Contribution of a Perennial Forage Grass to Carbon Storage in a Peanut-Cotton-Bahia Rotation in Ultisols.

The traditional cropping system in the Southeast US is an annual rotation of cotton and peanuts under conventional tillage.  This cropping system contributes to erosion and loss of nutrients in the soils of this region.  Incorporation of perennial grasses (e.g., bahiagrass) to this rotation has been suggested to improve soil organic carbon (SOC), particularly in conjunction with conservation tillage practices.  In addition, grazing by livestock provides another mechanism to maintain and recycle SOC and nutrients.  In order to determine the effect of perennial grasses and grazing activities on carbon sequestration in a peanut-cotton rotation, total SOC and its isotopic composition were evaluated on established (>10 years) crop rotation systems.  Systems evaluated included 1) peanut-cotton under conventional tillage, 2) peanut-cotton under strip tillage, 3) peanut-cotton-bahia-bahia under conventional tillage, 4) peanut-cotton-bahia under strip tillage and 5) peanut-cotton-bahia-bahia rotation under strip tillage with cattle grazing.  The isotopic ratio of ¹³C/¹²C was evaluated to assess the contribution of the perennial grass to SOC.  Plants with a C₄ photosynthetic pathway (e.g., bahia) are known to have greater ¹³C/¹²C ratio and higher d¹³C-values (-19 to -9‰) than C₃ plants (e.g., peanut, cotton, oat and  rye) (-35 to -20‰).  Results indicated that approximately 35% of the SOC was due to bahia, compared to <5% in systems without bahia.  The δ¹³C values in the 0-5 cm depth were highest following 2 years of bahia and lowest at the onset of the first year of bahia indicating that this pool of C is cycling.  Higher δ¹³C values were observed for the rotation with bahia, to a depth of 15-30 cm.  Use of a moldboard plow increased δ¹³C compared to strip tilled, but cattle grazing had no effect on δ13C values. Results suggest that bahia is relatively labile and concentrated near the surface.