Reducing cattle ranching is not enough to restore degraded soils in Brazil’s Caatinga, researchers say

The Caatinga is the only biome found exclusively in Brazil and is home to the largest contiguous area of seasonally dry tropical forest in the world. However, anthropogenic activities such as cattle ranching, agriculture, and firewood exploitation have caused large-scale environmental degradation processes that, together with climate change, could even lead to desertification.

A study of cattle ranches in the semi-arid region of the state of Pernambuco found that simply stopping grazing did not restore soil health. The researchers recommend complementary initiatives such as green manure and strategic tree planting, which have shown promise in other tropical biomes. This active management can accelerate the restoration of multiple soil functions, promoting the contribution of carbon and nitrogen to the system, as well as improvements in nutrient cycling and biodiversity.

This long-term research is being conducted in Pernambuco to assess the long-term effects of overgrazing (too many animals in the pasture), a typical condition in the region, and the effects of isolating plots to exclude these animals. The research is part of the Perennial Network of the National Observatory of Water and Carbon Dynamics in the Caatinga Biome (CBC). The results were published in the Journal of Environmental Management.

“We compared the soils of the three most common cover types in the region: preserved dense forests, open forests regenerating after deforestation and prolonged grazing, and pastures with a history of decades of overgrazing. The study showed that overgrazing compacts the soil and reduces the availability of nitrogen, carbon, microbial biomass, and glomalin proteins, affecting essential physical, chemical, and biological functions.

“We also found that removing animals from these areas [which were fenced off to prevent grazing] did not bring significant improvements, even after three years of spontaneous soil recovery,” says Wanderlei Bieluczyk, a researcher at the Center for Nuclear Energy in Agriculture at the University of São Paulo (CENA-USP) and first author of the article.

He says that on a regional scale, the transition from dense forest to degraded pasture has resulted in a loss of 14.7 tons of carbon per hectare. Removed from the soil at depths of up to 20 centimeters, this carbon ends up being oxidized and released into the atmosphere, contributing to global warming. In the same transition, the study found an 18% regional decline in a holistic soil health index.

“We developed a soil health index based on multiple edaphic functions [i.e. those belonging to or related to the soil], integrating physical, chemical, and biological indicators. In terms of biological indicators, such as microbial biomass carbon, beta-glucosidase, and glomalin, the conversion from forest to pasture reduced their values by more than 45%.”

It is worth remembering that “soil health” is a concept that refers to the ability of soil to support biodiversity, including microorganisms, plants, and animals, while maintaining essential functions such as water absorption and retention, resistance to erosion, and nutrient cycling, among others that were assessed in the study. This concept applies to a wide range of environments, from protected ecosystems to productive agricultural lands.

“It became clear that overgrazing causes severe degradation of soil health in the Caatinga, and that simply excluding animals is not enough to restore soil functions within a few years of isolating the area. The recovery of areas degraded by inappropriate pasture management, especially due to excessive grazing pressure, is a challenging process and probably requires additional practices, such as the use of green manure and strategic tree planting, to accelerate the ecological recovery of the soil,” Bieluczyk emphasizes.

Green manuring is the practice of sowing or planting plants known as green manures to improve soil fertility and structure. This technique has been used successfully in forest restoration in the Atlantic Rainforest biome, for example. The plants, which are usually composed of legumes or grasses, contribute biomass to nitrogen fixation, nutrient recycling, and soil protection.

Once they have reached a certain stage of development, they can be cut down and incorporated into the soil or allowed to senesce naturally in the shade of regenerating tree canopies. In this way, the soil is protected from erosion, retains more moisture, and receives a gradual supply of nutrients as the biomass decomposes.

Strategic tree planting involves planting fast-growing trees capable of forming a dense canopy in a short period of time. In this way, the soil is protected from excessive sunlight and a “forest environment” is gradually created under the canopy, favoring the germination and development of various regenerative species in this now more favorable environment.

All of this has to be considered in light of the great specificity of the Caatinga biome, as Ana Dolores Santiago de Freitas, a professor in the Agronomy and Soil Science Graduate Programs at the Federal Rural University of Pernambuco (UFRPE) and coordinator of the project in question, explains.

“The Caatinga is a different biome from the others in Brazil because of its semi-arid climate, which is only found in the Northeast. Here, the scarcity and irregularity of rainfall are combined with the great loss of moisture in the soil and plants due to the high temperatures throughout the year as a result of its proximity to the equator. The long coexistence with this climate has led to various plant adaptation processes.

“Most trees and shrubs shed their leaves in the dry season; because they are short-lived, the leaves are thin, with little biomass, and they form quickly when the rains come; there are many short-lived herbaceous plants that are only present in the rainy season, and many succulent plants, such as bromeliads, or thorny and leafless ones, such as cacti,” says Freitas.

The researcher adds that the little variation in temperature and light and the availability of water regulate the vegetation, creating a mosaic of situations. “The edges of the biome, with the Atlantic Rainforest to the east and the Cerrado [Brazilian savannah-like biome] to the west and south, get a little more rain, as do the higher areas in the semi-arid core. Throughout the region, topography has an influence, with valleys receiving water runoff from the hillsides.

“Added to this is the greatest variability of soil types in Brazil, from deep to shallow and from slightly to very fertile, with contrasting types in close proximity. All this environmental variability has resulted in a very diverse flora, with thousands of species, one of the richest among the world’s semi-arid regions,” she emphasizes.

According to Freitas, the Caatinga is also in a very high state of ecological degradation due to decades of inappropriate and unsustainable use of native vegetation as fodder. Bieluczyk describes the step-by-step process of degradation: “It all starts with the removal of native vegetation, logging, and burning. Then comes grazing and soil compaction from excessive trampling by cattle. Compacted soil makes it difficult for water to infiltrate and for roots to grow, resulting in many patches of exposed soil. And all of this leads to a loss of nutrients and soil health.”

The researcher emphasizes the need for public policies aimed at the recovery of the Caatinga and the sustainable management of agricultural activities that guarantee the conservation of the biome’s ecosystem services.

The study was conducted in areas located in three municipalities in the semi-arid region of Pernambuco: Araripina, Sertânia, and São Bento do Una. Soil and vegetation samples were taken to assess the impact of degradation and the first signs of recovery.