Rise in carbon dioxide changes soil structure

Abstract

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)¹. 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 soil². 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 CO₂fumigation, and we propose a mechanism for this that involves the production by fungi of the glycoprotein glomalin.

Main

The Jasper Ridge CO₂experiment in northern California³ exposed two natural annual grassland ecosystems (sandstone and serpentine) to increased atmospheric CO₂for six growing seasons by using cylindrical, open-top chambers (1 m tall, 0.33 m², n = 10). In both grasslands, a higher proportion of soil was found in aggregates 1-2 mm across in elevated CO₂, and the 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).

Table 1 Effects of increased CO₂on aggregates and glomalin in two annual grasslands

Although soil aggregation is a complex hierarchical process⁴, the soil concentration of the glycoprotein glomalin⁵ is tightly correlated with aggregate stability across many soils⁶. Glomalin is produced mainly by hyphae of arbuscular mycorrhizal fungi⁵, which form symbiotic associations with plant roots. The length of the hyphae in these fungi increases with elevated CO₂in the sandstone grassland, but not in the serpentine grassland, with root biomass and length showing the opposite pattern⁷. Total glomalin and immunoreactive glomalin concentrations in soil increased in both grasslands with elevated CO₂ (Table 1). Glomalin concentration in aggregates (from a separate extraction) increased under elevated CO₂for 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 CO₂study in southern California used 12 greenhouses (2×2×2 m) with controlled CO₂, ambient lighting and controlled temperature at six CO₂concentrations from a pre-industrial level of 250 μl l⁻¹ to 750 μl l⁻¹ at intervals of 100 μl l⁻¹ (n = 2). The chambers were built around Adenostoma fasciculatum (chamise) shrubs in chaparral vegetation recovering from an experimental burn. Soil samples were taken after three years of treatment and analysed for soil aggregation and glomalin concentration to see whether the patterns in the grasslands also existed in a different vegetation type. The proportion of soil mass in aggregates of 0.25-1 mm showed a linear increase (linear regression, P = 0.03, r ² = 0.74) along the CO₂gradient, 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 carbon stocks in these soils, and are on the order of 5% of the total calculated litter and soil accumulation under elevated CO₂on an annual basis⁸. 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 carbon sink itself.

Our results indicate that changes in soil structure in response to CO₂enrichment 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 severe⁹ and is accelerated by changes in many global factors, including climate and land use¹⁰. Our finding that an increase in soil aggregation could be brought about by atmospheric change may have implications for studies of soil stabilization in ecosystems.