elocation-id: e2970
A partir del uso intensivo del arado de discos y su acción de volteo en los suelos agrícolas del semiárido de México ha generado degradación severa de propiedades físicas y químicas. El objetivo de este estudio fue evaluar el estado estructural de un suelo (Xerosol) sometido a agricultura de conservación, para conocer los indicadores de calidad del suelo (ICS) e índices de sustentabilidad. En un experimento de largo plazo (1995-2020), bajo una rotación maíz-triticale en riego, se evaluaron dos sistemas de manejo de suelo: 1) labranza convencional y 2) agricultura de conservación. Los indicadores evaluados fueron: textura, densidad aparente, carbono orgánico del suelo, Índice de estabilidad estructural, estabilidad de los agregados mediante el diámetro peso medio, porosidad total, distribución de poros, porosidad ocupada por aire, constantes de humedad (ɵ s , CC y PMP), lámina de agua almacenada, conductividad hidráulica saturada, pH, conductividad eléctrica y rendimiento de grano y forraje. Los resultados mostraron diferencia estadística entre labranza convencional y agricultura de conservación (ɑ= 0.05) en 18 de los 19 ICS analizados. La mayor sustentabilidad estimada fue para AC con 85% en comparación con labranza convencional que fue de 59%. La agricultura de conservación presentó mayor estabilidad estructural con valores mayores de porosidad y menor densidad aparente, lo cual es favorable para la sostenibilidad de la estructura del suelo y los rendimientos del cultivo.
aggregate stability, conservation agriculture, MESMIS, soil organic carbon, sustainability.
Soil is a natural resource whose use is unsustainable under intensive agriculture and it has been shown that intensive agriculture negatively affects quality in its three main aspects: physical, chemical, and biological (Astier-Calderón et al., 2002; Dexter, 2004; Navarro et al., 2008). Conventional agricultural crop production systems practice intensive tillage and apply external inputs as strategies to increase soil fertility and yield.
The consequences of intensive tillage decrease soil quality, which is reflected in compaction problems, low water infiltration rates, poor aeration, loss of soil biodiversity, soil and water contamination due to the excessive use of agrochemicals and increased erosion (Verhulst et al., 2015).
On the contrary, conservation agriculture (CA) generates greater sustainability for crop production, with attributes of productivity, stability, and resilience, by positively impacting soil quality as a result of the improvement of its physical, chemical, and biological properties (Torres et al., 2006; Verhulst et al., 2015; FAO, 2016).
The main advantages of CA are increasing the content of organic matter (as well as carbon sequestration) in the soil surface, contributing to the structural development and stability of aggregates, increasing water retention, reducing runoff and soil erosion (Verhulst et al., 2015). This promotes an increase in the physical quality of the soil and the environment (Dexter, 2004).
To quantify the changes in the physical quality of the soil that CA produces in the long term, it is essential to measure qualitative indicators and indices through the evaluation of soil properties (physical, chemical and biological), which must be easy to measure, even the most sensitive changes generated by the set of management practices (zero tillage, retention of residues on the surface, and crop rotation) that CA integrates, to magnitudes that would explain soil quality, which is a practical step aimed at having sustainability and environmental resilience (Dexter, 2004; Navarro et al., 2008; Verhulst et al., 2015).
These changes in the physical attributes associated with tillage practices present symptoms that have a common cause: the deterioration of the soil structure (geometric and topological arrangement of the pores that form between the soil aggregates and their stability in time and space). Water and gas flows and root growth are associated with this attribute of the porous medium (Osuna et al., 2006; Martínez and Gómez, 2012).
This study aimed to evaluate the structural state of a soil (Xerosol) subjected to conservation agriculture to know the soil quality indicators (SQIs) and sustainability indices.
The trial was conducted at the San Luis Experimental Field, which is located at the geographical coordinates 22° 13’ 45.8” north latitude and 100° 51’ 01.5” west longitude at an altitude of 1 838 m. The average annual precipitation and temperature is 210 mm and 16.2 °C and the soil is a Xerosol (CGSNEGI, 1995) with a clay-sandy loam texture, with pH of 8.1, with 1.4% OM and EC of 0.81 dS m-1, with compaction problems throughout the profile. Water for irrigation registered an EC of 0.29 dS m-1 and SAR of 1.26, low in salinity and sodicity (Sarabia et al., 2011).
Since 1995, a long-term experiment (25 years) has been conducted under irrigation conditions, where two soil management systems were compared: 1) CT-conventional tillage fallow plus harrowing (Fa + Ha) and 2) conservation agriculture (CA) with a corn-triticale rotation. Each experimental unit had 240 m2 and two replications were used (Martínez et al., 2019).
The harvest of grain corn was carried out manually when the grain showed approximately 15% moisture. Two random samples of 6 m length per treatment were harvested in the two central furrows of each experimental unit. In the case of triticale, it was harvested when the grain was in a milky-doughy state and two samples of 1 m2 were taken per treatment.
In the corn harvest stage of the 2020 spring-summer (S-S) cycle, soil samples were collected at a depth of 0-10 cm, in which the following were determined: texture (% clay, silt and sand), MC- moisture constants (at saturation ɵ s , field capacity FC and permanent wilting point PWP), pH, electrical conductivity (EC), organic matter (OM) and soil organic carbon (SOC).
The following procedures were used: texture (Bouyoucos hydrometer), MC in pressure plate and membrane, EC in extract, pH in a water:soil ratio of 2.5:1 (Page et al., 1982), OM was performed with the Walkley and Black method (AS-07) and in the case of SOC, it was determined with soil samples prepared according to the AS-01 method (SEMARNAT, 2000).
The bulk density (p b ) was determined by the double-cylinder auger (Jury et al., 1991). Total porosity (f T ) was estimated based on the actual density (p a ), equal to 2.65 Mg m-3. The distribution of corresponding pores of the total pore space of the soil was determined from the moisture retention curves (Dexter, 2004).
The following was estimated: the stability of soil aggregates in water by means of the mean weight diameter (MWDa) according to Franzluebbers et al. (2000), the structural stability index (SSI) according to Duval et al. (2015), and saturated hydraulic conductivity (K s ) with the method of Reynols and Elrick (1990).
The following was performed: analysis of variance according to a completely randomized design with two replications of the measured variables, mean tests using Tukey’s criterion (0.05), and pairwise correlation of parameters of the measured attributes. The statistical analysis system, version 9.1.3 (SAS, 2013) was used and a sustainability analysis was carried out using the Ameba-type radial diagram (Masera et al., 2000).
A difference was detected between CA and CT (ɑ= 0.05) in the contents of sand and silt (Table 1); there was no statistical difference in clay, although this did not modify the textural clay-sandy loam classification (Verhulst et al., 2015). In bulk density (pa), there were statistical differences (ɑ= 0.05) between treatments (Table 1). The lowest value occurred in the treatment with CA + 33% C, which was attributed to the development of a better porous structure caused by the higher content of organic matter (OM) and the absence of compaction due to the transit of machinery (Hamza and Anderson, 2005).
The SOC values at 0-10 cm were statistically different between both management systems (ɑ= 0.05). The highest value of SOC is presented by the soil under CA compared to the soil with CT (Table 1). This reflected the greater mass of roots and the accumulation of plant residues in the topsoil that exist under the CA system, compared to the soil cultivated with CT (Duval et al., 2015).
The structural stability index (SSI) is an estimator of the ‘resilience capacity’ of soil structure, which relates SOC to soil texture (silt + clay). The values of the structural stability index (SSI) indicate a statistical difference (ɑ= 0.05) between treatments (Table 1), so a higher structural state of the soil was observed in the CA compared to the CT.
Table 1 showed that structural stability through MWDa exhibited differences (ɑ= 0.05) between treatments. CA presented a moderately stable MWDa value with an average value of 1.2 mm, compared to CT with an average value of 0.14 mm, considered a very unstable structural state (Le Bissonnais, 1996). MWDa increased over time in CA due to the contribution of residues, suggesting an effect of OM on increasing structural stability within the first 10 cm of depth. This was manifested after 25 years with CA, where the soil tends to present structural resilience.
Total porosity and its classification into macropores, mesopores, and micropores reported differences between treatments (ɑ= 0.05). CA presented the highest values compared to CT. This indicates that the continuous contributions of residues and their surface decomposition increase the incorporation of OM into the soil and promote the development of a more porous structure (Osuna et al., 2006). This study showed that continued tillage significantly decreased these different pore classes by approximately 12, 14, and 15%, respectively, compared to CA.
In the case of CT, fallow plus harrowing causes significant damage to structural stability, thus reducing porosity, water infiltration and gas exchange, and negatively affecting root growth and development and its contribution to the aerial part of the nutrients and water necessary for the development of the plant (Ceballos et al., 2010).
For air-filled porosity (ƒa), there were differences (ɑ= 0.05) between management systems. In the ƒa mean test (Table 1), CA had a higher volumetric air content than CT, which is 14% higher. This trend correlates with the decrease in ƒma and ƒme detected in the soil with the CT.
The value of saturated water content (ɵ S ) was higher for CA (0.496 cm3 cm-3) than for CT (0.419 cm3 cm-3) for the depth of 0-10 cm (ɑ= 0.05) (Table 2). The water contents at FC and PWP were different (ɑ= 0.05), also higher for CA than for CT, giving higher values of usable moisture in terms of sheet (Su), which was attributed to the fact that porosity and OM content were higher in CA compared to CT. Similar results have been reported by Rubio et al. (2008).
In relation to Ks, it was observed that it was higher in CA compared to CT, which confirms the degradation of the structure due to soil tillage. The analysis of the data shows that water mobility is clearly higher in soil with CA, evidencing its greater capacity to transport and redistribute water through the porous medium due to the creation of large, stable, and continuous pores that produce higher infiltration rates in the arable layer (Shukla et al., 2003; Navarro et al., 2008).
The mean pH values were 7.9 for CA and 8.3 for CT, with a significant difference (ɑ= 0.05). The tendency of this parameter to decrease in soil with CA is probably due to the accumulation of OM in the topsoil since it generates acidity due to the decomposition process or perhaps it may be due to the acidifying effect of fertilizers with nitrogen and phosphorus applied more superficially in CA than with CT (Verhulst et al., 2015; Báez et al., 2017). On the other hand, EC presented values of 0.76 and 1.4 dS m-1 and there was a difference between both treatments (ɑ= 0.05). The highest value occurred in the soil with CA; however, this parameter is still below the critical value indicated (< 3) (Shukla et al., 2003).
A correlation (p< 0.05) was found between 138 of the 171 pairs of soil attributes. pa was strongly and negatively correlated with fa, fT, fma, MWDa, and SSI (r ≥0.8). There were high positive correlations between MWDa and SSI, SOC, KS, ɵS, OM and fma (r >0.82) and negative correlations with pH and Su (r ≥0.72). Ks was highly and positively correlated with OM, SOC, SSI, ɵs, fma, fa, and fT (r ≥0.74) and negatively correlated with pH (r= -0.80). EC and pH were negatively correlated (r= -0.9). On the contrary, OM was significantly and positively correlated with SOC, SSI, fma, and fa (r ≥0.74) and Su was significantly correlated with fme, fmi, fT, fma, and fa (≥0.72).
The physical quality of the soil is defined based on its intrinsic properties, as well as its productive capacity, and environmental buffers (Astier-Calderón et al., 2002). CA produces an improvement in the physical quality of the soil since, in general, increases and decreases in the value of some attributes related to the structure and its stability are observed.
For example, the rate of infiltration or aeration may increase due to an increase in the number of macropores, a greater size and stability of aggregates, and a greater amount of OM, which produce increases in the transmission and availability of soil water for plants in the long term (25 years), which coincides with other authors (Navarro et al., 2008; García et al., 2018).
By means of the AMEBA-type radial diagram (Masera et al., 2000), it was possible to graphically visualize the deficiencies of each management system based on the selected indicators (Figure 1). CA had a sustainability value of 85%, while CT reached 59% (Table 3). CA tended towards the optimal value of sustainability in most attributes, while CT retracted towards the center of the graph (Figure 1). It is inferred that soil quality with CA is in an efficient and more sustainable condition than CT (Alonso, 2004; Altieri and Nicholls, 2005).
[i] Shukla et al. (2003).
The statistical analysis for grain and dry matter yield of corn and triticale reported a difference between management systems (ɑ= 0.05) (Table 4). The higher grain and forage yields of corn and triticale obtained in CA are attributed to the improvement of soil quality, associated with the stability and resilience of the soil structure. The CA/CT ratio indicates that the relative yield of both crops in CA was 54 and 34% higher than CT due to the sustainable improvement of the physical, chemical, and biological attributes of the soil (Martínez-Gamiño et al., 2019).
The soil studied is characterized by fragile structural stability. This problem is accentuated by the use of intensive tillage practices, such as ploughing and harrowing, which favor water and wind erosion in these semi-arid soils, making CA a more promising alternative for sustainable structural resilience in these soils with dry conditions.
The introduction of CA that combines the three management principles: zero tillage, retention of residues on the surface, and crop rotation, promotes the conservation and improvement of soil quality in the medium and long term, favors structural stability, and increases the content of SOC and the transmission and retention of water in the soil. This technique enables the development of sustainable agriculture in the semi-arid regions of Mexico.
In soil with CA, most of the attributes that represent physical, chemical, and water transmission properties were appropriate indicators to assess soil quality degradation since they showed sensitivity to the impact of tillage practices. This indicates that the structural system is susceptible to physical degradation; CA presented better structural stability and a greater increase in SOC, which is favorable for the sustainability of the soil structural system and crop yields.
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