Long-Term Grassland Intensiication Impacts on Particle-Size Soil Carbon Fractions: Evidence from Carbon-13 Abundance.

Proper management of grassland ecosystems for improved productivity can enhance their potential to sequester atmospheric CO₂ in the soil. However, the direction and extent of soil C changes in response to improved grass-land management or land-use conversion varies depending on the ecoregion or management practice. The objectives of this study were to: (i) assess the long-term (>20-yr) impact of grassland management intensiication on soil C fractions after conversion of native rangelands to silvopasture and sown pasture ecosystems; and (ii) determine the contribution of sown grass species to soil C sequestration in both the labile and more stable soil C fractions. Experimental sites consisted of a gradient of management intensities ranging from native rangeland (lowest), to silvopasture (intermediate), to sown pasture (highest). After 22 yr following land-use conversion from native rangeland to silvopasture or sown pasture, total soil C stocks (0-30-cm depth) were greater under silvopasture (69.2 Mg C ha^-1 ) and sown pasture (62.0 Mg C ha^-1 ) than native rangeland (40.9 Mg ha^-1 ). Conversion to sown pasture increased particulate organic C concentration (10.6 g C kg^-1 ) compared with native rangeland (6.3 g C kg^-1 ), while silvopasture increased the mineral-associated C fraction (5.7 vs. 10 g C kg^-1 for native rangeland and silvopasture, respectively). Isotopic analysis indicated that the C₄ grass component contributed significantly to soil C accumulation within these eco-systems. Data also showed that grassland management intensification has the potential to promote soil C sequestration, and the use of strategic management practices such as integration of trees can improve soil C stability under similar subtropical conditions. [ABSTRACT FROM AUTHOR]