Soil Carbon Change in Reconstructured Tallgrass Prairies.

Reconstructing former cropland to tallgrass prairie can increase soil carbon (C) and enhance C sequestration to mitigate increases in atmospheric CO₂. This large-scale study was conducted at Neal Smith National Wildlife Refuge (NSNWR) in Jasper County, south-central IA. Tracts of cropped land at NSNWR have been reconstructed to native prairie every year since 1992, creating a sequence of landscapes defined by prairie age (chronosequence). Our objectives were to (1) use the chronosequence of prairies to define the relationship between prairie age and soil C and (2) use this relationship to calculate the annual rate of soil C change when former cropland  is converted to tallgrass prairie. We used a stratified random sampling design to identify sampling locations within 20 reconstructed prairies on Tama/Otley silty-clay loam soil (Typic Argiudolls). Soil cores were collected to a depth of 120 cm in 5 depth increments using a hydraulically-driven soil coring tool in May 2000, 2005 and 2010. Soil C concentration (g C kg^-1) was quantified using dry combustion in a Fison NA 15000 Elemental Analyzer  (ThermoQuest Corp., Austin, TX). Soil C content (Mg C ha^-1) was calculated based on equivalent soil mass using a cubic spline approach. Relationships between soil C content and prairie age were developed through regression analysis. Regression parameters were used to estimate the rate of change in soil C content for prairies within the chronosequence.  Soil C content and prairie age were significantly related at 0-10 cm (p = 0.0434) in May 2000, when only 7% of the prairies were greater than 7 years old. In May 2005, when 68% of the prairies were greater than 7 years old,  soil C content and prairie age were significantly related at 0-10cm  (p = 0.0015) and 10-20 cm (p = 0.0506).  The rate of change in soil C content to a depth of 40 cm in May 2010, when 85% of the prairies were greater than 7 years old, was 0.73 Mg C ha^-1 y^-1 (p = 0.0220), with 48% of the change in soil C content occurring in the top 10 cm of soil.