CO2 and N-fertilization effects on fine-root length, production, and mortality: a 4-year ponderosa pine study

We conducted a 4-year study of juvenile Pinus ponderosa fine root (≤2 mm) responses to atmospheric CO2 and N-fertilization. Seedlings were grown in open-top chambers at three CO2 levels (ambient, ambient+175 μmol/mol, ambient+350 μmol/mol) and three N-fertilization levels (0, 10, 20 g m−2 year−1). Length and width of individual roots were measured from minirhizotron video images bimonthly over 4 years starting when the seedlings were 1.5 years old. Neither CO2 nor N-fertilization treatments affected the seasonal patterns of root production or mortality. Yearly values of fine-root length standing crop (m m−2), production (m m−2 year−1), and mortality (m m−2 year−1) were consistently higher in elevated CO2 treatments throughout the study, except for mortality in the first year; however, the only statistically significant CO2 effects were in the fine-root length standing crop (m m−2) in the second and third years, and production and mortality (m m−2 year−1) in the third year. Higher mortality (m m−2 year−1) in elevated CO2 was due to greater standing crop rather than shorter life span, as fine roots lived longer in elevated CO2. No significant N effects were noted for annual cumulative production, cumulative mortality, or mean standing crop. N availability did not significantly affect responses of fine-root standing crop, production, or mortality to elevated CO2. Multi-year studies at all life stages of trees are important to characterize belowground responses to factors such as atmospheric CO2 and N-fertilization. This study showed the potential for juvenile ponderosa pine to increase fine-root C pools and C fluxes through root mortality in response to elevated CO2.