Research Publications

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PRESENTATIONS

Conservation Chat: Implementing Iowa's Nutrient Reduction Strategy (Helmers) Video (2016).

Presentation: Nitrogen (Castellano, Helmers) Video.
 

PUBLICATIONS/HANDOUTS

 

Iowa Nutrient Reduction Strategy Annual Progress Report 2017-2018 (March 2019).

Iowa Nutrient Research Center Research Summary Handout (March 2016). 

Iowa Nutrient Research Center Accomplishments, 2013-2018 (November 2018).

Best Management Practices for Fall Grazing Cover Crops. Infographic - Iowa Beef Center. (November 2018). 

Iowa Prairie Seed Calculator. Tallgrass Prairie Center – University of Northern Iowa.

Linking Nutrient Reduction Practices with Biomass Energy: Quantifying Thermal Energy Demand and Supply Capacity for Representative Farms in Eastern Iowa. Tallgrass Prairie Center – University of Northern Iowa.

Reducing Nutrient Loss: Science Shows What Works. Iowa State University Extension and Outreach publication (SP435A/ rev. February 2017). 
 

PEER-REVIEWED JOURNAL ARTICLES

Land Use

Acharya, J., M. G. Bakker, T. B. Moorman, T. C. Kaspar, A. W. Lenssen, and A. E. Robertson. 2017. Time interval between cover crop termination and planting influences corn seedling disease, plant growth, and yield. Plant Disease 101(4): 591-600.

Acharya, J., T. Kasper, T. Moorman, A. W. Lenssen, and A. R. Robertson. 2016. Effect of cereal rye winter cover crop (Secale cereal) termination on corn seedling diseases. Phytopathology 106(1): S1-S1.

Basche, A. D., T. C. Kaspar, S. V. Archontoulis, D. B. Jaynes, T. J. Sauer, T. B. Parkin, and F. E. Miguez. 2016. Soil water improvements with the long-term use of a winter rye cover crop. Agricultural Water Management 172:40-50.           

Gassman, P. W., A. M. Valcu-Lisman, C. L. Kling, S. K. Mickelson, Y. Panagopoulos, R. Cibin, I. Chaubey, C. F. Wolter, and K. E. Schilling. 2017. Assessment of bioenergy cropping scenarios for the Boone River Watershed in North Central Iowa, United States. Journal of American Water Resources Association 53(6):1336-1354.

Jones, C.S., A. Seeman, P.M. Kyveryga, K.E. Schilling, A. Kiel, K.S. Chan, and C.F. Wolter. 2016. Crop rotation and Raccoon River nitrate. Journal of Soil and Water Conservation 71(3):206-219.

Lee, D., J. G. Arbuckle, Z. Y. Zhu, and L. Nowatzke. 2018. Conditional causal mediation analysis of factors associated with cover crop adoption in Iowa, USA. Water Resources Research 54(11):9566-9584.

Martinez-Feria, R. A., R. Dietzel, M. Liebman, M. J. Helmers, and S. V. Archontoulis. 2016. Rye cover crop effects on maize: A system-level analysis. Field Crops Research 196:145-159.

Roley, S. S., J. L. Tank, J. C. Tyndall, and J. D. Witter. 2016. How cost-effective are cover crops, wetlands, and two-stage ditches for nitrogen removal in the Mississippi River Basin? Water Resources and Economics 15:43-56.

Schilling, K., 2017. How much do golf courses contribute to Gulf of Mexico hypoxia. Golf Course Manag 17:76-81.

Schilling, K.E., and M.T. Streeter. 2018. Groundwater nutrient concentrations and mass loading rates at Iowa golf courses. JAWRA/ Journal of the American Water Resources Association 54(1):211-224.

Schulte, L. A., J. Niemi, M. J. Helmers, M. Liebman, J. G. Arbuckle, D. E. James, R. K. Kolka, M. E. O'Neal, M. D. Tomer, J. C. Tyndall, H. Asbjornsen, P. Drobney, J. Neal, G. Van Ryswyk, and C. Witte. 2017. Prairie strips improve biodiversity and the delivery of multiple ecosystem services from corn-soybean croplands. Proceedings of the National Academy of Sciences 114(42):11247-11252.

Streeter, M.T., and K.E. Schilling. 2018. Effects of golf course management on subsurface soil properties in Iowa. Soil 4(2):93-100.

Tomer, M.D., K.E. Schilling, and K.J. Cole. 2019. Nitrate on a slow decline: Watershed water quality response during two decades of tallgrass prairie ecosystem reconstruction in Iowa. Journal of Environmental Quality 48:579–585. 

Edge of Field

Amado, A.A., M. Politano, K.E. Schilling, and L. Weber. 2016. Investigating hydrologic connectivity of a drained Prairie Pothole Region wetland complex using a fully integrated, physically-based model. Wetlands.

Beck, W., T. Isenhart, P. Moore, K. Schilling, R. Schultz, and M. Tomer. 2018. Streambank alluvial unit contributions to suspended sediment and total phosphorus loads, Walnut Creek, Iowa, USA. Water 10(2):111.

Beck, W. J., P. L. Moore, K. E. Schilling, C. F. Wolter, T. M. Isenhart, K. J. Cole, and M. D. Tomer. 2019. Changes in lateral floodplain connectivity accompanying stream channel evolution: Implications for sediment and nutrient budgets. Science of the Total Environment 660:1015-1028.

Davis, M. P., T. A. Groh, T. B. Parkin, R. J. Williams, T. M. Isenhart, and K. S. Hofmockel. 2018. Portable automation of static chamber sample collection for quantifying soil gas flux. Journal of Environmental Quality 47(2):270-275.

Davis, M. P., T. A. Groh, D. B. Jaynes, T. B. Parkin, and T. M. Isenhart. 2019. Nitrous oxide emissions from saturated riparian buffers: Are we trading a water quality problem for an air quality problem? Journal of Environmental Quality 48(2):261-269.

Drake, C.W., C.S. Jones, K.E. Schilling, A.A. Amado, and L. J. Weber. 2017. Estimating nitrate retention in a large constructed wetland using high-frequency, continuous monitoring and hydrologic modeling. ASABE Annual International Meeting/American Society of Agricultural and Biological Engineers.

Drake, C.W., C.S. Jones, K.E. Schilling, A.A. Amado, and L.J. Weber. 2018. Estimating nitrate-nitrogen retention in a large constructed wetland using high-frequency, continuous monitoring and hydrologic modeling. Ecological Engineering 117:69-83.

Groh, T. A., M. P. Davis, T. M. Isenhart, D. B. Jaynes, and T. B. Parkin. 2019. In situ denitrification in saturated riparian buffers. Journal of Environmental Quality 48(2):376-384.

Hongthanat, N., J. L. Kovar, M. L. Thompson, J. R. Russell, and T. M. Isenhart. 2016. Phosphorus source-sink relationships of stream sediments in the Rathbun Lake watershed in southern Iowa, USA. Environmental Monitoring and Assessment 188(8).

Hoover, N. L., A. Bhandari, M. L. Soupir, and T. B. Moorman. 2016. Woodchip denitrification bioreactors: Impact of temperature and hydraulic retention time on nitrate removal. Journal of Environmental Quality 45(3):803-812.

Hoover, N. L., M. L. Soupir, R. D. Vandepol, T. R. Goode, and J. Y. Law. 2017. Pilot-scale denitrification bioreactors for replicated field research. Applied Engineering in Agriculture 33(1):83-90.

Jaynes, D. B., and T. M. Isenhart. 2019. Performance of saturated riparian buffers in Iowa, USA. Journal of Environmental Quality 48(2): 289-296.

Jones, C.S., and K.J. Kult. 2016. Use alkalinity monitoring to optimize bioreactor performance. Journal of Environmental Quality 45(3):855-865.

Law, J. Y., M. L. Soupir, D. R. Raman, and T. B. Moorman. 2018. Exploring multiple operating scenarios to identify low-cost, high nitrate removal strategies for electrically-stimulated woodchip bioreactors. Ecological Engineering 120:146-153.     

Law, J. Y., M. L. Soupir, D. R. Raman, T. B. Moorman, and S. K. Ong. 2018. Electrical stimulation for enhanced denitrification in woodchip bioreactors: Opportunities and challenges. Ecological Engineering 110:38-47.

Martin, E.A., M.P. Davis, T.B. Moorman, T.M. Isenhart, and M.L. Soupir (2019). Impact of hydraulic residence time on nitrate removal in pilot-scale woodchip bioreactors. Journal of Environmental Management 237:424-432.

Moustakidis, I.V., K.E. Schilling, and L.J. Weber. 2019. Soil total phosphorus deposition and variability patterns across the floodplains of an Iowa river. Catena 174:84-94.

Schilling, K.E., K. Kult, K. Wilke, M. Streeter, and J. Vogelgesang. 2017. Nitrate reduction in a reconstructed floodplain oxbow fed by tile drainage. Ecological Engineering 102:98-107.

Schilling, K.E., P.J. Jacobson, and C.F. Wolter. 2018. Using riparian Zone scaling to optimize buffer placement and effectiveness. Landscape Ecology 33(1):141-156.

Schilling, K.E., K. Kult, A. Seemon, K. Wilke, and C.S. Jones. 2018. Nitrate-N load reduction measured in a central Iowa restored oxbow. Ecological engineering, 124:19-22.

Schilling, K.E., M.T. Streeter, M.S. Clair, and J. Meissen. 2018. Subsurface nutrient processing capacity in agricultural roadside ditches. Science of The Total Environment 637:470-479.

Schilling, K.E., P.J. Jacobson, M.T. Streeter, and C.S. Jones. 2018. Groundwater hydrology and quality in drained wetlands of the Des Moines Lobe in Iowa. Wetlands, 38(2):247-259.

Schilling, K.E., B.J. Haines, C.S. Jones, and M. St. Clair. 2018. Effectiveness of a newly reconstructed floodplain oxbow to reduce NO3-N loads from a spring flood. Journal of Environmental Management 215:385-393.

Schilling, K.E., and S. Dinsmore. 2018. Monitoring the wildlife, hydrology and water quality of drained wetlands of the Des Moines Lobe, Northern Iowa: introduction to special feature. Wetlands 38(2):207-210.

Schilling, K.E., S.R. Then, and C.D. Ikenberry. 2019. Water balance modeling of temporary ponding in a drained prairie pothole wetland. Environmental Modeling & Assessment 24(1):37-48.

Streeter, M.T., K.E. Schilling, M. St. Clair, and Z. Demanett. 2019. Soil sedimentation and quality within the roadside ditches of an agricultural watershed. Science of The Total Environment 657:1432-1440.

Soupir, M. L., N. L. Hoover, T. B. Moorman, J. Y. Law, and B. L. Bearson. 2018. Impact of temperature and hydraulic retention time on pathogen and nutrient removal in woodchip bioreactors. Ecological Engineering 112:153-157.

N and P Management

Brandes, E., G.S. McNunn, L.A. Schulte, D.J. Muth, A. VanLoocke, and E.A. Heaton. 2017. Targeted subfield switchgrass integration could improve the farm economy, water quality, and bioenergy feedstock production. Global Change Biology Bioenergy 10:199-212.

Brendel, C. E., M. L. Soupir, L. A. M. Long, M. J. Helmers, C. D. Ikenberry, and A. L. Kaleita. 2019. Catchment-scale phosphorus export through surface and drainage pathways. Journal of Environmental Quality 48(1):117-126.

Cao, P. Y., C. Q. Lu, and Z. Yu. 2018. Historical nitrogen fertilizer use in agricultural ecosystems of the contiguous United States during 1850-2015: application rate, timing, and fertilizer types. Earth System Science Data 10(2):969-984.

Chen, X. W., R. M. Cruse, S. Niu, and X. Y. Zhang. 2019. Effect of loading time on soil structural failure. Soil & Tillage Research 186:87-93.

Hall, S. J., L. Reyes, W. Huang, and P. M. Homyak. 2018. Wet spots as hotspots: Moisture responses of nitric and nitrous oxide emissions from poorly drained agricultural soils. Journal of Geophysical Research-Biogeosciences 123(12):3589-3602.

Jones, C.S., C.W. Drake, C.E. Hruby, K.E. Schilling, and C.F. Wolter. 2018. Livestock manure driving stream nitrate. Ambio:1-11.

Jones, C.S., K.E. Schilling, and A. Seeman. 2019. Relating carbon and nitrogen transport from constructed farm drainage. Agricultural Water Management 213:12-23.

Kalcic, M., W. Crumpton, X. Liu, J. D'Ambrosio, A. Ward, and J. Witter. 2018. Assessment of beyond-the-field nutrient management practices for agricultural crop systems with subsurface drainage. Journal of Soil and Water Conservation 73(1):62-74.

Martinez-Feria, R. A., M. J. Castellano, R. N. Dietzel, M. J. Helmers, M. Liebman, I. Huber, and S. V. Archontoulis. 2018. Linking crop- and soil-based approaches to evaluate system nitrogen-use efficiency and tradeoffs. Agriculture Ecosystems & Environment 256:131-143.

Osterholz, W. R., M. Liebman, and M. J. Castellano. 2018. Can soil nitrogen dynamics explain the yield benefit of crop diversification? Field Crops Research 219:33-42.

Poffenbarger, H. J., J. E. Sawyer, D. W. Barker, D. C. Olk, J. Six, and M. J. Castellano. 2018. Legacy effects of long-term nitrogen fertilizer application on the fate of nitrogen fertilizer inputs in continuous maize. Agriculture Ecosystems & Environment 265:544-555.

Rizzo, G., J. I. R. Edreira, S. V. Archontoulis, H. S. Yang, and P. Grassini. 2018. Do shallow water tables contribute to high and stable maize yields in the US Corn Belt? Global Food Security-Agriculture Policy Economics and Environment 18:27-34.

Schilling, K. E., C. F. Wolter, T. M. Isenhart, and R. C. Schultz. 2015. Tile drainage density reduces groundwater travel times and compromises riparian buffer effectiveness. Journal of Environmental Quality 44(6):1754-1763.

Schilling, K.E., S.W. Kim, and C.S. Jones. 2017. Use of water quality surrogates to estimate total phosphorus concentrations in Iowa rivers. Journal of Hydrology: Regional Studies 12:111-121.

Schilling, K.E., S.W. Kim, C.S. Jones, and C.F. Wolter. 2017. Orthophosphorus contributions to total phosphorus concentrations and loads in Iowa agricultural watersheds. Journal of Environmental Quality, 46(4):828-835.

Schilling, K. E., M. T. Streeter, T. M. Isenhart, W. J. Beck, M. D. Tomer, K. J. Cole, and J. L. Kovar. 2018. Distribution and mass of groundwater orthophosphorus in an agricultural watershed. Science of the Total Environment 625:1330-1340.

Sharpley, A., M. J. Helmers, P. Kleinman, K. King, A. Leytem, and N. Nelson. 2019. Managing crop nutrients to achieve water quality goals. Journal of Soil and Water Conservation 74(5): 91A-101A.

Zhu, Q., M. J. Castellano, and G. S. Yang. 2018. Coupling soil water processes and the nitrogen cycle across spatial scales: Potentials, bottlenecks and solutions. Earth-Science Reviews 187:248-258.

Multi-Objective

Aller, D. M., S. V. Archontoulis, W. D. Zhang, W. Sawadgo, D. A. Laird, and K. Moore. 2018. Long term biochar effects on corn yield, soil quality and profitability in the US Midwest. Field Crops Research 227:30-40.

Amado, A.A., K.E. Schilling, C.S. Jones, N. Thomas, and L.J. Weber. 2017. Estimation of tile drainage contribution to streamflow and nutrient loads at the watershed scale based on continuously monitored data. Environmental Monitoring and Assessment 189(9):426.

Arenas, A., K.E. Schilling, J. Niemeier, and L. Weber. 2018. Evaluating the timing and interdependence of hydrologic processes at the watershed scale based on continuously monitored data. Water 10(3):261.

Ayers, J.R., G. Villarini, C. Jones, and K. Schilling. 2019. Changes in monthly baseflow across the US Midwest. Hydrological Processes 33(5):748-758.

Bates, H., and J. G. Arbuckle. 2017. Understanding predictors of nutrient management practice diversity in midwestern agriculture. Journal of Extension 55(6).

Beck, W.J., P.L. Moore, K.E. Schilling, C.F. Wolter, T.M. Isenhart, K.J. Cole, and M.D. Tomer. 2019. Changes in lateral floodplain connectivity accompanying stream channel evolution: Implications for sediment and nutrient budgets. Science of The Total Environment 660:1015-1028.

Enloe, S. K., L. A. Schulte, and J. C. Tyndall. 2017. Public-private partnerships working beyond scale challenges toward water quality improvements from private lands. Environmental Management 60(4):574-587.

Gardezi, M., and J. G. Arbuckle. 2019. Spatially representing vulnerability to extreme rain events using midwestern farmers' objective and perceived attributes of adaptive capacity. Risk Analysis 39(1):17-34.

Gassman, P.W., A.M. Valcu‐Lisman, C.L. Kling, S.K. Mickelson, Y. Panagopoulos, R. Cibin, I. Chaubey, C.F. Wolter,, and K.E. Schilling.   2017. Assessment of bioenergy cropping scenarios for the Boone River watershed in north central Iowa, United States. JAWRA/Journal of the American Water Resources Association 53(6):1336-1354.

Hall, S.J., L. Reyes, W. Huang, and P.M. Homyak. 2019. Wet spots as hotspots: Moisture responses of nitric and nitrous oxide emissions from poorly drained agricultural soils. Journal of Geophysical Research: Biogeosciences 123(12):3589-3602.

Hansen, E., K.S. Chan, C.S. Jones, and K. Schilling. 2016. Assessing the relative importance of nitrogen-retention processes in a large reservoir using time-series modeling. Journal of agricultural, biological, and environmental statistics, 21(1):152-169.

Hay, C. H., and M. J. Helmers. 2017. Evaluating drainage water recycling benefits for crop production in Iowa. World Environmental and Water Resources Congress/American Society of Civil Engineers.

Iqbal, J., M. Necpalova, S. V. Archontoulis, R. P. Anex, M. Bourguignon, D. Herzmann, D. C. Mitchell, J. E. Sawyer, Q. Zhu, and M. J. Castellano. 2018. Extreme weather-year sequences have nonadditive effects on environmental nitrogen losses. Global Change Biology 24(1):E303-E317.

Jones, C., S.W. Kim, and K. Schilling. 2017. Use of continuous monitoring to assess stream nitrate flux and transformation patterns. Environmental monitoring and assessment, 189(1):35.

Jones, C.S., B. Wang, K.E. Schilling, and K.S. Chan. 2017. Nitrate transport and supply limitations quantified using high-frequency stream monitoring and turning point analysis. Journal of Hydrology 549:581-591.

Jones, C.S., S.W. Kim, T.F. Wilton, K.E. Schilling, and C.A. Davis. 2018. Nitrate uptake in an agricultural stream estimated from high-frequency, in-situ sensors. Environmental Monitoring and Assessment 190(4):226.

Jones, C.S., C.A. Davis, C.W. Drake, K.E. Schilling, S.H. Debionne, D.W. Gilles, I. Demir, and L.J. Weber. 2018. Iowa statewide stream nitrate load calculated using in situ sensor network. JAWRA/Journal of the American Water Resources Association, 54(2):471-486.

Jones, C.S., J.K. Nielsen, K.E. Schilling, and L.J. Weber. 2018. Iowa stream nitrate and the Gulf of Mexico. PloS one, 13(4): p.e0195930.

Jones, C.S., K.E. Schilling, I.M. Simpson, and C.F. Wolter. 2018. Iowa stream nitrate, discharge and precipitation: 30-year perspective. Environmental Management 62(4):709-72

Keiser, D. A., C. L. Kling, and J. S. Shapiro. 2019. The low but uncertain measured benefits of US water quality policy. Proceedings of the National Academy of Sciences of the United States of America 116(12): 5262-5269.

Kling, C. L., I. Chaubey, R. Cibin, P. W. Gassman, and Y. Panagopoulos (2017). "Policy implications from multi-scale watershed models of biofuel crop adoption across the corn belt. Journal of the American Water Resources Association 53(6):1313-1322.

Kling, C. L., and D. J. Phaneuf. 2018. How are scope and adding up relevant for benefits transfer? Environmental & Resource Economics 69(3):483-502.

Mahal, N. K., M. J. Castellano, and F. E. Miguez. 2018. Conservation agriculture practices increase potentially mineralizable nitrogen: A meta-analysis. Soil Science Society of America Journal 82(5):1270-1278.

McLellan, E.L., K.E. Schilling, C.F. Wolter, M.D. Tomer, S.A. Porter, J.A. Magner, D.R. Smith, and L.S. Prokopy. 2018. Right practice, right place: A conservation planning toolbox for meeting water quality goals in the Corn Belt. Journal of Soil and Water Conservation 73(2): 29A-34A.

Puntel, L. A., J. E. Sawyer, D. W. Barker, P. J. Thorburn, M. J. Castellano, K. J. Moore, A. VanLoocke, E. A. Heaton, and S. V. Archontoulis. 2018. A systems modeling approach to forecast corn economic optimum nitrogen rate. Frontiers in Plant Science 9.

Rabotyagov, S. S., A. M. Valcu-Lisman, and C. L. Kling. 2016. Resilient provision of ecosystem services from agricultural landscapes: Trade-offs involving means and variances of water quality improvements. American Journal of Agricultural Economics 98(5):1295-1313.

Roesch-McNally, G. E., A. D. Basche, J. G. Arbuckle, J. C. Tyndall, F. E. Miguez, T. Bowman, and R. Clay. 2018. The trouble with cover crops: Farmers' experiences with overcoming barriers to adoption. Renewable Agriculture and Food Systems 33(4):322-333.

Rundhaug, T.J., G.R. Geimer., C.W. Drake, A.A. Amado, A.A. Bradley, C.F. Wolter, and L.J. Weber. 2018. Agricultural conservation practices in Iowa watersheds: Comparing actual implementation with practice potential. Environmental Monitoring and Assessment 190(11):659.

Schilling, K.E., C.S. Jones, C.F. Wolter, X. Liang, Y.K. Zhang, A. Seeman, T. Isenhart, D. Schnoebelen, and M. Skopec. 2017. Variability of nitrate-nitrogen load estimation results will make quantifying load reduction strategies difficult in Iowa. Journal of Soil and Water Conservation 72(4):317-325.

Schilling, K.E., M.T. Streeter, T.M. Isenhart, W.J. Beck, M.D. Tomer, K.J. Cole, and J.L. Kovar. 2018. Distribution and mass of groundwater orthophosphorus in an agricultural watershed. Science of the Total Environment, 625:1330-1340.

Schilling, K.E., C.S. Jones, R.J. Clark, R.D. Libra, X. Liang, and Y.K. Zhang. 2019. Contrasting NO 3-N concentration patterns at two karst springs in Iowa (USA): Insights on aquifer nitrogen storage and delivery. Hydrogeology Journal:1-12.

Schilling, K.E., P.W. Gassman, A. Arenas-Amado, C.S. Jones, and J. Arnold. 2019. Quantifying the contribution of tile drainage to basin-scale water yield using analytical and numerical models. Science of The Total Environment 657:297-309.

Streeter, M.T., K.E. Schilling, and C.F. Wolter. 2018. Sediment delivery and nutrient export as indicators of soil sustainability in an Iowa agricultural watershed. Journal of Soils and Sediments 18(4):1756-1766.

Thomas, N.W., A.A. Arenas, K.E. Schilling, and L.J. Weber. 2016. Numerical investigation of the spatial scale and time dependency of tile drainage contribution to stream flow. Journal of Hydrology, 538:651-666.

Upadhyay, P., L. O. S. Pruski, A. L. Kaleita, and M. L. Soupir. 2018. Evaluation of AnnAGNPS for simulating the inundation of drained and farmed potholes in the Prairie Pothole Region of Iowa. Agricultural Water Management 204:38-46.

Upadhyay, P., L. O. S. Pruski, A. L. Kaleita, and M. L. Soupir. 2019. Effects of land management on inundation of prairie pothole wetlands in the Des Moines Lobe using AnnAGNPS. Agricultural Water Management 213:947-956.

Vanloocke, A., T. E. Twine, C. J. Kucharik, and C. J. Bernacchi. 2017. Assessing the potential to decrease the Gulf of Mexico hypoxic zone with Midwest US perennial cellulosic feedstock production. Global Change Biology Bioenergy 9(5):858-875.

Villarini, G., K.E. Schilling, and C.S. Jones. 2016. Assessing the relation of USDA conservation expenditures to suspended sediment reductions in an Iowa watershed. Journal of Environmental Management 180:375-383.

Villarini, G., C.S. Jones, and K.E. Schilling. 2016. Soybean area and baseflow driving nitrate in Iowa’s Raccoon river. Journal of Environmental Quality 45(6):1949-1959.

Wilson, C.G., K.E. Schilling, et al. 2018. The intensively managed landscape critical zone observatory: A scientific testbed for understanding critical zone processes in agroecosystems. Vadose Zone Journal 17(1).