Soil Carbon and Nitrogen Accumulation and Vertical Distribution across a 74-Year Chronosequence

The majority of the world's terrestrial C and N is contained in soil; however, most soil research has been focused on the top 10 to 30 cm. We quantified the ecosystem C and N pools down to I M, including the aboveground biomass, litter, and roots in a 74-yr grassland chronoscquence of abandoned agricultural fields in Minnesota. Carbon accumulated at a rate of 34 g C m(-2) yr(-1); however, only the two top depths, 0 to 10 and 10 to 20 cm, showed a significant increase of 11.0 and 5.8 g C m(-2) yr(-1). Soil N also increased significantly only in the top 10 cm. Litter C, total root C, and total root N all increased significantly across the chronosequence but were not significantly related to sod C or N gains. Root C and N gains also occurred in the top 10 to 20 cm of the sod. Overall, the changes in vegetation pools were, at best, weakly correlated with soil C and N pools. Sod C accumulation represented 11% of the total C fixed (or sequestered) in the ecosystem and exceeded root C productivity, particularly below 20 cm. We argue that half of the sod C accumulation may be occurring below 20 cm, suggesting that deeper soil stocks are responsive to disturbances and should be included in assessments of a sod's C storage potential. Furthermore, the source of this C does not appear to be directly derived from recent plant inputs. The accumulation of soil C at all depths tended to be limited by N inputs, and our estimated rate of sod C accumulation does not appear to be sustainable because of N limitation.