Spatial and temporal patterns of nitrification rates in forested floodplain wetland soils of upper mississippi river pool 8

Overbank flooding is thought to be a critical process controlling nitrogen retention and cycling. Yet, studies aimed at quantifying these effects, specifically nitrification, are relatively few. In this study, we investigated the effects of season and flood frequency on soil nitrification rates in forested floodplains of Upper Mississippi River, Pool 8. Samples were collected from three plots within each site in April, August and November 2006. Plots were equally divided among three flood frequency categories as follows: rare, moderate and frequent based on elevation and flood probability model. We found a significant difference in nitrification rates among flood frequency categories as follows: rare>moderate>frequent (F=4.49, p<0.01) and over season: spring>summer>autumn (F=8.88, p<0.01). Regression for all samples showed that elevation, NH₄-N, bulk density and soil temperature explained a moderate amount of variation in nitrification rates (R²=0.29, p<0.01). Models for moderately flooded, spring, summer and autumn samples improved when analysed individually. The absence of a correlation between nitrification rates and hydrology limits our ability to predict rates based on hydrology alone. The model based on elevation and season allows us to estimate nitrification rates with moderate confidence (R²=0.27, p<0.01). A rough calculation of forest floodplain nitrification rates suggests that 473mt of NO₃-N are produced annually, about 0.5% of Pool 8 total annual NO₃-N budget.