Controls on timescales of soil organic carbon persistence across sub-Saharan Africa.

Given the importance of soil for the global carbon cycle, it is essential to understand not only how much carbon soil stores but also how long this carbon persists. Previous studies have shown that the amount and age of soil carbon are strongly affected by the interaction of climate, vegetation, and mineralogy. However, these findings are primarily based on studies from temperate regions and from fine-scale studies, leaving large knowledge gaps for soils from understudied regions such as sub-Saharan Africa. In addition, there is a lack of data to validate modeled soil C dynamics at broad scales. Here, we present insights into organic carbon cycling, based on a new broad-scale radiocarbon and mineral dataset for sub-Saharan Africa. We found that in moderately weathered soils in seasonal climate zones with poorly crystalline and reactive clay minerals, organic carbon persists longer on average (topsoil: 201 ± 130 years; subsoil: 645 ± 385 years) than in highly weathered soils in humid regions (topsoil: 140 ± 46 years; subsoil: 454 ± 247 years) with less reactive minerals. Soils in arid climate zones (topsoil: 396 ± 339 years; subsoil: 963 ± 669 years) store organic carbon for periods more similar to those in seasonal climate zones, likely reflecting climatic constraints on weathering, carbon inputs and microbial decomposition. These insights into the timescales of organic carbon persistence in soils of sub-Saharan Africa suggest that a process-oriented grouping of soils based on pedo-climatic conditions may be useful to improve predictions of soil responses to climate change at broader scales.

The uptake of selenium by perennial ryegrass in soils of different organic matter contents receiving sheep excreta.

Background and aims: The intake of selenium, an essential element for animals and humans, in ruminants is largely determined by selenium concentration in ingested forages, which take up selenium mainly from soil. Ruminant excreta is a common source of organic fertilizer, which provides both nutrients and organic matter. This study aims to unentangle the unclear effect of applying different types of ruminant excreta in soils of different organic matter contents on selenium uptake by forage. Methods: Perennial ryegrass (Lolium perenne) was grown in soils of different organic matter contents. Urine and/or feces collected from sheep fed with organic or inorganic mineral supplements, including selenium, were applied to the soils. The selenium in the collected samples were analyzed using ICP-MS. The associated biogeochemical reactions were scrutinized by wet chemistry. Results: The application of urine and/or feces resulted in either the same or lower selenium concentrations in perennial ryegrass. The excreta type did not affect total selenium accumulation in grass grown in low organic matter soil, whereas in high organic matter soil, feces resulted in significantly lower total selenium accumulation than urine, which was attributed to a possible interaction of selenium sorption in soil and microbial reduction of Se. Conclusion: This one-time excreta application did not increase, but further decrease in some treatments, selenium concentration and accumulation in the perennial ryegrass. Consequently, to increase ruminant selenium intake, supplementing selenium directly to animals is more recommended than applying animal manure to soil, which might drive selenium reduction and decrease selenium uptake by grass. Supplementary Information: The online version contains supplementary material available at 10.1007/s11104-023-05898-8.

The effect of soil type on yield and micronutrient content of pasture species.

The use of multispecies swards on livestock farms is growing due to the wide range of benefits they bring, such as improved biomass yield and animal performance. Preferential uptake of micronutrients by some plant species means the inclusion of legumes and forbs in grass-dominated pasture swards could improve micronutrient provision to livestock via careful species selection. However, although soil properties affect plant micronutrient concentrations, it is unknown whether choosing 'best-performing' species, in terms of their micronutrient content, needs to be soil-specific or whether the recommendations can be more generic. To address this question, we carried out an experiment with 15 common grass, forb and legume species grown on four soils for five weeks in a controlled environment. The soils were chosen to have contrasting properties such as texture, organic matter content and micronutrient concentrations. The effect of soil pH was tested on two soils (pH 5.4 and 7.4) chosen to minimise other confounding variables. Yield was significantly affected by soil properties and there was a significant interaction with botanical group but not species within a botanical group (grass, forb or legume). There were differences between botanical groups and between species in both their micronutrient concentrations and total uptake. Micronutrient herbage concentrations often, but not always, reflected soil micronutrient concentrations. There were soil-botanical group interactions for micronutrient concentration and uptake by plants, but the interaction between plant species (within a botanical group) and soil was significant only for forbs, and predominantly occurred when considering micronutrient uptake rather than concentration. Generally, plants had higher yields and micronutrient contents at pH 5.4 than 7.4. Forbs tended to have higher concentrations of micronutrients than other botanical groups and the effect of soil on micronutrient uptake was only significant for forbs.

Countering elevated CO2 induced Fe and Zn reduction in Arabidopsis seeds.

Growth at increased concentrations of CO2 induces a reduction in seed zinc (Zn) and iron (Fe). Using Arabidopsis thaliana, we investigated whether this could be mitigated by reducing the elevated CO2 -induced decrease in transpiration. We used an infrared imaging-based screen to isolate mutants in At1g08080 that encodes ALPHA CARBONIC ANHYDRASE 7 (ACA7). aca7 mutant alleles display wild-type (WT) responses to abscisic acid (ABA) and light but are compromised in their response to elevated CO2 . ACA7 is expressed in guard cells. When aca7 mutants are grown at 1000 ppm CO2 they exhibit higher transpiration and higher seed Fe and Zn content than WT grown under the same conditions. Our data show that by increasing transpiration it is possible to partially mitigate the reduction in seed Fe and Zn content when Arabidopsis is grown at elevated CO2 .

Changes in organic carbon to clay ratios in different soils and land uses in England and Wales over time.

Realistic targets for soil organic carbon (SOC) concentrations are needed, accounting for differences between soils and land uses. We assess the use of SOC/clay ratio for this purpose by comparing changes over time in (a) the National Soil Inventory of England and Wales, first sampled in 1978-1983 and resampled in 1994-2003, and (b) two long-term experiments under ley-arable rotations on contrasting soils in the East of England. The results showed that normalising for clay concentration provides a more meaningful separation between land uses than changes in SOC alone. Almost half of arable soils in the NSI had degraded SOC/clay ratios (< 1/13), compared with just 5% of permanent grass and woodland soils. Soils with initially large SOC/clay ratios (≥ 1/8) were prone to greater losses of SOC between the two NSI samplings than those with smaller ratios. The results suggest realistic long-term targets for SOC/clay in arable, ley grass, permanent grass and woodland soils are 1/13, 1/10, and > 1/8, respectively. Given the wide range of soils and land uses across England and Wales in the datasets used to test these targets, they should apply across similar temperate regions globally, and at national to sub-regional scales.

Portable X-ray fluorescence (pXRF) calibration for analysis of nutrient concentrations and trace element contaminants in fertilisers.

With the increasing popularity of local blending of fertilisers, the fertiliser industry faces issues regarding quality control and fertiliser adulteration. Another problem is the contamination of fertilisers with trace elements that have been shown to subsequently accumulate in the soil and be taken up by plants, posing a danger to the environment and human health. Conventional characterisation methods necessary to ensure the quality of fertilisers and to comply with local regulations are costly, time consuming and sometimes not even accessible. Alternatively, using a wide range of unamended and intentionally amended fertilisers this study developed empirical calibrations for a portable handheld X-ray fluorescence (pXRF) spectrometer, determined the reliability for estimating the macro and micro nutrients and evaluated the use of the pXRF for the high-throughput detection of trace element contaminants in fertilisers. The models developed using pXRF for Mg, P, S, K, Ca, Mn, Fe, Zn and Mo had R2 values greater or equal to 0.97. These models also performed well on validation, with R2 values greater or equal to 0.97 (except for Fe, R2val = 0.55) and slope values ranging from 0.81 to 1.44. A second set of models were developed with a focus on trace elements in amended fertilisers. The R2 values of calibration for Co, Ni, As, Se, Cd and Pb were greater than or equal to 0.80. At concentrations up to 1000 mg kg-1, good validation statistics were also obtained; R2 values ranged from 0.97-0.99, except in one instance. The regression coefficients of the validation also had good prediction in the range of 0-100 mg kg-1 (R2 values were from 0.78-0.99), but not as well at lower concentrations up to 20 mg kg-1 (R2 values ranged from 0.10-0.99), especially for Cd. This study has demonstrated that pXRF can measure several major (P, Ca) and micro (Mn, Fe, Cu) nutrients, as well as trace elements and potential contaminants (Cr, Ni, As) in fertilisers with high accuracy and precision. The results obtained in this study is good, especially considering that loose powders were scanned for a maximum of 90 seconds without the use of a vacuum pump.

Plastics in biosolids from 1950 to 2016: A function of global plastic production and consumption.

Plastics are ubiquitous contaminants that leak into the environment from multiple pathways including the use of treated sewage sludge (biosolids). Seven common plastics (polymers) were quantified in the solid fraction of archived biosolids samples from Australia and the United Kingdom from between 1950 and 2016. Six plastics were detected, with increasing concentrations observed over time for each plastic. Biosolids plastic concentrations correlated with plastic production estimates, implying a potential link between plastics production, consumption and leakage into the environment. Prior to the 1990s, the leakage of plastics into biosolids was limited except for polystyrene. Increased leakage was observed from the 1990s onwards; potentially driven by increased consumption of polyethylene, polyethylene terephthalate and polyvinyl chloride. We show that looking back in time along specific plastic pollution pathways may help unravel the potential sources of plastics leakage into the environment and provide quantitative evidence to support the development of source control interventions or regulations.

African soil properties and nutrients mapped at 30 m spatial resolution using two-scale ensemble machine learning.

Soil property and class maps for the continent of Africa were so far only available at very generalised scales, with many countries not mapped at all. Thanks to an increasing quantity and availability of soil samples collected at field point locations by various government and/or NGO funded projects, it is now possible to produce detailed pan-African maps of soil nutrients, including micro-nutrients at fine spatial resolutions. In this paper we describe production of a 30 m resolution Soil Information System of the African continent using, to date, the most comprehensive compilation of soil samples ([Formula: see text]) and Earth Observation data. We produced predictions for soil pH, organic carbon (C) and total nitrogen (N), total carbon, effective Cation Exchange Capacity (eCEC), extractable-phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), sodium (Na), iron (Fe), zinc (Zn)-silt, clay and sand, stone content, bulk density and depth to bedrock, at three depths (0, 20 and 50 cm) and using 2-scale 3D Ensemble Machine Learning framework implemented in the mlr (Machine Learning in R) package. As covariate layers we used 250 m resolution (MODIS, PROBA-V and SM2RAIN products), and 30 m resolution (Sentinel-2, Landsat and DTM derivatives) images. Our fivefold spatial Cross-Validation results showed varying accuracy levels ranging from the best performing soil pH (CCC = 0.900) to more poorly predictable extractable phosphorus (CCC = 0.654) and sulphur (CCC = 0.708) and depth to bedrock. Sentinel-2 bands SWIR (B11, B12), NIR (B09, B8A), Landsat SWIR bands, and vertical depth derived from 30 m resolution DTM, were the overall most important 30 m resolution covariates. Climatic data images-SM2RAIN, bioclimatic variables and MODIS Land Surface Temperature-however, remained as the overall most important variables for predicting soil chemical variables at continental scale. This publicly available 30-m Soil Information System of Africa aims at supporting numerous applications, including soil and fertilizer policies and investments, agronomic advice to close yield gaps, environmental programs, or targeting of nutrition interventions.

Plant Available Zinc Is Influenced by Landscape Position in the Amhara Region, Ethiopia.

Zinc (Zn) is an important element determining the grain quality of staple food crops and deficient in many Ethiopian soils. However, farming systems are highly variable in Ethiopia due to different soil types and landscape cropping positions. Zinc availability and uptake by plants from soil and fertilizer sources are governed by the retention and release potential of the soil, usually termed as adsorption and desorption, respectively. The aim of this study was to characterize the amount of plant available Zn at different landscape positions. During the 2018/19 cropping season, adsorption-desorption studies were carried out on soil samples collected from on-farm trials conducted at Aba Gerima, Debre Mewi and Markuma in the Amhara Region. In all locations and landscape positions, adsorption and desorption increased with increasing Zn additions. The amount of adsorption and desorption was highly associated with the soil pH, the soil organic carbon concentration and cation exchange capacity, and these factors are linked to landscape positions. The Freundlich isotherm fitted very well to Zn adsorption (r2 0.87-0.99) and desorption (r2 0.92-0.99), while the Langmuir isotherm only fitted to Zn desorption (r2 0.70-0.93). Multiple regression models developed by determining the most influential soil parameters for Zn availability could be used to inform Zn fertilizer management strategies for different locations and landscape positions in this region, and thereby improve plant Zn use efficiency.

A comparison of soil texture measurements using mid-infrared spectroscopy (MIRS) and laser diffraction analysis (LDA) in diverse soils.

Spectroscopic methods for the determination of soil texture are faster and cheaper than the standard methods, but how do the results compare? To address this question, laser diffraction analysis (LDA) and mid-infrared spectroscopy (MIRS) analysis have been compared to conventional sieve-pipette measurements of texture in diverse European and Kenyan soils. To our knowledge this comparison between LDA and MIRS has not been made previously. It has used soils with a broad range of organic carbon (OC) contents to investigate whether, as in other techniques, clay-OC aggregation affects the estimation of clay with MIRS. The MIRS predictions of clay content were much better than the LDA measurements, but both techniques gave good measurements of sand content. The MIRS over-estimated clay at low clay content and under-estimated at high clay content (calibration set R2 = 0.83). The LDA over-estimated clay by ~ 60% (calibration set R2 = 0.36), indicating that the widely used clay threshold of < 8 µm was too high, and < 4 µm was found to be more accurate. In samples with < 5% OC content, both the LDA and MIRS gave very good clay predictions (R2 = 0.88 and 0.81, respectively). But in predictions of clay content in samples with > 5% OC the LDA under-estimated (R2 = < 0.1) and MIRS over-estimated (R2 = 0.34) clay content. In soils with OC removed, the MIRS prediction of clay content improved, indicating interference between over-lapping spectral regions for organic and mineral constituents. Unlike granulometric measurements of texture such as the LDA, MIRS analysis is not subject to the limitations imposed by the shape and density of particles. It was concluded that in typical agricultural soils with < 5% OC and < 60% clay content, both techniques could be used for cheap, fast and reliable estimates of soil texture.

Assessing the evolution of wheat grain traits during the last 166 years using archived samples.

The current study focuses on yield and nutritional quality changes of wheat grain over the last 166 years. It is based on wheat grain quality analyses carried out on samples collected between 1850 and 2016. Samples were obtained from the Broadbalk Continuous Wheat Experiment (UK) and from herbaria from 16 different countries around the world. Our study showed that, together with an increase in carbohydrate content, an impoverishment of mineral composition and protein content occurred. The imbalance in carbohydrate/protein content was specially marked after the 1960's, coinciding with strong increases in ambient [CO2] and temperature and the introduction of progressively shorter straw varieties. The implications of altered crop physiology are discussed.

Comprehensive nutrient analysis in agricultural organic amendments through non-destructive assays using machine learning.

Portable X-ray fluorescence (pXRF) and Diffuse Reflectance Fourier Transformed Mid-Infrared (DRIFT-MIR) spectroscopy are rapid and cost-effective analytical tools for material characterization. Here, we provide an assessment of these methods for the analysis of total Carbon, Nitrogen and total elemental composition of multiple elements in organic amendments. We developed machine learning methods to rapidly quantify the concentrations of macro- and micronutrient elements present in the samples and propose a novel system for the quality assessment of organic amendments. Two types of machine learning methods, forest regression and extreme gradient boosting, were used with data from both pXRF and DRIFT-MIR spectroscopy. Cross-validation trials were run to evaluate generalizability of models produced on each instrument. Both methods demonstrated similar broad capabilities in estimating nutrients using machine learning, with pXRF being suitable for nutrients and contaminants. The results make portable spectrometry in combination with machine learning a scalable solution to provide comprehensive nutrient analysis for organic amendments.

Simulation of Phosphorus Chemistry, Uptake and Utilisation by Winter Wheat.

The phosphorus (P) supply from soils is crucial to crop production. Given the complexity involved in P-cycling, a model that can simulate the major P-cycling processes and link with other nutrients and environmental factors, e.g., soil temperature and moisture, would be a useful tool. The aim of this study was to describe a process-based P module added to the SPACSYS (Soil Plant and Atmosphere Continuum System) model and to evaluate its predictive capability on the dynamics of P content in crops and the impact of soil P status on crop growth. A P-cycling module was developed and linked to other modules included in the SPACSYS model. We used a winter wheat (Triticum aestivum, cv Xi-19) field experiment at Rothamsted Research in Harpenden to calibrate and validate the model. Model performance statistics show that the model simulated aboveground dry matter, P accumulation and soil moisture dynamics reasonably well. Simulated dynamics of soil nitrate and ammonium were close to the observed data when P fertiliser was applied. However, there are large discrepancies in fields without P fertiliser. This study demonstrated that the SPACSYS model was able to investigate the interactions between carbon, nitrogen, P and water in a single process-based model after the tested P module was implemented.

Plant-microbe networks in soil are weakened by century-long use of inorganic fertilizers.

Understanding the changes in plant-microbe interactions is critically important for predicting ecosystem functioning in response to human-induced environmental changes such as nitrogen (N) addition. In this study, the effects of a century-long fertilization treatment (> 150 years) on the networks between plants and soil microbial functional communities, detected by GeoChip, in grassland were determined in the Park Grass Experiment at Rothamsted Research, UK. Our results showed that plants and soil microbes have a consistent response to long-term fertilization-both richness and diversity of plants and soil microbes are significantly decreased, as well as microbial functional genes involved in soil carbon (C), nitrogen (N) and phosphorus (P) cycling. The network-based analyses showed that long-term fertilization decreased the complexity of networks between plant and microbial functional communities in terms of node numbers, connectivity, network density and the clustering coefficient. Similarly, within the soil microbial community, the strength of microbial associations was also weakened in response to long-term fertilization. Mantel path analysis showed that soil C and N contents were the main factors affecting the network between plants and microbes. Our results indicate that century-long fertilization weakens the plant-microbe networks, which is important in improving our understanding of grassland ecosystem functions and stability under long-term agriculture management.

Effective methods to reduce cadmium accumulation in rice grain.

Contamination of cadmium (Cd) in paddy soil is a serious environmental problem threatening food safety in some parts of southern China, where rice grain Cd concentration often exceeds the Chinese limit (0.2 mg kg-1). We tested the effect of CaCO3 liming combined with growing low Cd cultivars on Cd accumulation in rice grain in 2-year field trials. A liming model was used to predict the doses of lime required. Lime (2.25-7.5 t ha-1) was applied in 2016 only and the effect monitored in both 2016 and 2017. Soil pH was increased from the initial 5.5 to the target value 6.5 by 7.5 t ha-1 CaCO3. Liming greatly reduced CaCl2-extractable Cd in the rhizospheric soil. Grain Cd concentration in the control exceeded the limit by 2-5 times. Liming at 7.5 t ha-1 decreased grain Cd concentration by 70-80% in both seasons without affecting grain yield. Grain Cd concentration was below the limit in the 7.5 t ha-1 liming treatment in both seasons. Grain Cd concentration correlated closely with CaCl2-extractable Cd in rhizospheric soil at the grain maturing stage. Seasonal difference in grain Cd concentration was attributed to the soil water status at the grain maturing stage. Liming had no significant effect on grain arsenic concentration or speciation. A single application of CaCO3 to raise soil pH to 6.5, combined with low Cd cultivars and delayed drainage of paddy water during late grain filling stage, was highly effective at reducing Cd accumulation in rice grain.

Risk of Silver Transfer from Soil to the Food Chain Is Low after Long-Term (20 Years) Field Applications of Sewage Sludge.

The increasingly widespread usage of silver (Ag) nanoparticles has raised concerns regarding their environmental risk. The behavior of Ag and its transfer risk to the food chain were investigated using a long-term field experiment that commenced in 1942 in which Ag-containing sewage sludge was repeatedly applied to the soil (25 applications during 20 years). The speciation of the Ag in both the sludge and the soils retrieved from the long-term experimental archive was examined using synchrotron-based X-ray absorption spectroscopy, and extractable Ag concentrations from soils were determined using 0.01 M Ca(NO3)2 and 0.005 M DTPA. The total Ag in the sludge during the time period of 1942-1961 ranged from 155 to 463 mg kg-1. These values are 1-2 orders of magnitude higher than those in currently produced sludge (ca. 0.5-20 mg kg-1). Long-term repeated applications of these sludges resulted in an increase of Ag in soils from 1.9 mg kg-1 in the control to up to 51 mg kg-1. The majority (>80%) of the Ag in both the sludge and the sludge-treated soils was present as insoluble Ag2S, thereby markedly reducing the bioavailability of this Ag. Concentrations of Ag in the archived crop samples were generally <0.70 mg kg-1 in edible tissues, much less than those in diets that may cause an adverse effects in animals and humans (>100 mg kg-1). These data indicate that the transfer of Ag (derived from both traditional Ag industry and current nano Ag industry) to the terrestrial food chain is limited.

The Nodulin 26-like intrinsic membrane protein OsNIP3;2 is involved in arsenite uptake by lateral roots in rice.

Previous studies have shown that the Nodulin 26-like intrinsic membrane protein (NIP) Lsi1 (OsNIP2;1) is involved in arsenite [As(III)] uptake in rice (Oryza sativa). However, the role of other rice NIPs in As(III) accumulation in planta remains unknown. In the present study, we investigated the role OsNIP3;2 in As(III) uptake in rice. When expressed in Xenopus laevis oocytes, OsNIP3;2 showed a high transport activity for As(III). Quantitative real-time RT-PCR showed that the expression of OsNIP3;2 was suppressed by 5 µM As(III), but enhanced by 20 and 100 µM As(III). Transgenic rice plants expressing OsNIP3;2pro-GUS showed that the gene was predominantly expressed in the lateral roots and the stele region of the primary roots. Transient expression of OsNIP3;2:GFP fusion protein in rice protoplasts showed that the protein was localized in the plasma membrane. Knockout of OsNIP3;2 significantly decreased As concentration in the roots, but had little effect on shoot As concentration. Synchrotron microfocus X-ray fluorescence showed decreased As accumulation in the stele of the lateral roots in the mutants compared with wild-type. Our results indicate that OsNIP3;2 is involved in As(III) uptake by lateral roots, but its contribution to As accumulation in the shoots is limited.

Mineral Availability as a Key Regulator of Soil Carbon Storage.

Mineral binding is a major mechanism for soil carbon (C) stabilization, and mineral availability for C binding critically affects C storage. Yet, the mechanisms regulating mineral availability are poorly understood. Here, we showed that organic amendments in three long-term (23, 154, and 170 yrs, respectively) field experiments significantly increased mineral availability, particularly of short-range-ordered (SRO) phases. Two microcosm studies demonstrated that the presence of roots significantly increased mineral availability and promoted the formation of SRO phases. Mineral transformation experiments and isotopic labeling experiments provided direct evidence that citric acid, a major component of root exudates, promoted the formation of SRO minerals, and that SRO minerals acted as "nuclei" for C retention. Together, these findings indicate that soil organic amendments initialize a positive feedback loop by increasing mineral availability and promoting the formation of SRO minerals for further C binding, thereby possibly serving as a management tool for enhancing carbon storage in soils.