Nitrogen (N) fixation from trees is the largest natural input of new nitrogen into most forest ecosystems. Many anthropogenic global changes, especially N deposition, may alter the rates at which trees fix N in the future. However, our understanding of how N fixation rates are regulated across a gradient of N availability is based largely on agricultural herbaceous legumes, while natural N fixation inputs are dominated by woody fixers. To better understand the potential future responses of a variety of N-fixing trees to anthropogenic changes in N cycling, we quantified the regulation strategy of 11 N-fixing species, including five woody legumes, five woody actinorhizal fixers, and one herbaceous agricultural legume, in a greenhouse across a fine scale N gradient. We also grew a subset of these same tree species in the field to compare regulation strategies from the greenhouse to those of individuals in more natural conditions.
Results/Conclusions
Despite evidence that high N fertilization levels relieved N limitation, nearly all woody legumes continued to fix significant amounts of N across the full range of N fertilization levels. As expected, %Ndfa, or the percent of N in an individual that was derived from fixation, was nearly 100% for all species at low fertilization levels. However, only Glycine max, soybean, turned down fixation to nearly 0%Ndfa. For woody legumes, both actinorhizal and rhizobial, %Ndfa never dropped below 40%, indicating that these individuals were fixing large amounts of N despite very high levels of N fertilizer. This greenhouse result is confirmed by field-grown individuals: at the highest level of N fertilization in the field (150 kg/ha/yr), plants did not demonstrate N limitation but %Ndfa for all species was maintained around 40%. While our results are as expected for soybean, the same experimental setup yielded very different results for woody N fixers. We posit that access to high light conditions may allow woody fixers but not soybean to maintain high rates of N fixation even when soil N is not limiting to growth.
