Simulated Soil Respiration is Sensitive to Soil Hydraulic Properties from Intact vs. Repacked Cores

Andrew Townsend, Arjun Chakrawal, Odeta Qafoku, Mark L Rockhold, Tom Wietsma, John R Bargar, Emily Graham

Soil hydraulic properties, such as water retention and hydrodynamics, play a pivotal role in regulating belowground carbon (C) storage by influencing microbial activity and nutrient availability. However, empirical measurements of these properties are labor-intensive and often fail to replicate field conditions in laboratory settings. Standardizing and increasing the throughput of hydraulic property measurements is essential to improve model predictions of ecosystem-scale soil C dynamics. To address this need, we investigated the hydraulic properties of intact field-collected soil cores and laboratory-repacked soil columns to assess how soil structure impacts hydraulic measurements and subsequent model predictions of soil respiration. Using four distinct soil textures (sandy loam, silt loam, loam, and silt clay) sampled as part of the Molecular Observation Network (MONet), we compared water retention curves derived from both methods to publicly available HiHydroSoil v2.0 database values. We then applied the Millennial model to simulate soil respiration rates and total C stocks based on measured hydraulic properties. Our experimental results show the largest differences in hydraulic properties between intact and repacked approach for the silty soil texture, and the smallest differences in the sandy soil texture. The model predicted increased respiration in repacked sandy loam and loam cores (~6% and ~9%), while repacked silt loam and silty clay showed ~17% lower respiration vs. intact cores. Respiration in sandy loam and loam soils showed higher respiration in repacked associated with lower SOC stocks compared to intact. However, silt loam and silty clay in repacked cores had ~12–19% higher total C stocks as compared to intact, consistent with SOC protection by clay. These uncertainties in model predictions also varied with whether intact or repacked hydraulic properties were used, emphasizing the need for reliable measurements that account for environmental and anthropogenic impacts on soil structure, with implications for both empirical and modeling efforts.