Can rainfall seasonality trigger soil water repellency in a tropical riparian forest?

Though riparian areas generally have a shallow water table and higher soil moisture compared to upslope areas, climatic seasonality may trigger water repellency in tropical riparian forests, which, if persistent, could negatively affect essential ecosystem functions related to water resources protection such as reduced overland-flow, sediment transport, and nutrient filtration. The objective of this study was to answer the following: can tropical riparian forests develop water repellency? If so, does water repellency affect infiltration on a seasonal basis? For this, water repellency and infiltration were measured in a grid of 72 points during a dry and a wet month of a tropical riparian forest with a shallow water table in a region with highly marked climatic seasonality. Water repellency and infiltration were significantly different between the wet and dry months. Water repellency affected negatively infiltration in the dry month, its effect in the wet month was insignificant. As a result, a higher infiltration capacity was observed over the wet period. Previous research has claimed that the development and persistence of repellency in soils could promote other hydrological processes such as overland flow. The findings shown here demonstrate that such phenomenon does not persist longer than the dry season.


Introduction
Climatic seasonality affects ecosystem components in numerous ways, including flowering (Cascante-Marín et al. 2017), fruiting (Pages and Ehardt 2014), plant growth (Zalamea et al. 2013), animal migration (Bartzke et al. 2018), fruit predation (Oliveira et al. 2002), pollination (Mizunaga and Kudo 2017), seed dispersal (Sato 2013), species distribution (Ge et al. 2019), erosion (Evrard et al. 2010), and inflammability (Saha et al. 2019). For soil specifically, climatic seasonality, here defined as seasonal rainfall distribution, may basically reflect on low soil moisture during dry periods, which may influence important ecohydrological properties such as water retention, hydraulic conductivity, transpiration, and evaporation. One soil hydrological process highly influenced by rainfall seasonality is the ability to repel water during dry periods, i.e., its water repellency. Such phenomenon has been documented in many ecosystems (Jaramillo et al. 2000;Finley and Glenn 2010;Bonanomi et al. 2016;Vogelmann et al. 2017;Piyaruwan and Leelamanie 2020). Nevertheless, the detection of repellency in tropical forest soils, though existing (Elsenbeer et al. 1999), has only recently started to emerge as a research interest (Lozano-Baez et al. 2020). This topic is important for botanists, plant ecologists, ecohydrologists and others as it may have implications for several interrelated fields, including individual plant physiology and plant ecology (from individuals to ecosystems).
Abstract Though riparian areas generally have a shallow water table and higher soil moisture compared to upslope areas, climatic seasonality may trigger water repellency in tropical riparian forests, which, if persistent, could negatively affect essential ecosystem functions related to water resources protection such as reduced overland-flow, sediment transport, and nutrient filtration. The objective of this study was to answer the following: can tropical riparian forests develop water repellency? If so, does water repellency affect infiltration on a seasonal basis? For this, water repellency and infiltration were measured in a grid of 72 points during a dry and a wet month of a tropical riparian forest with a shallow water table in a region with highly marked climatic seasonality. Water repellency and infiltration were significantly different between the wet and dry months. Water repellency affected negatively infiltration in the dry month, its effect in the wet month was insignificant. As a result, a higher infiltration capacity was observed over the wet period. Previous research has claimed that the development and persistence of repellency in soils could promote other hydrological processes such as overland flow.

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Ecosystems are susceptible to the effects of rainfall seasonality depending on their position in the landscape. For example, ecosystems in upslope positions may experience higher soil water-deficits during the dry season compared to those on downslopes since, in this latter position, ecosystems are closer to watercourses, have shallow water tables and have a high contribution area (Salemi et al. 2012;Vidon 2012). In other words, ecosystems in downslope positions might experience effects of rainfall seasonality compared to those on upslopes. That may be the situation of tropical riparian forests along small streams in Brazilian tropical savanna domains. Such forests, referred to as "gallery forests" (Ribeiro and Walter 2008), resemble floristically seasonal tropical forests of the Atlantic and Amazon region (Oliveira-Filho and Fontes 2000). The climate is highly seasonal with regards to rainfall (Malaquias et al. 2010;Alvares et al. 2013). However, the influence of rainfall seasonality on soil properties such as water repellency and infiltration in tropical savanna ecosystems, including riparian forests, has not been studied. Their high soil moisture (Flores et al. 2021), generally shallow water tables (Vidon 2012), proximity to streams and a high contribution area (Agnew et al. 2006) might suggest that such riparian ecosystems would be unlikely to experience water repellency. However, this might not be the case in regions with pronounced rainfall seasonality. Such forests carry out essential ecosystem services related to water regulation, such as reducing runoff and sediment transport (Parron et al. 2011;Cordeiro et al. 2020), nutrient and chemical filtration (Christensen et al. 2013), as well as serving as wildlife corridors and habitats (Naiman and Décamps 1997;Naiman et al. 1998;Heise-Pavlov et al. 2018), sources of pollinators (Banks et al. 2013), the development of water repellency and reduced infiltration Jordán et al. 2013).
The objective of the present study was to address the following questions: (1) can tropical riparian forests develop water repellency? (2) If so, does water repellency affect infiltration on a seasonal basis?.

Study area
This study was carried out in the tropical riparian forest of the Paranoazinho stream, Sobradinho II, Distrito Federal (Brazil), 15°40′ S and 47°51′ W (Fig. 1). The average canopy cover estimated by Canopy Capture software is 84%, with a buffer zone width of 36 m. Like other tropical forests, numerous plant species occur. Some of the main species were Copaifera langsdorffii Desf., Matayba guianensis Aubl., Sclerolobium paniculatum var. rubiginosum (Mart. ex Tul.) Benth. and Tapirira guianensis Aubl. (Silva Júnior et al. 1998). Tree density and basal area were 1600-1900 trees ha −1 and 38-45 m 2 ha −1 , respectively (Silva Júnior 1999, Silva Júnior 2004, Silva Júnior, 2005. The elevation is approximately 1170 m with a tropical Aw climate (Köppen-Geiger) with two distinct seasons: wet and dry. The soil is classified as Organosol (following the Brazilian classification system by EMBRAPA). In addition to different textures associated with plant residues in different stages of decomposition, this soil is characterized by a histic horizon totally or partially saturated in the rainy season (Santos et al. 2018). The mean soil density, (estimated using 72 undisturbed soil cores), was 0.97 g cm −3 , had a texture which varied from clay to sandy loam.

Variables and sampling design
Rainfall was measured using a 674 cm 2 collector located 350 m from the forest site and 35 cm above the forest floor. The collected volume was measured with a graduated cylinder.
Water table depth was measured using two shallow wells (~ 80 cm depth) installed seven meters relative to the stream channel using a metric measuring tape (Fig. 2). Both rainfall and water table depth were measured weekly in the rainy season and biweekly during the dry period.
Soil water repellency and infiltration were sampled in a grid of 72 points (in both the wet and dry months) distributed across the tropical riparian forest (Fig. 2). Our sampling size is larger compared to others on the topic (see Buczko et al. 2005;Lozano-Baez et al. 2020). The mean distance between measurements was two meters. Both variables were measured in the dry (August) and wet (February) seasons of 2020. Water repellency was measured by the water drop penetration time (WDPT) test (De Bano 1981) using a Pasteur pipette. At each point, ten drops were added to the soil surface (Buczko et al. 2005;Fernández et al. 2019) and the time required to infiltrate completely was recorded. For this, 300 s (5 min) was established as the maximum time for monitoring. After calculating the mean for each 72 points, the level of repellency was classified based on Robichaud et al. (2016) as: wettable (< 5 s), slight (5-60 s), moderate (60-180 s), severe (180 s or more).
The infiltration capacity was measured using a minidisk infiltrometer (Decagon Devices Inc., Pullman, WA, USA) under a 0-cm suction pressure using the solution proposed by Zhang (1997). To increase the contact area between the equipment and the soil, two steps were carried out: (1) the litter layer was removed (Lozano-Baez et al. 2020;Murta et al. 2021), and (2) a thin layer of fine sand was added to the surface. Rates of water discharged through the infiltrometer, inferred from changes in water levels in the storage chamber, were recorded until steadystate rates were reached.

Data analysis
The residuals of all variables were examined using the Shapiro-Wilk normality test. Water repellency and infiltration showed a non-normal distribution, whereas monthly rainfall and mean water table depth followed a normal distribution. Monthly rainfall and depth of water table by season were compared using one-way ANOVA. Differences in water repellency and infiltration were examined using the Wilcoxon test (W). Spearman correlation was used to verify the degree of association between water repellency and infiltration. Pearson correlation verified the association between rainfall and mean water table depth. All statistical analyses were performed using the Paleontological statistics software-PAST version 3.25 (Hammer et al. 2001) at p < 0.05.
A shallow water table was observed throughout the study period (Fig. 4), and there was a significant difference between seasons (F = 22.73; df = 4.551; p < 0.05). Monthly rainfall had a significant inverse correlation with water table depth (r = − 0.76; p = 0.004).
The majority of the sampling units (96%) had no water repellency in the wet season. This was not the case in the dry season where 90% of the sampling showed a variety degrees of repellency (Table 1).
Water repellency was not associated with the infiltration capacity of the wet season (Fig. 8a). This was not the same during the dry season, i.e., water repellency was associated with infiltration in the dry season (Fig. 8b). An inverse relationship (R = − 0.4558; p < 0.01) was observed between these variables.

Discussion
Our tropical riparian forest showed a clear development of soil water repellency in the dry season with a substantial reduction in infiltration. The increase in the depth of the water table, the lack of rainfall, and the effect of evapotranspiration may together have promoted a decrease in soil moisture. Low moisture content, in turn, increased the action of organic compounds which drive soil repellency (Mao et al. 2019). However, soil water repellency was markedly decreased in the wet season, and as a consequence, infiltration capacity increased significantly.
Previous research has suggested that the development and persistence of repellency in soils could promote other hydrological processes such as overland flow Jordán et al. 2013). Our findings demonstrate that in tropical riparian forests with a highly marked seasonal climate, such  phenomenon does not persist longer than the dry season, i.e., it is a reversible process. Therefore, hydrological ecosystem services related to such tropical riparian forests might not be reduced. Though reversible, a threshold of soil moisture has to reach the soil to lessen water repellency (Doerr and Thomas 2000;Leighton-Boyce et al. 2005;Täumer et al. 2005;Vogelmann et al. 2017). Thus, in the time to reach such a threshold, problems such as erosion might arise in a given ecosystem (Witter et al. 1991;Piyaruwan and Leelamanie 2020;Lowe et al. 2021). This may likely be the case under climate change. For example, more frequent droughts and high-intensity rainfall are expected (Jehanzaib and Kim 2020;Tabari 2020), and water repellency could be greatly enhanced in extreme dry events (Goebel et al. 2011;Mao et al. 2019). The combination of extreme waterrepellent soils with high-intensity rainfall events may activate overland-flow (Mao et al. 2019). Therefore, under such situations, the ability to retain overland-flow, sediments, and nutrients may be reduced.
Climate change can favor increased wildfires (Liu et al. 2010), which is directly related to soil water repellency (De   Bano 1981;Zavala et al. 2009;Rodríguez-Alleres et al. 2012). Therefore, if riparian forests are affected by fire, repellency will be more intense, decreasing the infiltration capacity and increasing runoff. Some tropical riparian forests have already suffered damage due to fires (Flores et al. 2021), which leaves a concern regarding ecosystem services.
Despite their proximity to streams, shallow water table and high contribution area, tropical riparian forests may develop soil water repellency in the dry season. This is not the first time soil water repellency has been reported in riparian areas (Ruwanza et al. 2013;Moret-Fernández et al. 2019). However, to the best of our knowledge, this is the first time repellency has been detected in tropical riparian forests.

Conclusions
The initial questions may be answered as follows: (1) the soil of a tropical riparian forest developed water repellency in the dry season; and, (2) soil water repellency was associated with infiltration reduction in the dry season but not in the wet season.
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