Soil Carbon and Soil Organic Matter Quality In Soil Size Fractions From Crop and Livestock Systems In Texas.

Cotton based rotations and monocultures in the Southern High Plains have resulted in soil quality degradation because the semiarid environment combined with low crop residue returns has diminished soil C. Integrated crop-livestock systems and no-till based rotations can increase soil C when used as an alternative to intense cotton production. Soil aggregate size fractionations have allowed for the identification of separates such as microaggregates within macroaggregates that contain C protected from decomposition, and may serve as indicators of soil responses to changes in agricultural practices. Diffuse reflectance Fourier-transform infrared spectroscopy (MidIR) has the capacity to identify mineral and organic functional groups in soil size fractions and potentially help to quickly determine changes in C chemistry. We established an experiment comparing irrigated continuous cotton (CTN) with an integrated crop-livestock system that included a bluestem pasture (grazed and ungrazed) as well as wheat rye rotations.  The experiment was established in 1997. Soil samples (0-15 cm) were obtained in 1997 and 2010. The soils were fractionated and the whole soils and fractions analyzed for soil C, N, and MidIR. Free size fractions included large macroaggregates (LMacro), small Macroaggregates (SMacro), microaggregates (Micro), and silt+clay (S+C). Intra-aggregate fractions included POM (iPOM), microaggregate (iMicro), and silt+clay (iS+C). The MidIR data shows that the iPOM is very different from the whole soils and fractions.  As expected, the iPOM absorbs at the silica bands between 1790-2010 cm-1.  However, the iPOM also absorbs at the 1367 cm-1 region, possibly a CH bending band, at 1487 cm-1, possibly alkyl deformation and C-O single bond absorbance, and at 1600 cm-1 is possibly aromatic from humified materials. These last three organic bands seem to be showing the spectral properties of the protected OM in the iPOM, although the aromatic character suggests organics that have been processed to some extent. The rest of the non-iPOM fractions have higher clay influence. The CTN iPOM is different from  the rest of the iPOM due to less OM and more silicate, indicating that CTN, with the diminished residue return and tillage  may be protecting less soil C as iPOM. The Micro differs from the S+C and iS+C, in part because of the higher clay absorbance at 3620 cm-1 of the S+C and iS+C. The iMicro and sMacro have more absorbance at the aliphatic CH bands between 2930-2870 cm-1, as well as absorbance at 1660, 1554, and 1453 cm-1, but low clay and silicate absorbance relative to the Whole,  Micro, S+C and iS+C. This supports the soil C data, which shows that the iMicro and sMacro are among the fractions with the higher C content. Ongoing ashing subtractions of the MidIR data aim to enhance the organic derived spectral information and shed light on the different organic functional groups in the soil and soil fractions.