Abstract:
The main objectives of my thesis were to assess the importance of biological processes controlling the P cycle in forest ecosystems along a gradient of decreasing soil biological activity and soil P availability using multi-isotopic approaches (33P, δ18OPi, δ18OW) and to investigate the importance of soil organic matter turnover on P cycling under environmental change. Particularly, the research aims were to answer whether (i) the fate of P in a tree sapling-soil system differs between nutrient-poor and nutrient-rich sites, (ii) what influence the organic layers have on the fate of P in a tree sapling-soil system at either site, (iii) if purification procedures from soil used to determine the stable isotope composition of oxygen (O) of inorganic phosphate (δ18OPi) are adequately transferable to measure xylem sap δ18OPi and (iv) if the effect of site (BBR, LUE) i.e., a P rich versus a P poor forest ecosystem, and of fertilization (N, P, N+P) do alter Pi and Po concentrations as well as biologically cycled phosphate (inferred from the O isotope signature after adding an 18O label) in xylem sap.
We could show that (i) the mobilization of P likely mediated by the microbial community followed by efficient uptake in organic layers under nutrient-poor conditions (LUE) supported tree sapling growth while at the same time reducing the leaching of P to underlying mineral soil horizons. In contrast, the presence of organic layers did not significantly influence P uptake by beech saplings under nutrient-rich conditions (BBR). (ii) via DAX 8 we were able to remove Po before measuring δ18OPi. (iii) We did not see fertilization effects reflected in xylem sap δ18OPi either between the sites (BBR, LUE), nor the fertilization treatments (N;P; NP, Con). However, our investigations have shown that the 18O labeling approach pinpoints a decoupling of water and P uptake at the P-rich site BBR representing an acquiring ecosystem. By contrast, trees relied on the organic layer for both water and P uptake at the P-poor site LUE representing a recycling ecosystem.
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Such findings indicate the importance of the organic layers for P nutrition as well as water supply for beech trees at nutrient poor ecosystems. Since the distribution of most forest ecosystems in Europe is constrained to nutrient-poor soils, sustainable forest management should consider the formation and functioning of the forest floor. Foresters should consider that ecosystem resilience particularly at P-poor ecosystems might be strongly influenced by processes that deteriorate the organic matter and subsequently changes the forest floor structure and thereby its function as a place of water retention and nutrient retention from leaching to mineral soil layers. In this regard, ongoing N deposition and increasing soil temperatures especially in the forest floor due to climate change have the potential to considerably change forest floor turnover dynamics. My improved method of extracting xylem sap and subsequent silverphosphate purification for δ18OPi analysis in xylem sap enables new fields of application and was applied successfully in two different in situ experiments.
oxygen istopes