Evaluation of the effectiveness of riparian zone restoration in the southern Appalachians by assessing soil microbial populations
Microbial biomass, nitrifiers and denitrifiers in surface soil (0-2010 cm) were quantified in a riparian zone restoration project at Coweeta, North Carolina, USA. Four treatments are included in this study: ( I ) a degraded (+N) riparian zone with continued compaction, vegetation removal, and nutrient addition (mow, roll, and nutrient addition);(2) a degraded (-N) riparian zone with continued compaction and vegetation removal, but without nutrient addition (mow and roll only); (3) a restored riparian area (no grazing, cessation of manure and N fertilizer application and re-vegetation with natural regrowth for 2 years); and (4) reference riparian zone (no riparian zone degradation has occurred within the last 10 years). Mean microbial biomass C increased from winter (3 16 mg kg-') to summer (593 mg kg-1), and decreased from summer to fall (265 mg kg-1). There were no significant changes in microbial biomass C with the cessation of manure and chemical fertilizer application in the egraded - N plot and with re-vegetation in the restored plot as compared to the degraded + N plot. Microbial biomass C level in the restored plot was comparable to that in the reference plot for most seasons. Restored plots had significant greater populations of denitrifiers than reference plots in the spring. Nitrifier numbers were lower in the degraded - N and restored plots than the degraded + N. Ammonium oxidizers in summer were more abundant (25,000 g-1 soil) in the degraded + N plot compared to (1000g-1 soil) in the degraded - N and restored plots, and NO2 oxidizers in the same period were more abundant (130,000-1 soil) in the degraded + N plot than that in the degraded - N and restored plots about (40,000g-1 soil). The soil NO3 concentrations were considerably lower in the degraded - N and restored plots than the degraded + N plot. Our results imply either cessation of manure and N fertilizer application or cessation of manure and N fertilizer and re-vegetation could contribute to restoration of degraded riparian zone through reducing numbers of nitrifiers.