9.2 Grassland and Savanna Biogeochemistry

Grasslands and savannas are dynamic ecosystems shaped by fire, grazing, and complex soil-plant interactions. These processes drive nutrient cycling, carbon storage, and ecosystem structure, creating unique biogeochemical patterns that support diverse plant and animal communities.

Climate change poses significant challenges to grassland ecosystems, altering temperature and precipitation patterns. These shifts impact plant phenology, productivity, and carbon storage, while also influencing fire regimes and species composition, highlighting the need for adaptive management strategies.

Grassland and Savanna Biogeochemistry

Biogeochemical processes in grasslands

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• Fire dynamics drive ecosystem processes through frequent, low-intensity burns promoting nutrient cycling stimulating new growth and releasing stored nutrients from biomass (Tallgrass prairie)

• Herbivory impacts shape vegetation structure as grazing by large mammals accelerates nutrient cycling through dung and urine deposition affecting plant species composition (Serengeti)

• Soil-plant interactions characterized by extensive root systems of grasses enable rapid nutrient uptake and turnover with high belowground biomass allocation

• Nitrogen cycling involves biological fixation by legumes and other plants volatilization of ammonia during fires and nitrification and denitrification processes

• Phosphorus dynamics include weathering of parent material mycorrhizal associations for enhanced P uptake and P limitation in many grassland and savanna ecosystems

Effects of grazing on nutrient cycling

• Grazing intensity effects vary as moderate grazing stimulates productivity while overgrazing leads to soil degradation and nutrient loss changing plant community composition

• Nutrient redistribution concentrates nutrients in grazing hotspots creating spatial heterogeneity in soil fertility

• Soil compaction reduces infiltration and water holding capacity impacting soil microbial communities

• Fire management through controlled burns maintains ecosystems affects nutrient availability and cycling influences vegetation structure and diversity

• Fertilization practices alter nutrient ratios and availability potentially causing eutrophication in adjacent water bodies and shifting plant community composition

• Invasive species management impacts native plant communities alters nutrient cycling patterns and changes fire regimes (Cheatgrass)

Carbon storage in grassland ecosystems

• Soil organic carbon (SOC) dynamics show high SOC content in grassland soils with rapid turnover rates influenced by root systems on SOC accumulation

• Aboveground biomass carbon remains lower compared to forests with seasonal variations in carbon storage highlighting importance of perennial grasses

• Belowground biomass carbon contributes significantly to total ecosystem carbon with deep root systems of savanna trees

• Factors affecting carbon sequestration include climate (temperature and precipitation) soil texture and mineralogy and land-use history and management practices

• Carbon fluxes involve net ecosystem exchange (NEE) gross primary production (GPP) and ecosystem respiration

• Potential for carbon offsetting through improved grazing management and restoration of degraded grasslands and savannas (Loess Plateau, China)

Climate change impacts on grasslands

• Temperature effects alter plant phenology and growing seasons increase evapotranspiration rates and change soil microbial activity

• Precipitation changes shift vegetation composition impact primary productivity and alter fire regimes

• CO2 fertilization effect enhances water use efficiency in plants potentially increasing woody encroachment (African savannas)

• Land-use conversion through agricultural expansion intensification urbanization and habitat fragmentation changes soil structure and nutrient cycling

• Nitrogen deposition increases N availability in naturally N-limited systems potentially causing soil acidification and shifting plant community composition

• Extreme weather events such as droughts impact vegetation and soil processes increase frequency of wildfires and cause soil erosion and nutrient loss during intense rainfall events (Australian grasslands)