Soil microbial biomass: an estimator of soil development in reclaimed lignite mine soil

Abstract

A two-year study was conducted at the Big Brown lignite mine in Fairfield, Texas, to determine the rate and extent of recovery of the soil microbial biomass (SMB) in mixed overburden. The relationships between SMB carbon (SMBC), basal respiration and soil organic carbon (SOC) accretion was evaluated using the respiratory quotient (qCO2) and the ratio of the SMB to SOC (SMBC:SOC ratio). Newly leveled, 1-, 3-, 5-, 10-, 15-, and 23-year-old reclaimed mixed overburden as well as an unmined soil were sampled bimonthly to measure SMIBC and other parameters. Three methods [chloroform fumigation incubation (FI), chloroform fumigation extraction (FE), and substrate-induced respiration (SIR)] were used to measure SMB and compared as estimators of SMB in reclaimed mine soils. Basal respiration (CO2 evolved from untreated soil), metabolic quotient (i.e. specific respiratory activity; qCO2; C02 produced per unit mass of SMB), and the SMBC:SOC ratio (the abundance of SMB relative to SOC) were used to determine trends in microbial biomass dynamics relative to SOC accumulation. A nearly linear increase in SMB was observed over the chronosequence of mine soils (r--O.98 to 0.99) for each of the three biomass methods. Mean values of SMB from 12 sample dates ranged from 41 pg SMIBC g-1 at the 0-year site to 291 ptg SMBC g-' at the 23-year site. The unmined reference soil averaged 84 jig SMBC g-1 through the period of the study. The qCO2 declined from 0.24 to 0. 12 Mg C02-C Mg SMBC d-' during the first year and tended to stabilize near 0.06 to 0.09 as reclaimed sites matured. The ratios of SMBC:SOC increased linearly with age of site through 23 years (r--O. 97). A substantial amount of seasonal variation in SMB was observed during the two-year study. Older sites (15-and 23-years) showed significant fluctuations of SMB that correlated well with the growing season of Coastal bermudagrass. Microbial biomass peaked during mid to late summer and declined to a minimum during the cold, wet winter months. Younger sites were less affected by seasonal influences, and changes at these sites appeared more related to changes in soil moisture.