The Significance of Dissolved Organic Carbon (DOC) to Deep Soil Carbon Storage

Abstract

<p>Deep soil organic carbon plays a vital role in total soil organic carbon (TOC) storage in the soil profile. However, the quantity of TOC storage through the whole soil profile differs significantly in response to land-use and management practices. A key mechanism for the transport of organic carbon into the soil profile is via dissolved organic carbon (DOC) and the movement of DOC potentially contributes significantly to the carbon balance of terrestrial ecosystems.</p> <p>In this project the source, mobilisation-transport and storage of DOC and TOC were investigated through the soil profile to a depth of 1.0 m of selected Australian soils and landuse systems. The project aims were to: i) determine the quantity of DOC and TOC in different land-use systems and to compare the amount of DOC and TOC in different land-use systems at 0-1.0 m soil depth, ii) examine and compare the pattern of vertical distribution of DOC and TOC in each land-use system and depth, iii) compare the quantity and vertical distribution of DOC with TOC under different land-use systems and depths, iv) estimate the potential effect of land-use changes on the storage of DOC and TOC in the whole soil profile, v) determine the source of DOC in the soil profile under different soil conditions (e.g., moist and dry), and vi) explore the mechanisms of organic carbon (C) input into the deeper soil profile under various land-use systems.</p> <p>Three contrasting land-uses (cultivated, improved pasture and woodland) were investigated to study TOC and DOC dynamics, located on the Northern Tablelands, NSW, Australia. The soils of the area were intermediate between red/black Ferrosol and Dermosol soils (Australian soil classification). The effect of land-use conversion on TOC and DOC dynamics was also investigated at the Gunnedah Research Centre, located in northwest New South Wales Australia. In the research area the soil was Dermosols (Australian soil classification), and soils were collected from the sites on which sub-tropical perennial pasture had been established was previously cultivated until 1992. </p> <p>Under contrasting land-uses (cultivated, improved pasture and woodland), TOC and DOC consistently had higher concentrations near to the soil surface (0-20 cm). The concentrations of both TOC and DOC decreased with increasing soil depth. Woodland soils had higher concentrations of TOC and DOC followed by improved pasture and cultivated in most of the sites studied. Land-use and management practices had strong effects on TOC and the production of DOC depended largely on the quantity of TOC at low concentrations (<3%). However, the quality of organic matter had an impact on DOC production above this concentration (>3%) with a diminishing proportion of DOC with further increase in TOC. Moreover, this study highlighted that the distribution and storage of organic C in the deeper soil profile is strongly determined by inputs from the soil surface and DOC makes a significant contribution to deep soil C storage. </p> <p>Conversion of land-use from cropland to sub-tropical perennial pasture has a profound influence on TOC storage in the whole soil profile. The concentrations of TOC and DOC were significantly higher in the surface layers (0-10 cm) of pasture soil compared with adjacent cultivated land. Proportionally more DOC has been found in the deeper soil profile in respect to TOC suggests the importance of DOC in the deeper soil layers. The δ¹³C values of TOC and DOC were less ¹³C depleted at all soil depths of a C₄-pasture system compared with that under C₃-dominated cultivated soil. These results suggest that addition of organic carbon in the soil profile through C₄-carbon inputs were higher than C₃-carbon inputs. Overall, this study confirmed that C₄-carbon inputs can increase TOC and DOC in the surface soil as well as deeper soil layers in a C-depleted system.</p> <p>To study the quantity, distribution and source of DOC in the soil under different land-use systems (woodland, improved pasture and cultivated), DOC was extracted from fresh soils and from 40oC oven dry soils. DOC concentrations were higher in surface soils under woodland followed by improved pasture and cultivated that diminished with depth in the soil profile. Moreover, DOC concentrations were significantly higher in 40oC oven dry soils than fresh soil samples in every land-use systems and site studied. DOC samples were analysed using ¹Hnuclear magnetic resonance (NMR) spectroscopy to determine the source of DOC in soils. While none of the ¹H-NMR spectra were generated from DOC samples extracted from fresh soil, ¹H-NMR spectra generated from DOC samples extracted from 40oC oven dried soils consistent with carbohydrates and aliphatic biomolecules suggest a microbial origin DOC in the soil. Thus, these results suggest that soil conditions have a great influence on microbialorigin DOC and the quantity of DOC may change due to the contribution of microbial residue carbon in the soil that are regularly subject to drying periods.</p> <p>A subsequent study was conducted using radiocarbon (¹⁴C) analysis to explore the mechanisms of organic C transport in to the deeper soil profile under three contrasting landuse systems. TOC of woodland soils had higher ¹⁴C content (i.e., contained younger C) than TOC of other land-uses at all soil depths in most of the soils studied. In addition, ¹⁴C age of DOC in woodland soil indicated continuous transport of younger and fresh C down the entire soil profile. These results indicate that different land-uses, and their management practices might lead to differing supply of young C down the soil profile, supporting the concept that organic C storage in soils is predominantly surface input driven. The pattern of young DOC transport from the surface soil (i.e. continual transport) of woodland further suggest that DOC transport occurred via translocation of rainfall supporting the mechanisms of organic C transport via DOC.</p>