Biogeochemical cycling of phosphorus in subsoils of temperate forest ecosystems

Andrei Rodionov,Federica Tamburini,Jens Kruse,Wulf Amelung,Sven Marhan,David Uhlig,Christian Von Sperber,Sara L Bauke,Friedhelm Von Blanckenburg,Friederike Lang,Hans Lewandowski,Margaux Simon,Carsten W Mueller,Carmen Hoeschen,Ellen Kandeler

doi:10.1007/s10533-020-00700-8

Abstract

Tree roots penetrate the soil to several meters depth, but the role of subsoils for the supply of nutrient elements such as phosphorus (P) to the trees is poorly understood. Here, we tested the hypothesis that increased P deficiency in the topsoil results in an increased microbial recycling of P from the forest subsoil. We sampled soils from four German temperate forest sites representing a gradient in total P stocks. We analyzed the oxygen isotopic composition of HCl-extractable phosphate (δ18OP) and identified differences in P speciation with increasing soil depth using X-ray absorption near-edge structure (XANES) spectroscopy. We further determined microbial oxygen demand with and without nutrient supply at different soil depths to analyse nutrient limitation of microbial growth and used nanoscale secondary ion mass spectrometry (NanoSIMS) to visualize spatial P gradients in the rhizosphere. We found that δ18OP values in the topsoil of all sites were close to the isotopic signal imparted by biological cycling when oxygen isotopes in phosphate are exchanged by enzymatic activity. However, with increasing soil depth and increasing HCl-P concentrations, δ18Ο values continuously decreased towards values expected for primary minerals in parent material at depths below 60 cm at sites with high subsoil P stocks and below more than 2 m at sites with low subsoil P stocks, respectively. For these depths, XANES spectra also indicated the presence of apatite. NanoSIMS images showed an enrichment of P in the rhizosphere in the topsoil of a site with high P stocks, while this P enrichment was absent at a site with low P stocks and in both subsoils. Addition of C, N and P alone or in combination revealed that microbial activity in subsoils of sites with low P stocks was mostly P limited, whereas sites with high P stocks indicated N limitation or N and P co-limitation. We conclude that subsoil P resources are recycled by trees and soil microorganisms. With continued weathering of the bedrock and mobilisation of P from the weathered rocks, P cycling will proceed to greater depths, especially at sites characterised by P limitation.

Highlights

In terrestrial ecosystems the availability of phosphorus (P) for plants is often limiting primary productivity (Gradowski and Thomas 2006; Elser et al 2007; Vitousek et al 2010)
The d18OP values of HCl-extractable P in the top 60 cm of all soil profiles were above or within the range expected for isotopic equilibrium mediated by intracellular pyrophosphatases (Fig. 1)
The d18OP values continuously decreased until a soil depth of about 2 m at the high P site Bad Bruckenau (BBR) and at the intermediate site VES, whereas at intermediate site MIT and low P site CON, the d18OP values remained within the range of isotopic equilibrium

Summary

Introduction

In terrestrial ecosystems the availability of phosphorus (P) for plants is often limiting primary productivity (Gradowski and Thomas 2006; Elser et al 2007; Vitousek et al 2010). For forests it has been suggested that at sufficient P supply in the soil the acquisition of P provided from chemical weathering of primary minerals may sustain tree growth (P acquiring systems; Lang et al 2016). At P deficient conditions, P supply from the mineral phase may not. Mueller Department of Geosciences and Natural Resource Management, Section for Geography, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen, Denmark. F. Tamburini Institute of Agricultural Sciences, ETH Zurich, Eschikon 33, 8315 Lindau, Switzerland

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