Effects of Biochar Types and Atmospheric CO2 Concentrations on C and N Cycling in Mine Site Spoils under Growth Chamber Conditions

Abstract

Increasing global population is magnifying demand for raw resources from mining, with the resulting mine soil overburden low in the carbon (C) and nitrogen (N) necessary for plant rehabilitation. Unabated growth in anthropogenic carbon dioxide (CO2) emissions is changing plant and spoil C and N cycles. The full extent of these changes is unknown, adding further complexity to mine spoil rehabilitation. Biochar is a promising soil rehabilitation product, with a stable aromatic structure derived from the anoxic pyrolysis of organic matter. This study aimed to investigate the C and N pools in mine soil, and plant physiological response to the use of biochar produced at different temperatures under elevated and ambient atmospheric CO2. An experiment was established in four biochambers, with controlled temperature, CO2 and UV, in the University of Western Sydney. Five species of plants were grown in mine spoil pots treated with three types of pinewood biochar, slow pyrolised at 650 oC, 750 oC, and 850 oC respectively, with CO2 concentrations at 400 ppm and 700 ppm. Plant foliar, stem, and root samples and soil samples were collected and analysed for total C (TC), total N (TN), C isotope composition (δ13C) and N isotope composition (δ15N) plant biomass and soil moisture. The results showed that elevated CO2 resulted in the further depletion of δ13C, with the 700 ppm CO2 reducing foliar δ13C compared to the 400 ppm CO2 treatments, at -36.8‰ and -30.9‰ respectively. Elevated CO2 also created a CO2 fertilisation effect, with the 700 ppm CO2 increasing plant foliar TC compared to the 400 ppm CO2 treatment, at 1.35 g and 1.05 g respectively. Evidently, elevated CO2 increased plant water use efficiency (WUE) and plant photosynthesis. Biochar amendment to the mine soil significantly increased soil moisture, plant biomass, and further depleted plant δ13C and δ15N. The biochar treatment significantly increased soil moisture and reduced plant δ13C which, in combination, provides strong evidence for the ability of biochar to increase the water holding capacity of soil, and water bioavailability to plants. Reduced δ15N provides evidence that biochar amendment increased the availability of N sources with a depleted 15N value in the soil with significant N limitation. Complementing the growing literature on the benefits of biochar, these results showed that the amendment of soil with biochar was able to increase plant growth success in a poor C and N environment, reducing the leaching of soil N and increasing the bioaccumulation of soil C and N compared to the soil without the biochar treatment.