Long-Term Integrated Crop-Livestock Agroecosystems and the Effect on Soil Carbon.

An integrated crop-livestock system (ICL) was implemented in 1997 at the Texas Tech Experimental Farm in the semi-arid Southern High Plains region as an alternative agricultural practice to monoculture cotton (CTN).  The ICL incorporated stocker steer grazing in three paddocks consisting of two small grain forage-cotton rotation paddocks and a W.W.B.Dahl-Old World Bluestem (OWB) pasture.  The objective of this study was to examine the long-term (13 years) effects of ICL management on total soil carbon (TC) in comparison to CTN management.  Archived baseline soil samples (0-15 cm) collected in 1997 and samples collected in 2010 were isolated into macroaggregate (M, >250 µm), microaggregate (m, 53-250 µm), silt+clay (S+C, <53 µm), intra-aggregate particulate organic matter (iPOM, >250 µm), intra-aggregate microaggregate (iMm, 53-250 µm), and intra-aggregate silt+clay (iS+C, <53 µm) fractions.  Aggregate distribution changed significantly in the M, S+C, iPOM, and iMm fractions regardless of management system.  Within the ICL the change in distribution is driven by the vegetation management of individual paddocks.  OWB paddocks produced a greater quantity of M and iMm compared to small grain paddocks and CTN as a result of the decrease in disturbance and increased vegetative input associated with grassland management.  Disturbances associated with CTN resulted in a decrease in the quantity of M while the iPOM fraction increased.  Total C measured in 1997 (8.92 mg g^-1) increased significantly (~25%) under ICL management (11.18 mg g^-1) in 2010 however no change was detected under CTN (8.99 mg g^-1).  This increase of whole soil C was the most pronounced by the increase of C in the m and iMm fractions from 1997 to 2010.  In 2010, differences were also detected between the CTN and ICL in the m (2.47 and 6.53 mg g^-1 soil, respectively) and iMm (5.36 and 8.50 mg g^-1 soil, respectively) fractions.