Abstract
Studies on the effect of high atmospheric N deposition report inconsistent results on forest productivity and N cycling which might be related to P availability in soil and subsequently affect tree P nutrition. We wanted to test the effects of (i) site i.e., a P-poor versus a P-rich site and of (ii) fertilization (N, P, N+P) on inorganic P (Pi) and organic P (Po) concentrations as well as on biologically cycled phosphate (inferred from the O isotope signature after adding an 18O-enriched label) in xylem sap. We measured Pi and Po concentrations and the O isotope signature in phosphate (δ18OPi) in xylem sap of beech (Fagus sylvatica L.) trees two and 14 days after addition of 18O-enriched water to the organic layer in a full factorial fertilization experiment (control, +N, +P, +NP) at two sites differing in P availability. Higher P concentrations in xylem sap at the P-rich than at the P-poor site originated from accelerated biological P cycling indicated by incorporation of 18O from the isotope label into phosphate in xylem sap shortly after labeling. At this site, δ18OW values of xylem sap after label application remained close to background δ18OW values of soil solution. We speculate that in contrast to P uptake, trees took up water from deeper (non 18O-labeled) soil layers. At the P-poor site, the 18O label was recovered both in xylem sap water and phosphate in xylem sap, the latter only after 14 days. These results imply that trees relied on the organic layer for P acquisition and water uptake. However, biological processes associated with an incorporation of 18O from the label were slower at the P-poor than at the P-rich site. P addition (P, NP) increased Pi concentrations in xylem sap at the P-rich site. Based on δ18OPi values in xylem sap, the additional P originated both from the fertilizer and from accelerated biological P cycling in soil. We conclude that P-poor sites likely suffer more from climate change in case of an increased frequency of droughts because as opposed to P-rich sites both water and nutrient uptake will be affected.
Highlights
Phosphorus (P) is an essential agent in a variety of vital processes like the build-up of DNA, RNA, and cell membranes, energy transfer via free nucleotides and carbon metabolism
We found Pi concentrations of 1.1 ± standard error (SE) 0.4 mg P l−1 and Po concentrations of 3.4 ± SE 0.6 mg P l−1
The variability of P concentrations in xylem was high at both sites
Summary
Phosphorus (P) is an essential agent in a variety of vital processes like the build-up of DNA, RNA, and cell membranes, energy transfer via free nucleotides and carbon metabolism. Continuously high deposition of atmospheric N and increased CO2 concentrations due to climate change accelerate forest growth, resulting in enhanced P demands by trees (Jonard et al, 2015; Talkner et al, 2015). Gaudio et al (2015) describe N limitation in forests to become crucial under climate scenarios where the turnover rate of organic matter is slow and N deposition continues to decrease. Other nutrients such as P could become limiting in the future or are already limiting in nutrient−poor ecosystems (Jonard et al, 2012). Missing effects of N fertilization might be due to the fact that elements other than N limit forest growth
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
