@article{xiao_kuang_sauer_heitman_horton_2015, title={Bare Soil Carbon Dioxide Fluxes with Time and Depth Determined by High-Resolution Gradient-Based Measurements and Surface Chambers}, volume={79}, ISSN={["1435-0661"]}, DOI={10.2136/sssaj2015.02.0079}, abstractNote={Soil CO2 production rates and fluxes vary with time and depth. The shallow near-surface soil layer is important for myriad soil processes, yet knowledge of dynamic CO2 concentrations and fluxes in this complex zone is limited. We used a concentration gradient method (CGM) to determine CO2 production and effluxes with depth in shallow layers of a bare soil. The CO2concentration was continuously measured at 13 depths in the 0to 200-mm soil layer. For an 11-d period, 2% of the soil CO2 was produced below a depth of 175 mm, 8% was produced in the 50to 175-mm soil layer, and 90% was produced in the 0to 50-mm soil layer. Soil CO2concentration showed similar diurnal patterns with temperature in deeper soil layers and out-of-phase diurnal patterns in surface soil layers. Soil CO2 flux from most of the soil layers can be described by an exponential function of soil temperature, with temperature sensitivity (Q10) ranging from 1.40 to 2.00 (1.62 ± 0.17). The temperature-normalized CO2 fluxes are related to soil water content with a positive linear relationship in surface soil layers and a negative relationship in deep soil layers. The CO2 fluxes from CGM and chamber methods had good agreement at multiple time scales, which showed that the CGM method was able to estimate near-surface soil CO2 fluxes and production. The contrasting patterns between surface and deep layers of soil CO2 concentration and fluxes suggest the necessity of intensive CO2concentration measurements in the surface soil layer for accurate determination of soil-atmosphere CO2 flux when using the CGM. Disciplines Agriculture | Hydrology | Soil Science Comments This article is published as Xiao, Xinhua, X. Kuang, T. J. Sauer, J. L. Heitman, and R. Horton. "Bare soil carbon dioxide fluxes with time and depth determined by high-resolution gradient-based measurements and surface chambers." Soil Science Society of America Journal 79, no. 4 (2015): 1073-1083. doi: 10.2136/ sssaj2015.02.0079. Rights Works produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted. This article is available at Iowa State University Digital Repository: https://lib.dr.iastate.edu/agron_pubs/428}, number={4}, journal={SOIL SCIENCE SOCIETY OF AMERICA JOURNAL}, author={Xiao, X. and Kuang, X. and Sauer, T. J. and Heitman, J. L. and Horton, R.}, year={2015}, pages={1073–1083} }
 @article{daigh_sauer_xiao_horton_2015, title={Comparison of Models for Determining Soil-Surface Carbon Dioxide Effluxes in Different Agricultural Systems}, volume={107}, ISSN={["1435-0645"]}, DOI={10.2134/agronj14.0423}, abstractNote={Models of instantaneous soil‐surface CO2 efflux (SCEins) are critical for understanding the potential drivers of soil C loss. Several simple SCEins models have been reported in the literature. Our objective was to compare and validate selected soil temperature (Ts)‐ and water content (θv)‐based equations for modeling SCEins among a variety of cropping systems and land management practices. Soil‐surface CO2 effluxes were measured and modeled for grain‐harvested corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotations, grain‐ and stover‐harvested continuous corn systems with and without a cover crop, and reconstructed prairies with and without N fertilization on soils with subsurface drainage. Soil‐surface CO2 effluxes, Ts, and θv were measured from 2008 to 2011. Models calibrated with weekly measured SCEins, Ts, and θv throughout the growing season produced lower root mean squared error (RMSE) than models calibrated with several weeks of hourly measured data. Model selection significantly affected SCEins estimations, with models that use only Ts parameters having lower RMSE than models that use both Ts and θv. However, the model that produced the lowest RMSE during validation estimated growing‐season SCE that did not significantly differ from numerical integration of weekly measured SCEins. All models had similar residual errors with autocorrelated trends at monthly, weekly, and hourly scales. Autoregressive moving average functions were able to precisely describe the temporal errors. To accurately model SCEins and scale across time, improvement of temporal errors in Ts– and θv–based SCEins models is needed to obtain accurate and precise closure of C balances for managed and natural ecosystems.}, number={3}, journal={AGRONOMY JOURNAL}, author={Daigh, Aaron L. and Sauer, Thomas and Xiao, Xinhua and Horton, Robert}, year={2015}, pages={1077–1086} }
 @article{xiao_zhang_ren_horton_heitman_2015, title={Thermal Property Measurement Errors with Heat-Pulse Sensors Positioned near a Soil-Air Interface}, volume={79}, ISSN={["1435-0661"]}, DOI={10.2136/sssaj2014.12.0493n}, abstractNote={Heat‐pulse sensor measurements analyzed with pulsed infinite line source (PILS) theory have been widely used to measure soil properties. The PILS theory assumes that the measured soil medium is uniform and infinite. When the sensors are positioned near the soil surface, the effects of the heterogeneity associated with the soil–air interface should not be ignored. In 1999, Philip and Kluitenberg (PK99) proposed an analytical solution using an instantaneous heating model to analyze the effects of the soil–air interface on soil thermal property measurements with heat‐pulse sensors. The purpose of this study is to test the PK99 instantaneous heat source solution under controlled laboratory conditions. Soil thermal properties including volumetric heat capacity, thermal diffusivity, and thermal conductivity were measured with a commercially available dual‐needle heat‐pulse sensor buried at different depths beneath the soil surface. Three soil materials, sand, loamy sand, and sandy clay loam, were tested at both air‐dry and saturated moisture conditions. With shallow sensor burial, measured thermal properties were underestimated by up to 50%, similar to the predicted thermal properties from the PK99 analytical solution, due to the effects of the soil–air interface. Using PK99 to adjust thermal property values obtained from shallow sensors has potential to improve estimates of water content, evaporation, and other soil measurements derived from heat‐pulse sensors.}, number={3}, journal={SOIL SCIENCE SOCIETY OF AMERICA JOURNAL}, author={Xiao, X. and Zhang, X. and Ren, T. and Horton, R. and Heitman, J. L.}, year={2015}, pages={766–771} }
 @article{xiao_heitman_sauer_ren_horton_2014, title={Sensible Heat Balance Measurements of Soil Water Evaporation beneath a Maize Canopy}, volume={78}, ISSN={["1435-0661"]}, DOI={10.2136/sssaj2013.08.0371}, abstractNote={ing period. The SHB evaporation estimates varied among R, I, and IE, with cumulative totals of 4.4, 7.4, and 7.9 mm, respectively, during a 12-d drying period. Lower soil water contents from plant water uptake reduced evaporation rates at R more appreciably with time than at the other positions; I and IE provided similar evaporation patterns. The SHB evaporation estimates at R and I were compared with microlysimeter data on 8 d. Correlation between approaches was modest (r 2 = 0.61) but significant (p < 0.001) when compared separately at R and I positions. Correlation was improved (r 2 = 0.81) when evaporation estimates were combined across positions, with differ ences between SHB and microlysimeters typically within the range of values obtained from microlysimeter replicates. Overall, the results suggest good potential for using SHB and modified SHB approaches to determine soil water evaporation in a cropped field. The SHB approach allowed continuous daily estimates of evaporation, separate from evapotranspiration and without destructive sampling.}, number={2}, journal={SOIL SCIENCE SOCIETY OF AMERICA JOURNAL}, author={Xiao, X. and Heitman, J. L. and Sauer, T. J. and Ren, T. and Horton, R.}, year={2014}, pages={361–368} }
 @article{daigh_sauer_xiao_horton_2014, title={Spatial and Temporal Dynamics of Soil-Surface Carbon Dioxide Emissions in Bioenergy Corn Rotations and Reconstructed Prairies}, volume={78}, ISSN={["1435-0661"]}, DOI={10.2136/sssaj2014.02.0072}, abstractNote={The interest in bioenergy crops has raised questions as to the potential of management strategies to preserve soil C pools and soil quality. Since soil-surface CO2 effluxes are a major fate of soil C, knowledge of CO2 efflux’s spatial and temporal trends among bioenergy crops will facilitate advances in research on improving terrestrial C-cycle models as well as decision support tools for policy and land-management. Our objective was to evaluate spatial and temporal dynamics of soil-surface CO2 effluxes in bioenergy-based corn (Zea mays L.) and reconstructed prairie systems. Systems evaluated included continuous corn (harvested for grain and 50% of the corn stover) with and without a cover crop, mixed prairies (harvested for aboveground biomass) with and without N fertilization, and corn–soybean [Glycine max (L.) Merr.] rotations harvested for grain. Soil-surface CO2 effluxes, soil temperature, and soil water contents were monitored weekly from July 2008 to September 2011 and hourly during portions of 2010 and 2011. Annual soil-surface CO2 effluxes were greater in prairies than row crops and are attributed to greater plant root respiration. Soil-surface CO2 effluxes spatially varied among intra-crop management zones only for continuous corn with stover removal. However, the cover crop reduced CO2 efflux spatial variability 70% of the time as compared to stover removal without a cover crop. Spatial variability of effluxes was not explained by soil physical properties or conditions. Temperature-induced diurnal fluctuations of CO2 effluxes were not evident during apparent soil–water redistribution. Further research on the mechanisms behind this process is needed followed by incorporation of mechanisms into CO2efflux models. Disciplines Agriculture | Agronomy and Crop Sciences | Natural Resources and Conservation | Soil Science Comments This article is published as Daigh, Aaron L., Thomas J. Sauer, Xinhua Xiao, and Robert Horton. "Spatial and temporal dynamics of soil-surface carbon dioxide emissions in bioenergy corn rotations and reconstructed prairies." Soil Science Society of America Journal 78, no. 4 (2014): 1338-1350. doi: 10.2136/ sssaj2014.02.0072. Posted with permission. Rights Works produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted. This article is available at Iowa State University Digital Repository: https://lib.dr.iastate.edu/agron_pubs/400}, number={4}, journal={SOIL SCIENCE SOCIETY OF AMERICA JOURNAL}, author={Daigh, Aaron L. and Sauer, Thomas J. and Xiao, Xinhua and Horton, Robert}, year={2014}, pages={1338–1350} }