Temporal patterns of inorganic nitrogen uptake by mature sugar maple (Acer saccharum Marsh.) and red spruce (Picea rubens Sarg.) trees using two common approaches

Abstract

Background: Plant uptake of nitrogen influences many ecosystem processes, yet uptake by trees in northern forests of the United States has not been quantified throughout the growing season. Aims: To measure NH4 + and NO3 − uptake by mature sugar maple (Acer saccharum) and red spruce (Picea rubens) trees during the early, mid and late growing season. Methods: At Hubbard Brook Experimental Forest, New Hampshire, we used two approaches to measure nitrogen uptake capacity by mature trees: an in situ depletion method using intact roots and an ex situ 15N tracer method using excised roots. Results: NH4 + uptake was greater than NO3 − for both methods and tree species (P < 0.05). NH4 + uptake was lowest during the early growing season, while NO3 − uptake was lowest during the late growing season. Measured rates of NH4 + uptake were 2–3 orders of magnitude greater using the in situ depletion method compared with the ex situ 15N tracer method. Conclusions: These results demonstrate seasonal differences in nitrogen uptake by two dominant tree species in a northern forest and show that the method employed can significantly impact measured rates of uptake, which could have implications for understanding the magnitude of plant nitrogen uptake and for cross-study comparisons of this process.