Soil Sustainability as Measured by Carbon Sequestration using Carbon Isotopes from Crop-Livestock Management Systems.

Soil Organic Carbon (SOC) is an integral part of maintaining and measuring soil sustainability. This study was undertaken to document and better understand the relationships between two livestock-crop-forage systems and the sequestration of SOC with regards to soil sustainability and was conducted on the Texas High Plains, a semiarid region.  The two systems included in the study were a cotton (Gossypium hirsutum) monoculture system and a cotton and beef cattle grazing system. The cotton and beef cattle grazing system is 53.6% WW-B Dahl old world bluestem (Bothriochloa bladhii), a perennial warm-season grass and the other 46.4 % of the system is divided in two paddocks of equal size where no-till cotton is grown alternately with cereal rye (Secale cereale). With a livestock-crop rotation system that has varying cover crop residues from different forage species and input returns from the livestock, it becomes difficult to understand the main contributing factor to the increase or decrease of SOC sequestered within the system. Each system is replicated three times. Seven soil core samples from each paddock from depths of 0-5, 5-10, and 10-20 cm were obtained. d^­13C isotope ratios were used to trace the percentage of SOC to its source. C₃ (trees, shrubs, and cool seasoned grasses) and C₄ plants (warm season grasses) have a specific isotopic ratio variation of d^­13C in SOC.  Warm season grasses produce about twice as much biomass as cool season grasses and it is hypothesized that the cropping system with better management practices integrating warm season grasses and livestock return higher inputs of C to the soil improving soil sustainability. We believe that the old world bluestem is the more significant contributor to higher carbon returns followed by cereal rye, no-tilled cotton and conventionally-tilled cotton, which decreases soil sustainability.