Phosphate oxygen isotope ratios in vegetated riparian buffer strip soils

Abstract

The oxygen isotopic composition of phosphate (δ 18 O P ) in soils and surface water bodies has been used to trace terrestrial P inputs into aquatic ecosystems. However, enhanced biological activity in vegetated riparian buffer strips (VBSs) may lead to an alteration of δ 18 O P values. The objective of this study was to assess whether enhanced biological cycling of P in VBS soils can be identified using δ 18 O P values. For this purpose, we sampled temperate grassland soils at various depths along a VBS to grassland transect. Here, we combined sequential P soil extracts with an analysis of δ 18 O P values. Soil P pool concentrations tended to decrease significantly along the transect from the VBS to the grassland soils; the strength of this relationship varied with P extract, sample depth, and inorganic or organic bonding form. For the δ 18 O P values of the 1 M HCl-extractable P we observed a significant negative trend along the VBS to grassland transect, indicating a tendency for accelerated rates of biological cycling of P within the VBS soil profile compared with the upslope soils. We conclude that oxygen isotope-based assessments of P source contributions to freshwater bodies should consider the enhanced biological turnover of P in VBS soils. INTRODUCTIONSurface waters worldwide receive excessive inputs of nutrients such as N and P from agricultural production and other human activities adjacent to rivers, lakes, and coastal estuaries. These nutrient inputs trigger eutrophication of waterAbbreviations: AIC, Akaike information criteria; IRMS, isotope ratio mass spectrometer; P i , inorganic phosphorus; P o , organic phosphorus; VBS, vegetated riparian buffer strip; δ 18 O P , standardized oxygen isotope ratio in phosphate.This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Oxygen isotope ratios of phosphates in the soil‐plant system: Limitations and future developments

“…Due to the high reaction rate, the complete oxygen exchange between phosphate and water and the large isotope fractionation (approx. 23‰ at 20°C), the activity of pyrophosphatase is often assumed to dominate the oxygen isotope composition of phosphate in environmental systems (Bauke et al, 2022; Helfenstein et al, 2018; Tamburini et al, 2012). Indeed, multiple studies have found one or more phosphate pools in the environment (soils, waters and plants) that carried the isotope equilibrium value.…”

“…All these studies concluded that the isotope equilibrium values in environmental phosphates can essentially be assigned to the activity of intracellular pyrophosphatases. However, there are frequently found deviations from oxygen isotope equilibrium, particularly in the soils (Bauke et al, 2022; Helfenstein et al, 2021; Pistocchi et al, 2020; Rodionov et al, 2020; Wang et al, 2021).…”

“…All these studies concluded that the isotope equilibrium values in environmental phosphates can essentially be assigned to the activity of intracellular pyrophosphatases. However, there are frequently found deviations from oxygen isotope equilibrium, particularly in the soils (Bauke et al, 2022;Helfenstein et al, 2021;Rodionov et al, 2020;Wang et al, 2021). The δ 18 O p values below the isotopic equilibrium could be attributed to the contribution of other endmembers or processes in the soil-plant system, for example, the hydrolysis of organic phosphate by extracellular phosphatases, geogenic sources of phosphates (Smith et al, 2021) or fertilizers (Granger et al, 2017;Tamburini et al, 2014).…”

Soil water status shapes nutrient cycling in agroecosystems from micrometer to landscape scales

“…Hydrological flow paths in riparian zones mix these nutritional elements with C leading to biogeochemical processes that either promote or impede the export of nutrients (Bauke et al., 2022; Hedin et al., 1998; Vidon et al., 2010). Having generally high but also strongly fluctuating water tables, redox processes at the microbial scale (see previous chapters) are especially critical in soils of riparian zones.…”

“…Due to the complete oxygen exchange, the high reaction rate, and the large isotope fractionation, the activity of pyrophosphatase is often assumed to dominate the oxygen isotope composition of phosphate in the soil–plant system (Bauke et al., 2021; Helfenstein et al., 2018; Tamburini et al., 2012). Hence, oxygen isotope exchange in phosphates can be used to locate hotspots of P cycling, for example, within different soil compartments (Bauke et al., 2017; Siebers et al., 2018), plant compartments (Bauke et al., 2021; Pfahler et al., 2013), or within catchments (Bauke et al., 2022; Pistocchi et al., 2017).…”