Soil carbon and nitrogen accumulation in a 15-year prairie restoration experiment

Background/Question/Methods

Prairie restoration following agricultural abandonment can lead to the accumulation of soil carbon (C) and nitrogen (N).  Understanding the rate and patterns of C and N accumulation is important for determining how quickly these systems recover from agriculture and the capacity for restored prairies to act as a carbon sink.  Studies on the dynamics of C and N accumulation following prairie restoration from agriculture are generally based on chronosequence studies.  However, few studies have compared the results of using a chronosequence to direct measures of accumulation over time.   We present the results of a 15-year study to determine the rate of C and N accumulation in restored tallgrass prairies in the Cowling Arboretum of Carleton College, Northfield, Minnesota, USA.  Restored prairies were established annually from 1995-2007. In 2000 and again in 2010, we sampled for soil C and N content at different soil depths.  Soil cores were taken from 12 permanent plots in each field and pooled to determine average C and N content for each restoration age.  In 2000 we sampled both restored prairies and those planning to be restored, but at the time still in agriculture. Thus, we are able to determine both the effects of prairie age using the chronosequence of restored fields as well as the change in pool sizes within a specific location over time.

Results/Conclusions

In both 2000 and 2010, there was no effect of field age across the chronosequence on C and N pools at any either 0-10 or 10-20 cm soil depth.  However, when we compared the accumulation C and N in each field over time there was a significant positive effect of field age on N content (p = 0.037; r² = 0.61) and a marginally significant positive effect on C content (p = 0.109; r² = 0.217) at the 0-10 cm depth.  Furthermore, C accumulation rate (g C m^-2 y^-1) increased with the number of years between sampling dates that a field was planted in prairie (p <0.001; r² = 0.80).  Our findings suggest that both soil C and N content increases significantly with prairie age, even in the early stages of successional development and that chronosequence studies may not accurately capture C and N accumulation in restored tallgrass prairie ecosystems.