Soil Water Matric Potential Research Articles

Performance of Hydra Probe and MPS-1 Soil Water Sensors in Topsoil Tested in Lab and Field

Soil water sensors are commonly used to monitor water content and matric potential in order to study hydrological processes such as evaporation. Finding a proper sensor is sometimes difficult, especially for measurements in topsoil, where changes of temperature and soil water dynamics occur generally with greater intensity compared to deeper soil layers. We assessed the perfor-mance of Hydra Probe water content sensors and MPS-1 matric potential sensors in topsoil in the laboratory and in the field. A common soil-specific calibration function was determined for the Hydra Probes. Measurement accuracy and sensor-to-sensor variation were within the manufacturer specification of ±0.03 m3·m-3. Hydra Probes can operate from dry to saturated conditions. Sensor-specific calibrations from a previous study were used to reduce sensor-to-sensor variation of MPS-1. Measurement accuracy can be expressed by a mean relative error of 10%. According to the manufacturer, the application range of matric potential readings is from -10 kPa to -500 kPa. MPS-1 delivered also values beyond this range, but they were not reliable. Sensor electronics of the MPS-1 were sensitive to ambient temperature changes. Beyond instrument effects, field measurements showed substantial temperature-driven fluctuations of soil water content and matric potential, which complicated data interpretation.

Survivability ofAporrectodea caliginosain Response to Drought Stress in a Colorado Soil

The distribution of earthworms is limited in many areas in the semiarid western United States by the availability of water and low soil organic matter; however, earthworms have been shown to adapt to periods of low soil moisture by making small chambers and entering estivation to protect themselves from declining soil moisture. The objective of this study was to investigate the effects of varying lengths of drought stress on the survival of earthworms in a low organic matter soil from eastern Colorado that was amended with biosolids to increase organic matter. The earthworms were exposed to constant water content or cycles of 1-, 2-, or 3-wk drought stress periods, which resulted in average soil water matric potentials of −0.061, −0.085, −0.13, and −0.19 MPa, respectively. Replicate pots were destructively sampled at 21, 42, and 63 d. At sampling, the earthworms were classified as either active, in estivation, or dead. Drought did not affect earthworm mass, but drought lasting 2 or 3 wk increased the number of A. caliginosa in estivation. Three weeks of drought resulted in a mortality rate of 14%, with all other drought treatments having negligible mortality rates. The average number of A. caliginosa in estivation decreased with time, from 1.71 at 21 and 42 d to 0.75 at 63 d. Based on these results, A. caliginosa has the potential to survive up to 3-wk drought periods and after introduction, can adapt to this Colorado soil with time, but field studies would need to confirm these results.

Measurement of the matric potential of soil water in the rhizosphere

The availability of soil water, and the ability of plants to extract it, are important variables in plant research. The matric potential has been a useful way to describe water status in a soil-plant system. In soil it is the potential that is derived from the surface tension of water menisci between soil particles. The magnitude of matric potential depends on the soil water content, the size of the soil pores, the surface properties of the soil particles, and the surface tension of the soil water. Of all the measures of soil water, matric potential is perhaps the most useful for plant scientists. In this review, the relationship between matric potential and soil water content is explored. It is shown that for any given soil type, this relationship is not unique and therefore both soil water content and matric potential need to be measured for the soil water status to be fully described. However, in comparison with water content, approaches for measuring matric potential have received less attention until recently. In this review, a critique of current methods to measure matric potential is presented, together with their limitations as well as underexploited opportunities. The relative merits of both direct and indirect methods to measure matric potential are discussed. The different approaches needed in wet and dry soil are outlined. In the final part of the paper, the emerging technologies are discussed in so far as our current imagination allows. The review draws upon current developments in the field of civil engineering where the measurement of matric potential is also important. The approaches made by civil engineers have been more imaginative than those of plant and soil scientists.

Effects of soil water matric potential (SWMP) on growth,yield and quality of two potato cultivars

Effects of soil water matric potential (SWMP) on growth,yield and quality of two potato cultivars

Consequências hidrológicas da mudança de uso da terra de floresta para pastagem na região da floresta tropical pluvial atlântica

The Atlantic rain forest is the most endangered ecosystem in Brazil. Its degradation has started since 1500 when the European settlers arrived. Despite of all land use changes that have occurred, hydrological studies carried out in this biome have been limited to hydrological functioning of rain forests only. In order to understand the hydrological consequences of land-use change from forest to pasture, we described the hydrological functioning of a pasture catchment that was previously covered by tropical rain forest. To reach this goal we measured the precipitation, soil matric potential, discharge, surface runoff and water table levels during one year. The results indicated that there is a decrease in surface soil saturated hydraulic conductivity. However, as low intensity rainfall prevails, the lower water conductivity does not necessarily leads to a substantially higher surface runoff generation. Regarding soil water matric potential, the pasture presented higher moisture levels than forest during the dry season. This increase in soil moisture implies in higher water table recharge that, in turn, explain the higher runoff ratio. This way, land-use change conversion from forest to pasture implies a higher annual streamflow in pasture catchments. Nonetheless, this increase in runoff due to forest conversion to pasture implies in losses of biological diversity as well as lower soil protection.

Measurement systems of soil water matric potential and evaluation of soil moisture under different irrigation depths

The development of new methodologies and tools that enable to determine the water content in soil is of fundamental importance to the practice of irrigation. The objective of this study was to evaluate soil matric potential using mercury tensiometer and puncture digital tensiometer, and to compare the gravimetric soil moisture values obtained by tensiometric system with gravimetric soil moisture obtained by neutron attenuation technique. Four experimental plots were maintained with different soil moisture by irrigation. Three repetitions of each type of tensiometer were installed at 0.20 m depth. Based on the soil matric potential and the soil water retention curve, the corresponding gravimetric soil moisture was determined. The data was then compared to those obtained by neutron attenuation technique. The results showed that both tensiometric methods showed no difference under soil matric potential higher than -40 kPa. However, under drier soil, when the water was replaced by irrigation, the soil matric potential of the puncture digital tensiometer was less than those of the mercury tensiometer.

Influence of different amounts of irrigation water on salt leaching and cotton growth under drip irrigation in an arid and saline area

In order to evaluate the effects of different amounts of water, applied by drip irrigation, to a saline–sodic soil (surface ECe>40dS/m; SAR>40), on cotton growth and soil salinity, a three-year experiment was conducted on a saline wasteland in Xinjiang Northwest China during 2008–2010. Five water treatments were used for this experiment based on the soil–water matric potential (SMP) measured 20cm beneath a drip emitter located close to the plant: the SMP levels used to determine when to irrigate were −5kPa (S1), −10kPa (S2), −15kPa (S3), −20kPa (S4), and −25kPa (S5). After three years, both the soil salinity (ECe) and sodicity (SAR) declined significantly in 0–120cm depth and more reduction were achieved in 0–40cm soil depth than in 40–80 and 80–120cm depths. Moreover, the reductions of SAR were smaller than those of ECe. Additionally, the amount of salt removed from the 0 to 80cm depth decreased with decreasing SMP threshold. The S1 treatment resulted in the highest lint yields in 2009 and 2010. Considering the effects of leached salts on the environment of deep soil layer and the yield of cotton, an SMP of −10kPa can be used to trigger irrigation for cotton in the first three years for saline wasteland reclamation in Xinjiang Northwest China.

Hydrologic conditions controlling runoff generation immediately after wildfire

We investigated the control of postwildfire runoff by physical and hydraulic properties of soil, hydrologic states, and an ash layer immediately following wildfire. The field site is within the area burned by the 2010 Fourmile Canyon Fire in Colorado, USA. Physical and hydraulic property characterization included ash thickness, particle size distribution, hydraulic conductivity, and soil water retention curves. Soil water content and matric potential were measured indirectly at several depths below the soil surface to document hydrologic states underneath the ash layer in the unsaturated zone, whereas precipitation and surface runoff were measured directly. Measurements of soil water content showed that almost no water infiltrated below the ash layer into the near‐surface soil in the burned site at the storm time scale (i.e., minutes to hours). Runoff generation processes were controlled by and highly sensitive to ash thickness and ash hydraulic properties. The ash layer stored from 97% to 99% of rainfall, which was critical for reducing runoff amounts. The hydrologic response to two rain storms with different rainfall amounts, rainfall intensity, and durations, only ten days apart, indicated that runoff generation was predominantly by the saturation‐excess mechanism perched at the ash‐soil interface during the first storm and predominantly by the infiltration‐excess mechanism at the ash surface during the second storm. Contributing area was not static for the two storms and was 4% (saturation excess) to 68% (infiltration excess) of the catchment area. Our results showed the importance of including hydrologic conditions and hydraulic properties of the ash layer in postwildfire runoff generation models.

Thermal Properties of Peaty Soils: Effects of Liquid‐Phase Impedance Factor and Shrinkage

Soil thermal conductivity (λ) and heat capacity (C) control heat transport and the thermal environment for biogeophysical processes in the vadose zone. Accurate λ and C predictions for peaty soils with high organic contents are particularly important for assessing emissions of greenhouse gases formed during microbial activity in wetlands. In this study, we measured the λ and C at different soil‐water matric potentials on undisturbed samples for three peaty soil profiles at the Hokkaido Bibai marsh in Japan, representing a total of 10 different soil horizons. The thermal properties under air‐dried conditions, λdry and Cdry, were measured separately at changing volumetric solids contents (σ). For each sample, volume shrinkage was observed to varying degrees during the drying process. Measured λ and C increased linearly with increasing volumetric water content (θ). Applying the concept of a three‐phase mixing model and incorporating the λ–θ or C–θ and the λdry–σ or Cdry–σ relations, predictive λ and C models were developed as functions of σ and θ. The new mixing model is represented by λ = λdry + fλθλw and C = Cdry + fCθCw, where λw and Cw are the thermal conductivity and heat capacity of water, respectively, and f is an impedance factor that takes into account the liquid‐phase tortuosity. The new mixing model predicted literature λ–θ data on peaty and highly organic soils under variable saturation well. The probable ranges of λ and C under variable saturation were proposed based on the sensitivity analysis.

Solute Diffusivity in Undisturbed Soil: Effects of Soil Water Content and Matric Potential

Solute diffusivity in soil plays a major role in many important processes with relation to plant growth and environmental issues. Soil solute diffusivity is affected by the volumetric water content as well as the morphological characteristics of water‐filled pores. The solute diffusivity in intact soil samples from two different tillage treatments (soil from below the depth of a harrow treatment and soil from within a moldboard plowed plow layer) was estimated based on concentration profiles using a newly developed method. The method makes use of multiple tracers (two sets of counterdiffusing tracers) for a better determination of the diffusivity. The diffusivity was higher in the below‐till soil than the plowed soil at the same soil water matric potential due to higher water content but also due to higher continuity and lower tortuosity of the soil pores. We measured identical solute diffusivities independent of the tracer set used. We analyzed the whole data set using Archie's law and found a linear relation between Archie's exponent and the logarithm of the soil water matric suction in centimeters of water (pF). An analysis of seven data sets from the literature showed that this was a general trend for soils with moderate to low clay contents.

Linking Particle and Pore Size Distribution Parameters to Soil Gas Transport Properties

Accurate estimation of soil gas diffusivity (Dp/Do, the ratio of gas diffusion coefficients in soil and free air) and air permeability (ka) from basic texture and pore characteristics will be highly valuable for modeling soil gas transport and emission and their field‐scale variations. From the topsoil of two Danish arable fields representing two natural clay gradients, Dp/Do and ka were measured at soil water matric potentials between −1 and −100 kPa on undisturbed soil cores. The Rosin–Rammler particle size distribution parameters α and β (characteristic particle size and degree of sorting, respectively) and the Campbell water retention parameter b were used to characterize particle and pore size distributions, respectively. Campbell b yielded a wide interval (4.6–26.2) and was highly correlated with α, β, and volumetric clay content. Both Dp/Do and ka followed simple power‐law functions (PLFs) of air‐filled porosity (εa). The PLF tortuosity–connectivity factors (X*) for Dp/Do and ka were both highly correlated with all basic soil characteristics, in the order of volumetric clay content = Campbell b > gravimetric clay content > α > β. The PLF water blockage factors (H) for Dp/Do and ka were also well (but relatively more weakly) correlated with the basic soil characteristics, again with the best correlations to volumetric clay content and b. As a first attempt at developing a simple Dp/Do model useful at the field scale, we extended the classical Buckingham Dp/Do model (εa2) by a scaling factor based on volumetric clay content. The scaled Buckingham model provided accurate predictions of Dp(εa)/Do across both natural clay gradients.

A new method of applying a controlled soil water stress, and its effect on the growth of cotton and soybean seedlings at ambient and elevated carbon dioxide

While numerous studies have shown that elevated CO 2 can delay soil water depletion by causing partial stomatal closure, few studies have compared responses of plant growth to the same soil water deficits imposed at ambient and elevated CO 2. We applied a vacuum to ceramic cups in pots filled with soil to reduce the soil water matric potential to −0.10 MPa. This system resulted in uniform soil water content throughout the pot, and was used to maintain a constant mild stress for seven days. In cotton, the soil water stress treatment reduced stomatal conductance at both 380 and 560 μmol mol −1 CO 2, but the reduction was relatively smaller at the higher CO 2. No reduction of photosynthesis measured under the daytime growth conditions occurred at elevated CO 2 in stressed cotton plants, while photosynthesis was reduced by the stress in the lower CO 2 treatment. The soil water stress treatment reduced the leaf area and biomass of cotton at the lower, but not at the higher CO 2. In soybean, the soil water stress treatment reduced stomatal conductance, photosynthesis and growth at both CO 2 levels, but the effect of water stress was not less at elevated than ambient CO 2. In neither species nor CO 2 level did the soil water stress treatment cause a detectable change in daytime leaf water potential. In both species, the stomatal closure with the soil water stress may have resulted from the lower soil to leaf hydraulic conductivity. The failure of high CO 2 to protect soybean growth from the soil water stress might be related to the lower hydraulic conductivity of stressed soybeans grown at elevated compared with ambient CO 2.

Enhanced Biogeochemical Cycling and Subsequent Reduction of Hydraulic Conductivity Associated with Soil‐Layer Interfaces in the Vadose Zone

Biogeochemical dynamics in the vadose zone are poorly understood due to the transient nature of chemical and hydrologic conditions but are nonetheless critical to understanding chemical fate and transport. This study explored the effects of a soil layer on linked geochemical, hydrological, and microbiological processes. Three laboratory soil columns were constructed: a homogenized medium-grained sand, a homogenized organic-rich loam, and a sand-over-loam layered column. Upward and downward infiltration of water was evaluated during experiments to simulate rising water table and rainfall events, respectively. In situ collocated probes measured soil water content, matric potential, and Eh. Water samples collected from the same locations were analyzed for Br, Cl, NO, SO, NH, Fe, and total sulfide. Compared with homogeneous columns, the presence of a soil layer altered the biogeochemistry and water flow of the system considerably. Enhanced biogeochemical cycling was observed in the layered column over the texturally homogeneous soil columns. Enumerations of iron- and sulfate-reducing bacteria showed 1 to 2 orders of magnitude greater community numbers in the layered column. Mineral and soil aggregate composites were most abundant near the soil-layer interface, the presence of which likely contributed to an observed order-of-magnitude decrease in hydraulic conductivity. These findings show that quantifying coupled hydrologic-biogeochemical processes occurring at small-scale soil interfaces is critical to accurately describing and predicting chemical changes at the larger system scale. These findings also provide justification for considering soil layering in contaminant fate and transport models because of its potential to increase biodegradation or to slow the rate of transport of contaminants.

A modification of the mixed form of Richards equation and its application in vertically inhomogeneous soils

Abstract. Recently, new soil data maps were developed, which include vertical soil properties like soil type. Implementing those into a multilayer Soil-Vegetation-Atmosphere-Transfer (SVAT) scheme, discontinuities in the water content occur at the interface between dissimilar soils. Therefore, care must be taken in solving the Richards equation for calculating vertical soil water fluxes. We solve a modified form of the mixed (soil water and soil matric potential based) Richards equation by subtracting the equilibrium state of soil matrix potential ψE from the hydraulic potential ψh. The sensitivity of the modified equation is tested under idealized conditions. The paper will show that the modified equation can handle with discontinuities in soil water content at the interface of layered soils.

Slumping dynamics in tilled sandy soils under natural rainfall and experimental flooding

Compaction of tilled layers under the single effect of rainfall or irrigation was called slumping. Slumping affects strongly root development and plant biomass production. It has been observed in different soil types, but sandy soils appear particularly prone to this physical degradation. Our objectives in this study were (i) to measure in the field the changes in soil structure and water status simultaneously, (ii) to study the effects of rainfall and management practices on slumping, and (iii) to propose a conceptual model for sandy soil slumping. An experimental site was selected in Northeast Thailand and we studied the effect of tillage depth and initial water content on slumping dynamic. Plots (9 m × 15 m each) were tilled at (i) two depths (20 and 40 cm, called S and D respectively) in dry conditions, (ii) at 20 cm depth in dry or wet conditions (called Y and W respectively). These plots were submitted to natural rainfall for 20 or 61 days to get different total rainfall amounts (114 and 212 mm respectively). In addition, smaller plots (0.24 m 2 each) were used for experimental flooding irrigation (similar to measured rainfalls, i.e. 100 and 200 mm). Soil bulk density, soil surface elevation, soil water content and matric potential were measured. A decrease in soil elevation was observed in all treatments. In the absence of erosion it was interpreted as a loss of porosity which resulted from slumping. Bulk density increased in all layers of the tilled profile (from 1.38 to 1.57 g cm −3). In the surface layer (0–5 cm) this increase was systematically higher compared to deeper layer. No significant difference in final bulk density was found between the S and W treatments, and between the Y and W treatments. Bulk density increased more rapidly in the Y and W treatments than in the S and D treatments, even though the cumulative rainfall was lower. After the flooding experiments, bulk density was higher than after natural rainfall despite similar amounts of water added to the soil.

Demanda hídrica e coeficientes de cultura (Kc) para macieiras em Vacaria, RS

O objetivo do presente trabalho foi determinar a demanda hídrica e os coeficientes de cultura (Kc) para macieiras cultivadas na região de Vacaria, RS. O trabalho foi desenvolvido na Estação Experimental de Fruticultura Temperada (EEFT) da Embrapa Uva e Vinho, localizada em Vacaria, RS, em plantas da cultivar 'Royal Gala' (Malus domestica) sobre porta-enxerto M9. Foram determinados os valores do potencial matricial da água no solo, empregando-se tensiômetros de punção. Com base nesses valores, determinou-se a umidade volumétrica e o balanço hídrico mensal. O consumo hídrico da cultura variou entre 0,3mm dia-1 a 4,5mm dia-1, com média de 1,9mm dia-1. O coeficiente da cultura (Kc) apresentou tendência quadrática, variando entre 0,19 e 0,88, com média igual a 0,58.

Testing Some Pedo-Transfer Functions (PTFs) in Apulia Region. Evaluation on the Basis of Soil Particle Size Distribution and Organic Matter Content for Estimating Field Capacity and Wilting Point

The knowledge of soil water retention vs. soil water matric potential is applied to study irrigation and drainage scheduling, soil water storage capacity (plant available water), solute movement, plant growth and water stress. To measure field capacity and wilting point is expensive, laborious and is time consuming, so, frequently, matemathic models, called pedo-transfer functions (PTFs) are utilized to estimate field capacity and wilting point through physical-chemical soil characteristics. Six PTFs have been evaluated (Gupta and Larson, 1979; Rawls et al., 1982; De Jong et al., 1983; Rawls and Brakensiek, 1985; Saxton et al., 1986; Vereecken et al., 1989) by comparing measured soil moisture values with estimated ones at soil water matric potential of -33 and -1500 kPa. Soil samples were collected (361) from 185 pedons of Apulian Region (Southern Italy). Accuracy of the soil moisture predictions is quantified with Root Mean Square Deviation (RMSD) between estimated and measured water retention values. In Apulia Region the tested PTFs give different results on soils grouped on the basis of textural composition and organic matter (O.M.) content both at the Field Capacity (FC) and Wilting Point (WP). At the FC, Gupta and Larson model has given the best performance in Clayey (C), Sandy clay loam (SaCL), Sandy loam (SaL) and Silty (Si) soil, in loamy and tendency silty soils with O.M. content less than 1.9% and in tendency sandy soils with O.M. content less than 1.5% and greater than 2%; the Rawls model in Silty clay (SiC) and Silty loam (SiL) soils, in tendency clayey soils with O.M. less than 2.3% and in loamy and tendency silty soils with O.M. greater than 1.9%; the Rawls and Brakensiek model in tendency sandy soils with O.M. content between 1.5 and 2%; the Saxton model in Silty clay loam (SiCL), Loamy sand (LSa) soils and in tendency clayey soils with O.M. content greater than 2.3% and the Vereecken model in Sandy clay (SaC), Loamy (L), Clay loam (CL) and Sandy (Sa) soils. At the WP, the Gupta and Larson model has resulted the best in SiL, Si soils and, in general, in loamy and tendency silty and in tendency sandy soils with O.M. content greater than 1.9% and 2%, respectively; the Rawls model in Loamy soils and in loamy and tendency silty soils with O.M. between 1.0 and 1.9%; the De Jong model in C soils; the Rawls and Brakensiek model in SiC, SaC, CL, SiCL, SaCL soils and generally in tendency clayey soils with whatever O.M. content and in tendency sandy soils with O.M. content between 0.8 and 2%; the Saxton model in loamy and tendency silty soils with O.M. content less than 1% and in tendency sandy soils with O.M. less than 0.8%; the Vereecken model in SaL, Sa and LSa soils.

Estimating plant root water uptake using a neural network approach

Water uptake by plant roots is an important process in the hydrological cycle, not only for plant growth but also for the role it plays in shaping microbial community and bringing in physical and biochemical changes to soils. The ability of roots to extract water is determined by combined soil and plant characteristics, and how to model it has been of interest for many years. Most macroscopic models for water uptake operate at soil profile scale under the assumption that the uptake rate depends on root density and soil moisture. Whilst proved appropriate, these models need spatio-temporal root density distributions, which is tedious to measure in situ and prone to uncertainty because of the complexity of root architecture hidden in the opaque soils. As a result, developing alternative methods that do not explicitly need the root density to estimate the root water uptake is practically useful but has not yet been addressed. This paper presents and tests such an approach. The method is based on a neural network model, estimating the water uptake using different types of data that are easy to measure in the field. Sunflower grown in a sandy loam subjected to water stress and salinity was taken as a demonstrating example. The inputs to the neural network model included soil moisture, electrical conductivity of the soil solution, height and diameter of plant shoot, potential evapotranspiration, atmospheric humidity and air temperature. The outputs were the root water uptake rate at different depths in the soil profile. To train and test the model, the root water uptake rate was directly measured based on mass balance and Darcy's law assessed from the measured soil moisture content and soil water matric potential in profiles from the soil surface to a depth of 100 cm. The ‘measured’ root water uptake agreed well with that predicted by the neural network model. The successful performance of the model provides an alternative and more practical way to estimate the root water uptake at field scale.

Determination of critical soil water content and matric potential for wind erosion

Purpose Soil strength and thus stability concerning wind erosion are controlled by the soil water content. The concept of soil critical water content (Θcrit.) for deflation was extended to include matric potential (Ψcrit.) as well. The focus of this paper is to quantify the Θcrit. and Ψcrit. as the upper boundary for wind erosion or as the lower boundary for soil strength, to model the Ψcrit. at the immediate soil surface (0–0.2 cm) and to evaluate the effect of soil moisture upon erosion as a function of time and sampling height.

Hydrological and biogeochemical controls on the timing and magnitude of nitrous oxide flux across an agricultural landscape

AbstractAnticipated increases in precipitation intensity due to climate change may affect hydrological controls on soil N2O fluxes, resulting in a feedback between climate change and soil greenhouse gas emissions. We evaluated soil hydrologic controls on N2O emissions during experimental water table fluctuations in large, intact soil columns amended with 100 kg ha−1 KNO3‐N. Soil columns were collected from three landscape positions that vary in hydrological and biogeochemical properties (N= 12 columns). We flooded columns from bottom to surface to simulate water table fluctuations that are typical for this site, and expected to increase given future climate change scenarios. After the soil was saturated to the surface, we allowed the columns to drain freely while monitoring volumetric soil water content, matric potential and N2O emissions over 96 h. Across all landscape positions and replicate soil columns, there was a positive linear relationship between total soil N and the log of cumulative N2O emissions (r2= 0.47; P= 0.013). Within individual soil columns, N2O flux was a Gaussian function of water‐filled pore space (WFPS) during drainage (mean r2= 0.90). However, instantaneous maximum N2O flux rates did not occur at a consistent WFPS, ranging from 63% to 98% WFPS across landscape positions and replicate soil columns. In contrast, instantaneous maximum N2O flux rates occurred within a narrow range (−1.88 to −4.48 kPa) of soil matric potential that approximated field capacity. The relatively consistent relationship between maximum N2O flux rates and matric potential indicates that water filled pore size is an important factor affecting soil N2O fluxes. These data demonstrate that matric potential is the strongest predictor of the timing of N2O fluxes across soils that differ in texture, structure and bulk density.