Ecosystem changes during restoration using an extended chronosequence of restored grasslands

The spatial and temporal extent of restored grasslands dominated by C₄ grasses throughout the historic extent of the tallgrass prairie ecosystem presents an opportunity to gain a better mechanistic understanding of factors influencing the recovery of soil organic matter pools, belowground microbial communities, soil structure and ecosystem-wide nutrient cycling.  We used an extended chronosequence approach to quantify changes in pools and fluxes of C and N in the surface 10 cm of soil.  A temporal sequence of independent sites consisting of 3 agricultural fields, 19 restored grasslands, and 3 native prairies were sampled in 1999 and 2006, resulting in restoration chronosequences of  2-12 and 8-18 years.  Changes in total soil C and N remained linear over time, and extending the chronosequence improved model fitting and predicted slightly higher rates of total soil C (35.74 g/m²/yr, r²=0.40, P<0.001) and N (2.48 g/m²/yr, r²=0.25, P=0.004) accrual.  Microbial biomass C also continued to increase linearly (2.79 g/m²/yr, r²=0.30, P=0.001), but microbial biomass N showed no change over time due to N limitations.  Potential net N mineralization rates declined linearly across the original 2-12 year chronosequence (r²=0.21, P=0.073), and lower rates observed across the extended chronosequence (-2.83 mg/m²/d, r²=0.30, P=0.001) demonstrate a sustained N limitation, characteristic of native grasslands.  Our results are consistent with other chronosequence studies on similar soils and reinforce that dominant grasses drive ecosystem recovery, but more information is needed on mechanisms underlying accrual and dynamics of stable soil C and N pools to maintain long-term ecosystem productivity.