Dynamics of Soil Aggregation and Carbon in Long-Term Integrated Crop-Livestock Agroecoystems in the Southern High Plains.

In semi-arid regions of the Southern High Plains, where monoculture cotton (CTN) production is common, alternative land management practices are gaining interest for their ability to maintain productivity in this water-limited environment.  Five systems were selected to examine effects of alternative management practices on soil organic matter (SOM), carbon functional groups, nutrient status, aggregate distribution, and total nitrogen and carbon (TN/TC).  These systems included three long-term (>5 years) integrated crop-livestock systems (ICL) with a variety of vegetation and irrigation practices in conjunction with stocker steer grazing and two subsurface drip irrigated CTN systems (CTN_1; CTN_2). The first system (FRG-CTN) included pastures of perennial native grasses, foxtail millet and cotton (planted in rotation), and buffer-irrigated W.W.B.Dahl-Old World Bluestem (OWB).  The second system (OWB-BER), a subsurface drip deficit-irrigated system, included pastures of bermudagrass and OWB.  The third system (OWB-RC) included pastures of center-pivot irrigated OWB and corn.  Soil samples were collected at depths of 0-5cm and 5-20cm in July 2010, and isolated into macroaggregate (M, >250µm), microaggregate (m, 53-250µm), silt+clay (S+C, <53µm), intra-aggregate particulate organic matter (iPOM, >250µm), intra-aggregate microaggregate (iMm, 53-250µm), and intra-aggregate silt+clay (iS+C, <53µm) fractions.  The two OWB dominated systems (OWB_BER and OWB_RC) contained the greatest quantities of SOM while the two CTN systems contained the least.  Across the five systems the OWB_RC whole soil contained the greatest TC (12.74 mg g^-1)  content, however it was only significantly greater than the OWB_BER (8.78 mg g^-1) and CTN_2 (7.14 mg g^-1) systems. The OWB_RC also produced the greatest quantity of M regardless of depth (28% at 0-5 cm; 20% at 5-20 cm) and was the only system to produce large M (>2000 µm) associated with the OWB portion of the system.  Soil fractionation allowed for a deeper understanding of C dynamics and the potential for these systems to sequester C.