Biochemical inventory as a tool to assay ecosystem carbon dynamics

Background/Question/Methods

Global soil carbon (C) stocks (2 × 10¹⁸ g C) are large enough that a minor change in soil C dynamics would constitute a major climate feedback. The responses of soil C stocks to experimental manipulations of atmospheric carbon dioxide concentration ([CO₂]) and temperature vary widely in direction and magnitude across different ecosystems. The reasons for these differences are not well understood. Biochemical inventories in biomass and soil C pools give a new perspective on ecosystem C dynamics. A comparison of biochemical stocks in the soil  to the fluxes from roots and shoots (e.g. kg carbohydrate/m²·yr) provide insight to C cycling mechanisms. We performed biochemical inventories for native grassland ecosystems on 3 different soil types (Mollisol, Alfisol, and Vertisol) and under varying [CO₂] at the USDA CO₂ Tunnel Experiment (Temple, TX). We used ¹³C nuclear magnetic resonance to study the chemical structure of plant tissues and soil organic matter and applied a molecular mixing model (Baldock et al., 2004) to estimate biochemical stocks.

Results/Conclusions

Biochemical fluxes to the soil surface were affected by soil order, through differences in net primary productivity (NPP) and plant species distribution within the community. Of the 3 soil orders, the Mollisol, a clay-rich alkaline soil, had the lowest NPP (278 g/m²·yr) and the lowest carbohydrate fluxes to the soil surface (205 g/m²·yr). Despite the lower inputs, the Mollisol had the highest carbohydrate stock in the soil (34% of the soil C). This suggests that the mineralogy and/or structure of the Mollisols preserve labile organic matter more efficiently than the Alfisol and Vertisol. Conversely, the Vertisol grassland ecosystem had the highest lignin production (91 g/m²·yr) and the lowest soil lignin stocks (660 g lignin/m²·yr)), perhaps indicating higher fungal activity than in the Mollisol and Alfisol. The Alfisol showed the largest response to [CO₂]. Ecosystem carbohydrate production increased from 92 g carbohydrate/m²·yr), at 292 ppm [CO₂] to 417 g/m²·yr) at 476 ppm [CO₂]. Despite this, soil carbohydrate stocks declined from 1300 g/m² to ~600 g/m², perhaps suggesting a [CO₂]-induced priming of microbial activity in sandy, acidic soils. Here we will further explore the utility of biochemical inventories as a tool for understanding ecosystem C dynamics.