Introduction

The soil is one of the most important resources in agricultural production, its state determines the type of activities that can be performed and necessary corrections to achieve desired production levels. Various physical, chemical and biological properties, give the soil the quality necessary to sustain life and maintain their productive capacity, functions that are affected negatively by degradation phenomena such as erosion and loss of vital components such as fertility and biodiversity (^{UNCCD, 1996}).

In places with advanced levels of degradation has generated a decrease of up to 50% of the productive capacity of the land, triggering a major risk to food safety of people (^{Eswaran et al., 2001}). For the specific case of Mexico, the Secretaría de Marina y Recursos Naturales (SEMARNAT) and the Colegio de Postgraduados (CP) (²⁰⁰³) report that the total area affected by degenerative processes (44.9%), 88% have loss soil erosion caused by water, while 92.7% showed a chemical degradation generated specifically for the loss of fertility.

It is known that water erosion occurs by the impact of rain drops, or the flow of water through the soil surface, and in this process appears, remove and drag material bare soils containing organic matter and the nutrients required for plant growth, generating long-term loss of fertility (^{Unger, 1996}; ^{Singer and Munns, 1999}). Aggravating phenomenon that has reduced vegetation cover, which can be recognized as the trigger of a circle of degradation, due to the disturbance in the water cycle that is generated.

Thus in a soil that is not protected erratic and heavy rains, water cannot infiltrate, generating surface runoff that carries erosion during wet periods, which also induces a reduction of water storage in the soil , events with high evaporation that occurs in areas without coverage may limit the availability of water for plants during dry seasons (^{UNCCD, 2009}). In addition to soil loss, lack of vegetation can affect biological properties, as populations roots, microbes and animals are important to maintaining fertility due to perform processes such as consumption and destruction of organic matter, making humus and nutrient recycling (^{Singer and Munns, 1999}).

The agroforestry represents the art and science of growing trees in interactive combination with crops and/or animals in the same unit of land for multiple purposes (^{Krishnamurthy and Ávila, 1999}). Which promotes a number of positive effects on the soil and therefore crops that are in the system, according to Nair (¹⁹⁹³), the trees produce biomass that maintains and improves the levels of organic matter. Some species are nitrogen fixers, others protect against erosion caused by water and wind, reducing nutrient loss while favoring an increase in soil fertility because they can take nutrients from the deeper layers of Likewise, they can also improve different physical properties, create a favorable microclimate under the canopy of trees and encourage an increase in activity by the microorganisms responsible for breaking down the organic matter. However, there are also side effects that are generated with poor agro-forestry planning, these include competition for water and nutrients and growth inhibition due to excessive shade and difficulty performing mechanized harvests crops (^{Mahecha, 2003}).

In this vein, different scenarios that promote the ability of agroforestry to maintain or increase soil fertility are generated, reducing their rates of water erosion and improve or maintain crop yields, which necessitates research in areas needed to verify or refute the claims that are generated around agroforestry. Therefore, this study aims to assess the loss and properties physical chemical and biological soil in an agroforestry system where corn is produced in association with olive trees distributed in alleys, while the influence of the arrangement is determined on production of the agricultural component, compared to a conventional system in monoculture corn.

Materials and methods

Location of the experiment. The research was conducted in an agroforestry system in alleys composed of olive trees (Olea europaea) and corn, located in Texcoco, Estado de Mexico, 19° 28' 15.17" north latitude and 98° 52' 26.02" west longitude. The town lies at an altitude of 2 265 m, has an average temperature of 15.9 °C, an annual rainfall of 650 mm and a climate that is characterized by semi-dry temperate (^{Moreno, 2007}). It was selected an area of 6 hectares in which they have olive trees with an age of 60 years (who no longer produce olive), under which native maize is grown as the main crop, which is sometimes broken with beans and oats agroforestry system mode; the site also features an area characterized by the total absence of trees, which also corn is planted as monocultures, land entirely receives the same management tasks and the same fertilization treatments, ideal situation to establish the experiment and evaluate the effect of trees on maize production, as their yields thanks to the homogeneity of the place and the proximity of the areas without trees, presents similar conditions that allow us to infer that possible differences in production are effect the presence or not of trees.

The agroforestry system is characterized by rows of trees planted at a distance of 8 x 8 m leaving an alley in which seven rows of corn planted at 80 cm, in north-south direction are installed.

As mentioned above, the entire property was managed under the same agricultural work, which consisted of a manual fertilization at a rate of 92 kg ha^-1 of nitrogen and 46 kg ha^-1. And mechanized farm work.

Experimental design. Two treatments were established: the agroforestry system (SAF) and monoculture (MC). As conditions for the selection of the installation site of treatment, it was taken into account that both were similar in slope (2%) and effective depth (30 cm); in the case of treatment with trees, a homogeneous distribution of them was considered, and the monoculture was taken into account a suitable distance from the trees to avoid an edge effect and the possible influence there of.

In each treatment comprised three replicates useful plots of 16 m², in which different variables were measured to evaluate the influence of agroforestry system on fertility (physical, chemical and biological), loss of soil and corn production were located. Additionally daily measurements of precipitation and temperature, under trees and in monoculture were made.

Soil physical properties. They were made determinations of different physical properties of the soil, with the aim of characterizing the system; these were carried out in the laboratory of soil physics of Universidad Autónoma Chapingo (UACH), under the standards and methodologies of the Official Mexican Standard NOM-021-RECNAT-2000 (^{SEMARNAT, 2000}). Finally, soil temperature records were obtained throughout the duration of the investigation. Additionally, a determination of the volumetric soil moisture was performed by using reflectometry time domains (TDR), specifically with the use of a TDR 300, in order to establish the ability of water conservation in each treatment according the behavior of precipitation.

Chemical properties. Measurement of chemical soil parameters was performed to characterize the system and establish the nutritional conditions available to plants. The determinations were performed in the central laboratory of the Universidad Autónoma Chapingo, under the procedures established by the Mexican Official Standard NOM-021- RECNAT-2000 (^{SEMARNAT, 2000}).

Biological properties. The sampling for the determination of biological indicators was performed three times in order to evaluate their behavior during the production period of maize. For this investigation were chosen as parameters to evaluate the microbial activity and mesofauna. The microbial activity was determined by the technique of soil respiration proposed by ^{Anderson (1982}), in which the release of CO2 (expressed as mg CO2 kg^-1 of soil) from the decomposition of the organic matter is measured microorganisms present in the system.

The mesofauna was determined by the method Berlese modified Tullgren ^{Karyanto et al. (2012}) in which a bulb 60 W heated 500 g of sieved soil by a maya 2 mm (maximum size mesofauna) which are arranged in a funnel, causing the individuals present descend toward the tip thereof, looking cooler areas, then fell into a jar with 70% ethyl alcohol. Individuals obtained were quantified and classified, thus establishing the number and type of organisms per square meter.

Runoff and soil loss. To corroborate runoff and soil loss were established plots of 16 m² (useful area of each repetition) that captured runoff water in tubs of 144 cubic liters; in turn, the plots had an edge of 15 cm high, covered with plastic, to delimit and maintain within the collected water. Daily (from June 30 to September 5 the agricultural cycle spring-summer 2014), was revised to record the rain gauge precipitation and with each rain event the presence or absence of drained water collected in tubs observed. When precipitation caused runoff water collected sheet was recorded and a sample of 110 ml was taken to quantify soil loss by water erosion.

Development and production of maize. To determine the influence of the presence of trees on development and production of corn plants, it was recorded weekly up to 20 plants selected at random within each useful plot from the fifth week of age, until presented in 100% flowering plants of each treatment. Subsequently, the total production of plants present in each useful plot (16 m²) was quantified. The ears were threshed harvested and dried in the open, then grain production despite each repetition, which was extrapolated grain kg ha^-1.

Statistical analysis. The data obtained were analyzed with SAS 9.1 statistical software using analysis of variance (ANOVA) and Tukey tests to determine significant differences (p< 0.05) between treatments. Additionally, using the same software, testing correlation between precipitation and runoff, precipitation and soil loss and runoff and soil loss, for the two treatments were performed.

Results and discussion

Physical properties. Despite the closeness of the two treatments and the apparent homogeneity of the terrain and soil conditions through the results showed similarities in terms of real and apparent density, the presence of trees has significantly affected (p< 0.05) features structural determinations as CC, PMP, hydraulic conductivity and soil moisture (Table 1), which were definitive when making an assessment of the quality of the soil, as they modified the behavior of water in the system and its availability for the plants.

Table 1 Soil physical properties belonging to agroforestry system (SAF) and monoculture (MC) to 20 cm deep. 

^z valores sobre cada renglón con diferentes letras son diferentes (p< 0.05) mediante

la prueba de Tukey.

A greater contribution of organic matter to the soil can influence improvement of structural conditions (^{Nair, 1993}). However, observing the results of chemical analysis, no significant differences were found in the contents of this parameter in the two treatments, which is confirmed by studying the records of real and apparent density, which are similar in both the SAF and the MC. When analyzing the biggest records of field capacity and wilting point was observed by a graph of moisture retention (Figure 1), the agroforestry system had a lower retention force of water relative to monoculture as it increased pressure, which coincides with the increased infiltration rate which owns the ground under the presence of trees, a situation that may be favorable to prevent runoff in the presence of heavy rains, but would result in a risk for sustaining crop production if the precipitation was scarce.

Figure 1 Curve soil moisture retention under the treatments SAF and MC. 

According to de la Rosa (2008), soils with a similar real and apparent density present near porosity values, however, the distribution of macro and micro pores may vary for each, where one with a higher proportion of large spaces incident in increased infiltration rate and a lower water holding capacity. In the present study found that the soil under the influence of olive trees has a significantly greater presence of macropores in relation to monoculture, a situation which explains the lower water holding capacity of agroforestry system (Table 2).

Table 2 List of macro and micro pores in the soil under the treatments SAF and MC. 

^z valores sobre cada columna con diferentes letras son diferentes (p< 0.05) mediante la prueba de Tukey.

The soil porosity can be maintained by increasing vegetation cover, which protects the system from disruption, which causes the impact of drops of rain, by canceling or decrease its ability to break soil aggregates and separating the fine particles, thus generating little or no blockage of pores in the soil surface (^{Shaxson and Barber, 2005}). Similarly these authors also report that the roots of the trees or cover crops act as biological subsoilers that penetrate different horizons, generating) more stable than those formed by mechanically channels, since the roots extrudate organic substances which stabilize the surface thereof. Once the roots have died and have shrunk, they said pores are large and stable enough to facilitate water infiltration.

^{Melloni et al. (2008)} reported in a study conducted in Minas Gerais, Brazil, a value of bulk density on a f loor covered by a typical forest of the Atlantic Forest Brazilian 0.94 kg dm3, which was lower than those recorded by a plantation of A. angustifolia, a plantation of Eucalyptus grandi and pasture in monoculture of Brachiaria decumbens, with values of 1.25, 1.06 and 1.36 kg dm3 respectively. In addition, the Atlantic Mata scored a percentage of macroporosity of 34.31%, which was significantly higher than that obtained by treating treeless (B. decumbens) which recorded a value of 13.53%.

The higher records room temperature and soil temperature for monoculture were 35 and 28 °C respectively, while the agroforestry system introduced ambient temperatures and maximum soil of 33 and 28 °C, which allows us to infer that the shadow generated by the woody perennial he had a direct impact on soil heating to 20 cm deep. For a month was assessed volumetric moisture same in both treatments determination that allowed us to observe a lower loss of water in the SAF, mainly conditioned by the lower ambient temperature and soil present in this treatment (Figure 2), this can note that with a system reload mediated precipitation, the moisture content in the SAF and MC was similar, however, when precipitation decreased, the shadow generated by the trees prevented soil water is lost more rapidly in relation to monoculture thus favoring greater conservation and availability thereof to plants.

Figure 2 Soil moisture volumetric registered for treatments SAF and MC in relation to precipitation. 

These data are consistent with those obtained by Matoso et al. (²⁰⁰⁷) who found in a growing coffee in the shade of various timber, moisture content was maintained between 0.5 to 2% higher, in relation to coffee monoculture, at a depth of 20-40 cm.

Chemical properties. Both the SAF as the MC had similar content (no statistical difference) in total parameters evaluated (Table 3), points out that despite the presence of trees the percentage of organic matter was similar in both treatments, indicating that olive trees do not provide enough biomass to increase its content in the soil, hence the nutrients included in the assessment showed no significant differences.

Table 3 Soil chemical properties belonging to agroforestry system (SAF) and monoculture (MC) to 20 cm deep. 

^z valores sobre cada renglón con diferentes letras son diferentes (p< 0.05) mediante la prueba de Tukey.

Evaluated in different agroforestry systems, the presence of trees has not been causing a higher concentration of nutrients in the soil, give example of this Ávila et al. (²⁰⁰⁴), who in an agroforestry system Eucalyptus deglupta associated with coffee in Costa Rica, showed a lower content of nitrates (68 kgNO3 ha^-1) compared to a coffee plantation monoculture (115 kgNO3 ha^-1) indicating that the trees reduce the availability of nitrogen for growing coffee; in another study ^{Barreto et al. (2006)} found higher content of potassium in a pasture in monoculture (0.20 cmol dm-3), compared with cocoa SAF associated with timber (0.12 cmol dm^-3); finally, in a study by Bertalot et al. (2014), no significant differences were found in the fertility of a SAF composed of trees Leucaena diversifolia and corn compared to a monoculture of corn, in this study the nutritional analysis determined the following: an identical pH treatments (5.2); organic matter 17 and 16% for the SAF and MC respectively; phosphorus content of 17 and 14 mg dm^-3 in the same order and finally a potassium concentration similar in the two treatments SAF (0.4 mmolc dm^-3) and MC (0.3 mmolc dm^-3).

Biological properties

A trend toward greater was observed in microbial activity expressed in mg kg of CO2 in the agroforestry system, however, it did not show a significant difference between treatments for the first two samples (Figure 3).

Figure 3 Microbial activity registered in the treatments SAF and MC. 

The presence of individuals was high in the first sample, while still not giving start the rainy season, in this case, the population was represented mostly by mites, which abounded in both the MC and the SAF, and springtails who had a significantly greater presence in the SAF; for the second sampling, the presence of individuals per m² fell sharply, from around 16 000 mites m^-2 on MC to 4 682 mites m^-2, on the same treatment, a situation that probably gave the high percentage of water in the soil and low temperatures thereof, that force individuals to seek warmer areas, why the presence of these organisms was higher in the MC because without the influence of the shade of the trees the soil temperature was 0.5 - 1.5 °C higher than in the SAF. For the third sampling, soil conditions in terms of temperature and humidity, allowed the presence of a greater number of individual’s m², except that this time was characterized by a greater presence of springtails and mites in the SAF.

Soil loss and runoff. The evaluation was conducted from 29 june to 4 september period presented 35 storm events, of which 15 (42%) caused runoff and of these, 100% were erosive, minimal rainfall that caused a flow of surface water particle entrainment was 7.63 mm, and the maximum precipitation recorded in the analysis was 26.24 mm, although twice events with a higher intensity that caused runoff occurred, however the magnitude of these exceeded the capacity of vats water, so it was impossible to perform a measurement of the sheet present, which led to such rainfall was not taken into account in the study, so the number of storm events analyzed dropped from 15 to 13. overall, agroforestry system, thanks to the protection provided by the trees had an impact on lower rates of runoff and soil loss throughout the study, compared with those observed in which were statistically higher monoculture (Table 4).

Table 4 Runoff and soil loss in the SAF and MC during the duration of study. 

^z valores sobre cada columna con diferentes letras son diferentes (p< 0.05) mediante la prueba de Tukey.

^{Pinese et al. (2008)} evaluated laminar in soil under different mulches in Minas Gerais, Brazil, subject to rainfall of 457.5 mm, where they showed a runoff of 19.5 L m^-2 in a monoculture of corn erosion, which was higher to that observed by a native forest, which presented 0.5 L m^-2, relationship remained as to the loss of soil which was higher in the corn crop compared to native forest (144.3 and 0.08 g m^-2); Results allow us to infer that with greater vegetation cover, runoff and the magnitude of the erosion process are presented in smaller amounts.

Development and production of corn. Contrary to expectations, both production and development of corn were lower in the agroforestry system, in which the plants showed slower growth and expressed a lower height relative to those present in monoculture situation lies in the fact that olive trees generated an intense and constant shadow on the crop, which greatly hampered its development. In general, the agricultural component of the SAF presented various limitations to express their productive capacity; first, even though the planting of the two treatments was made the same day, the seeds under the trees, on average, took longer to germinate, then its growth was at a slower rate; excess soil moisture, yellowing symptoms generated by oxygen deficiency and plants to be weaker than monoculture were blight at different times. The differences in growth and development observed in the figure above, are confirmed by height determinations were made weekly; from the fifth week of culture age (Figure 4), this one can see that from the first measurement to f lowering 100% crop plants grown in full sunlight presented a greater height than those located at the shade of olive trees.

Figure 4 Height of corn plants through crop development in the treatments SAF and MC. 

Being a plant C4 type, solar radiation is vital for the development and production of maize parameter, and that a reduction in the photosynthetically active radiation of 30- 40% for long periods may delay the ripening of the grains, slow growth and cause a fall in production (^{Cruz et al., 2006}). For this reason, agroforestry systems have to be set in this direction east-west and thus allow entry of light on the agricultural component ensuring optimal development of photosynthesis (^{Nair, 1993}).

Conclusions

The association of maize with olive trees, favored an improvement of features that together contribute to the conservation of the resource, thus manifesting, an environmental benefit product of agroforestry practice, which is made noticeable by reducing the amount of soil that lost due to water erosion. However, the environmental benefits are overshadowed by contrasting them with productive maize yields, which were higher in monoculture. For this reason it is necessary to generate technologies that express production in intercropping, the potential of agroforestry.