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SOIL & WATER MANAGEMENT & CONSERVATION

Soil Water Hysteresis in Water-Stable Microaggregates as Affected by Organic Matter

^a Institute for a Secure and Sustainable Environment, Center for Environmental Biotechnology, Dep. of Earth and Planetary Sciences, Univ. of Tennessee, Knoxville, TN 37996
^b Darling Marine Center, Univ. of Maine, Walpole, ME 04573
^c Biosciences Division, Argonne National Lab., Argonne, IL 60439

^* Corresponding author (jzhuang{at}utk.edu).

Interaction of water and organic matter (OM) in pores affects the capacity for long-term C sequestration and water retention in soils. This study investigated the effects of the amount and distribution of OM in microstructures on water retention in soil microaggregates. The focus of the study was on the hysteretic soil water characteristic (i.e., the difference between the main drying and wetting curves), which has been largely ignored in previous studies on water–OM relations. The main drying and wetting branches of the water retention curve were measured using a water activity meter on water-stable microaggregates (53–250 µm), which were collected from surface soils (0–15 cm) subject to different management practices: tallgrass prairie restoration on a Mollisol, and tillage with and without N fertilization on an Alfisol. The results show that the presence of OM in <5-µm-diameter pores increases water retention in microaggregates, while management practices that either increased or decreased the abundance of OM-filled pore volume in the microaggregates promoted hysteresis of water retention characteristics due to changes in soil pore structure. Comparison of the drying and wetting curves between intact and combusted (OM removed) microaggregates indicated a strong retention of water by pore-filling OM. This retention suggests that the high water saturation in microaggregates from soils under management practices that increased OM-filled porosity, which also offered physical protection of the OM, may be a positive feedback mechanism that retards OM decomposition in micropores. The water retention data measured on intact and combusted microaggregates were consistent with results from ultra-small-angle x-ray scattering, and indicated that management practices have a great potential for facilitating the synergistic retention of water and organic C in soils.

Abbreviations: ANCOVA, analysis of covariance • OC, organic carbon • OM, organic matter • SSA, specific surface area