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Climate controls on C₃ vs. C₄ productivity in North American grasslands from carbon isotope composition of soil organic matter
- von FISCHER, JOSEPH C., TIESZEN, LARRY L., SCHIMEL, DAVID S.
- Global change biology 2008 v.14 no.5 pp. 1141-1155
- B horizons, C4 plants, carbon, climate, databases, field experimentation, fine roots, grasses, grasslands, isotopes, prediction, rain, soil organic matter, temperature, Great Plains region, United States
- We analyzed the δ¹³C of soil organic matter (SOM) and fine roots from 55 native grassland sites widely distributed across the US and Canadian Great Plains to examine the relative production of C₃ vs. C₄ plants (hereafter %C₄) at the continental scale. Our climate vs. %C₄ results agreed well with North American field studies on %C₄, but showed bias with respect to %C₄ from a US vegetation database (statsgo) and weak agreement with a physiologically based prediction that depends on crossover temperature. Although monthly average temperatures have been used in many studies to predict %C₄, our analysis shows that high temperatures are better predictors of %C₄. In particular, we found that July climate (average of daily high temperature and month's total rainfall) predicted %C₄ better than other months, seasons or annual averages, suggesting that the outcome of competition between C₃ and C₄ plants in North American grasslands was particularly sensitive to climate during this narrow window of time. Root δ¹³C increased about 1[per thousand] between the A and B horizon, suggesting that C₄ roots become relatively more common than C₃ roots with depth. These differences in depth distribution likely contribute to the isotopic enrichment with depth in SOM where both C₃ and C₄ grasses are present.