Site description

Las Cañadas Agroecological Center is located in the municipality of Huatusco, Veracruz, located in the central area of the state on the eastern Sierra Madre, at the geographical coordinates of 19° 09’ north latitude and 96° 58’ west longitude, at a height between the 1 300 and 1 500 masl. It comprises an area of 306 ha, of which 265 ha are destined for forestry use, the rest for agriculture, a space in which the study agroforestry systems are developed.

The soils are of volcanic origin classified as Andosol molic + Luvisol chromic, with a frank texture, dark in color, slightly stony and acidic. The relief is steep, rugged and slopes (^{Rey and Bustamante, 1982}; ^{Cisneros, 2000}). The climate of the study region is humid semi-warm with an average temperature of 19.1 °C, mean annual rainfall of 1 763 mm (^{Hernández, 2006}).

Description of ecological systems

The production of food and satisfaction of human needs while maintaining the health of natural resources is the main objective of the Agroecological Center, which is why they have implemented various production techniques: silvopastoral systems to meet the needs of dairy, cultivation in alleys and with little tillage of the soil for the production of corn, beans and tubers. As well as firewood, a biointensive method (^{John et al., 2006}) to obtain vegetables and carbon; edible forest for fruits, seeds, spices and medicinal plants.

Table 1 describes the ecological farming systems practiced in the Agro-ecological Center. Likewise, the management history of each production system was recorded (Table 2).

In 2015, composite soil samples (from 15 to 20 subsamples) were collected from the ten production systems; as well as two areas of vegetation, acahual and natural, to determine their chemical and physical properties. Soils were analyzed for the following chemical properties: organic matter (Walkley and Black), olsen extractable P; interchangeable K, Ca and Mg in 1N ammonium acetate neutral pH; S extractable with ammonium acetate 0.05 M NH₄O and determination by turbidimetry; Zn, Cu, Fe and Mn extracted with DTPA, B extracted with CaCl₂ 1.0 M, according to the methodologies described in Alvarez and Marin (2015).

The following physical properties were also determined: texture (Bouyoucos hygrometer), bulk density (test tube method), total porosity, macro and microporosity (^{Flores, 2010}), moisture retention (membrane method); stable water aggregates (sieve method), hydraulic conductivity (permeameter method) according to the methodologies indicated by ^{Elrick and Reynolds (1992)} and ^{USDA (1999)}. The local inputs used in the fertilizer of the crops were also chemically characterized according to the methodologies for the analysis of plant material (Alvárez and Marín, 2015).

Chemical characteristics of the agricultural inputs used in Las Cañadas

Table 3 shows the concentrations of the elements considered essential for the development of plants in agricultural inputs that are used in La Cañada to fertilize crops.

As can be seen in Table 3, the concentrations of N, P, Ca, Mg and in general of micronutrients in agricultural inputs that are used in La Cañada to fertilize crops, in general, are very low and would require enormous amounts compost to cover the needs of crops. In addition, nutrients such as Ca and P could not be covered, since these in themselves are deficient in the system due to the genesis of the soil and climate conditions.

Agroecological management and changes in the chemical properties of the soil

In the Table 4 shows that with most types of agroecological management, not only has soil resilience been achieved, the original levels of indicated soil chemical properties in the cloud forest have also been exceeded. It can be seen that the pH from being moderately acidic in mature and acahual forest became neutral in the biointensive system, King Grass bed and giganton bed.

This due to the continuous addition of compost prepared from kitchen waste and ground bone ash resulting in a pH of 7, the application of phosphoric rock in these systems also generates an alkaline effect (^{Chien, 2003}) with the passage of weather. In none of the systems did salt problems appear, which varied in the range of 80.9 µS in mature forest to 142.3 µS in the biointensive.

The OM content is an indicator that strongly reflects the effects of management in different systems. The mature forest that can be considered as the witness of the original natural conditions in equilibrium (soil-climate-vegetation), shows an organic matter content of 7.42%, with the different managements throughout approximately 20 years of its establishment, it’s have substantially exceeded this content.

This did not occur in the gigantic bed, management with which it has contributed to accelerate the oxidation of native organic matter; these beds are solely for carbon production and aerial biomass is continuously being extracted for use as a carbon source in composting, with almost no return; this system can be illustrative of what happens in the production of a monoculture in a conventional system, with the consequences of exhausting the reserves of organic matter even below the mature forest.

Inorganic nitrogen levels were medium to low. This is to be expected due to the rainfall in the area, which is high (1763 mm per year), which promotes inorganic nitrogen leaching (NH₄+NO₃) even when good management practices are carried out (^{Stopes et al., 2002}). Regarding the availability of phosphorus, with the exception of the acahual forest, biointensive o. and giganton bed, the phosphorus levels in the soil are low (<5.5-11 ppm).

Depending on the values of iron available in the soil, which far exceed the value considered adequate (> 4.5 ppm), this could be the causal factor of P fixation (^{Jensen et al., 1992}). This limiting factor could be overcome at a mean P level with the management and addition of phosphoric rock plus ground bone in the biointensive system; in the giganton bed a process of availability may be occurring thanks to the relation of the giganton (Thitonia diversifolia) with the fractions of the fixed phosphorus (^{Eckert, 1987}; ^{Jama et al., 2000}).

Those nutrients identified as deficient, in part due to the climatic conditions and genesis of the soil, have been introduced sporadically and in insufficient quantities to cover this need (Table 2). For example, in the corn-araucaria system (Table 2) with the human compost, only 5.8 kg ha^-1 of MgO would be applied from a corn fertilization need of 29 kg ha^-1 MgO, in terms of nitrogen deficit it would be 65 kg ha^-1.

The fertilization recommendations that were estimated for each production system (not presented in this document), indicate that in addition to local inputs, it must be complemented with external products for better crop performance.

Agroecological management and changes in the physical properties of the soil

In contrast to chemical properties, different agroecological management has been less consistent in restoring the physical properties of the soil to the equilibrium level represented by mature forest. This system presented the highest percentage of porosity, (66.87%) and although it is lower in the rest of the systems, including acahual (Table 5), there is a relationship between the content of organic matter derived from management and total porosity as shown in Figure 1a.

Figure 1 Relationship between organic matter content (OM) and a) total porosity; b) macroporosity f; d) field capacity and microporosity f; d) OM and usable humidity in different agroecological production systems. 

On average, from approximately 7% of OM, porosity increases as its content increases in production systems, as a result of the continuous addition of organic fertilizers, confirming that agroecological management tends to reduce the negative effect due to the change in land use from forest to agriculture (^{Chauveau et al., 2015}).

The total pore space is made up of macropores (Macroƒ) and micropores (Microƒ). The former is responsible for the drainage and aeration of the soil, also constituting the main space in which the roots develop (^{Prasad and Power, 1997}). The mature forest presents the highest macroporosity with a percentage of 21.3 and none of the management conditions has restored this property (Figure 1b).

This effect is also reflected in hydraulic conductivity, where mature and acahual forests maintain the highest values at 25.7 and 25.1 cm h^-1, respectively. Studies carried out in systems with tillage and without tillage, show that macro-porosity is the property most affected by cultivation conditions and with it water conduction (^{Soracco et al., 2012}; ^{Dal Ferro et al., 2014}).

According to ^{Dexter (1987}, ²⁰⁰⁴⁾ the volume occupied by a root corresponds to a decrease of equal magnitude in the volume of the pore space surrounding the root, the soil adjacent to it is compressed to the minimum possible porosity, which is a constant for a given soil, between this zone of minimum porosity and the body of the soil, the porosity increases exponentially, the distance from the root to which the soil density is affected is proportional to its diameter.

Consequently, continuous cultivation can be said to promote root growth that leads to soil compression (^{Dexter, 2004}), favoring microporosity to the detriment of macroporosity, as observed in all cultivation systems.

Micropores (Microƒ) are responsible for water retention, part of which is available to plants. With the exception of biointensive o. and silvopastoral, all systems have contributed to increasing the water-holding capacity of the soil in terms of CC and HA (Figures 1c and 1d). This effect is largely attributed to the considerable contributions of organic matter.

In the form of compost that have also favored the microporosity of the soil (^{Prasad and Power, 1997}), responsible for capillary water (^{Salcedo-Pérez et al., 2007}), which is confirmed by an increase in the stability of aggregates still above of the mature forest (Table 5). Agroecological management of production systems have failed to restore the apparent density to its original level (0.79 g cm^-3) of mature forest.

It is important to note that silvopastoral systems 1 and 2 present densities similar to those of the corn crop (0.91 g cm^-3), in which the tillage of the soil is very reduced. According to ^{Touchton et al. (1989)}; ^{Dal Ferro et al. (2014)}. From basal area and body weight data, it is possible to estimate that grazing animals apply pressures on the ground in the range between 150 (300 kg steer) and 350 kPa (adult sheep), values notoriously higher than those corresponding to agricultural tractors, which exert pressures of the order of 80 (high flotation tires) to 160 kPa (single radial tires) (^{Wood et al., 1991}).

Consequently, the degree and extent of soil densification are expected to be greater when caused by animals (^{Sánchez et al., 1989}) than by tractors; however, the effect between systems has been similar. The percentage of stable aggregates was higher in the biointensive o. system (82.08%) compared to the value in mature forest (70.03%) and the lowest in King Grass bed (62.55 %).

The OM participates in the formation and stability of the different sizes of aggregates, a process where the maintenance of the level of aggregation depends on the way and the frequency with which the OM is incorporated, in addition to this, the dimension of the soil aggregates would be a function of the size, geometry and mode of deposition of the same (^{Golchin et al., 1998}; ^{Dexter, 2004}).