Seasonal variation of water uptake of a Quercus suber tree in Central Portugal

Hydraulic redistribution (HR) is the phenomenon where plant roots transfer water between soil horizons of different water potential. When dry soil is a stronger sink for water loss from the plant than transpiration, water absorbed by roots in wetter soil horizons is transferred toward, and exuded into dry soil via flow reversals through the roots. Reverse flow is a good marker of HR and can serve as a useful tool to study it over the long-term. Seasonal variation of water uptake of a Quercus suber tree was studied from late winter through autumn 2003 at Rio Frio near Lisbon, Portugal. Sap flow was measured in five small shallow roots (diameter of 3–4 cm), 1 to 2 m from the tree trunk and in four azimuths and at different xylem depths at the trunk base, using the heat field deformation method (HFD). The pattern of sap flow differed among lateral roots as soil dried with constant positive flow in three roots and reverse flow in two other roots during the night when transpiration ceased. Rain modified the pattern of flow in these two roots by eliminating reverse flow and substantially increasing water uptake for transpiration during the day. The increase in water uptake in three other roots following rain was not so substantial. In addition, the flux in individual roots was correlated to different degrees with the flux at different radial depths and azimuthal directions in trunk xylem. The flow in outer trunk xylem seemed to be mostly consistent with water movement from surface soil horizons, whereas deep roots seemed to supply water to the whole cross-section of sapwood. When water flow substantially decreased in shallow lateral roots and the outer stem xylem during drought, water flow in the inner sapwood was maintained, presumably due to its direct connection to deep roots. Results also suggest the importance of the sap flow sensor placement, in relation to sinker roots, as to whether lateral roots might be found to exhibit reverse flow during drought. This study is consistent with the dimorphic rooting habit of Quercus suber trees in which deep roots access groundwater to supply superficial roots and the whole tree, when shallow soil layers were dry

Nanoscale Metallic Iron for Environmental Remediation: Prospects and Limitations

The amendment of the subsurface with nanoscale metallic iron particles (nano-Fe0) has been discussed in the literature as an efficient in situ technology for groundwater remediation. However, the introduction of this technology was controversial and its efficiency has never been univocally established. This unsatisfying situation has motivated this communication whose objective was a comprehensive discussion of the intrinsic reactivity of nano-Fe0 based on the contemporary knowledge on the mechanism of contaminant removal by Fe0 and a mathematical model. It is showed that due to limitations of the mass transfer of nano-Fe0 to contaminants, available concepts cannot explain the success of nano-Fe0 injection for in situ groundwater remediation. It is recommended to test the possibility of introducing nano-Fe0 to initiate the formation of roll-fronts which propagation would induce the reductive transformation of both dissolved and adsorbed contaminants. Within a roll-front, FeII from nano-Fe0 is the reducing agent for contaminants. FeII is recycled by biotic or abiotic FeIII reduction. While the roll-front concept could explain the success of already implemented reaction zones, more research is needed for a science-based recommendation of nano- Fe0 for subsurface treatment by roll-front

Data requirements for identifying macroscopic water stress parameters: A study on grapevines

We have tested the inverse modeling approach to derive macroscopic water stress parameters (MWSP) using different types of information, such as soil water content, pressure head, and transpiration rate. This testing was performed by numerical experiments considering a multilayered soil growing grapevines under three different irrigation regimes and two contrasted water stress scenarios. The results indicate that measurements of the soil water content alone do not contain enough information to estimate MWSP. Nonuniqueness is likely to occur, and the MWSP estimates may contain large uncertainties. However, the incorporation of only transpiration measurements into the objective function does allow the deriving of accurate MWSP. This contrast is mainly due to the difference of sensitivity to the MWSP, which is much higher for the transpiration than for the soil water content. Results obtained using only soil water pressure head measurements are similar to or poorer than those obtained using transpiration data. Moreover, visual inspection of response surfaces of the objective function suggests that the incorporation of further information in addition to transpiration into the objective function is not of great value for the identification of MWSP. Uncertainties for the MWSP estimated using the three types of information combined are in most cases only 1.3 times smaller than when transpiration measurements alone are incorporated into the objective function. Beyond the specific results obtained for the estimation of MWSP we find that the parameters estimates and their associated uncertainties are strongly dependent upon the type, quantity, and quality of the information included into the objective function. Hence inverse modeling may provide a means to design better experiments

Transpiration of squash under a tropical maritime climate

We present the measurement and modelling of transpiration from squash (Cucurbita maxima Duchesne) growing in the field under a tropical maritime climate. Measurements were carried out on Tongatapu (175 degrees 12' W, 21 degrees 08' S), a coral atoll located in the Pacific Ocean. Transpiration was determined from heat-pulse measurements of sap flow in the vine stem using the T-max method. Steady-state porometry was used to monitor stomatal conductance (g(S), mm s(-1)). The data were used to derive parameters for a functional model of conductance that includes response functions for light, air temperature and vapour pressure deficit of the air, and a novel response function for soil moisture. Leaf area development was monitored through the growing season using a point quadrant approach. The maximum leaf area was about 2.7 m(2) per plant and the effective ground area was about 1 m(2) for each plant. Transpiration losses were calculated using a 2-layer big-leaf model in combination with modelled stomatal response and measured leaf area. In general, the sap flow measurements were in good agreement with the calculations of plant water use. Peak water use was between 3 and 5 L per plant per day. Daily transpiration measurements from heat-pulse were used to derive a crop factor, K-C, for squash in this tropical maritime climate. The derived seasonal pattern of K-C was similar to the FAO recommended crop factor for squash. However, the growing season was a little shorter. Measured sap flow also revealed periods of short-term drought and leaf fungal disease that reduced the actual transpiration losses, and there was often a rapid recovery from water stress following rainfall events

Can minor compaction increase soil carbon sequestration? A case study in a soil under a wheel-track in an orchard

In an organic apple ('Malus domestica' Borkh., Braeburn on MM.106 rootstock) orchard in New Zealand we observed that the soil's carbon concentration in 0-0.1 m depth directly below a wheel-track in the grassed alley-way was significantly higher than under the grassed tree-row. By using a literature review and direct measurements, we systematically exclude two explanations for the possible accumulation of soil carbon under the wheel-track. Firstly, we discard the possibility of higher carbon inputs to the wheel-track compared to the tree-row area. The tree-row received more carbon inputs by the regular application of compost, and the growth of roots was not impeded by compaction. Secondly, we directly measured the monthly dynamics of microbial biomass and respiration rates per unit of soil carbon over a year, and based on 3D X-ray computed tomography (CT) images we modelled the gas exchange close to water saturation. A restricted gas exchange under wet conditions would be an indication that soil carbon mineralization is reduced under wet conditions. We found that both the microbial carbon decomposition dynamics and the gas exchange under wet conditions were similar in the tree-row and under the wheel-track. The most probable explanation for the enhanced carbon sequestration under the wheel-track is a reduction in carbon losses. The water infiltration rates into the wheel-tracks were significantly smaller than in the tree-row. This means that the carbon loss with water flow in the form of dissolved carbon is probably smaller under the wheel-track. Additionally, a reduction in meso- and macro-faunal activities exporting particulate organic matter from the soil volume is probable under minor compaction. We conclude that a minor compaction like under a wheel-track can be a physical protection mechanism for soil carbon

Evaluation of drainage from passive suction and nonsuction flux meters in a volcanic clay soil under tropical conditions

Root zone drainage measurements are needed to improve fertilizer management in areas where agriculture may be impacting ground-water supplies. We present results of field tests where drainage was with two types of suction (resolution of 0.16 and 1.6 mm (-1)) and a nonsuction (resolution of 0.22 mm tip(-1)) water flux meter (WFM). The soil was a microstructured weathered volcanic ash located on a coral atoll subject to intense rainfall and located in the Kingdom of Tonga. Our objectives were to evaluate water flux measurements by comparing them with (i) simple water balance estimates of cumulative fluxes, (ii) cumulative fluxes deduced from soil moisture content changes, and (iii) simulated fluxes using HYDRUS-1D. Soil hydraulic properties were obtained at five soil depths. During the 60-d evaluation period rainfall totaled 340 mm. The WFMs were installed in duplicate using disturbed soil. The consistency of the shape of the drainage curves measured with the WFMs, those derived from soil moisture changes, and those obtained with modeling led us to conclude that soil disturbance during WFM installation did not severely influence measurements. This was attributed to the strong microaggregation and disturbance introduced by plowing. Water balance and HYDRUS model estimates of drainage corresponded well with the measurement by nonsuction WFMs. Suction WFMs overestimated drainage, possibly due to flow convergence created by wick and divergence barrier lengths being not properly sized for the observed flow conditions. After the evaluation period some of the WFMs failed to respond. Nevertheless, flux meters are seen as promising tools to provide remote and continuous measurement of root zone drainage

An improved solution for the infiltration advance problem in irrigation hydraulics

The irrigation advance problem in irrigation hydraulics has been spread across the engineering and soil science literature over a number of decades. The Lewis-Milne framework has been used extensively, but one problem has been to find a suitable infiltration equation. The infiltration advance solutions of Philip and Farrell, and Collis-George and Freebairn are compared to a new solution based on the linear soil infiltration equation. It is shown that the linear soil solution is able to give similar results to the Philip and Farrell solution at early stages of infiltration when this is valid, and the Collis-George and Freebairn solution at longer times when this is valid. The linear soil infiltration advance solution presented here is the first using physically meaningful parameters which is able to give adequate infiltration and advance behaviour over all time scales. To further test the linear soil concept, we inversely fit irrigation advance data to get the sorptivity, saturated hydraulic conductivity and infiltration rate behaviour of the soil using all three infiltration equations. The linear soil is shown to give the best fit for the infiltration behaviour to the measured results with an average r of 0.98 compared to 0.84 for Philip and Farrell and 0.77 for Collis-George and Freebairn. The linear soil model was also the best fit using other statistical tests such as RMSE and RSR