No evidence that chronic nitrogen additions increase photosynthesis in mature sugar maple forests

Abstract

Atmospheric nitrogen (N) deposition can increase forest growth. Because N deposition commonly increases foliar N concentrations, it is thought that this increase in forest growth is a consequence of enhanced leaf-level photosynthesis. However, tests of this mechanism have been infrequent, and increases in photosynthesis have not been consistently observed in mature forests subject to chronic N deposition. In four mature northern hardwood forests in the north-central United States, chronic N additions (30 kg N ha(-1) yr(-1) as NaNO3 for 14 years) have increased aboveground growth but have not affected canopy leaf biomass or leaf area index. In order to understand the mechanism behind the increases in growth, we hypothesized that the NO3(-) additions increased foliar N concentrations and leaf-level photosynthesis in the dominant species in these forests (sugar maple, Acer saccharum). The NO3(-) additions significantly increased foliar N. However, there was no significant difference between the ambient and +NO3(-) treatments in two seasons (2006-2007) of instantaneous measurements of photosynthesis from either canopy towers or excised branches. In measurements on excised branches, photosynthetic nitrogen use efficiency (micromol CO2 s(-1) g(-1) N) was significantly decreased (-13%) by NO3(-) additions. Furthermore, we found no consistent NO3(-) effect across all sites in either current foliage or leaf litter collected annually throughout the study (1993-2007) and analyzed for delta 13C and delta 18O, isotopes that can be used together to integrate changes in photosynthesis over time. We observed a small but significant NO3(-) effect on the average area and mass of individual leaves from the excised branches, but these differences varied by site and were countered by changes in leaf number. These photosynthesis and leaf area data together suggest that NO3(-) additions have not stimulated photosynthesis. There is no evidence that nutrient deficiencies have developed at these sites, so unlike other studies of photosynthesis in N-saturated forests, we cannot attribute the lack of a stimulation of photosynthesis to nutrient limitations. Rather than increases in C assimilation, the observed increases in aboveground growth at our study sites are more likely due to shifts in C allocation.

Invaders do not require high resource levels to maintain physiological advantages in a temperate deciduous forest

“…Responses may have become significant if we had reduced N availability below ambient levels (Cassidy et al 2004). In a related study, this species showed increased growth from fertilization, but not photosynthetic rates, suggesting soil N might affect C allocation, rather than C assimilation (Talhelm et al 2011). This result is in contrast with previous long-term N addition studies including similar native species that reported increases in leaf N following fertilization (e.g., Magill et al, 2004;Bauer et al 2004).…”

Anthropogenic nitrogen deposition ameliorates the decline in tree growth caused by a drier climate

“…However, Talhelm et al. () did not observed differences in photosynthesis between treatments at one of our sites. Another mechanism plausibly explaining our results could be related to tree conduit architecture.…”

Plant nitrogen uptake drives responses of productivity to nitrogen and water addition in a grassland

“…However, others found no relationship between NUE and soil N availability due to the inherent trade-offs between the two components of NUE, N productivity and mean residence time of N1428. Decreased NUE at the community level in response to N addition may result from lower photosynthesis under enriched N conditions29. Nitrogen addition may stimulate the growth of annual forbs at the expense of grasses as found in a similar ecosystem30, because grasses generally have higher NUE than forbs as reflected by the lower N status in grasses (Fig.…”