Introduction

Soil is a natural resource on which civilizations depend. Population growth and the pressure exerted on it, mainly for food production, leads to its degradation when recovery strategies are not applied. This decrease in the capacity to produce goods and services is known as degradation, which is a limiting factor with severe negative impacts, as a result of the periodic change in the use that is made of the product (^{Yinka et al., 2022}).

Land use change is the modification of natural vegetation cover to an induced ecosystem, such as cropland and grassland, which usually leads to a loss of soil fertility (^{Gong et al., 2022}), as their physical, chemical, and biological properties are affected (^{Yinka et al., 2022}). However, when these changes are focused on land cover recovery or conversion, such as with reforestation following the abandonment of cropland, they can increase fertility over time (^{Smith et al., 2016}), in addition to the positive effects due to the increase in the number of fauna species (^{Nardi and Marini, 2021}).

Soil properties, as indicators of soil quality, reflect its ability to function within the limits of the ecosystem of which it is a part (^{Bautista et al., 2004}); although they can be studied in isolation, they are all interrelated (^{López and Estrada, 2015}). The physical, chemical and biological properties indicate the behavior, constitution, and fertility of the soil and are an inherent product of the soil formation process.

In Mexico, the National Institute of Statistics and Geography (Inegi) cites 25 of the 32 soil units classified by the World Reference Base for Soil Resources (WRB), in which shallow soils such as Leptosol and Regosol predominate, covering 41.5 % of the national territory (^{Semarnat, 2016}). Regosols are a type of soil that develops on unconsolidated materials poor in organic matter; they are widely distributed worldwide and at all altitudes, mainly in arid areas, in the dry tropics and in mountain regions (^{Semarnat, 2016}) and are used in agricultural and livestock activities.

In Mexico, soil deterioration affects many components of the social and natural environment; its management involves different agencies, international organizations and academic and civil institutions (^{Semarnat, 2000}). Environmental restoration programs are focused on vegetation, which involves actions for the recovery of the natural capacity and potential of forest soils, depending on their structure, the management practices applied, and the time they have been under disturbance conditions (^{de Andrade et al., 2017}); it should be noted that organic matter is one of the most useful properties as an indicator of fertility (^{Salehi et al., 2011}; ^{Cantú and Yáñez, 2018}).

The objective of this research was to evaluate the physical (texture, bulk density, mechanical resistance to penetration) and chemical properties (pH, electrical conductivity, and organic matter) of a Regosol with pine forest vegetation and to compare it with areas of reforested forest, agricultural use and grassland.

Materials and Methods

Study area

The study area was located in the Congregación de la Laguna ejido, in Galena municipality, Nuevo León, Mexico, at 24°49′59.88″ N and 100°04′01″ W. ‎The dominant climate is temperate sub-humid with summer rains, total annual precipitation fluctuates between 600 and 800 mm, and the average annual temperature between 12 and 18 °C (^{INEGI, 2008}). The dominant soils in the area belong to the Leptosol (38.8 %) and Regosol (20 %) calcareous types. The vegetation in the valleys and lower areas is a desert rosette scrub and in the higher areas pines cover considerable areas (^{INEGI, 2007}).

Within the area, four 100 m² sites were randomly selected. S1: Pine forest (PF) dominated by Pinus cembroides Zucc. and P. greggii Engelm., S2: A reforestation area (REF) with pine (P. cembroides) with level edges established in 2008, S3: A rainfed agricultural area (AGRI) with corn (Zea mays L.) and wheat (Triticum aestivum L.) crops, and S4: An abandoned grassland area (PAST) with the presence of grasses, approximately 10 years of abandonment, previously used for rainfed agriculture (Figure 1).

Ejido Congregación de la Laguna = Congregación de la Laguna ejido.

Figure 1 Location of sampling sites in the Congregación de la Laguna ejido, Galeana municipality, Nuevo León, Mexico. 

Results and Discussion

The chemical properties indicative of the quality and the results for the four types of use (Table 1) evidenced the effect of the changes in soil use leading to changes in fertility. The pH values correspond to a slightly alkaline soil (^{Semarnat, 2002}), with an average of 7.70 pH units in a range of 7.71 (agricultural 0-5 cm) to 7.84 (grassland 5-30 cm); the highest values were registered for the second depth, with differences between the two depths. This is in agreement with ^{Martínez et al. (2008)}, who reported that after four years of soil management by tillage, there was a decrease in the pH on the surface of the soil. The EC registered low salinity values within a range of 0.159 dS m^-1 (agricultural 0-5 cm) to 0.187 dS m^-1 (grassland 5-30 cm) and without significant differences between soil uses for each depth (Table 1). Generally, coarse-textured soils have macropores that hinder the accumulation of salt (^{Encina and Valinotti, 2000}).

Organic matter represents the fertility of soils ―an important product of the combination of physical and chemical properties―, in addition to the contribution of organic residues in the process of decay, and constitutes 5 % of the composition of an ideal soil (^{Porta et al., 2003}).

In the analyzed Regosol, the OM values at depth 0-5 cm ranged from 2.56 % for the agricultural area to 4.43 % for the grassland. For the 5-30 cm depth, pine forest and grassland decreased about 1 % in relation to the total content; in the reforestation site it remained similar (with a difference of 0.1 %), while in the agricultural use it increased slightly (0.5 %). Higher values in the first depth indicate a higher content of active carbon and nitrogen, due to the accumulation of plant residues (^{Gong et al., 2022}).

Soil is an active reservoir of organic carbon and can act as a source or reservoir of carbon (C) for release into the atmosphere, depending on the management applied (^{Lal, 1997}). Soils with agricultural systems generally have lower carbon stocks than soils with natural vegetation (^{Kátai et al., 2022}); in grasslands, 70 Mg ha^-1 are estimated, this is a value similar to that documented for forest soils (^{Céspedes et al., 2012}).

In this study, the increase in OM in the abandoned grassland, as opposed to the agricultural area, involves a greater potential for carbon sequestration, and evidences the effect of good land use management practices (rest); in this regard, ^{Sahoo et al. (2019)} note that a decrease in the OM stock in land uses other than forests, implies C depletion, a situation that is ameliorated only when better management practices are applied that include addition of plant residues and lower disturbance.

Ordóñez et al. (2022) compared the effect of land use changes in forests; these authors reported a lower impact in the conversion to natural grasslands compared to that observed in crops, due to the differences between the two uses in the time required for soil microbial communities to recover; good management practices and the effect of climatic conditions, such as humidity, influence nutrient content and soil microbial processes (^{Kátai et al., 2022}). According to ^{Martínez et al. (2008)}, the OM content not only depends on local environmental conditions but also is affected by the management.

Physical soil properties are the way in which the soil behaves in processes that do not involve chemical modifications and have a direct or indirect influence on all the functions it performs (^{Narro, 1994}); this influence is expressed by the structural organization, i. e., its aggregation status characterized by the shape, stability, and resilience of the structure (^{Álvarez et al., 2006}) and is determined by the activities carried out on its surface that affect the reduction of the porous system and its continuity (^{González et al., 2008}). Bulk density (BD) and mechanical resistance to penetration (MRP) are two of the physical properties that reflect the affectation of the porosity of the soil as a result of changes in land use. In this research, the BD was maintained at an average of 1.00 g cm^-3, with no significant differences between land uses in relation to the pine forest (Figure 2).

Different values for each property indicate significant differences (p≤0.05) between the land uses. PF = Pine forest; REF = Reforestation; AGRI = Agricultural; PAST = Grassland.

Figure 2 Average values for bulk density (BD) and mechanical resistance to penetration (MRP) for shallow depths. 

In the grassland, the presence of grasses covering most of the area stood out; the BD registered low values, with no significant differences between uses, however, the grassland had the lowest value with 0.97 g cm^-3 compared to the other sites, where the reforestation area had values of up to 1.04 g cm^-3. In areas with 25 years of conversion to buffel grass, ^{Celaya et al. (2015)} observed an impact on the physical properties and on the nitrogen and water reservoirs in the spaces between soil particles, which they related to cattle trampling. ^{Mongil-Manso et al. (2022)} point out that the recovery time on some properties is slow after reforestation and long periods are required for significant changes in soil properties, even in 20-year-old reforestation. Conversely, ^{Hernández-Vigoa et al. (2018)} point out that biological variables, and certain physical and chemical ones, such as bulk density and total organic carbon, are very susceptible to change in the very short term due to land use intensity.

The MRP was the property most sensitive to land use change. There were significant differences between land uses in the surface layer; the pine forest had the highest MRP with an average value of 22.5 kg cm^-2, in contrast to the agricultural area where the effect of tillage breaks the surface layer, reducing its compaction to only 8.25 kg cm^-2 on average (Figure 2). This is in agreement with the findings of ^{Navarro et al. (2000)}, who assessed the effect of five tillage treatments on sandy loam and clay loam soils; according to them, the BD and MRP were modified independently of the type of tillage and remain unaltered only with a no-tillage system.

Texture is one of the most stable physical properties. Regosols are unconsolidated, fine-grained, little-evolved mineral soils (^{FAO, 2016}), with high silt and sand contents. In the present study, texture was classified as loamy for all soil uses, with mean values of 40.9 % sand, 40.3 % silt, and 18.8 % clay for depth 0-5 cm, while at the second depth, the values for sand, silt, and clay were 38.3 %, 42.1 %, and 19.6 %, respectively.

Nevertheless, differences in sand content were obtained according to the analysis of variance between the two depths (Table 2), the comparison of means between land uses for each of the depths showed differences for the percentage of silt in the 5-30 cm depth; in this respect, reforestation had contents of 39 % and pine forest 45.5 %. In the grassland and agricultural sites, there were no differences between them, their values were 41.8 and 42.2 %, respectively (Figure 3). ^{Novillo et al. (2018)} obtained differences in physical and chemical properties with the change from native forest to monocultures, mainly in texture, which affects the properties that contribute to the decrease of aggregates, thereby influencing the vertical movement of water.

Different letters for the same component indicate significant differences based on Tukey's test (p≤0.05) between the land uses. PF = Pine forest; REF = Reforestation; AGRI = Agricultural; PAST = Grassland.

Figure 3 Graphs of average texture values (sand, silt, and clay) for the two depths (0-5 and 5-30 cm). 

High silt contents do not favor stable aggregation (^{Rollán and Bachmeier, 2015}), however, in the presence of low clay contents, the silt fraction plays a key role in OM stabilization, especially when clays are saturated with carbon (^{Matus et al., 2016}).

It should be noted that among the four land uses: edged reforestation, abandoned grassland, agricultural areas, and pine forest, there were no significant differences in the physical and chemical properties evaluated, with the exception of the MRP. Table 2 summarizes the results of the two-way analysis of variance: land use (Factor A) only had significant differences for mechanical resistance to surface layer penetration, where land use change mainly affected the porous system, which is correlated with water infiltration, root growth, and crop productivity (^{Cerana et al., 2005}).

The depth factor (Factor B), which evaluates the effect on soil properties at the two depths, showed significant differences in pH and sand content. ^{Domínguez-Calleros et al. (2017)} explain that the surface layer of the soil presents a greater release of nutrients due to the accumulation of organic residues, with a reduction of soil properties at greater depths.

The soil properties evaluated in the interaction between land use factors by depth (Factor A×Factor B) exhibited no significant differences (Table 2). However, some authors report that changes in soil properties vary according to the use to which the land is subjected (^{Dieckow et al., 2009}; Ordóñez et al., 2022). Others, such as ^{Mongil-Manso et al. (2022)} indicate the opposite; they report the same organic matter content and bulk density for the various uses (shrubland, oak woodland, grassland). ^{Geissen et al. (2009)} evaluated the effect of changes in soil use on physical and chemical properties with different conversion times; their results showed that the evaluated grasslands, with up to 15 years of change, had lower pH, increased BD and MRP, but maintained fertility due to their high clay content and the influence of water, due to the high precipitation that promotes the inhibition of organic matter mineralization processes.

The deterioration and absence of forest cover cause loss of soil fertility, which is why actions are taken to protect ecosystems and contribute to their conservation, protection, restoration, and sustainable use (^{Conafor, 2010}); thus, the evaluated reforestation area was created for the purpose of protecting forest areas. The improvement of its properties unlike in the agricultural area evidences the environmental importance of recovering and improving the soil properties; however, the joint effect takes time to manifest itself. Although the results did not show significant differences, the improvement of physical properties such as bulk density, in addition to the increase in organic matter content, indicated that is in the process of recovery.

On the other hand, although no information was obtained regarding the type of management applied to the pine forest, it is likely that because it is a common area of the ejido, it is used for grazing or forestry; this is reflected in the high values of MRP compared to other uses, or simply in the fact that the physical and chemical properties of the soil of this forest correspond to the intrinsic and dynamic properties of the Regosol.

Conclusions

Most of the physical and chemical properties of the analyzed soil show no significant differences between the evaluated land uses. The most susceptible property to land use change is MRP, for which agricultural use has the lowest value due to the effect of tillage, as the tools utilized for this purpose disperse the soil and reduce compaction. Sand content shows differences when only depths are compared; significant differences in silt content between uses are observed at the 5-30 cm depth, and although the OM content does not exhibit significant differences, it does show a higher accumulation at both depths, in the following order: grassland>pine forest>reforestation>agricultural.

Finally, fallow grasslands after years of agricultural use and reforestation with edges favor the recovery of the properties of Regosol and eventually improve the soil quality, which is maintained in the natural forests due to the intrinsic properties of the Regosol.