The surface particle distribution is dominated by fine and very fine sand. However, the soil profile is mainly composed by fine and medium sand. There is almost no silt along the whole soil profile, and clay is scarce on the surface, but appears in higher quantities in deeper layers, evidencing a washed soil. Soil porosity (Table 2
) is equilibrated between macro and micro pores, as medium to very fine sand are the dominant grain sizes, increasing porosity with depth. The soil profile has a moderate to rapid hydraulic conductivity, with low water retention between field capacity (−0.33 bar) and wilting point (−15.00 bar).
Not all rainfall events produced runoff (Figure 3
), and runoff did not always occur on both treatments. Of the 122 rainfall events (1459 mm) in the first year, only 48 events (totalling 1326 mm) generated runoff from the pasture, and 41 events (1259 mm) from the sugarcane. In the second year, there were 131 rainfall events (1389 mm), of which 34 (1073 mm) produced runoff from the pasture, and 33 events (977 mm) from the sugarcane (Figure 4
). The lowest amount of rainfall to produce runoff was 6 mm. All rainfall events with an intensity over 5 mm within 10 min or more than 15 mm in total produced runoff. The first two rainfall events after the dry season in 2012 did not produce runoff (9.8 and 14.4 mm).
presents the average runoff results per event with the precipitation that generated them. The column represents an individual event, with the exception of successive events, when the 6-h interval occurred during the night, making it impossible to collect independent samples. This occurred on 10 December 2011, 11 January, 14 May, 8 June, 3 and 17 December 2012, 4 February, 11 March, 29 May, and 1 October 2013. The events with the most runoff occurred in the following situations: (i) high level of rainfall (total per event) during the rainy season (10 December 2011, 23 January and 28 December 2012, 13 January and 29 May 2013); (ii) frequent rainy periods (16 to 22 January 2012); or (iii) high intensity events (12 February, 21 April, 25 October 2012, 6 February, 12 March, 3 April and 2 October 2013). The runoff values recorded for sugarcane during high intensity events were similar among themselves during the first year (1 to 3 mm) due to the absence of crop mulch in the soil. However, this did not happen during the second year.
The amount of runoff under sugarcane was larger than under pasture in the first year, except for the first two rainfall events, when soil and grass were disturbed due to plot construction. In the second year, the opposite occurred, as sugarcane generated less runoff. This was attributed to the effect of the sugarcane mulch which presented on the soil surface, with the influence of the established cane root system. In the first year, 40.5 mm of runoff from the pasture was recorded (equivalent to 2.8% of the total rainfall), while in the sugarcane, the amount was 56.1 mm (3.8%). In the second year, the pasture generated 56.8 mm of runoff (4.1% of the rainfall) while sugarcane produced only 13.1 mm of runoff (0.9% of the rainfall).
The highest runoff (12.5 mm runoff from a rainfall of 51.8 mm, 37.4%, on 23 February 2012) occurred early in the first year, when the sugarcane soil was bare and without residue mulch. In the following year, the sugarcane residue mulch was not uniformly distributed in the field, but was accumulated in every third row, emulating the operation of the harvester. Nevertheless, this accumulation of residue—combined with the sugarcane canopy and roots—decreased the amount of runoff. With the mulch on the soil, the highest total and percentage runoff figure reduced to 3.1 mm and 9.0%, respectively, for the event of 34.4 mm on 12 March 2013. During the same event, the pasture produced its highest runoff percentage for that period (10.3 mm, 37%). This high runoff resulted from the high intensity of this event (10 mm in 10 min), after two consecutive days of rainfall. However, the highest absolute runoff amount (14.9 mm, 26% of the annual total) for the pasture was recorded during the second year (29 May 2013) due to a combination of high rainfall (115.4 mm), long duration (42.5 h), and the event intensity (5.2 mm 10 min−1). Soil compaction from cattle grazing may also have contributed to a larger runoff from pasture than from second-year sugar cane.
Based on these results, it can be said that switching pasture by sugarcane increased runoff by 15.6 mm (1.1% of precipitation) during the first year, but reduced runoff by 43.7 mm (3.1% of precipitation) during the second year. Analysing the treatment results with the Mann–Whitney test shows statistical differences in the first year measurements (p = 0.008). The second year did not result in significant differences (p = 0.2287). In the second year, sugarcane runoff was reduced, but the pasture results exhibited higher values and higher variability of runoff. Within each treatment, there were significant differences between the first and second year for sugarcane (p = 0.031), but not for the pasture (p = 0.095).
The erosion results exhibited a declining trend subsequent to plot construction and sugarcane planting. Erosion for the pasture was lower than for the sugarcane, except during the first two events following the soil disturbance by the plot construction process. The amount of pasture sediment then decreased and remained between 0 and 0.05 Mg·ha−1
The erosion process for the plots with sugarcane presented three phases during the first year. In the first phase—from planting to January 2012—erosion reached its highest levels with little runoff. The runoff was contained between the contour rows of the sugarcane plants (except in December 2011 under successive events of 82 and 51 mm), where sediments were transported by splash and wash from the uncovered soil close to the collectors.
The second phase (from January to March 2012) occurred when the crops started to grow. An increase in runoff due to crusting and sealing of the soil surface was observed in the field as described in the literature [23
], due to the decreased roughness of the soil [28
] and due to sedimentation and break-overs of the contour beds. Sediment was transported along the plots by the concentrated runoff, especially during events with high amounts of rainfall, high intensity, or high frequency, when runoff accumulated along the plot length.
The third phase began in March 2012 and was associated with a reduction in the precipitation frequency leading to reduced erosion, along with the fact that the crop canopy coverage was now developed. The plants protected the soil from the rainfall impact and created a barrier of leaves that reduced runoff. In this phase, the average erosion was 0.01 Mg·ha−1 per event, which remained the same until harvest.
In the first year, the crop is referred to as plant-cane, and along the successive years as shoot-cane, with the cane re-growing from the previous roots. Erosion was reduced from 2.58 Mg·ha−1·year−1 for plant-cane, to 0.50 Mg·ha−1·year−1 for shoot-cane. This reduction is attributed to several factors: the growth of new canopy is faster with shoot-cane than with plant-cane, a perennial root system is already established, and mulch from the previous harvest remains on the soil every three rows, thus considerably reducing erosion.
In the first year, the pasture soil loss was 0.58 Mg·ha−1·year−1, although 0.32 Mg·ha−1·year−1 of this occurred during the first events that occurred when the soil was disturbed by the plot construction process. In the second year, erosion decreased to 0.06 Mg·ha−1·year−1. The results show that the substitution of pasture by sugarcane increased the production of sediments by 2.32 Mg·ha−1 for plant-cane (first year) and by 0.43 Mg·ha−1 for shoot-cane (second year).
In Table 3
, the events that generated the highest runoff and soil loss rates, accompanied by information about the precipitation events (rainfall depth, maximum intensity, and duration), are presented. In general, during the first year erosion is more likely to occur, after the construction of the plots on the pasture area (methodological distortion, not representative of the land cover). In the sugarcane plots, the highest values were caused by the soil tillage and its initial condition as fallow.
The Mann–Whitney test gave significant differences between levels of sediment production from the two treatments for the first and the second year (p < 0.001). Within each treatment, there were differences between plant-cane and shoot-cane (p < 0.001) and a narrow difference between the first and second year for the pasture (p = 0.049), due to the higher erosion rates after plot construction.