Long-term effects of grazing on the biological, chemical, and physical soil properties of the Caatinga biome

Abstract

Soil degradation is a global issue that affects both plant productivity and human life. Intensive grazing practices can accelerate this process, mainly due to rapid removal of biomass from the soil surface. However, the long-term effects of grazing on biological, chemical, and physical properties remain poorly understood, particularly in tropical drylands, such as the Caatinga biome. Our aim was to evaluate the soil properties and combine both culture-dependent and -independent analyses to assess metabolic activity and bacterial community structure. We collected samples (0-20 cm) of three different types of soil in the Caatinga biome: secondary Caatinga forest (NC), grazing exclusion (GE), and degraded areas by overgrazing (OG). We sought to investigate how grazing affects soil properties to determine the effectiveness of grazing exclusion in the restoration of soil fertility/functions. Redundancy analysis demonstrated NC were positively correlated with organic carbon (λ = 0.18, p = 0.0012) and total nitrogen (λ = 0.16, p = 0.0011), while OG was correlated with harmful soil parameters such as Na⁺ (λ = 0.08, p = 0.0400), electric conductivity (λ = 0.13, p = 0.0060) and exchangeable acidity (λ = 0.11, p = 0.0030). In addition, GE showed lower aluminum content and saturation, reducing these harmful parameters by 48 % and 34 %, respectively. Also, GE showed the highest values for the β-glucosidase (63.62 mg ρ-nitrophenol kg^-1 h^-1) and arylsulfatase (5.8 mg ρ-nitrophenol kg^-1 h^-1) activities. Changes in bacterial community structure were significant (p = 0.0096), with a higher difference comparing GE and OG (p = 0.0135). The GE area showed 20 % more phosphate solubilizers than OG, but there were no differences for siderophores production. All isolates were halotolerant and had at least 60 % nitrogen fixers. Our findings indicate that while soil recovery is slow, with grazing-exclusion areas presenting 18 years of implantation, it seems to improve in subsequent years. Finally, our results provide evidence that microbe-based technologies can mitigate soil degradation in the Caatinga biome.

Keywords: Drylands; Microbial ecology; Plant-Growth promoting bacteria; Soil enzymes.