Phosphorus (P) is an essential plant nutrient; however, there is a growing concern for future P scarcity due to an impending depletion of geological P resources. P processes in the soil environment are complex, and P availability is often low due to strong P sorption and slow diffusion of P in soil. However, excess P in leachates and P in runoff materials can cause severe environmental problems through eutrophication of surface waters. Thus, sustainable use and reuse of P in agriculture is a necessity to sustain future crop production and protect the environment.
In recent years, increasing attention has been drawn to biochar, the carbonaceous material produced by thermal conversion of biomass. Biochar can improve soil fertility and sequester carbon (C) in soil. In addition, biochar application can affect both chemical and biological soil properties. Thus, biochar may affect important P processes in soil consequently affecting P availability. Furthermore, biochar contains some P that may be taken up by plants. Therefore, this Ph.D. study aimed to investigate how biochar application may affect chemical and biological P processes in soil and plant P uptake. Furthermore, to assess the broader agronomic value of biochar plant uptake of other essential nutrients were investigated. Accordingly, three research themes were framed:
Research theme 1: Biochar effects on P sorption and P bioavailability in different soil types.
In research theme 1, the aim was to examine the properties of different biochars exhibiting contrasting P sorption abilities and further to investigate how these biochars would affect P sorption and P bioavailability when amended to different soil types. Accordingly, biochar mineral structures and surface functionality were investigated using X-ray diffraction and Fourier Transform Infrared Spectroscopy (FTIR), respectively. Furthermore, P sorption curves were produced for different soil and soil/biochar mixtures. Finally, a pot experiment (pot experiment 1) with maize was used to assess P bioavailability in biochar-amended soils. The results indicated that the mineral phases rather than the C structures controlled the P sorption capacity of the investigate biochars. Furthermore, biochar could either increase or decrease the P sorption in soil. However, the initial P sorption capacity of the soil was an
important factor controlling the effects of biochar application on P availability. The results of this study is reported in Manuscript 1 (Appendix I).
Research theme 2: Biochar effects on dynamic P pools and biological P processes in soil
In research theme 2, the objectives were to 1) investigate the effects of biochar on short-term and long-term P pools in soil and 2) to investigate biochar effects on enzyme activity and exudates of citrate and sugars in soil. Biochar effects on P pools were investigated using a modified sequential P fractionation procedure in soil samples from a a pot experiment with maize (pot experiment 2) (short-term effects), and in a field experiment (long-term effects). The results showed that crop residue biochars increased the readily available P pools in soil, whereas, wood biochar had not affect (Manuscript 2, Appendix II). However, in the long-term experiment, there were no effects on P pools six years after application of 40 t biochar ha-1. Enzyme activity and exudates of citrate and sugars were investigated in pot experiment 2. The results suggested that biochar could stimulate biological P processes by increasing the activity of alkaline phosphatases and increase the concentration of citrate in the rhizosphere. It was concluded, that biochar-induced changes to the soil environment such as pH partly control the stimulation of biological P processes (Manuscript 2, Appendix II).
Research theme 3: Biochar effects on the uptake of essential macro- and micronutrients
Biochar effects on the uptake of essential nutrients were investigated in pot experiment 1. The results showed that biochar affected several aspects of maize nutrition. However, the combined effect of biochar and soil type controlled the extent of these effects (Manuscript 3, Appendix III).
In conclusion, biochar can affect important chemical and biological P processes in soil by affecting P sorption processes, increasing the available P content, and potentially affecting P mineralization and solubilization processes. However, the combined effect of biochar and soil type control these effects. Furthermore, understanding the long-term effects of biochar on P processes in soil is needed for the optimal use of biochar as a P management tool in agriculture.