Dynamics of soil nitrogen and carbon accumulation for 61 years after agricultural abandonment

We used two independent methods to determine the dynamics of soil carbon and nitrogen following abandonment of agricultural fields on a Minnesota sand plain. First, we used a chronosequence of 19 fields abandoned from 1927 to 1982 to infer soil carbon and nitrogen dynamics. Second, we directly observed dynamics of carbon and nitrogen over a 12-yr period in 1900 permanent plots in these fields. These observed dynamics were used in a differential equation model to predict soil carbon and nitrogen dynamics. The two methods yielded similar results. Resampling the 1900 plots showed that the rates of accumulation of nitrogen and carbon over 12 yr depended on ambient carbon and nitrogen levels in the soil, with rates of accumulation declining at higher carbon and nitrogen levels. A dynamic model fitted to the observed rates of change predicted logistic dynamics for nitrogen and carbon accumulation. On average, agricultural practices resulted in a 75% loss of soil nitrogen and an 89% loss of soil carbon at the time of abandonment. Recovery to 95% of the preagricultural levels is predicted to require 180 yr for nitrogen and 230 yr for carbon. This model accurately predicted the soil carbon, nitrogen, and carbon: nitrogen ratio patterns observed in the chronosequence of old fields, suggesting that the chronosequence may be indicative of actual changes in soil carbon and nitrogen. Our results suggest that the rate of carbon accumulation was controlled by the rate of nitrogen accumulation, which in turn depended on atmospheric nitrogen deposition and symbiotic nitrogen fixation by legumes. Our data support the hypothesis that these abandoned fields initially retain essentially all nitrogen and have a closed nitrogen cycle. Multiple regression suggests that vegetation composition had a significant influence on the rates of accumulation of both nitrogen and carbon; legumes increased these rates, and C₃ grasses and forbs decreased them. C₄ grasses increased the C:N ratio of the soil organic matter and thereby increased the rate of carbon accumulation, but not nitrogen accumulation.