Effects of nitrogen fertilization and tillage on soil carbon dynamics in Iowa soils

As the atmospheric CO2 concentration continues to increase, more attention is being focused on the soil as a possible sink for atmospheric CO2. This study was conducted to examine the short-term effects of N fertilization and tillage on soil carbon dynamics. There were two main objectives in this study. Objective one was to determine the impact of N fertilization on soil CO2 emission and soil carbon (C) fractions change in central Iowa soils. Objective two was to determine the impact of tillage on changes in soil C fractions in Iowa soils. Soil CO2 emissions were measured during the growing seasons of 2003 and 2004 from plots fertilized with three N rates in continuous corn (Zea mays L.) and a corn-soybean (Glycine max L.) rotation. Soil samples were collected in the spring of 2002 and 2004 to determine soil C and N contents in different soil C fractions. A laboratory soil incubation and a soil microbial biomass determination were conducted to examine the effects of N fertilization on microbial biomass and microbial activity. Objective two was accomplished by collecting soil samples in the spring of 2002 and 2004 from no-tillage and chisel plow plots at six sites to determine soil C and N fractions changes. Nitrogen fertilization increased soil CO2 emissions by 25% in the corn-soybean rotation in 2003. The laboratory soil incubation produced contrary results with the highest N rate producing the greatest CO2 evolution. This was attributed to the increased N in the soil at the start of the soil incubation. Even though N fertilization consistently increased crop biomass and had minimal effects on soil CO2 emission, the soil C fractions only showed small increases. Tillage had minimal effects on the soil C fractions. No-tillage treatment had 7-17% greater SOC content than that of the chisel plow treatment in the Marshall, GPS, and CNW soil associations. Soil C fractions were most affected in the Marshall and GPS soil associations. We hypothesize these soils are more sensitive to tillage intensity due to more well balanced soil air and moisture conditions than the poorly drained soils in Iowa