last update February 29, 2000
Carbon in soil affects the formation and stabilization of aggregates (groups of primary particles that adhere to each other more strongly than to surrounding soil particles). Soil aggregation is important for preventing soil loss through wind and water erosion, and the size distribution and abundance of water-stable aggregates influences a range of physical, chemical, biological, and agricultural properties of soil. The effects on soil biota and nutrient cycling of increases in soil carbon availability, brought about by increased CO2, are well studied, but the consequences for soil aggregation and structure have not been examined. Here we show for three ecosystems that the water stability and size distribution of aggregates is affected by long term CO2 fumigation, and we propose a mechanism for this that involves the production by fungi of the glycoprotein glomalin.
The Jasper Ridge CO2 experiment in northern California exposed two natural annual grassland ecosystems (sandstone and serpentine) to increased atmospheric C02, for six growing seasons by using cylindrical, open-top chambers (1m tall, 0.33 m2, n=10). In both grasslands, a higher proportion of aggregates of 0.25-1 mm was significantly increased in the sandstone grassland (Table 1). The water stability of both size classes followed a pattern similar to the mass of aggregates. This suggested that the higher mass of aggregates could be explained by an increase in the water stability of aggregates (Table 1). Although soil agitation is a complex hierarchical process, the soil concentration of the glycoprotein glomalin is tightly correlated with aggregate stability across many soils. Glomalin is produced mainly by hyphae or arbuscular mycorrhizal fungi, which form symbiotic associations with plant roots. The length of the hyphae in these fungi increases with elevated CO2 in the sandstone grassland, with root biomass and length showing the opposite pattern. Total glomalin and immunoreactive glomalin concentrations in soil increased in both grasslands with elevated CO2 (Table1). Glomalin concentration in aggregates of 0.25-1 mm in both communities, but this was not the case for those of 1-2 mm (Table 1). The water stability of that fraction may be under different control.
The Sky Oaks CO2 study in southern California used 12 greenhouses (2x2x2m) with controlled CO2, ambient lighting and controlled temperature at six CO2 concentrations from a pre-industrial level of 250ull^-1 to 750ull^-1 at intervals of 100ull^-1 (n=2). The chambers were built around Adenostama fasciculatum (chamise) shrubs in chaparral vegetation recovering from an experimental burn. Soil samples were taken after three years of treatment and analyzed for soil aggregation and glomalin concentrations to see whether the patterns in the grasslands also existed in a different vegetation type. The proportion of soils mass in aggregates of 0.25-1 mm showed a linear increase (linear regression, p=0.03, r^2=0.74) along the CO2 gradient, but the 1-2 mm aggregate mass did not (p=0.68, r^2=0.04). Glomalin concentrations followed a pattern similar to that of the small aggregate size class (P=0.03, r^2=0.71).
The carbon sink represented by glomalin over the experimental period for Jasper Ridge was 8.29 g C m^-2 in the serpentine and 4.25 g C m^-2 in the sandstone grassland. These are very small amounts compared with the large organic stocks in these soils, and are on the order of 5% of the total calculated litter and soil accumulation under elevated CO2 on an annual basis. Glomalin therefore seems to be more important in carbon sequestration by virtue of its function in soil aggregation (which has been linked with carbon stabilization) than by acting as a caron sink itself.
Our results indicate that changes in soil structure in response to CO2 enrichment should be incorporated into global research because soil structure has a strong effect on soil processes and organisms. On a global scale, the extent of soil degradation and erosion is severe and is accelerated by changes in many global factors, including climate and land use. Our finding that an increase in soil agitation could be brought about by atmospheric change may have implications for studies of soil stabilization in ecosystems.