1. Kate Scow
Russell Ranch
Adina Paytan
Isotope Geochemistry
Asmeret Berhe
Soil Biogeochemistry
Phosphorus Cycling in Soils
Assessing the Impact of Agricultural
Practices on Phosphorous Availability and
Loss Using Oxygen Isotopes of Phosphate

2. Phosphorus (P) is a key limiting nutrient in terrestrial ecosystems
because most soil P is found in pools of low plant availability:
bound to calcium, aluminum or iron minerals, or in low lability
organic compounds.
Increasing P cycling rates can increase P availability, including in
agricultural soils that receive external P inputs.
In intensive agriculture, external inputs such as mineral
fertilizers, manures, and composts can increase phosphorus
availability to crops, but a significant fraction of these inputs is
not taken up by plants and ends up “fixed” in soil pools of low
plant availability or lost through runoff.
Background

3. Determine soil P availability and mobility and how these
characteristics vary with soil type and agriculture practices.
This should help reduce P loss from agriculture systems and
contribute fundamental understanding to inform science based
management plans.
Tracking P cycling, mobility in soils, and determining soils’ P
availability to plants is challenging because adsorption-
desorption, immobilization (occlusion by or precipitation as
minerals), mineralization (conversion of organic P compounds to
Pi), and uptake all occur simultaneously in the soil
Goals

4. Test a newly developed isotope system to shed light on soil P
cycling under different management practices.
Oxygen isotopic composition of phosphate (d18Op) which is
associated with various pools of P in soil (soil solution, loosely
adsorbed, Fe and Al associated, and Ca associated) is used to
elucidate some of the P transformations that take place within the
soil system under different agriculture management practices.
Does P cycling in the soil change over time?
Approach

5. Oxygen Isotopes in Phosphate

6. P Sources and Cycling – d18Op
The P-O bond is resistant to inorganic hydrolysis
(Longinelli, 1976)
The P-O bond is broken in enzyme-mediated biochemical
reactions (Dahms and Boyer, 1973; Boyer, 1978)
Intracellular oxygen isotopic exchange between phosphate and
water is rapid and involves equilibrium fractionation
(Paytan et al., 2002; Blake et al., 2005).
Kinetic fractionations associated with hydrolysis of specific
organic P enzymatic processes
(Blake et al., 2005; Liang and Blake 2006, 2009).
Oxygen Isotopes in Phosphate

7. d18OP a tracer for source and cycling
Recycling
in the biomass
Source A
d18Op IP
Source B
d18Op IP
d18Op IPsources
d18Op IP
Mixture of
source signature
and cellular
turnover
Simple case – Only intracellular cycling
Intracellular P cycling by
pyrophosphatase results in a
temperature-dependent equilibrium
oxygen isotope fractionation

8. d18OP a tracer for source and cycling
IPRecycling
in the biomass
Source A
d18Op OP d18Op IP
Source B
d18Op IP and OP
d18Op IPsource
Extracellular hydrolysis
Extracellular OP hydrolysis is
accompanied by kinetic isotope effects
and incorporation of water oxygen. The
fractionation depends on substrate type
and the enzymes involved and results in
disequilibrium isotope effects
d18Op OPsource
d18Op - Mixture of
source signature,
cellular turnover
and regenerated IP
d18Op IPregenerated
OP

9. d18OP a tracer for source and cycling
Equilibrium isotope fractionation associated with pyrophosphatase P cycling
inside cells – both DIP and DOP from cell lysis or excreation will have an
equilibrium signature (equilibrium equation, Temp. d18Ow).
Kinetic fractionation P-monoesters hydrolysis by Alk-Pase -30‰
Kinetic fractionation P-monoesters hydrolysis by 5’Nase -10‰
Kinetic fractionation P-monoesters hydrolysis by Acid-Pase -7 to -10‰
Kinetic fractionation RNA hydrolysis by PDase + Alk-Pase +20‰ and -30‰
Kinetic fractionation RNA hydrolysis by PDase + 5’Nase +20‰ and -10‰
Kinetic fractionation DNA hydrolysis by PDase + Alk-Pase -20‰ and -30‰
Kinetic fractionation DNA hydrolysis by PDase + 5’Nase -20‰ and -10‰
Kinetic fractionation P-monoesters hydrolysis by Phytase -10 ‰
Offset from equilibrium suggests hydrolysis of organic P
The deviation from equilibrium can be used in a simple isotope mass balance
model to estimate the fraction of phosphate derived from DOP remineralization.

10. Work takes place at the Russell Ranch Experimental LTRAS, a
long-term comparison of 10 conventional, organic and alternative
cropping systems, both irrigated and non-irrigated.
Select treatments are compared and the changes in d18Op
among treatments and within each treatment plot with depth and
over time are compared.
Experimental Setup
Russell Ranch’s ongoing
experiments is a 100-year study
on Agricultural Sustainability,
which is comprised of 72 one-
acre plots.

11. https://www.youtube.com/watch?v=Y4tUa8lUmJ4#action=share

12. The Century Experiment measures the long-term impacts of crop rotation, farming
systems and inputs of water and nitrogen on agricultural sustainability. The
cropping systems include rain-fed and irrigated systems, organic (chicken manure
87 lbs/year P) and conventional (45 lbs/year P) systems, and different forms and
quantity of nitrogen inputs. The study began in 1993. Currently, the Century
Experiment contains ten systems that are two-year rotations and include
corn/tomato, wheat/tomato, wheat/fallow and wheat/legume rotations. Additionally,
a perennial native grass system and a 6-year alfalfa-corn-tomato rotation were
initiated in 2012.

13. Soil cores from several replicate plots used:
Organic fertilizer, full irrigation, corn/tomato alternating plots
Organic fertilizer, full irrigation, tomato/corn alternating plots
Mineral fertilizer, full irrigation, tomato/ corn alternating plots
Mineral fertilizer, full irrigation, corn/tomato alternating plots
Depths: 0-15cm, 15-30cm, 30-60cm, and 150-200cm
Organic 87 lb ac-1 year-1 P
Mineral 45 lb ac-1 year-1 P
Experimental Setup
Fertilization calculated based on N requirements
P use is inefficient (low recovery of inputs in crops, excess P in soil)
Organic treatments less efficient than Mineral treatments (higher surplus)
Corn - %P in crop similar across treatments; hence yields for Mineral > Organic
Tomatoes - %P Organic > Mineral; hence yields similar across treatments
Soil P accumulation less than expected from mass balance (loss?)
Maltais-Landry et al., 2015

14. -250
-200
-150
-100
-50
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
depth(cm)
PO4-P and TP (ppm)
H2O extraction across treatments
org C/T H2O PO4-P (ppm) org C/T H2O TP (ppm)
org T/C H2O PO4-P (ppm) org T/C H2O TP (ppm)
min C/T H2O PO4-P (ppm) min C/T H2O TP (ppm)
min T/C H2O PO4-P (ppm) min T/C H2O TP (ppm)
-250
-200
-150
-100
-50
0
0.0 0.2 0.4 0.6 0.8
depth(cm)
PO4-P and TP (ppm)
NaHCO3 extraction across treatments
org C/T NaHCO3 PO4-P (ppm) org C/T NaHCO3 TP (ppm)
org T/C NaHCO3 PO4-P (ppm) org T/C NaHCO3 TP (ppm)
min C/T NaHCO3 PO4-P (ppm) min C/T NaHCO3 TP (ppm)
min T/C NaHCO3 PO4-P (ppm) min T/C NaHCO3 TP (ppm)
-250
-200
-150
-100
-50
0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
depth(cm)
PO4-P and TP (ppm)
NaOH extraction across treatments
org C/T NaOH PO4-P (ppm) org C/T NaOH TP (ppm)
org T/C NaOH PO4-P (ppm) org T/C NaOH TP (ppm)
min C/T NaOH PO4-P (ppm) min C/T NaOH TP (ppm)
min T/C NaOH PO4-P (ppm) min T/C NaOH TP (ppm)
-250
-200
-150
-100
-50
0
0 2 4 6 8
depth(cm)
PO4-P and TP (ppm)
HCl extraction across treatments
org C/T HCl PO4-P (ppm) org C/T HCl TP (ppm)
org T/C HCl PO4-P (ppm) org T/C HCl TP (ppm)
min C/T HCl PO4-P (ppm) min C/T HCl TP (ppm)
min T/C HCl PO4-P (ppm) min T/C HCl TP (ppm)
Blue shades – Organic
Red shades – Mineral
Filled – TP
Empty – SRP
SRP&TP decrease with
depth; Impact seen in
upper 50cm.
SRP & TP are higher in
the Organic treatments
for all fractions.
P Concentration Results

15. Isotope Results
Extraction Manure (‰) Fertilizer (‰)
H2O 18.38 15.88
NaHCO3 18.40 19.15
NaOH 19.42 11.35
HCl 14.37 11.28
Source Isotopic Signature (‰)
Soil/bedrock P 10
Fertilizer P 10
Manure P 20
Equilibrium Value 16
15.88‰ = All recycled extensively by biomass
19 ‰ = 75% of P from organic sources not utilized
18 ‰ = 60% of P from organic sources not utilized
14 ‰ = 60% of P from bedrock and 40% organic sources not recycled
11 ‰ = Less available pools ~20% recycled 80% not biologically used

16. Thank you!