Long-term biosolids application alters the composition of soil microbial groups and nutrient status in a pine plantation

Tree nutrient concentrations, root biomass, soil nitrogen (N) and phosphorus (P) contents, and enzyme activities were determined in a radiata pine plantation (Pinus radiata D. Don) on a sandy loam soil receiving 19 years of repeated biosolids applications, and the composition of soil microbial groups was assessed using phospholipid fatty acid analysis. Biosolids application significantly increased soil organic N, available P and foliar N concentration, but had no significant impact on foliar P concentration over the long term. Biosolids application improved above-ground tree growth but reduced fine-root biomass. Repeated biosolids applications also altered the composition of soil microbial community and increased the potential activities of N-acetyl-beta-glucosaminidase and acid phosphomonoesterase. Changes in soil N and P availability were better explained by variations in the composition of bacterial groups than those of fungal groups. The fungi-to-bacteria ratios were not changed, but arbuscular mycorrhizal fungi were suppressed possibly due to elevated soil P caused by biosolids application. The ratios of Gram-positive to Gram-negative bacteria were significantly increased by biosolids application and were linked to increased N mineralisation rates. In addition, it is evident that variations in soil P availability and the composition of soil microbial groups following biosolids application were not significantly related to foliar P status. These findings contribute to a better understanding of the effects of biosolids application on microbial-mediated soil N and P processes.