Microbial responses to biosolids treatment of rangelands

Abstract

Biosolids application to semiarid rangelands is a change in soil management that may introduce large quantities of organic C and N and varying concentrations of trace elements to soils. Since changes in the physical and chemical characteristics of soil may alter the size and activity of the soil microbial community, this study was initiated to determine how soil microorganisms respond to biosolids treatment. Biosolids were applied once in 1991 at 0 and 40 Mg ha-1 to a shrubland soil and at 0 and 30 Mg ha-1 to a grassland soil in Colorado. Biosolids application increased the basal respiration rates of shrubland soil by 400% over unamended soil (control soil) 5 yr following treatment, with a return to lower respiration rates, similar to control soil, 6 yr after treatment. Basal respiration rates also increased by 62% 6 yr following biosolids amendment of grassland soils as compared to control soil. Metabolically active biomass-C (SIR-Cmicr) showed a 25% increase in biosolids-amended shrubland soil in 1996 and 13 and 12% increases, respectively, in shrubland and grassland biosolids-amended soil relative to control soils in 1997. The specific respiration rate (qCO2) of biosolids-amended soil increased 300% over control soil at the shrubland site 5 yr after treatment, but returned to a lower value, similar to unamended soil by the sixth year. The qCO2 value for the grassland biosolids-amended soil also increased by 45% in 1997. The large increase in basal respiration and qCO2 values for biosolids-amended shrubland soil in 1996 is an indication that microbial efficiency in C cycling was decreased as a result of treatment. The higher biosolids application rate at the shrubland site in combination with a lower mean annual temperature and shorter mean frost-free period may have increased the adversity index (includes both stress and disturbance) for soil microflora. The biosolids increased potential C mineralization by 130% over control soil for the shrubland and 71% over control soil for the grassland 6 yr following biosolids addition. An increase of 440% in potential net N mineralization was also found for biosolids-amended shrubland soil in 1997. No significant difference in net N mineralization was shown for biosolids-amended grassland soil in 1997, possibly due to the short duration of the incubation. It appears that biosolids C and N were mineralized to a greater extent during the 6 yr following treatment at the grassland site due to the lower biosolids application rate, coarser soil texture, warmer mean annual temperature, and a longer growing season. These results indicate that when field environmental conditions are optimal for microbial activity, increases in C and N mineralization may still be expected after 6 yr for these biosolids-amended rangeland soils. In 1997, western wheatgrass (Pascopyrum smithii (Rydb.) A. Love) at the shrubland site showed a 33% increase in the percentage of root samples colonized by arbuscular mycorrhizal (AM) fungi as a result of biosolids amendment A 23% increase was shown in the percentage of blue grama (Bouteloua gracilis (H.B.K.) Lag. ex steud) root samples colonized by AM fungi in biosolids-treated soil at the grassland site 6 yr following treatment The most likely reason for the increase in AM fungal colonization of these economically important forage grasses on biosolids-amended plots is fungal proliferation into soil locations with increased organic C content In general, biosolids quality did not impair the size of the active microbial biomass, but biosolids quality and/or quantity lowered microbial efficiency in cycling of C substrates. Biosolids application greatly increased total soil organic C at the shrubland site (130%) and the grassland site (120%) and total soil organic N at the shrubland site (110%) and the grassland site (260%) 6 yr after addition indicating that decomposition of biosolids may not yet be complete. Large additions of biosolids to two semiarid rangelands may alter soil nutrient cycling and C storage longer than previously thought.