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Recent European Common Agricultural Policy (CAP) forces Flemish farmers in the Belgian loess belt to combat soil erosion on high erosion sensitive parcels. A possible measure against erosion is the application of reduced tillage operations. To evaluate the efficiency of reduced tillage practices, erosion rates of a maize field under conventional tillage, non-inversion tillage and strip-till were compared, all with and without wheel track compaction. To assess erosion rates, 20 min rainfall simulations in combination with fallout radionuclide (FRN) 7Be measurements were used. At the small plot scale (5 m²), runoff coefficient, suspended sediment concentration and soil loss were measured directly during rainfall simulation in order to compare the effect of the different tillage treatments and the effect of wheel track compaction. After the rainfall simulations soil samples were taken for 7Be analysis to explore spatial patterns in soil redistribution within the different plots. Furthermore, a total soil redistribution budget was obtained based on the 7Be measurements and validated with the direct erosion measurements during the rainfall simulations. Direct measurements showed a significant decrease in runoff coefficient, suspended sediment concentration and total soil loss in the non-inversion and strip-till treatments compared to the conventional tillage treatment. No difference was observed between the non-inversion and strip-till treatment. For all treatments, wheel track compaction increased runoff coefficients and soil loss. The 7Be measurements confirmed overall trends observed during the rainfall simulations, with high soil losses in the conventional treatment (5.96 ± 1.37 kg–6.23 ± 2.36 kg) and a decrease in soil loss with the non-inversion (1.50 ± 0.34 kg–1.95 ± 0.54 kg) and the strip-till treatment (−0.19 ± 0.60 kg–0.10 ± 2.50 kg). However, 7Be-derived net erosion estimates overestimated total soil loss per plot compared to the direct measurements. The appropriateness of correction factors, like particle size correction and variations in relaxation mass depth, to improve the tracer based assessment of absolute soil loss values was evaluated. Spatial patterns in soil redistribution clearly reflected the sediment and runoff buffering capacity of the inter-row area in the strip-till treatment, while higher erosion rates were observed in the plant rows. Yet, sample representativeness to construct the soil erosion budget is a key consideration in light of discrepancies between tracer and rainfall simulation results. The different rainfall simulations support the preference of non-inversion tillage and strip-till over conventional tillage in order to reduce soil loss in the Belgian loess belt. The increased soil loss due to wheel track compaction implies the need to till favorable soil moisture conditions to avoid soil compaction.

The natural fallout radionuclide 7Be is used as a tracer allowing estimates of soil redistribution on an event-based time scale. The observed 7Be inventory in the soil is converted to soil redistribution by comparing the 7Be inventory at a stable reference site to the location of interest and taking the 7Be depth distribution in the surface soil into account. The relaxation mass depth (h0), describing this depth distribution, is assumed to be uniform across the study area, whereas the 7Be inventory at the reference site represents the balance of atmospheric 7Be input and radioactive decay. The 7Be reference inventory should, therefore, not be influenced by soil redistribution or by any variation in physico-chemical characteristics of the soil within the entire study area. Most studies to date use 7Be to monitor soil redistribution on the spatial scale of a hillslope. However, if the assumptions of spatially uniform fallout and rapid and irreversible sorption of 7Be to soil particles can be extended over a larger area, 7Be could be used to monitor soil redistribution at the catchment scale. In this paper, the variability in 7Be distribution in the soil at hillslope and catchment scales is explored and possible sources of this variability are identified. To assess the impact of variability in soil hydraulic conductivity on the depth penetration of 7Be in surface soil, a rainfall simulation experiment with 9Be spiked rainfall was performed on artificially compacted soil cores. These rainfall simulation experiments indicated a significant positive correlation between the saturated hydraulic conductivity (Ksat) and the relaxation mass depth and, thus, demonstrated that the assumption of a spatially constant relaxation mass depth is likely to be invalid. An empirical correction factor is proposed to circumvent this problem. This work demonstrates the importance of assessing variability in soil hydrological properties across a study area and is also relevant to studies concerning the vertical transport of fallout contaminants in surface soil. To assess spatial variability in fallout across a catchment and across soil types, soil trays containing different soil types were placed at three reference locations in a small catchment of 8 km2 across a nine month period. The spatial variation in 7Be reference inventory between the sites in the catchment was not larger than the variation within one reference site (37% and 36% respectively), indicating that the uncertainty on the reference inventory will be similar over the small catchment. However, the different soil types displayed diverse 7Be depth profiles and total 7Be inventories, suggesting that a clear understanding of sorption behavior across the soil types present in a catchment is needed prior to the use of 7Be as a catchment-scale sediment tracer.

Beryllium-7 (7Be) has been used as a sediment tracer to evaluate soil redistribution rates at hillslopes and as a tool to estimate sediment residence time in river systems. A key assumption for the use of 7Be as a sediment tracer is the rapid and irreversible sorption of 7Be upon contact with the soil particles. However, recent studies have raised questions about the validity of these assumptions. Seven soil types were selected to assess the adsorption rate of 7Be on the soil particles, subsequently an extraction experiment was performed to assess the rate of desorption. Next, different treatments were applied to assess the impact of soil pH, fertilizer, humic acid and organic matter on the adsorption of Be. Finally, the influence of regularly occurring cations present on the soil complex on the adsorption of Be on pure clay minerals was evaluated. The adsorption rate experiment showed a rapid and nearly complete sorption of Be for Luvisols and Cambisols under agriculture. For a temperate climate Stagnosol under forest and two highly weathered tropical Ferralsols sorption of Be was less rapid and less complete. This may result in an incomplete adsorption of 7Be on these three soils when runoff initiates, which could lead to an overestimation of erosion rates and sediment residence time. Additional observations were made during the extraction experiment, showing a significant loss of Be from the forest Stagnosol and a stable binding of Be to the arable soils. Of the different treatments applied, only pH showed to be of influence. Finally, Ca2+ and NH4+ on the soil complex had only a limited effect on the adsorption of Be, while Al3+ in combination with a low pH inhibits the adsorption of Be on the exchange complex of the pure clay minerals. All these findings more rigorously support the use of 7Be as a soil redistribution tracer in arable soils in a temperate climate at a hillslope scale. The use of 7Be in highly weathered Ferralsols or forest rich environments should be limited to avoid overestimations of erosion rates. The spatially extended use of 7Be to evaluate residence times of sediments should be avoided in catchments with rapid changing environmental parameters as they might influence the sorption behavior of 7Be.

There is growing interest in the application of the natural fallout radionuclide 7 Be as a soil erosion and sediment tracer. Development of robust datasets is, however, hampered by unquantified variability in its vertical distribution within surface soil. Models that convert 7 Be inventory measurements to soil erosion estimates are all based on the observed depth distribution of 7 Be, described by the relaxation mass depth (h 0 ) parameter. Previous work, however, has not considered potential spatial variation in h 0 linked to natural variability in soil physical properties, which could have major implications for the reliability of soil erosion estimates. Two complementary experiments were designed to study the variability in depth distribution within and between potential reference sites. First, a field sampling programme was carried out whereby two reference sites with variable degree of compaction were sampled using two different sectioning techniques, i.e. by use of a fine increment soil collector (FISC) and the scraping methodology. During a laboratory rainfall simulation experiment, water spiked with stable 9 Be was used to study the variability in 9 Be soil depth distribution within and between the two reference sites. In the field experiment, variations in the 7 Be depth distribution, and thus in h 0 , were limited between both reference sites (13 to 16%). In contrast, the impact of the sectioning technique was remarkable, with scraping resulting in a higher h 0 (up to 60%) compared to the estimates based on the use of a FISC. The rainfall simulation experiment offered the opportunity to study the variation in 9 Be depth distribution in more detail. With an average h 0 of 4.66 kg m − 2 , 9 Be penetrated deeper in the non-compacted (NC) reference site cores, while the compacted (C) cores showed an average h 0 of 2.42 kg m − 2 . The reported h 0 values at the former site were also characterized by a larger coefficient of variation (24%) than those at the latter site (11%). Lower bulk density, higher infiltration rates and a pore network characterized by a higher macroporosity and connectivity, as revealed by the X-ray Computed Tomography (CT) scans, explained the deeper penetration of Be into the topsoil of reference site NC. The results indicate the importance of selecting appropriate reference sites and for ensuring an adequate sampling strategy to encompass local variability in soil physical properties. Hydraulic conductivity assessment could be a useful tool to properly assess suitable reference sites and the number of samples needed to assess the reference inventory.

Field water flow processes can be precisely delineated with proper sets of soil hydraulic properties derived from in situ and/or laboratory experiments. In this study we analyzed and compared soil hydraulic properties obtained by traditional laboratory experiments and inverse optimization tension infiltrometer data along the vertical direction within two typical Podzol profiles with sand texture in a potato field. The main goal was to identify proper sets of hydraulic parameters and to evaluate their relevance on hydrological model performance for irrigation management purposes. Tension disc infiltration experiments were carried out at four and five different depths for both profiles at consecutive negative pressure heads of 12, 6, 3 and 0.1 cm. At the same locations and depths undisturbed samples were taken to determine Mualem-van Genuchten (MVG) hydraulic parameters (theta(r), residual water content, theta(s), saturated water content, alpha and n, shape parameters and K-ls, saturated hydraulic conductivity) in the laboratory. Results demonstrated horizontal differences and vertical variability of hydraulic properties. The tension disc infiltration data fitted well in inverse modeling using Hydrus 2D/3D in combination with final water content at the end of the experiment, theta(f). Four MVG parameters (theta(s), alpha, n and field saturated hydraulic conductivity K-fs) were estimated (theta(r) set to zero), with estimated K-ls and alpha values being relatively similar to values from Wooding's solution which used as initial value and estimated theta(s) corresponded to (effective) field saturated water content, theta(f). The laboratory measurement of K-ls yielded 2-30 times higher values than the field method K-fs from top to subsoil layers, while there was a significant correlation between both K-s values (r = 0.75). We found significant differences of MVG parameters theta(s), n and alpha values between laboratory and field measurements, but again a significant correlation was observed between laboratory and field MVG parameters namely K-s, n, theta(s) (r >= 0.59). Assessment of the parameter relevance in 1-D model simulations, illustrated that the model over predicted and under predicted top soil-water content using laboratory and field experiments data sets respectively. The field MVG parameter data set resulted in better agreement to observed soil-water content as compared to the laboratory data set at nodes 10 and 20 cm. However, better simulation results were achieved using the laboratory data set at 30-60 cm depths. Results of our study do not confirm whether laboratory or field experiments data sets are most appropriate to predict soil water fluctuations in a complete soil profile, while field experiments are preferred in many studies. (C) 2016 Elsevier B.V. All rights reserved.

Biochar is often proposed to increase soil quality and crop yield, while sequestering carbon. Despite the growing number of studies in temperate regions, the claimed positive effects are still unsure for northwestern European soils. Moreover, there is a need to upscale results from lab and pot studies in these soil types to field experiments. The objectives of this study were therefore to investigate the effect of biochar application to a temperate agricultural soil on soil chemical, physical and biological properties, and on crop growth and nutrient uptake under field circumstances. A field trial, located in Merelbeke (Belgium), was established in October 2011 and monitored until August 2013. The biochar applied was produced from a mixture of hard- and softwood at 480 °C. The biochar dose was 0 (control) or 20 t ha−1 (on dry weight basis). Over two years, biochar addition to soil did not affect soil chemical properties, except for organic carbon content and C:N ratios. Effects on bulk density, porosity and soil water retention curves were non-consistent over time, possibly due to interaction with tillage operations. Biochar increased soil water content in 2012, although mostly not significantly. However, in 2013, when soil water content was overall lower compared to 2012, it was not affected by biochar addition. Soil temperature, as measured at a soil depth interval of 8–20 cm, was not changed by biochar addition. Furthermore, biochar addition to soil did only slightly influence soil microbiological community structure during the first year after biochar application, as only certain bacterial biomarker PLFAs were significantly affected by biochar addition, but no fungal biomarker PLFAs. Hence, it was not surprising that biochar addition did not affect crop yield, N or P uptake during the first two years after biochar application.

In situ water and soil conservation (WSC) practices are a promising intervention to improve rainwater management particularly in the semi-arid to dry sub-humid tropics. This study applies a fully coupled surface–subsurface process-based model (HydroGeoSphere) to simulate in detail rainwater partitioning as affected by two in situ WSC practices [terwah+ (TER+) and derdero+ (DER+)] currently under study on Vertisols in Tigray, Ethiopia and to evaluate the treatments in terms of rainwater partitioning. In the TER+ practice, contour furrows of 0·2 m wide and 0·1 m deep are created at 1·5 m intervals between permanent broad beds, whereas in DER+, permanent raised beds 0·6 m wide with furrows 0·2 m wide and 0·1 m deep are created, to minimize runoff and water logging. The model accurately reproduced measured surface runoff (e.g. in DER+: Nash–Sutcliffe model efficiency E = 0·6 for calibration and 0·7 for verification) and soil moisture content (DER+: E = 0·6 for calibration and 0·8 for verification). Runoff depth was lowest under DER+ (50 mm) followed by TER+ (67 mm) and significantly higher in conventional tillage (CT) (160 mm). Simulated transpiration, evaporation and drainage out of the root zone were all higher under DER+ and TER+ compared with CT. The effects of DER+ and TER+ practices on rainwater partitioning were more pronounced in wet years than in dry years. The model proved to be a promising and versatile tool to assess the impact of WSC practices on rainwater partitioning at the field scale. Copyright © 2014 John Wiley & Sons, Ltd.