Improved accounting of carbohydrate carbon from plants and soils

Soils have a great potential to accumulate carbon from atmospheric sources, but we possess few quantitative tools to predict and understand the conversion of C from plant sources into stable soil organic matter. Evaluation of present methods used for analyzing the major form of carbon present in plants, carbohydrates, found that development of ‘total carbohydrate’ methods were originally based on colorimetric or gravimetric tests whose accuracy is questionable. Use of ion chromatography found that total carbohydrate extraction and hydrolysis methods based on previously published H2SO4 solublization and hydrolysis techniques released from pure cellulose as little as 0.4% and a maximum of 22% of the glucose equivalents due to a failure to solublize the substrate. Optimum solublization conditions with concentrated H2SO4 (18 M) for 15–30 min followed by autoclave hydrolysis (1–1.5 M H2SO4) resulted in 82–97% recovery of purified cellulose–glucose and accounted for a major portion of the plant glucose that was not released by previously published methods. Application of this methodology to soils resulted in additional release of glucose equivalents if the hemicellulose fraction was first extracted with 1.0 M H2SO4 (30 min autoclave digestion). Prolonged hydrolysis times (16 h) recommended by previous methodology resulted in increased formation of carbohydrate degradation products as furan derivatives. Use of improved solublization procedures with autoclave hydrolysis (30 min) accounted for up to 99% of the theoretical carbohydrate content from plant biomass and recovered an additional 2.5–3.5 times of soil carbon content as carbohydrate forms while minimizing formation of furan derivatives.