Abstract:

To investigate the carbon release characteristics of biochar derived from horticultural waste and its effects on soil carbon fractions, nine common types of horticultural waste in South China were selected: tree materials (eucalyptus, camphor tree, guava, palm leaves, and palm shells), shrub (Cassia fistula), and herbaceous plants (broadleaf carpet grass, petunia, and balsam). Biochar was produced from each material, and their basic physicochemical properties, dissolved organic carbon (DOC) release characteristics, and composition were characterized. A typical lateritic red soil from a subtropical broad-leaved forest urban park was used as the test soil. Soil incubation experiments were conducted to examine the effects of different application rates (0.2% and 1.0%, m/m) of biochar from broadleaf carpet grass, C. fistula, petunia, and eucalyptus on soil physicochemical properties, microbial biomass carbon (MBC), particulate organic carbon (POC), mineral-associated organic carbon (MAOC), and easily oxidizable organic carbon (EOC). The results showed that biochar derived from woody plants such as eucalyptus had higher specific surface areas and more compact pore structures, while those from herbaceous species like broadleaf carpet grass exhibited higher pH, electrical conductivity (EC), and ash content. DOC release followed a pattern of rapid initial release followed by gradual decline, with the release amount ranking as: herbaceous>shrub>woody species. Under different extraction methods, DOC release was in the order of hot water extraction>cold water extraction>weak alkaline extraction, with palm shell biochar showing the highest release and eucalyptus biochar the lowest. The 1.0% biochar application rate most significantly increased soil pH, EC, and organic matter content. After 14 days of incubation, except for the treatment with 0.2% C. fistula biochar, all other treatments showed a significant increase in soil DOC content by 47.12% to 111.32%. After 28 days, the soil DOC content in the 0.2% broadleaf carpet grass biochar treatment increased significantly by 69.92%. By day 56, DOC levels in all treatments were lower than those in the control group. Except for the 0.2% petunia biochar treatment, MBC in all other treatments increased significantly by 14.90% to 72.61%. The treatment with biochar also significantly increased the POC content by 31.66% to 364.65%, and the contents of MAOC and EOC also increased to varying degrees. At an addition rate of 1%, the contents of MBC, POC, MAOC and EOC were all higher than those at an addition rate of 0.2%. The addition of herbaceous biochar such as broadleaf carpet grass is beneficial for the rapid release of soil active carbon, the addition of shrub-like biochar such as C. fistula is helpful for activating the activity of soil microorganisms, and the addition of tree-like biochar such as eucalyptus is more conducive to the accumulation of stable carbon components in the soil.

Key words: garden waste, biochar, dissolved organic carbon, release characteristics, soil carbon fractions, lateritic red soil