Factors affecting runoff and erosion under simulated rainfall in Mediterranean vineyards

Data on surface runoff and soil loss on gentle slopes with vineyards are analysed. Using a rainfall simulator, 22 rainstorms with varied intensities from 30 to 117.5 mm h⁻¹ and return periods from 2 to 127 years were reproduced. The experimental plots were installed on vineyards planted in straight rows and oriented with the slope direction having a mean gradient of 3.8°. The texture of soils was loamy, with a very heterogeneous surface gravel cover. Values of measured surface runoff varied from 7.2 mm h⁻¹ for low rainfall intensities (30 mm h⁻¹) and short return periods (2 years) to 41.9 mm h⁻¹ with simulation experiments of higher rainfall intensity (104 mm h⁻¹) and long return periods (68 years). Runoff increased linearly with rainfall intensity resulting in soil losses that also increased with rainfall intensity (18.2 g m⁻² h⁻¹ with storms of 30 mm h⁻¹, and 93.2 g m⁻² h⁻¹ with storms of 104 mm h⁻¹); however, r² explains only 36% of the variance. It was necessary to add other factors to improve the coefficient of determination (0.74; p = 0.001) and the predictive function of the equation. These variables were rainfall intensity, kinetic energy of the storm, runoff, soil resistance to drop detachment, surface gravel cover, and gradient. The equation obtained was validated with the USLE-M. In comparison with similar experiments in other regions, the results obtained for soil loss were very moderate, especially those caused by rainstorms of intermediate and low intensity.     

Erosion rates and nutrient losses affected by composted cattle manure application in vineyard soils of NE Spain

Land preparation for mechanisation in vineyards of the Anoia–Alt Penedès region, NE Spain, has required major soil movements, which has enormous environmental implications not only due to changes in the landscape morphology but also due to soil degradation. The resulting cultivated soils are very poor in organic matter and highly susceptible to erosion, which reduces the possibilities of water intake as most of the rain is lost as runoff. In order to improve soil conditions, the application of organic wastes has been generalised in the area, not only before plantation but also every 3–4 years at rates of 30–50 Mg ha^− 1 mixed in the upper 30 cm.These organic materials are important sources of nutrients (N and P) and other elements, which could reduce further fertilisation cost. However, due to the high susceptibility to sealing of these soils, erosion rates are relatively high, so a higher nutrient concentration on the soil surface increases non-point pollution sources due to runoff.The aim of this study is to analyse the influence of applied composted cattle manure on infiltration, runoff and soil losses and on nutrients transported by runoff in vineyards of the Alt Penedès–Anoia region, NE Spain. In the two plots selected for the analysis, composted cattle manure had been applied in alternate rows 1 year previous to the study. In each plot soil surface samples (0–25 cm) were taken and compared to those of plots without manure application. The study was carried out at laboratory scale using simulated rainfall. Infiltration rates were calculated from the difference between rainfall intensity and runoff rates, and the sediment and total nitrogen and phosphorus were measured for each simulation. In addition, the influence of compost was investigated in the field under natural rainfall conditions by analysing the nutrient concentration in runoff samples collected in the field (in the same plots) after seven rainfall events, which amount different total precipitation and had different erosive character.Compost application increases infiltration rates by up to 26% and also increases the time when runoff starts. Sediment concentration in runoff was lower in treated (13.4 on average mg L^− 1) than in untreated soils (ranging from 16.8 to 23.4 mg L^− 1). However, the higher nutrient concentration in soils produces a higher mobilisation of N (7–17 mg L^− 1 in untreated soils and 20–26 mg L^− 1 in treated soils) and P (6–7 mg L^− 1 in untreated soils and 13–19 mg L^− 1 in treated soils). A major part of the P mobilised was attached to soil particles (about 90% on average) and only 10% was dissolved. Under natural conditions, higher nutrient concentrations were always recorded in treated vs. untreated soils in both plots, and the total amount of N and P mobilised by runoff was higher in treated soils, although without significant differences. Nutrient concentrations in runoff depend on rainfall erosivity but the average value in treated soils was twice that in untreated soils for both plots.     

Modeling daily soil water dynamics during vertical drainage using the incoming flow concept

Crop management models require simulation of daily soil water dynamics. The objective of this study was to develop a model to simulate the daily soil water dynamics during vertical drainage with reasonable accuracy using the incoming flow concept. The execution of this model, which has been developed based on the conservation of mass law, consists of two steps. First, calculating the potential daily change of soil water content (Δθp) for each soil layer in the profile assuming each one receives no water from the above layer. Then, calculating the actual daily change of soil water (Δθa) for each soil layer in the profile by adjusting Δθp using the incoming water flow, which can be defined as the amount of drainage water that reaches a layer in a soil profile from the above layer. The model was compared with the Suleiman and Ritchie [Suleiman, A.A., Ritchie, J.T., 2004. Modifications to the DSSAT vertical drainage model for more accurate soil water dynamics estimation. Soil Sci. 169 (11), 745–757] vertical drainage model (SRVDM) and HYDRUS-1D for diverse soils and was tested using drainage experimental data of a Eutric Regosol in Bekkevoort, Belgium and a sandy soil in Georgia, U.S. The difference in Δθp between the new model and HYDRUS-1D for diverse soils ranged from − 0.01 to 0.016 m³ m^− 3 for the first day and from − 0.005 to − 0.025 m³ m^− 3 for the second day while the difference in Δθp between the SRVDM and HYDRUS-1D for these soils ranged from 0.014 to 0.062 m³ m^− 3 for the first day and from − 0.01 to 0.026 m³ m^− 3 for the second day. The relative maximum absolute errors in Δθa between the new model and HYDRUS-1D was 10% while the relative maximum absolute errors in Δθa between the SRVDM and HYDRUS-1D was 112%. In the experiments, the root mean square difference of the soil water content for the new model was lower than that for the SRVDM at the different soil depths. These results indicated that the new model outperformed the SRVDM in simulating Δθp and Δθa for diverse soil. It can be concluded that the new model was robust and reasonably accurate for diverse soils at different soil depths. The implementation of such model will improve the accuracy and applicability of regional soil water dynamics simulation and will reduce considerably the computational time and the required inputs.     

Magnetic properties of the urban soils in Shanghai and their environmental implications

To study the feasibility of using magnetic techniques for monitoring soil pollution in Shanghai, magnetic properties and heavy metals in the topsoils in an urban site (Songnan Town) and a less-urbanized agricultural site (Luojing Town) in Baoshan District, Shanghai, were studied. Compared with the background, magnetic signals of the urban topsoils are extremely enhanced with magnetic susceptibility (χ_lf) from 127.3–1959 × 10^− 8 m³ kg^− 1; while those of the agricultural topsoils are only slightly increased. However, both the urban and agricultural topsoils contain few pedogenic SP grains, as indicated by their low χ_fd% (< 3.6%). Ratios of χ_arm/SIRM, χ_arm/χ_lf and SIRM/χ_lf indicate that the grain size of magnetic minerals in the urban topsoils is significantly coarser than that in the background and the agricultural topsoils. Furthermore, the urban topsoils show low coercivity and magnetic soft behaviors, as indicated by higher SOFT%, lower HARD%, higher IRM_300 mT/SIRM (close to 1) and lower IRM_− 200 mT/SIRM (close to − 1). It suggests that the urban topsoils have received some coarse ferrimagnetic particles. Heavy metals are highly enriched in the magnetic fractions of the topsoils. Geochemical properties of the magnetic fraction of the urban topsoils are significantly different from those of the agricultural topsoils, further indicating that the extra magnetic minerals accumulated in the urban topsoils are neither inherited from soil parent materials nor from pedogenic processes, but originate from anthropogenic activities. The significant correlations between heavy metals and χ_lf, χ_arm, SIRM, SOFT and HIRM of the topsoils in the district indicate that the magnetic techniques can be used for monitoring soil pollution in Shanghai. The soils with χ_lf from 39–50 × 10^− 8 m³ kg^− 1 in the district are tentatively defined as “slightly polluted soils”; those with χ_lf > 50 × 10^− 8 m³ kg^− 1 are defined as “polluted soils”.     

Contour furrows for in situ soil and water conservation, Tigray, Northern Ethiopia

In Tigray (Northern Ethiopia), soil moisture has been identified as the most limiting factor in agricultural production; on the other hand, loss of rain water through runoff as well as the induced soil loss has been determined as a critical problem in the region in the last two to three decades. To alleviate the above paradox, the government has mobilized communities and resources for the construction of physical soil and water conservation structures (stone bunds, terraces) in almost all land uses. However, yield improvement was mainly concentrated within the vicinity of the structures and runoff continued to overtop the structures, as no measures for in situ soil conservation were taken. The terwah system, consisting of traditional ploughing followed by making every 1.5–2 m contour furrows, and permanent raised beds with contour furrows at 60–70 cm interval treatments, were considered and evaluated as practices that could increase the efficiency of in situ water utilization and soil conservation. An experiment was started in Gum Selasa, which is one of the drought prone areas in Tigray, whereby runoff volume and sediment load were measured after every rain event. Permanent raised beds with contour furrows at 60–70 cm interval significantly (P < 0.05) reduced runoff volume, runoff coefficient and soil loss as compared to traditional ploughing: 255, 381 and 653 m³ ha⁻¹ runoff was recorded from permanent bed, terwah and traditional ploughing, respectively during the whole cropping season. The above runoff induced 4.7 t ha⁻¹ soil loss from permanent bed, 7.6 t ha⁻¹ from terwah and 19.5 t ha⁻¹ from traditional ploughing. Overall, contour furrows and permanent raised beds can be part of the ongoing intensification process which includes physical soil and water conservation, slope reforestation, irrigation development and agro forestry in crop lands. Moreover, the use of permanent raised beds if combined with crop mulching and crop diversification is an important component for the development of sustainable conservation agriculture practices in the region.     

Relationships among dust outbreaks, vegetation cover, and surface soil water content on the Loess Plateau of China, 1999–2000

We analyzed relationships among dust outbreaks, Normalized Difference Vegetation Indices (NDVI), and surface soil water content (0 to 2 cm depth) on the Loess Plateau, a significant dust source area of East Asia. World Surface Data for wind speed and current weather, coarse-resolution data for NDVI, and a three-layer soil model for surface soil water content were used. The threshold NDVI for preventing dust outbreaks was about 0.2 when the wind speed ranged from 7 to 8 m s^− 1. This threshold NDVI corresponds to a vegetation cover of 18%. The threshold ratio of surface soil water content to the field capacity (θ_r) was about 0.2. Conditions facilitating dust outbreaks on the Loess Plateau are when NDVI is less than 0.2 with wind speed  7 m s^− 1 and θ_r < 0.2, and when NDVI is greater than 0.2 with wind speed  9 m s^− 1 and θ_r < 0.2.     

The cost of soil erosion in vineyard fields in the Penedès–Anoia Region (NE Spain)

On-site and off-site environmental impacts of runoff and erosion are usually stressed in order to bring to the public's attention the importance and implications of soil erosion. However, few studies are aimed at calculating the economic implications of erosion, this being the message that farmers and/or policy makers understand best. In this current work we estimated the cost of erosion in vineyards in the Penedès–Anoia region (NE Spain), in which high intensity rain storms (> 80–100 mm h^− 1) are frequent. Modern plantations in the region consist of trained vines, usually planted perpendicular to the maximum slope direction. Broadbase terraces are interspersed between vine rows to intercept surface runoff and convey it out of the field. Part of the sediment generated above these terraces is deposited in them and other parts are either deposited beyond the boundaries of the fields or are exported to the main drainage network. High intensity rainfall produces heavy soil losses (up to 207 Mg ha^− 1 computed in an extreme event in June 2000, which had a maximum intensity in 30-min periods of up to 170 mm h^− 1). To estimate the cost of erosion in vineyard fields of this region, two important aspects were considered. These were a) the cost incurred by the maintenance of the broadbase terraces, drainage channels and filling of ephemeral gullies and b) the cost incurred by the loss of fertilisers (mainly N and P) caused by erosion. According to farmers' records, the former was estimated at 7.5 tractor-hour ha^− 1 year^− 1 (as average), which comprises 5.4% of the income from grape sales. Regarding N and P losses, nutrients exported by runoff were 14.9 kg ha^− 1 N and 11.5 kg ha^− 1 of P, which, if compared to the annual intakes, represent 6% and 26.1% of the N and P respectively. In economic terms, the replacement value of the N and P lost represents 2.4% for N or 1.2% for P of the annual income from the sale of the grapes.     

The role of hydropedologic vegetation zones in greenhouse gas emissions for agricultural wetland landscapes

Net greenhouse gas (GHG) source strength for agricultural wetland ecosystems in the Prairie Pothole Region (PPR) is currently unknown. In particular, information is lacking to constrain spatial variability associated with GHG emissions (CH₄, CO₂, and N₂O). GHG fluxes typically vary with edaphic, hydrologic, biologic, and climatic factors. In the PPR, characteristic wetland plant communities integrate hydropedologic factors and may explain some variability associated with trace gas fluxes at ecosystem and landscape scales. We addressed this question for replicate wetland basins located in central North Dakota stratified by hydropedologic vegetation zone on Jul 12 and Aug 3, 2003. Data were collected at the soil-atmosphere interface for six plant zones: deep marsh, shallow marsh, wet meadow, low prairie, pasture, and cropland. Controlling for soil moisture and temperature, CH₄ fluxes varied significantly with zone (p < 0.05). Highest CH₄ emissions were found near the water in the deep marsh (277,800 μg m^− 2 d^− 1 CH₄), which declined with distance from water to − 730 μg m^− 2 d^− 1 CH₄ in the pasture. Carbon dioxide fluxes also varied significantly with zone. Nitrous oxide variability was greater within zones than between zones, with no significant effects of zone, moisture, or temperature. Data were extrapolated for a 205.6 km² landscape using a previously developed synoptic classification for PPR plant communities. For this landscape, we found croplands contributed the greatest proportion to the net GHG source strength on Jul 12 (45,700 kg d^− 1 GHG-C equivalents) while deep marsh zones contributed the greatest proportion on Aug 3 (26,145 kg d^− 1 GHG-C equivalents). This was driven by a 30-fold reduction in cropland N₂O–N emissions between dates. The overall landscape average for each date, weighted by zone, was 462.4 kg km^− 2 d^− 1 GHG-C equivalents on Jul 12 and 314.3 kg km^− 2 d^− 1 GHG-C equivalents on Aug 3. Results suggest GHG fluxes vary with hydropedologic soil zone, particularly for CH₄, and provide initial estimates of net GHG emissions for heterogeneous agricultural wetland landscapes.     

Impacts of soil tilth on the effectiveness of biological geotextiles in reducing runoff and interrill erosion

The effectiveness of a surface cover material (e.g. geotextiles, rock fragments, mulches, vegetation) in reducing runoff and soil erosion rates is often only assessed by the fraction of the soil surface covered. However, there are indications that soil structure has important effects on the runoff and erosion-reducing effectiveness of the cover materials. This study investigates the impact of soil pre-treatment (i.e. fine tilth versus sealed soil surface) on the effectiveness of biological geotextiles in increasing infiltration rates and in reducing runoff and interrill erosion rates on a medium and steep slope gradient. Rainfall was simulated during 60 min with an intensity of 67 mm h⁻¹ on an interrill erosion plot having two slope gradients (i.e. 15 and 45%) and filled with an erodible sandy loam. Five biological and three simulated geotextiles with different cover percentage were tested on two simulated initial soil conditions (i.e. fine tilth and sealed soil surface). Final infiltration rates on a sealed soil surface (7.5–18.5 mm h⁻¹) are observed after ca. 10 min of rainfall compared to ca. 50 min of rainfall on an initial seedbed (16.4–56.7 mm h⁻¹). On the two tested slope gradients, significantly (α = 0.05) smaller runoff coefficients (RC) are observed on an initial seedbed (8.2% < RC < 59.8%) compared to a sealed soil surface (75.7% < RC < 87.0%). On an initial seedbed, decreasing RC are observed with an increasing simulated geotextile cover. However, on an initial sealed soil surface no significant effect of simulated geotextile cover on RC is observed. On a 15% slope gradient, calculated b-values from the mulch factor equation equalled 0.054 for an initial fine tilth and 0.022 for a sealed soil surface, indicating a higher effectiveness of geotextiles in reducing interrill erosion on a fine tilth compared to a sealed soil surface. Therefore, this study demonstrates the importance of applying geotextiles on the soil surface before the surface tilth is sealed due to rainfall. The effect of soil structure on the effectiveness of a surface cover in reducing runoff and interrill erosion rates, as indicated by the results of this study, needs to be incorporated in soil erosion prediction models.     

N2O and NO fluxes between a Norway spruce forest soil and atmosphere as affected by prolonged summer drought

Global change scenarios predict an increasing frequency and duration of summer drought periods in Central Europe especially for higher elevation areas. Our current knowledge about the effects of soil drought on nitrogen trace gas fluxes from temperate forest soils is scarce. In this study, the effects of experimentally induced drought on soil N₂O and NO emissions were investigated in a mature Norway spruce forest in the Fichtelgebirge (northeastern Bavaria, Germany) in two consecutive years. Drought was induced by roof constructions over a period of 46 days. The experiment was run in three replicates and three non-manipulated plots served as controls. Additionally to the N₂O and NO flux measurements in weekly to monthly intervals, soil gas samples from six different soil depths were analysed in time series for N₂O concentration as well as isotope abundances to investigate N₂O dynamics within the soil. N₂O fluxes from soil to the atmosphere at the experimental plots decreased gradually during the drought period from 0.2 to −0.0 μmol m⁻² h⁻¹, respectively, and mean cumulative N₂O emissions from the manipulated plots were reduced by 43% during experimental drought compared to the controls in 2007. N₂O concentration as well as isotope abundance analysis along the soil profiles revealed that a major part of the soil acted as a net sink for N₂O, even during drought. This N₂O sink, together with diminished N₂O production in the organic layers, resulted in successively decreased N₂O fluxes during drought, and may even turn this forest soil into a net sink of atmospheric N₂O as observed in the first year of the experiment. Enhanced N₂O fluxes observed after rewetting up to 0.1 μmol m⁻² h⁻¹ were not able to compensate for the preceding drought effect. During the experiment in 2006, with soil matric potentials in 20 cm depth down to −630 hPa, cumulative NO emissions from the throughfall exclusion plots were reduced by 69% compared to the controls, whereas cumulative NO emissions from the experimental plots in 2007, with minimum soil matric potentials of −210 hPa, were 180% of those of the controls. Following wetting, the soil of the throughfall exclusion plots showed significantly larger NO fluxes compared to the controls (up to 9 μmol m⁻² h⁻¹ versus 2 μmol m⁻² h⁻¹). These fluxes were responsible for 44% of the total emission of NO throughout the whole course of the experiment. NO emissions from this forest soil usually exceeded N₂O emissions by one order of magnitude or more except during wintertime.     

Evaluation of ultrasonic aggregate stability and rainfall erosion resistance of disturbed and amended soils in the Lake Tahoe Basin, USA

Application of organic soil amendments to disturbed soil has been shown to improve aggregate stability and reduce soil susceptibility to erosion. Employing ultrasonic aggregate stability assessment techniques described earlier [Fristensky, A. and Grismer, M.E., 2008. A simultaneous model for ultrasonic aggregate stability assessment. Catena, 74: 153–164.], we assess the effect of two experimental organic soil amendments – a compost and a woodchip mulch incorporated at a rate of 2000–6000 kg ha^− 1 N-equivalence – on soil aggregation and aggregate stability at four drastically disturbed sites within the Lake Tahoe Basin, USA. Experimental plots were established 1–3 years prior to testing. The soils were of granitic or volcanic origin, and disturbed by either ski run or road development. Soil treatments were observed to significantly (p < 0.05) increase both aggregation (300% average increase) and ultrasonic aggregate stability (600% average increase) relative to the untreated soil. However, at the two sites disturbed by ski run development, the control treatment (tilling and surface application of pine–needle mulch) performed comparably to the two incorporated compost treatments, suggesting that the effects of the experimental amendments on aggregation were negligible at these sites, or their effective duration was shorter than the evaluation period.Rainfall simulations (72–120 mm h^− 1) were performed on the treatment plots, and results were compared with the ultrasonic aggregate stability indices. Significant (p < 0.05) positive correlations were obtained between the measurements of aggregate instability and indices of soil susceptibility to runoff, including steady-state infiltration rate (measured values between 1 and 120 mm h^− 1), and the level of kinetic energy of applied rainfall at which runoff commences (EBR, measured values between 12 and 224 J m^− 2). However, no correlation was found between the ultrasonic aggregate stability indices and observed soil erosion variables. Interestingly, positive relationships (p < 0.05) were observed between both infiltration rate and EBR and the proportion of 2–20 μm and < 2 μm particles liberated from the largest aggregates detected in each soil. Our results suggest that ultrasonic aggregate stability indices may be useful indicators of soil susceptibility to runoff and erosion under rainfall.     

Management of a previously eroded tropical Alfisol with herbaceous legumes: Soil loss and physical properties under mound tillage

A study was carried out on a previously eroded Oxic Paleustalf in Ibadan, southwestern Nigeria to determine the extent of soil degradation under mound tillage with some herbaceous legumes and residue management methods. A series of factorial experiments was carried out on 12 existing runoff plots. The study commenced in 1996 after a 5-year natural fallow. Mound tillage was introduced in 1997 till 1999. The legumes – Vigna unguiculata (cowpea), Mucuna pruriens and Pueraria phaseoloides – were intercropped with maize in 1996 and 1998 while yam was planted alone in 1997 and 1999. This paper covers 1997–1999. At the end of each year, residues were either burned or mulched on respective plots. Soil loss, runoff, variations in mound height, bulk density, soil water retention and sorptivity were measured. Cumulative runoff was similar among interactions of legume and residue management in 1997 (57–151 mm) and 1999 (206–397 mm). However, in 1998, cumulative runoff of 95 mm observed for Mucuna-burned residue was significantly greater than the 46 mm observed for cowpea-burned residue and the 39–51 mm observed for mulched residues of cowpea, Mucuna and Pueraria. Cumulative soil loss of 7.6 Mg ha⁻¹ observed for Mucuna-burned residue in 1997 was significantly greater than those for Pueraria-mulched (0.9 Mg ha⁻¹) and Mucuna-mulched (1.4 Mg ha⁻¹) residues whereas in 1999 it was similar to soil loss from cowpea treatments and Pueraria-burned residue (2.3–5.3 Mg ha⁻¹). There were no significant differences in soil loss in 1998 (1–3.2 Mg ha⁻¹) whereas Mucuna-burned residue had a greater soil loss (28.6 Mg ha⁻¹) than mulched cowpea (6.9 Mg ha⁻¹) and Pueraria (5.4 Mg ha⁻¹). Mound heights (23 cm average) decreased non-linearly with cumulative rainfall. A cumulative rainfall of 500 mm removed 0.3–2.3 cm of soil from mounds in 1997, 3.5–6.9 cm in 1998 and 2.3–4.6 cm in 1999, indicating that (detached but less transported) soil from mounds was far higher than observed soil loss in each year. Soil water retention was improved at potentials ranging from −1 to −1500 kPa by Mucuna-mulched residue compared to the various burned-residue treatments. Also, mound sorptivity at −1 cm water head (14.3 cm h^−1/2) was higher than furrow sorptivity (8.5 cm h^−1/2), indicating differences in hydraulic characteristics between mound and furrow. Pueraria-mulched residues for mounds had the highest sorptivity of 17.24 cm h^−1/2, whereas the least value of 6.96 cm h^−1/2 was observed in furrow of Mucuna-burned residue. Pueraria phas eoloides was considered the best option for soil conservation on the previously eroded soil, cultivated with mound tillage.     

Slope sediment yield in arid lowland continuous permafrost environments, Canadian Arctic Archipelago

Surface wash erosion was measured at runoff plots on low to moderate slopes in clayey and sandy silts underlain by continuous permafrost on the Fosheim Peninsula, Ellesmere Island. Due to snow redistribution in winter, total precipitation on the plots varied from 34 to 150 mm, with corresponding surface runoff values of 0 to 102 mm. Where runoff occurred, at least 80% of it was derived from snowmelt. Suspended sediment removal was <75 g m⁻² a⁻¹ at relatively well-vegetated sites but averaged more than 1200 g m⁻² a⁻¹ at a plot where the vegetation had been removed by landsliding. Niveo-aeolian deposition was greater than suspended sediment removal at some plots, indicating net accumulation. Solute removal ranged up to 80 g m⁻² a⁻¹ and exceeded clastic sediment transport at one vegetated site. Elevated rates of erosion at the sites of detachment slides that pre-date 1950 demonstrated that terrain disturbance in permafrost environments can affect slopewash processes for at least several decades.Plot data (precipitation, vegetation and surface grain-size) from the Fosheim Peninsula and Banks Island were used to develop a statistical model of suspended sediment removal by surface wash on undisturbed slopes. For any given grain-size, the model predicts a rise in erosion from zero precipitation (because of an absence of runoff) to a peak at about 50 mm, a decline as precipitation increases to 300 mm and a further increase in erosion beyond this inflection point. This non-linear response is due to the complex interaction of moisture (primarily snow) and vegetation cover. Erosion at any given precipitation value varies through three orders of magnitude depending on surface grain-size. The maximum erosion predicted is 1 kg m⁻² a⁻¹ for a runoff plot with 1100 mm of precipitation, a corresponding vegetation cover of 77% and a median surface grain-size of 7 φ.     

Assessing spatial variability of soil enzyme activities in pasture topsoils using geostatistics

The purpose of this study was to assess the spatial variability of the activity of three hydrolytic enzymes, i.e. urease activity (UAc), alkaline phosphatase activity (APAc), and arylsulfatase activity (ASAc), in pasture topsoils using geostatistics. Enzyme activities along a transect in a 1.35-ha pasture were determined using 77 soil samples from the upper 20 cm of soil. UAc varied from 101.0 to 182.7 μg N g⁻¹ soil h⁻¹; APAc varied from 1.56 to 3.62 μg p-nitrophenol g⁻¹ soil h⁻¹; and ASAc varied from 1.50 to 3.26 μg p-nitrophenol g⁻¹ soil h⁻¹. The linear models fit the best semivariogram models for UAc, APAc, and ASAc. Semivariograms for enzyme activities exhibited spatial dependence with ranges of influence of approximately 124.7 m.     

Soil and sediment properties affecting the accumulation of mercury in a flood control reservoir

Mercury accumulations in some fish species from Grenada Lake in north Mississippi exceed the Food and Drug Administration standards for human consumption. This large flood control reservoir serves as a sink for the Skuna and Yalobusha River watersheds whose highly erodible soils contribute to excessively high sediment yields and impaired water quality. This study was conducted to characterize the distribution of total Hg in watershed soils and determine the relationship between the easily transportable clay, organic C (OC), and Fe oxide fractions and the movement of Hg from upland sources to reservoir sinks. Cores were collected from soils, of different land-use, representative of the three soil orders (Alfisols, Entisols, and Vertisols) found in the watersheds. Sediment cores were collected from the Yalobusha River and Grenada Lake. In the laboratory, soil cores were sampled by horizon while sediment cores were sampled in 10 cm increments. These samples were characterized for total Hg, particle size distribution, OC, Fe oxide contents, and pH. Mercury concentrations ranged from 10 to 112 µg kg^− 1 in the soil profiles, with average regression coefficient (r²) values of 0.104, 0.362, and 0.06 for Hg versus clay, OC, and Fe oxides, respectively. River sediment cores had Hg concentrations ranging from 0 to 38 µg kg^− 1, and significant (1% level) r² values of 0.611, 0.447, and 0.632 versus clay, OC, and Fe oxides, respectively. Mercury concentrations in the lake sediment ranged from 0 to 125 µg kg^− 1. The r² values for Hg versus clay, OC, and Fe oxides in the lake sediment were 0.813, 0.499, and 0.805, respectively, all significant at the 1% level. These results indicate that total Hg is poorly correlated with the clay, OC, and Fe oxide fractions at depth in the soil profiles because maximum Hg concentrations occur in the surface horizons due to atmospheric in-fall. The statistically significant r² values for Hg versus these components in the sediment cores are the result of particulate clay, Fe oxides, and finely divided OC sorption of Hg from solution during the runoff and sediment transport process. The higher correlations for the lake sediment reflect an enrichment of the Hg-laden clay fraction relative to stream sediment through flocculation and sedimentation processes in the slack-water environment of the reservoir.     

Heterogeneity in soil hydrological response from different land cover types in southern Spain

This paper reports results from the analysis of the soil hydrological response to simulated rainfall in a cork oak forest in Los Alcornocales Natural Park (SW Spain). Four different soil/vegetation units were selected for the field experiments: [1] cork oak woodland, [2] heathland, [3] grassland, and [4] cork oak/olive tree mixed forest. Rainfall simulations tests were performed on circular plots of 1256.6 cm² at an intensity of 56.5 mm h^− 1 for 30 min.Marked differences in the hydrological behavior of the studied vegetation types were observed after the rainfall simulations. The soils under woodland showed low runoff rates and coefficients. The highest runoff rates were measured on the heath and grass-covered parts of the hillslope. Water repellency of the soil, measured from water drop penetration tests, reduced infiltration (especially under the heathland), and seems to be the cause of fast ponding and runoff generation during the first stages of rainstorms.The mosaic of different patterns of hydrological response to rainfall, such as runoff generation or infiltration, is governed by the spatial distribution of vegetation and its influence on the soil surface.     

Comparison of modeling approaches to quantify residue architecture effects on soil temperature and water

RZ-SHAW is a hybrid model, comprised of modules from the Simultaneous Heat and Water (SHAW) model integrated into the Root Zone Water Quality Model (RZWQM) that allows more detailed simulation of different residue types and architectures that affect heat and water transfer at the soil surface. RZ-SHAW allows different methods of surface energy flux evaluation to be used: (1) the SHAW module, where evapotranspiration (ET) and soil heat flux are computed in concert with a detailed surface energy balance; (2) the Shuttleworth–Wallace (S–W) module for ET in which soil surface temperature is assumed equal air temperature; and (3) the PENFLUX module, which uses a Penman transformation for a soil slab under incomplete residue cover. The objective of this study was to compare the predictive accuracy of the three RZ-SHAW modules to simulate effects of residue architecture on net radiation, soil temperature, and water dynamics near the soil surface. The model was tested in Akron, Colorado in a wheat residue-covered (both standing and flat) no-till (NT) plot, and a reduced till (RT) plot where wheat residue was incorporated into the soil. Temperature difference between the soil surface and ambient air frequently exceeded 17 °C under RT and NT conditions, invalidating the isothermal assumption employed in the S–W module. The S–W module overestimated net radiation (R_n) by an average of 69 Wm⁻² and underestimated the 3-cm soil temperature (T_s3) by 2.7 °C for the RT plot, attributed to consequences of the isothermal assumption. Both SHAW and PENFLUX modules overestimated midday T_s3 for RT conditions but underestimated T_s3 for NT conditions. Better performances of the SHAW and PENFLUX surface energy evaluations are to be expected as both approaches are more detailed and consider a more discretized domain than the S–W module. PENFLUX simulated net radiation slightly better than the SHAW module for both plots, while T_s3 was simulated the best by SHAW, with a mean bias error of +0.1 °C for NT and +2.7 °C for RT. Simulation results for soil water content in the surface 30 cm (θ_v30) were mixed. The NT conditions were simulated best by SHAW, with mean bias error for θ_v30 within 0.006 m³ m⁻³; RT conditions were simulated best by the PENFLUX module, which was within 0.010 m³ m⁻³.     

Modelling multi-year coupled carbon and water fluxes in a boreal aspen forest

In order to test two hypotheses: (i) that carbon (C) and energy exchanges between terrestrial ecosystems and the atmosphere are closely constrained by soil water availability, and (ii) that vegetation is able to optimize soil water uptake from different soil layers; two model simulations were conducted. The Boreal Ecosystem Productivity Simulator (BEPS) model was run to simulate an aspen forest in Saskatchewan, Canada during the period 1997–2004. In Simulation 1, the effect of soil water availability in different soil layers on stomatal conductance was weighted only by root fraction. In Simulation 2, the influence of soil water availability in different soil layers on stomatal conductance was weighted according to both the root fraction and soil water availability, in order to allow easier access of roots to soil layers containing more water.Comparison against measured fluxes showed that Simulation 2 was an improvement over Simulation 1 in predicting C, water and energy fluxes at different time scales in dry years. In Simulation 1, the daytime C and water fluxes were underestimated during the transition from adequate to insufficient soil water content in the upper layers. In this run, the model captured 92, 79 and 91% of the daily variances in gross primary productivity (GPP), net ecosystem productivity (NEP), and ecosystem respiration (R_e) during 1997–2004. In Simulation 2, the daily variances of GPP, NEP, and R_e explained by the model increased to 93, 82 and 92%, respectively. In Simulation 1, the annual NEP was considerably underestimated in the dry years and years with dry periods, with a root mean square error (RMSE) of 45 g C m⁻² year⁻¹ (n = 8) from 1997 to 2004. In Simulation 2, the RMSE value of simulated annual NEP was reduced to 14 g C m⁻² year⁻¹, a relatively small value compared with the average NEP of 157 g C m⁻² year⁻¹ during 1997–2004. This suggested that the ability of plant roots to extract water from deep soil layers is critical for the forest to maintain growth when surface layers dried out. Our model results showed that NEP was very sensitive to water conditions at this site. In wet years, heterotrophic respiration was enhanced and NEP was reduced.