Jan Jacob Keizer

Extraction of compounds associated with water repellency in sandy soils of different origin

After an initial evaluation of several solvents, the efficiency of Soxhlet extractions with isopropanol/ammonia (s.g. 0.88) (70 : 30 v : v; 24 h) in extracting compounds associated with water repellency in sandy soils was examined using a range of repellent and wettable control soils (n = 15 and 4) from Australia, Greece, Portugal, The Netherlands, and the UK. Extraction efficiency and the role of the extracts in causing soil water repellency was examined by determining extract mass, sample organic carbon content and water repellency (after drying at 20 ◦ C and 105 ◦ C) pre- and post-extraction, and amounts of aliphatic C-H removed using DRIFT, and by assessing the ability of extracts to cause repellency in acid-washed sand (AWS). Key findings are: (i) none of organic carbon content, amount of aliphatic C-H, or amount of material extracted give any significant correlation with repellency for this diverse range of soils; (ii) sample drying at 105 ◦ Ci s not necessarily useful before extraction, but may provide additional information on extraction effectiveness when used after extraction; (iii) the extraction removed repellency completely from 13 of the 15 repellent samples; (iv) extracts from all repellent and wettable control soils were capable of inducing repellency in AWS. The findings suggest that compounds responsible for repellency represent only a fraction of the extract composition and that their presence does not necessarily always cause repellency.

Polyacrylamide application versus forest residue mulching for reducing post-fire runoff and soil erosion

For several years now, forest fires have been known to increase overland flow and soil erosion. However, mitigation of these effects has been little studied, especially outside the USA. This study aimed to quantify the effectiveness of two so-called emergency treatments to reduce post-fire runoff and soil losses at the microplot scale in a eucalyptus plantation in north-central Portugal. The treatments involved the application of chopped eucalyptus bark mulch at a rate of 10-12 Mg ha(-1), and surface application of a dry, granular, anionic polyacrylamide (PAM) at a rate of 50 kg ha(-1). During the first year after a wildfire in 2010, 1419 mm of rainfall produced, on average, 785 mm of overland flow in the untreated plots and 8.4 Mg ha(-1) of soil losses. Mulching reduced these two figures significantly, by an average 52 and 93%, respectively. In contrast, the PAM-treated plots did not differ from the control plots, despite slightly lower runoff but higher soil erosion figures. When compared to the control plots, mean key factors for runoff and soil erosion were different in the case of the mulched but not the PAM plots. Notably, the plots on the lower half of the slope registered bigger runoff and erosion figures than those on the upper half of the slope. This could be explained by differences in fire intensity and, ultimately, in pre-fire standing biomass.

Overland flow generation processes, erosion yields and solute loss following different intensity fires

Fire induces important changes in vegetation and soil structure, which can have major impacts on overland flow generation processes, runoff amounts and erosion yields. This paper presents the results of an investigation into the impact of different fire intensities on water, sediment and solute yields. Wildfire, prescribed fire and experimental fire impacts were investigated at two scales: (1) a temporary circular bounded plot of 0.24 m2 (rainfall simulation plot); (2) a permanent bounded runoff plot of 16 m2 (natural rainfall conditions). In addition, the spatial patterns of the soil water repellency, vegetation, litter cover and stone cover for each land use and fire intensity were assessed using a transect system.The results of this study show a significant difference in the spatial distribution patterns of soil water repellency. Wildfire sites were characterized by more intense and by more spatially contiguous repellency than the experimental and prescribed fire sites. These differences have important effects on overland flow generation processes and runoff amounts. At the prescribed and experimental fire sites, the discontinuous distribution of repellency provides greater potential for water to infiltrate via non-repellent soils, whereas there are far fewer infiltration pathways at the wildfire sites because of their spatially contiguous repellency. This spatial discontinuity (alternation between hydrophobic and hydrophilic soil patches) is therefore responsible for the lower overland flow amounts, smaller erosion rates and nutrient yields at broader scales, when compared with the wildfire burned areas where soil water repellence is more intense and spatially contiguously distributed.

Post-fire overland flow generation and inter-rill erosion under simulated rainfall in two eucalypt stands in north-central Portugal ☆

The aim of this study was to improve the existing knowledge of the runoff and inter-rill erosion response of forest stands following wildfire, focusing on commercial eucalypt plantations and employing field rainfall simulation experiments (RSEs). Repeated RSEs were carried out in two adjacent but contrasting eucalypt stands on steep hill slopes in north-central Portugal that suffered a moderate severity fire in July 2005. This was done at six occasions ranging from 3 to 24 months after the fire and using a paired-plot experimental design that comprised two pairs of RSEs at each site and occasion. Of the 46 RSEs: (i) 24 and 22 RSEs involved application rates of 45-50 and 80-85 mm h(-1), respectively; (ii) 22 took place in a stand that had been ploughed in down slope direction several years before the wildfire and 24 in an unploughed stand. The results showed a clear tendency for extreme-intensity RSEs to produce higher runoff amounts and greater soil and organic matter losses than the simultaneous high-intensity RSEs on the neighbouring plots. However, there existed marked exceptions, both in space (for one of the plot pairs) and time (under intermediate soil water repellency conditions). Also, overland flow generation and erosion varied significantly between the various field campaigns. This temporal pattern markedly differed from a straightforward decline with time-after-fire and rather suggested a seasonal component, reflecting broad variations in topsoil water repellency. The ploughed site produced less runoff and erosion than the unploughed site, contrary to what would be expected if the down slope ploughing had occurred after the wildfire instead of several years before it. Finally, sediment losses at both study sites were noticeably lower than those reported by other studies involving repeat RSEs, i.e. in Australia and western Spain. This possibly reflected a history of intensive land use in the study region, including in more recent times after the widespread introduction of eucalypt plantations.

Impacts of climate and land use changes on the hydrological and erosion processes of two contrasting Mediterranean catchments.

The impacts of climate and land use changes on streamflow and sediment export were evaluated for a humid (São Lourenço) and a dry (Guadalupe) Mediterranean catchment, using the SWAT model. SWAT was able to produce viable streamflow and sediment export simulations for both catchments, which provided a baseline for investigating climate and land use changes under the A1B and B1 emission scenarios for 2071-2100. Compared to the baseline scenario (1971-2000), climate change scenarios showed a decrease in annual rainfall for both catchments (humid: -12%; dry: -8%), together with strong increases in rainfall during winter. Land use changes were derived from a socio-economic storyline in which traditional agriculture is replaced by more profitable land uses (i.e. corn and commercial forestry at the humid site; sunflower at the dry site). Climate change projections showed a decrease in streamflow for both catchments, whereas sediment export decreased only for the São Lourenço catchment. Land use changes resulted in an increase in streamflow, but the erosive response differed between catchments. The combination of climate and land use change scenarios led to a reduction in streamflow for both catchments, suggesting a domain of the climatic response. As for sediments, contrasting results were observed for the humid (A1B: -29%; B1: -22%) and dry catchment (A1B: +222%; B1: +5%), which is mainly due to differences in the present-day and forecasted vegetation types. The results highlight the importance of climate-induced land-use change impacts, which could be similar to or more severe than the direct impacts of climate change alone.

The role of soil water repellency in overland flow generation in pine and eucalypt forest stands in coastal Portugal

Soil water repellency is now known to occur in diverse soils in various parts of the world. One of the possible adverse effects of soil water repellency is that it can reduce infiltration capacity and hence, on sloping terrain, enhance overland flow and soil erosion. The main aim of the present work is to assess the effects of soil water repellency on surface runoff production in the inner coastal dune areas of central Portugal. This was done for a pine and a eucalypt forest stand and, within each stand, for 2 slopes with contrasting aspect and somewhat different slope angles. Overland flow was measured for 4 pairs of unbounded plots of about 5 m2 at fortnightly intervals from February to October 2001. Over the same period, soil water repellency at and immediately below the soil surface was measured next to the plots at monthly intervals. The runoff–repellency relationship was also studied by carrying out rainfall simulation experiments on 0.24-m2 plots and associated repellency measurements. The effect of soil water repellency was most clearly demonstrated by statistically significant higher runoff coefficients under strong-to-extremely than under none-to-slightly hydrophobic conditions immediately below the soil surface. Such a difference in runoff over the measurement period was, however, restricted to 2 unbounded plots, both of which were located on the eucalypt slope with a southerly aspect and the greater slope angle. At the scale of these plots, the increase in runoff coefficient due to soil water repellency is moderate, when integrated over the entire period of strong–extremely repellent conditions, but can be quite substantial for individual 2-weekly periods. With respect to the observed differences in runoff between plots, be it plots on the same slope or not, it has proved difficult to distinguish the effect of soil water repellency from that of other factors likely to affect overland flow generation.

Runoff and inter-rill erosion in a Maritime Pine and a Eucalypt plantation following wildfire and terracing in north-central Portugal

Abstract The purpose of this study was to assess how terracing affected overland flow and associated sediment losses, at the micro-plot scale (0.25 m2), in recently burnt stands of the two principal forest types in north-central Portugal, i.e. mono-specific stands of Maritime Pine and Eucalypt. Terracing is an increasingly common practice of slope engineering in the study region but its impacts on runoff and erosion are poorly studied. Non-terraced plots at the Eucalypt and the Pine site revealed similar median runoff coefficients (rc: 20-30%) as well as comparable median sediment losses (15-25 g m-2) during the first seven months following wildfire. During the ensuing, slightly wetter 18-month period, however, non-terraced plots at the Pine site lost noticeably more sediments (in median, 90 vs. 18 g m-2), in spite the runoff response had remained basically the same (median rc: 33 vs. 28%). By contrast, terraced plots at the same Pine site lost hugely more sediments (in median, 1,200 g m-2) during this 18-month period. Terraced plots at the Eucalypt site even lost three times more sediments (in median, 3,600 g m-2). Ground cover and resistance to shear stress seemed to be key factors in the observed/inferred impacts of terracing.

Soil water repellency under dry and wet antecedent weather conditions for selected land-cover types in the coastal zone of central Portugal

This paper reports on the first systematic inventory of soil water repellency in Portuguese coastal dune sand areas. Since water repellency is widely associated with certain vegetation types or individual plant species, this inventory concerned arable land as well as 6 natural and semi-natural land-cover types representative for the vegetation zonation in the study area. Since water repellency further is a feature that commonly varies through time, disappearing when soils become wet, initial sampling was carried out during late summer 2000 and later repeated, at 1 of the 2 sites per land-cover type, during early spring 2001. Water repellency was principally measured in the field using the Molarity of an Ethanol Droplet (MED) test. Under the dry summer conditions, water repellency was a widespread phenomenon at and immediately below, the soil surface and numerous significant differences in ethanol classes existed between the land-cover types. The transient nature of water repellency was confirmed by many instances of significantly lower spring than summer ethanol classes. These significant differences were in general accompanied by a significant negative correlation of the summer and spring ethanol classes with volumetric soil moisture content. The sites’ overall repellency levels under dry antecedent weather conditions were significantly correlated with their overall levels of soil organic matter.

The impact of soil water repellency on soil hydrological and erosional processes under Eucalyptus and evergreen Quercus forests in the Western Mediterranean

Forest areas of the Mediterranean regions of Portugal, Morocco, and Tunisia are suffering major land use changes, with the replacement of traditional evergreen Quercus species (i.e. Quercus suber and Quercus ilex) by fast-growing Eucalyptus species. Since Eucalyptus species are amongst those with a higher impact on soil water repellency, this study examined the effect of the replacement on soil properties, water repellency, and on soil hydrological processes and erosion rates. Measurements were performed in areas that correspond to the climatic distribution of evergreen Quercus suber: at Macao and Portel in Portugal; Ben Slimane in Morocco; and Cap Bon, Sousse, and Ain Snoussi in Tunisia. Soil superficial characteristics, including vegetation and litter cover, organic matter content, soil compaction and shear strength, and water repellency were measured for evergreen oak and Eucalyptus stands and related to soil erosion rates and soil hydrological processes. The data are based on the spatial distribution of properties assessed through the use of intensive spatial sampling and on rainfall simulations to address soil hydrological and erosional processes. The results show very different wetting patterns for some of the Eucalyptus stands during dry and moist periods, as a result of strong hydrophobic characteristics following dry spells. Nevertheless, the Eucalyptus stands in semi-arid climate show no sign of water repellency, which contradicts the theory that water repellency is purely a result of dry conditions. The experiments show no significant increases on overland flow amounts and erosion rates as direct result of soil water repellence (hydrophobicity) characteristics.

Mid-term and scaling effects of forest residue mulching on post-fire runoff and soil erosion

Mulching is an effective post-fire soil erosion mitigation treatment. Experiments with forest residue mulch have demonstrated that it increased ground cover to 70% and reduced runoff and soil loss at small spatial scales and for short post-fire periods. However, no studies have systematically assessed the joint effects of scale, time since burning, and mulching on runoff, soil loss, and organic matter loss. The objective of this study was to evaluate the effects of scale and forest residue mulch using 0.25m2 micro-plots and 100m2 slope-scale plots in a burnt eucalypt plantation in central Portugal. We assessed the underlying processes involved in the post-fire hydrologic and erosive responses, particularly the effects of soil moisture and soil water repellency. Runoff amount in the micro-plots was more than ten-fold the runoff in the larger slope-scale plots in the first year and decreased to eight-fold in the third post-fire year. Soil losses in the micro-plots were initially about twice the values in the slope-scale plots and this ratio increased over time. The mulch greatly reduced the cumulative soil loss measured in the untreated slope-scale plots (616gm-2) by 91% during the five post-fire years. The implications are that applying forest residue mulch immediately after a wildfire can reduce soil losses at spatial scales of interest to land managers throughout the expected post-fire window of disturbance, and that mulching resulted in a substantial relative gain in soil organic matter.