Biochar as a soil amendment and productivity stimulus for Eucalyptus nitens plantations

Biochar is a carbon-rich material produced by pyrolysis (heating in the absence of oxygen) of biomass to capture combustible gases and generate heat and electricity. It can be added to soils as a means to sequester carbon and to maintain or improve soil functions. The physical and chemical properties of biochars determine their function as a tool for environmental management. Soil changes and plant response have been analysed on diverse soil types under various climatic conditions, water regimes, chemo-physical environments and on different species. Yet, the exact mechanisms behind the effects of biochar application are not fully understood and productivity gains vary greatly depending on the type of biochar, application rates, crop species and environmental conditions. Producing biochar from different organic waste materials appears to be a promising method of achieving greater levels of certainty and flexibility for integrating carbon sequestration, managing waste disposal costs and introducing a new solution into soil and yield management in the conventional agricultural and forestry production systems. Biochar, however, is not widely used by farmers or foresters in Australia, mainly due to the lack of certainty concerning long-term consequences, yield gains and a lack of 'know-how' in the field of quality certification, transportation, logistics and cost efficiency. Forestry is a significant industry in Tasmania, with large scale plantations of radiata pine (Pinus radiata, D. Don) and Eucalyptus (E. globulus and E. nitens, H.Deane & Maiden) which play an increasingly important role in supplying for national and international demand for timber. Propagating robust seedlings for planting in the field is an important part of plantation establishment as it influences potential yield, while also being a significant budget component. As biochar has been reported to positively affect desirable soil characteristics (e.g. increased nutrient efficiency, improved water holding capacity or reduced bulk density) and enhance crop productivity, it was hypothesised that it can bring benefits to Eucalyptus seedling growth. The main objectives of this project were to investigate chemical changes of the soil, plant material and soil solution following biochar application; and to determine the optimum biochar dose required to positively influence eucalyptus growth under Tasmanian conditions; both in a controlled nursery environment and during establishment in the field. A secondary aim was to determine if commercial fertiliser rates could be reduced via biochar application to the growing medium. The final assumption was that biochar can be profitably made from forest residues and utilized within the forest production systems of Tasmania. The macadamia shell biochar used in this research was characterised as high in potassium and sodium, relatively high in total carbon content and low in total nitrogen (N) and phosphorus (P) content relative to other biochars described in literature. Two experiments were conducted: a pot trial in which Eucalyptus seedlings were observed from sowing to 9 months; and a field experiment in the Florentine valley in South West Tasmania where seedling establishment was monitored for 14 months. In both experiments the agronomic characteristics of the seedlings and trees was monitored on a regular basis. Each experiment had 4 sample collection periods when plant material and soil or potting mix samples were collected and analysed. Percolating water was collected from custom-built lysimeters installed in the field plantation. Agronomic monitoring revealed that in both experiments fertiliser combined with certain doses of biochar influenced the tree growth. The height of seedlings and young trees was comparable between full fertiliser treatment (i.e. when no biochar was applied) and treatments where medium biochar rates were combined with reduced fertiliser amounts. However, biochar application did not result in significantly taller plants. Other agronomic features were not influenced by biochar application in either experiment. The potting mix was high in organic matter and the fertiliser applied at rates reflected industry standards. While fertiliser rates were reduced to simulate a nutritionally poor soil, all other environmental parameters were optimal in the pot trial. As previous reports have indicated that biochar has more noticeable effects on poor quality soils, it is possible that the quality of the potting mix masked the efficacy of biochar in relation to agronomic productivity, although both soil and leachate parameters were influenced. In contrast, under field conditions the biochar doses were applied at rates below that at which soil and leachate parameters were modified in the potting experiment, thus the doses were possibly too low to show a significant effect. The application method could also have influenced biochar efficiency under field conditions. Chemical and physical changes in the analysed mediums used implied a number of different, in some cases contradictory, mechanisms. Biochar in both experiments increased growing medium pH and released potassium and sodium to the soil. In the plant tissues, biochar induced changes, yet no clear trends were evident that macadamia biochar has any sizable effect on the nutritional status of E. nitens in Tasmania. In most cases, the changes in leaf tissue were correlated with changes in the soil with no evidence that biochar directly influenced plant nutrient uptake. Biochar did not have a clear effect on percolating water nutritional changes and the only thing that can be concluded with certainty is that application of biochar increased potassium infiltration from the soil. This is most likely connected with significant amount of potassium introduced to the soil when biochar was applied. While the mechanisms for the reported changes remain unclear, in many cases biochar was responsible for changes in soil and plant material chemical characteristics and a limited agronomic response. It appears that biochar is able to influence nutrients transformations in the soil and therefore influence the soil environment for the E. nitens in Tasmania. It also shows significant potential for reduced commercial fertiliser rates both in the forestry plantations and in the forest nurseries. The financial analysis was based on the trial outcomes and the local operating environment in Tasmania; including current forestry procedures used for managing plantations in Tasmania; and benefits resulting from biochar production and incorporation into Tasmanian soils. The Biochar Scenario assumed on-site biochar making, out of post-harvest forestry residues, and different methods of biochar utilization. The model was built in Microsoft Excel¬¨vÜ with help from Forestry Tasmania experts. A number of assumptions were considered concerning: a) production costs, b) savings enjoyed by traditional operations following biochar scenario implementation and c) biochar sale. The analysis revealed a potential annual income of $179,514 resulting from introducing the Biochar Scenario on 270 ha. The sensitivity analysis identified the crucial factors responsible for scenario profitability, namely biochar price and final product distribution. The findings of this work supported the hypothesis that reducing common fertiliser rates used for seedling establishment in forestry plantations is viable. It has also provided insight into changes in both soil and plant tissue following macadamia shell biochar application to Tasmanian soils. The financial analysis served as a solid background for the realistic implementation of the Biochar Scenario for forestry industry in Tasmania.