Ecological Modelling 222 (2011) 1913–1921 Contents lists available at ScienceDirect Ecological Modelling journal homepage: www.elsevier.com/locate/ecolmodel Integrating soil carbon cycling with that of nitrogen and phosphorus in the watershed model SWAT: Theory and model testing Armen R. Kemanian a,∗ , Stefan Julich b,1 , Valipuram S. Manoranjan c , Jeffrey R. Arnold d a Department of Crop and Soil Sciences, The Pennsylvania State University, 116 ASI Building, University Park, PA 16802-3504, United States b Institute of Landscape Ecology and Resources Management, Justus-Liebig-Universität Gießen, Germany c Department of Mathematics, Washington State University, 208 Morrill Hall, Pullman, WA 99164-3520, United States d Grassland Soil and Water Research Laboratory, United States Department of Agriculture – Agricultural Research Service, 808 E. Blackland Road, Temple, TX 76502, United States article info Article history: Received 24 September 2010 Received in revised form 11 March 2011 Accepted 17 March 2011 Available online 15 April 2011 Keywords: Soil carbon and nutrient cycling modeling Soil carbon saturation Soil tillage abstract In this paper we describe and test a sub-model that integrates the cycling of carbon (C), nitrogen (N) and phosphorus (P) in the Soil Water Assessment Tool (SWAT) watershed model. The core of the sub-model is a multi-layer, one-pool soil organic carbon (S C ) algorithm, in which the decomposition rate of S C and input rate to S C (through decomposition and humification of residues) depend on the current size of S C . The organic N and P fluxes are coupled to that of C and depend on the available mineral N and P, and the C:N and N:P ratios of the decomposing pools. Tillage explicitly affects the soil organic matter turnover rate through tool-specific coefficients. Unlike most models, the turnover of soil organic matter does not follow first order kinetics. Each soil layer has a specific maximum capacity to accumulate C or C saturation (S x ) that depends on texture and controls the turnover rate. It is shown in an analytical solution that S x is a parameter with major influence in the model C dynamics. Testing with a 65-yr data set from the dryland wheat growing region in Oregon shows that the model adequately simulates the S C dynamics in the topsoil (top 0.3 m) for three different treatments. Three key model parameters, the optimal decomposition and humification rates and a factor controlling the effect of soil moisture and temperature on the decomposition rate, showed low uncertainty as determined by generalized likelihood uncertainty estimation. Nonetheless, the parameter set that provided accurate simulations in the topsoil tended to overestimate S C in the subsoil, suggesting that a mechanism that expresses at depth might not be represented in the current sub-model structure. The explicit integration of C, N, and P fluxes allows for a more cohesive simulation of nutrient cycling in the SWAT model. The sub-model has to be tested in forestland and rangeland in addition to agricultural land, and in diverse soils with extreme properties such high or low pH, an organic horizon, or volcanic soils. © 2011 Elsevier B.V. All rights reserved. Abbreviations: C, carbon; d, index of agreement; fE, combined effect of soil temperature, moisture, and aeration on soil organic matter, residue, and manure decompo- sition; fO, aeration factor controlling soil organic matter, residue, and manure decomposition; fT, temperature factor controlling soil organic matter, residue, and manure decomposition; fW, moisture factor controlling soil organic matter, residue, and manure decomposition; fp, power factor that affects fE; fcm, cumulative f mix ; f mix , mixing coefficient associated to different tillage tools; f tool , tillage factor controlling soil organic matter decomposition rate; GLUE, generalized likelihood uncertainty estimation; h, humification; hR, hM, humification coefficient of residue and manure; hx, maximum humification coefficient for a given soil texture; HRU, hydrologic response unit; IRC, IMC input of organic C through residue and manure; k, maximum apparent soil organic matter decomposition rate multiplied by fE and f tool ; kM, optimum manure decomposition rate; kR, optimum residue decomposition rate; kS, apparent soil organic matter decomposition rate; kx, maximum apparent soil organic matter decomposition rate; MC, MN, MP, manure organic C, N, and P mass; MCN, MCP, manure C:N and C:P ratios; MINMN, net mineralization rates from decomposing manure; MINRN, net mineralization rates from decomposing residues; N, nitrogen; N min , mineral N in the soil layer; P, phosphorus; RC, RN, RP, residue organic C, N, and P mass; RCN, RCP, residue C:N and C:P ratios; SC, SN, SP, soil organic carbon C, N, and P mass; Sx, reference or saturation soil organic C mass; SCN, SCP, soil organic C:N and C:P ratios; SOM, soil organic matter; SWAT, Soil Water Assessment Tool; Z l , soil layer thickness. ∗ Corresponding author. E-mail address: akemanian@psu.edu (A.R. Kemanian). 1 Current address: Resource Centre for Environmental Technologies (CRTE), Public Research Centre Henri Tudor Luxembourg. 0304-3800/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ecolmodel.2011.03.017