Climate controls on C3vs. C4productivity in North American grasslands from carbon isotope composition of soil organic matter

We analyzed theδ13C 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 C3vs. C4plants (hereafter %C4) at the continental scale. Our climate vs. %C4results agreed well with North American field studies on %C4, but showed bias with respect to %C4from 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 %C4better than other months, seasons or annual averages, suggesting that the outcome of competition between C3and C4plants in North American grasslands was particularly sensitive to climate during this narrow window of time. Rootδ13C increased about 1‰ between the A and B horizon, suggesting that C4roots become relatively more common than C3roots with depth. These differences in depth distribution likely contribute to the isotopic enrichment with depth in SOM where both C3and C4grasses are present.