Experimental design

The experimental design involved using a randomized block with three repetitions. Four dosages of hydrogel were tested: 0, 5, 10 and 15 kg ha^-1 as well as two vermicompost dosages: 0 and 40 t ha^-1 based on dry weight, corresponding to a 4x2 factorial, with 8 repeat treatments. The experimental unit was a micro-basin of 2 m² (2 m long x 1 m wide), built with machinery. The 8 treatments of each repetition were randomized in a north-south direction, with three rows for the three repetitions (one per repetition) with a distance of 4 m between rows and 4 m between each micro-basin within each row. In the center of each micro-basin, three plants were randomly selected that intercepts a 30 cm transect along the length of the micro-basin. From these plants, growth and development measurements were obtained.

The vermicompost was obtained from composting action of the Californian red worm (Eisenia fetida) processed at the University of Arid Zones Regional Unit of the Autonomous University of Chapingo with a predetermined nutritional content¹⁰. The hydrogel used was a granulated commercial product, which had a dry matter content of 85 to 90 %, bulk density of 0.85 g ml^-1, specific gravity of 1.10 g cm^-3, and pH of 8.1. The product has an absorption maximum of 150 times its own volume and retention capacity of 980 ml water L^-1, with an availability of 95 % and a productive life of 5 yr. Dosages that are recommended are 5 to 25 kg ha^-1, depending on the type of soil, crop and climate⁷.

Once micro-basins were built with use of pick and shovel, it was made a 20 cm bed floor where the vermicompost was mixed manually in the corresponding dosages per experimental unit. Later, grasses were planted simultaneously with the application of the hydrogel. The first at a density of 150 seeds m^-2 and second, corresponding to the dosage: 0, 5, 10 or 15 kg ha^-1. For this exercise, a rake was used to remove the surface layer of soil, then the seed and hydrogel were "scattered", trying to leave the seed and hydrogel slightly covered by a thin layer of moist soil not more than 5 cm after raking. The experiment was begun on June 21^st, 2012 and evaluations were conducted during 2012 and 2013.

Variables

Soil moisture content (%) at a depth of 20 cm were measured at different times: 14th and 29th of August and on the 4th and 19^th of December 2012, corresponding to 84, 99, 196 and 211 d after planting (dap), average readings were taken by measuring three sites within the micro-basin (two at the extreme ends and a site in the middle along the rectangle) by using a moisture analyzer Brand Lutron Model PMS-714, with real-time digital measurement readout. Percentage of seedling emergence of grass seedlings were obtained by counting germinated seeds, dividing the value by the total of seeds sown per unit area (m²) 10 d after planting, multiplied by 100. Plant height (cm) was measured for three plants in each micro-basin, with use of tape graduated in centimeters. Coverage area was recorded by using a 1 m² square decimeter graded mesh. Vigor, specifically referring to the condition of relative plant turgidity, on a scale of 0 to 5, where 0 corresponded to the plant almost wilting and 5 to a turgid plant and the other correspond to intermediate values. The last four variables were measured six times on different dates: 4^th of September, 10^th of October, and the 25^th of November 2012, and corresponded to 74, 110 and 156 dap, and on the 29^th of May, 11^th of September and the 7^th of October 2013, which corresponded to 241, 346 and 372 dap, respectively; while the dry matter in g m^-2 oven-dried to a constant weight was measured only once at 241 dap. Additionally, in the first week of November 2013 photosynthesis was measured in units of mmol m^-2 s^-1, conductance in mol m^-2 s^-1 and transpiration in mmol H₂O m^-2 s^-1, using an Infrared gas analyzer IRGA (for its acronym in English) Model LI-6400. For this end, leaf readings were taken from the upper middle third from each plant, in each of the micro-basins.

Data processing

SAS version 9.0 package was used for analysis of variance and Tukey mean multiple range test to determine the effect of treatments, and regression analysis to identify the rate of plant growth. Additionally, an analysis of variance and regression and multiple range test of means was carried out by sampling dates by arranging data in plots as a function of time¹¹. To do so, this technique averages the values of the first variation factors arising in the study (dosage of hydrogel and dosage of vermicompost) and then they are classified by repetition and sampling date.

Moisture content in soil

At 241 dap (29 May, 2013), the moisture content was significantly higher (P<0.05), and then decreased at 346 dap (September 11) and 372 dap (October 7), with average values of 22.5, 16.8 and 8.2 %, respectively (Figure 1). This trend is related to precipitation events during May and June, followed by a period of drought during July and August, which is accompanied by a significant lack of rainfall¹⁴. Although there was no significant difference in precipitation between 2012 and 2013, that the grass was already established the previous year, allowed for a greater expression of treatments applied to the micro-basins in 2013. Although the curve declined and showed significant differences at all three sampling dates, the soil moisture content was always higher (P<0.05) when the hydrogel was applied at least for the first two sampling times (241 and 346 dap), with values on the first date (241 dap) of 25, 23.2 and 23.4 % when applying 5, 10 and 15 kg ha^-1, respectively, vs 17.5 % in the control. A similar pattern was observed in the second sampling date (346 dap); while at 372 dap, the effect of moisture retention ceased to manifest itself, where different dosages of hydrogel were equal to the control (P>0.05) (Figure 2). This means that after about 5.23 mo (157 d after the first sample) without a significant rain event, the effect of soil moisture retention of the hydrogel is diluted, as soil moisture approaches the wilting point. Based on the above, the effect of the hydrogel was identified, at least in the first two samples in any of the dosages tested, indicating that the product has benefits from 5 to 10 kg ha^-1, without applying higher dosages. This is partially agreed with what Idrobo et al¹⁵ identified, where they assessed dosages of 0, 20, 30 and 40 g of hydrogel in 130 g of sand in each treatment, finding that with higher dosages of hydrogel they obtained better moisture retention effect, at least in the dosages used in their study. There was no consistent effect when vermicompost was applied.

^abc Values with different letters are different (P<0.05).

Figure 1 Soil moisture content at different sampling dates in days after planting (2013) 

dap = days after planting.

^abc Values with different letters in the same line, are different (P<0.05).

Figure 2 Soil moisture content with different dosages of hydrogel and sampling dates (2013) 

Seedling emergence, growth and plant development

Seedling emergence was 47.7 %, when 15 kg ha^-1 of hydrogel was applied, compared to 29 % for the control (P<0.05), which was higher by 18.7 %. The dosages of 5 and 10 kg ha^-1, had intermediate values, with no statistical difference compared to the control (Figure 3). Although the lowest dosage (5 kg ha^-1) tends to manifest an effect, it fails to differentiate itself from the control, and when applying 10 kg, a lack of an effect was confirmed since measured values are closer to the control. This can be considered contradictory, since with increasing dosages of 5-10 kg hydrogel, should confirm the effect of soil moisture retention and therefore have a greater impact on seedling emergence; nevertheless, the opposite was found, there was a decrease in the effect, which could be interpreted as a lack of consistency in response effect, which already manifests in a defined manner by applying 15 kg, with a significantly greater effect in terms of seedling emergence based on seed germination, which in previous laboratory evaluation was on average 92.5 %. The above results are consistent with those reported by Rojas et al¹⁶ regarding the use of hydrogel as soil moisture retainer, it has a positive effect on the capacity of germination of tomato (Lycopersicon esculentum Mill)¹⁶. The dosages of 5 and 10 kg hydrogel used in this study were less expressive and inconsistent in seedling emergence, which could mean that at this stage of seed germination of buffel grass high moisture content is required in the soil, which is expressed to a dose of 15 kg. There was no effect with the vermicompost on seed germination of grasses.

^ab Values with different letters between columns are different (P<0.05).

Figure 3 Effect of hydrogel dosages on germination percentage of buffel grass in rainwater capturing watersheds during August, 2012 

Even with the low rainfall regime in 2012 and the absence of treatment effect on the moisture content in the soil, it was identified that the growth of buffel grass remained constant, with significant growth in each evaluation date, at an exponential rate of 1.7 units per unit time (Figure 4A). This is an indicator of the high adaptability of this forage species, even in drought years^17,18. A similar behavior was observed with pasture development during 2013, with an exponential growth rate of 1.5 (Figure 4B).

^abc Values with different letter are different (P<0.05).

Figure 4 Growth of buffel grass in rainwater capturing watersheds, at different evaluation times during 2012 (A) and 2013 (B) 

The height of the plant was significantly higher (P<0.05) at 241 dap in either dosages of hydrogel, without any statistical difference between them, but statistically different from the control, with an average of 44.7 cm for the first and 34.6 cm for the second; consequently the dry matter weight behaved very similarly, significantly higher compared to any dosages of hydrogel with an average of 108.2 g m^-2, vs 81.7 g m^-2 for the control.

There was no significant difference in plant coverage and vigor in terms of hydrogel dosages (Table 1). This means that, under this system for capturing rainwater, soil moisture is retained by the hydrogel, affecting greater plant height and weight of dry matter; although it is not reflected both in terms of coverage and plant vigor. This is in line with that reported by Beltran et al¹⁹ where they indicate that applying water conservation practices increased soil infiltration and thus productivity in rangeland sites.

For this same sampling date and in relation to the effect of vermicompost, no statistical difference was identified in terms of coverage, vigor and plant height, although in terms of dry matter 125.1 g m^-2 was obtained by applying vermicompost, compared to 88.8 g m^-2 obtained in the control, 28.9 % more for the first compared to the second (Table 2). The effect of vermicompost was less consistent in relation to the hydrogel, especially in plant responses, without showing the benefits that are attributed to this component²⁰.

According to the results obtained, the development of buffel grass depends largely on the amount water retained in the ground, the latter is affected by the rate of evaporation due to high temperatures²¹. From the foregoing, it follows that hydrogel and somewhat vermicompost retained moisture in the soil, the first showed to have positive effects at any dosage, and improved development and plant growth and hence, results in increased production of biomass, which is consistent with reportes for Swiss chard (Beta vulgaris var. cycla)²² and tomato (Lycopersicon esculentum Mill)²³.

Photosynthetic activity and transpiration

Photosynthesis was significantly higher (P<0.05) in dosages of 15 kg ha-¹ of hydrogel at a rate of assimilation mmol 6.67 m^-2 s^-1 CO₂, compared to the assimilation rate obtained when 10, 5 and 0 kg ha^-1 were applied, with values of 4.05, 3.82 and 3.72, respectively. Similarly, at higher photosynthesis, increased conductance and transpiration, and vice versa (Table 3). Thus, the presence of moisture in the soil promotes plant photosynthetic activity, while water deficits decreases it²⁴. The photosynthetic activity of the grass was strongly influenced by the condition of soil moisture; this was identified on November 6, 2013, when the average soil moisture was 18.4 % when the hydrogel was applied, with no statistical differences between the dosages, but significantly higher than the control (16.4 %). All this indicates that the moisture content in the soil is influenced by hydrogel, which effected positively the physiology of the plant (Table 4) and some variables of growth and development, at least in plant height and plant coverage, with significant correlation values of 0.5669 and 0.5452, respectively. Similarly, a significant positive correlation was found between plant height and plant coverage with the amount of dry matter produced (Table 4). In the case of buffel grass the physiological activity such as photosynthesis, transpiration and conductance, as well as the biological condition of the plants, depends also on light and other environmental conditions, particularly the condition of the water in the soil and somewhat on temperature²⁵.