Tree species and diversity effects on soil water seepage in a tropical plantation

Plant diversity has been shown to influence the water cycle of forest ecosystems by differences in water consumption and the associated effects on groundwater recharge. However, the effects of biodiversity on soil water fluxes remain poorly understood for native tree species plantations in the tropics. Therefore, we estimated soil water fluxes and assessed the effects of tree species and diversity on these fluxes in an experimental native tree species plantation in Sardinilla (Panama). The study was conducted during the wet season 2008 on plots of monocultures and mixtures of three or six tree species. Rainfall and soil water content were measured and evapotranspiration was estimated with the Penman-Monteith equation. Soil water fluxes were estimated using a simple soil water budget model considering water input, output, and soil water and groundwater storage changes and in addition, were simulated using the physically based one-dimensional water flow model Hydrus-1D. In general, the Hydrus simulation did not reflect the observed pressure heads, in that modeled pressure heads were higher compared to measured ones. On the other hand, the results of the water balance equation (WBE) reproduced observed water use patterns well. In monocultures, the downward fluxes through the 200 cm-depth plane were highest below Hura crepitans (6.13 mm day−1) and lowest below Luehea seemannii (5.18 mm day−1). The average seepage rate in monocultures (±SE) was 5.66 ± 0.18 mm day−1, and therefore, significantly higher than below six-species mixtures (5.49 ± 0.04 mm day−1) according to overyielding analyses. The three-species mixtures had an average seepage rate of 5.63 ± 0.12 mm day−1 and their values did not differ significantly from the average values of the corresponding species in monocultures. Seepage rates were driven by the transpiration of the varying biomass among the plots (r = 0.61, p = 0.017). Thus, a mixture of trees with different growth rates resulted in moderate seepage rates compared to monocultures of either fast growing or slow growing tree species. Our results demonstrate that tree-species specific biomass production and tree diversity are important controls of seepage rates in the Sardinilla plantation during the wet season.

Tracking water pathways in steep hillslopes by d18O depth profiles of soil water

Assessing temporal variations in soil water flow is important, especially at the hillslope scale, to identify mechanisms of runoff and flood generation and pathways for nutrients and pollutants in soils. While surface processes are well considered and parameterized, the assessment of subsurface processes at the hillslope scale is still challenging since measurement of hydrological pathways is connected to high efforts in time, money and personnel work. The latter might not even be possible in alpine environments with harsh winter processes. Soil water stable isotope profiles may offer a time-integrating fingerprint of subsurface water pathways. In this study, we investigated the suitability of soil water stable isotope (d18O) depth profiles to identify water flow paths along two transects of steep subalpine hillslopes in the Swiss Alps. We applied a one-dimensional advection–dispersion model using d18O values of precipitation (ranging from _24.7 to _2.9‰) as input data to simulate the d18O profiles of soil water. The variability of d18O values with depth within each soil profile and a comparison of the simulated and measured d18O profiles were used to infer information about subsurface hydrological pathways. The temporal pattern of d18O in precipitation was found in several profiles, ranging from _14.5 to _4.0‰. This suggests that vertical percolation plays an important role even at slope angles of up to 46_. Lateral subsurface flow and/or mixing of soil water at lower slope angles might occur in deeper soil layers and at sites near a small stream. The difference between several observed and simulated d18O profiles revealed spatially highly variable infiltration patterns during the snowmelt periods: The d18O value of snow (_17.7 ± 1.9‰) was absent in several measured d18O profiles but present in the respective simulated d18O profiles. This indicated overland flow and/or preferential flow through the soil profile during the melt period. The applied methods proved to be a fast and promising tool to obtain time-integrated information on soil water flow paths at the hillslope scale in steep subalpine slopes.

Geographic information system-based decision support for soil conservation planning in Tajikistan

Soil erosion on sloping agricultural land poses a serious problem for the environment, as well as for production. In areas with highly erodible soils, such as those in loess zones, application of soil and water conservation measures is crucial to sustain agricultural yields and to prevent or reduce land degradation. The present study, carried out in Faizabad, Tajikistan, was designed to evaluate the potential of local conservation measures on cropland using a spatial modelling approach to provide decision-making support for the planning of spatially explicit sustainable land use. A sampling design to support comparative analysis between well-conserved units and other field units was established in order to estimate factors that determine water erosion, according to the Revised Universal Soil Loss Equation (RUSLE). Such factor-based approaches allow ready application using a geographic information system (GIS) and facilitate straightforward scenario modelling in areas with limited data resources. The study showed first that assessment of erosion and conservation in an area with inhomogeneous vegetation cover requires the integration of plot-based cover. Plot-based vegetation cover can be effectively derived from high-resolution satellite imagery, providing a useful basis for plot-wise conservation planning. Furthermore, thorough field assessments showed that 25.7% of current total cropland is covered by conservation measures (terracing, agroforestry and perennial herbaceous fodder). Assessment of the effectiveness of these local measures, combined with the RUSLE calculations, revealed that current average soil loss could be reduced through low-cost measures such as contouring (by 11%), fodder plants (by 16%), and drainage ditches (by 53%). More expensive measures such as terracing and agroforestry can reduce erosion by as much as 63% (for agroforestry) and 93% (for agroforestry combined with terracing). Indeed, scenario runs for different levels of tolerable erosion rates showed that more cost-intensive and technologically advanced measures would lead to greater reduction of soil loss. However, given economic conditions in Tajikistan, it seems advisable to support the spread of low-cost and labourextensive measures.

Modelling Lake Kivu water level variations over the last seven decades

This study aimed at analysing the hydrological changes in the Lake Kivu Basin over the last seven decades with focus on the response of the lake water level to meteorological factors and hydropower dam construction. Historical precipitation and lake water levels were acquired from literature, local agencies and from global databases in order to compile a coherent dataset. The net lake inflow was modelled using a soil water balance model and the water levels were reconstructed using a parsimonious lake water balance model. The soil water balance shows that 370 mm yr−1 (25%) of the precipitation in the catchment contributes to the runoff and baseflow whereas 1100 mm yr−1 (75%) contributes to the evapotranspiration. A review of the lake water balance resulted in the following estimates of hydrological contributions: 55%, 25%, and 20% of the overall inputs from precipitation, surface inflows, and subaquatic groundwater discharge, respectively. The overall losses were 58% and 42% for lake surface evaporation and outflow discharge, respectively. The hydrological model used indicated a remarkable sensitivity of the lake water levels to hydrometeorological variability up to 1977, when the outflow bed was artificially widened.

Soil and water conservation: LforS guidelines for trainers

The guidelines discuss the following topics: - Towards a common understanding of Soil & Water Conservation - Disturbances in the water and biomass cycle lead to a decrease in soil fertility - Diagnosis of the local water and biomass cycle and their links - Assessment of S&W Conservation measures - Implementation of S&W Conservation measures

Direct and indirect associations between plant species richness and productivity in grasslands: regional differences preclude simple generalization of productivity-biodiversity relationships

Plant species richness of permanent grasslands has often been found to be significantly associated with productivity. Concentrations of nutrients in biomass can give further insight into these productivity- plant species richness relationships, e.g. by reflecting land use or soil characteristics. However, the consistency of such relationships across different regions has rarely been taken into account, which might significantly compromise our potential for generalization. We recorded plant species richness and measured above-ground biomass and concentrations of nutrients in biomass in 295 grasslands in three regions in Germany that differ in soil and climatic conditions. Structural equation modelling revealed that nutrient concentrations were mostly indirectly associated with plant species richness via biomass production. However, negative associations between the concentrations of different nutrients and biomass and plant species richness differed considerably among regions. While in two regions, more than 40% of the variation in plant species richness could be attributed to variation in biomass, K, P, and to some degree also N concentrations, in the third region only 15% of the variation could be explained in this way. Generally, highest plant species richness was recorded in grasslands where N and P were co-limiting plant growth, in contrast to N or K (co-) limitation. But again, this pattern was not recorded in the third region. While for two regions land-use intensity and especially the application of fertilizers are suggested to be the main drivers causing the observed negative associations with productivity, in the third region the little variance accounted for, low species richness and weak relationships implied that former intensive grassland management, ongoing mineralization of peat and fluctuating water levels in fen grasslands have overruled effects of current land-use intensity and productivity. Finally, we conclude that regional replication is of major importance for studies seeking general insights into productivity-diversity relationships.

Modeling tree water deficit from microclimate: an approach to quantifying drought stress

Tree water deficit estimated by measuring water-related changes in stem radius (DeltaW) was compared with tree water deficit estimated from the output of a simple, physiologically reasonable model (DeltaW(E)), with soil water potential (Psi(soil)) and atmospheric vapor pressure deficit (VPD) as inputs. Values of DeltaW were determined by monitoring stem radius changes with dendrometers and detrending the results for growth, We followed changes in DeltaW and DeltaW(E) in Pinus sylvestris L. and Quercus pubescens Willd. over 2 years at a dry site (2001-2002; Salgesch, Wallis) and in Picea abies (L.) Karst. for 1 year at a wet site (1998; Davos, Graubuenden) in the Swiss Alps. The seasonal courses of DeltaW in deciduous species and in conifers at the same site were similar and could be largely explained by variation in DeltaW(E). This finding strongly suggests that DeltaW, despite the known species-specific differences in stomatal response to microclimate, is mainly explained by a combination of atmospheric and soil conditions. Consequently, we concluded that trees are unable to maintain any particular DeltaW. Either Psi(soil) or VPD alone provided poorer estimates of AWthan a model incorporating both factors. As a first approximation of DeltaW(E), Psi(soil) can be weighted so that the negative mean Psi(soil) reaches 65 to 75% of the positive mean daytime VPD over a season (Q. pubescens: similar to65%, P abies: similar to70%, P sylvestris: similar to75%). The differences in DeltaW among species can be partially explained by a different weighting of Psi(soil) against VPD. The DeltaW of P. sylvestris was more dependent on Psi(soil) than that of Q. pubescens, but less than that of P. abies, and was less dependent on VPD than that of P. abies and Q. pubescens. The model worked well for P. abies at the wet site and for Q. pubescens and P. sylvestris at the dry site, and may be useful for estimating water deficit in other tree species.

Drought avoidance and the effect of local topography on trees in the understorey of Bornean lowland rain forest

The water relations of two tree species in the Euphorbiaceae were compared to test in part a hypothesis that the forest understorey plays an integral role in drought response. At Danum, Sabah, the relatively common species Dimorphocalyx muricatus is associated with ridges whilst another species, Mallotus wrayi, occurs widely both on ridges and lower slopes. Sets of subplots within two 4 -ha permanent plots in this lowland dipterocarp rain forest, were positioned on ridges and lower slopes. Soil water potentials were recorded in 1995-1997, and leaf water potentials were measured on six occasions. Soil water potentials on the ridges (-0.047 MPa) were significantly lower than on the lower slopes (-0.012 MPa), but during the driest period in May 1997 they fell to similarly low levels on both sites (-0.53 MPa). A weighted 40-day accumulated rainfall index was developed to model the soil water potentials. At dry times, D. muricatus (ridge) had significantly higher pre-dawn (-0.21 v. -0.57 MPa) and mid-day (-0.59 v. -1.77 MPa) leaf water potentials than M. wrayi (mean of ridge and lower slope). Leaf osmotic potentials of M. wrayi on the ridges were lower (-1.63 MPa) than on lower slopes (-1.09 MPa), with those for D. muricatus being intermediate (-1.29 MPa): both species adjusted osmotically between wet and dry times. D. muricatus trees were more deeply rooted than M. wrayi trees (97 v. 70 cm). M. wrayi trees had greater lateral root cross-sectional areas than D. muricatus trees although a greater proportion of this sectional area for D. muricatus was further down the soil profile. D. muricatus appeared to maintain relatively high water potentials during dry periods because of its access to deeper water supplies and thus it largely avoided drought effects, but M. wrayi seemed to be more affected yet tolerant of drought and was more plastic in its response. The interaction between water availability and topography determines these species' distributions and provides insights into how rain forests can withstand occasional strong droughts.

Plant diversity effects on the water balance of an experimental grassland

In the literature, contrasting effects of plant species richness on the soil water balance are reported. Our objective was to assess the effects of plant species and functional richness and functional identity on soil water contents and water fluxes in the experimental grassland of the Jena Experiment. The Jena Experiment comprises 86 plots on which plant species richness (0, 1, 2, 4, 8, 16, and 60) and functional group composition (zero to four functional groups: legumes, grasses, tall herbs, and small herbs) were manipulated in a factorial design. We recorded meteorological data and soil water contents of the 0·0–0·3 and 0·3–0·7 m soil layers and calculated actual evapotranspiration (ETa), downward flux (DF), and capillary rise with a soil water balance model for the period 2003–2007. Missing water contents were estimated with a Bayesian hierarchical model. Species richness decreased water contents in subsoil during wet soil conditions. Presence of tall herbs increased soil water contents in topsoil during dry conditions and decreased soil water contents in subsoil during wet conditions. Presence of grasses generally decreased water contents in topsoil, particularly during dry phases; increased ETa and decreased DF from topsoil; and decreased ETa from subsoil. Presence of legumes, in contrast, decreased ETa and increased DF from topsoil and increased ETa from subsoil. Species richness probably resulted in complementary water use. Specific functional groups likely affected the water balance via specific root traits (e.g. shallow dense roots of grasses and deep taproots of tall herbs) or specific shading intensity caused by functional group effects on vegetation cover. Copyright © 2013 John Wiley & Sons, Ltd.

Transport of iodide in structured clay–loam soil under maize during irrigation experiments analyzed using HYDRUS model

Transport of radioactive iodide 131I− in a structured clay loam soil under maize in a final growing phase was monitored during five consecutive irrigation experiments under ponding. Each time, 27 mm of water were applied. The water of the second experiment was spiked with 200 MBq of 131I− tracer. Its activity was monitored as functions of depth and time with Geiger-Müller (G-M) detectors in 11 vertically installed access tubes. The aim of the study was to widen our current knowledge of water and solute transport in unsaturated soil under different agriculturally cultivated settings. It was supposed that the change in 131I− activity (or counting rate) is proportional to the change in soil water content. Rapid increase followed by a gradual decrease in 131I− activity occurred at all depths and was attributed to preferential flow. The iodide transport through structured soil profile was simulated by the HYDRUS 1D model. The model predicted relatively deep percolation of iodide within a short time, in a good agreement with the observed vertical iodide distribution in soil. We found that the top 30 cm of the soil profile is the most vulnerable layer in terms of water and solute movement, which is the same depth where the root structure of maize can extend.

Induced Jasmonate Signaling Leads to Contrasting Effects on Root Damage and Herbivore Performance

Induced defenses play a key role in plant resistance against leaf feeders. However, very little is known about the signals that are involved in defending plants against root feeders and how they are influenced by abiotic factors. We investigated these aspects for the interaction between rice (Oryza sativa) and two root-feeding insects: the generalist cucumber beetle (Diabrotica balteata) and the more specialized rice water weevil (Lissorhoptrus oryzophilus). Rice plants responded to root attack by increasing the production of jasmonic acid (JA) and abscisic acid, whereas in contrast to in herbivore-attacked leaves, salicylic acid and ethylene levels remained unchanged. The JA response was decoupled from flooding and remained constant over different soil moisture levels. Exogenous application of methyl JA to the roots markedly decreased the performance of both root herbivores, whereas abscisic acid and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid did not have any effect. JA-deficient antisense 13-lipoxygenase (asLOX) and mutant allene oxide cyclase hebiba plants lost more root biomass under attack from both root herbivores. Surprisingly, herbivore weight gain was decreased markedly in asLOX but not hebiba mutant plants, despite the higher root biomass removal. This effect was correlated with a herbivore-induced reduction of sucrose pools in asLOX roots. Taken together, our experiments show that jasmonates are induced signals that protect rice roots from herbivores under varying abiotic conditions and that boosting jasmonate responses can strongly enhance rice resistance against root pests. Furthermore, we show that a rice 13-lipoxygenase regulates root primary metabolites and specifically improves root herbivore growth.

A proposed framework to operationalize ESS for the mitigation of soil threats

Despite various research activities in the last decades across the world, many challenges remain to integrate the concept of ecosystem services (ESS) in decision-making, and a coherent approach to assess and value ESS is still lacking. There are a lot of different – often context-specific – ESS frameworks with their own definitions and understanding of terms. Based on a thorough review, the EU FP7 project RECARE (www.recare-project.eu) suggests an adapted framework for ecosystem services related to soils that can be used for practical application in preventing and remediating degradation of soils in Europe. This lays the foundation for the development and selection of appropriate methods to measure, evaluate, communicate and negotiate the services we obtain from soils with stakeholders in order to improve land management. Similar to many ESS frameworks, the RECARE framework distinguishes between an ecosystem and human well-being part. As the RECARE project is focused on soil threats, this is the starting point on the ecosystem part of the framework. Soil threats affect natural capital, such as soil, water, vegetation, air and animals, and are in turn influenced by those. Within the natural capital, the RECARE framework focuses especially on soil and its properties, classified in inherent and manageable properties. The natural capital then enables and underpins soil processes, while at the same time being affected by those. Soil processes, finally, are the ecosystem’s capacity to provide services, thus they support the provision of soil functions and ESS. ESS may be utilized to produce benefits for individuals and human society. Those benefits are explicitly or implicitly valued by individuals and human society. The values placed on those benefits influence policy and decision-making and thus lead to a societal response. Individual (e.g. farmers’) and societal decision making and policy determine land management and other (human) driving forces, which in turn affect soil threats and natural capital. In order to improve ESS with Sustainable Land Management (SLM) – i.e. measures aimed to prevent or remediate soil threats, the services identified in the framework need to be “manageable” (modifiable) for the stakeholders. To this end, effects of soil threats and prevention / remediation measures are captured by key soil properties as well as through bio-physical (e.g. reduced soil loss), socio-economic (e.g. reduced workload) and socio-cultural (e.g. aesthetics) impact indicators. In order to use such indicators in RECARE, it should be possible to associate the changes in soil processes to impacts of prevention / remediation measures (SLM). This requires the indicators to be sensitive enough to small changes, but still sufficiently robust to provide evidence of the change and attribute it to SLM.

Alteration of natural ³⁷Ar activity concentration in the subsurface by gas transport and water infiltration

High ³⁷Ar activity concentration in soil gas is proposed as a key evidence for the detection of underground nuclear explosion by the Comprehensive Nuclear Test-Ban Treaty. However, such a detection is challenged by the natural background of ³⁷Ar in the subsurface, mainly due to Ca activation by cosmic rays. A better understanding and improved capability to predict ³⁷Ar activity concentration in the subsurface and its spatial and temporal variability is thus required. A numerical model integrating ³⁷Ar production and transport in the subsurface is developed, including variable soil water content and water infiltration at the surface. A parameterized equation for ³⁷Ar production in the first 15 m below the surface is studied, taking into account the major production reactions and the moderation effect of soil water content. Using sensitivity analysis and uncertainty quantification, a realistic and comprehensive probability distribution of natural ³⁷Ar activity concentrations in soil gas is proposed, including the effects of water infiltration. Site location and soil composition are identified as the parameters allowing for a most effective reduction of the possible range of ³⁷Ar activity concentrations. The influence of soil water content on ³⁷Ar production is shown to be negligible to first order, while ³⁷Ar activity concentration in soil gas and its temporal variability appear to be strongly influenced by transient water infiltration events. These results will be used as a basis for practical CTBTO concepts of operation during an OSI.