The effect of vegetal mulching on soil surface temperature in semiarid Brazil
Article Category: Research Article
Publicado en línea: 30 may 2021
Páginas: 185 - 195
Recibido: 28 ene 2019
Aceptado: 07 dic 2020
DOI: https://doi.org/10.2478/boku-2020-0016
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© 2021 João L.M.P. de Lima et al., published by Sciendo
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License.
Water scarcity and soil erosion are major natural barriers to the economic development of semiarid regions of the Brazilian Northeast, contributing to severe social problems and inequality. These regions are highly susceptible to desertification processes, since they are characterized by soils with low infiltration capacities and irregular rainfall patterns, with high intensity and low frequency rainfall events occurring mainly in the beginning of the rainy season when soil is more susceptible to evaporation and erosion (e.g., Montenegro and Ragab, 2010; Oliveira et al., 2010; de Lima et al., 2013). Moreover, these development barriers are even more intensified by the growing demand for agricultural and forestry products, the inadequate agricultural management, the unplanned soil use and soil occupation, the uncontrolled deforestation of native vegetation (e.g., Caatinga) and the extensive forest fires (e.g., Albuquerque et al., 2001; Brasileiro, 2009; Montenegro and Ragab, 2010). The climatic conditions of this region together with the land use changes and the lack of conservative agricultural practices may lead to reduction of crop productivity and desertification.
Among the different hydroclimatic factors that can affect agriculture, soil temperature plays a key role in crop productivity, especially in semiarid regions characterized by soils with low thermal diffusivity and conductivity. Understanding the temporal and spatial dynamic of soil temperature is crucial, since it acts as a control mechanism for water evaporation and soil biological activity, influencing seed germination, crop initial growth, root development and water and nutrients absorption by crops (e.g., Silans and Werlang, 2011; Poll et al., 2013; Huang et al., 2014), and ultimately the runoff process.
Soil temperature depends on incident radiation, soil type, water content, air temperature, land use and land cover (e.g., Ozgener et al., 2013; Huang et al., 2014; Odriozola, et al., 2014). Several studies have shown that mulching is one of the most efficient soil and water conservation method used to promote water infiltration, reduce surface runoff and prevent soil erosion in agricultural areas (Cook et al., 2006; Jordán et al., 2010; Zonta et al., 2012; Montenegro et al., 2013b). However, mulching of different types, coverage and applied area densities also play a role as a controlling technique of soil temperature fluctuations and water evaporation (Tripathi and Katiyar, 1984; Jin et al., 2008; Li et al., 2013; Montenegro et al., 2013a). In the semiarid region of Plateau Loess in the Northwest of China, Jun et al. (2014) verified that mulching reduced maximum soil temperature values and increased the minimum values. It is of crucial importance to study the effect of different types of mulch, applied with different densities, on soil temperature, and evaluate the technical and economic viability of this controlling technique. No data is available for the Brazilian semiarid region, therefore, this experiment aimed at evaluating the effect of vegetal mulching on soil surface temperatures. The mulches used in this study were: elephant grass straw, cashew tree leaves and coconut powder (coconut coir dust). Several mulch densities were used to monitor the temporal dynamic of soil surface temperature, aiming at contributing to improve soil and water conservation management, under field conditions, of the Brazilian semiarid region. Several day/night cycles were monitored to support future modelling of processes at the soil surface.
This study follows others that made use of infrared cameras to study soil surface hydrological processes (Abrantes and de Lima, 2014; de Lima and Abrantes, 2014a, 2014b; de Lima et al., 2014a, 2014b, 2014c, 2015; Abrantes et al., 2016, 2017, 2018a, 2018b).
The study area is located in the representative catchment of Mimoso, with an area of 149 km2, which is part of the Alto Ipanema catchment, with an area of 183 km2, located in the Brazilian semiarid region in the Pernambuco State (Figure 1). The study area consists essentially of ephemeral streams, where soils with thickness less than 2 m at the hillslopes are dominant. Communal rainfed agriculture is developed in these hillslopes.
Figure 1
Location of the study area in the Alto Ipanema catchment (Pernambuco, Brazil)
Abbildung 1. Lage der Untersuchungsfläche im Einzugsgebiet des Alto Ipanema (Pernambuco, Brasilien)
According to the Köppen classification, the climate of the study area is within the BSsh category (hot, semiarid), typical of the Caatinga, with a mean annual precipitation of 700 mm, a mean annual evapotranspiration of 1600 mm and a mean annual temperature of around 25°C.
Experiments presented in this study were conducted on a hillslope with a 5% slope on an Abruptic Eutrophic Yellow Argisol (Santos et al., 2010). At a depth between 0.00–0.17 m, the soil texture was comprised of 34% of sand, 24% of clay, 37% silt content and 5% organic matter (loam on USDA Soil Texture Triangle), a soil bulk density of 1.73 kg m−3, and a saturated hydraulic conductivity of 0.13 m h−1.
Soil temperature was measured using two methods:
A conventional method comprising a group of ten T model thermocouple sensors, resting on the soil surface and connected to a data logger (CR1000 from Campbell Scientific). Sensors had a measuring temperatures range of −67 to +400°C and the accuracy was < 0.5°C. Soil temperature fluctuated from nearly +50°C to around +15°C at the time of the experiments. A thermographic technique using a portable hand-held infrared camera (Model E6 from Flir Systems) with an IR pixel resolution of 19200 (160×120), a field of view of 45° × 34° and a thermal sensitivity of < 0.06°C. The thermocouple sensors provided high temporal resolution data during day period from 6h00 to 17h59 and night period from 18h00 to 5h59, while the infrared camera provided the spatial distribution of the temperature during the day.
Soil surface temperature monitoring started on 31st October 2014, at 10:00 a.m., and went to the 4th November 2014, at 8:00 a.m., during a dry spell, after many days without rainfall, with the soil moisture content at wilting point. During this period, considered to be representative of the situation in the semiarid “Agreste” region of Pernambuco State, Brazil, soil surface temperature was monitored with both the thermocouple sensors and with the infrared camera.
Thermocouple sensors registered the soil surface temperature in 10 min intervals. Data was stored in the data logger and then downloaded for statistical analyses.
Thermograms of the soil surface, on bare soil and beneath the structures covered with mulch, were recorded using the infrared camera, positioned at 1.5 m height. Thermograms were recorded at every hour between 10:00 a.m. and 5:00 p.m., and were later analyzed. Statistical analysis of the data set was carried out applying Tukey test at 5% significance. The field experiments provided a detailed dataset to be used by modelers.
Ten different soil cover treatments were considered, as shown in Figure 2: a bare soil treatment (without mulch) and nine mulching treatments that included three types of mulch with three different area densities (2, 4 and 8 t ha−1 dry weight; dry mass applied per unit area). Mulch types used in this study consisted of elephant grass straw (
Figure 2
Schematic representation of the monitoring procedure: thermocouple sensors, infrared camera, soil cover treatments (dried elephant grass straw, dried cashew tree leaves and coconut powder on removable frames as well as the bare soil)
Abbildung 2. Schematische Darstellung des Monitoringverfahrens: Thermoelementsensoren, Infrarotkamera und Bodenbedeckungsvarianten (Elefantengrasstroh, Kaschubaumblätter und Kokusnusspulver auf entfernbaren Rahmen sowie der vegetationslose Boden)
Mulch was not directly spread over the soil surface. Instead, mulch was uniformly spread over nine square frames of 1 m2 with plastic meshes of 20×20 mm2 aperture, which in turn were placed over the soil surface (Figure 2). This setup resulted in a movable structure that allowed obtaining several thermograms of the soil surface throughout the experiments with the infrared camera, by previously removing the structures covered with mulch and then repositioning it again, without affecting the soil surface and mulch distribution. The structures were positioned side by side, next to each other, in order to obtain the same experimental conditions for all mulch densities (e.g., soil characteristics, slope, and incident radiation). Thermocouple sensors were installed on the soil surface for all soil cover treatments and the bare soil. The thermocouples were placed beneath the structures covered with mulch, as schematized in Figure 2.
Soil surface temperature variation over time, for the bare soil and for the nine different mulching treatments can be seen in Figure 3. Mulching provided a buffer zone dampening soil surface temperature fluctuations over time, reducing soil surface temperature during the day and increasing it during the night. Mulch regulate soil temperature as verified, for example, by Li et al. (2013).
Figure 3
Soil surface temperature recorded with thermocouples for the bare soil and for the three types of mulch: elephant grass straw (top), cashew tree leaves (middle) and coconut powder (bottom).
Abbildung 3. Mittels Thermoelementen gemessene Oberflächentemperatur des Bodens vom nackten Boden und drei Mulchvarianten: Elefantengrasstroh (oben), Kaschubaumblätter (Mitte) und Kokusnusspulver (unten).
In general, the lowest soil surface temperature fluctuations were observed for the coconut coir dust at the area density of 8 t ha−1. Bare soil presented the highest daily soil surface temperature fluctuations, for example, on the 3rd of November 2014, it was observed a thermal variation of 34.7°C, with a maximum of 53.1°C at 1:00 p.m. and a minimum of 18.4°C at 05:00 a.m. Variation of soil surface temperatures, for the different soil mulching covers and respective densities, for both day and night periods, can be seen in Figure 4.
Figure 4
Soil surface temperatures of the different soil mulching covers and respective densities, and for bare soil, for both day and night periods
Abbildung 4. Oberflächentemperatur des Bodens bei verschiedenen Mulchauflagen und Mengen sowie dem nackten Boden während des Tages und der Nacht
Figure 5 further illustrates the effect of mulching in daytime and night-time soil surface temperature (same sampling time of Figure 3). It can be seen that, for the mulch densities of 2 and 4 t ha−1, cashew tree leaves were the most efficient alternative in reducing day-time soil surface temperature, with an average reduction of 7.1 and 9.7°C, respectively, compared to bare soil. For the same mulch densities, elephant grass straw showed a mean reduction of 2.4 and 6.3°C, while coconut powder showed an average reduction of 5.0 and 7.9°C. For the higher mulch density of 8 t ha−1, the following was observed: a mean reduction of day-time soil surface temperature of 10.9°C for the coconut powder, 9.1°C for the elephant grass straw and 8.8°C for the cashew tree leaves.
Figure 5
Soil surface temperature variations for both day and night periods compared to the bare soil (recorded with the thermocouple), of the different mulching treatments. Mean values from data recorded between 10:00 a.m. of October 31, 2014 and 8:00 a.m. of November 4, 2014.
Abbildung 5. Die Abweichungen der Oberflächentemperatur des Bodens der Mulchvarianten vom nackten Boden während des Tages und der Nacht (mittels Thermoelement gemessen). Es werden Mittelwerte der vom 31. Oktober 2014, 10:00 Uhr, bis zum 4. November 2014, 8:00 Uhr, aufgenommen Daten gezeigt.
The night-time buffering effect induced by the mulching, which increased soil surface temperature during the night, did not show any clear relation with the mulch density or mulch type. However, in average, the highest increase in night-time soil surface temperature was observed for the mulch of cashew tree leaves at a density of 4 t ha−1.
Based on measured soil surface temperature data (day and night periods; hourly data) comparison is made between the means for the different mulching covers scenarios (Tukey test to a significance level of 5%).
According to Table 1, there exists a significant difference between the bare soil and the other treatments, stressing the importance of vegetable mulch cover in semiarid environments. Nevertheless, the only case where a significant difference to bare soil was not observed was for the 2 t ha−1 elephant grass straw cover (day period). Differences due to mulch density were noticeable (e.g., elephant grass straw covers and coconut powder, 2 t ha−1 and 8 t ha−1 showed significant differences for the day period).
Mean values of soil surface temperatures for different mulching covers, for day and night periods
Table 1. Mittelwerte der Oberflächentemperatur des Bodens der verschiedenen Bodenbedeckungsvarianten für die Tag- und Nachtperioden
| Soil cover treatment | Mulch density (t ha−1) | Mean temperature (°C) | |
|---|---|---|---|
| Day | Night | ||
| Bare soil | 0 | 39.13a | 22.18f |
| Elephant grass straw | 2 | 36.69ab | 25.77bcd |
| 4 | 32.87cd | 23.71e | |
| 8 | 30.02def | 26.64ab | |
| Cashew tree leaves | 2 | 32.01cde | 26.13abc |
| 4 | 29.41ef | 27.01a | |
| 8 | 30.31def | 26.56ab | |
| Coconut powder (coir dust) | 2 | 34.09bc | 26.25abc |
| 4 | 31.26cdef | 25.32cd | |
| 8 | 28.22f | 24.18d | |
Mean temperature values followed by different letters on the same column do not differ in 5% level of significance, by Tukey test. The day period was considered from 6h00 to 17h59 and night period from 18h00 to 5h59.
Figure 6 shows the box plot of soil surface temperature recorded with the thermocouples, for the ten soil cover treatments (see also Figure 4 and Table 2). The thermal data present a positive asymmetric distribution, that is, the median values are closer to the first quartile. The bare soil showed the highest data variability, where 50% of the recorded values ranged from 24.6 to 40.3°C. In general, higher mulch densities reduced the thermal data dispersion for all the different mulch types. Through statistical descriptive analysis, it can be observed that lower standard-deviation occurred for the higher area density of 8 t ha−1 (Table 2).
Figure 6
Box plot of soil surface temperatures recorded with the thermo-couples of the different soil cover treatments
Abbildung 6. Box-Plots der Oberflächentemperatur des Bodens der verschiedenen Bodenbedeckungsvarianten
a) Basic statistics of soil surface temperature register from October 31 to November 4, 2014; b) Data obtained from the soil surface thermograms of the different soil cover treatments, recorded at 10:00 a.m. and 2:00 p.m. of October 31, 2014 (see Figure 7)
Table 2. a) Grundlegende Statistiken der Temperaturdaten vom 31. Oktober 2014 bis zum 4. November 2014, 8:00 Uhr; b) Daten der Thermogramme der Bodenoberfläche der verschiedenen Bodenbedeckungsvarianten, aufgenommen vom 31. Oktober 2014, 10:00 Uhr, bis zum 4. November 2014, 8:00 Uhr (vgl. Abbildung 7)
| a) | |||||||
|---|---|---|---|---|---|---|---|
| Soil cover treatment | Mulch density (t ha−1) | Temperature (°C) | |||||
| Mean | Mode | Median | Max. | Min. | S.D. | ||
| Bare soil | 0 | 30.4 | 24.0 | 24.6 | 53.1 | 18.4 | 10.2 |
| 2 | 31.1 | 25.3 | 27.7 | 52.0 | 23.0 | 7.3 | |
| Elephant grass straw | 4 | 28.2 | 34.4 | 25.7 | 41.4 | 21.0 | 5.7 |
| 8 | 28.2 | 29.9 | 27.5 | 35.9 | 24.2 | 2.7 | |
| 2 | 29.0 | 32.5 | 27.6 | 39.5 | 24.0 | 3.9 | |
| Cashew tree leaves | 4 | 28.1 | 29.2 | 27.3 | 34.0 | 25.0 | 2.1 |
| 8 | 28.3 | 30.6 | 28.1 | 36.2 | 24.6 | 2.8 | |
| 2 | 30.0 | 23.1 | 26.8 | 47.2 | 23.1 | 5.9 | |
| Coconut powder (coir dust) | 4 | 28.2 | 23.1 | 25.6 | 43.0 | 22.4 | 4.5 |
| 8 | 26.4 | 24.6 | 24.6 | 33.0 | 22.9 | 2.6 | |
| Max stands for maximum. Min stands for minimum. S.D. stands for standard deviation. | |||||||
| b) | ||||||
|---|---|---|---|---|---|---|
| Soil cover treatment | Mulch density (t ha−1) | Temperature (°C) | ||||
| 10:00 a.m. | 2:00 p.m. | T14:00−T10:00 | ||||
| T10:00 | TM-TBS | T14:00 | TM-TBS | |||
| Bare soil | 37.9 | 54.1 | 16.2 | |||
| Elephant grass straw | 2 | 32.6 | −5.3 | 40.2 | −13.9 | 7.6 |
| 4 | 29.4 | −8.5 | 37.2 | −16.9 | 7.8 | |
| 8 | 28.9 | −9.0 | 33.5 | −20.6 | 4.6 | |
| Cashew tree leaves | 2 | 28.0 | −9.9 | 33.7 | −20.4 | 5.7 |
| 4 | 29.6 | −8.3 | 33.0 | −21.1 | 3.4 | |
| 8 | 29.2 | −8.7 | 31.3 | −22.8 | 2.1 | |
| Coconut powder (coir dust) | 2 | 28.2 | −9.7 | 31.3 | −22.8 | 3.1 |
| 4 | 28.6 | −9.3 | 29.9 | −24.2 | 1.3 | |
| 8 | 27.8 | −10.1 | 29.4 | −24.7 | 1.6 | |
| T10:00 stands for temperature at 10:00 a.m. T14:00 stands for temperature at 14:00 p.m. TM stands for temperature of mulching treatment. TBS stands for temperature of bare soil. | ||||||
Thermograms of the soil surface (i.e., beneath the mulch cover) of the ten different soil cover treatments, recorded at 10:00 a.m. and 2:00 p.m. of October 31, 2014, are shown in Figure 7, considered representative for the studied period. Thermal data obtained from the thermograms is presented in Table 2. It can be seen that mulching cooled the soil surface (darker blue colors). This is more evident in the thermograms obtained at 2:00 p.m., where the mean soil surface temperature of the bare soil was higher. At 10:00 a.m. a reduction of the soil surface temperature between 5.3 and 10.1°C was observed, and at 2:00 p.m. one between 13.9 and 24.7ºC. Thus, mulching is an efficient alternative soil protection technique, by reducing soil surface temperature during the day (see Figure 7 and Table 2). Zang et al. (2009) also investigated the effects of mulching on soil temperature, soil moisture and wheat yield on the Loess Plateau of China, and observed that mulch promoted reduction of soil temperature for the warmer periods, and kept soil warmer for cooler times when compared to bare soil. Montenegro et al. (2013a) verified the positive impact of mulch on soil moisture, temperature and runoff, under controlled laboratory conditions, using rice straw mulch density of 4 t ha−1. In this study, it can be observed that mulching during the day (warm period) reduced soil temperature, and in the night (coldest period) maintained the soil temperature compared to the bare soil.
Figure 7
Thermograms of the soil surface for the different soil cover treatments, recorded at 10:00 a.m. and 2:00 p.m. of October 31, 2014.
Abbildung 7. Thermogramme der Bodenoberfläche der verschiedenen Bodenbedeckungsvarianten, aufgenommen vom 31. Oktober 2014, 10:00 Uhr, bis zum 4. November 2014, 8:00 Uhr.
Figure 8 shows the daily fluctuations of the difference between the soil surface temperature of the mulching treatments (TMS) and the bare soil (TBS), obtained from the thermograms recorded with the infrared camera. It can be seen that the soil surface temperature was attenuated by the presence of the mulch, especially during the hottest periods of the day.
Among the different types of mulch, the density effect was more evident for the elephant grass straw. At the hottest moment of the day (11:00 a.m.), mulch of elephant grass straw decreased soil surface temperature in 12.1, 18.7 and 22.7°C for the densities of 2, 4 and 8 t ha−1, respectively. Mulch from cashew tree leaves provided a reduction of 22.8, 22.8 and 22.5°C for the densities of 2, 4 and 8 t ha−1, respectively, and for the same densities, the mulch with coconut powder provided a reduction of 23.9, 23.9 and 25.4°C, respectively.
Figure 8
Daily fluctuations of the difference between the soil surface temperature of mulching treatments (TMS) and bare soil (TBS). Thermal data (spatially averaged) obtained from the thermograms recorded with the infrared camera.
Abbildung 8. Die täglichen Schwankungen der Differenz von der Oberflächentemperatur der Mulchvarianten (TMS) und des nackten Bodens (TBS). Die Daten der Thermogramme wurden mittels Infrarotkamera aufgenommen.
The results of this field study clearly show that vegetative mulching (e.g., dried elephant grass straw, dried cashew tree leaves and coir dust), a commonly used low cost soil and water conservation technique, strongly influences soil surface temperature, constituting an effective controlling technique of soil temperature fluctuations in semiarid regions. Mulching provided a buffer zone, dampening soil surface temperature fluctuations over time, reducing soil surface temperature during the day, especially in the hottest periods, and increasing it during the night. However, mulch was more competent in reducing temperature during the day than increasing it during the night. All types of mulch studied had a satisfactory equivalent performance in controlling soil surface temperature; application of 8 t ha−1 was most effective during the day; elephant grass straw was less effective for the lower mulch density. In general, higher mulch densities are more efficient in reducing soil surface temperature fluctuations.
In future work, soil surface temperature dynamics should consider soil moisture variations and incorporate surface hydrologic processes, such as evaporation, evapotranspiration, infiltration and even runoff. Also, extension of the field work to different geographical regions and considering dry and wet spells could give a better insight on how effective mulch can be in controlling soil surface temperatures.