Does canopy nitrogen uptake enhance carbon sequestration by trees?

Abstract

Temperate forest (15) N isotope trace experiments find nitrogen (N) addition-driven carbon (C) uptake is modest as little additional N is acquired by trees; however, several correlations of ambient N deposition against forest productivity imply a greater effect of atmospheric nitrogen deposition than these studies. We asked whether N deposition experiments adequately represent all processes found in ambient conditions. In particular, experiments typically apply (15) N to directly to forest floors, assuming uptake of nitrogen intercepted by canopies (CNU) is minimal. Additionally, conventional (15) N additions typically trace mineral (15) N additions rather than litter N recycling and may increase total N inputs above ambient levels. To test the importance of CNU and recycled N to tree nutrition, we conducted a mesocosm experiment, applying 54 g N/(15) N ha(-1) yr(-1) to Sitka spruce saplings. We compared tree and soil (15) N recovery among treatments where enrichment was due to either (1) a (15) N-enriched litter layer, or mineral (15) N additions to (2) the soil or (3) the canopy. We found that 60% of (15) N applied to the canopy was recovered above ground (in needles, stem and branches) while only 21% of (15) N applied to the soil was found in these pools. (15) N recovery from litter was low and highly variable. (15) N partitioning among biomass pools and age classes also differed among treatments, with twice as much (15) N found in woody biomass when deposited on the canopy than soil. Stoichiometrically calculated N effect on C uptake from (15) N applied to the soil, scaled to real-world conditions, was 43 kg C kg N(-1) , similar to manipulation studies. The effect from the canopy treatment was 114 kg C kg N(-1) . Canopy treatments may be critical to accurately represent N deposition in the field and may address the discrepancy between manipulative and correlative studies.

Keywords: 15N labelling; C sequestration; Nitrogen deposition; Picea sitchensis; canopy fertilization; canopy nitrogen uptake; isotope trace; soil fertilization.

Figure 1

Treatment descriptions for the six experimental treatments. Each treatment received 0–1 sources of enriched ¹⁵N, either no enrichment (CONTROL), ¹⁵N‐enriched litter (^LITTERCONTROL,^LITTERSNU,^LITTERCNU) or 98% double‐labelled ¹⁵N in deposition, that is ¹⁵ NH ₄ ¹⁵ NO ₃ (SNU, CNU). All deposition treatments received a total of 54 g N ha⁻¹ yr⁻¹ in deposition (as ¹⁵ NH ₄ ¹⁵ NO ₃ or NH ₄ NO ₃).

Figure 2

δ ¹⁵N (%) of needles older than the 2013 cohort from ¹⁵N‐labelled deposition treatments (a) and ¹⁵N‐labelled litter treatments (b). CONTROL is shown on both plots (white circles); on (a), plot treatments are CNU (red circles) and SNU(orange circles); and on (b), plot treatments are ^LITTERCNU (dark blue triangles), ^LITTERSNU (light blue triangles) and ^LITTERC (grey triangles). Error bars show standard error of the mean (n = 5).

Figure 3

N content by dry mass (%) of needles from all treatments from 2013 cohort (a) and 2011–2012 cohort (b). While a yearly cycle is observed, this does not differ between treatments. Treatments are shown with same symbology as Fig. 2. Error bars show standard error of the mean (n = 5).

Figure 4

δ ¹⁵N (%) of 2013 needle cohort from ¹⁵N‐labelled deposition (a) and ¹⁵N‐labelled litter treatments (b). CONTROL is shown on both plots (white circles); on (a), treatments are CNU (red circles), and SNU (orange circles); and on (b), plot treatments are ^LITTERCNU (dark blue triangles), ^LITTERSNU (light blue triangles) and ^LITTERC (grey triangles). Error bars show standard error of the mean (n = 5).