Distribution and retranslocation of 15N lodgepole pine over eight growing seasons

Abstract

We studied the distribution and retranslocation of N in 11-year-old Pinus contorta Dougl. trees following a winter application of N at 100 kg ha(-1) as (15)N-urea, (15)NH(4)NO(3) or NH(4) (15)NO(3). In all treatments, there was little uptake of (15)N after the first growing season although labeled N was still present in the soil. In subsequent years, (15)N in the trees was partly retranslocated, and, at the same time, it was diluted by uptake of unlabeled N from the soil. Between Years 1 and 8 after N fertilization, net retranslocation of (15)N from the lower crown (branches formed before fertilization) was 14%, and 18-25% of the (15)N in the trees was translocated to the upper and mid-crown. Overall, uptake of (15)N from nitrate was less than from urea or ammonium. However, when compared with the urea- and ammonium-N sources, (15)N from the nitrate source initially moved as rapidly into the foliage, but a greater proportion of it was retranslocated from the foliage during the second growing season. Nitrogen in foliage and wood formed in the growing season following fertilization was more highly labeled (measured as % N derived from the fertilizer) than in recently formed tissues. Labeling was substantially higher in foliage formed before fertilization than in wood of a similar age. In contrast, N in foliage formed after fertilization had only slightly higher labeling than wood of a similar age, indicating a relatively stable labeling throughout the trees once (15)N uptake had ceased. The concentrations of total and labeled N were substantially higher in foliage than in either wood or bark. There was evidence of N movement into wood tissues formed before fertilization, presumably along rays, and also of N retranslocation out of xylem cells as they matured. This study of internal N cycles was facilitated by the use of (15)N labeling because there was little uptake of labeled N after the first growing season, whereas interpretation based on total N was obscured by substantial uptake of N from the soil. We conclude that retranslocation studies based on measurements of total N content should be avoided.

Reconstructing terrestrial nutrient cycling using stable nitrogen isotopes in wood

“…Fertilizer and tracer studies recovered labeled N in rings formed prior to N application, indicating that trees redistribute N from recently formed rings into older rings via rays (Nommik 1966;Mead and Preston 1994;Schleppi et al 1999;Elhani et al 2003). In fact, studies have shown N mobility throughout all plant tissues over the course of numerous growing seasons, indicating that N redistribution is a continual process (Mead and Preston 1994) (Elhani et al 2003;Kwak et al 2009).…”

“…In fact, studies have shown N mobility throughout all plant tissues over the course of numerous growing seasons, indicating that N redistribution is a continual process (Mead and Preston 1994) (Elhani et al 2003;Kwak et al 2009). Under high pollutant inputs, the amount of extracted N was higher in recent rings, suggesting that increases in [N] in recent rings reflect increases in mobile N as opposed to structural N components (Doucet et al 2010).…”

“…Drake et al (2011) are the only authors to analyze extracted compounds, and reported that extracted sap exhibited δ 15 N values nearly 20 ‰ lower than extracted wood, although the mechanism for this difference was not determined. Considering the fact that N forms experience different translocation trajectories within the tree (Mead and Preston 1994), that trees exhibit speciesspecific preferences for certain forms of N over others (ex. (Templer and Dawson 2004), and that extraction can increase or decrease wood δ 15 N signatures (Drake et al 2011) A given extraction technique would need to meet three requirements before its use could be justified.…”

“…First, rings formed prior to N fertilization also show increased δ 15 N indicating translocation does occur (Elhani et al 2003, Hart andClassen 2003). Second, mass-balance studies indicate the majority of labeled-N uptake occurs in the first year following application (Mead and Preston 1994). Since the majority of 15 N-labeled compounds remain in the soil profile (Koopmans et al 1996, Nadelhoffer et al 1999a, Schleppi et al 1999 ranging from +6 to +13 ‰ (Heaton 1990) although subsequent atmospheric alterations may mask differences in point-source emission values under a broader deposition footprint (Freyer et al 1993).…”

Nutrient distribution in Picea likiangensis trees growing in a plantation in West Sichuan, Southwest China

“…On the other hand, nutrient translocation during the ageing of tissues is also an important mechanism for maintaining tree growth. Translocation of nutrients in foliage of trees during senescence has been intensively studied (Mead and Preston 1994, Zhang and Allen 1996, Saur et al 2000, Zas and Serrada 2003, but studies dealing with nutrient translocation in other tree compartments are relatively few.…”