Banana (Musa spp.) is one of the most widely grown fruits in the world and a major food source for the planet’s population. Its cultivation in the semiarid usually requires irrigation to meet crop water requirements (e.g., [29]). Moreover, several authors have reported high water consumption for a range of banana cultivars in different soil and climate conditions (e.g., [10-12]).

[25] emphasize the importance of studying soil temperature, since it will be affected by predicted changes in global temperature. Recent studies show that every 1 °C increase in air temperature raises the associated soil temperature by 1.5 °C. Changes in soil temperature may impact the regulation of the plant physiological process. [24] state that temperature regulates gas exchange on the surface, affects the movement, viscosity and density of soil water and, consequently, water and nutrient absorption by plants, thus influencing most soil physical, chemical and biological processes.

The Brazilian semiarid is subject to frequent droughts. In particular, the present study was conducted during the 2014-2015 drought, which was severe and affected a large area of Northeast Brazil, with significant impacts on the population and economic activities, as reported by [5].

Mulching provides greater hydraulic roughness, increasing water infiltration into the soil and reducing surface runoff velocities and, consequently, soil losses. This protects the soil surface from the direct impact of raindrops, thereby reducing surface crusting, compaction, splash erosion and water evaporation and controlling soil temperature fluctuations, particularly during dry periods (e.g., [1-8,17,20-22,27,39]).

Among the different organic materials used as mulch, coconut coir dust, a by-product from the coconut (Cocos nucifera L.) industry, is known to be efficient in controlling runoff and soil loss (e.g., [17,20,38]). Coir is an inexpensive, highly durable natural fiber that biodegrades slowly.

In semiarid locations, mulching can benefit irrigated crops by improving water use efficiency and fruit physical and chemical properties (e.g., [9,35]). Moreover, irrigation contributes to temperature regulation, helping boost agricultural production [16,33] as a direct result of soil moisture availability for evapotranspiration.

[24] report that temperature regulates gas exchange at the soil surface, affecting water and nutrient absorption by plants and influencing chemical and biological processes. Mulch cover lowers the maximum soil temperature and temperature range when compared to a bare surface [6].

As such, mulching may be relevant for soil physical amelioration, soil temperature control, and water conservation. However, the type and cover density of mulch may have different effects, depending on crop stage, soil type, and climate conditions. [14] adopted 4-6 t ha^-1 straw mulching in semiarid Nigeria and observed an improvement in soil physical properties. [37] used 9 t ha^-1 bean straw mulching in a highly heterogeneous alluvial valley in northeastern Brazil (the same soils used in the present study) for carrot (Daucus carota) production, with micro sprinkler irrigation. Mulching at a 9 t ha^-1 cover rate was efficient in conserving soil moisture and decreasing soil moisture spatial variability. Later, [21] used soil flumes in a laboratory and found that 2 and 4 t ha^-1 of rice straw mulch was efficient at lowering soil temperature and raising soil water content. Although the conditions described in this paragraph differ from those of the aforementioned study (e.g., in terms of mulch type, soil physical characteristics, plantation pattern and water application regime), they show a general trend in terms of mulch effects.

Comparing different mulch materials (coconut coir and cashew leaves, among others) and densities, [16] conducted an in-depth field study in semiarid Brazil and found that 4 t ha^-1 of coconut mulch was effective at controlling temperature fluctuations and enhancing soil moisture under natural conditions. On the other hand, in a detailed investigation in the same alluvial valley studied here, [26] used geostatistics and Beerkan Infiltration Tests and observed high spatial cross dependence between soil physical characteristics and hydraulic properties. The authors recommended adopting soil management techniques to support irrigation management in areas with low infiltrability. In this respect, the present study aimed to analyze the impact of mulching cover as a management alternative in low infiltrability zones.

Thus, an in-depth short-term field experiment was conducted in an irrigated plot in the semiarid (near the location studied by [16] and [26]), to investigate the impact of i) different coconut coir mulch densities on surface temperatures and ii) temperature and soil moisture dynamics for different soil depths under 4 and 8 t ha^-1 mulch treatments, in a drip irrigated banana plantation with low rainfall input in an area with low infiltrability. Although several studies have been conducted on drip irrigation under artificial and natural mulch (e.g., [34,35,40]), loose coir mulch (e.g. coir dust) applied on bare soil has not been sufficiently investigated in terms of controlling soil temperature and moisture, since most research focuses on soil erosion issues. This type of thermal/moisture study had yet to be conducted on banana plantations in Brazilian semiarid alluvial valleys, particularly during a period of severe drought.

The study was carried out on Mimosa farm in the Mimoso District of the municipality of Pesqueira, Pernambuco state (Brazil), at 08° 10' S and 35° 11' W and an altitude of 613 m (Fig. 1). The farm is located in the alluvial valley of the Ipanema River, where small-scale irrigated community farming is carried out. Climate in the region is BSh (extremely hot, semiarid) according to the Köppen classification, with average annual rainfall, evapotranspiration and temperature ~700 mm, 1600 mm and around 25°C, respectively (e.g., [19]). Monitoring was carried out during a severe prolonged drought, according to [4].

Figure 1 Source: The authors

Figure 2 Source: The authors

Soil in the study area is characterized as Fluvic Neosol. Based on the [30] soil analysis manual, the soil profile is classified as sandy clay loam (USDA Soil Texture Triangle), consisting of ~59% sand, ~24% clay and ~17% silt. Soil bulk density is ~1.47 kg m^-3, porosity ~0.52 m³ m^-3, hydraulic conductivity ~0.05 mm s^-1, and soil moisture content at field capacity ~0.28 m³m^-3. Detailed infiltration tests have been carried out in the area [26]. Hydraulic conductivity in the zone selected for the irrigated plot was ~0.03 mm s^-1.

The Prata banana cultivar was grown in 4 m × 4 m regular spacing. At the time of the study, the plantation was already in the production phase (Fig. 2).

A drip irrigation system was used, with simple rows and 0.20 m spacing between drippers, flow rate of 1.25 L h^-1 and working pressure of 98.06 KPa. Irrigation began on November 27, 2014, one day before the soil temperature and moisture measurements. Water was pumped from a shallow water table well (large diameter well) in an unconfined aquifer with 0.90 dS m^-1. The water table was estimated once during the study period ~2.5 m below ground level, using an electronic measuring tape.

The irrigation depths applied were based on crop evapotranspiration (ETc), estimated from daily readings in Class A evaporation tank (Class A), using a tank coefficient of ~0.75 and crop coefficients (Kc) in accordance with [9].

Six thermocouples and 3 soil moisture probes were installed and connected to a Datalogger. Soil temperature was monitored by the thermocouples every 10 minutes. Soil moisture dynamics was measured with CS616 sensors (Campbell Scientific), also at 10-minute intervals. The experiments were carried out from November 28, 2014 to March 24, 2015, during the dry season and under a severe multiannual regional drought. Average temperature during the experimental period was ~25 °C.

Temperature variability was evaluated under 3 ground cover conditions: bare soil, and coconut coir dust at 4 and 8 t ha^-1. The mulch was not spread directly onto the ground surface. Instead, two 1 m² wooden frames with a thin flexible mesh screen were used to prevent the material from passing through and facilitate access to the sensors. The different coir dust densities were placed on the screens and the frames positioned over the soil surface. Coir dust was pre-washed and dried at ambient temperature. Fig. 3 shows a diagram of the field setup. Both mulch support frames had approximately the same sun exposure.

Figure 3 Source: the authors

Soil temperature for the 4 t ha^-1 experiments was measured on the surface (T_s), and at depths of 0.10 m (T₁), 0.30 m (T₂) and 0.50 m (T₃). For the 8t ha^-1 treatment, temperature was only measured at the surface (T_s8). Air temperature data were obtained from an automated weather station close to the study site. Soil moisture content was measured at depths of 0.10 (θ₁), 0.30 (θ₂) and 0.50 m (θ₃), but only for the 4t ha^-1 treatment, and as such, the CS616 sensors were placed alongside the thermocouples to establish the relationship between soil moisture and temperature. The probes used to monitor soil moisture were calibrated in the field using a gravimetric method. The methodology and calibration equations for the CS616 probes are in line with [32].

The daytime (6:00 a.m. to 5:00 p.m.) and nighttime periods (5 p.m. to 6 a.m.) were established based on solar radiation data from the weather station (approximated to the hour).

Statistical analysis was performed to assess the significance of the soil moisture and temperature variations and relate them to the daytime and nighttime periods at the different depths studied. The experimental results were submitted to analysis of variance (ANOVA), and when significant, means were compared using Tukey’s test at 5% significance. The variables were submitted to regression analyses, with significance determined by the F-test (p< 0.05).

Surface temperatures were lower during the day under coconut coir dust mulch when compared to bare soil (up to a 9% reduction for 8 t ha^-1).