Impacts of Fertilization on Biologically Cycled P in Xylem Sap of Fagus sylvatica L. Revealed by Means of the Oxygen Isotope Ratio in Phosphate

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 18 O-enriched label) in xylem sap. We measured Pi and Po concentrations and the O isotope signature in phosphate (δ 18 O Pi) in xylem sap of beech (Fagus sylvatica L.) trees two and 14 days after addition of 18 O-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 18 O from the isotope label into phosphate in xylem sap shortly after labeling. At this site, δ 18 O W values of xylem sap after label application remained close to background δ 18 O W values of soil solution. We speculate that in contrast to P uptake, trees took up water from deeper (non-18 O-labeled) soil layers. At the P-poor site, the 18 O 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 18 O 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 δ 18 O Pi 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.

Leaching of inorganic and organic phosphorus and nitrogen in contrasting beech forest soils – seasonal patterns and effects of fertilization

“…Laboratory fertilization experiments with mineral soils from the same sites indicated mostly microbial C and P limitation for the low-P soil, and N and N-P co-limitation for the high-P soil (Chen et al, 2019;Rodionov et al, 2020). However, a study on P concentrations in xylem sap on the fertilized plots likewise observed the antagonistic interaction on P concentrations in xylem after the +N+P treatment at the low-P site, 495 but a significant synergistic effect of the combined +N+P fertilization at the high-P site (Hauenstein et al, 2020), supporting the theory of a co-limitation of N and P at the low-P site.…”

“…They were replicated three times and arranged in blocks, resulting in a total of 12 plots per site (details on experimental set-up are given in e.g. Hauenstein et al, 2020). 5.0 g P m -2 were applied as KH2PO4 in 110 a single dose in 2016 (0.6 and 3.0% of total P stock at the high-P and the low-P site, Table 5).…”

“…Nevertheless, N leaching increased more than P leaching (Table 5 and 6). Most probably, a large fraction of added P was either rapidly sorbed to reactive minerals, taken up by plants (Hauenstein et al, 2020), or immobilized by soil microorganisms. Microbial 465 biomass in the Oe/Oa horizons at two study sites had very low microbial C:P ratios of 12 before the nutrient addition (Siegenthaler et al (2021, submitted).…”

Phosphorus fluxes in two contrasting forest soils along preferential pathways after experimental N and P additions

“…The nutrients were applied in dissolved form on the plots as NH 4 NO 3 (N and P?N plots) and as KH 2 PO 4 (P and P?N plots). The N and control plots were treated additionally with KCl dissolved in water to account for potassium (K) addition in the P and P?N plots (Hauenstein et al 2020).…”

“…As a result, P concentrations in seepage water and associated P translocation fluxes through the soil profile should be very low. For an input of additional P only, it can be assumed, that P transport in these soils will increase because the systems are not adapted to high P availability and therefore cannot rapidly integrate it into the tight recycling (Hauenstein et al 2020). Mesocosm experiments indicated that N addition affected downward translocation (first 20 cm of a soil column) stronger in a P-poor soil as compared to a P-rich soil (Holzmann et al 2016).…”