Browsing by Author "Kirk, Guy J. D."
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Item Open Access An anticipatory life cycle assessment of the use of biochar from sugarcane residues as a greenhouse gas removal technology(Elsevier, 2021-06-02) Lefebvre, David; Williams, Adrian; Kirk, Guy J. D.; Meersmans, Jeroen; Sohi, Saran; Goglio, Pietro; Smith, PeteGreenhouse gas removal technologies are needed to reach the targets of the UNFCCC Paris Agreement. Among existing technologies, the use of biochar is considered promising, particularly biochar derived from the large quantities of sugarcane residues available in South America and elsewhere. However, the net greenhouse gas removal potential of sugarcane biochar has not been assessed hitherto. We use a scenario-based anticipatory life cycle assessment to investigate the emissions associated with a change from the combustion of sugarcane residues in a combined heat and power plant to the pyrolysis of these residues for biochar production and field application in São Paulo State, Brazil. We define scenarios based on different mean marginal electricity production and biochar production share. The results indicate that emissions from covering the electricity deficit generated by partial combustion of biomass during biochar production is the main emitting process. Overall, the processes associated with biochar production lower the net greenhouse gas benefits of the biochar by around 25%. Our analysis suggests that allocating 100% of the available sugarcane residues to biochar production could sequester 6.3 ± 0.5 t CO2eq ha−1 yr−1 of sugarcane in São Paulo State. Scaled up to the entire State, the practice could lead to the removal of 23% of the total amount of GHGs emitted by the State in 2016.Item Open Access Assessing the carbon capture potential of a reforestation project(Nature Publishing Group, 2021-10-07) Lefebvre, David; Williams, Adrian G.; Kirk, Guy J. D.; Burgess, Paul J.; Meersmans, Jeroen; Silman, Miles R.; Román-Dañobeytia, Francisco; Farfan, Jhon; Smith, PeteThe number of reforestation projects worldwide is increasing. In many cases funding is obtained through the claimed carbon capture of the trees, presented as immediate and durable, whereas reforested plots need time and maintenance to realise their carbon capture potential. Further, claims usually overlook the environmental costs of natural or anthropogenic disturbances during the forest’s lifetime, and greenhouse gas (GHG) emissions associated with the reforestation are not allowed for. This study uses life cycle assessment to quantify the carbon footprint of setting up a reforestation plot in the Peruvian Amazon. In parallel, we combine a soil carbon model with an above- and below-ground plant carbon model to predict the increase in carbon stocks after planting. We compare our results with the carbon capture claims made by a reforestation platform. Our results show major errors in carbon accounting in reforestation projects if they (1) ignore the time needed for trees to reach their carbon capture potential; (2) ignore the GHG emissions involved in setting up a plot; (3) report the carbon capture potential per tree planted, thereby ignoring limitations at the forest ecosystem level; or (4) under-estimate tree losses due to inevitable human and climatic disturbances. Further, we show that applications of biochar during reforestation can partially compensate for project emissions.Item Open Access Below-ground plant-soil interactions affecting adaptations of rice to iron toxicity(Wiley, 2021-10-09) Kirk, Guy J. D.; Manwaring, Hanna R.; Ueda, Yoshiaki; Semwal, Vimal K.; Wissuwa, MatthiasIron toxicity is a major constraint to rice production, particularly in highly weathered soils of inland valleys in sub-Saharan Africa where the rice growing area is rapidly expanding. There is wide variation in tolerance of iron toxicity in the rice germplasm. However, introgression of tolerance traits into high-yielding germplasm has been slow owing to the complexity of the tolerance mechanisms and large genotype-by-environment effects. We review current understanding of tolerance mechanisms, particularly those involving below-ground plant–soil interactions. Until now these have been less studied than above-ground mechanisms. We cover processes in the rhizosphere linked to exclusion of toxic ferrous iron by oxidation, and resulting effects on the mobility of nutrient ions. We also cover the molecular physiology of below-ground processes controlling iron retention in roots and root-shoot transport, and also plant iron sensing. We conclude that future breeding programmes should be based on well-characterized molecular markers for iron toxicity tolerance traits. To successfully identify such markers, the complex tolerance response should be broken down into its components based on understanding of tolerance mechanisms, and tailored screening methods should be developed for individual mechanisms.Item Open Access Can we model observed soil carbon changes from a dense inventory? A case study over England and Wales using three version of Orchidee ecosystem model (AR5, AR5-PRIM and O-CN)(Copernicus Publications, 2013-07-12T00:00:00Z) Guenet, B.; Moyano, F. E.; Vuichard, N.; Kirk, Guy J. D.; Bellamy, Patricia H.; Zaehle, Sönke; Ciais, P.A widespread decrease of the top soil carbon content was observed over England and Wales during the period 1978-2003 in the National Soil Inventory (NSI), amounting to a carbon loss of 4.44 Tg yr-1 over 141 550 km2. Subsequent modelling studies have shown that changes in temperature and precipitation could only account for a small part of the observed decrease, and therefore that changes in land use and management and resulting changes in soil respiration or primary production were the main causes. So far, all the models used to reproduce the NSI data did not account for plant-soil interactions and were only soil carbon models with carbon inputs forced by data. Here, we use three different versions of a process-based coupled soil-vegetation model called ORCHIDEE, in order to separate the effect of trends in soil carbon input, and soil carbon mineralisation induced by climate trends over 1978-2003. The first version of the model (ORCHIDEE-AR5) used for IPCC-AR5 CMIP5 Earth System simulations, is based on three soil carbon pools defined with first order decomposition kinetics, as in the CENTURY model. The second version (ORCHIDEE-AR5-PRIM) built for this study includes a relationship between litter carbon and decomposition rates, to reproduce a priming effect on decomposition. The last version (O-CN) takes into account N-related processes. Soil carbon decomposition in O-CN is based on CENTURY, but adds N limitations on litter decomposition. We performed regional gridded simulations with these three versions of the ORCHIDEE model over England and Wales. None of the three model versions was able to reproduce the observed NSI soil carbon trend. This suggests that either climate change is not the main driver for observed soil carbon losses, or that the ORCHIDEE model even with priming or N-effects on decomposition lacks the basic mechanisms to explain soil carbon change in response to climate, which would raise a caution flag about the ability of this type of model to project soil carbon changes in response to future warming. A third possible explanation could be that the NSI measurements made on the topsoil are not representative of the total soil carbon losses integrated over the entire soil depth, and thus cannot be compared with the model output.Item Open Access Carbon losses from all soils across England and Wales 1978−200(Nature Publishing Group, 2005-09-08T00:00:00Z) Bellamy, Patricia H.; Loveland, Peter J.; Bradley, R. Ian; Lark, R. Murray; Kirk, Guy J. D.Most terrestrial carbon is held in soils, more than twice as much as in vegetation or the atmosphere 1 , and changes in soil carbon content can have a large effect on the global carbon budget. The possibility that climate change is being reinforced by increased carbon dioxide emissions from soils with rising temperature is the subject of a continuing debate 29 . But to date evidence for the suggested feedback mechanism has come solely from small-scale laboratory and field experiments and modelling studies 29 . Here we use data from the National Soil Inventory of England and Wales obtained between 1978 and 2003 to show that carbon was lost from soils across England and Wales over the survey period at a mean rate of 0.6 per cent per year (relative to the existing soil carbon content). We find that the relative rate of carbon loss increased with soil carbon content and was more than two per cent per year in soils with carbon contents greater than 100 grams per kilogram. The relationship between rate of loss and carbon content held across the whole country and across all forms of land use suggesting a link to climate change. Our findings indicate that losses of soil carbon in England and Wales, and by inference other temperate regions, are likely to have been offsetting absorption of carbon by terrestrial siItem Open Access Changes in organic carbon to clay ratios in different soils and land uses in England and Wales over time(Springer Nature, 2022-03-25) Prout, Jonah M.; Shepherd, Keith D.; McGrath, Steve P.; Kirk, Guy J. D.; Hassall, Kirsty L.; Haefele, Stephan M.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.Item Open Access Climate change: carbon losses in the Alps(Nature Publishing Group, 2016-06-13) Kirk, Guy J. D.The response of the terrestrial carbon cycle to global change is one of the main uncertainties in current climate change predictions1. Most terrestrial carbon is held in soils as organic matter derived from the decay of plant material (Fig. 1). Soil organic matter accounts for roughly three times more carbon than living vegetation, and for more carbon than vegetation and the atmosphere combined. Because elevated atmospheric CO2 concentrations have a fertilizing effect on plant growth, anthropogenic CO2 emissions have triggered increases in the land carbon sink2. However, models predict that other factors — such as water and nutrients — will eventually become limiting to plant growth, and hence to the land carbon sink. In contrast, the turnover of soil organic matter producing CO2 is expected to increase as the Earth warms. As a result, simulations using coupled carbon–climate models predict that the land surface will become a net source of CO2 before the end of the century, leading to a feedback loop between climate and soil carbon losses: increased emissions of CO2 from soil organic matter will lead to enhanced warming, which may then feedback to cause further soil organic matter losses. Prietzel and colleagues3, writing in Nature Geoscience, now provide evidence that warming has already caused a decline in soil organic matter in the German Alps.Item Open Access Digital soil assessment for quantifying soil constraints to crop production: a case study for rice in Punjab, India(Wiley, 2018-09-24) Okonkwo, Ezekiel Iloabuchi; Corstanje, Ronald; Kirk, Guy J. D.Assessments of land capability for particular functions such as food production need to allow for uncertainties both in the criteria used to specify the function and in information on relevant soil properties. In this paper, we evaluate the use of digital soil assessment (DSA) for dynamic assessment of soil capability allowing for both uncertainties and spatial variability in soil properties and flexibility in the values of assessment criteria. We do this for soil constraints to rice production in the state of Punjab, India, where soil salinity and alkalinity are potentially important constraints to cropping. In DSA, spatial predictions of soil properties and associated uncertainties made with digital soil mapping (DSM) are used to assess soil functions. We use a combination of DSM and Monte Carlo simulation methods to estimate the spatial variation in soil electrical conductivity (ECe) and pH to 20 cm depth in soils across Punjab. We then use the estimates and associated uncertainties to assess the likelihood that soil salinity or alkalinity or both could constrain rice production. Results show that allowing for prediction uncertainties of soil attributes results in far smaller areas affected by salinity (1.2 vs. 2.0 Mha) and alkalinity (3.0 vs. 3.2 Mha). Results also show the importance of correctly setting threshold values for constraint criteria and the flexibility of the DSA approach for setting thresholds.Item Open Access The effect of non-uniform microscale distribution of sorption sites on solute diffusion in soil(Wiley, 2016-06-23) Masum, Shakil; Kirk, Guy J. D.; Daly, K. R.; Roose, T.Conventional models of solute transport in soil consider only soil volumes large enough to average over microscale heterogeneities, and it is assumed that microscale variations are unimportant at the macroscale. In this research we test this assumption for cases in which the microscale distribution of solute-sorbing sites is patchy. We obtain a set of equations at the macroscale that allow for the effect of the microscale distribution with the mathematical technique of homogenization. We combine these equations with an image-based model that describes the true microscale pore geometry in a real, structured soil measured with X-ray computed tomography. The resulting models are used to test the microscale averaging assumptions inherent in conventional models. We show that, in general, macroscale diffusion is little affected by microscale variation in the distribution of sorption sites. Therefore, for most purposes the assumption of microscale averaging used in conventional models is justified. The effects of microscale heterogeneity are noticeable only when (i) the rate of sorption is slow compared with diffusion, but still fast enough to affect macroscale transport and (ii) the defined macroscale volume approaches the microscale. We discuss the effects when these conditions are metItem Open Access Effects of soil type and composition of rhizodeposits on rhizosphere priming phenomena(Elsevier, 2016-10-13) Lloyd, D.; Ritz, Karl; Paterson, E.; Kirk, Guy J. D.Inputs of fresh plant-derived C may stimulate microbially-mediated turnover of soil organic matter (SOM) in the rhizosphere. But studies of such ‘priming’ effects in artificial systems often produce conflicting results, depending on such variables as rates of substrate addition, substrate composition, whether pure compounds or mixtures of substrates are used, and whether the addition is pulsed or continuous. Studies in planted systems are less common, but also produce apparently conflicting results, and the mechanisms of these effects are poorly understood. To add to the evidence on these matters, we grew a C4 grass for 61 d in two contrasting soils – an acid sandy soil and a more fertile clay-loam – which had previously only supported C3 vegetation. We measured total soil respiration and its C isotope composition, and used the latter to partition the respiration between plant- and soil-C sources. We found SOM turnover was enhanced (i.e. positive priming) by plant growth in both soils. In treatments in which the grass was clipped, net growth was greatly diminished, and priming effects were correspondingly weak. In treatments without clipping, net plant growth, total soil respiration and SOM-derived respiration were all much greater. Further, SOM-derived respiration increased over time in parallel with increases in plant growth, but the increase was delayed in the less fertile soil. We conclude the observed priming effects were driven by microbial demand for N, fuelled by deposition of C substrate from roots and competition with roots for N. The extent of priming depended on soil type and plant growing conditions. In a further experiment, we simulated rhizodeposition of soluble microbial substrates in the same two soils with near-continuous additions for 19 d of either C4-labelled sucrose (i.e. a simple single substrate) or a maize root extract (i.e. a relatively diverse substrate), and we measured soil respiration and its C isotope signature. In the more fertile soil, sucrose induced increasingly positive priming effects over time, whereas the maize root extract produced declining priming effects over time. We suggest this was because N and other nutrients were provided from the mineralization of this more diverse substrate. In the less-fertile soil, microbial N demand was probably never satisfied by the combined mineralization from added substrate and soil organic matter. Therefore priming effects were approximately constant over time. We conclude that the chemical nature of putative priming compounds can greatly influence priming phenomena.Item Open Access Experimental determination of zinc isotope fractionation in complexes with the phytosiderophore 2′-deoxymugeneic acid (DMA) and its structural analogues, and implications for plant uptake mechanisms(American Chemical Society, 2016-10-17) Marković, Tamara; Manzoor, Saba; Humphreys-Williams, Emma; Kirk, Guy J. D.; Vilar, Ramon; Weiss, Dominik J.The stable isotope signatures of zinc and other metals are increasingly used to study plant and soil processes. Complexation with phytosiderophores is a key reaction and understanding the controls of isotope fractionation is central to such studies. Here, we investigated isotope fractionation during complexation of Zn2+ with the phytosiderophore 2′-deoxymugeneic acid (DMA), and with three commercially available structural analogues of DMA: EDTA, TmDTA, and CyDTA. We used ion exchange chromatography to separate free and complexed zinc, and identified appropriate cation exchange resins for the individual systems. These were Chelex-100 for EDTA and CyDTA, Amberlite CG50 for TmDTA and Amberlite IR120 for DMA. With all the ligands we found preferential partitioning of isotopically heavy zinc in the complexed form, and the extent of fractionation was independent of the Zn:ligand ratio used, indicating isotopic equilibrium and that the results were not significantly affected by artifacts during separation. The fractionations (in ‰) were +0.33 ± 0.07 (1σ, n = 3), + 0.45 ± 0.02 (1σ, n = 2), + 0.62 ± 0.05 (1σ, n = 3) and +0.30 ± 0.07 (1σ, n = 4) for EDTA, TmDTA, CyDTA, and DMA, respectively. Despite the similarity in Zn-coordinating donor groups, the fractionation factors are significantly different and extent of fractionation seems proportional to the complexation stability constant. The extent of fractionation with DMA agreed with observed fractionations in zinc uptake by paddy rice in field experiments, supporting the possible involvement of DMA in zinc uptake by rice.Item Open Access A field system for measuring plant and soil carbon fluxes using stable isotope methods(Wiley, 2020-06-21) McCloskey, Christopher S.; Otten, Wilfred; Paterson, Eric; Ingram, Benjamin R.; Kirk, Guy J. D.There is a lack of field methods for measuring plant and soil processes controlling soil organic matter (SOM) turnover over diurnal, seasonal, and longer time-scales with which to develop datasets for modelling. We describe an automated field system for measuring plant and soil carbon fluxes over such time-scales using stable isotope methods, and we assess its performance. The system comprises 24 large (1-m deep, 0.8-m diameter) cylindrical lysimeters connected to gas-flux chambers and instruments. The lysimeters contain intact, naturally-structured C3 soil planted with a C4 grass. Fluxes of CO2 and their 13C isotope composition are measured 3-times daily in each lysimeter, and the isotope composition is used to partition the fluxes between plant and soil sources. We investigate the following potential sources of error in the measurement system and show they do not significantly affect the measured CO2 fluxes or isotope signatures: gas leaks; the rate of gas flow through sampling loops; instrument precision and drift; the concentration-dependence of isotope measurements; and the linearity of CO2 accumulation in the chambers and associated isotope fractionation resulting from different rates of 13CO2 and 12CO2 diffusion from the soil. For the loamy grassland soil and US prairie grass (Bouteloua dactyloides) tested, the precision of CO2 flux measurements was ± 0.04 % and that of the flux partitioning ± 0.40 %. We give examples of diurnal and seasonal patterns of plant and soil C fluxes and soil temperature and moisture. We discuss the limitations of the isotope methodology for partitioning fluxes as applied in our system. We conclude the system is suitable for measuring net ecosystem respiration fluxes and their plant and soil components with sufficient precision to resolve diurnal and seasonal patternsItem Open Access Grain Zn concentrations and yield of Zn-biofortified versus Zn-efficient rice genotypes under contrasting growth conditions(Elsevier, 2019-02-13) Goloran, J. B.; Johnson-Beebout, S. E.; Morete, M. J.; Impa, S. M.; Kirk, Guy J. D.; Wissuwa, M.Higher grain Zn concentration in ‘biofortified’ rice genotypes, bred for high grain Zn concentration, should not be at the expense of reduced grain yield. This study examined the grain yield and grain Zn concentration of Zn-biofortified genotypes in field experiments in the Philippines. Zinc-biofortified genotypes (high grain Zn concentration in Zn-sufficient soil) were compared with efficient genotypes (tolerant of soil Zn deficiency), inefficient genotypes (sensitive to soil Zn deficiency) and check genotypes (popular local varieties) at four sites (Bay, Bohol, Bukidnon and IRRI) with differing types and degrees of Zn deficiency, over five cropping seasons (wet season 2012, 2014 and 2015 and dry season 2013 and 2015). A common experimental design and plot size were used with treatments (genotypes and Zn fertilization) arranged in a two-factorial randomized complete block design. The results showed that biofortified genotypes achieved both the Philippine grain yield target (4.0 t ha−1) and grain Zn biofortification target (30 mg kg−1 for brown rice) only when grown under Zn-sufficient conditions. In Zn-deficient soils, most Zn-biofortified and deficiency-tolerant genotypes reached the Zn concentration target but not the yield target, suggesting the need to correct the soil Zn-deficiency to prevent yield penalty. Further, results from IRRI showed that only Zn-fertilized plants were able to achieve the Zn biofortification target during the wet season; whereas during the dry season, when the soil was less chemically-reduced and therefore the soil Zn probably more plant-available, grain Zn levels were all above the threshold, with or without Zn fertilizer. This suggests that Zn fertilization may not be needed during the dry season in soils with sufficient potentially plant-available Zn.Item Open Access The greenhouse gas impacts of converting food production in England and Wales to organic methods(Nature Publishing Group, 2019-10-22) Smith, Laurence G.; Kirk, Guy J. D.; Jones, Philip J.; Williams, Adrian G.Agriculture is a major contributor to global greenhouse gas (GHG) emissions and must feature in efforts to reduce emissions. Organic farming might contribute to this through decreased use of farm inputs and increased soil carbon sequestration, but it might also exacerbate emissions through greater food production elsewhere to make up for lower organic yields. To date there has been no rigorous assessment of this potential at national scales. Here we assess the consequences for net GHG emissions of a 100% shift to organic food production in England and Wales using life-cycle assessment. We predict major shortfalls in production of most agricultural products against a conventional baseline. Direct GHG emissions are reduced with organic farming, but when increased overseas land use to compensate for shortfalls in domestic supply are factored in, net emissions are greater. Enhanced soil carbon sequestration could offset only a small part of the higher overseas emissions.Item Open Access Long-term increases in soil carbon due to ecosystem fertilization by atmospheric nitrogen deposition demonstrated by regional-scale modelling and observations(Nature Publishing Group, 2017-05-15) Tipping, Edward; Davies, J. A. C.; Henrys, P. A.; Kirk, Guy J. D.; Lilly, Allan; Dragosits, U.; Carnell, Edward J.; Dore, A. J.; Sutton, M. A.; Tomlinson, S. J.Fertilization of nitrogen (N)-limited ecosystems by anthropogenic atmospheric nitrogen deposition (Ndep) may promote CO2 removal from the atmosphere, thereby buffering human effects on global radiative forcing. We used the biogeochemical ecosystem model N14CP, which considers interactions among C (carbon), N and P (phosphorus), driven by a new reconstruction of historical Ndep, to assess the responses of soil organic carbon (SOC) stocks in British semi-natural landscapes to anthropogenic change. We calculate that increased net primary production due to Ndep has enhanced detrital inputs of C to soils, causing an average increase of 1.2 kgCm−2 (c. 10%) in soil SOC over the period 1750–2010. The simulation results are consistent with observed changes in topsoil SOC concentration in the late 20th Century, derived from sample-resample measurements at nearly 2000 field sites. More than half (57%) of the additional topsoil SOC is predicted to have a short turnover time (c. 20 years), and will therefore be sensitive to future changes in Ndep. The results are the first to validate model predictions of Ndep effects against observations of SOC at a regional field scale. They demonstrate the importance of long-term macronutrient interactions and the transitory nature of soil responses in the terrestrial C cycle.Item Open Access Magnesium supply alleviates iron toxicity-induced leaf bronzing in rice through exclusion and tissue-tolerance mechanisms(Frontiers, 2023-07-21) Rajonandraina, Toavintsoa; Ueda, Yoshiaki; Wissuwa, Matthias; Kirk, Guy J. D.; Rakotoson, Tovohery; Manwaring, Hanna; Andriamananjara, Andry; Razafimbelo, TantelyIntroduction: Iron (Fe) toxicity is a widespread nutritional disorder in lowland rice causing growth retardation and leaf symptoms referred to as leaf bronzing. It is partly caused by an imbalance of nutrients other than Fe and supply of these is known to mitigate the toxicity. But the physiological and molecular mechanisms involved are unknown. Methods: We investigated the effect of magnesium (Mg) on Fe toxicity tolerance in a field study in the Central Highlands of Madagascar and in hydroponic experiments with excess Fe (300 mg Fe L-1). An RNA-seq analysis was conducted in a hydroponic experiment to elucidate possible mechanisms underlying Mg effects. Results and discussion: Addition of Mg consistently decreased leaf bronzing under both field and hydroponic conditions, whereas potassium (K) addition caused minor effects. Plants treated with Mg tended to have smaller shoot Fe concentrations in the field, suggesting enhanced exclusion at the whole-plant level. However, analysis of multiple genotypes showed that Fe toxicity symptoms were also mitigated without a concomitant decrease of Fe concentration, suggesting that increased Mg supply confers tolerance at the tissue level. The hydroponic experiments also suggested that Mg mitigated leaf bronzing without significantly decreasing Fe concentration or oxidative stress as assessed by the content of malondialdehyde, a biomarker for oxidative stress. An RNA-seq analysis revealed that Mg induced more changes in leaves than roots. Subsequent cis-element analysis suggested that NAC transcription factor binding sites were enriched in genes induced by Fe toxicity in leaves. Addition of Mg caused non-significant enrichment of the same binding sites, suggesting that NAC family proteins may mediate the effect of Mg. This study provides clues for mitigating Fe toxicity-induced leaf bronzing in rice.Item Open Access Measured and modeled nitrogen balances in lowland rice-pasture rotations in temperate South America(Frontiers, 2023-04-03) Castillo, Jesús; Kirk, Guy J. D.; Rivero, M. Jordana; Fabini, Guillermo; Terra, José A.; Ayala, Walter; Roel, Alvaro; Irisarri, Pilar; Haefele, Stephan M.Rotational rice systems, involving pastures, other crops and/or livestock, are common in temperate South America, exemplified by the rice-pasture-livestock system of Uruguay which combines very high rice yields with tight nitrogen (N) balances. The generally good nutrient use efficiency in these systems provides a template for nutrient management in other mixed farming systems, if the underlying processes can be sufficiently well quantified and understood. Here, we studied N balances in rice–non-rice rotations in a long-term experiment in Uruguay, with the aim of parameterizing and testing the DNDC model of N dynamics for such systems for use in future work. The experiment includes three rotations: continuous rice (RI-CONT), rice-soybean (RI-SOY) and rice-pasture (RI-PAST). We considered 9 years of data on N balances (NBAL), defined as all N inputs minus all N outputs; N surplus (NSURP), defined as all N inputs minus only N outputs in food products; and N use efficiency (NUE), defined as the fraction of N inputs removed in food products. We parameterized DNDC against measured yield and input and output data, with missing data on N losses inferred from the N balance and compared with literature values. The model performance was assessed using standard indices of mean error, agreement and efficiency. The model simulated crop yields and rice cumulative N uptake very well, and soil N reasonably well. The values of NBAL were +45 and−20 kg N ha−1 yr−1 in RI-CONT and RI-SOY, respectively, and close to zero in RI-PAST (−6 kg N ha−1 yr−1). Values of NSURP decreased in the order RI-CONT >> RI-SOY > RI-PAST (+115, +25 and +13 kg N ha−1 yr−1, respectively). Values of NUE (84, 54, and 48% for RI-SOY, RI-PAST, and RI-CONT, respectively) decreased as NBAL increased. The sensitivity of DNDC's predictions to the agronomic characteristics of the different crops, rotations and water regimes agreed with expectations. We conclude that the DNDC model as parameterized here is suitable for exploring how to optimize N management in these systems.Item Open Access Measurement of isotopically-exchangeable Zn in Zn-deficient paddy soil(Wiley, 2015-10-27) Izquierdo, M.; Impa, S. M.; Johnson-Beebout, S. E.; Weiss, D. J.; Kirk, Guy J. D.The changes in soil chemistry following submergence of a soil for rice production result in zinc (Zn) being immobilized in very insoluble forms. Consequently, Zn deficiency is widespread in rice crops and in human populations that subsist on rice. We explored the use of stable isotopic dilution assays for assessing Zn dynamics in submerged paddy soil with two types of strongly Zn-deficient soil for rice cultivation in the Philippines. We optimized the isotope enrichment, electrolyte and equilibration time to measure isotopically-exchangeable Zn (E-values) without changing redox conditions. Available Zn was rapidly and strongly immobilized following submergence, which was controlled by CO2 accumulation. Addition of the isotopic tracer before submergence produced unreliable E-values because irreversible immobilization of the tracer progressed faster than isotopic exchange. Addition of the tracer to already reduced soil produced stable E-values for tracer–soil contact of up to 1 week. Longer periods produced unreliable E-values because of continuing irreversible fixation of the tracer. We discuss the implications for applications of isotopic dilution methods to measure trace-element dynamics in submerged soil.Item Open Access Mechanisms of genotypic differences in tolerance of iron toxicity in field-grown rice(Elsevier, 2023-04-28) Rajonandraina, Toavintsoa; Rakotoson, Tovohery; Wissuwa, Matthias; Ueda, Yoshiaki; Razafimbelo, Tantely; Andriamananjara, Andry; Kirk, Guy J. D.Iron (Fe) toxicity is a major constraint to rice yields in much of the world due to the greater solubility of reduced ferrous Fe in paddy soils compared with ferric Fe in aerobic soils and resulting excess uptake into the plants. There is genotypic variation in tolerance in Oryza gene pools, but so far only weak-effect alleles have been identified, largely because multiple critical physiological processes determine the tolerance. Most past research has been done in nutrient solution screens at the seedling stage, and not under field conditions over the full life cycle. We investigated tolerance mechanisms in a diverse set of genotypes under field conditions in a highly iron toxic soil in the Central Highlands of Madagascar. We made repeated plant samplings of young and old tissues throughout the growth period until maturity. Multiple mechanisms were involved, and the importance of different mechanisms changed between growth stages. Higher grain yields were mainly due to healthy vegetative growth, achieved either by reducing Fe uptake (exclusion) or by minimizing the effect of excess uptake through compartmentalization in older tissues and tissue tolerance. Exclusion mechanisms were relaxed during reproductive growth, leading to increased Fe accumulation in shoots. But tolerant genotypes were nonetheless able to grow well through a combination of Fe compartmentalization and tissue tolerance, so that grain filling could proceed relatively unimpeded. Tissue phosphorus (P) and potassium (K) concentrations were close to or below deficiency limits throughout growth. Exclusion by ferrous Fe oxidation in the rhizosphere will impede access of P and K ions to roots, but the differences in their tissue concentrations were much smaller than differences in growth rates, so growth rates evidently drove the uptake differences and responses to Fe toxicity were the more important constraints. There was no relation between grain yield and visual symptoms. To identify useful donors and markers for breeding it is important to develop screening protocols that capture the individual tolerance mechanisms, allowing for the effects of growth stage on their relative importance and expression, and possible interactions with other factors such as mineral nutrition. Selection for tolerance based on visual symptoms, particularly at the seedling stage, is overly simplistic, though it can be useful in the study of specific tolerance mechanisms.Item Open Access A model of uranium uptake by plant roots allowing for root-induced changes in the soil(American Chemical Society, 2018-02-21) Boghi, Andrea; Roose, Tiina; Kirk, Guy J. D.We develop a model with which to study the poorly understood mechanisms of uranium (U) uptake by plants. The model is based on equations for transport and reaction of U and acids and bases in the rhizosphere around cylindrical plant roots. It allows for the speciation of U with hydroxyl, carbonate, and organic ligands in the soil solution; the nature and kinetics of sorption reactions with the soil solid; and the effects of root-induced changes in rhizosphere pH. A sensitivity analysis showed the importance of soil sorption and speciation parameters as influenced by pH and CO2 pressure; and of root geometry and root-induced acid–base changes linked to the form of nitrogen taken up by the root. The root absorbing coefficient for U, relating influx to the concentration of U species in solution at the root surface, was also important. Simplified empirical models of U uptake by different plant species and soil types need to account for these effects.