Increases in soil organic carbon in semi-arid integrated crop-livestock systems

Background/Question/Methods

Agricultural systems may sequester carbon (C) and offset greenhouse gas (GHG) emissions but the degree and direction of impact depends on management. In the semi-arid Texas High Plains, incorporation of perennial grasses and cattle into the cropping system has been shown to conserve water, decrease soil erosion, and increase soil organic matter. However, a detailed inventory of soil organic C (SOC) and GHG monitoring has not occurred.  Therefore, two integrated crop-livestock systems were selected to measure SOC in whole soil and multiple aggregate pools, and soil CO₂ and N₂O fluxes.  One system contained perennial native grasses (PNG), and millet and cotton planted in rotation all managed under dryland conditions. The other system was deficit irrigated and contained bermudagrass (BER) and old world bluestem (OWB).  CO₂ fluxes via a Li-COR LI-8100 system, and N₂O fluxes via static chambers, were measured weekly from July-September 2010 and monthly October 2010-June 2011.  Soil samples (0-5 and 5-20 cm) were collected in July 2010 and processed through a physical fractionation method to isolate C pools.  Of particular interest are intra-aggregate POM (iPOM) and micro-aggregate (iMicro) fractions as these have been associated with labile and recalcitrant C pools, respectively.

Results/Conclusions

Fluxes of CO₂ and N₂O averaged 113 mg CO₂-C m^-2h^-1 and 0.006 mg N₂O-N m^-2h^-1, respectively and were most responsive to irrigation or precipitation events. Greatest fluxes were measured under BER, followed by PNG and OWB, with the lowest fluxes produced in cotton-millet rotations. Only two significant N₂O fluxes occurred within 48-hours of precipitation events (0.059 mg N₂O-N m^-2h^-1 on 30-June-2010 and 0.113 mg N₂O-N m^-2h^-1 6-August-2010). From December 2010-April 2011 CO₂ fluxes averaged 37 mg CO₂-C m^-2h^-1 corresponding to a dormant period for all vegetation types.  As of mid-August 2010 N₂O fluxes had decreased to an average 0.005 mg N₂O-N m^-2h^-1 with no significant variations detected.  Soils from perennial grasses contained significantly higher greater quantities of SOC (11.8 g kg^-1) in comparison to annual vegetation (7.6 g kg^-1), which may offset increased GHG fluxes measured in grassland vegetation.  Intra-aggregate fractions, and their SOC, were the most responsive to systems management with the exception of the large macro-aggregate fraction found only in BER.  Although, BER had higher activity as indicated by increased CO₂ and N₂O flux rates, these systems also have the potential to increase protected SOC pools; a result of low disturbance rates and the increased vegetative inputs demonstrated by high OM and SOC contents.