Soil Water Characteristic Curve Measurement and Field Capacity Estimation Influences Daycent Predicted N2O Emissions.

The soil water characteristic curve (SWCC) relates soil volumetric water content, õ, to soil matric potential, ã, and is used to estimate soil hydraulic parameters such as field capacity (FC) and hydraulic conductivity that are critical inputs to environmental models. DayCent is a widely used process-based, biogeochemical ecosystem model that is used to estimate nitrous oxide (N2O) emissions from agricultural systems by the U.S. Environmental Protection Agency. The objective of this research was to evaluate the effect of different FC estimations taken from laboratory and in situ SWCC measurement approaches on the N2O emissions predicted by DayCent. Measurements were conducted on a well-structured, prairie derived, silt loam soil in three systems: the corn phases of a corn-soy rotation (CS2) and a corn followed by three years of alfalfa rotation (CS4), and a rotationally grazed pasture (CS6) at the Wisconsin Integrated Cropping Systems Trial (WICST) in southern Wisconsin, USA. Laboratory measurements reported greater õ for a given ã than in situ measurements. When FC was defined as the õ at ã=33 kPa, laboratory-derived FC estimates ranged from 28 to 32% and in situ derived FC estimates ranged from 23-24% across the treatments. Using FC estimates derived from in situ SWCC (23, 24%) led to underestimations of õ at 15 cm by the DayCent model compared to observed õ. Laboratory derived FC estimates (28, 32%) resulted in slightly better õ predictions by DayCent, however differences were not striking. DayCent simulated cumulative N2O emissions over the growing season were 1.5-2.3 times greater when a laboratory-derived FC was input to DayCent rather then an in situ derived FC, but all simulated N2O emissions were of the same order of magnitude as previously measured N2O emissions at WICST. These results suggest that how SWCC are determined and the hydraulic parameters that are estimated from SWCC can significantly influence modeled processes that depend on soil water status, highlighting the importance of the fundamental SWCC relationship and questioning the validity of assumptions surrounding the classic paradigm of field capacity.