Effect of Management Intensification on Deeper Soil Carbon in Subtropical Rangeland.

Grassland ecosystems are known for their significant potential as a net sink of atmospheric CO₂ and soil carbon sequestration. Conversion of native rangeland to improved pastures can affect the soil organic carbon in different soil horizons and their loss as CO₂ in the atmosphere. We hypothesized that the grassland conversion can be used to estimate the availability of C4-derived and C3-derived carbon sources to soil respiration using 13C natural abundance. The experimental design consisted two ecosystems: native rangelands with saw-palmetto and improved pastures with bahia grass in Ona, Florida. Soil CO₂ flux was measured in situ by using the automated soil CO₂ flux system (LICOR-8100A infrared gas analyzer) and stable isotope ratio (d13C) was measured using mass spectrometer. Significant effect of management practices was observed in soil CO₂ flux with high SOC content in the improved pasture that enhanced rapid decomposition. Greater CO₂ flux was observed in the improved pasture (4.4 µmol CO₂ m^-2 s^-1) because of higher soil microbial activity and root respiration. The average flux from native rangeland was 1.44 µmol CO₂ m^-2 s^-1 which is 29% lower than the improved pasture. A1 horizon soil in improved pasture had δ13C value of -18‰ which indicated the contribution of newly added C4 derived C inputs to SOC decomposition. Bh horizon had higher negative δ13C (-22‰) value which explained the origin of soil respiration that is mainly composed of C originating from C3 native plants. Contribution of C4-derived and C3-derived carbon sources in Bh horizon was 30% and 70% to soil CO₂ release respectively. Addition of C4-derived carbon in improved pasture increased soil respiration rate by approximately 2 times in Bh horizon that has higher storage of C3-derived carbon. These results indicated that the conversion of native rangeland to improved pastures enhanced soil carbon loss in Bh horizon in subtropical grasslands.