High frequency irrigations as means for reduction of pollution hazards to soil and

water resources and enhancement of nutrients uptake by plants

Avner SILBER Institute of Soil, Water and Environmental

Sciences, Agricultural Research Organization, Volcani Centre, P.O. Box 6, Bet Dagan 50250,

Israel

Plant adaptation to low nutrient availability: different physiological mechanisms

Increasing acquisition efficiency through

modification of

Proteoid roots

Root hair

Root Shoot ratio Allocation roots

to shallow soil horizons

Rhizosphere modification with:

organic acids, protons and enzymes

Mycorrhizal symbioses

Regulation the transcript of nutrient transporters

In our world nothing is gratis; you have to pay

for goodies you get

Benefits: Improved nutrient

acquisition

Costs: Photosynthesis

products (up to 50%)

Photosynthesis improvement

Mechanisms of plant adaptation to nutrient deficiency

Enhancement of biomass production

Carbon Carbon

Carbohydrates allocated to: roots, fungus, root exudation, etc.

Lynch and Ho, 2004. Rhizoeconomics: Carbon costs of phosphorus acquisition

Alternative approach: optimising nutrient application:

Location: Nutrients are applied

in the vicinity of the roots

Plant demand: Amount and concentration of nutrient can be adjusted to crop requirements

Teaspoon feeding

Schematic presentation of the depletion zone

Water and nutrients acquisition by roots leads to differences in water and nutrients concentration between the rhizosphere and the bulk soil.

Rhizosphere Bulk soil

The depletion zone

Depletion zone

Bulk soil

Nutrient transport from the soil solution to the root surface takes place by two simultaneous processes:

Convection in the water flow (mass flow)

Diffusion along the concentration

gradient Soil-grown plant Soilless-grown plant

The grower’s dilemma: irrigation scheduling

High irrigation frequency was defined in the 1980s and the 1990s to be less than seven days intervals (Martin et al., 1990).

Nowadays?

Background

Daily cycle of plant activity in semi-arid climates is 10-14 hours

Daily transpiration is usually 5-10 mm (Shalhevet et al., 1981)

Daily cycle of irrigation is usually (using standard device) 1-3 hours

Dynamic of nutrient concentration in the root zone

Plant demand

Chemical equilibrium

Excessive rate

Deficiency rate

Irrigation Fast surface reaction

(adsorption)

Slow chemical reaction

Time

Nut

rien

t con

cent

ratio

n

Dynamic of nutrient availabilty in the root zone

Fast (hours) time-dependent processes governs nutrient concentration in the

media • Electrostatic surface reactions:

Adsorption/desorption • Precipitation\dissolution of insoluble

compounds • Microbial activity

The effect of time on solution-Zn concentration in perlite suspensions

Step I Step II

Step I: Adsorption on external surfaces Step II: Solid-state diffusion to internal binding sites

The effect of time on solution-P concentration in soil suspensions

Step I: Adsorption on external surfaces Step II: precipitation

The effect of time on solution Mn

concentrations in perlite media

pH 7.2-7.5; media height: 15-20 cm, high irrigation frequency Solution flow through the medium: 10-15 min

Biotic oxidation of Mn(II): effect of time on solution Mn(II) concentration in used perlite

suspensions

µ

Step I: Mn(II) adsorption onto the external surfaces of the bacteria

Step I Step II

Step II Extracellular Mn((II) oxidation

How to prevent nutrient deficiency in the rhizosphere during the day ?

raising nutrient concentrations

Not recommended

Environmental problems

Increases of excessive rate

Formation of insoluble compounds

dS/dt=k(Ct-Ce) (Enfield et al., 1981)

Alternative I: Force

Alternative II

• Supplying water and nutrients at a similar rate of plant uptake throughout the potential transpiration cycle.

Reducing discharge rate of emitter

Increasing the frequency of irrigation

Plant demand

Chemical equilibrium

Excessive rate

Deficiency rate

Time

Irrigation N

utri

ent c

once

ntra

tion

High irrigation frequency may affect the uptake of nutrients by plants through:

• Increased temporal water content (θ):

• Increased nutrient availability:

Enhanced the convection flow

Frequent replenishment of nutrients in the depletion zone

Enhanced the diffusive movement

Diffusion coefficient of nutrient ion in water (Di) and order of magnitude in soil (De; cm2 s-1) (from Barber, 1995)

NO3-

K+ H2PO4

-

Di (250 C)

1.9x10-5

2.0x10-5

0.9x10-5

De (soil)

10-6-10-7

10-7-10-8

10-8-10-11

Diffusive movement (cm/day)

1.3

0.13

0.004

θ=moisture content De = Diθf(dCi/dCs) f=tortuosity f (θ)

Increased temporal water content (θ)

Decreased water suction (ψ)

Increased hydraulic conductivity (K)

Enhanced transport of nutrients by convection (mass flow)

Measured

Calculated

Ψ

Irrigation

Bulk soil

Depletion zone

Replenishment of nutrients

After irrigation: formation of depletion zone induced by

water and nutrients acquisition by roots

Hypothesis

Continuous application of water and nutrients at a similar rate as plant uptake throughout the potential transpiration cycle may reduce fertilizer quantities needed to achieve optimum yield

Results: • Increasing irrigation frequency improved time-

averaged water availability • The effect of irrigation frequency on water

uptake (per unit of leaf area) or on leaf conductance was meager

• Increasing irrigation frequency improved yield • The main effect of irrigation frequencies was on

the uptake of nutrients • Differences in leaf-P concentration between

treatments were accounted for the majority of variations in DW production

Results: • Adjustment of the NH4/NO3 ratio under

high irrigation frequency is necessary • Irrigation frequency significantly affected

the rhizosphere pH • Irrigation frequency significantly affected

root system and the root/shoot ratio • Irrigation frequency may have a negative

role on diseases incidence

Results:

• Increasing irrigation frequency improved time-averaged water availability

• The main effect of irrigation frequencies was on the uptake of nutrients

• Differences in leaf-P concentration between treatments were accounted for the majority of variations in DW production

• Irrigation frequency significantly affected root system and the root/shoot ratio

Soilless-grown bell pepper: effect of irrigation frequency on daily variations of water tension

Soil-grown bell pepper: effect of irrigation frequency on daily variations

of matric potential in soil (0-20 cm)

Irrigation

Irrigation

Water stress Water uptake under non-stress condition

Soilless-grown bell pepper: effect of irrigation frequency on water uptake

(rate per leaf unit area)

Water uptake (per unit of leaf area) was not affected by irrigation frequency

Soilless-grown bell pepper: effect of irrigation frequency on

leaf conductance

Effect of irrigation frequency on leaf area (m2/plant)

Soilless-grown bell pepper: effect of irrigation frequency on water uptake

(rate per plant)

The increases of water uptake resulted from higher DW production

Soilless-grown lettuce: effect of irrigation frequency and P concentration on yield

General • As long as water availability did not

limit plant growth, yield improvement can be primary attributed to enhances availability of nutrients

• Effect of irrigation frequency on nutrient concentration in plant followed the order: P>K>N

Soilless-grown bell pepper: effect of irrigation frequency on leaf-P

concentration

Soilless-grown bell pepper: effect of irrigation frequency on fruit-Mn

concentration

µ

Relationship between fruit-Mn content and blossom-end rot incidence?

Multiple stepwise regression analysis: pot-grown lettuce

Multiple stepwise regression analysis: soilless-grown bell pepper

Relationships between DW production and leaf-P concentrations, as

determined by irrigation frequency

Combination of high irrigation frequency and NH4

+ nutrition

Negative outcome Increase the hazards

of NH4 toxicity

High transient NH4 concentrations in the rhizosphere

Bell pepper: effects of irrigation frequency and irrigation-NH4-N concentration on the

vegetative growth

Bell pepper: effects of irrigation frequency and irrigation-NH4-N concentration on the

yield

The effects of irrigation frequency on rhizosphere-pH of wax flower plants

Mechanism

Nitrification decreases the temporal concentrations of NH4 between consecutive fertigation

NH4++2O2 NO3

-+H2O+2H+

Increasing irrigation frequency

Increasing temporal concentration of NH4

+

Mechanism

NH4+ NO3

- H+ OH-

Increasing NH4+

concentration Increasing irrigation frequency

Reducing soil pH

Effect of N source on pH in the vicinity of roots (rape plant)

Based on Gahoonia and Nielsen, (1992a)

Unplanted

soil

Possible effects of irrigation frequency on root system

Increased irrigation frequency

Enhancing P uptake by plant

Decreasing root/shoot ratio

Changing wetting patterns and water distribution in soil volume Shallower root system

Indirect effect Direct effect

Soil-grown bell pepper: root distribution

Soilless grown bell pepper: integrated effect of irrigation frequency and P level

on root/shoot ratio

The effect of leaf-P concentration on root/shoot ratio

Soil-grown melon: negative effect of irrigation frequency

The increases of θ value which is beneficial for the uptake of water and nutrients, may have a negative role on diseases incidence, especially on soilborne pathogens.

Based on Pivonia et al. (2004)

Conclusions • High-frequency irrigation regimes enhance

the time-averaged moisture content in the root region.

• The main beneficial effect of high fertigation frequency may be related to an improvement of P, K and micronutrients mobilisation and uptake.

Conclusions

• An increase in fertigation frequency enables the concentrations of immobile elements in irrigation water to be reduced, so reducing environmental pollution.

• Frequent irrigation, in combination with NH4 nutrition, may be very effective for modifying the pH and, consequently, nutrients availability in the rhizosphere.

Caution

• High irrigation frequency may cause severe damage to crops as a result of soilborne pathogens.

• Adjustment of fertilisation regime, especially that of NH4 concentration is recommended.

Thank you

Integrated effect of irrigation frequency and P level on leaf-starch concentrations

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Under normal P condition the product of photosynthesis process (carbon) is converted to hexose-P. Under P deficiency starch is accumulated.