Effect of Biochar on Strawberry Growth, Soil Properties and Ecosystem Gas Exchange

Biochar creates a resistant soil carbon pool that is carbon-negative, provides long-lasting improvements in soil fertility and serves as a net withdrawal of atmospheric carbon dioxide stored in highly stable soil carbon stocks. The enhanced nutrient retention, improved soil fertility and water holding capacity of biochar-amended soil not only reduces the total fertilizer requirements, but also the climate and environmental impact of croplands with generally increased production. I hypothesized that biochar increases plant growth by ameliorating negative soil physicochemical, and enhancing microbial, properties with special relation to nutrient availability and contributes actively to modify ecosystem gas exchange. Moreover, I hypothesized that the rate of biochar application influences the rate of biochar surface oxidation, nature and mineralization of functional groups, when it was added to soil for a long period of time in a controlled environment. The present study focused on determining the potential of a wood-based, high temperature (1100°C) biochar, to increase strawberry plant growth and ecosystem gas exchange in topsoil and its influence on soil quality. The results discussed in this thesis were obtained from a long-term investigation conducted under controlled conditions and is novel because of its duration (18 months), and because of the use of biochar derived from demolition wood. There is currently much interest in utilising biochar as a soil amendment for increased soil health and for carbon sequestration and European and International voluntary standards for biochar safety are under review in the UK post-Brexit. All work on biochar to date, has utilised biochar from virgin wood or agricultural residues. To the best of my knowledge, this is the first study to quantify effects of biochar derived from demolition wood on soil health. The importance of this is twofold; firstly, the stock of virgin wood for biochar production is limited, therefore it is important to be aware of any dangers of ‘diluting’ virgin wood with unapproved feedstock during production, and secondly, it is possible that biochar from such feedstocks might be acceptable for restoration programmes of already contaminated land. Biochar (0, 2.5, 5, 10 and 15% w/w) was mixed with topsoil, added to 14 L pots and maintained in a growth room at 20/16°C (16 hours day/night) and 50 % relative humidity for 18 months. Pots were either planted or left bare and soils in planted and unplanted pots were regularly sampled for microbiological and soil chemical determinations and plant growth measured. Biochar addition did not affect strawberry shoot growth or carbon or nitrogen content, but the 2.5% addition of biochar slightly increased root biomass, whilst the highest concentration (15%), reduced biomass relative to the 2.5% amendment, but not to the control. In the strawberry shoot, K, P, Zn, Cu, and As concentrations increased with biochar addition, while Pb content decreased with increasing rate of biochar compared to the control. Other than these, none of the shoot or root elements analysed exhibited clear biochar-driven trends. Neither leaf conductance nor leaf temperature were affected by biochar amendment. However, biochar amendment generally reduced ecosystem respiration (Re), net ecosystem exchange (NEE), gross ecosystem exchange (GEE) and soil enzyme activities. Biochar had no effect on microbial biomass nitrogen and carbon. CO2 and CH4 fluxes in soil were generally reduced by biochar amendment, but presence or absence of strawberry plants had no effect. However, soil water content, pH and Olsen P concentrations all increased with biochar amendment, as did soil nitrate concentrations in unplanted soils (but not as markedly in the presence of plants). Bulk density of the soil deceased in line with increasing biochar addition. Results from FTIR analysis showed that when this high temperature wood biochar was applied to soil, due to microbial and plant mediated transformation, it becomes more aromatic because of the loss of aliphatic and labile compounds and broadening of aromatic bands. The maximum number of functional groups (aliphatic, aromatic and carbohydrates) was recorded in the control soil (0 % biochar) both with and without plants. Aromatics (C-C and CH) were more prevalent than oxygen containing compounds (carboxyl and carbonyl), or aliphatic compounds and there were very few hydrocarbons. Shifts in the spectra for all wave numbers were observed in planted biochar-amended soils compared to control (0 % biochar). After 12 months, a marked decrease in spectral bands between 500 and 4000 cm−1 was noted in treatments with 2.5 %, 10 % and 15 % biochar. Overall, the use of biochar made from demolition wood ought to be avoided in agricultural settings. However, in contaminated areas, concentrations up to the lowest used in this study may be beneficial if pH changes or improvements in bulk density are desired