Worldwide climate change is having severe consequences in the biosphere, and present concentration of atmospheric carbon dioxide has driven global surface temperature up nearly one degree centigrade (IPCC, 2021). Climate change forecasts for Mexico contemplate an increase in average annual temperature of 1.5°C by 2030, 2.7°C by 2060 and 3.7° by 2090, when compared to weather records taken in the 1961-1990 period (Sáenz-Romero et al., 2010). The rising detrimental consequences that climate change will have on Mexico's temperate forests are only one of the many repercussions it will have on the country. As temperatures increase and rainfall decreases, drier conditions will result, thus causing loss of climatic habitat across Mexican ecosystems (Sáenz-Romero et al., 2010). These changes in the climatic niche are consistent with climatic habitat models for individual species, including coniferous such as: Pinus hartwegii Lindl., Abies religiosa (Kunth) Schltdl. & Cham., Pinus pseudostrobus Lindl., Pinus devoniana Lindl., and Pinus oocarpa Schiede ex Schltdl. (Gómez-Pineda et al., 2020).

Soil erosion is one of the major environmental problems worldwide, and it has been identified as the main factor in soil degradation (Zhou et al., 2021). In severely eroded soils microbiota is disrupted or suffers considerable loss, humidity retention is diminished, and soil structure is modified; in the absence of a protective plant cover, rain falls directly on the soil, leading to the disintegration of unconsolidated soil material and triggering sedimentation when soil particles are dispersed by runoff (Pérez-Nieto et al., 2012), and contributing to global warming by freeing carbon dioxide into the atmosphere (Lal, 2004). Iron-rich soils are sensitive and prone to degradation, especially when plant cover has been lost. In this scenario such soils present high erosion rates and the formation of gullies (Lindig-Cisneros et al., 2019). Gullies is a form of advanced soil erosion caused by water runoff when water flows into channels or rills after intense rainfall. Andosol and Acrisol in humid climates follow an evolutionary pathway that start with the former and ends with the latter (Solleiro-Rebolledo et al., 2019).

Restoration of degraded soils has a positive effect on ecosystems, the quality of water bodies and C sequestration, thus contributing to counteract the impact of climate change. Soil restoration management practices allow annual C accumulations of 50 to a 1000 kg/ha (Lal, 2004). There are large tracts of land worldwide where soil loss has been severe, such as areas dominated by gullies, where soil fertility for agricultural use cannot be restored but that can be restored for C sequestration and prevention of further catchment degradation.

The aim of this study was to assess potential C sequestration in Acrisol and Andosol-dominated sites with gullies in Mexico, on the basis of GIS analysis and field restoration trials which began fifteen and eleven years ago. Sites with Andosol and Acrisol were selected due to the evolutionary link between these two types of soil. The potential sites were chosen due to the presence of gullies and elevation (between 1500 and 3500 m a.s.l.). This elevation range correspond to that of most Pinus species recognized for Mexico: P. leiophylla, P. herrerae, P. hartwegii, P. pseudostrobus, P. montezumae, P. devoniana, P. oocarpa, P. teocote, and P. greggii (Farjon et al., 1997). Moreover, there are other species, such as P. caribea, currently growing at elevations below 1500 m a.s.l. which may be made to migrate to higher elevation in the future as a management option.

Based on the layers used, we performed a geographical analysis of the 1,960,189 km²continental area of Mexico, and the results indicated that water erosion occurs in around 40% of Acrisols and 42% in Andosols. At the national level, 36,034 km²(1.83%) are Acrisols and 23,306 km²(1.18%) are Andosols. Acrisols are found at elevations in the range of 0 to 2,500 m a.s.l., located mainly in the south and southeastern regions of Mexico in the states of Oaxaca, Chiapas, Veracruz and Tabasco. Andosols are found at elevations from 2000 to 4500 m a.s.l., located mainly along the Transvolcanic Belt, in the states of Jalisco, Michoacán, the State of Mexico, Mexico City, Morelos, Tlaxcala, Puebla, Hidalgo, Veracruz, Guerrero, and Chiapas. Upon fulfilling all inclusion requirements, at the national scale, we discovered a potential restoration area for C sequestration of 938.57 km²(2.60%) of Acrisols and 1,871.55 km²(8.03%) of Andosols (Fig. 2), which correspond to areas dominated by gullies. These areas would represent, in terms of soil condition, controlling erosion in 6.37% of the Acrisol affected area and 19.07% of the Andosol area. At the state scale, Acrisol areas are located in the states of Chiapas (527.87 km²), and Oaxaca (358.09 km²), at an elevation range of 1500-2000m asl. Andosol lands are located in the state of Michoacán (926.60 km²), Puebla (331.19 km²), Chiapas (315.43 km²), Veracruz (198.11 km²) and Estado de México (80.04 km²) at elevations between 2000 and 3000 m a.s.l.

The two restored sites' aerial biomass had a C content of 1.41 t/ha for the older site and 1.15 t/ha for the more recent site, with an average of 1.27 t/ha. The sites were evaluated 15 and 11 years after they were established. In terms of soil carbon content, the top 10 cm had higher C accumulation, while deeper soil had lower C accumulation (Fig. 3). Carbon content for the entire profile was 4.25 t/ha. Gullies had no vegetation cover prior to the restoration trials and C content under these conditions was below the detection limit of the total C analyzer equipment (0.1%). However, the soil organic C content was very low in comparison with the soil C content of mature pine forest fragment in Acrisols (0-10 cm depth) or Andosols (0-20 cm depth): 115 ± 0.7 t/ha and 98 ± 13.7 t/ha, respectively (Galicia et al., 2016). These results suggest that the reforested gullies could capture significant amounts of C.

We estimated that potential C sequestration for an average period of 13 years for an area of 2810 km² was 3.947 megatons of C (MtC). A large percentage (89%) of potential capture is concentrated in four states in Mexico, Michoacán in the first place with 1.35 MtC in an area of 965.80 km², followed by Chiapas, with 1.18 MtC in 843.31 km²(Fig. 4). In calculating potential sequestration for each land type, we found that for Acrisols, the state with the greatest potential was Chiapas with 0.74 MtC in 527.87 km², followed by Oaxaca, with 0.50 MtC in 358.09 km². For Andosols, the state of Michoacán had the greatest potential with 1.30 MtC in 926.60 km², followed by Puebla, with 0.46 MtC in 331.19 km², and Chiapas, 0.44 MtC in 315.47 km².

Climate change will bring to Mexico an increase in temperatures and a decrease in rainfall (Sáenz-Romero et al., 2010). This might suggest that conditions for high erosion rates might decrease. But not only the amount of rainfall should be considered, but also the intensity of rainfall events. Worldwide data (Seneviratne et al., 2021) shows that extreme weather conditions are more common as a consequence of climate change, including heavy precipitation. For Mexico, our own field data from conifer forest restoration sites shows that in recent years the rainy season has shortened and that annual precipitation has decreased, but a higher volume of water can fall in a shorter period of time (Gómez-Pineda et al., 2021). Consequently, erosion processes might worsen, in particular if heavy precipitation is coupled with losses of vegetation cover caused by plant mortality during the dry season.

Restoration of severely degraded sites is usually very limited, except in cases when it is undertaken as part of mitigation projects mandated by law, due to the cost involved, and because recovery of ecosystem structures and functions is at best limited. Nevertheless, inaction in addressing this problem contributes to the increase in the degradation of whole watersheds. Our results show that severely degraded sites where gullying is dominant have a high potential for C sequestration of up to 0.3 MtC per year in the most severely degraded Acrisol and Andosol lands, representing 0.14% of the Mexican territory. Potential C sequestration would rise to 2.7 MtC annually if we take into account the approximately 24,000 km² of degraded Andosol and Acrisol areas that are located in Mexican territory (representing 1.25% of the country). Furthermore, severely degraded sites are an attractive option for restoration for C sequestration because: (i) social barriers associated with the restoration of these sites are few because there are no competing land uses; (ii) controlling gully formation prevents greater soil loss, and (iii) by applying strategies of assisted migration, these sites may serve as refuges for species whose habitats are disappearing due to the negative effects of climate change.