Study sites

Site 1. Ejido Gustavo Díaz Ordaz (GDO). Is located in the Emiliano Zapata municipality (19°32'31.6'' N and 98°08'58.0'' W) at 2 415 masl, with a total area of 470 ha. Subhumid temperate climate, 6.5-22.1 °C temperature, 6.8-140.8 mm rainfall, and predominant soil order Leptosol (30.11 %) (^{INEGI, 2010a}). The dominant vegetation is pine-oak forest (Pinus pseudostrobus Lindl., P. greggii Engelm. ex Parl., Quercus laurina Bonpl. and Q. rugosa Née) (^{Conafor, 2012}) and among the conservation works there is a notable presence of pastures.

In 2012, different conservation works were established in the Peña del Chivo area, whose soils show moderate degradation, due to deforestation and water erosion; in 30 ha, trench ditches and board ditches were built alternately, with a 40 ° slope (^{Conafor, 2012}). The specific study area was 2.5 ha, including conservation works and the control (without conservation works).

Site 2. Zacapexco (ZAC). Private property within the San Pedro Ecatepec community, Atlangatepec municipality (19°32'31.6'' N and 98°08'58.0'' W) with an altitudinal interval between 2 480 and 2 600 m. Subhumid temperate climate with summer rains, 12-14 °C temperature, 600-700 mm rainfall, and Phaeozem soil order (54 %) (^{INEGI, 2010b}).

In 2015, 40 ha of trenches and 50 ha of individual terraces were built. There are slopes of 15 to 20 % at the place and soil degradation was caused, mainly, by overgrazing and laminar water erosion (^{Conafor, 2015}). The predominant vegetation is Juniperus deppeana Steud. Forests and where the conservation works were established, seedlings of Pinus pseudostrobus were planted.

Site 3. San Bartolomé Matlalohcan (SBM). Community located in the Tetla de La Solidaridad municipality (19°28'11.68" N and 98°07'16.65" W) at an average elevation of 2 516 masl. Subhumid temperate climate with rains in summer, 12-14 °C temperature, 600-900 mm rainfall, and soil order Phaeozem (61.8 %), with a slope of 15-20 % (^{INEGI, 2010c}).

In this site, board-ditch and terraces were built in 7 ha between 1976 and 1978, as experimental plots of the National Institute of Forest, Agriculture and Livestock Research (INIFAP) to seek the rehabilitation of the tepetate (porous rocky soil). During sampling, the presence of introduced species such as eucalyptus (Eucalyptus L'Hér. spp.) and casuarina (Casuarina equisetifolia L.) was observed.

Soil sampling and sample preparation

At the GDO site, a grid with quadrats of 158 m×158 m was drawn on 2.5 ha of land, using the Schweizer´s systematic soil sampling method (2011). Two plots with a trench ditch (18 samples) and two with a board ditch (18 samples) were selected, in addition to six samples in a plot as a control (without conservation work). Within each quadrat, soil samples were collected between ditches at a depth of 0-30 cm. In ZAC, 3 ha were sampled with a trench (9 samples) and 1 ha as a control (without conservation work, 5 samples). For SBM, the sampling was in 1.5 ha with a ditch along the edge (9 samples) and in 0.5 ha without conservation work considered a witness (9 samples).

For the microbiological analysis of the soil, 100 g of each of the soil samples were taken and transferred to the Natural Resources Laboratory of the Genetics and Environment Research Center-UATx, cold, inside a Coleman^® cooler with gel bags 250g POWERICE^® coolant. Composite samples were made for each site using the quartering method, based on NOM-021-RECNAT-2000 (^{Semarnat, 2002}). For GDO, 14 composite samples were obtained, in ZAC 7 and SBM 6, later stored at 4 °C in an Imbera^® refrigerator to inhibit microbial activity. The rest of the soil from each sample was placed on Kraft paper to dry at room temperature and in the shade. The soil was sieved in a 2 mm mesh, all of the above following NOM-021-RECNAT-2000 (^{Semarnat, 2002}).

Soil analysis

Each soil sample was determined texture, Bd (bulk density), EC (electrical conductivity); OM (organic matter); OC (organic carbon); CEC (cation exchange capacity), pH, total N, phosphorus, potassium, calcium and magnesium by methods established in NOM-021-RECNAT-2000 (^{Semarnat, 2002}). Microbial respiration through the quantification of CO₂, the activity of the dehydrogenase enzyme and the bacterial biomass by the methods described by ^{García et al. (2003)}.

Data analysis

Data were analyzed by site using one-way analysis of variance and a Tukey mean comparison test (p=0.05). Previously, the data were analyzed to verify compliance with the assumptions of normality (Kolmogorov) and homogeneity of variance (Levene). The model used for the analysis was the one described in Equation 1 (^{InfoStat, 2008}).

Where:

µ = General average

τi = Effect of the i ^-th treatment

ε_ij = Experimental error

From the soil properties of the three sites with and without conservation works, a Pearson correlation analysis (p=0.05) and a principal component analysis (PCA) were carried out to determine their contribution to the total variation of the data. All of the above was done with the statistical software InfoStat free version 2020 (Balzarini et al., 2008).

Gustavo Díaz Ordaz

The soils between the board-ditch and trench-ditch treatments show significant differences (p<0.05) in bacterial biomass and total N with respect to the control. Based on the values observed in these two variables, it is clear that the soil conservation works and the vegetation present did not increase the values, since they are lower than the control (Table 1). The above agrees with ^{Cotler et al. (2013)}, who report that the construction of trench-ditches did not allow an increase in the total N content, even with vegetation; they state that the mineralization of the litter to inorganic forms (ammonium and nitrates) will depend on the amount that can be converted by the microbial biomass, which in this case, was also lower with the conservation works, which suggests low effectiveness of the trench-ditch. ^{Beltrán et al. (2018)} point out that the establishment of plant species allows a greater diversity of microorganisms and microbial activity, in such a way that trophic relationships are established that contribute to the improvement of soil quality.

The establishment of trenches coupled with the vegetation did not modify the physical properties of the soil for eight years, the sand fraction predominates by 73 % on average and the bulk density without variation. On the other hand, even without significant differences, the chemical properties show that the presence of trees tends to increase the acidity of the soil (^{Khalaji et al., 2021}). With the trenches on board, an increase in the content of OM, OC, Ca, Mg and CEC was observed, which is high [>40 Cmol (⁺) kg^-1] based on NOM-021-RECNAT-2000 (Table 1). ^{Vazquez-Alvarado et al. (2011)} refer that soil conservation works and an adequate selection of plant species are important to obtain positive results, due to the fact that there is an increase in the OM content and, therefore, a greater retention of moisture and soil particles, as well as a greater number of microorganisms (^{Doria et al., 2022}). However, this was not observed in this site for bacterial biomass, which was significantly higher in the control soil.

Zacapexco

In the soil between the board-ditches, the sand and clay fraction, the pH, OM, OC, microbial respiration and the dehydrogenase enzyme presented significant differences (p<0.05) with respect to the control soil (Table 2). At this site, the construction of the board-ditches probably allowed an increase in soil moisture, which favors the presence of herbaceous plants and grasses associated with J. deppeana and P. pseudostrubos, which improves the chemical and biological properties of the soil.

Five years after the establishment of the conservation works, there is a decrease in pH, an increase in OM and OC, in CEC, phosphorus, calcium and magnesium, as well as a notable improvement in biological activity. This highlights the importance of maintaining and incorporating plant cover with the correct species together with conservation works, since plants supply OC as an energy source for soil organisms to carry out their metabolic activity, which will depend on the type of soil, time of year, climate, nutriments, topographic conditions of the site and the type of vegetation (^{Yáñez et al., 2017}; ^{Khalaji et al., 2021}; ^{Doria et al., 2022}).

San Bartolomé Matlalohcan

The average values of Bd, EC, CEC, the content of Ca, Mg and K were significantly different (p<0.05) between the treatments (Table 3). On the other hand, the values of the content of OM, OC and of the biological properties, although without significant differences, reflect a slight increase. At this site, conservation works were complemented by reforestation with eucalyptus trees. It has been indicated that although eucalyptus trees are fast growing, they have an allelopathic effect that negatively impacts the soil (^{Murillo et al., 2005}), because they modify their characteristics and decrease the biodiversity of fungi, lichens, and herbaceous plants. In the same way, the functioning of the ecosystem is altered in processes such as the decomposition of litter, due to the decrease in soil microorganisms (^{Munguía et al., 2004}; ^{García-Osorio et al., 2020}; ^{Solís-Vargas et al., 2021}), what is observed in this particular site.

Leaf litter is the most important source of nutrients. Particularly, the decomposition of eucalyptus leaves releases a greater amount of K (0.96 %) than P (0.06 %) and N (0.89 %), which depends on soil conditions (^{Munguía et al., 2004}). The cation exchange capacity was verified, mainly, by the clay content, which was higher with the presence of the board-ditch, since the organic matter is low, from the slow decomposition of the litter (^{Welemariam et al., 2018}; ^{García-Osorio et al., 2020}).

Relationship between soil properties

In this part of the results, bulk density (Bd) is negatively correlated with OM, potassium, calcium and microbial respiration, which means that if Bd decreases, there will be less compaction; this will increase the pore space and, consequently, there will be an increase in the water storage capacity, which, in turn, will impact the functions of soil microorganisms (^{Notaro et al., 2018}; ^{Rosero et al., 2019}; ^{Barajas et al., 2020}). ^{Cotler (2015)} evaluated the construction of trench-ditches and reported that Bd increased, and explained that this material which is deposited undergoes an alteration that is reflected in OM and microbial activity, as observed in GDO.

The OM content is a measure of soil quality since it is considered a source of energy and a reserve of nutriments, contributes to the resilience of the soil-plant system, favors CEC, improves the availability of phosphorus, influences the increase in water retention capacity and attenuates thermal variations in the soil, in addition to microbial diversity and activity (^{Fischer and Dubis, 2019}; ^{Frugoni et al., 2020}). This situation was reflected in GDO and ZAC with the establishment of the board-ditch and the vegetation, and there was an improvement in the chemical and biological properties of the soil. On the other hand, OM also had a positive influence on EC, P, K and Ca. Total N with bacterial biomass and CEC, which was higher in the three sites, is positively associated with Mg, but negatively with K and Ca (Table 4).

The principal component analysis (PCA) (Figure 1) showed that 74.5 % of the total variability of the data is explained by the PC1 (52.1 %) and PC2 (22.4 %) components. The variables with the greatest weight in CP1 were cation exchange capacity (0.32), calcium (0.32), organic carbon (0.32) and organic matter (0.31), which presented significant correlations (Table 4) and influence the biological activity of the soil. Similar results were reported by ^{Rosero et al. (2019)}, who through a PCA recognized that in forest, grassland and silvopastoral system soils, PC1 explains that 48.48 % of the variability is determined by CEC (0.97) and calcium (0.86), while, in PC2 (21.69 %), the sand and clay particles, the pH and total N stood out.

Figure 1 Biplot graph of soil properties in three sites in the state of Tlaxcala. 

On the other hand, in forest soils with native vegetation, ^{Notaro et al. (2018)} obtained values in PC1 of 79.01 % for variables such as enzymatic activity, carbon of microbial biomass and soil OC, which is directly related to OM and humification processes.

In addition, the PC2 was explained by the sand particle (0.47), clay (-0.41), pH (-0.41) and total N (-0.41), being associated with the Bd and OM values. ^{Alvarez-Arteaga et al. (2020)} recorded 10.36 % variability associated with the mineral fraction of the soil, true density, silt, sand, and CEC. This suggests that the correlations and importance of the soil variables can be modified depending on the specific conditions of each locality. In the soils of the sites under study there is a predominance of the sand fraction, which implies a greater infiltration and, with it, a decrease in the water retention capacity that impacts the absorption by the roots of the plants and, therefore, in the dynamics of organic matter and biological activity of the soil (^{Díaz et al., 2018}).

^{Garcia-Osorio et al. (2020)} mention that as the age of a reforestation increases, there will be a gradual integral recovery of the ecosystem, due to the fact that the OM incorporation process increases until reaching amounts similar to the original site. This assertion is only partially reflected in this study, since in SBM, at 40 years, there is a notable difference in terms of soil quality with respect to GDO and ZAC. Clearly, in SBM the established tree species plays a relevant role in the lack of recovery of soil properties, unlike ZAC where there are native species with conservation works.

With the results obtained, the relevance of evaluating the impact of establishing soil conservation works through soil properties is confirmed, since they provide more detailed information for planning and management of works and soil conservation or recovery practices.