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This Special Issue presents a collection of papers commissioned to celebrate the UK Centre for Doctoral Training (CDT) in soil science known as 'STARS' (Soils Training And Research Studentships). The collection was written by emerging scientists and their collaborating supervisory teams. The call for papers was principally aimed at STARS students, but submissions were also encouraged from students in the wider UK soils community, who are also represented. The STARS CDT was originally commissioned in 2015 as part of a funding collaboration between the UK Natural Environment Research Council and the UK Biotechnology and Biological Science Research Council (BBSRC). The paper by Haygarth et al. (2021) provides a reflection on the STARS CDT experience, exploring what can be learnt from the new pedagogic approach. It describes how a discipline-focussed doctoral centre is a novel and unique way of teaching and learning soil science, proposing that this might be a model for future post graduate soil science teachers to learn from, in order to continually improve and innovate in the way we train our PhD students. In total, the Special Issue contains 17 papers, of which 15 appear in hard copy (incidentally, the last hard copy issue of the European Journal of Soil Science before conversion to solely on-line publication). All but one of the papers (Haygarth et al., 2021) are student-led, with three by non-STARS students (Jonah Prout, Yan Ma and Melanie Armbruster). The papers loosely fall into three categories: (1) Those involving local-scale controlled experiments and method development (seven papers), (2) those focussed on empirical analyses of large-scale datasets (six papers) and (3) those that involve a meta-analyses of previously published literature (three papers). In the first category, Chris McCloskey contributes two papers as lead author. In the first paper (McCloskey et al., 2020), a field system is presented for measuring plant and soil carbon fluxes using stable isotope methods, with sufficient precision to resolve diurnal and seasonal patterns. In the companion paper, the field system is used to demonstrate the importance of allowing for transient variation in plant and soil δ13C end members in partitioning fluxes from net ecosystem respiration (McCloskey et al., 2021). Ma et al. (2021) assessed the relative efficacy of nitrification inhibitors in a highly nitrifying soil and, like McCloskey, used carbon-labelling techniques to help achieve this. Dan Evans takes us to the core of soil formation, using a new technique of cosmogenic radionuclide analysis (Evans et al., 2021) with a conclusion that questions the accuracy of our existing soil formation knowledge, arguing that we must consider the bulk density profile of the overlying soil. At a slightly larger scale, 'Bee' Burak et al. (2020) uses an inductive mesocosm-based assessment to study how root hairs affect soil erosion by simulated rainfall. In similar controlled conditions, Corina Lees et al. (2020) used a growth room to control her climate change study in selecting plant traits for soil erosion control in grassed waterways. Marta Cattin et al. (2021) also uses controlled conditions with a 21-day laboratory microcosm incubation in order to assess the fate of soil carbon following the application of anaerobic digestate. In the second category, Fiona Seaton et al. (2020) uses an empirical data-driven analysis based on national (Welsh) monitoring of soil indicators to reflect on soil health, using large-scale data analyses (over 1350 topsoils) to understand the state and change of soils at a national scale. Her work shows the importance of land-use management in determining the soil health and functional capacity of soils. In an empirical approach not dissimilar, Armbruster et al. (2020) has a focus on bacterial and archaeal taxa as potential indicators of soil restoration across grasslands. The work highlights that microbial taxon are among the most sensitive indicators of soil restoration. Paul George et al. (2020) studied anaerobes and sulphate-reducing bacteria in relation to pH across varied land uses, using a nationwide 'metabarcoding' dataset from 436 sites belonging to seven contrasting temperate land uses. Andrew Tweedie et al. (2021) tested the hypothesis that phosphorus forms and functions in agricultural soils have changed over a period of 50–70 years, using topsoils from 35 agricultural sites in Northeast Scotland, compared at 'original' and 'resampled' timepoints. The paper by Hannah Cooper (2021) uses a broadly similar 'long-term' sampling approach and considers how long-term zero-tillage enhances the protection of soil carbon in tropical agriculture, studying soil samples collected from experimental fields in Botucatu, Brazil, which had been under zero-tillage for 2, 15 and 31 years. The paper by Prout et al. (2020) focusses on soil organic matter and proposes an index approach based on organic carbon-to-clay ratio. Again, like the above papers this work was empirically based, using 3809 sites from the National Soil Inventory of England and Wales. In the third category, Harry Barrat studied the impact of drought and rewetting on nitrous oxide emissions from soil in temperate and Mediterranean climates (Barrat et al., 2020). The method used the first meta-analysis and synthesis of the literature. Anchen Kehler et al. (2021) also applied a literature-based approach to her work, trying to predict how soil phosphorus will react to climate change. Finally in a wonderful collaborative article between the STARS students, Mihai Cimpoiasu et al. (2021) was first author on a reflective consideration of future priorities for soil science: 'Comparing perspectives from scientists and stakeholders'. Soil science has never seemed so topical, providing services underpinning our existence (Haygarth & Ritz, 2009) while addressing sustainable development and global grand challenges (Lal et al., 2021). It is thus timely to see this collection led by early career scientists on Innovations in Soil Science to Address Global Grand Challenges. Moreover, it is encouraging, not only because of the diversity of research findings themselves, but because of the promise it shows for our future capacity to deliver the discipline. Data sharing is not applicable to this article as no new data were created or analyzed in this study.
Background Monitoring the properties of dissolved organic carbon (DOC) in soil water is frequently used to evaluate changes in soil quality and to explain shifts in freshwater ecosystem functioning. Methods Using >700 individual soils (0–15 cm) collected from a 209,331 km2 area we evaluated the relationship between soil classification (7 major soil types) or vegetation cover (8 dominant classes, e.g. cropland, grassland, forest) and the absorbance properties (254 and 400 nm), DOC quantity and quality (SUVA, total soluble phenolics) of soil water. Results Overall, a good correlation (r2 = 0.58) was apparent between soil water absorbance and DOC concentration across the diverse range of soil types tested. In contrast, both DOC and the absorbance properties of soil water provided a poor predictor of SUVA or soluble phenolics which we used as a measure of humic substance concentration. Significant overlap in the measured ranges for UV absorbance, DOC, phenolic content and especially SUVA of soil water were apparent between the 8 vegetation and 7 soil classes. A number of significant differences, however, were apparent within these populations with total soluble phenolics giving the greatest statistical separation between both soil and vegetation groups. Conclusions We conclude that the quality of DOC rather than its quantity provides a more useful measure of soil quality in large scale surveys.
Low molecular weight organic acids such as citrate and oxalate have been hypothesized to play a key role in rhizosphere ecology and pedogenesis. A mathematical site-specific model, DYNLOW, was constructed to describe the temporal and spatial dynamics of these organic acids in coniferous forest soils using the modelling software STELLA®. Experimentally derived values for biodegradation, adsorption, and daily values of soil temperature, moisture and hydrological flow were used to parameterize the model. The model describes the dynamics and downward movement of oxalate and citrate through the horizons (O, AE, E, Bhs, Bs) of three podzolic soil profiles in Sweden. After calibration, the model predicted average soil solution organic acid concentrations ranging from <1 to 90μM, which was in agreement with experimental measurements (<1 to 116μM). The model results indicated that microbial degradation of organic acids was in quantitative terms the biggest process regulating soil solution concentrations. Primary production rates of organic acid in the soil were predicted to be high (<1 to 1250nmolg−1 soil d−1) in comparison to the amount present at steady state in the soil solution pool (<0.1 to 240nmolg−1 soil). The downward transfer of organic acids between soil horizons due to mass flow was predicted to be a small flux (<0.1 to 3% of the total loss) compared to that lost by microbial biodegradation. The model predicted that the amount of basal soil respiration that could be attributable to the microbial turnover of organic acids was on average 19±22% of the basal CO2 production across all sites and horizons for citrate and 7±7% for oxalate. The model results are discussed in the context of pedogenesis, forest soil respiration and organic matter production.
Proteins represent a significant reservoir of organic nitrogen in most terrestrial ecosystems and therefore comprise a key component of the soil N cycle. Consequently, there is a critical need to develop robust methodologies for quantifying the abundance of proteins in soil. In this study we evaluated the performance of five commercially available total protein assay kits in a contrasting range of soils. All the kits were based on the detection of proteins after conjugation with either chromophores or fluorophores. Overall, we found that all the kits suffered significant signal interference from humic substances present in solution, resulting in either a quenching or enhancement of the protein response. Inter-comparison of the kits yielded no agreement when quantifying the total amount of protein present in soil solution, with differences in concentration for individual samples ranging 20–500-fold when using the different assay kits. We concluded that none of the commercial assay kits can provide a reliable indicator of soil solution protein content. Although it may detect some non-proteinaceous material (e.g. peptidoglycan), total protein quantitation is currently best undertaken by acid hydrolysis of proteins in solution with subsequent determination of the amino acids liberated. However, there is an imperative need to develop robust methods for protein extraction, purification and analysis in soil.
The external morphology of Acremoniella atra, as observed in a scanning electron microscope, is described and compared with that of a related soil fungus, Acremoniella velata.
Summary The loss of carbon from roots (rhizodeposition) and the consequent proliferation of microorganisms in the surrounding soil, coupled with the physical presence of a root and processes associated with nutrient uptake, gives rise to a unique zone of soil called the rhizosphere. In this review, we bring together evidence to show that roots can directly regulate most aspects of rhizosphere C flow either by regulating the exudation process itself or by directly regulating the recapture of exudates from soil. Root exudates have been hypothesized to be involved in the enhanced mobilization and acquisition of many nutrients from soil or the external detoxification of metals. With few exceptions, there is little mechanistic evidence from soil‐based systems to support these propositions. We conclude that much more integrated work in realistic systems is required to quantify the functional significance of these processes in the field. We need to further unravel the complexities of the rhizosphere in order to fully engage with key scientific ideas such as the development of sustainable agricultural systems and the response of ecosystems to climate change. Contents I. Introduction 460 II. What is rhizodeposition? 460 III. Regulation of rhizodeposition 460 IV. How large is the root exudation C flux? 463 V. How responsive is the root exudation C flux? 463 VI. How responsive is the microbial community to root exudation? 464 VII. The role of root exudates in nutrient acquisition 464 VIII. Mycorrhizal fungi and rhizodeposition 471 IX. Future thoughts 474 Acknowledgements 474 References 474
