Climate controls on C3 vs. C4 productivity in North American grasslands from carbon isotope composition of soil organic matter

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‰ 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.