Soil water extractable organic matter under long-term dryland cropping systems on the Texas High Plains

Authors: Waldrip, Heidi; Schwartz, Robert; He, Zhongqi; Todd, Richard; Baumhardt, Roland - Louis; ZHANG, MINGCHU - University Of Alaska; Parker, David; Brauer, David; Min, Byeng Ryel

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/12/2022
Publication Date: 5/11/2022
Citation: Waldrip, H., Schwartz, R.C., He, Z., Todd, R.W., Baumhardt, R.L., Zhang, M., Parker, D.B., Brauer, D.K., Min, B. 2022. Soil water extractable organic matter under long-term dryland cropping systems on the Texas High Plains. Soil Science Society of America Journal. 1-15. https://doi.org/10.1002/saj2.20432.
DOI: https://doi.org/10.1002/saj2.20432

Interpretive Summary: Healthy and productive soils generally contain large amounts of organic matter from the breakdown of plants, microorganisms and other compounds. A small part of this organic matter can be dissolved in water, which makes this water dissolved organic matter important to reactions in the soil. The Texas High Plains is a region known for its hot summers and limited rainfall. Over the years, different dryland cropping systems have been developed that capture and retain rainfall. The effects of these dryland systems on water soluble organic matter in soils are not well understood. Scientists from ARS (Bushland, Texas) and the University of Alaska-Fairbanks conducted a study to investigate the long-term effects of different winter wheat cropping systems on the amount and stability of labile water-soluble organic matter in soils and compared it to native grassland. Using soils taken from a long-term study (1941 to 2013), we looked at properties of water-soluble carbon in soils collected in 1977 and 2013. Results showed that water soluble carbon content decreased over time by 13 to 60% with all cropping systems. The small amount of water-soluble carbon in cropped soils in 2013 contained more carbon compounds that are difficult to degrade. In contrast, the water-soluble carbon in cropped soils from 1977 and native grassland contained more humic substances than cropped soils in 2013. These results indicated that current dryland systems may be insufficient to maintain this important water-soluble form of soil organic matter. Better systems need to be developed that improve levels of water-soluble soil carbon but these maybe difficult to develop because of the regional high temperatures and limited soil water.

Technical Abstract: Water extractable organic matter (WEOM) is critical for soil fertility. In the semi-arid Texas High Plains, dryland cropping systems are essential because of limited precipitation and irrigation water. This study examined the legacy effects of long-term dryland winter wheat (Triticum aestivum) cropping systems [continuous wheat (CW) and wheat-fallow rotations (WF)] and tillage methods [disk plow (DP), stubble-mulch tillage (ST), and delayed stubble-mulch tillage (DST)] on WEOM characteristics, compared to native grassland. Concentrations of dissolved organic carbon (DOC), a measure of WEOM content, were depleted regardless of cropping system, with reductions up to 13% in 1977 and 60% in 2013, compared to native grassland. Other parameters of WEOM changed over time, where cropped soils from 2013 had proportionally more aromatic and phenolic compounds, and lignin-like organic matter (attributed to crop residue input over time). In contrast, WEOM in cropped soils from 1977 and native grassland (2013) contained more functional groups and humic substances than cropped soils in 2013. Typical dryland systems may be insufficient to maintain/replenish WEOM after grassland conversion. More vigorous conservation systems should be adopted to improve WEOM stocks, enhancing long-term sustainability. This may be difficult to achieve in regions where high temperature and limited rainfall/irrigation water limit biomass inputs and promote C mineralization.