Partitioning of carbon and carbon dioxide in plant-soil systems

Soil organic matter is an important component of ecosystems. How this ecologically and agronomically important component responds to land management is critical for understanding the global C cycle. In order to answer this question, plant and soil C pools were measured in two ecosystems, wheat and native tallgrass prairie. Biological soil C pools, plant root biomass, and aboveground biomass were determined for wheat and tallgrass prairie from 1998 through 2001. Mineralizable C and N (C_o and N_o) was two times greater in prairie than wheat, but wheat had a proportionally greater mineralizable pool. Thus, soil C and N under wheat was more dynamic with a greater turnover rate compared to prairie. A second study determined the contributions of CO₂ to total soil surface CO₂ flux during wheat (Triticum aestivum L.) growth. This second study occurred under greenhouse and field conditions. In the greenhouse study, the relative contributions of root and rhizomicrobial respiration to total CO ₂ flux were determined at three different growth stages (tillering, flag leaf, and flowering). Generally, root-rhizomicrobial respiration accounted for 60% of total CO₂ flux with the remaining 40% from heterotrophic respiration. Rhizomicrobial respiration accounted for 18–25% of total CO₂ flux. Three techniques (with shoot, shade, no-shoot) to measure root respiration were statistically similar. In the field study, soil surface CO₂ flux was measured from wheat planted and unplanted plots under no-till (NT) and conventional tillage (CT). Soil microbial activity at the surface (0–5 cm) was higher in planted plots during the growing season. The relative contributions of root-rhizomicrobial respiration to total CO ₂ flux were the greatest at boot stage in both tillage systems. Generally, root-rhizomicrobial respiration represented 50% of the total soil surface CO₂ flux from both NT and CT. Conventional tillage caused 20% higher CO₂ flux to the atmosphere in both the planted and unplanted plots. Therefore, intensive tillage increased the proportional distributions of labile soil C and turnover rate and the change of soil management from CT to NT could potentially increase C sequestration by 20%.