Cambio de uso y degradación de los suelos en la Pampa Medanosa Cordobesa: Evolución, impactos y escenarios futuros

Abstract

In Argentina, the expansion of the agricultural frontier over marginal environments of great ecological fragility has occurred without sustainable land use policies. In the south-west of Córdoba province, in the sandy plains known as Pampa Medanosa, the agricultural expansion has exerted high pressure on the soil. This is evidenced by recent studies reporting the reactivation of wind erosion processes. However, little is known about the land use change impact on soil quality, represented mainly by the replacement of extensive livestock by purely agricultural systems and the possible effects of the introduction of peanut cultivation, crop whose harvest removes the top soil; and there are no previous studies of peanut as a component of cultivation sequences with perennial pastures under mixed agricultural-livestock systems. In the predominant agricultural systems in the Pampa Medanosa, even under direct sowing, the crop sequences show low contributions of vegetal remains and a high proportion of annual legumes, whose stubble is promptly degraded, leaving the soils devoid of cover and exposed to wind erosion events, which casts doubt on the sustainability of the current soil management practices. The main objective of this thesis was to study the evolution of the land use change in the Pampa Medanosa region, characterized by the replacement of mixed agricultural-livestock production systems by purely agricultural systems. In particular, the introduction of peanut cultivation was analyzed to identify its effects on soil quality indicators and soil attributes related to wind erosion, generating basic information for the identification of macro environments with different susceptibility to degradation by the wind action that allow guidance on the need to implement sustainable management alternatives to the soils of the region. In the first place, through the analysis of satellite images, the reactivation of wind erosion processes in the Pampa Medanosa was confirmed. A total of 66,448 hectares of active foci were identified. The 4.75% of the region shows erosive processes, indicating the need to rehabilitate degraded lands and to stop the desertification process. In addition, Holocene dunes and older megadunes extend to 73,671.6 hectares of environments that require protection or conservation practices, attending to their fractionation for agricultural use. In a second stage, employing remote sensing and supervised classifications algorithms, it was observed that the change in land use in Pampa Medanosa due to the advance of agriculture over mixed agricultural-livestock activities has increased since the 2006/07 season and continued up to the present. The replacement of mixed agricultural-livestock production systems by purely agricultural systems, and the introduction of peanut cultivation in the Pampa Medanosa, has affected soil quality indicators and attributes related to wind erosion. The evaluated indicators reflected the degradation in soil quality under both agricultural and mixed uses, suggesting that the current uses exert an amount of pressure on the soil that is exceeding the capacities of its potential use. Soils under two purely agricultural uses were compared, one including peanut cultivation (A1) and another excluding this crop (A2); and soils under two mixed agricultural-livestock uses, one with peanut cultivation as component of the crop sequence (Mx1) and the other without its presence (Mx2). Soils under A1 use presented the greatest loss of organic carbon (CO), 45% in the surface layer of the soil in comparison to a natural reference condition (N), increasing their susceptibility to wind erosion. The soils under Mx2 use were superior in terms of physical indicators behavior -erodible fraction (FE) and dry structural stability (DSS)- compared to agricultural ones, which places them in a better position against the risk of erosion. Regarding chemical soil quality indicators, no effects of soil use were observed on pH, whereas the mixed systems with and without peanut exceeded the agricultural ones in CEC values. The use of the soil explained variations in the concentrations of Ca, Mg and K, being the use Mx2 the one that showed the highest availability for the three bases. The soils under mixed systems exhibited reductions in the concentration of extractable phosphorus (P) of between 27 and 30% relative to N, while the soils of agricultural systems without and with peanut presented reductions of 50 and 60% respectively, in the first 5 cm of the profile. The reduction of P concentration in the soils under A1 use increased with depth, showing a loss of 73% from 5-10 cm and 77% from 10-20 cm, demonstrating the importance of using stratified sampling. The indicators with the highest sensitivity to land use in the study area were CO, FE, CIC and K. The spatial variability of the soil quality indicators surveyed in the Pampa Medanosa allowed us to observe that this is a heterogeneous environment that can be zoned according to its susceptibility to wind erosion. The region was subdivided into four sub-regions (SR). Towards the east of the region, SR-I was identified with soils of the best aptitude (258700 ha), whose wind erosion potential rate (WEP) was 21.6 t. ha-1. year -1. The largest SR was SR-II (496,900 ha), which has a WEP of 41 t. ha-1. year-1. Two SRs that require erosion mitigation practices and adequate planning for the management of their soils were observed on the north and west. These two sub-regions together occupy 47% of the sandy plains. SR-III (445,000 ha) whose WEP is 50.9 t. ha-1. year-1 and SR-IV that showed the highest risk of erosion (215600 ha) with an WEP of 66.5 t. ha-1. year -1. The results have made it possible to understand that the Pampa Medanosa is a heterogeneous region in which, in order to achieve sustainable agricultural production systems, it is necessary to implement mitigation techniques for wind erosion and taking into account the potential of land use in each sub-region. The use of the Wind Erosion Equation –EWEQ- made it possible to compare different uses and rotations to guide the selection of sustainable alternatives. An agricultural crop sequence with peanut cultivation without cover crops (CC) was simulated. This was the one that presented the highest erosion rates, widely exceeding the threshold value of 8-12 tons of allowable soil loss per hectare per year. An agricultural rotation without peanuts that included CC after each summer crop showed soil losses due to wind erosion higher than the threshold only in extremely dry and windy years, suggesting that, given these adverse conditions, the mere inclusion of this technique would not be enough to mitigate the degradation process. The modification of crop sequences and the implementation of perennial pastures is of greater importance in reducing the annual rates of soil loss in dry and windy years. Even in extreme years, the mixed use without peanut cultivation and with cover crops after the summer crops did not show soil losses due to the action of the wind. The simulations for mixed agricultural-livestock use including the cultivation of peanuts together with cover crops did not reach the threshold value of annually admissible soil loss. Carrying out peanut production in mixed systems with perennial pastures is presented as an alternative that could contribute to solving the problem of wind erosion in unfavorable years in the Pampa Medanosa.