Impact of LaBC on microbiological processes in coarse textured soil

Biosolids are the by-product of wastewater treatment plant and are also commonly known as treated sewage sludge. Countries with sewage treatment plants produce approximately 27 kg of dry biosolids person-1 year-1. Agricultural land application is the most popular way of biosolids disposal because of its plant nutrient properties. The Water Corporation of Western Australia has proposed to recycle most of its biosolids on agricultural land, after blending with lime as the soil amendment LAB (lime-amended biosolids) with further dilution with clay to form LaBC®. The use of LaBC® as a soil amendment was investigated in laboratory, glasshouse and field scale studies with an emphasis on soil biological processes, either alone or in combination with inorganic-fertilisers and an N absorbing amendment, biochar. The soil used for most of the experiments was a water repellent, coarse-textured, acid sandy soil with poor plant productivity in the field; the land was used for grazing. A preliminary field study demonstrated high spatial variability of LaBC® after application to soil (Chapter 3). A laboratory incubation study (Chapter 4) investigated whether the presence of clay in LaBC® altered soil microbial processes over a 30 week period. Aerobic-incubation and CO2 respiration assays were used to monitor water repellence, chemical and soil biological properties in amended soil. LaBC® was applied at 50, 100 and 150 t ha-1 (wet weight equivalent) as dry material. In addition, dry components of LaBC® (lime, clay, LAB (lime+biosolids) and LAC (lime+clay)) were applied separately at rates equivalent to their fractions within LaBC®. Inclusion of clay in LaBC® was effective in eliminating soil water repellence even at 50 t ha-1 and temporarily suppressed N release from the biosolids (by 58%), even at this lowest amount. LaBC® consistently suppressed soil microbial respiration compared with LAB alone at the highest rate (150 t ha-1) which appeared to be associated with protection of organic matter. There was no significant N release from soils treated with lime and clay, alone or in combination, in the absence of the biosolids. There may be a complex interaction between, clay, lime and organic matter from LaBC®, but each may have played a dominant role in altering N release from biosolids at different times. Further, for this 30 week laboratory incubation study, it was attempted to sequence the bacteria from the water repellent soils amended with LaBC® using the Ion Torrent Personal Genomic Machine (PGM Platform). Water repellence over time was monitored and water holding capacity was measured and compared with bacterial community structure in soil amended with LaBC® (Chapter 5).The dominant phyla detected were Proteobacteria and Actinobacteria. Addition of LaBC® decreased the relative abundance of Actinobacteria and increased the relative abundance of Firmicutes, Chloroflexi, Bacteroidetes and Proteobacteria. Acidobacteria were rare. The residual effect of LaBC® in coarse-textured sandy soil, when previously co-applied with macro-nutrient fertilisers over 5 plant growth cycles, was assessed in a glasshouse experiment (Chapter 6). LaBC® was re-applied to soil which had different histories of application of fertiliser and/or LaBC®. Ryegrass was grown for 2 further cycles in pots, and each was harvested 6 weeks after the seedling emergence. The re-applied LaBC® (50 t ha-1) increased soil pH from 5.1 to 6.7 regardless of soil history. Plant dry biomass was significantly increased (p>0.05) by fertiliser history after 5 cycles of ryegrass, but there was no residual effect of previously applied LaBC®. There was an increase in shoot biomass with re-applied LaBC® (cycles 6 and 7). Finally, the application of LaBC® was further investigated in 4 soils that varied in pH and nutrient status in a glasshouse experiment (Chapter 7). In this experiment, biochar was added to the soil to determine whether there was an interaction with LaBC®. The nutrient release due to the application of LaBC® alone and when co-applied with biochar on subterranean-clover productivity was observed for 2 cycles of plant growth and root nodulation. The results were inconsistent across soil types but the addition of biochar decreased the N release from LaBC® in all soils. Application of LaBC® had the largest effect on bacterial community structure in the sandy soils compared with the clay soil. Therefore, combined application of LaBC® and biochar could be a practical option for soils with heavy leaching conditions. Overall, the application of LaBC® has potential beneficial effects on soil microbial processes and could be used as a process for disposal of biosolids (up to 150 t ha-1 wet weight equivalent) in water repellent, acidic-sandy soils. However, considering the optimum pH values for plant growth, the lowest rate of 50 t ha-1 would be sufficient for pH amelioration of acid agricultural soil. The higher application rates could be further combined with biochar wherever N leaching is considered as a serious threat to the environment.