Rodger P. White

The dynamics of arsenic in four paddy fields in the Bengal delta

Irrigation with arsenic contaminated groundwater in the Bengal Delta may lead to As accumulation in the soil and rice grain. The dynamics of As concentration and speciation in paddy fields during dry season (boro) rice cultivation were investigated at 4 sites in Bangladesh and West Bengal, India. Three sites which were irrigated with high As groundwater had elevated As concentrations in the soils, showing a significant gradient from the irrigation inlet across the field. Arsenic concentration and speciation in soil pore water varied temporally and spatially; higher As concentrations were associated with an increasing percentage of arsenite, indicating a reductive mobilization. Concentrations of As in rice grain varied by 2-7 fold within individual fields and were poorly related with the soil As concentration. A field site employing alternating flooded-dry irrigation produced the lowest range of grain As concentration, suggesting a lower soil As availability caused by periodic aerobic conditions.

Long-term management changes topsoil and subsoil organic carbon and nitrogen dynamics in a temperate agricultural system

Summary Soil organic carbon (SOC) and nitrogen (N) contents are controlled partly by plant inputs that can be manipulated in agricultural systems. Although SOC and N pools occur mainly in the topsoil (upper 0.30 m), there are often substantial pools in the subsoil that are commonly assumed to be stable. We tested the hypothesis that contrasting long‐term management systems change the dynamics of SOC and N in the topsoil and subsoil (to 0.75 m) under temperate conditions. We used an established field experiment in the UK where control grassland was changed to arable (59 years before) and bare fallow (49 years before) systems. Losses of SOC and N were 65 and 61% under arable and 78 and 74% under fallow, respectively, in the upper 0.15 m when compared with the grass land soil, whereas at 0.3–0.6‐m depth losses under arable and fallow were 41 and 22% and 52 and 35%, respectively. The stable isotopes 13C and 15N showed the effects of different treatments. Concentrations of long‐chain n‐alkanes C27, C29 and C31 were greater in soil under grass than under arable and fallow. The dynamics of SOC and N changed in both topsoil and subsoil on a decadal time‐scale because of changes in the balance between inputs and turnover in perennial and annual systems. Isotopic and geochemical analyses suggested that fresh inputs and decomposition processes occur in the subsoil. There is a need to monitor and predict long‐term changes in soil properties in the whole soil profile if soil is to be managed sustainably. Highlights Land‐use change affects soil organic carbon and nitrogen, but usually the topsoil only is considered. Grassland cultivated to arable and fallow lost 13–78% SOC and N to 0.6 m depth within decades. Isotopic and biomarker analyses suggested changes in delivery and turnover of plant‐derived inputs. The full soil profile must be considered to assess soil quality and sustainability.

Methods to estimate changes in soil water for phenotyping root activity in the field

Background and aimsThere is an urgent need to develop new high throughput approaches to phenotype roots in the field. Excavating roots to make direct measurements is labour intensive. An alternative to excavation is to measure soil drying profiles and to infer root activity.MethodsWe grew 23 lines of wheat in 2013, 2014 and 2015. In each year we estimated soil water profiles with electrical resistance tomography (ERT), electromagnetic inductance (EMI), penetrometer measurements and measurements of soil water content. We determined the relationships between the measured variable and soil water content and matric potential.ResultsWe found that ERT and penetrometer measurements were closely related to soil matric potential and produced the best discrimination between wheat lines. We found genotypic differences in depth of water uptake in soil water profiles and in the extent of surface drying.ConclusionsPenetrometer measurements can provide a reliable approach to comparing soil drying profiles by different wheat lines, and genotypic rankings are repeatable across years. EMI, which is more sensitive to soil water content than matric potential, and is less effective in drier soils than the penetrometer or ERT, nevertheless can be used to rapidly screen large populations for differences in root activity.

The effect of impedance to root growth on plant architecture in wheat

Background and aimsWe were interested in the effect of impedance to root growth on root and shoot architecture of wheat. It is known that Rht-1 semi-dwarfing alleles decrease the degree of leaf stunting due to root impedance. We compared commercial wheat cultivars containing different Rht-1 alleles to determine whether leaf stunting caused by root impedance differed between cultivars. We investigated effects of impedance to root growth on the angular spread of roots.MethodsThe wheat cultivars Avalon, Robigus and Battalion, carrying semi-dwarfing alleles of Rht-1, and cv. Cadenza, carrying the tall, wild-type allele, were grown under two levels of soil strength in a sand culture system designed to allow the mechanical impedance of the root growth environment to be adjusted independently of water and nutrient availability.ResultsImpeded roots grew more steeply than non-impeded roots: the angular spread of roots decreased from 55° to 43° from the vertical, but the genotypic effects were weak. Root impedance reduced leaf elongation and the number of tillers. Leaf area and total root length provided a common relationship across all genotype x treatment combinations. Leaf stunting in Cadenza was more severe.ConclusionOur data support the hypothesis that the severity of leaf stunting due to root impedance is related to the Rht allele. Impeded roots had a smaller angular spread.

Genotypic variation in the ability of wheat roots to penetrate wax layers

Background and aimsThe role of the root system in mediating crop yields has recently been emphasised, resulting in several laboratory approaches for phenotyping root traits. We aimed to determine the existence of, and reasons for, genotypic variation in wheat (Triticum aestivum L.) root penetration of strong wax layers.MethodsThree contrasting groups (UK elite lines, CIMMYT lines and near-isogenic lines of cv Mercia containing dwarfing and semi-dwarfing Rht alleles) comprising 18 different genotypes with contrasting phenologies were studied. We determined the ability of roots of these genotypes to penetrate strong wax layers and the angular spread of the root systems.ResultsThere were no intrinsic differences in root system ability to penetrate strong wax layers (consistent with the similar root diameter of all lines) since greater root penetration was simply related to more root axes. Recording root penetration of concentric zones of the wax layer demonstrated that cv. Battalion had a root system with a smaller angular spread than cv. Robigus, which had the root system with the greatest angular spread.ConclusionsThere was limited genotypic variability in root penetration of strong layers within the wheat cultivars studied. A key challenge will be to determine the physiological and agronomic significance of the variation in root angular spread.

Major limitations to achieving “4 per 1000” increases in soil organic carbon stock in temperate regions: Evidence from long-term experiments at Rothamsted Research, United Kingdom

Abstract We evaluated the “4 per 1000” initiative for increasing soil organic carbon (SOC) by analysing rates of SOC increase in treatments in 16 long‐term experiments in southeast United Kingdom. The initiative sets a goal for SOC stock to increase by 4‰ per year in the 0–40 cm soil depth, continued over 20 years. Our experiments, on three soil types, provided 114 treatment comparisons over 7–157 years. Treatments included organic additions (incorporated by inversion ploughing), N fertilizers, introducing pasture leys into continuous arable systems, and converting arable land to woodland. In 65% of cases, SOC increases occurred at >7‰ per year in the 0–23 cm depth, approximately equivalent to 4‰ per year in the 0–40 cm depth. In the two longest running experiments (>150 years), annual farmyard manure (FYM) applications at 35 t fresh material per hectare (equivalent to approx. 3.2 t organic C/ha/year) gave SOC increases of 18‰ and 43‰ per year in the 23 cm depth during the first 20 years. Increases exceeding 7‰ per year continued for 40–60 years. In other experiments, with FYM applied at lower rates or not every year, there were increases of 3‰–8‰ per year over several decades. Other treatments gave increases between zero and 19‰ per year over various periods. We conclude that there are severe limitations to achieving the “4 per 1000” goal in practical agriculture over large areas. The reasons include (1) farmers not having the necessary resources (e.g. insufficient manure); (2) some, though not all, practices favouring SOC already widely adopted; (3) practices uneconomic for farmers—potentially overcome by changes in regulations or subsidies; (4) practices undesirable for global food security. We suggest it is more realistic to promote practices for increasing SOC based on improving soil quality and functioning as small increases can have disproportionately large beneficial impacts, though not necessarily translating into increased crop yield.

Changes in soil organic matter over 70 years in continuous arable and ley–arable rotations on a sandy loam soil in England

The sequestration in soil of organic carbon (SOC) derived from atmospheric carbon dioxide (CO2) by replacing arable crops with leys, has been measured over 70 years on a sandy loam soil. The experiment was designed initially to test the effect of leys on the yields of arable crops. A 3‐year grazed grass with clover (grass + clover) ley in a 5‐year rotation with arable crops increased percentage organic carbon (%OC) in the top 25 cm of the soil from 0.98 to 1.23 in 28 years, but with little further increase during the next 40 years with all‐grass leys given fertilizer nitrogen (N). In this second period, OC inputs were balanced by losses, suggesting that about 1.3% OC might be near the equilibrium content for this rotation. Including 3‐year lucerne (Medicago sativa) leys had little effect on %OC over 28 years, but after changing to grass + clover leys, %OC increased to 1.24 during the next 40 years. Eight‐year leys (all grass with N or grass + clover) in 10‐year rotations with arable crops were started in the 1970s, and after three rotations %OC had increased to ca. 1.40 in 2000–2009. Over 70 years, %OC declined from 0.98 to 0.94 in an all‐arable rotation with mainly cereals and to 0.82 with more root crops. Applications of 38 t ha−1 farmyard manure (FYM) every fifth year increased %OC by 0.13% by the mid‐1960s when applications ceased. Soil treated with FYM still contained 0.10% more OC in 2000–2009. Changes in the amount of OC have been modelled with RothC‐26.3 and estimated inputs of C for selected rotations. Little of the OC input during the 70 years has been retained; most was retained in the grazed ley rotation, but 9 t ha−1 only of a total input of 189 t ha−1. In other rotations more than 98% of the total OC input was lost. Despite large losses of C, annual increases in OC of 4‰ are possible on this soil type with the inclusion of grass or grass + clover leys or the application of FYM, but only for a limited period. Such increases in SOC might help to limit increases in atmospheric CO2. Highlights Can leys sequester significant amounts of atmospheric CO 2 in SOM and contribute to the 4‰ initiative? Changes in the percentage and amount of OC were measured and modelled over 70 years and OC losses estimated. Three‐year grass or grass + clover leys increased %OC, but only to an equilibrium level that was then maintained. Despite large losses, sequestering CO 2‐C at 4‰ year−1 by growing grass or grass + clover leys is possible.

Root growth in field-grown winter wheat: Some effects of soil conditions, season and genotype

Highlights • We present evidence to support the hypothesis that the general and well-documented shape of the relationship between root length density and soil depth in UK grown winter wheat is related to the increase in soil strength with depth.• Effects of the soil environment on root length distribution were greater than genetic effects and this was most likely related to soil saturation.• In a dry season, there was genotypic variation in rooting depth.• Greater root length at depth in the dwarf NILs suggests that deep rooting is not simply related to plant height.

Effects of break crops, and of wheat volunteers growing in break crops or in set-aside or conservation covers, all following crops of winter wheat, on the development of take-all (Gaeumannomyces graminis var. tritici) in succeeding crops of winter wheat.

Experiments on the Rothamsted and Woburn Experimental Farms studied the effects on take-all of different break crops and of set-aside/conservation covers that interrupted sequences of winter wheat. There was no evidence for different effects on take-all of the break crops per se but the presence of volunteers, in crops of oilseed rape, increased the amounts of take-all in the following wheat. Severity of take-all was closely related to the numbers of volunteers in the preceding break crops and covers, and was affected by the date of their destruction. Early destruction of set-aside/conservation covers was usually effective in preventing damaging take-all in the following wheat except, sometimes, when populations of volunteers were very large. The experiments were not designed to test the effects of sowing dates but different amounts of take-all in the first wheats after breaks or covers apparently affected the severity of take-all in the following (second) wheats only where the latter were relatively late sown. In earlier-sown second wheats, take-all was consistently severe and unrelated to the severity of the disease in the preceding (first) wheats. Results from two very simple experiments suggested that substituting set-aside/conservation covers for winter wheat, for 1 year only, did not seriously interfere with the development of take-all disease or with the development or maintenance of take-all decline (TAD). With further research, it might be possible for growers wishing to exploit TAD to incorporate set-aside/conservation covers into their cropping strategies, and especially to avoid the worst effects of the disease on grain yield during the early stages of epidemics.

Evaluation of the stability of soil nanoparticles: the effect of natural organic matter in electrolyte solutions

Nanoparticles are ubiquitous in soil, and their quantity and stability play important roles in the fate, transport and bioavailability of contaminants in the environment. In this research, four soil nanoparticles (SNPs) were extracted from typical soil of different regions in China. Three different kinds of natural organic matter (NOM) were subsequently added to suspensions of SNPs to determine the effects of NOM on the stability of SNPs. The critical coagulation concentration (CCC), one of the most important indicators of the stability of the colloidal suspension in electrolyte solutions, was measured. The increase in CCC values indicated that the stability of SNPs was enhanced by NOM; however, the stability varied with the different electrolytes, properties of SNPs and properties of NOM. A coating of NOM increased the negative charges on the surface of SNPs. Therefore, the electrostatic force had a greater effect on the stability of SNPs than the van der Waals force, which contributed to their stability. The effect of NOM on the stability of SNPs was more evident in electrolytes with a monovalent cation (Na+) than in those with a divalent (Ca2+) or trivalent cation (La3+). The NOM increased the stability of less stable SNPs in relation to the composition and properties of SNPs. Further research is required to understand better how the behaviour and fate of SNPs are affected by NOM in the environment. Highlights The effect of organic matter on the stability of soil nanoparticles is evaluated. Stability of soil nanoparticles was enhanced by a surface coating of organic matter. The electrostatic force contributed to the stability of soil nanoparticles with the addition of organic matter. Change in stability was also related to types and properties of soil nanoparticles and organic matter.