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INTRODUCTION
In Cuba, rice is the most common food in the diet of Cubans, with a national demand of 700 thousand tons and an average consumption rate of more than 70 kg per person per year. However, national production only guarantees 40 percent of these demand, so the government is obliged to import more than 400,000 tons of rice annually (Reyes, 2019).
In that context, since 2012, a comprehensive development program has been carried out that anticipates, before 2030, guaranteeing 85 % of the national demand, with the incorporation of new areas, the introduction of modern technology and the gradual increase of yield in the fields, with the purpose of replacing imports. That effort contributed to reach in 2018 more than 300,000 tons, the largest historical record of rice production in our country (Reyes, 2019).
The rice is mainly cultivated in Cuba by traditional way, cultivation based on the disc harrow and permanent flooding, which contributes to soil degradation. In the current conditions of climate change, rice production must adapt to provide food to the population in a sustainable way and, at the same time, it needs to conserve and improve the soils.
Los Palacios Municipality, in Pinar del Río Province, is one of the main rice producers in the country. In that region, the crop is affected by the low physical and chemical fertility of the soils, their compaction and bad drainage, among other factors (Pozo et al., 2017 and Pérez et al., 2018).
A viable alternative may be the adoption of a climate-smart agricultural system like CA. The FAO cited by Kassam, (2018) defines CA as an agricultural system that is characterized by three fundamental principles: keep the soil permanently covered with crop residues or vegetation covers for at least 30%, a minimum disturbance of soil and a diversification of species grown in rotation.
Globally, CA is used with good results in approximately 180.4 million hectares worldwide, mainly in countries such as the United States, Brazil, Argentina, Canada and Australia, in soils that vary from 90 % of sand in Africa and Australia, to 80 % of clay in Brazil, and it can be applied to all crops. Even some South American countries like Argentina, Brazil, Paraguay and Uruguay, are using it in more than 70 % of their total cultivation area with experiences of transforming degraded areas into productive agricultural land (Kassam et al., 2018).
But, in the cultivation of irrigated rice, there are still few studies (Martins et al., 2017) and CA adoption rates in the leading countries are low. On the other hand, although there is a good acceptance in researchers and farmers in Brazil, China and India (Huang et al., 2018; Kaur and Singh, 2017 and Landers, 2018), there is a significant variability in the information on the effect of CA in different types of soil, and in Cuba, there is no practical experience on that way.
However, the determination of properties sensitive to soil use and management practices, such as organic matter, bulk density, porosity and resistance to penetration (Soracco et al 2018, Gómez et al., 2018, Singh et al., 2017 and Issaka et al., 2019) can be an effective tool to evaluate the effect of CA in the soil. The objective of the work is to evaluate the effect of CA on some physical and chemical properties of a Gleysol Nodular Ferruginoso soil dedicated to irrigated rice cultivation.
MATERIALS AND METHODS
The research was carried out between 2017 and 2019, in an area of 2,63 ha, in The Unidad Científica Tecnológica de Base, Los Palacios, belonging to Instituto Nacional de Ciencias Agrícolas (INCA), Pinar del Río Province, in a Gleysol Nodular Ferruginoso Soil (Hernández et al., 2015) with more than 20 years under intensive rice cultivation.
The soil is characterized by a texture of sandy loam on the surface horizon, grayish brown in color, 15 cm thick and abundant pellets. The effective depth of the soil is 17 cm, where a ferruginous hard-pan appears. It is a soil with very low natural fertility and poor internal drainage, suitable for growing rice (Díaz et al., 2009).
To implement the basic principles of conservation agriculture in January 2017, a traditional tillage was carried out, based on the disc harrow, which consisted of breaking, crossing and fluffing the soil and leveling. Subsequently, the soil was scarified and left fallow for 170 days, then an herbicide (glyphosate at a rate of 3 l / ha) was applied. The existing plant material was settled and after 7 days corn was seeded. Subsequently, the sowing of rice was carried out for two consecutive seasons, followed by corn (Table 1).
TABLE 1 Till technology used for rice seeding
| No. | Crops | Work | Aggregation | Working depth, cm | |
|---|---|---|---|---|---|
| Tractor | Implement | ||||
| 1 | Rice-Corn | Herbicide application | MTZ- 80 | Spraying | - |
| 2 | Rice-Corn | Crop residues covering | Belarus 572 | Rollo GENOVESE | - |
| 3 A | Rice | Sowing | YTO 1204 | Baldan seed drill SPD-5000 | 2.0 |
| 3 B | Corn | Sowing | Belarus 572 | VENCE TUDO seed drill | 2.0 |
The bulk density of the soil was determined in undisturbed samples, taken 98 cm3 cylinders, following the Cuban standard procedures (NC ISO 10272, 2003). The real density was determined following the Cuban standard procedures (NC ISO 11508, 2000). For each horizon, three replicates were taken for both determinations. The total porosity was estimated from the bulk density and the real density according to the Cuban standard procedure (NC 20, 2010). The natural humidity of the soil was determined by the gravimetric method, according to the Cuban standard procedure (NC 110, 2010). The organic matter was determined by the colorimetric method described in NC 51, 1999.
The penetration resistance (Rp) was determined by Eijkelkamp manual penetrometer Model: P.O. Box 4, 6987 ZG Giesbeek, Dutch-made, equipped with an 11.28 mm diameter conical tip and 1cm2 base area. The instrument is capable of registering values of 100-1000 N, from 1 to 50 cm in depth, with an appreciation of ± 8 %. Measurements of resistance to soil penetration were carried out together with moisture and density tests.
RESULTS AND DISCUSSION
Figure 1 shows the bulk density values obtained per soil depth for the different moments of the study. After two years and four planting cycles under the principles of CA, it is observed a trend of increasing bulk density as soil depth increases and decreasing soil bulk density values over time, for the three depth levels evaluated.
The bulk density values obtained are higher than those obtained by Díaz et al. (2009) in the same soil (1,18 g cm-3), but with more than 15 years of non-disturbance. They suggest for the good development of the crop of rice in the soil Hydromorphic Gley Nodular Ferruginous, bulk density values below 1,20 g m-3, with a critical value of 1,26 g m-3, which they associated with yield losses from 15 to 70 %. On the other hand, they are lower than those found by Pozo et al., (2017) when studying the same type of soil, under intensive cultivation of rice for more than 50 years, which were 1,6 g cm-3 for the open texture horizons and 1,83 g cm-3 in those with clay texture.
The increase in the values of bulk density with the increase in depth is a normal behavior in cultivated soils, and coincides with results published by other authors in different types of soil (Becerra, 2005 and Herrera et al., 2017). Similarly, Sánchez et al. (2003), suggest that the density of the soil increases with the crop cycle, which justifies the existence of such high values in 2017, even after two and a half years of the soil being fallowed, and also corroborates the state of degradation of this rice soil.
On the other hand, the behavior of bulk density in no-tillage systems, as well as during the transition process from a traditional system to a no-tillage system, has produced contradictory results. Generally, in long-term experiments with more than 5 years, an increase in bulk density is reported in rice soils planted with zero tillage. In this regard, Bonilla and Murillo (1998) affirm that the opposite can occur when the soil has a clayey texture or a high content of organic matter. However, Singh et al. (2017), in a similar three-year study, in northeast India on loamy soil, reported significantly lower soil bulk density and concluded that the adoption of CA can improve the productivity of the system, and the sequestration of C and N in the rice fields. Analogous behavior, obtained by Issaka et al., (2019) in Ghana, evaluating the effect of zero tillage on a Gleysol soil, for two years and they report a decrease in bulk density from 1,56 to 1,32 g.cm-3.
Total Porosity (Pt)
The behavior of soil Pt is showed in Figure 2. As a tendency, an increase in Pt values is observed at different depth levels, with respect to base line, which indicates a favorable effect of CA on this property. Furthermore, it can be seen that Pt decreases abruptly with increasing soil depth up to 15 cm. This behavior may be associated with the intensive cultivation of irrigated rice for more than 20 years, where efforts are made to reduce the infiltration of water into the soil to maintain a layer of water permanently.
The behavior observed in the Pt coincides with a decrease in the bulk density for the different moments and depths of evaluation. This corroborates the relationship between the two properties described by other authors.
Although the results differ from the optimal Pt conditions suggested by Díaz et al. (2009) for rice cultivation (between 54 and 57 %), the positive effect over time of the implementation of the CA, also coincides with results obtained by Selau, (2017) in similar studies of medium and long duration, in irrigated rice production systems in Rio Grande, Brazil. Also Sasal, (2012) when studying the structural evolution of soils under the principles of CA, appreciated a general increase in total porosity. On the other hand, Becerra (2005) describes that, as the soil depth increases, the porosity decreases, which is in correspondence with the results obtained.
These results are lower than those found by Pozo et al. (2017) of 0-14 cm deep (54,3 %) and Nelson et al. (2012), while evaluating the effect of zero tillage on irrigated rice in Brazil, in a medium textured soil, determined 48 % in the depth of 0-50 cm. Instead they are similar to those found by Muñoz (2016) in Casanare, Colombia in soils under intensive rice cultivation, as monoculture for more than 20 years, with an average porosity values between 44,09 and 46,72 % and to those described by Díaz (2004) in similar soils (41,13 %).
Penetration Resistance (Rp)
Figure 3, represents the behavior of penetration resistance in relation to soil depth. The results show as a tendency to decrease in Rp values respect to 2017. The Rp decreases from 4,29 MPa to 3,01 MPa, evidencing the favorable effect of CA on the soil. Increasing Rp with increasing depth reveals soil compaction problems, which corresponds to the results of the properties described above.
The results obtained, in the years 2017 and 2018 and at a depth greater than 12 cm in 2019, show compaction problems, which constitutes a great disadvantage, considering the low fertility of these soils and their low effective depth (15 cm), so it will be necessary to keep the humidity in the soil as close as possible to the field capacity. Botta et al. (2015) obtained a similar result in Argentina.
Rp values greater than 2,5 MPa could produce difficulties for the root development of the crop, and may remain impeded at depths greater than 8 to 10 cm, with values greater than 3,5 MPa, according to Salazar (2002) and Micucci and Toboada (2006). However, the values observed in 2019 up to a depth of 12 cm are considered specifically for rice cultivation.
Penetration resistance (Rp) depends on texture, bulk density, total porosity, organic matter content, and soil moisture content, and is specifically correlated with tillage systems (Afzalinia and Zabihi, 2014). The abrupt change showed in 2019 for the depth from 0 to 10 cm, may be associated with higher moisture content in the soil, increasing in organic matter, soil kept in untilled condition and 90% covered with crop residues, which favors water retention.
Organic Matter (OM)
The OM content in the soil at different moments of evaluation is shown in Figure 4. An increase in organic matter from 2,41 to 3,66 % was observed, respect to 2017, reflecting a favorable effect of CA on the soil. Although, the values corresponding to 2017 and 2018 are similar to those found by Pozo et al. (2017) at the depth of 0 -14 cm (from 2,5 to 2,7 %), which they considered adequate for this type of soil, according to clay content. In reference to the Technical Instructions for the Cultivation of Rice (2014), values above 3 % are considered normal for the development of the cultivation, so the result obtained in 2019 can be considered very good.
Other authors have also found significant increases in OM in CA practices. Espinosa (2010) appreciated an increase in OM content by 16 %, below zero tillage. Da Silva et al. (2002) in soils with hydromorphic condition, dedicated to the cultivation of irrigated rice in southern Brazil, showed that with the adoption of zero-tillage systems, there was a significant accumulation of OM in the soil, in the layer 0 to 2,5 cm, whose contents observed under grasses and legumes were 66 and 48 % higher. This behavior is in correspondence with the results obtained and confirms that the tillage system is one of the management practices that can significantly influence this soil property.
CONCLUSIONS
The results show that after four sowing cycles, the CA produces an increase in organic matter from 2,41 to 3,66 % with respect to the base line, the total porosity was increased in 9,51 %, the bulk density decreased in 4 g.cm-3 and the penetration resistance was reduced from 4,29 to 3,01 MPa.
Considering that the best physical, chemical and biological conditions for crops are preferably found in soils with a high OM content, the implementation of CA in the cultivation of irrigated rice can contribute to increase soil fertility.
