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Phosphorus (P) species in colloidal and dissolved soil fractions may have different distributions. To understand which P species are potentially involved, we obtained water extracts from the surface soils of a gradient from Cambisol, Stagnic Cambisol to Stagnosol from temperate grassland in Germany. These were filtered to dissolved P (31P-nuclear magnetic resonance (NMR) spectroscopy. The total P of soil water extracts increased in the order Cambisol 43−/pyrophosphate were absent in nano-sized (1–20 nm) colloids from the Cambisol but not in the Stagnosol. The 31P-NMR results indicated that this was accompanied by elevated portions of organic P in the order Cambisol > Stagnic Cambisol > Stagnosol. Across all soil types, elevated proportions of inositol hexakisphosphate (IHP) species (e.g., myo-, scyllo- and D-chiro-IHP) were associated with soil mineral particles (i.e., bulk soil and small-sized soil colloids), whereas other orthophosphate monoesters and phosphonates were found in the dissolved P fraction. We conclude that P species composition varies among colloidal and dissolved soil fractions after characterization using advanced techniques, i.e., AF4 and NMR. Furthermore, stagnic properties affect P speciation and availability by potentially releasing dissolved inorganic and ester-bound P forms as well as nano-sized organic matter–Fe/Al–P colloids.

An increasing number of studies demonstrate the potential of Fe isotope ratios as a tool to identify soil formation processes, to pinpoint Fe translocation processes within soils and to determine the geologic source of Fe (1, 2). Fe isotope ratios of the suspended river load have recently been used to determine the geologic source of Fe in the Amazon river system (dos Santos Pinheiro et al., 2013) and the correlation of dissolved organic carbon (DOC)/[Fe] with δ56Fe of the dissolved load was identified as an indicator for authigenic Fe input (3).The Wüstebach catchment is a small forested subcatchment (~38.5 ha) located within the National Park Eifel at the southernmost outreach of the Rur catchment. It is part of the TERENO Eifel/Lower Rhine Valley Observatory (4) which aims at monitoring the long-term impacts of environmental change at a regional scale. The site is equipped with in situ sensors monitoring constantly a variety of parameters (5). At this catchment we aim to identify the magnitudes of different Fe stocks as well as sources and sinks of the Fe cycle. Evaluation of the Fe concentration data in the dataset paper of Gottselig et al. (6) revealed a stock of 33.6 t of plant available Fe (extracted with CaCl2/pentatic acid) in the top 30 cm of soil. The total stock was estimated to be approx. 2770 t (extracted by pressurized microwave digestion) in the top 30 cm of soil. The vegetation consisted mainly of Norway spruce (90%). The Fe stock contained in the vegetation was estimated to be approx. 1.6 t. Monitoring of the Fe concentration in the stream water over 5 years revealed that, depending on hydrological conditions, the catchment loses between 18 and 57 kg dissolved Fe per year. First Fe isotope measurements of environmental samples (stream water, soil horizons, spruce needles and beech leaves) revealed a range of δ56Fe values from -1.82 to 0.78 ‰ (values relative to IRMM-14). Vegetation samples represent the lightest reservoir with values of -0.61 ‰ (spruce needles) and -1.82 ‰ (beech leaves). In contrast, soil samples ranged from -1.63 to 0.61 ‰ showing the effects of litter recycling in the top soil and redox variations in the subsoil, respectively. The stream water therewith represents a sink of Fe from different sources and liberation processes.References:(1) Fekiacova Z., Pichat S., Cornu S., Balesdent J., 2013. Geoderma 209-210, 110-118.(2) Dos Santos Pinheiro, Poitrasson F., Sondag F., Cruz Vierira L., Martins Pimentel M., 2013. Journal of South American Earth Sciences, 44, 35-44.(3) Chen J-B., Busigny V., Gaillardet F., Louvat P., Wang Y-N., 2014. Geochimica et Cosmochimica Acta 128, 128-143.(4) Bogena, H., Kunkel, R., Pütz, T. et al., 2012. TERENO - Long-term monitoring network for terrestrial environmental research. Hydrol. Wasserbewirts. 56, 138-143.(5) Bogena, H.R., Bol, R., Borchard, N. et al., 2015. A terrestrial observatory approach to the integrated investigation of the effects of deforestation on water, energy, and matter fluxes. Sci China Earth Sci 58, 61-75.(6) Gottselig, N., Bol, R., Nischwitz, V., Vereecken, H., Amelung, W., Klumpp, E., 2014. Distribution of phosphorus-containing fine colloids and nanoparticles in stream water of a forest catchment. Vadose Zone J. 13.

Iron is one of the most abundant elements in soil and analysis of iron isotopes can be a powerful tool to elucidate the biogeochemical cycle of Fe in nature. Over the last two decades, studies of stable isotope variations of Fe have markedly increased, as there are multiple pedogenic processes which induce iron fractionation like continental weathering including dissolution, precipitation, biological processes and redox transformations. Up to date, there is only a limited number of studies that have investigated iron isotope fractionation in soil and, except for one study on a paddy field, none of them are related to agriculture soils. We will present first-time data of the stable iron isotope compositions in different depths (down to 100 cm) of anthropogenically impacted agricultural fields. We were able to assess that long-term management practices like irrigation and fertilization changed the isotope composition of iron pools in soil. Under long-term irrigation we detected a depletion of light Fe isotopes in deeper soil horizons, while the plough horizons showed an accumulation in light Fe isotopes. By comparison, the non-irrigated plots displayed a relatively uniform isotope composition across all depths. Hence, Fe isotopes respond sensitively to management, and we may use these signatures also to trace the origin of Fe in plants. Current research is now running to clarify to which degree we may interprete our data as indicator for the long-term effect of a more active Fe uptake in irrigated field plots by the plants, or whether other processes like redox-induced discrimination processes mask the origin of Fe in plants. To highlight these issues, ongoing research focuses on assessing the fractionation during plant uptake, especially under different deficiency scenarios, as well as on the analyses of the plants.

Magnesium (Mg) is the fourth most abundant element in the earth and widely distributed in the silicate earth, hydrosphere and biosphere. Moreover, Mg is one of the macro essential elements for most plants and of great importance to biological functions. With the increasing use of non-traditional stable isotopes systems in recent years, isotope fractionations in 26Mg/24Mg have been observed in many geological and biological processes. Analyses of Mg isotopes could be an innovative and powerful tool to elucidate the biogeochemical cycle of Mg in natural ecosystems. However, to date most investigations of Mg isotope fractionation are centered in geologic studies. Studies of Mg isotope fractionation in agriculture soil-plant systems have so far not been reported. In this study, we present for the first time Mg isotope signatures in soil profiles (down to 100 cm) from several anthropogenically impacted arable fields in Germany. The effect of long-term soil managements like liming and fertilization were assessed by analysing the isotope compositions of Mg pools in soil. The soil δ26Mg compositions showed that the isotopic ratio 26Mg/24Mg increased along with soil profile, indicating a depletion of heavier Mg in the topsoil while compared with heavier Mg accumulation in the subsoil. Magnesium in the no-liming fields was found isotopically lighter than that in liming fields across all depths, which highlighted a positive response to the soil managements. Thus, the isotope signature could potentially yield significant information on elemental transports in soils and help to clarify important nutrients uptake processes by crops. We aim to explore that to what extent the Mg isotope compositions in soil-plant systems could be affected by plants as well as some other long-term soil managements, through which we could illuminate these Mg isotope data as an indicator of nutrient use efficiency by plants from different soil depth. Therefore, Mg isotope fractionations that occurred during the whole process including nutrient uptake and transport inside plant under various soil conditions would be explored.

Magnesium (Mg) is the fourth most abundant element in the earth and widely distributed in the silicate earth, hydrosphere and biosphere. Moreover, Mg is one of the macro essential elements for most plants and of great importance to biological functions. With the increasing use of non-traditional stable isotopes systems in recent years, isotope fractionations in 26Mg/24Mg have been observed in many geological and biological processes. Analyses of Mg isotopes could be an innovative and powerful tool to elucidate the biogeochemical cycle of Mg in natural ecosystems. However, to date most investigations of Mg isotope fractionation are centered in geologic studies. Studies of Mg isotope fractionation in agriculture soil-plant systems have so far not been reported. In this study, we present for the first time Mg isotope signatures in soil profiles from several anthropogenically impacted arable fields in Germany. The effect of long-term soil managements like liming and fertilization were assessed by analysing the isotope compositions of Mg in soil. The soil δ26Mg compositions showed that the isotopic ratio 26Mg/24Mg increased along with soil profile, indicating a depletion of heavier Mg in the topsoil while compared with heavier Mg accumulation in the subsoil. Magnesium in the no-liming fields was found isotopically lighter than that in liming fields across all depths, which highlighted positive response to the soil managements. Thus, the isotope signature could potentially yield significant information on elemental transports in soils and help to clarify important nutrients uptake processes by crops. We aim to explore that to what extent the Mg isotope compositions in soil-plant systems could be affected by plants as well as some other long-term soil managements, through which we could illuminate these Mg isotope data as an indicator of nutrient use efficiency by plants from different soil depth. Therefore, Mg isotope fractionations that occurred during the whole process including nutrient uptake and transport inside plant under various soil conditions would be explored.

Iron is one of the most abundant elements in soil and analysis of iron isotopes can be a powerful tool to elucidate the biogeochemical cycle of Fe in nature. Over the last two decades, studies of stable isotope variations of Fe have markedly increased, as there are multiple pedogenic processes which induce iron fractionation like continental weathering including dissolution, precipitation, biological processes and redox transformations. Up to date, there is only a limited number of studies that have investigated iron isotope fractionation in soil and, except for one study on a paddy field, none of them are related to agriculture soils. We present first-time data of the stable iron isotope compositions in different depths (down to 100 cm) of agricultural fields. Under long-term irrigation we detected a depletion of light Fe isotopes in deeper soil horizons. By comparison, the non-irrigated plots displayed a relatively uniform isotope composition across all depths. After 50 years of irrigation, significant Fe losses could be detected in the irrigated plots. The Fe isotopes composition of the plant available Fe is also investigated. In all investigated layers, the irrigated plots displayed a greater plant available fraction (9 - 23 % total Fe) than their non-irrigated counter-parts (8 – 17 % total Fe). We will compare the isotope composition of plant available Fe to that of the strategy II plant Triticum aestivum L. grown on this field. We will hence present a comprehensive overview of the Fe isotope fractionation in a lomg-term irrigated agricultural soil-plant system.

Elemental contents in catchment headwaters are indicative of the load of nutrients and minerals cycled or released from ecosystems, yet little is known about natural colloids (1–1000 nm) and especially natural nanoparticles (NNP, 1–100 nm) as nutrient carriers in forested headwater streams. We hypothesize that the majority of P is bound to NNP in forest streams but that their size and composition varies for different forested headwater systems. Four forested sites in Germany and one in Norway, which differ in total P content, were sampled for stream water and analyzed for colloids. The samples were fractionated using field flow fractionation coupled to inductively coupled plasma–mass spectrometry and an organic C detector. The results showed that NNP and colloids from all sites could be separated into three distinct fractions (approximately 1–20 nm, >20–60 nm, and >60 nm). The elemental concentrations of P, organic C, Al, Si, Fe, and Mn in the fractions differed among the five sites. However, cluster analysis showed that each fraction had unique elemental signatures with different preferential P binding partners. Phosphorus was preferentially associated with Fe in the smallest size fraction, with an increasing contribution of organic-C-associated P as the fraction size increased. The largest fraction was dominated by clay minerals. Also, the data indicated that the relative contribution of the NNP and colloidal fractions for ecosystem nutrient supply rises as total P concentrations decline. The study highlighted the still underestimated importance of NNP for matter transport in forest streams and thus P cycling. This is an open access article under the CC BY-NC-ND license. Proper attribution is required for reuse. No permissions are needed for reuse unless it is derivative or for commercial purposes.

As an application-oriented part of the AlgalFertilizer Project, we cultivated Chlorella vulgaris, IPPAS C1 in 3.6 m3 suspension in V-shaped plastic bag photobioreactors (NovaGreen GmbH) in Jülich, Germany. The culture was grown in batch regime in a P-rich medium. The thus produced algal biomass was analyzed for nutrient content with special regard to phosphorus and used as fertilizer to grow wheat in a pot experiment under controlled phosphorus levels on different substrates: sand and nutrient-poor ‘Null-Erde’ for controlled nutrient levels as well as a nutrient-rich substrate (Dachstaudensubstrat, SoMi 513). The algal biomass was mixed into the soil substrates either in form of spray-dried powder or as a suspension of fresh cells obtained by centrifugation to estimate P-exchange properties of pre-treated and fresh biomass. Aiming at comparison to standard Hoagland mineral fertilizer, we applied algae at two different P-levels: high, corresponding to 45 kg(P)/ha and low, corresponding to 4.5 kg(P)/ha. 150 pots were prepared for the experiment. Each experimental variant was represented by 10 pots of identical substrate and fertilization treatment. One pre-germinated seed of wheat (Triticum aestivum, var. Scirocco) was placed in each pot. The plants were placed in a greenhouse on a robotic platform that randomized position of individual pots and also ensured uniform watering. Individual plants were moved regularly with their pots into an imaging system that served to quantify the plant growth. We also analyzed the dynamics of nutrients in the soil. The results of the AlgalFertilizer experiment lead us to conclude that microalgae are not only very potent in sequestering phosphorus from nutrient rich streams (Part 1 of this presentation) but that algal biomass can be used as a very effective fertilizer and ameliorant of nutrient-poor soils (Part 2 of this presentation).

The contents and properties of soil organic phosphorus (Po) largely drive ecosystem productivity with increasing development of natural soil. We hypothesized that soil Po would initially increase with paddy management and then would persist under steady-state conditions. We analyzed soils from a 2000-year chronosequence of a rice-wheat rotation and an adjacent non-paddy 700-year chronosequence in Bay of Hangzhou (China) for their Po composition using solution 31P-NMR after NaOH-EDTA extraction. Land reclamation promoted Po accumulation in both paddy and non-paddy topsoils (depths ≤ 18 cm) until steady-state equilibria were reached within 200 years of land use. Greater Po concentrations were found, however, in the non-paddy subsoils than in those under paddy management. Apparently, the formation of a dense paddy plough pan hindered long-term Po accumulation in the paddy subsoil. The surface soils showed higher proportions of orthophosphate diesters under paddy than under non-paddy management, likely reflecting suppressed decomposition of crop residues despite elevated microbial P compounds stocks under anaerobic paddy-rice management. Intriguingly, the composition of Po was remarkably stable after 194-years of paddy management and 144-years of non-paddy management, suggesting novel steady-state equilibria of P dynamics had been reached in these man-made ecosystems after less than two centuries.

In sparsely rooted subsoils secondary minerals, such as iron and aluminum oxides are predominant sorption sites for phosphorus (P) and thus important sinks for P fertilizers. P supply from these oxides can contribute to effective strategies for a sustainable future use of P fertilizers. Here we are going to combine (1) P radio-isotope tracing and imaging techniques, (2) diffuse gradient in thin films technology (DGT), and (3) magnetic resonance imaging (MRI) with the objective to assess the P accessibility and use efficiency from stable P reservoirs.