Study sites

The study was conducted in the Cuitzeo Basin, located between 19° 59ʼ and 19° 30ʼ north latitude and 101° 00ʼand 101° 30ʼ west longitude. It has an approximate area of 1 050 km², and occupies an important area of the Morelia-Queréndaro Irrigation District (Figure 1). The altitude is over 2 000 meters above sea level. Four Vertisols were evaluated in this site. Another soil was evaluated at INIFAP, Bajío Experimental Field in Celaya, Guanajuato, located at 20º 3ʼ north latitude and 100º 0ʼwest longitude, at an altitude of 1 754 m. The climate (^{García, 1984}) is BS1hw(w)(e)g with an average annual temperature of 20.6 °C and annual rainfall of 597 mm.

Figure 1 Location of the study area. Morelia-Queréndaro Valley, Michoacán.  

The soils

The textural classification of soils, except one, corresponded to clayey (Table 1), which is characteristic of Vertisols (Table 2). These soils were characterized by being dark and deep (>1 m) and possess expandable clays of smectic type (^{USDA, 1999}).

Table 1 Textural classification of soils evaluated under LC. 

Table 2 Agronomic management under conservation tillage in four localities of the Queréndaro-Morelia Valley and Bajío Experimental Field. 

O-I= otoño-invierno; P-V= primavera-verano.

Agronomic management

Soils were cultivated for four to eight continuous cropping cycles (two to four years) with conservation agriculture practices (CAP) adding 30 to 100% of crop residues on the soil surface.The agronomic history is presented in Table 2. In the Morelia-Queréndaro valley, farmers regularly plant wheat in beds of 1.6 m wide during autumn-winter seasons, re-marking the furrow to facilitate the water irrigation; and in spring-summer they plant maize in a double row, on the same beds. At the end of each crop cycle, after harvesting the grains, the crop residues are crushed and then spread in a mulch-like form as homogeneously as possible on the soil surface.

The soils located in Indaparapeo and Álvaro Obregón, Michoacán, are continuously irrigated with sewage from Morelia city, while the soils of Queréndaro are irrigated with water from the dam. In all soils, the total amount of crop residues on the soil surface was added, except in the soil called Queréndaro II, where approximately three quarters are packed for sale. The amount of crop residues that were added in this soil, after each crop cycle, was variable (Table 2).

In the Experimental Field Bajío wheat is sown in autumn-winter on grooves of 0.76 cm wide, in double row, while maize is sown in spring-summer in the same grooves in single row. Well water was used for irrigation and all crop residues were left on the soil surface in a mulch-like form.

Sampling and processing of soil samples

Four samples were taken in each soil evaluated, since the conservation agriculture practices (CAP) were implemented. In the Morelia-Queréndaro valley these were carried out at 0, 960, 1 200 and 1 440 days of the implementation of PACs; and in the Bajío Experimental Field at 0, 180, 360 and 540 days. The samples consisted of 22 sub samples each, and were collected randomly within each plot. A stainless steel bit was used and three depths were considered: 0-5, 5-15 and 15-30 cm. Samples then were dried in the shade and at room temperature, milled with a wooden mallet, sieved in a 2 mm diameter mesh and perfectly homogenized. For the analytical determination of the organic matter (MOS) a sub-sample of 100 g was prepared, ground and sieved in a number 30 mesh.

The determination was made by the Walkley and Black method, described by ^{Jackson (1976)}. For the analytical determination of C total, a sub-soil sample of 10 g was used, which was ground and sieved in a number 100 mesh. The C total was measured by a Shimadzu automatic carbon determinator, model TOC-5050^a. It was used to measured inorganic C. The COS was calculated by subtracting the inorganic C to the total C. The pH of the soil was measured in wáter at 1:2 ratio. The P-extractable was evaluated by the Olsen method, described by ^{Jackson (1976)}. Results were related to cultivation time, in order to evaluate the behavior and accumulation tendency of organic reserves by the effect of conservation agriculture practices.

Organic material

The accumulation of MOS in Vertisols, after four to eight cycles of continuous cultivation with the addition of crop residues on the soil surface and its minimal removal, was significant (p≤ 0.05). The increase was linear according to the cultivation time and the amount of agricultural wastes. However, when producers break the conservation tillage system due to the need to level their cropland, MOS reserves falls drastically in a single crop cycle.

This was the case of the Queréndaro II soil where the farmer, after four continuous cycles of CAPs, performed all the tillage practices to level his plot. It was observed that after two years with CAPs the MOS increased up to 5% in the 0-5 cm depth stratum, but when the culture system was broken the MOS decreased drastically to a little more than 3% (Figure 3).

Figure 3 Evolution in the accumulation of MOS and COS in five Vertisols submitted to conservation tillage in the Bajío.  

In this soil, about 38 t ha^-1 were added to the surface for 4 continuous crop cycles. The harvest residues in the soil surface until that moment were incorporated to the soil with the fallow, later it was resumed with the CAP; however, due to soil disturbance the MOS accumulation rate was altered and decreased.

Incorporation of crop residues into the soil by fallowing involves flipping the arable layer and exposing the lower layer to the surface, which has a lower MOS content than the upper layer. The latter being buried and in direct contact with soil microorganisms, increases the rate of mineralization of the MOS and the organic reserves decrease. The spraying of soil aggregates by the work of tracking, leveling and furrowing also favors the oxidation of COS (^{Elliot, 1986}; ^{Oades, 1988}).

In the soils irrigated with sewage, in Álvaro Obregón and Indaparapeo, after eight cycles of cultivation with CAPs, MOS increased by about 2%, in the stratum 0-5 cm deep, with respect to the content of MOS before implementing The CAPs (Figure 2). This level of high accumulation is explained by a continuous intake of organic waste with the irrigation water. To achieve this increase in MOS, after nearly four years of cultivation, about 50 t ha^-1 of crop residues were introduced into the soil. At the depth of 5-15 cm the MOS accumulation was 0.5% smaller, compared to the previous depth.

In the stratum of 15-30 cm the MOS accumulation did not show changes with respect to the amount of organic matter found before starting with the CAP. It was evident that the accumulation of MOS was lower as the depth increased, because the contact with the crop residues with the deeper strata is more distant. In order for a substantial increase of MOS at a greater depth to occur, longer cultivation time is required for the leaching, intemperisation, mineralization and humification processes, among others, to affect the deeper horizons.

In the Queréndaro I soil the percentage of MOS slightly increased in the first 5 cm of depth, in relation to the content it had before implementing the PACs. In this soil about 30% of the crop residues were incorporated after each crop cycle, which was equivalent to approximately 28 t ha^-1 (Table 2). This can be explained because the mineralization rate of the MOS is slightly lower than the accumulation rate. In other words, a higher input of crop residues is required to significantly increase the MOS content in this soil (Reycosky et al., 1995).

In the soil evaluated in Celaya, with only four cycles of cultivation with PACs, there was an approximately linear trend in the MOS accumulation, reaching a content greater than 3%; i.e 1.2% more than year zero. For the depth of 5-15 cm this increase was 0.7%. It was estimated that the amount of crop residues added to the soil was about 46 t ha^-1 (Table 2). In the stratum 15-30 cm deep the MOS content did not increase (Figure 3).

Organic carbon

The evolution in the accumulation of COS reserves in the evaluated Vertisols showed a significant increase, especially in the 0-5 cm depth stratum, for the soils of Indaparapeo, Álvaro Obregón and Celaya. This increase varied from 0.5 to 2%, which depended on the cultivation time with the PAC and the amount of crop residues added to the soil (Figure 3). The behavior in the accumulation dynamics of the COS was similar to that reported for MOS, which is explained by the close relation (R²< 0.7) between both parameters. Soil MOS contains 58% of C (^{Jackson, 1976}) and its rate of mineralization depends on the amount of lignin, comminution degree, soil distribution, agronomic and edaphoclimatic conditions (^{Curtin et al., 2000}).

In the second stratum, 5 to 15 cm deep, the COS accumulation was more discrete and in some cases was not even evident, whereas in the depth of 15-30 cm in most cases, it was not evident. This is because there is no direct contact with the crop residues, as explained above.

The COS accumulation rate in the Vertisol of Indaparapeo, Michoacán, was the highest of the soils that were evaluated in the valley Morelia-Queréndaro. It entered on average 7 t ha^-1 year^-1 in the first 30 cm of depth (Figure 4). Meanwhile, in the Vertisols of Álvaro Obregón and Queréndaro II, 1.5 and 2.5 t ha^-1 year^-1 of this element were accumulated, respectively. But on the Queréndaro I soil there was practically no carbon accumulation by adding only 30% of the crop residues. In the soil of Celaya, where there was an accumulation of greater quantities of agricultural wastes, the accumulation rate of COS was the highest, with 9.3 t ha^-1 year^-1.

Figure 4 COS accumulation rate in five Vertisols submitted to conservation tillage in the Bajío.  

The accumulation of COS in the soil occurs when the income (organic waste) is greater than the losses (erosion, mineralization, leaching) (^{Lal, 2004}). ^{Báez-Pérez et al. (2009)} concluded that the CO2 emission rate (TEC) of the soil is a function of the retained moisture and the COS content. Therefore, soils subject to conservation tillage have a higher content of MOS that is potentially mineralizable, compared to soils where traditional tillage is practiced. This implies a higher CO2 emission as the MOS content increases, as has been reported in another study in these same Vertisols soils (^{Báez-Pérez et al., 2011}).

In traditional tillage systems, in the Vertisols soils of Morelia-Queréndaro Valley, the amount of crop residues incorporated in the soil varies from 1 to 2 t ha^-1 and its volumetric moisture in the first 30 cm of depth during spring-summer can vary from 12% in dry season, to 42% in wet season, with a CO2 emission of 0.2 to 1.2 g m^-2 h^-1, respectively. In CAP systems the volumetric humidity fluctuates from 20% to 52% and CO2 emissions, in the same regard, from 0.4 to 2.6 g m^-2 h^-1. The above shows that even when there is a constant income of MOS in the culture systems with CAP, the TEC is also larger, so the accumulation of COS can also be limited. In order to achieve a COS accumulation rate greater than the COS loss, additional strategies are required in addition to the incorporation of organic waste. The use of crops with wide vegetation coverage and crop rotation could be additional alternatives to reduce COS losses due to mineralization and erosion.

Extractable phosphorus

The P-extractable content (P) in the evaluated soils showed a low correlation with the soil MOS content (R²= 0.2). The continued contribution of crop residues to the soil surface necessarily contributes to significant reserves of P; however, the constant application of phosphate fertilizers, which farmers apply cycle by cycle to crops, has provided a large reservoir of this element available in the short term to meet the requirement of cereal crops.

No trend was observed in the accumulation of this element that was related to the culture time by effect of the CAP. Because P is an element with low soil mobility and the forms assimilable by plants (H2PO4^- y HPO4⁼) are stable in a medium with slightly acidic pH (^{Castellanos, 2000}), it could be inferred that there is a high availability of this element in the ground. However, the values of alkaline pH that present the majority of the soils in this evaluation could influence the solubility of this element, and therefore the availability for the plants nutrition.

The highest concentration of P-extractable in the evaluated Vertisols was found in the 0-5 cm depth stratum, with the exception of the Indaparapeo soil (Table 3), where strata of 5-15 and 15-30 cm depth were more rich in this element, 42 and 65 ppm, respectively, which is extremely high.

Table 3 Amount of P-extractable Olsen (ppm) in the evaluated soils of the Bajío. 

This is explained by the continuous use of sewage from Morelia city, for irrigation purposes, as explained above. The concentration of P-extractable in the soils of Michoacán can be classified from high to very high, suggesting that it would not be necessary to apply phosphate fertilizers in several crop cycles. According to ^{Castellanos (2000)}, the requirement of P-extractable for a yield potential in 10 t ha^-1 maize for the Bajío region is 90 to 100 kg ha^-1.

The available reserve of this element in the soils of Michoacán was estimated to be 169 to 680 kg ha^-1, which can satisfy the nutritional needs of two to six consecutive cycles of grasses (Table 3). This can constitute a significant saving in the cost of chemical fertilizer. However, the alkaline pH present in some soils could influence its availability.

The soil of Celaya, Guanajuato, with only two years with CAP, showed the lowest P-extractable values, from 17 to 26 ppm, with respect to the other soils; however, its content is classified as medium to moderately high. The reserves of P-extractable in this soil were estimated at more than 200 kg ha^-1.