Nitrogen mineralization and nitrification in four Minnesota old fields

Nitrogen availability and its response to fertilizer amendments was measured by in situ incubation in four old fields ranging in age from 16 to >100 years at Cedar Creek Natural History Area. Net nitrogen mineralization in control plots increased with field age, from 4.4 g/m² in the youngest field to 6.5 g/m² in the oldest field. The proportion of total N mineralized decreased with field age, from 6.2% of total N mineralized in the youngest field to 4.8% mineralized in the oldest field, suggesting a decrease in organic matter quality with time. Unlike many forests in the region, nitrogen mineralization was correlated with total soil nitrogen content. Greater than 90% of the mineralized N was nitrified in most months. Analyses of variance indicate significant effects of field age and month of year on N mineralization and nitrification, but not effect of fertilizer treatment except in the oldest field. Fertilizer additions did not significantly increase standing pools of mineral N in the youngest or oldest fields but did in the 26 and 50 year old fields. However, changes in mineral N pools did not account for the amount added in fertilizer. Strong plant and microbial sinks for fertilizer and possibly leaching losses may be the reasons why fertilizer additions did not stimulate N mineralizations during the first two years in most fields.