Nutrient use and nutrient use efficiency of crops in a high CO2 atmosphere

Abstract Atmospheric CO2 concentrations [CO2] are continually increasing and are predicted to reach ~550 μmol mol-1 by 2050, about a 40 % increase from 2013 levels. Such a large increase in one of the key resources for plant growth will have significant effects on all plants, as carbon assimilation and, consequently, growth and yield is stimulated by the so-called ‘CO2 fertilisation effect’. The one sided increase in carbohydrate acquisition leads to changes in the chemical composition of plants: despite decreases in nutrient concentrations in plant tissues, the greater biomass developed by crops under elevated [CO2] could lead to increased nutrient demand. Nutrient use efficiency in terms of yield divided by available nutrient may improve, but grains or vegetative plant parts have decreased protein and mineral nutrient concentrations, which can diminish market and nutritious value. A number of hypotheses have been proposed to explain the decreases in nutrient concentra- tions, among them: (1) Dilution by increased biomass, (2) decreased mass flow, (3) changes in root architecture and function, (4) decreased nitrate eduction, and (5) changes in nutrient allocation and remobilisation. In addition, elevated [CO2] is likely to change soil processes, including nutrient supply. The extent to which some or all of these contribute to changes in crop nutrition and yield quality is currently unknown because most have not been sufficiently tested under relevant field conditions. This chapter gives an overview of the changes in plant nutrition and trade-offs under elevated [CO2] to point out that current and future efforts towards improved plant nutrient efficiency should explicitly take into consideration rising [CO2]. In particular, field testing of putative nutrient use efficiency traits and nutrient management strategies should include elevated [CO2] as a relevant factor in suitable exposure systems such as Free Air CO2 Enrichment (FACE) technology

Can intra-specific differences in root traits of wheat increase nitrogen use efficiency (NUE) under elevated CO2?

The atmospheric CO2 concentration is predicted to reach ~550 ppm by the middle of this century. The observed negative effect of increased CO2 concentration on grain quality is of major concern. One strategy to compensate for this effect is to apply more N fertiliser. However, increased fertiliser costs as well as associated environmental concerns have stimulated growers to reevaluate their fertiliser application strategies with the aim of optimizing Nitrogen Use Efficiency (NUE) while maintaining crop yields and minimising N losses. Improving NUE through selection of specific root characteristic will be crucial to the adaptation of future climates. Because roots are the first plant organ that receive nutrients such as N from the soil, traits like root biomass, length and rooting depth can significantly affect N uptake rates and therefore NUE in crops. This study investigated the NUE and root biomass of two contrasting wheat (Triticum aestivum L.) cultivars (Scout and Yitpi) grown under two atmospheric CO2 concentrations (ambient, ~400 ppm, and elevated, ~550 ppm) and two N treatments (low, 21 mg N/core, and high, 250 mg N/core) at the Australian Grains Free Air CO2 Enrichment (AGFACE) facility in south-eastern Australia. Results showed that at anthesis, Scout had more N (g) in aboveground and whole plant while Yitpi recorded more N (g) in root biomass. At maturity, Yitpi still had more N in roots, but Scout recorded greater N in grains. While both cultivars uptake almost the same amount of N from the soil, Scout was more able to transfer this N from roots to aboveground biomass at anthesis and then to grains at maturity, which is a very important characteristic that can allow wheat to maintain grain quality under elevated CO2

Carbon sink strength of nodules but not other organs modulates photosynthesis of faba bean (Vicia faba) grown under elevated [CO2 ] and different water supply

Photosynthetic stimulation by elevated [CO2 ] (e[CO2 ]) may be limited by the capacity of sink organs to use photosynthates. In many legumes, N2 -fixing symbionts in root nodules provide an additional sink, so that legumes may be better able to profit from e[CO2 ]. However, drought not only constrains photosynthesis but also size and activity of sinks, and little is known about the interaction of e[CO2 ] and drought on carbon sink strength of nodules and other organs. To compare carbon sink strength, faba bean was grown under ambient (400 ppm) or elevated (700 ppm) atmospheric [CO2 ] and subjected to well-watered or drought treatments, and then exposed to 13 C pulse-labelling using custom-built chambers to track the fate of new photosynthates. Drought decreased 13 C uptake and nodule sink strength, and this effect was even greater under e[CO2 ], and associated with an accumulation of amino acids in nodules. This resulted in decreased N2 fixation, increased accumulation of new photosynthates (13 C/sugars) in leaves, which in turn can feed back on photosynthesis. Our study suggests that nodule C sink activity is key to avoid sink limitation in legumes under e[CO2 ], and legumes may only be able to achieve greater C gain if nodule activity is maintained

Can intra-specific differences in root traits of wheat increase nitrogen use efficiency (NUE) under elevated CO2?

The atmospheric CO2 concentration is predicted to reach ~550 ppm by the middle of this century. The observed negative effect of increased CO2 concentration on grain quality is of major concern. One strategy to compensate for this effect is to apply more N fertiliser. However, increased fertiliser costs as well as associated environmental concerns have stimulated growers to reevaluate their fertiliser application strategies with the aim of optimizing Nitrogen Use Efficiency (NUE) while maintaining crop yields and minimising N losses. Improving NUE through selection of specific root characteristic will be crucial to the adaptation of future climates. Because roots are the first plant organ that receive nutrients such as N from the soil, traits like root biomass, length and rooting depth can significantly affect N uptake rates and therefore NUE in crops. This study investigated the NUE and root biomass of two contrasting wheat (Triticum aestivum L.) cultivars (Scout and Yitpi) grown under two atmospheric CO2 concentrations (ambient, ~400 ppm, and elevated, ~550 ppm) and two N treatments (low, 21 mg N/core, and high, 250 mg N/core) at the Australian Grains Free Air CO2 Enrichment (AGFACE) facility in south-eastern Australia. Results showed that at anthesis, Scout had more N (g) in aboveground and whole plant while Yitpi recorded more N (g) in root biomass. At maturity, Yitpi still had more N in roots, but Scout recorded greater N in grains. While both cultivars uptake almost the same amount of N from the soil, Scout was more able to transfer this N from roots to aboveground biomass at anthesis and then to grains at maturity, which is a very important characteristic that can allow wheat to maintain grain quality under elevated CO2

Grain mineral quality of dryland legumes as affected by elevated CO2 and drought: A FACE study on lentil (Lens culinaris) and faba bean (Vicia faba)

Stimulation of grain yield under elevated [CO] grown plants is often associated with the deterioration of grain quality. This effect may be further complicated by the frequent occurrence of drought, as predicted in most of the climate change scenarios. Lentil (Lens culinaris Medik.) and faba bean (Vicia faba L.) were grown in the Australian Grains Free Air CO Enrichment facility under either ambient CO concentration ([CO], ∼400 mol mol-1) or elevated [CO] (e[CO], ∼550 mol mol-1), and with two contrasting watering regimes (for faba bean) or over two consecutive seasons contrasting in rainfall (for lentil), to investigate the interactive effect of e[CO] and drought on concentrations of selected grain minerals (Fe, Zn, Ca, Mg, P, K, S, Cu, Mn, Na). Grain mineral concentration (Fe, Zn, Ca, K, S, Cu) increased and grain mineral yield (i.e. g mineral per plot surface area) decreased in dry growing environments, and vice versa in wet growing environments. Elevated [CO] decreased Fe, Zn, P and S concentrations in both crops however, the relative decrease was greater under dry (20-25) than wet (4-10) growing conditions. Principal component analysis showed that greater grain yield stimulation under e[CO] was associated with a reduction in Fe and Zn concentrations, indicating a yield dilution effect, but this was not consistently observed for other minerals. Even if energy intake is kept constant to adjust for lower yields, decreased legume micronutrients densities under e[CO] may have negative consequences for human nutrition, especially under drier conditions and in areas with less access to food. © 2019 CSIRO

Free air CO2 enrichment (FACE) improves water use efficiency and moderates drought effect on N 2 fixation of Pisum sativum L.

Background and aims: Legume N 2 fixation is highly sensitive to drought. Elevated [CO 2 ] (e[CO 2 ]) decreases stomatal conductance (g s ) and improves water use efficiency (WUE), which may result in soil water conservation and allow N 2 fixation to continue longer under drought. Using a Free-Air CO 2 Enrichment (FACE) approach, this study aimed to elucidate whether e[CO 2 ] improves N 2 fixation of Pisum sativum L. under drought. Methods: In a FACE system, plants were grown in ambient [CO 2 ] (~400 ppm) or e[CO 2 ] (~550 ppm) and subjected to either terminal drought or well-watered treatments. Measurements were taken of photosynthesis, soil water dynamics, water soluble carbohydrates (WSC), amino acids (AA) and N 2 fixation. Results: Lower g s under e[CO 2 ] increased water use efficiency at leaf and plant level, and this translated to slower soil water depletion during drought. Elevated [CO 2 ] increased WSC and decreased total AA concentrations in nodules, and increased nodule activity under drought. N 2 fixation was stimulated (+51%) by e[CO 2 ] in proportion to biomass changes. Under e[CO 2 ] a greater proportion of plant total N was derived from fixed N 2 and a smaller proportion from soil N uptake compared to a[CO 2 ]. Conclusion: This study suggests that e[CO 2 ] increased WUE and this resulted in slower soil water depletion, allowing pea plants to maintain greater nodule activity under drought and resulting in appreciable increases in N 2 fixation. Our results suggest that growth under e[CO 2 ] can mitigate drought effects on N 2 fixation and reduce dependency on soil N resources especially in water-limited agro-ecosystems. © 2019, Springer Nature Switzerland AG

Elevated CO2 improves yield and N2 fixation but not grain N concentration of faba bean (Vicia faba L.) subjected to terminal drought

Legumes grown in Mediterranean environments frequently experience terminal drought which reduces yield and N2 fixation processes. Decreased N2 fixation during reproductive phases may constrain seed nitrogen concentrations ([N]), reducing protein concentration of grain legumes. Plants grown under elevated atmospheric CO2 concentrations ([CO2]) have greater water use efficiency. This may result in reduced use of conserved/stored soil water, potentially helping to reduce soil water deficits later during grain filling. The extent that this process applies to drought sensitive grain legumes, which are extensively cultivated in Mediterranean environments is unclear. The objectives of this study were to investigate yield, N2 fixation and seed N response of faba bean (Vicia faba L. cv. ‘Fiesta’) grown in a dryland Mediterranean-type environment under elevated [CO2]. Plants were grown in soil columns under ambient [CO2] (˜400 ppm) or elevated [CO2] (e[CO2], ˜550 ppm) in a Free-Air CO2 Enrichment (FACE) facility in the field. One sub-group was continuously well-watered (80% field capacity, FC), whereas a second sub-group was exposed to a drought treatment (water was withheld until 30% FC was reached, which was then maintained during the reproductive phases). Biomass, gas exchange, 13C isotopic discrimination, N2 fixation by the natural abundance 15N method, nodulation and soil water content were assessed throughout the crop developmental stages. Initially, plants grown under elevated [CO2] depleted soil water more slowly in the drought treatment than those under ambient [CO2], but as plants grown under elevated [CO2] produced more biomass they used soil water more rapidly, especially towards the critical pod-filling phase. Water savings during the first phase of the drought treatment, through flowering up to the start of pod-filling, were associated with increased yield (+25%) and N2 fixation (+15%) under drought. Elevated [CO2]-induced stimulation of nodulation and nodule density helped maintain N2 fixation under drought, even though nodule activity decreased under the combined effect of e[CO2] and drought from pod-filling onwards. This later stage decrease was associated with decreased carbohydrate and increased amino acid concentrations in nodules, indicating a down-regulation of N2 fixation. Associated with the decrease of N2 fixation during pod-filling, seed N concentration was lower under the combination of e[CO2] and drought. We propose a conceptual model to explain the importance of N2 fixation during the grain filling stage to maintain seed N concentration under e[CO2]. These findings suggest that e[CO2]-induced savings in soil water may mitigate negative effects of drought on yield and N2 fixation of faba bean, without fully compensating the effect of prolonged drought on seed N concentration. © 2019 Elsevier B.V

Yield of canola (Brassica napus L.) benefits more from elevated CO2 when access to deeper soil water is improved

This study investigated the interactive effects of atmospheric CO2 concentration ([CO2]) and water availability on yield, root growth and water use of two canola cultivars with contrasting growth and vigour (vigorous hybrid cv. Hyola 50 and non-hybrid cv. Thumper). Plants were grown under ambient [CO2] (a[CO2], ∼400 μmol mol−1) or elevated [CO2] (e[CO2], ∼700 μmol mol−1) in a glasshouse. Two water treatments (well-watered and drought) were established in each [CO2] treatment. During the growing season leaf gas exchange parameters were measured. Leaf area was measured at 80 days after sowing. Aboveground biomass, seed yield, yield components and root biomass in four different soil layers (Layer 1: 0–20 cm, Layer 2: 21–40 cm, Layer 3: 41–60 cm and Layer 4: 61–80 cm depth) were measured at maturity. Weekly water use was determined gravimetrically. Elevated [CO2] stimulated seed yield (38%), aboveground biomass (34%), root biomass (42%), leaf area (42%) and leaf biomass (41%). Whilst e[CO2] stimulated root biomass in all soil layers, this stimulation was greater in the deeper than upper soil layers, and was associated with greater extraction of deeper soil water under e[CO2]. The cultivar with greater stimulation of deeper root biomass under e[CO2] showed greater yield benefit from the ‘CO2 fertilisation effect’. Under well-watered conditions, e[CO2]-induced reductions of stomatal conductance (gs) balanced the effect of increased leaf area on water use, resulting in similar water use compared to a[CO2]. In contrast, under drought conditions, water use was greater under e[CO2] than a[CO2]. The ‘CO2 fertilisation effect’ depended on cultivar and water treatment. Under well-watered conditions, aboveground biomass of the hybrid cultivar benefitted more from the ‘CO2 fertilisation effect’. However, under drought both aboveground biomass and seed yield of the non-hybrid cultivar benefitted more from the ‘CO2 fertilisation effect’. These findings show that interactions between environmental conditions (here experimental water treatments) and expression of genotypic traits (here differences between cultivars) play a decisive role in determining potential yield and growth benefits from rising [CO2]. © 2018 Elsevier B.V

Water use dynamics of dryland canola (Brassica napus L.) grown on contrasting soils under elevated CO2

Background and aims: Increasing atmospheric carbon dioxide concentration ([CO 2 ]) stimulates the leaf-level (intrinsic) water use efficiency (iWUE), which may mitigate the adverse effects of drought by lowering water use in plants. This study investigated the interactive effect of [CO 2 ] and soil type on growth, yield and water use of canola (Brassica napus L.) in a dryland environment. Methods: Two canola cultivars (vigorous hybrid cv. ‘Hyola 50’ and non-hybrid cv. ‘Thumper’) were grown in large intact soil cores containing either a sandy Calcarosol or clay Vertosol under current ambient (a[CO 2 ]) and future elevated [CO 2 ] (e[CO 2 ]), ∼550 μmol mol −1 ). Net assimilation rates (A net ), stomatal conductance (g s ) and leaf area were measured throughout the growing season. Seed yield and yield components were recorded at final harvest. Water use was monitored by lysimeter balances. Results: Elevated [CO 2 ]-stimulation of iWUE was greater than the effect on leaf area, therefore, water use was lower under e[CO 2 ] than a[CO 2 ], but this was further modified by soil type and cultivar. The dynamics of water use throughout the growing season were different between the studied cultivars and in line with their leaf development. The effect of e[CO 2 ] on seed yield was dependent on cultivar; the non-hybrid cultivar benefitted more from increased [CO 2 ]. Although textural differences between soil types influenced the water use under e[CO 2 ], this did not affect the ‘CO 2 fertilisation effect’ on the studied canola cultivars. Conclusion: Elevated [CO 2 ]-induced water savings observed in the present study is a potential mechanism of ameliorating drought effects in high CO 2 environment. Better understanding of genotypic variability in response to water use dynamics with traits affecting assimilate supply and use can help breeders to improve crop germplasm for future climates. © 2019, Springer Nature Switzerland AG

Trade-offs between water-use related traits, yield components and mineral nutrition of wheat under Free-Air CO2 Enrichment (FACE)

This study investigated trade-offs between parameters determining water use efficiency of wheat under elevated CO2 in contrasting growing seasons and a semi-arid environment. We also evaluated whether previously reported negative relationships between nutrient content and transpiration efficiency among wheat genotypes will be maintained under elevated CO2 conditions. Two cultivars of wheat (Triticum aestivum L.), Scout and Yitpi, purportedly differing in water use efficiency related traits (e.g. transpiration efficiency) but with common genetic backgrounds were studied in a high yielding, high rainfall (2013), and in a low yielding, very dry growing season (2014) under Free-Air CO2 Enrichment (FACE, CO2 concentration of approximately 550 μmol mol-1) and ambient (approximately 390 μmol mol-1) CO2. Gas exchange measurements were collected diurnally between stem elongation and anthesis. Aboveground biomass and nutrient content (sum of Ca, K, S, P, Cu, Fe, Zn, Mn and Mg) were determined at anthesis. Yield, yield components and harvest index were measured at physiological maturity. Cultivar Scout showed transiently greater transpiration efficiency (measured by gas exchange) over cultivar Yitpi under both ambient and elevated CO2 conditions, mainly expressed in the high yielding but not in the low yielding season. Nutrient content was on average 13% greater for the lower transpiration efficiency cultivar Yitpi than the cultivar with higher transpiration efficiency (Scout) in the high yielding season across both CO2 concentrations. Elevated CO2 stimulated grain yield to a greater extent in the high yielding season than in the low yielding season where increased aboveground biomass earlier in the season did not translate into fertile tillers in cultivar Yitpi. Yield increased 27 and 33% in the high yielding and 0 and 19% in the low yielding season for cultivars Yitpi and Scout, respectively. Intraspecific variation in CO2 responsiveness related mechanisms of grain yield were observed. These results suggest CO2-driven trade-offs between traits governing water use efficiency are related to both growing season and intraspecific variations, and under very dry finishes, the trade-offs may even reverse. The negative relationship between nutrient content and transpiration efficiency among wheat genotypes will be maintained under elevated CO2 conditions. © 2016 Elsevier B.V