| Agricultural sources account for significant proportions of global anthropogenic production of some major greenhouse gases (GHG), such as nitrous oxide and methane. The effects of these gases have been strongly linked to past and present climate change, and are predicted to correlate with future climate change based on the predictions of model simulations. Soil-based fluxes of GHG are produced primarily through plant and microbial processes and are affected by soil physical, chemical, and biological properties. The lower Missouri River Floodplain (MRF) region encompasses many different land use systems including agriculture and riparian forest. The effects of these different land use systems in the MRF on soil GHG (i.e., carbon dioxide, nitrous oxide and methane) emissions has been little studied possibly partly because of the challenges of measuring GHG fluxes due to the occurrence of periodic flooding and the high spatial variability in soil properties in this region. The goal of this study was to investigate the influences of lower MRF land use systems on the spatial and temporal variations of soil GHG emissions. The specific objectives were to evaluate soil GHG emissions (CO2, CH4, N2O) in floodplain soils under agroforestry, row-crop agriculture, and forested systems in response to differences in soil water content, temperature, land use, and N inputs. The project contains two distinct portions, a laboratory and in situ field study.;For the laboratory incubation study intact soil surface cores (0 – 10 cm depth) were obtained from row crop agricultural, agroforestry and riparian forest sites within the MRF near New Franklin, Missouri and incubated for a period of 94 days. Cores were subjected to three water regime treatments: Flooded (FLD), Optimal for CO2 efflux (OPT), and Fluctuating (FLX). For each of 20 sampling dates, the cores were placed in sealed 2 L plastic jars and the headspace flushed with He gas. The soil CO2, CH 4, and N2O fluxes were determined by sampling the head space at three time points (0, 20, 40 min. after sealing).;Soil CO2 emissions were associated over time by both the MRF land use system and the soil moisture regime. Riparian forest (FOR) soils had higher cumulative CO2 emission rates than those in fertilized agriculture (AG-N) despite no significant differences (p ≤ 0.05) in initial soil particulate organic matter (POM) or KMnO4- oxidizable C. OPT sample cumulative emissions were significantly higher (p ≤ 0.05) than those of FLD and FLX. Soil CH4 emissions showed no influence of land use, but were influenced by the water regime and length of time under flooding conditions. FLD samples showed a significant difference in cumulative soil CH4 emissions from other samples at Day 51 onward when Eh had significantly decreased. FLD CH4 efflux rates were significantly higher than under the OPT and FLX soil moisture regimes. Time, land use, and water regime all showed independent and interactive influences on the surface cumulative N2O emissions. AG-N and AF-N emitted the most N 2O among the land use and N treatments. Loss of N through soil N 2O emissions decreased as the incubation progressed. These results suggest that land use management, the soil moisture regime, and soil N content can influence the amounts of GHG emitted in the MRF.;The in situ study component was comprised of three spatially intensive samplings through the 2011 growing season. Soil GHG (CO2, CH 4, N2O) flux and soil properties were measured from agriculture, agroforestry, and riparian forested areas. Within each land use type, a representative 0.10 ha area was delineated and 45 sample chambers were installed for assessment of spatial variation in soil GHG flux. Samplings occurred in May, June, and August with the resulting range of coefficient of variation values for CH 4, 157-496, CO2, 47-132.6, and N2O, 66-351%. Spatial interpolation displayed the GHG efflux rate variations across the seasonal land use sampling, and the relative importance of individual hot spot efflux rates upon total emissions. No significant relationships were found between soil properties and gas emissions. Limited significant relationships at p ≤ 0.05 were found between soil properties and log transformed GHG emissions. Considerations of the closed chamber methodologies for GHG assessment need to occur to expand field studies within the MRF region. Specifically, the dimensions and deployment strategy of the chambers must be adapted to individual site characteristics due to the widespread temporal and spatial variation among soil conditions. |
