Cumulative Soil Carbon Dioxide Emissions: Accounting for Intra-Crop Spatial and Temporal Variability.

Since soil-gas emissions are dynamic, high-resolution data are needed to capture their spatial and temporal relations and variability.  The objectives of this study are to 1) quantify and test relationships among soil-surface CO₂ effluxes to soil physical conditions both spatially and temporally and 2) evaluate soil-surface CO₂ effluxes among various cropping systems.  Near Ames, Iowa, six no-till cropping systems of mixed prairie (nitrogen fertilized and unfertilized), continuous maize with 50 % stover removal (with and without cover crop), and maize-soybean rotation (each crop type grown each year) was initiated with four replications in a randomized complete block design.  Various cropping systems and intra-crop management positions were instrumented with sensors to measure soil-surface CO₂ efflux, soil water content and soil temperature.  Soil-surface CO₂ effluxes differed among intra-crop management positions at times mid and late in the growing season in continuous maize systems but did not differ throughout the growing season in maize-soybean systems.  Soil-surface CO₂ efflux was affected by physical conditions especially soil temperature and water content and/or management practice events.  The temporal structure of soil-surface CO₂ efflux associated with soil temperature variations greatly differ when soil water content is either stationary or non-stationary over time.  Cumulative soil-surface CO₂ emissions were greatest for the unfertilized prairie releasing 5.2 Mg CO₂-C ha^-1 from the soil.  All other cropping system soils released 4.0 (+/- 0.3) Mg CO₂-C ha^-1.  These data are a critical component of the carbon balance and thus their intra-crop spatial and temporal trends need to be considered when attempting to determine net carbon gain or loss among cropping systems.