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Predicting the impact of land use and climate change on the Earth system hinges on credible representation of soil hydrological processes (SHP), adequate availability of parameters and hydrological states and inclusion of key soil properties. There is increasing evidence that extreme events such as droughts and high intensity precipitation, and land use changes, affect fundamental hydrological processes such as infiltration and runoff generation. In this review, we analyse the influence of soil structure on SHP, critically evaluate the parameterization of soil hydrologic properties and their importance in representing the terrestrial water cycle and highlight the key role of soil hydrology in the functioning of carbon-rich soils and in linking the water and carbon cycles. It emerges that linking soil hydrology and pedology will lead to better understanding critical zone processes, especially in tropical regions. Further, we discuss the role of local scale hydrological processes in understanding root water uptake, vegetation and groundwater dynamics and feedbacks. These processes control and modulate the impact of extreme events such as droughts, floods and heatwaves and they are essential to assess drought and flooding. Finally, new emerging technologies such as wireless and automated sensing approaches, soil moisture observation through novel synthetic aperture radars satellites, big data analysis and machine learning approaches offer unique opportunities to advance soil hydrology.

Iron (Fe) is a key element with multiple interaction mechanisms in the cycles of all three major nutrients (carbon, nitrogen and phosphorus). As large deforestation actions are expected to influence ecosystem nutrient cycling, we aimed to determine the impact of the clear-cut on Fe reservoirs and cycling. We further hypothesized that the main source of riverine Fe would change with seasons in the Wüstebach catchment as two distinct hydrological regimes prevailed in summer and winter. We determined Fe content and isotopic composition of different compartments (soil, plant, water) in order to: (i) estimate Fe pool sizes in the catchment and (ii) identify the main seasonal sources of the riverine Fe.

Nitrous oxide (N2O) emission is a negative side effect of modern agriculture and a serious issue for global climate change. The combined application of nitrogen (N) fertilizer and high organic carbon soil amendments (HCA) has been regarded as an alternative to promote fertilizer-related N immobilization and enhance nitrogen use efficiency. The effect of HCA on N2O emission and N immobilization highly depends on its chemical composition, as it controls carbon (C) supply to soil microbes and reactivity of lignin-derived phenols to fertilizer-derived N species. Here we present a 127-d laboratory incubation study to explore the N2O emission and N immobilization after combined application of N fertilizer and HCA (wheat straw, spruce sawdust, and commercial alkali lignin) differing in their chemical composition. The 15N labeling technique was used to trace the transformation of fertilizer-N in ammonium (NH4+), nitrate (NO3–), soil organic nitrogen (SON), and N2O. The amendment of wheat straw and spruce sawdust greatly promoted N immobilization and N2O emission, while lignin amendment enhanced the immobilization of fertilizer N. The chemical composition of HCA explained 26% of the total variance of fertilizer-derived N2O emission and N retention via soil microbial biomass, composition of lignin-derived phenols, and nitrification. The holocellulose/lignin ratio of HCA could be used as an indicator for predicting HCA decomposition, microbial N immobilization and N2O emission. In addition, the composition of lignin-derived phenols was affected by HCA amendment and significantly related to N2O emission and N retention. The varying chemical composition of HCA could thus be a promising tool for controlling N2O emission and N immobilization in environment-friendly and climate-smart agriculture.

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., Biogeochemistry, 150 (3), ISSN:0168-2563, ISSN:1573-515X

The cycling of iron (Fe) is often closely linked with that of carbon, nitrogen, phosphorus and manganese. Therefore, alterations in the Fe cycle may be indicative of concurrent overall changes in the biogeochemistry of terrestrial and aquatic ecosystems. Biogeochemical processes taking part in the Fe cycle frequently fractionate stable Fe isotopes, leaving soil, plant and other compartments of the ecosystems with varied Fe isotopic signatures. In this work, we reviewed the Fe isotope fractionation processes that have been reported so far for terrestrial ecosystems. While parent materials vary in Fe isotope compositions, pedogenic processes can further fractionate Fe isotope signatures, resulting in soil profiles with δ56Fe values (relative to isotope standard IRMM-014) from -0.52 to +0.72‰. Different soil Fe pools, as a result of cycling processes, show an even broader range of δ56Fe values, with secondary Fe oxides being isotopically the lightest, and with Fe sequestered in silicate minerals being the heaviest, due to preferential release of light Fe isotopes during dissolution of minerals. Actually and potentially plant-available Fe in soil can be extracted by 0.5 M HCl, which includes pools of water-extractable and exchangeable Fe, organically bound or adsorbed Fe, and poorly crystalline Fe oxides, altogether showing a depletion of heavy Fe isotopes with δ56Fe values down to -1.08‰. Depending on the Fe speciation and concentration present in the growth medium, plants can adapt their uptake strategy for Fe. Plants of the strategy I type especially take up light iron isotopes, while strategy II plants fractionate less towards light isotopes. Aboveground tissues usually show lighter Fe isotope signatures than the roots, with flowers (δ56Fe: -2.15 to -0.23‰) being isotopically the lightest. In freshwater systems, the most distinct Fe isotope fractionation is usually found at the oxic-anoxic interface, where redox conditions change and thus Fe speciation controls the degree of Fe isotope fractionation. Similar to soils, the δ56Fe values of unfiltered water mainly reflect averaged Fe isotope compositions across fractions with different particle sizes. In filtered freshwater (

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.