Comparison of soil sutainability measured by carbon sequestration using carbon isotopes from cotton (Gossypium Hirustum) - forage - integrated management systems



Journal Title

Journal ISSN

Volume Title



Soil Organic Carbon (SOC) is an integral part of maintaining and measuring soil sustainability. This study was undertaken to document and better understand the effect of agro ecosystems on SOC. The methods used were to compare SOC content and 13C isotopes from the total soil and particulate organic matter fraction (POM) of two agro ecosystems. This research was conducted on the Texas High Plains, a semiarid region. The two systems included in the study were a continuous cotton (Gossypium hirsutum L.) monoculture system and cotton and beef cattle (Bos Taurus L.) grazing system. The cotton and beef cattle grazing system is comprised of 53.6% WW-B Dahl old world bluestem [Bothriochloa bladhii (Retz) S.T. Blake; OWB], 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 cereal L.) and wheat (Triticum aestivum L.). 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. Because C3 (trees, shrubs, and cool season grasses) and C4 plants (warm season grasses) have a specific isotopic ratio variation of ä¬13C (‰) in SOC, the ä-13C isotope ratios were used to trace the percentage of SOC to its source. An investigation was made of both the total soil organic carbon (TSOC) and the particulate organic carbon (POC). The POC is the carbon found within the sand size fraction, also known as the particulate organic matter carbon (POM) and is comprised of partially decomposed organic matter. This fraction revealed dynamics regarding the more current management system. Warm season grasses produce about twice as much biomass as cool season grasses and it was hypothesized that the cropping system that integrated warm season grasses and livestock with cotton would return higher inputs of C to the soil and would have lower carbon turnover rates than a continuous cotton monoculture, thus improving soil sustainability. The integrated livestock-crop system sequestered 30% more SOC than the continuous cotton monoculture system. The WW B-Dahl old world bluestem system component was the more significant contributor to higher carbon returns having 19.6 Mg ha-1 of SOC and sequestering 43% more soil organic carbon than the continuous cotton monoculture. The no-till cotton/ cereal rye/ wheat rotation (RCW/WFR) system component had 10 Mg ha-1 ±1 SOC and sequestered 16 ±3% more SOC than the conventionally-tilled cotton. The conventional tilled continuous cotton monoculture decreased soil sustainability and resulted in SOC loss on the surface with a vertical distribution of 1.1,1.4, and 1.5 Mg ha-1 at the 0-5, 5-10, and 10-20 cm depth, respectively, for the continuous cotton monoculture. There was an increase in POC with depth in the soil profile in the continuous cotton monoculture system. This suggests current management is effecting changes deeper in the soil plow layer for the continuous cotton monoculture than for the integrated system. Vertical distribution of POC within paddocks displayed a significant decrease in carbon 0-5 >5-10>10-20 cm for all paddocks except the conventional-till continuous cotton monoculture. POC for the 0-5, 5-10, 10-20 cm depth were 1.97, 0.53, and 0.23 Mg ha-1 cm-1 for OWB, 1.38, 0.42 and 0.27 Mg ha-1 cm-1 for RCW, 0.74, 0.33, and 0.16 Mg ha-1 cm-1 for the WFR and 0.26, 0.36, and 0.32 Mg ha-1 cm-1 for the continuous cotton monoculture. Carbon isotope signatures from soils sampled in 1997 at the initiation of the field experiment compared with soils sampled from 2007 showed that soil became less enriched with 13C in paddocks with C3 plant inputs. The 1997 ä¬13C values were ~-15.0‰ a result of C4 native grasses and more recent C4 plant inputs. Current soil ä¬13C values range from -21.1‰ to -16.1‰ and values decreased with depth for the integrated cotton-forage-livestock system and remained consistently less enriched in 13C isotopes throughout the 0-20 cm depth. The highest value (21.1 ‰) was found in the continuous cotton monoculture system demonstrating that it is having more of an effect on carbon turnover. The integrated cotton-forage-livestock system had an overall increase in SOC while retaining the carbon in the system from past land management practices and a decrease in C3/C4 carbon turnover. A system that increases SOC and also protects carbon that is already in the system may provide economic incentives for land managers if carbon credit systems are implemented. In addition, a system with slower carbon turnover rates sequesters more carbon if carbon inputs are higher than conventional monoculture systems.



Livestsock systems, Carbon sequestration, Integraded