Soil water extractable organic matter under long‐term dryland cropping systems on the Texas high plains

Water extractable organic matter (WEOM) is critical for soil fertility because of its role as a readily available C substrate for microbial metabolism. In the semi-arid Texas High Plains, dryland cropping systems are common because of limited precipitation and irrigation water. Little is known about the long-term effects of dryland cropping on soil WEOM content and composition, and how these differ from native grassland. The objective of this study was to use soils collected near Amarillo, TX, in 1977 and 2013 to examine the long-term (72 yr total cropping length) effects of dryland cropping systems [continuous wheat and wheat-fallow rotations] and tillage methods (disk plow, stubble-mulch tillage, and delayed stubble-mulch tillage) on WEOM characteristics, compared with native grassland. In cropped plots, water-extractable organic carbon (WEOC) concentrations, a measure of WEOM content, were depleted regardless of tillage or crop, with reductions up to 13% in 1977 and 60% in 2013, compared with native grassland in 2013. Other parameters of WEOM changed over time, where cropped soils from 2013 had proportionally more WEOM in aromatic, phenolic, and lignin-like forms, attributed to recently incorporated crop residues. In contrast, WEOM in cropped soils from 1977 and native grassland (2013) contained more functional groups (e.g., aliphatic, aromatic, polysaccharide, phenol, etc.) and humic-like substances than cropped soils in 2013. This suggests that long-term dryland cropping led to depletion of stable, humic-like WEOM. Enhancement of WEOM stocks and long-term sustainability in the Texas High Plains may require more vigorous conservation systems than currently used.