Carbon and nitrogen cycling during old-field succession: Constraints on plant and microbial biomass

Soil C and N dynamics were studied in a sequence of old fields of increasing age to determine how these biogeochemical cycles change during secondary succession. In addition, three different late-successional forests were studied to represent possible "steady state" conditions. Surface soil samples collected from the fields and forests were analyzed for total C, H₂O-soluble C, total N, potential net N mineralization, potential net nitrification, and microbial biomass. Above-and belowground plant biomass was estimated within each of the old field sites. Temporal changes in soil organic C, total N and total plant biomass were best described by a gamma function [y =at^b e^ctd +f] whereas a simple exponential model [y =a(l - e^-bt) + c] provided the best fit to changes in H₂O-soluble C, C:N ratio, microbial C, and microbial N. Potential N mineralization and nitrification linearly increased with field age; however, rates were variable among the fields. Microbial biomass was highly correlated to soil C and N pools and well correlated to the standing crop of plant biomass. In turn, plant biomass was highly correlated to pools and rates of N cycling. Patterns of C and N cycling within the old field sites were different from those in a northern hardwood forest and a xeric oak forest; however, nutrient dynamics within an oak savanna were similar to those found in a 60-yr old field. Results suggest that patterns in C and N cycling within the old-field chronosequence were predictable and highly correlated to the accrual of plant and microbial biomass.