Efficient nutrient use - lessons from roots Flashcards
480 million metric tons of milled rice is produced annually. China and India alone account for around 50% of the rice grown and consumed.
Rice provides up to 50% of the dietary caloric supply for millions living in poverty in Asia and there is a critical food security crop. It is becoming just as important in Latin America and Africa
Rice paddy fields are highly productive- no limitation by water, rice has little competition from weeds and nutrients are made available to rice by waterlogging - up to three rice crops can be planted in a year
But paddy fields are contributing >10 % of global methane emissions, and have made increasing contributions to N2O fluxes too.
Populations of low-income countries increased by 90% between 1966 and 2000, paddy rice production has increased 130% from 1960’s to 2000. About 84% of the rice-production growth is due to semi-dwarf, early-maturing rice varieties that can be planted up to three times per year and are responsive to nitrogen fertilizers.
These new rice varieties grown in irrigated land contribute to nearly three-quarters of the world’s total rice production.
Need to manage water in paddy fields
SEE SLIDES FOR REFS
Nitrous oxide emissions from intermittently flooded fields could be 30-45 times higher than reported under continuous flooding. Climate impacts of rice cultivation could be reduced 90% through co-management of water, nitrogen and carbon
Chemical properties
Redox potential (eH) – oxidation-reduction status
Controlled by the water saturation of soil
Related to the soil structure, drainage and water inputs
Soil saturated with water quickly runs out of available O2
Redox
Strong gradients of redox potential occur even in freely draining soil due to biological oxygen demand.
(Sheppard & Lloyd 2002)
Plants adapted to wetland conditions supply oxygen to their roots and increase the redox potential of soil
(Justin & Armstrong, 1987)
Redox reactions in soil are mainly biologically-driven.
Organisms turn to to alternative electron acceptors when O2 is scarce.
Redox reactions have important biological consequences.
Reduction removes free H+ ions therefore pH increases. Oxidation releases H+ there pH falls.
Reduction produces potentially toxic products, Fe2+, NO2-, CH4
63% of methane sources
are anthropogenic (Conrad 2009)
Global warming potentials:
N2O is 296 X greater than an equal mass of CO2
CH4 is 20 x greater than an equal mass of CO2
Vertical distribution of roots reflects capacity to take up water (and nutrients) from soil by depth
Increasing interest in the use of diverse herbal leys (for 3-4 years) in crop rotations including deep rooting species and nitrogen fixing legumes that have potential to improve soil structure and nutrient status of arable soils.
Mean residence time of root C is 2.4 x that of shoot C.
Lignin % in roots is typically 2.2 x that of shoots.
Lignin/N ratio in roots is 2.4 x that of shoots.
Over 80% of plant species form mycorrhizal associations to assist uptake of P and N.
How do the 15-20% of species that do not form mycorrhizas cope?
Root adaptations: very long root hairs and secretion of large amounts of organic acids to facilitate P uptake e.g.
Brassicaceae: cabbage, cauliflower, sprouts, turnips, oilseed rape, mustard
Chenopodiaceae: Sugar Beet
Plants that lack mycorrhizas depend upon root hairs to increase effectiveness of root P capture.
But note- the more phosphorus is available the less efficient becomes the roots.
Adding fertilizer requires more fertilizer to be added?
(Ma et al., 2001)
Cropping systems remove P in the harvested crop- many cereal crops put most of the P in above ground biomass into the grain and leave very little in the straw and senesced leaves and roots.
The proportion of the total P ending up in grain is normally >50%, and often as high as 90%.
(Richardson et al., 2011)
However, much of the P in grain (e.g. 88% in wheat) can be stored in phytate or
phytic acid (inositol hexakisphosphate) which is not readily digested and ends up in manure.
Strategies for improving P efficiency in cropping systems.
Greater use of crop species with cluster roots in environments with large amounts of total P but low P availability – e.g. Lupinus albus grown on young volcanic soils in Chile.
Potential of native species with P-efficient traits for use in crops or pastures. ?New crops?
Breeding of greater P-use efficiency in existing crop species.
Better understanding of the molecular basis of P-efficient traits
Intercropping and crop rotations to provide benefits of P-efficient crops to less efficient ones.
Specialised root functioning –
Root adaptations to secure critical limiting nutrients:
N, P and Fe
Some plants that do not form mycorrhizal associations produce specialised root adaptations
Non-mycorrhizal Proteaceae (with cluster roots)
and mycorrhizal Myrtaceae in Western Australia
Erik Veneklaas, unpublished
Increased surface for nutrient absorption?
Absorptive surface is enormous, but far greater than functional: overlapping depletion zones!
However, cluster roots release organic compounds into the soil
Increased surface is ideal to locally enhance the concentration of the exudates.
Plants that form cluster roots often have N fixing symbiosis-
Rhizobium nodules or
actinorrhiza (Frankia)
but rarely have mycorrhiza.
Cluster-root development is primarily an adaptation to uptake P- these specialized roots are suppressed at a high P supply
(Shane et al., 2005; Physiol. Plant.)
Root organic acid exudation as a ‘burst’ coincident with attainment of maximum of root surface area.
Proteoid root production is stimulated by P limitation-
but also by limitations of some other elements e.g. Fe
(Liang & Li 2003)
Iron (Fe) is very insoluble in soils of neutral and alkaline pH unless the soil is anaerobic.
Fe2+ is much more soluble than Fe3+.
In response to Fe deficiency plants can acidify the rhizosphere, and produce reductase enzymes that reduce Fe3+ to Fe2+.
(Romheld & Marschner 1981)
Dauciform roots are characterised by exceptionally long root hairs produced at a very high frequency per unit root length
Rates of P uptake in Schoenus unispiculatus are related to the proportion of the root system that is of dauciform roots
Dauciform roots release exudates, in an pulse coupled to root maturity, just like cluster roots in Proteaceae and Lupinus albus
(Shane et al., 2005)
NOTE- many legume trees support both N fixing symbiosis (typically arbuscular mycorrhiza) plus nodulating N fixing bacteria.
Ribeiro-Barros et al., (2018)
Sustainable agro-ecosystems
Potential to breed crops for greater nutrient use efficiency
Root hairs
Root branching
Root branching angles
Root fineness
Specialized roots for “mining” P and Fe from soils
Crop rotations
Intercropping plants with different rooting traits- especially N fixing
Use of deep rooted perennial intercrops / field margin plants to capture and recycle nutrients from deeper than annual crop roots
Breeding to promote symbiotic associations with N fixing organisms and mycorrhizas.
Management to promote mycorrhizas and N fixers
Microbial inoculants to increase nutrient use
Reduced nutrient demand- selection for reduced phytate accumulation in grains
