Impact of restoration and management on aggregation and organic carbon accumulation in urban grasslands

Understanding the patterns of soil organic carbon (SOC) accumulation in restored grasslands is necessary to determine their potential to sequester atmospheric CO₂. Patterns of SOC accrual after grassland restorations are generally based on studies in former agricultural fields, and these studies show an increase in SOC and aggregated-associated OC over time. In this study, we quantified SOC accrual in a series of restored grasslands situated within the matrix of the Chicago metropolitan region. Soil samples were collected from unmanaged (no restoration), short-term (<7 yr), and longer-term restorations (>10 yr) and from native prairie remnants located on Mollisols or Alfisols and analyzed using a combination of physical fractionation (aggregate size) and chemical measurements (organic C [OC], ¹³C/¹²C natural abundance). We found that (i) total SOC declined after restoration and remained significantly lower in restored than unmanaged soils, (ii) patterns of aggregate OC in longer-term restorations were more similar to those in unmanaged than to prairie sites, and (iii) microaggregate OC levels and C₄-C inputs into aggregate fractions were significantly higher in short-than longer-term restorations. Thus, an increasing time under management did not enhance SOC accrual. For all sites, path models showed that SOC accrual was correlated with soil N, texture, and moisture, whereas C₄-C inputs were significant model terms only in short-term restorations and prairie sites. Taken together, these results indicate the need to consider inter-relationships among vegetation and soil factors as influences on SOC accrual rather than simply time under management.