Authored by Abdul Khalil Gardezi*
Introduction
Soil is the main support for vegetation, infrastructure and habitat of biodiversity and participates in an essential way in the functioning of any ecosystem [2]. However, modern human societies have conceived soils as simple mechanical supports for plants or as sites for establishing human settlements, ignoring their biological, ecological, physical-chemical, socio-economic and cultural importance [3,4]. This conception has contributed, together with other factors, to the processes of destruction and degradation that affect the edaphic resource. One of the areas most affected by soil degradation in Mexico is the Mixteca Alta region located in the northern part of the state of Oaxaca which presents advanced processes of degradation and loss of soil by erosion. Tending some areas to include desertification [5]. The problem of degradation and loss of productivity of soils extends, in many cases, beyond the effects on this resource. When areas with forest cover or other natural ecosystems are transformed into crop fields, beside the damage to biodiversity, there is a large loss of carbon stored in both biomass and soil. Therefore, the emission of gases type greenhouse into the atmosphere. Faced with this situation, the National Forestry Commission (CONAFOR), through the National Forestry Program 2014-2018 (PRONAFOR), objective is to promote the recovery of forest cover and the restoration of soils in degraded forest lands devoid of vegetation [6]. In the Mixtec region and specifically in the study area, the actions of CONAFOR focus on the integral restoration. That involves the planting of tree species, as well as various actions focused on the recovery of soils as soil conservation works. The reforestation seen as a strategy to achieve the restoration of the degraded lands that have lost the vegetation cover by the different causal agents. Was established with the purpose to protect and contribute to the stabilization and restoration of lands with strong problems of vegetation loss and soil erosion [7]. In addition to the environmental services to provide support and regulation that offers reforestation, its importance is also due to the function of reforestations to capture carbon in its aerial biomass, which is generally studied in natural forests [8-10]. From the economic point of view, these generates income through payment for environmental services [11-14]. Likewise, it contributes to the accumulation of SOC in the soil, but there are few studies on the accumulation dynamics of SOC recently incorporated into the soil as a result of the establishment of reforestation [15]. The objective of this research was to estimate the accumulation of aerial carbon and SOC in reforestations with plowing of soil of different ages (1 to 8 years) under two conditions of slope in the northern zone of the Mixteca Alta of Oaxaca, Mexico.Materials and Methods
Study area
The study was conducted in the northern portion of the Mixteca Alta region of the state of Oaxaca (Figure 1), located between the parallels 17°47’ and 17°57’ North latitude and the meridians 97°20’ and 97° 31’ West longitude with an altitudinal variation of 2 000 and 2 200 m. According to the climatic classification of Köppen modified by Enriqueta García [16] the climate is BS1kw, semi-arid, temperate, with average annual temperature between 12° C and 18° C, summer rainfall regime, with an average annual precipitation of 474.7 mm. According to the baseline world reference of the soil resource of the FAO World Reference Base (WRB) used by INEGI the predominant soils in the study area are regosols, vertisols and leptosols; and to a lesser extent phaeozem, luvisols and cambisols. The predominant use of land is induced pasture, there are extensions where vegetation has been lost due to felling or continuous grazing of livestock, which prevents new plants from growing again. The herbaceous stratum, mainly of low height, and some specimen of maguey (Agave sp.), cover the land partially (Figure 1).Sampling sites
Due to the degradation conditions of the place, the best adapted species is Pinus greggii, which is a native species of Mexico and is distributed naturally in isolated populations along the Sierra Madre Oriental. This species has a great potential to adapt to moisture and nutrient limiting conditions, which makes it a species widely used in reforestation programs for the recovery of degraded soils in different parts of Mexico [17,18]. Reforestation was established in a planting frame of 3 m between rows x 3 m between plants, with an average density of 1,100 plants per hectare. The soil preparation consisted of plowing soil in curves at a level 3 m apart. A fence was also established to serve as protection against livestock, with maintenance after one year of its establishment. Which consists mainly of the reconstruction of a bowl, a work that promotes the capture of water. Reforestation was selected with an average of 1, 2, 3, 4, 5, 6 and 8 years of age and an area with scarce vegetation (ASV). Under two conditions of slope (1-5° and 5-15°) where 3 sampling sites of 400 m2 of circular shape (radius of 11.28m) were randomly established.Biomass and C content in trees
All plants that were found in the sampling sites were measured, the total height (TH) with the help of a telescopic rod and the normal diameter (ND) and diameter to the base (DB) with a precision vernier (0.1mm). To calculate aerial biomass and total air carbon of P. greggii, models generated for the same species proposed by Pacheco et al., [19].
Once the biomass and carbon data of each tree were obtained, they were added to obtain estimates at plot level and finally, by extrapolation, the biomass per unit area was estimated.
Content of C in soil
At each site, the sampling was carried out in a transect oriented towards the slope, in which, with the help of a straight shovel, 3 subsamples were taken and a composite sample of soil of approximately 500g was formed, which was deposited in a plastic bag and properly labeled. Subsequently, the samples were sent to the Central University Laboratory of the Autonomous University of Chapingo, Mexico for analysis. Where the percentage of M.O. [1] and the Dap (paraffin method), based on the Official Mexican Standard NOM-021-SEMARNAT-2000 [20]. Once the percentage of M.O. was determine by the Van Benmelen factor of 1,724 a correction was applied, resulting from the assumption that soil organic matter contains 58% Carbon (1 / 0.58 = 1.724). Finally, the SOC content per unit area (Mg ha-1) was calculated based on the equation proposed by González et al., [21].
where: SOC is the total organic carbon in soil per surface (Mg ha- 1), SOC is the total organic carbon in%, Dap is the apparent density (g cm-3) and Ps is the soil depth in cm.
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