may be improved by incorporating knowledge about the process of evapotranspiration. In this paper we combine a physical-deterministic evapotranspiration model with space–time geostatistical interpolation to predict soil water storage in the upper 30 cm of soil (SWS 30) for a 0.5 ha plot in a mixed stand of Norway spruce ( Picea abies (L.) Karst.) and European beech ( Fagus sylvatica L.) in Kreisbach, Lower Austria. Soil water storage was measured at 198 locations by permanently installed wave guides. This was repeated 28 times, about every two weeks during the growing seasons of 2000 and 2001. Incorporation of a process-based model in space–time prediction of SWS 30 reduced the effect of precipitation on SWS 30 predictions prior to precipitation. Spatial patterns of SWS 30 between the permanent wilting point and field capacity depend on the precipitation and drying history, which is affected by vegetation. Early in the growing season spruce starts to transpire markedly, which is common for coniferous trees. During dry periods, spruce reduces transpiration earlier than beech. Overall beech transpires more than spruce during the growing season. The greater transpiration rates of beech are compensated for by greater soil water recharge after precipitation because less rainfall is intercepted. At low water contents near the permanent wilting point SWS 30 was spatially quite uniform. This was also the case at water contents nearfield capacity, probably because the soil physical parameters varied little. Space–time interpolation of SWS 30 and the prediction of soil water discharge and soil water recharge during periods of drying and rewetting demonstrate the important role of vegetation on the spatial patterns of SWS 30.

HYDRAULIC REDISTRIBUTION THROUGH THE ROOT SYSTEMS OF SENESCED PLANTS

Hydraulic redistribution, the movement of water from soil layers of higher water potential to layers of lower water potential through the root systems of plants, has been documented in many taxa worldwide. Hydraulic redistribution is influenced principally by physical properties of roots and soils, and it should occur whenever root systems span soil layers of different water potential. Therefore, hydraulic redistribution should occur through the root systems of plants with aboveground tissue removed or through the root systems of fully senesced plants as long as roots remain intact and hydrated. We examined our hypothesis in field and greenhouse studies with the annual grass Bromus tectorum. We used soil psychrometry to measure soil water potential and performed 2H-labeling exper- iments. In the field, following senescence of B. tectorum, we show substantial changes in soil water potential consistent with both upward and downward movement of water through roots. The amount of water redistributed represents a significant proportion of that which can be stored in the rooted zone. We also experimentally demonstrated upward movement of a 2H label by roots of senesced plants and by roots of plants without aboveground tissues. In the greenhouse, we further demonstrated redistribution by senesced individuals using a 2H label. Hydraulic redistribution through the roots of senesced plants should receive further attention because it may have important ecological consequences for soil water recharge, survival of plants through drought, and agricultural practices.

Soil water dynamics differ among rangeland plant communities dominated by yellow starthistle (Centaurea solstitialis), annual grasses, or perennial grasses

California's interior grasslands have undergone dramatic changes during the last two centuries. Changes in land-use patterns and plant introductions after European contact and settlement resulted in the conversion of perennial-dominated grasslands to exotic annual grasses. More recently, the annual grasslands have been heavily invaded by the deeply rooted late-maturing forb yellow starthistle. This series of invasions and conversions has changed the community structure and phenology of the grasslands. We hypothesized that these changes have resulted in significant differences in soil water–use patterns in the grasslands. We studied soil water depletion and recharge patterns of three grassland community types dominated by perennial grasses, annual grasses, or yellow starthistle with contrasting phenology and rooting depths for 4 yr. Soil moisture measurements were taken every month from March to December in 1998, 1999, and 2000 and every other month in 2001. Measurements were taken with a neutron probe at depths of 30 to 150 cm at 30-cm intervals. The results indicate that the yellow starthistle community maintained a significantly drier soil profile than the annual grass community. The perennial grass community maintained an intermediate soil water content that was not significantly different from either of the other two communities. Significant time by community and depth by community interactions indicated that the yellow starthistle community continued depleting soil moisture later into the season and at deeper depths than the other grass communities. This study demonstrates the effect of plant invasion on soil water recharge and depletion patterns in California grasslands.

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.

Temporal and spatial patterns of soil water following wildfire-induced changes in plant communities in the Great Basin in Nevada, USA

A widespread conversion of sagebrush-dominated plant communities to post-fire communities dominated by herbaceous annual species has occurred in the Great Basin of North America's Intermountain West during the last century, mainly driven by invasions of alien species and a subsequent increase in fire frequency. Little experimental data, however, are available on the potential consequences of wildfire and post-fire plant community alteration on key ecosystem functions such as ecosystem hydrology. The objectives of our study were to (1) quantify the effects of this vegetation transformation on the spatial and temporal distribution of soil water in the rooting zone; (2) quantify the effects of wild-fire on surface soil water distribution (upper 20 cm) to infer about potential consequences on re-establishment of the native plant community; and (3) examine which processes (e.g., soil water recharge or replenishment, plant water uptake, and soil evaporation) may be responsible for observed changes soil water distribution. Continuous measurement of soil water content using a segmented time domain reflectrometry system in sixteen locations in an intact sagebrush ecosystem (eight shrub and eight inter-shrub locations) and in eight locations in an adjacent post-fire ecosystem from March 2001 to October 2002 indicated that soil water contents in the upper 75 cm of the soil were similar and very low during summers, but that substantially lower water recharge in post-fire ecosystems after large snowfalls – possibly due to decreased snow deposition and higher sublimation – caused large differences in water storage between the two ecosystems in winter. These differences declined with the onset of the vegetation period in March, probably due to higher plant water uptake in the more densely vegetated intact sagebrush ecosystem. The presence of patchiness of near-surface soil water (0–20 cm), quantified among 80 points of nested grid systems placed in each ecosystem, appears to benefit native perennial shrub re-generation in the intact ecosystem. Taken together, the results of our study indicate that vegetation transition from shrublands to post-fire successional plant communities may (1) decrease water recharge in winters leading to lower water storage in winter and spring and potentially reduce groundwater recharge; (2) decrease the amount of plant-available soil water in the root zone during phenologically important times of the years; and (3) reduce the lateral patchiness in surface soil water, which may impede the re-establishment of sagebrush or other patch-dependent native perennials.

Influence of roots and climate on mineral and trace element storage and flux in tropical mangrove soils

The storage and flux of various mineral and trace elements in soils (0-30 cm depth) were examined in relation to monsoonal rains and fine root biomass in four mangrove forests of different age and type in southern Thailand. The onset of the wet SW monsoon resulted in the percolation and dilution of porewater solutes by rainwater and by less saline tidal water, as indicated by shifts in Eh, pH and porewater SO4/Cl ratios. This is contrary to temperate intertidal environments where seasonal patterns of porewater constituents, and biological and biogeochemical activities, are strongly cued to temperature. Fluxes across the soil-water interface were most often not statistically significant. Concentration of dissolved porewater metals were dominated by Fe, Mn, Al, Mo and Zn. The decreasing order of solid- phase element inventories in these soils, on average, was: Al, S, Fe, Na, Mg, K, Ca, N, P, Mn, V, Zn, Cr, Ni, As, Co, Cu, Pb, Mo, Cd and Hg. There were no gradients in concentrations of dissolved or solid- phase elements with increasing soil depth. This phenomenon was attributed to physical and biological processes, including the presence and activities of roots and tidal recharge of soil water. Fine dead roots were storage sites for most mineral and trace elements, as some elements in roots composed a significant fraction (� 5%) of the total soil pool. Analysis of S and Fe concentration differences between live and dead roots suggested extensive formation of pyrite associated with dead roots; correlation analysis suggested that trace metals coprecipitated with pyrite. An analysis of inventories and release/uptake rates indicate turnover of the N, P, Na and Ca soil pools equivalent to other tropical forests; turnover was slow (decades to centuries) for S, Fe, K and trace elements. Our results indicate that mineral and trace element cycling in these soils are characterized by net storage, with net accumulation of most elements much greater than uptake and release by tree roots.

Modifying the 'pulse-reserve' paradigm for deserts of North America: precipitation pulses, soil water, and plant responses.

The 'pulse-reserve' conceptual model--arguably one of the most-cited paradigms in aridland ecology--depicts a simple, direct relationship between rainfall, which triggers pulses of plant growth, and reserves of carbon and energy. While the heuristics of 'pulses', 'triggers' and 'reserves' are intuitive and thus appealing, the value of the paradigm is limited, both as a conceptual model of how pulsed water inputs are translated into primary production and as a framework for developing quantitative models. To overcome these limitations, we propose a revision of the pulse-reserve model that emphasizes the following: (1) what explicitly constitutes a biologically significant 'rainfall pulse', (2) how do rainfall pulses translate into usable 'soil moisture pulses', and (3) how are soil moisture pulses differentially utilized by various plant functional types (FTs) in terms of growth? We explore these questions using the patch arid lands simulation (PALS) model for sites in the Mojave, Sonoran, and Chihuahuan deserts of North America. Our analyses indicate that rainfall variability is best understood in terms of sequences of rainfall events that produce biologically-significant 'pulses' of soil moisture recharge, as opposed to individual rain events. In the desert regions investigated, biologically significant pulses of soil moisture occur in either winter (October-March) or summer (July-September), as determined by the period of activity of the plant FTs. Nevertheless, it is difficult to make generalizations regarding specific growth responses to moisture pulses, because of the strong effects of and interactions between precipitation, antecedent soil moisture, and plant FT responses, all of which vary among deserts and seasons. Our results further suggest that, in most soil types and in most seasons, there is little separation of soil water with depth. Thus, coexistence of plant FTs in a single patch as examined in this PALS study is likely to be fostered by factors that promote: (1) separation of water use over time (seasonal differences in growth), (2) relative differences in the utilization of water in the upper soil layers, or (3) separation in the responses of plant FTs as a function of preceding conditions, i.e., the physiological and morphological readiness of the plant for water-uptake and growth. Finally, the high seasonal and annual variability in soil water recharge and plant growth, which result from the complex interactions that occur as a result of rainfall variability, antecedent soil moisture conditions, nutrient availability, and plant FT composition and cover, call into question the use of simplified vegetation models in forecasting potential impacts of climate change in the arid zones in North America.