Processes and Fluxes of Trace Metals in Agricultural Soil-Fertilizer-Plant Systems Investigated with Isotope Applications
Embargoed until 2019-10-19
- Doctoral Thesis
Abstract Trace elements occur naturally in soils but are also introduced into soils through human activities. In agroecosystems, fertilizers are applied to soils to promote plant growth. However, as a side effect, trace metals such as cadmium (Cd), copper (Cu), and zinc (Zn) can be introduced into these systems with such fertilizers. Cd is a toxic element for humans and is added to arable soils with P-fertilizer applications. Although Cd is toxic for plants, they take up Cd which can be then transported into edible parts such as grains and enter the food chain. Cu and Zn are essential micro nutrients for all living organisms, but become toxic when they are present in excess. Farmers often feed their piglets with these micro-nutrients to prevent health disorders. This practice leads to an enrichment of Cu and Zn in pig manure. The application of pig manure on the field can cause an accumulation of Cu and Zn in grassland soils and threatens soil fertility. The fate of Cd, Cu, and Zn that are applied with fertilizers into agricultural soils and the biogeochemical processes that distribute these metals in soil pools and plant parts need to be understood well in order to maintain soil fertility and crop quality. This PhD-thesis aimed to investigate biogeochemical processes of Cd and Zn in soil-plant systems and pathways of Cd, Cu, and Zn soil-fertilizer-plant systems that are representative for Swiss agriculture. For that purpose, stable isotope and radioisotope approaches were applied in pot studies under controlled conditions to improve the understanding of the processes and the fate of these elements in agricultural systems. In the first Chapter, the pathway of Cd from soil to grain of wheat was studied by measuring Cd concentrations and Cd isotope ratios in soil-wheat systems. It was shown that most of the Cd was retained in root and shoots (20 to 54 %) and only a minor part of the Cd taken up in the plant reached the grains (13 to 31 %). The Cd isotopes were markedly fractionated within the different plant parts: straw was isotopically heavier than roots (Δ114/110Cdstraw−root = 0.21 to 0.41 ‰), and grains were heavier than straw (Δ114/110Cdgrain−straw = 0.10 to 0.51 ‰). We ascribed the enrichment of heavy isotopes in the wheat grains to mechanisms that avoid the accumulation of Cd into grains, such as the chelation of light Cd isotopes by thiol-containing peptides in roots and straw. In the second Chapter, we investigated the processes that transport the essential element Zn into wheat grains during the grain filling period. Furthermore, we aimed to identify the processes that discriminate Zn from the biotoxic element Cd. For that purpose, we investigated Zn isotope fractionation in the identical soil-plant systems that we used to study Cd isotope fractionation. To determine the Zn isotope fractionation during grain filling, we harvested wheat at flowering and at full maturity. During the grain filling period, Zn concentrations significantly decreased in straw (by -9 to -17 µg g-1) while a strong enrichment of heavy Zn isotopes occurred (Δ66Znfullmaturity-flowering = 0.21 to 0.31 ‰). Three quarters of the Zn in the wheat accumulated in the grains where Zn was enriched in light isotopes compared with the straw (Δ66Zngrain−straw = -0.21 to -0.31 ‰). In general, the light Zn isotopes accumulated in phloem sinks (grains, spikelets) while the heavy isotopes were retained in phloem sources (straw organs) which was ascribed to apoplastic retention and compartmentalization. The opposing isotope fractionation of Zn and Cd between straw and grain might be caused by the different affinities of Zn and Cd to oxygen (O), nitrogen (N) and sulfur (S) ligands. In the third Chapter, the fate of Cd applied with fertilizer was traced by measuring stable Cd isotope at natural abundance or by adding a radioactively labeled P-fertilizer (109Cd) to a soil-wheat system. The Cd stable isotope ratios at natural abundance could not be used to trace the Cd applied with fertilizer since the isotope ratios in the two Cd sources (soil, fertilizer) were not distinct enough. The 109Cd source-tracing experiment revealed that the Cd in the P-fertilizer contributed to 5 to 11 % to the total Cd in the wheat shoots. Furthermore, the fertilizer recovery in the wheat shoot was < 2%. Therefore, most of the Cd applied with fertilizer remains in the soils and might either be leached into the groundwater or accumulate in the soils. We suppose that a plant available residual fertilizer Cd pool has been built up during the past 50 years of P-fertilization that partly determines the Cd fluxes in the current soil-wheat systems. In the fourth Chapter, a stable isotope method was used to trace the fate of pig slurry derived Cu and Zn in soil-slurry-ryegrass systems. For that purpose, the isotopically exchangeable soil pools of Cu and Zn were labeled with stable isotopes of Cu and Zn. Moreover, consecutive tests were conducted to measure Cu and Zn isotopes in plants using High Resolution ICP-MS (HR-ICP-MS) without a time consuming sample purification. It revealed that the proportion of Cu and Zn derived from the fertilizer could be determined with a relative standard deviation (2RSD) of 2.4 %. Whether this precision is sufficient for the purpose of tracing Cu and Zn applied with fertilizer in soil-fertilizer-plant systems depends on three factors: on the labeling technique, on the extent of isotope enrichment of the labeled source, and on the expected range of Cudff and Zndff values. The source-tracing experiment showed that a single addition of Cu and Zn with pig slurry did not lead to a significant increase of Cu and Zn concentrations in ryegrass, suggesting that Cu and Zn either accumulate in the grassland soils or that these elements are washed into surface or groundwater. The isotope source-tracing failed because of an untimely addition of pig slurry to the soils labeled with 65Cu and 67Zn. Recommendations for future studies using enriched stable isotope approaches to trace Cu and Zn applied withorganic fertilizers are discussed. This PhD thesis demonstrated that isotope applications contribute to a better understanding of processes and fluxes of trace metals in agriculture. The implications of these results on Swiss agriculture and the research field of trace metals in agriculture were discussed. For future process tracing studies, we recommend (i) to complement experiments on a soil-plant level with experiments on a molecular scale to investigate the isotope fractionation between free Cd and Zn and complexed Cd and (ii) to combine Cd and Zn stable isotope analysis with Cd and Zn speciation data in order to improve the understanding of the role O, N, and S ligands of Cd and Zn regarding the separation of Cd and Zn in the roots and straw of cereals. For source-tracing studies, we recommend to (iii) enlarge the scale of observation from a soil-plant level to lysimeters, subplots in the field, and micro catchments and to (iv) investigate the long-term impact of trace metals applied with fertilizers to soils. For that purpose, isotope source-tracing can be applied on soils from long-term fertilization trials with distinct fertilization histories. Show more
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ContributorsExaminer: Frossard, E.
Examiner: Sarret, G.
Examiner: Wilcke, W.
Examiner: Bigalke, M.
SubjectCadmium, Zinc, Copper, Iosotopes, Isotope fractionation, soil, plant, wheat, ryegrass, fertilizer, manure
Organisational unit03427 - Frossard, Emmanuel / Frossard, Emmanuel
145195 - Stable metal isotopes as tools to assess enrichment and sources of trace metals in soils and crops to improve sustainability of agricultural systems (SNF)
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