Five decades of modeling supporting the systems ecology paradigm

Authors: PARTON, WILLIAM - COLORADO STATE UNIVERSITY; Del Grosso, Stephen - Steve; CAMPBELL, ELEANOR - COLORADO STATE UNIVERSITY; HARTMAN, MELANNIE - COLORADO STATE UNIVERSITY; HOBBS, N - COLORADO STATE UNIVERSITY; MOORE, JOHN - COLORADO STATE UNIVERSITY; SWIFT, DAVID - COLORADO STATE UNIVERSITY; SCHIMEL, DAVID - COLORADO STATE UNIVERSITY; OJIMA, DENNIS - COLORADO STATE UNIVERSITY; COUGHENOUR, MIKE - COLORADO STATE UNIVERSITY; BOONE, RANDALL - COLORADO STATE UNIVERSITY; PAUSTIAN, KEITH - COLORADO STATE UNIVERSITY; HUNT, H - COLORADO STATE UNIVERSITY; WOODMANSEE, R - COLORADO STATE UNIVERSITY

Submitted to: Cambridge University Press
Publication Type: Book / Chapter
Publication Acceptance Date: 1/2/2020
Publication Date: 2/26/2021
Citation: Parton, W.J., Del Grosso, S.J., Campbell, E.E., Hartman, M.D., Hobbs, N.T., Moore, J.C., Swift, D.M., Schimel, D.S., Ojima, D.S., Coughenour, M., Boone, R.B., Paustian, K., Hunt, H.W., Woodmansee, R.G. 2021. Five decades of modeling supporting the systems ecology paradigm. In: Woodmansee, R.G., Moore, J.C., Ojima, D.S., Richards, L., editors. Natural Resource Management Reimagined: Using the Systems Ecology Paradigm. Cambridge: Cambridge University Press. p. 90-130.

Interpretive Summary: Nitrogen (N) fertilizer additions are typically necessary to maintain high crop yields but are also largely responsible for agricultural soils being the primary source of anthropogenic nitrous oxide (N2O) emissions. N2O is a greenhouse gas and also is the primary anthropogenic stratospheric ozone-depleting substance. Management practices such as N fertilizer type and amount, tillage intensity, and crop rotation have various effects on N2O emissions, nutrient losses, soil carbon stock changes and crop yields. As interest in mitigating the negative environmental impacts of crop and livestock productions systems increases, it is important to better quantify how environmental factors interact with management decisions to control agronomic and environmental outcomes. Models of varying complexity have been developed to represent processes such as denitrification that transform N and contribute to N2O emissions. The DayCent ecosystem model is of intermediate complexity and simulates the complete plant soil system. DayCent represents common management strategies including fertilizer amendments, irrigation, tillage, crop rotation, residue management, etc. and has been widely used to estimate N2O emissions under conventional and alternative management scenarios. In this paper, we focus on how processes responsible for N2O emissions are represented in DayCent, compare model outputs with field observations, and describe recent improvements in model algorithms. Specific improvements include accounting for how soil volume changes in response to wetting and representing freeze/thaw dynamics which affect soil oxygen status and denitrification rates. Including a pH impact also improved model performance. In sum, more complete implementation of current understanding of process controls should improve model performance, although increased complexity does not always imply better results so it is crucial to compare model outputs with high quality field observations.

Technical Abstract: Nitrogen (N) fertilizer additions are typically necessary to maintain high crop yields but are also largely responsible for agricultural soils being the primary source of anthropogenic nitrous oxide (N2O) emissions. N2O is a greenhouse gas and also is the primary anthropogenic stratospheric ozone-depleting substance. Management practices such as N fertilizer type and amount, tillage intensity, and crop rotation have various effects on N2O emissions, nutrient losses, soil carbon stock changes and crop yields. As interest in mitigating the negative environmental impacts of crop and livestock productions systems increases, it is important to better quantify how environmental factors interact with management decisions to control agronomic and environmental outcomes. Because it is not feasible to directly measure N loss vectors and others factors of concern at the farm level models of varying complexity have been developed to represent processes such as denitrification that transform N and contribute to N2O emissions. The DayCent ecosystem model is of intermediate complexity and simulates the complete plant soil system. DayCent represents common management strategies including fertilizer amendments, irrigation, tillage, crop rotation, residue management, etc. and has been widely used to estimate N2O emissions under conventional and alternative management scenarios. In this paper, we focus on how processes responsible for N2O emissions are represented in DayCent, compare model outputs with field observations, and describe recent improvements in model algorithms. Specific improvements include accounting for how soil volume changes in response to wetting and representing freeze/thaw dynamics which affect soil O2 status and denitrification rates. Including a pH impact also improved model performance. In sum, more complete implementation of current understanding of process controls should improve model performance, although increased complexity does not always imply better results so it is crucial to compare model outputs with high quality field observations.