Effects of phytolithic rice-straw biochar, soil buffering capacity and pH on silicon bioavailability

Abstract

Supplying phytolith-rich biochar in agrosystems increases soil pH, CEC and nutrient availability, adding to the impact of Si uptake on plant growth. Here we studied this specific impact as influenced by soil properties, and assessed the role of phytoliths to provide plant available Si. We used a young Cambisol and a highly weathered, poorly buffered, desilicated Nitisol. The biochars were produced from rice plants respectively enriched (Si+) and depleted (Si-) in Si. They had identical pH and nutrient contents, but largely differed in Si content (51.3 g Si kg−1 in Si + vs 0.3 g Si kg−1 in Si-). We compared their effects to that of wollastonite (CaSiO3) on the biomass and mineralomass of wheat plants in a soil:solution:plant device. The contents of soil bioavailable Si and biogenic Si were assessed through an original CaCl2 kinetic extraction and the DeMaster Na2CO3 alkaline dissolution, respectively. The DeMaster technique dissolved Si from phytolith as well as from wollastonite. The soil buffering capacity (cmolc kg−1) was 31 in the Cambisol and 0.2 in the Nitisol. An identical supply of phytolithic biochar increased pH from 4.5 to 4.8 in the Cambisol, and from 4.8 to 7.4 in NI. It further increased the content of bioavailable Si (from 55 to 97 mg kg−1 in the Cambisol, and 36 to 209 mg kg−1 in the Nitisol), as well as plant Si uptake, biomass and Si mineralomass. That increase was largest in the Nitisol. The DeMaster technique did not specifically quantify the phytolith pool. This pool was the main source of plant available Si in both the Cambisol and Nitisol amended with phytolithic biochar. At identical phytolithic Si supply, however, soil pH and soil buffering capacity controlled the transfer of Si in the soil-plant system, which was largest in the poorly buffered Nitisol. The effect of phytolithic biochar on Si bioavailability was depending on soil constituents and properties, and thus on soil type.