Long-term impacts of infrequent biosolids applications on chemical and microbial properties of a semi-arid rangeland soil

A plot study was conducted to quantify long-term (>12 years) impacts of a single biosolids application, and short-term impacts (<2 years) of a repeated application, on semi-arid rangeland soil chemical and microbial parameters. In 2003 and 2004, plots which had received 0, 2.5, 5, 10, 21, or 30 Mg biosolids ha−1 once in 1991 (long-term plots), or again in 2002 (short-term plots), were sampled and analyzed for soil chemical parameters, microbial biovolumes, C and N mineralization activities, Biolog EcoPlate substrate utilization potential, and plant productivity and tissue quality. Repeated applications temporarily exacerbated differences in soil chemical properties among treatments, but after 2 years, soil chemistry trends were similar between short-term and long-term plots. Soils which received a repeated application of 21 or 30 Mg biosolids ha−1 had greater bacterial biovolumes and C and N mineralization activities. In long-term plots, mineralization activities were stimulated only at the highest rate. Biosolids-amended soil communities also utilized Biolog substrates more quickly compared to communities from control plots. Plant biomass increased, whereas plant diversity and plant C/N ratio decreased with increasing application rate for both short- and long-term plots. Infrequent biosolids application had positive ecosystem effects in terms of site management objectives, with relatively low extractable metal levels in soil and greater plant biomass and tissue quality despite reduced species richness.