Fire impact on soil-water repellency and functioning of ...

Geomorphology 280 (2017) 67–75

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Fire impact on soil-water repellency and functioning of semi-arid croplands and rangelands: Implications for prescribed burnings and wildfires Ilan Stavi a,⁎, Daniel Barkai b, Yaakov M. Knoll b, Hiam Abu Glion b, Itzhak Katra c, Anna Brook d, Eli Zaady b a

Dead Sea and Arava Science Center, 88820 Yotvata, Israel Department of Natural Resources, Agricultural Research Organization, Gilat Research Center, 85280 Negev, Israel c Department of Geography and Environmental Development, Ben-Gurion University of the Negev, 84105 Beer Sheva, Israel d Department of Geography and Environmental Studies, University of Haifa, 3498838 Haifa, Israel b

a r t i c l e

i n f o

Article history: Received 27 December 2015 Received in revised form 24 November 2016 Accepted 14 December 2016 Available online 16 December 2016 Keywords: Environmental pollution Mixed farming systems Nutrient availability Soil quality

a b s t r a c t An unintended fire outbreak during summer 2015 in the semi-arid Israeli Negev resulted in the burning of extensive croplands and rangelands. The rangelands have been managed over the long term for occasional grazing, while the croplands have been utilized for rainfed wheat cropping. Yet, during the studied year, the croplands were left fallow, allowing the growth of herbaceous vegetation, which was harvested and baled for hay before the fire outbreak. The study objectives were to investigate the impacts of fire, land-use, and soil depth on water-repellency and on the status and dynamics of some of the most important organic and mineral soil resources. Additionally, we aimed to assess the severity of this fire outbreak. The soil-water repellency was studied by measuring the soil's water drop penetration time (WDPT) and critical surface tension (CST). A significant effect of fire on soil hydrophobicity was recorded, with a slight increase in mean WDPT and a slight decrease in mean CST in the burnt sites than in the non-burnt sites. Yet, soil hydrophobicity in the burnt lands was rather moderate and remained within the water repellency's lowest class. A significant effect of land-use on the means of WDPT and CST was also recorded, being eleven-fold greater and 7% smaller, respectively, in the rangelands than in the croplands. This is consistent with the almost eightfold greater mean above-ground biomass recorded in the non-burnt rangelands than in the non-burnt post-harvest croplands, revealing the positive relations between available fuel load and soil-water repellency. The effect of soil depth was significant for CST but not for WDPT. Overall, the gathered data suggest that fire severity was low to moderate. Fire was also found to significantly affect the b250 μm particle size fraction of the unconsolidated material cover, its mass being twofold to threefold greater in the non-burnt than in the burnt sites. Yet, soil organic carbon and ammonium-N were also studied, and generally showed higher values for the burnt lands. Overall, this study shows that the low- to moderate-fire severity only slightly increased the soil water repellency, and at the same time, increased on-site availability of some important soil resources. Nevertheless, it is acknowledged that such fires could impose risks to off-site air and water source quality. This study has implications for the assessment of geo-ecosystem functioning, as well as for the status and dynamics of soil resources following prescribed burnings or wildfires. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Controlled burnings of farmland crop residue or rangeland aboveground biomass have been prevalent as a common management practice in extensive areas around the world (Brooks et al., 2006). The

⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (I. Stavi), [email protected] (D. Barkai), [email protected] (Y.M. Knoll), [email protected] (H.A. Glion), [email protected] (I. Katra), [email protected] (A. Brook), [email protected] (E. Zaady).

http://dx.doi.org/10.1016/j.geomorph.2016.12.015 0169-555X/© 2016 Elsevier B.V. All rights reserved.

objectives of prescribed burnings are varied, ranging between the control of weeds (Koski et al., 2011) and pests in croplands (DeFrancesco and Murray, 2011), the control of woody vegetation and other invasive plant species in grasslands and chaparral rangelands (Veach et al., 2014), and as means to control wildfire in shrublands and tree plantations (Shakesby et al., 2015). However, while possibly fulfilling these aims, similar to wildfires, burning management practices can have some detrimental impacts on soils, These include the uneven wetting patterns of the soil profile, the development of preferential flow, the decreased availability of water for vegetation, and the increased susceptibility to overland flow generation and soil erosion (Doerr et al., 2000; Bachmann et al., 2002).

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I. Stavi et al. / Geomorphology 280 (2017) 67–75

These effects have been widely attributed to fire-induced soil-water repellency (DeBano and Krammes, 1966), which stems from the decreased mutual attraction forces (adhesion) between the water and solid surfaces (Doerr et al., 2000). The soil-water repellency feature is derived from the fire's high temperatures that cause the vaporization of the soil organic compounds, which condense and coat soil particles that become hydrophobic (Letey, 2001). Regardless, assuming similar fuel load availability, differences in fire impacts between different land-uses (e.g., croplands vs. rangelands) are attributed to vegetation species composition. For example, in Alicante in southeastern Spain, it was found that that Rosmarinus officinalis (L.) induced greater soil water repellency than that of Pinus halepensis (Miller), and that this soil feature under these two species was greater than that under Brachypodium retusum (Pers. Beauv.) (Arcenegui et al., 2007). At the same time, Ansley et al. (2006) highlighted the importance of the fire timing. For a temperate mixed-grass savanna in Texas, United States, Ansley and colleagues reported that late-summer and winter fires, occurring when (highly productive) C3 grasses and forbs are active and (lesser productive) C4 grasses are dormant, have a large impact on soil organic carbon and black carbon dynamics. In addition, water repellency was reported to be generally related to the soil texture, and positively related to the soil organic carbon content (DeBano, 1981; Bisdom et al., 1993). This is particularly relevant in coarse textured soils, where macro-aggregation processes are relatively little, and the non-aggregate encapsulated organic carbon becomes available for vaporization and vertical redistribution. Additionally, the development of water repellency is negatively related to the soil moisture content, because dry soil is a poor conductor of heat (DeBano, 1981). No less important than the absolute temperature is the temperature gradient which develops across the soil profile (DeBano, 1981). That way, water repellency tends to increase with soil depth, because the organic compounds' vapors move down throughout the decreasing temperature gradient across the soil profile – from the hottest surface layer to the cooler deeper layers – where they condense and coat mineral particles. Depending on the burning severity, the water repellent soil layer can be developed in a wide range of depths, ranging between few mm to several cm from the surface (DeBano, 1981; Letey, 2001). In addition to the fire effects on the solid soil, burning also affects the dynamics of unconsolidated material cover on the ground surface. These materials include both organic matters such as plant litter and ash, as well as mineral substances, such as dust and non-aggregated substances (Parsons et al., 2010). Specifically, fire outbreaks can eliminate such materials through their burning (Miesel et al., 2007), or deposit the flying, partially-burnt materials (Rau et al., 2008), resulting in on-site depletion or insertion, respectively, of soil resources, and with the successive impact on the soil quality and fertility. This study's major objective was to assess the impact of fire outbreak on the geo-ecosystem functioning of rangelands and croplands, as determined by soil-water repellency, as well as by the status and dynamics of some of the most important, organic and mineral soil resources. The secondary objectives were to examine the effects of land-use, i.e., croplands vs. rangelands, and soil depth, on these soil features. Additionally, we aimed at assessing the severity of this fire outbreak. The study's major hypothesis was that the fire induced water repellency in the fire-prone lands, and decreased the on-site availability of soil resources. The impact on soil water repellency is attributed to the burning of above-ground biomass, which results in the deposition of some ash on the ground surface, as well as the ‘baking’ of the ground surface, with the resultant vaporization of the vegetation- and soil-organic matter. These combined effects lead to the condensation of some of the organic matter, with the resultant coating of soil particles, which become hydrophobic. At the same time, the impact on on-site availability of soil resources, despite the deposition of some ash on the ground surface, is attributed to the elimination of the largest part of above-ground biomass by the burning. The second hypothesis was that the fire-induced

repellency increased with soil depth. This expected effect is attributed to the descending temperature gradient across the soil profile, which enables the condensation of the organic matter vapors at relatively deeper depths. The third hypothesis was that the fire-derived repellency impact was greater in the rangelands than in the croplands. This is due to the (easily observable) greater fuel load in the rangelands, with the expected higher fire severity and greater rates of vaporization and condensation of organic matter, than that in the (fallow but) harvested croplands. 2. Materials and methods 2.1. Regional settings The study region extends across the north-western semi-arid Negev in southern Israel (31° 35′ N, 34° 59′ E, 93–101 m.a.s.l.). Mean daily temperatures in the coldest (January) and warmest (July) months are 12 °C and 26 °C, respectively (Bitan and Rubin, 1991), and long-term mean annual cumulative precipitation is 230 mm, occurring between November and March (Israel Meteorological Service). Lithology of the region is comprised of calcareous sandstone, and topography is comprised of extensive flat plains transected by wide and deep wadis (ephemeral rivers). Soil is classified as loessial Calcic Xerosol (FAO, 2015). The study site consisted of the southern edges of the Agricultural Research Organization's Migda Farm for croplands, and the nearby Patish Wadi's wide shoulders for rangelands (Fig. 1). Similar to the extensive rainfed croplands across the region, the studied farmlands have been continuously cultivated and sown with wheat (Triticum aestivum L.) over the long term. However, during the cropping year of fall 2014 to spring 2015, the farm's croplands were left fallow, allowing the spontaneous growth of a range of native herbaceous vegetation species, including: Sinapis alba L., Chrysanthemum coronarium L., Erucaria hispanica L. Druce,Malva sylvestris L., Erodium moschatum L. L'HÉR., Raphanus raphanistrum L., Centaurea iberica Trevir. & Spreng., and Carthamus tenuis Boiss. & Blanche Bornm. During March 2015, the herbaceous vegetation was harvested, baled for hay, and taken off the field. As practiced in extensive cropping systems elsewhere, the combine harvester table was set to 10-cm height, leaving vegetation stems of this height, which remained attached to the ground surface. Rangelands across the study region have mainly encompassed marginal lands, such as wadi shoulders. The rangelands have been prone to occasional grazing by flocks of sheep and goats. Main herbaceous vegetation species in the rangelands have included grasses, forbs, and legumes, including: S. alba, C. coronarium, E. hispanica, C. iberica, C. tenuis, Trigonella arabica Delile, Schismus arabicus Nees, Stipa capensis Thunb., and Cynodon dactylon L. An uncontrolled fire – breaking out following the expansion of an intentional burning of agricultural residues on a day with a heat wave, where temperatures exceeded 40 °C – took place on May 18th 2015, emerging from the farmlands south of Patish Wadi. The fire outbreak advanced northward, towards Patish Wadi, and further north, towards the southern parts of the Migda Farm. On its track northward, the fire burnt approximately 210 ha of open lands in Patish Wadi and an additional approximately 20 ha of the Migda's farmlands, as well as approximately 160 ha of the surrounding croplands. About two weeks after the fire outbreak, a survey (of the 0–1, 1–2, and 2–3 cm depths) of the study site's croplands and rangelands was undertaken in order to determine the soil texture. The survey revealed a sandy loam texture (63.8% ± 2.3 sand, 18.5% ± 1.4 silt, and 17.7% ± 1.5 clay), with no significant effect of any of the land-use or depth. 2.2. Remote sensing-based assessment of fire severity Fire severity was calculated by spectral indices that track the reflectance changes in burnt areas, mainly related to vegetation


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Block C

Block A

Block B

Fig. 1. Aerial picture of the study region. Photographed in early May 2015, about two weeks before the fire outbreak. The post-fire sampling blocks are indicated in yellow. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

removal, soil exposure, water content, and the deposition of organic matter and ash (Jakubauskas et al., 1990). Selected Landsat 8 Operational Land Imager (OLI images, pre-processed by the Google Earth Engine (GEE)) was utilized to convert digital number (DN) to topof-atmosphere reflectance. Further, the multispectral eight images (spatial resolution 30 m) across visible-, near-, and short-wave infrared regions were pan-sharpened using the panchromatic band (spatial resolution 15 m) and the High Pass Filter (HPF) algorithm (Bourgeau-Chavez et al., 2007). The spatial information injection was performed by adding, pixel by pixel, the filtered image that results from subtracting filtered PAN from the original PAN image, to the multispectral images (González-Audícana et al., 2004). In this study, the modulation transfer function (MTF) of the sensor as the low-pass filter (Khan et al., 2008) was utilized. Remotely sensed measurements of fire severity aggregated the fire effects at the spatial grain of the sensor; for fire severity measurement it was aggregated at the spatial grain of a Landsat pan-sharpening pixel of 15 m2. The fire severity spectral index was based on the Normalized Burn Ratio (NBR) (López García and Caselles, 1991; Key and Benson, 2006). For a given area, NBR was calculated from an image prior to the fire and a second NBR was calculated for an image after the fire. Fire extent and severity was determined by calculating the difference between these two index layers – also known as dNBR index (Brewer et al., 2005; Epting et al., 2005; Thode, 2005): dNBR ¼ NBRprefire −NBRpostfire

ð1Þ

The USGS Fire Effects Monitoring and Inventory (FireMon) protocol was utilized as the approximation for interpreting the dNBR: values between − 0.1 to + 0.1 for non-burnt, 0.1 to 0.27 for low fire severity, 0.27 to 0.44 for moderate-low fire severity, 0.44 to 0.66 for moderate-high fire severity, and above 0.66 for high fire severity (USGS, https://www.frames.gov/partner-sites/firemon/firemonhome/).

2.3. Aboveground biomass and soil sampling and monitoring at field Field work was implemented in early June 2015, a couple of weeks after the fire outbreak. Along Patish Wadi's rangelands, stretching approximately from east to west, three plots were delineated in the wadi shoulders, with the roughly plain topography. The plots – each covering approximately 0.1 ha – encompassed both burnt and non-burnt sites. Burnt sites were identified by the occurrence of partially-burnt organic materials and fresh ash attached to the surface soil. In parallel to the rangeland plots, an additional set of three plots of approximately 0.1 ha each were delineated in the Migda's farmlands. Each of the farmland plots also encompassed both burnt and non-burnt sites. This allowed us to establish the study scheme in a 3-block design, where each block contained one rangeland plot and one cropland plot, with an aerial cover of approximately 0.2 ha. The distance between each two adjacent blocks was at least 500 m. In each plot, five spots were randomly selected for the burnt site, and an additional five spots for the non-burnt site. In each spot, a 20 × 20 cm quadrate was laid down on the surface soil, and all the ground-attached aboveground biomass within it was harvested and put in a paper bag. Then, for each of these quadrates, all the unconsolidated material – including both the mineral and organic components – was collected and put in a separate paper bag. At the next stage, for each of these quadrates, we measured the soil-water repellency using water drop penetration time (WDPT: Letey, 1969) and critical surface tension (CST) procedures. In the surface soil (0 cm depth), these measurements were conducted after the solid soil surface was carefully cleared of any vegetation and unconsolidated organic and mineral material. CST was conducted by using a set of solutions, with 0, 1, 3, 5, 9, 14, 19, 24, 34, 48, and 60% concentrations of ethanol by volume. Five drops of the 0-solution concentration were applied to the soil. If all the drops were not absorbed to the soil within 5 s, then a solution with the next-higher ethanol concentration was tested. The recorded CST values corresponded with the lowest ethanol concentration absorbed by the soil (Letey,

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1969; MacDonald and Huffman, 2004). In addition to the surface soil, these measurements were also conducted at the 1- and 2-cm depths after the soil layers were peeled to these depths. WDPT was determined by using deionized water, kept at the field in a food hamper under shade, at a temperature of 30 °C (±2 °C). Also the ethanol's set of solutions for determining CST was kept in the same way. Because of the relatively short mean time of water drop penetration to soil, and in order to increase the accuracy of measuring the water repellency, WDPT was not based on 5 s as the limit of soil classification as water repellant (Bisdom et al., 1993), but rather, on measuring the actual time (seconds) until penetration. Finally, for each of the quadrates and each of the three depths, a sample of the solid soil, of approximately 300 g was obtained and put in a sealed plastic bag for laboratory analyses. 2.4. Laboratory analyses Aboveground biomass samples were put in a drying oven set to 60 °C for 48 h to determine their dry weight. The solid-soil samples were utilized to determine the (hygroscopic level's) gravimetric moisture content (Gardner, 1965). Then, total soil organic matter concentration was determined by the loss-on-ignition method (Nelson and Sommers, 1996) after fumigation with diluted hydrochloric acid (Harris et al., 2000). The results were then divided by 1.724 to calculate for total soil organic carbon. Labile (readily oxidizable) soil organic carbon concentration was determined by the mild potassium permanganate oxidation method (Weil et al., 2003). In addition, ammonium-N (NH+ 4 ) concentration of soil was determined by the Nessler method (Sahrawat and Prasad, 1975). In addition, samples of the unconsolidated material were weighed to determine their total mass, and a separate sub-sample of each of them was set aside to determine gravimetric moisture content (for determining the dry weight) and concentrations of total organic matter and ammonium. The main samples of unconsolidated material were then placed on top of a set of sieves of 2000, 1000, 500, 250, 106, and 63 μm from top to bottom, for simultaneously determining their (primary and secondary) particle size distribution. 2.5. Statistical analysis Statistical analysis was conducted with the General Linear Model (GLM) procedure of SAS (SAS Institute, 1990). A split-plots type analysis of variance (ANOVA) was conducted with fire (burnt vs. non-burnt) and block as the main plots, and land-use (croplands vs. rangelands) and soil depth (0, 1, and 2 cm) as the sub-plots. For each of the variables of aboveground biomass and the unconsolidated material (and its concentrations of total organic carbon and ammonium), factors in the model were: fire (1 df), block within fire (2 df, error term for fire), land-use (1 df), and the interaction fire × land-use (1 df). For each of the solidsoil variables, factors in the model were: fire (1 df), block within fire (2 df, error term for fire), land-use (1 df), fire × land-use (1 df), soil depth (2 df), fire × soil depth (2 df), and land-use × soil depth (2 df). Statistically significant interactions were subjected to further ANOVA with the SLICE command of PROC GLM. Separation of means was determined by Tukey's honestly significant difference (HSD) at the 0.05 probability level. Pearson correlation coefficients were calculated to examine the relations between pairs of the analyzed variables. 3. Results and discussion 3.1. Fire effect On-ground monitoring of ash and partially-burnt organic materials in the burnt sites revealed high variability in space. Yet, as determined by both the WDPT and CST data, soil at the burnt sites experienced soil-water repellency to some extent, as opposed to no repellency at

all in the non-burnt sites (Table 1). This is consistent with the study's major hypothesis regarding the positive effect of fire on soil-water repellency. Yet, despite the significant and almost ninefold greater WDPT and approximately 6% smaller CST in the burnt than in the non-burnt sites, also the burnt sites' WDPT and CST had overall mean low values (Table 1) – lying within the water repellency's lowest class (Letey, 1969) – and suggesting that the fire severity was relatively low. This is also consistent with visual observations under field conditions of the: (i) relatively large quantities of slightly burnt or partiallycharred plant litter, with recognizable morphology; (ii) no signs of burning of belowground biomass (herbaceous vegetation roots); (iii) no change in the topsoil structure; and (iv) no modified color in the mineral particles of topsoil, altogether indicating an overall low to moderate fire severity (Parsons et al., 2010). The field observations were also consistent with the dissimilarities between spectral signatures of pre- and post-wildfire satellite-driven images (Fig. 2). As vegetation and organic matter burnt, the visible-tonear-infrared reflectance was strongly reduced, and the short infrared surface reflectance increased due to extreme surface dryness and the formation of an ash layer on the ground. The burn severity was classified into four levels: high to moderate, moderate, low, and non-burnt. One week after the fire, the areas of moderate to high and moderate fire severities together amounted to 5.7 ha (approximately 2.5% of the total burnt area), while the area of low fire severity covered 8.82 ha (approximately 4% of the total burnt area). No evidence was recorded for the high fire severity. Regardless of the fire characteristics, soil moisture content is also important in determining the soil-water repellency. For example, in the grasslands and shrublands of southern California, Hubbert et al. (2012) reported negative relations between the soil moisture content and water repellency. However, DeBano (1981) proposed that the least severe water repellency occurs when light-intense fires burn over wet soils. Nonetheless, he suggested that the lower the soil moisture content is, the smaller the soil-heat conductivity, with the negated repellency being established in the deeper soil layers. The obtained results in our study are consistent with DeBano (1981) and negate the study's secondary hypothesis, suggesting that the generally low (hygroscopic) soil moisture content at the time of the fire outbreak prevented the deepening of the water-repellency front to deeper depths. Despite having no data on soil moisture content at the time, it was expected that the soil moisture level (of the non-burnt lands: Table 1) had not faced any changes during the couple of weeks between the time of fire outbreak and that of sampling. This is due to the region's season characteristics of hot and rainless summers. Also, this trend concurs with the CST results, revealing a general trend of increasing mean values with soil depth, suggesting a greater water-repellency of the surface soil than that in the 1-cm and 2-cm soil depths. Moreover, despite the no significant effect of soil depth on WDPT, the decreasing mean value with increasing soil depth (Table 2) also accords with this suggestion. One way or another, the significantly smaller CST values for each of the surface layer and 1-cm depth under the burnt sites than those under the non-burnt sites (Table 3) suggests some degree of fire-induced water repellency also in the middle depth (but not in the deepest depth). Regardless, the marginally significant and approximately 8% lower moisture content of soil in the burnt than that in the non-burnt sites (Table 1) could be attributed to the increase in evaporation loss due to the drying effect of fire (Craine, 2013). As reported in other studies, the soil organic carbon concentration positively impacts the emergence of soil-water repellency, as well as the advance of the hydrophobicity front to deeper soil depths (DeBano, 1981; Bisdom et al., 1993; Letey, 2001). Regardless, our results reveal 31% greater mean soil concentration of total organic carbon in burnt than that in non-burnt sites. A similar effect was reported by Ansley et al. (2006), who found that for a mixed-grass savanna in Texas, USA, fires occurring during the summer, when the plant species


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Table 1 Fire effects on vegetation, solid soil, and unconsolidated material. Fire

P value

Burnt

Non-burnt

Above-ground biomass (Mg ha−1) Water drop penetration time (WDPT: s) Critical surface tension (CST: dyn cm−1) Hygroscopic moisture content (%) Total organic carbon concentration (g kg−1) Labile organic carbon concentration (ppm) Ammonium-N concentration (mg ml−1) Total unconsolidated material (Mg ha−1) Unconsolidated material N 2000 μm weight (kg ha−1) Unconsolidated material 2000–1000 μm weight (kg ha−1) Unconsolidated material 1000–500 μm weight (kg ha−1) Unconsolidated material 500–250 μm weight (kg ha−1) Unconsolidated material 250–106 μm weight (kg ha−1) Unconsolidated material 106–63 μm weight (kg ha−1) Unconsolidated material 63–0 μm weight (kg ha−1) Total organic carbon pool in unconsolidated material (Mg ha−1) Ammonium-N pool in unconsolidated material (kg h−1)

0.0061 0.0008 b0.0001 0.0505 b0.0001 0.2390 b0.0001 0.0667 0.9700 0.7803 0.3991 0.3133 0.0262 0.0041 0.0141 0.5159 0.7132

0.82 b (0.24) 7.8 a (2.1) 66.9 b (0.9) 1.27 a (0.04) 23.5 a (1.1) 437.7 a (4.1) 18.7 a (1.2) 2.93 a (0.72) 46.0 a (14.8) 401.4 a (69.0) 757.0 a (154.6) 541.3 a (145.8) 394.3 b (117.3) 520.3 b (160.7) 268.7 b (82.4) 0.54 a (0.15) 27.2 a (8.1)

6.23 a (2.09) 0.9 b (0.1) 71.2 a (0) 1.37 a (0.03) 18.0 b (0.6) 431.1 a (4.4) 12.3 b (0.7) 4.88 a (0.95) 46.7 a (12.1) 377.7 a (58.5) 619.0 a (87.5) 719.9 a (134.7) 832.0 a (182.2) 1633.9 a (390.6) 651.0 a (155.1) 0.43 a (0.08) 23.8 a (5.0)

Notes: Bold P value indicates a significant effect. Means within the same row followed by a different letter differ at the 0.05 probability level according to Tukey's honestly significant difference (HSD). Numbers within parentheses are standard error of the means.

a

b

c

d

May 5th 2015

e

May 26th 2015

f

g

h

June 8th 2015

Fig. 2. Modified Normalized Burn Ratio (NBR) spectral index applied on the space-borne multispectral image (Landsat 8): panels a, c, and f are RGB (true color) images; panels b, d, and g are thematic maps on modified NBR; panels e and h are stretched thematic results (Low Moderate to High severity burn).

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Table 2 Soil depth effects on the solid soil. Depth

P value

0–1 cm

1–2 cm

2–3 cm

Water drop penetration time (WDPT: s)a Critical surface tension (CST: dyn cm−1)a Hygroscopic moisture content (%) Total organic carbon concentration (g kg−1) Labile organic carbon concentration (ppm) Ammonium-N concentration (mg ml−1)

0.5302 0.0006 b0.0001 b0.0001 0.1425 b0.0001

5.8 a (1.5) 67.0 b (1.1) 1.15 c (0.04) 24.1 a (1.5) 436.2 a (4.5) 23.6 a (1.3)

4.0 a (1.9) 69.5 a (0.8) 1.30 b (0.03) 20.2 b (0.9) 440.1 a (4.9) 13.0 b (0.9)

3.1 a (2.2) 70.6 a (0.5) 1.49 a (0.03) 18.0 c (0.7) 426.8 a (6.0) 9.9 c (0.7)

Notes: Bold P value indicates a significant effect. Means within the same row followed by a different letter differ at the 0.05 probability level according to Tukey's honestly significant difference (HSD). Numbers within parentheses are standard error of the means. a WDPT and CST were tested in the soil depths of 0 (surface soil), 1, and 2 cm.

community is dominated by C3 species, tend to increase the soil organic carbon content. This is consistent with our study site, where plant community – both in the rangelands and fallow croplands – was almost exclusively composed of C3 species, with only the C. dactylon the exception. The greater ammonium concentration in the burnt than that in the non-burnt sites (Table 1) is consistent with Augustine et al. (2014), who found that fires of relatively low temperatures (ranging between 126 °C and 148 °C) do not cause nitrogen volatization, but rather onsite deposition of nitrogen embedded in partially-combusted organic matter or as highly-available ammonium-N. This matches our visual observations in the field, which suggest low to moderate fire severity across the study site. At the same time, no significant effect of fire was recorded for the mean concentration of the labile fraction of soil organic carbon (Table 1). Regardless of fire, mean soil concentrations of total organic carbon and ammonium decreased with soil depth (Table 2). This consists with DeBano (1981), who suggested that under low fire-temperatures, the soil organic matter and its contained nutrients do not translocate to deeper depths, and further supports our observations regarding the low to moderate fire severity. However, no effect of soil depth was recorded for the mean soil concentrations of the labile organic carbon (Table 2). The interaction between fire and soil depth revealed – for each of the shallowest and middle depths – greater ammonium concentration in the burnt than that in the non-burnt sites (Table 3). The (marginally significant and) 67% greater mean total unconsolidated material weight in the non-burnt than that in the burnt sites (Table 1) revealed the considerable elimination of detached matters from the fire-prone lands. The eliminated material has presumably been burnt, eroded by wind and deposited off-site, or a combination of both of these processes. Sieving the unconsolidated material to (primary + secondary) particle size distribution revealed that a significant effect was limited to the fractions of the b 63, 63–106, and 106–250 μm only. This suggests that within the unconsolidated material cover, only Table 3 Effect of the interaction fire × soil depth on the solid soil. Fire ∗ depth P value Burnt × shallowest deptha Burnt × middle depthb Burnt × deepest depthc Non-burnt × shallowest deptha Non-burnt × middle depthb Non-burnt × deepest depthc

Critical surface tension (CST: dyn cm−1) 0.0006

Ammonium-N concentration (mg ml−1) 0.0037

62.9 c (2.1) 67.9 b (1.6) 70.0 ab (0.9) 71.2 a (0)

28.6 a (2.1) 16.6 b (1.4) 11.0 c (0.9) 18.7 b (1.1)

71.2 a (0)

9.5 c (0.6)

71.2 a (0)

8.8 c (0.9)

Notes: Means within the same column followed by a different letter differ at the 0.05 probability level according to Tukey's honestly significant difference (HSD). Numbers within parentheses are standard error of the means. Only significant interactions are presented. a Shallowest depth for CST is 0 cm, and for ammonium-N is 0–1 cm. b Middle depth for CST is 1 cm, and for ammonium-N is 1–2 cm. c Deepest depth for CST is 2 cm and for total ammonium-N is 2–3 cm.

the fine fraction of b 250 μm is susceptible to elimination by fire. One way or another, the twofold to threefold greater mass of these particle size fraction of unconsolidated material in the non-burnt than in the burnt sites (Table 1), suggests the greater susceptibility of soil resources to depletion in the burnt sites, with the successive lesser soil quality and fertility. However, overall pools of soil organic carbon and ammonium in the unconsolidated material were similar in the burnt and nonburnt sites (Table 1), indicating no deterioration of the soil quality and fertility following the fire outbreak. Therefore, in general terms, no adverse impact of the low- to moderate-fire severity on geo-ecosystem functioning was recorded for this study site. This is due to the only slight increase in soil-water repellency detected for the burnt lands. Moreover, an increase in on-site availability of soil resources was observed for the fire-prone lands. Therefore, in events where prescribed burnings encompass an integral practice in the routine management of lands, low- to moderate-severity fires – for the control of pests (DeFrancesco and Murray, 2011), weeds (Koski et al., 2011), or invasive woody vegetation (Veach et al., 2014) – could be considered advantageous. At the same time, despite these on-farm advantages, prescribed fires could impose serious off-farm environmental risks, of which the pollution of air and water sources is of special concern (Goldammer et al., 2009). 3.2. Land-use effect Highly variable amounts of ash and partially-burnt organic material were observed in the rangelands. Yet, as revealed by both of the WDPT and CST data, the rangeland soil faced water repellency to some degree, as opposed to no water repellency at all in the cropland soil (Table 4). The general similarity of herbaceous vegetation species composition between the rangelands and (the studied year's) fallow croplands negated the possible effect of plant community on the emergence of water repellency. Therefore, these results are attributed to the differences in fuel load, and consist with the almost eightfold greater mean aboveground biomass in the rangelands than that in the post-harvested croplands (Table 4). This concurs with the study hypothesis and also with previous studies that revealed the importance of vegetation biomass cover to the emergence and degree of soil water-repellency (DeBano, 1981; Moench and Fusaro, 2012). This also consists with the produced Landsat remote sensing images, showing that dNBR was higher for Patish Wadi than that for the Migda farmlands (Fig. 2). The variability in dNBR values in areas of high fire severity is related to the amount of live and dry pre-fire vegetation in each pixel. In this figure, panels e and h – the threshold between moderate and high severity categories – could be optimally placed to omission errors for the high severity category. It is shown that one month after the fire event, the locations with the highest severity remained highlighted, while locations with the lowest severity were no longer shown on the thematic maps (Fig. 2g, h). Nonetheless, the absolute absence of soilwater repellency in the croplands is unexpected, as clear visual indications of ash and partially-burnt organic residues were located at the farm's burnt sites. Regardless, even in the rangelands, water repellency was rather low (Table 4).


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Table 4 Land-use effects on vegetation, solid soil, and unconsolidated material. Land-use

P value

Croplands

Rangelands

Above-ground biomass (Mg ha−1) Water drop penetration time (WDPT: s) Critical surface tension (CST: dyn cm−1) Hygroscopic moisture content (%) Total organic carbon concentration (g kg−1) Labile organic carbon concentration (ppm) Ammonium-N concentration (mg ml−1) Total unconsolidated material (Mg ha−1) Unconsolidated material N 2000 μm weight (kg h−1) Unconsolidated material 2000–1000 μm weight (kg h−1) Unconsolidated material 1000–500 μm weight (kg h−1) Unconsolidated material 500–250 μm weight (kg h−1) Unconsolidated material 250–106 μm weight (kg h−1) Unconsolidated material 106–63 μm weight (kg h−1) Unconsolidated material 63–0 μm weight (kg h−1) Total organic carbon pool in unconsolidated material (Mg ha−1) Ammonium-N pool in unconsolidated material (kg h−1)

0.0057 0.0004 b0.0001 b0.0001 b0.0001 0.0008 b0.0001 0.5299 0.7283 0.0957 0.9910 0.6967 0.5718 0.4095 0.7894 0.7118 0.1490

0.79 b (0.29) 0.7 b (0.1) 71.2 a (0) 1.42 a (0.04) 17.6 b (0.5) 444.0 a (3.3) 13.3 b (0.7) 4.24 a (0.84) 43.0 a (10.2) 461.2 a (47.5) 687.1 a (69.4) 665.0 a (116.7) 668.0 a (161.2) 1233.3 a (346.8) 480.3 a (136.9) 0.46 a (0.07) 18.4 a (2.8)

6.25 a (2.08) 7.9 a (2.1) 66.8 b (0.9) 1.21 b (0.03) 23.9 a (1.1) 424.8 b (4.9) 17.8 a (1.2) 3.57 a (0.88) 49.7 a (18.4) 317.9 a (85.7) 689.0 a (196.7) 596.2 a (181.3) 558.3 a (144.2) 921.0 a (199.5) 439.5 a (101.3) 0.52 a (0.19) 32.5 a (9.9)


The significantly and approximately 17% greater moisture content of the soil in the croplands than that of the rangelands (Table 4) could be attributed to the cropland harvest in the spring, assumed to decrease soil-water loss through transpiration, as opposed to the herbaceous vegetation in the rangelands, which assumed to keep high transpiration rates (Dohnal et al., 2006) for at least one month later on. Regardless, mean soil concentrations of total organic carbon and ammonium were 36% and 34%, respectively, greater in the rangelands than those in the croplands (Table 4). To some extent, this consists with Stavi et al. (2015), who reported for the same region greater rates of soil organic carbon sequestration in low-intensity managed rangelands than that in relatively high-intensity managed croplands. However, the opposite trend for the mean soil concentrations of labile organic carbon – being approximately 5% greater in the croplands than that in the rangelands (Table 4) – is unexpected. The interaction between fire and land-use was highly significant for both WDPT and CST (Table 5). For WDPT, mean value in the burnt rangelands was 18 times greater than that in the burnt croplands, while rather similar mean values were recorded in the non-burnt sites of the two land-uses. For CST, mean value in the burnt rangelands was approximately 14% smaller than that in the burnt croplands, while mean values for both of the non-burnt rangelands and croplands were

identical to those in the burnt croplands, suggesting no water repellency at all. Overall, these results are consistent with the mean aboveground biomass data, ranging between eightfold to 36 times greater in the non-burnt rangelands than those in any of the other combinations of fire and land-use (Table 5), further highlighting the crucial impact of fuel load on the emergence of soil-water repellency. The effect of this interaction was also significant for the soil concentrations of total organic carbon and ammonium, which had greater mean values under the burnt rangelands than those under any other combination of fire and land-use. This stresses the positive impact of biomass cover and the resultant fire severity on contents of soil organic carbon and ammonium in burnt lands. At the same time, no significant effect of this interaction was recorded for the labile soil organic carbon (Table 5). The effect of land-use on the total unconsolidated material mass, on any of its particle-size fractions separately, and on its pools of organic carbon and ammonium, was not significant (Table 4). Yet, the interaction between fire and land-use was significant for the total unconsolidated material mass (Table 5), being significantly greater in the nonburnt croplands than that under any of the non-burnt rangelands and burnt croplands, and similar to that in the burnt rangelands. The decrease in unconsolidated material mass in the burnt croplands could be attributed to its burning or erosion by winds (occurring between

Table 5 Effect of the interaction fire × land-use on vegetation, solid soil, and unconsolidated material. Fire ∗ land-use −1

Above-ground biomass (Mg ha ) Water drop penetration time (WDPT: s) Critical surface tension (CST: dyn cm−1) Hygroscopic moisture content (%) Total organic carbon concentration (Mg ha−1) Labile organic carbon concentration (ppm) Ammonium-N concentration (mg ml−1) Total unconsolidated material (Mg ha−1) Unconsolidated material N 2000 μm weight (kg ha−1) Unconsolidated material 2000–1000 μm weight (kg ha−1) Unconsolidated material 1000–500 μm weight (kg ha−1) Unconsolidated material 500–250 μm weight (kg ha−1) Unconsolidated material 250–106 μm weight (kg ha−1) Unconsolidated material 106–63 μm weight (kg ha−1) Unconsolidated material 63–0 μm weight (kg ha−1) Total organic carbon pool in unconsolidated material (Mg ha−1) Ammonium-N pool in unconsolidated material (kg h−1)

P value

Burnt × croplands

Burnt × rangelands

Non-burnt × croplands

Non-burnt × rangelands

0.0218 0.0009 b0.0001 0.1355 0.0020 0.0947 0.0014 0.0007 0.0209 0.0012 0.0011 0.0002 0.0015 0.0075 0.0044 0.1344 0.9091

0.31 b (0.15) 0.8 b (0.2) 71.2 a (0) 1.41 a (0.04) 18.9 b (0.8) 442.6 a (3.3) 14.9 b (1.1) 1.41 b (0.14) 20.1 b (3.2) 329.6 ab (40.2) 479.0 ab (44.1) 225.6 b (24.2) 130.0 b (19.6) 159.3 b (30.6) 65.2 b (12.5) 0.30 a (0.28) 19.7 a (6.2)

1.33 b (0.39) 14.7 a (4.0) 62.7 b (1.7) 1.13 b (0.05) 28.1 a (1.7) 432.8 ab (7.6) 22.5 a (1.9) 4.45 ab (1.72) 71.8 a (36.0) 473.2 ab (162.8) 1034.9 a (373.3) 857.0 ab (349.0) 658.6 ab (279.5) 881.3 b (382.6) 472.2 ab (194.1) 0.78 a (0.37) 34.8 a (16.9)

1.28 b (0.51) 0.7 b (0.1) 71.2 a (0) 1.44 a (0.08) 16.3 b (0.6) 445.4 a (5.6) 11.6 b (0.8) 7.07 a (1.46) 65.9 a (19.3) 592.7 a (78.1) 895.1 ab (118.4) 1104.4 a (195.5) 1206.1 a (283.6) 2307.2 a (623.2) 895.3 a (247.4) 0.61 a (0.13) 17.3 a (2.4)

11.17 a (3.48) 1.1 b (0.2) 71.2 a (0) 1.30 ab (0.04) 19.7 b (0.9) 416.7 b (6.0) 13.1 b (1.1) 2.70 b (0.41) 27.5 b (5.9) 162.6 b (26.3) 343.0 b (69.1) 335.3 b (70.3) 458.0 b (80.0) 960.7 ab (134.1) 406.7 ab (67.7) 0.26 a (0.06) 30.2 a (13.1)


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Table 6 Effect of the interaction land-use × soil depth on the solid soil. Land-use ∗ depth

Critical surface tension (CST: dyn cm−1)

Total organic carbon concentration (g kg−1)

Ammonium-N concentration (mg ml−1)

P value Cropland × shallowest deptha Cropland × middle depthb Cropland × deepest depthc Rangelands × shallowest deptha Rangelands × middle depthb Rangelands × deepest depthc

0.0006 71.2 a (0) 71.2 a (0) 71.2 a (0) 62.9 b (2.1) 67.9 a (1.6) 70.0 a (0.9)

b0.0001 18.3 bc (1.0) 17.6 c (0.8) 17.0 c (0.9) 29.9 a (2.3) 22.7 b (1.4) 19.0 bc (1.1)

0.0052 19.2 b (1.2) 11.8 cd (0.9) 8.8 d (0.8) 28.1 a (2.1) 14.3 c (1.5) 11.0 cd (1.1)

Notes: Means within the same column followed by a different letter differ at the 0.05 probability level according to Tukey's honestly significant difference (HSD). Numbers within parentheses are standard error of the means. Only significant interactions are presented. a Shallowest depth for CST is 0 cm, and for total organic carbon and ammonium is 0–1 cm. b Middle depth for CST is 1 cm, and for total organic carbon and ammonium is 1–2 cm. c Deepest depth for CST is 2 cm and for total organic carbon and ammonium is 2–3 cm.

the time of fire and that of field work), while the increase in unconsolidated material mass in the burnt rangelands is attributed to the deposition of flying materials during the fire event. This contradicting trend between the two land-uses may be attributed to the greater surface roughness in the rangelands, augmenting the trapping of flying ash. At the same time, the smoother surface in the croplands seems to lessen this process. This accords with the fourfold greater above-ground biomass in the burnt rangelands than that in the burnt croplands (Table 5), assumed at lowering wind speed and increasing aeolian deposition. Despite some discrepancies, a similar trend was also recorded for the different particle size fractions of unconsolidated material. A significant effect of this interaction was recorded for the pools of organic carbon and ammonium, whose concentrations were significantly greater under the burnt rangelands than those under any other combination of fire and land-use (Table 5). The interaction between land-use and soil depth, despite being significant for CST, showed a difference only for the shallowest depth (ground surface), where mean value was 13% smaller for the rangelands than for the croplands. For the rest of the combinations of land-use and soil depth, CST was similar to that in the ground surface of croplands. No significant effect for this interaction was recorded for WDPT (Table 6). This interaction demonstrated a limited fire-affected soil-water repellency, and only for the rangelands' shallowest depth, further strengthening indications of the low- to moderate-fire severity in the rangelands and the low fire severity in the croplands. For the total soil organic carbon concentration, this interaction showed greater mean values for the rangelands than that for the croplands, under each of the shallowest (a difference of 63%) and middle depths (a difference of 29%). This consists with the general trend of accumulation over time of organic carbon in rangelands soil, as opposed to its decomposition in croplands soil due to the frequent tillage (Stavi et al., 2015). A similar effect, though not significant, was also recorded under the deepest depth. The same trend was also recorded for the soil ammonium concentration, but with a significant effect for the shallowest depth only (Table 6). 3.3. Data integration, general implications, and future research needs No strong correlation coefficient was recorded between the soil moisture content and any of WDPT (r = 0.12) and CST (r = 0.22), nor between the total concentration of soil organic carbon and these measures of hydrophobicity (r = 0.01 and 0.03, respectively). Regardless, the greatest and smallest soil-water repellency in the ground surface and 2-cm depth, respectively (Table 2), is presumably attributed to the combination of the low (hygroscopic-level) moisture content of soil, poorly conducting heat to deeper depth (DeBano, 1981), and the relatively small fuel load at the time of the fire outbreak. Apart from that, the relatively moderate correlation coefficient (r = 0.58) between WDPT and CST could be explained by the very high variability of the ground surface's attributes of the burnt sites, with the resultant large differences in (intra-treatment) reactions to the water repellency tests.

Some gaps still exist in understanding the impacts of low- to moderate-severity fires. For example, the contradiction in reaction of total unconsolidated material mass to fire in croplands vs. rangelands – i.e., the decreased mass in croplands as opposed to the increased mass in rangelands – requires additional investigations in order to track the involved mechanisms. Another related question is the fate of the eliminated unconsolidated material, i.e., whether it gets burnt, deposited offsite, or a combination of the two processes. Also, additional research is needed to examine the persistence of soil-water repellency in burnt semi-arid croplands and rangelands, over time, after a fire outbreak. A recent study implemented in Californian grasslands and chaparral shrublands reported a decreasing trend of soil-water repellency over time (Hubbert et al., 2012). Unfortunately, such an experiment was impossible in our study site due to the yearly tillage of the studied croplands. Another particular focus for additional investigations could be the overall nutrient (N-P-K, as well as micro-elements) content of the unconsolidated material as an indication of the fire-induced impact on the geo-ecosystem functioning. Despite the unintended and uncontrolled nature of the fire outbreak in this study, it could have implications for the prescribed burning practice. As such, the obtained data is of high relevance for land management, as well as for its impacts on environmental quality. 4. Conclusions Unintended fire outbreak, spreading in extensive rangelands and fallow croplands in the semi-arid Israeli Negev region, has resulted in slight to moderate soil-water repellency in the rangelands, but not caused any water repellency in the soil of croplands. The difference in water repellency between these two land-uses is attributed to the available fuel load at the time of the fire outbreak, being much greater in the occasionally-grazed rangelands than that in the post-harvested croplands. Regardless, the emergence of fire resulted in the increase in on-site soil concentrations of organic carbon and ammonium-N. It is therefore concluded that the low- to moderate-fire severity has not adversely affected the functioning of the geo-ecosystem. Yet, it is acknowledged that such fires could jeopardize environmental quality, and specifically, impose risks to off-site, air and water source quality. The obtained data of this study can be applied for the understanding of agronomic and environmental impacts of both prescribed burnings and wildfires on geo-ecosystem functioning of semi-arid croplands and rangelands. Acknowledgments The authors gratefully acknowledge the helpful comments made by Lorenzo Marchi and an additional anonymous reviewer, which allowed the considerable improvement of the manuscript's original version. The study was supported by the Israeli Ministry of Science, Technology and Space.


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