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 an d 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 obser ved dynamics of carbon and nitrogen over a 12-yr period in 1900 permanent p lots in these fields. These observed dynamics were used in a differential e quation model to predict soil carbon and nitrogen dynamics. The two methods yielded similar results. Resampling the 1900 plots showed that the rates o f accumulation of nitrogen and carbon over 12 yr depended on ambient carbon and nitrogen levels in the soil, with rates of accumulation declining at h igher carbon and nitrogen levels. A dynamic model fitted to the observed ra tes of change predicted logistic dynamics for nitrogen and carbon accumulat ion. On average, agricultural practices resulted in a 75% loss of soil nitr ogen 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 nitro gen and 230 yr for carbon. This model accurately predicted the soil carbon, nitrogen, and carbon : nitrogen ratio patterns observed in the chronoseque nce 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 ni trogen deposition and symbiotic nitrogen fixation by legumes. Our data supp ort the hypothesis that these abandoned fields initially retain essentially all nitrogen and have a closed nitrogen cycle. Multiple regression suggest s that vegetation composition had a significant influence on the rates of a ccumulation of both nitrogen and carbon; legumes increased these rates, and C-3 grasses and forbs decreased them. C-4 grasses increased the C:N ratio of the soil organic matter and thereby increased the rate of carbon accumul ation, but not nitrogen accumulation.