Atmospheric CO2, soil-N availability, and allocation of biomass and nitrogen by Populus tremuloides

Our ability to predict whether elevated atmospheric CO2 will alter the cycl ing of C and N in terrestrial ecosystems requires understanding a complex s et of feedback mechanisms initiated by changes in C and N acquisition by pl ants and the degree to which changes in resource acquisition (C and N) alte r plant growth and allocation. To gain further insight into these dynamics, we grew six genotypes of Populus tremuloides Michx. that differ in autumna l senescence (early vs, late) under experimental atmospheric CO2 (35.7 and 70.7 Pa) and soil-N availability (low and high) treatments. Atmospheric CO2 concentrations were manipulated with open-top chambers, and soil-N availab ility was modified in open-bottom root boxes by mixing different proportion s of native A and C horizon soil. Net N mineralization rates averaged 61 ng N.g(-1).d(-1) in low-N soil and 319 ng N.g(-1).d(-1) in high-N soil. After 2.5 growing seasons, we harvested above- and belowground plant components in each chamber and determined total biomass, N concentration, N content, a nd the relative allocation of biomass and N to leaves, stems, and roots. Elevated CO2 increased total plant biomass 16% in low-N soil and 38% in hig h-N soil, indicating that the growth response of P. tremuloides to elevated CO2 was constrained by soil-N availability. Greater growth under elevated CO2 did not substantially alter the allocation of biomass to above- or belo wground plant components. At both levels of soil-N availability, elevated C O2 decreased the N concentration of all plant tissues. Despite declines in tissue N concentration, elevated CO2 significantly increased whole-plant N content in high-N soil (ambient = 137 g N/chamber; elevated = 155 g N/chamb er), but it did not influence whole-plant N content in low-N soil (36 g N/c hamber). Our results indicate that plants in high-N soil obtained greater a mounts of soil N under elevated CO2 by producing a proportionately larger f ine-root system that more thoroughly exploited the soil. The significant po sitive relationship between fine-root biomass and total-plant N content we observed in high-N soil further supports this contention. In low-N soil, el evated CO did not increase fine-root biomass or production, and plants unde r ambient and elevated CO2 obtained equivalent amounts of N from soil. In h igh-N soil, it appears that greater acquisition of soil N under elevated CO 2 fed forward within the plant to increase rates of C acquisition, which fu rther enhanced plant growth response to elevated CO2.