Root carbon inputs under moderately diverse sward and conventional ryegrass-clover pasture: implications for soil carbon sequestration.

Background and aims: A strategy to increase soil C under pasture-based systems is to increase the root mass inputs or increase rooting depth of plants. Our objective in this study was to measure the seasonal dynamics of root mass and C inputs under two different pasture types (ryegrass-clover vs moderately diverse) that differ in plant diversity and which are commonly used in New Zealand agriculture. Methods: This study was carried out on an existing plant diversity field trial containing six replicate paddocks of both moderately-diverse and ryegrass-clover pastures. Soil cores (0-100-200-300 mm sections) were collected seasonally across 1 year and individual root traits assessed from all species. Results: The moderately diverse pasture had greater root mass (5320-9350 kg ha) than the ryegrass-clover pasture (3810-5700 kg ha) for all seasons and had greater root mass lower in the soil profile. A secondary objective demostrated no significant difference in root mass between high and low sugar ryegrass cultivar. Increased root mass results in an estimated increase of C input to the soil of about 1203 kg C ha (0-300 mm depth) under the moderately diverse pasture, excluding root exudates. Root trait measurements demonstrated a greater diversity of root traits in the moderately diverse sward compared to the ryegrass-clover pasture. Conclusions: Moderately diverse pasture systems offer scope to increase soil C under grazed pastures through increased root mass inputs and rooting depth. [ABSTRACT FROM AUTHOR]