Climate controls on C-3 vs. C-4 productivity in North American grasslands from carbon isotope composition of soil organic matter

摘要

We analyzed the delta C-13 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 C-3 vs. C-4 plants (hereafter %C-4) at the continental scale. Our climate vs. %C-4 results agreed well with North American field studies on %C-4, but showed bias with respect to %C-4 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 %C-4, our analysis shows that high temperatures are better predictors of %C-4. In particular, we found that July climate (average of daily high temperature and month's total rainfall) predicted %C-4 better than other months, seasons or annual averages, suggesting that the outcome of competition between C-3 and C-4 plants in North American grasslands was particularly sensitive to climate during this narrow window of time. Root delta C-13 increased about 1 parts per thousand between the A and B horizon, suggesting that C-4 roots become relatively more common than C-3 roots with depth. These differences in depth distribution likely contribute to the isotopic enrichment with depth in SOM where both C-3 and C-4 grasses are present.