Talk summary. ⢠Basics of soil N ... Arctic. ⢠New techniques for N tracing in the rhizosphere ... How much N is there stored in soil? ... What does this tell us about N cycling? ..... Intense competition exists between plants and microbes for nitrogen.
Nitrogen dynamics in the rhizosphere Modelling and experimental approaches
Davey Jones, Paul Hill, John Farrar, Mark Farrell, Paula Roberts, Liz Boddy, Liz York, Helen Glanville Kevin Newsham, David Hopkins, Tiina Roose, Andrea Schnepf, Richard Bardgett, Dan Murphy, Liz Stockdale, Matina Christou
Talk summary • • • • • •
Basics of soil N cycling Ecosystem scale models What types of N exist in soil Regulation of N cycling Rhizosphere scale models Examples – Antarctic – Europe – Arctic
• New techniques for N tracing in the rhizosphere
How many papers are there on soil nitrogen? In June, 2013: 251,357 papers Have we got enough information to model soil nitrogen well?
Using modelling as a tool to understand rhizosphere N flow …the last resort…..until recently
Soil nitrogen cycling “101” 1. How much N is there stored in soil? Soil (0-50 cm) = 5000 t /ha 5% soil organic matter = 250 t/ha C = 40%, C:N ratio = 12.5 Soil organic N = 8 t N/ha (8000 kg N/ha)
2. How much does a growing crop need? Yield 10 t/ha Grain N = 1.5% 150 kg N/ha/y
What does this tell us about N cycling?
Soil nitrogen cycling “101” 3. What is the main soluble form of N in soil? Soil water = 10 mg NO3- / l 5000 t soil / ha, 20% moisture =1,000,000 litres water/ha =10 kg NO3- / ha
4. What are the main inputs/outputs of N from soil? Inputs = rainfall, canopy throughfall, inorganic fertilizer, manure, roots & microbial death Outputs = leaching, runoff, gaseous losses (N2O, N2, NH3, NO3-)
Soil nitrogen cycling “101” 5. How much microbial N is there in soil? 1% of soil organic matter is microbial biomass 2.5 t biomass / ha 1.25 t microbial C / ha C:N ratio = 8:1 ≈150 kg microbial-N / ha
Soil microbial N in different land uses
Soil microbial N (kg N/ha)
Mean = 175 kg N/ha 1000
8000
600
400
300
Box/process modelling
Stella
Modelmaker
Crop N demand (150)
N2O, N2
Michaelis Menten kinetics V = (𝑉𝑚𝑎x . C C + Km )
Organic matter N (8000)
Soil water N First order kinetics
(10)
Microbial N (150)
V =k.C
𝐶 = 𝐶0 × exp−k.t … … .
NO3-
The Soil Nitrogen Cycle
DNDC: DeNitrification-DeComposition model
Previous work with DNDC
How good is the DNDC model?
Different crops, climates, locations, model versions
Variable climate data (East Anglia, UK) 500
25.0
450 20.0 400 350
250
10.0
200 5.0 150 100 0.0 50
Year
10
9
8
7
6
5
4
3
-5.0 2
0 1
Rainfall
300
Temperature
15.0
Different versions of DNDC: N2O emission prediction Version 82
Version 86
Version 90
Version 91
Version 92
Version 95
Different crops
Beet Peas Lettuce Beans Wheat Potatoes Brassicas
DNDC conclusions • Partly mechanistic model • Works well for maize in North Carolina • International acceptance
• • • • •
Parameter overload No validation in UK soils Changes in model not documented Needs ‘extra’ factors for other locations Implies our understanding of C and N cycling is poor
Back to the drawing board “Parameterisation and validation”
What forms of soluble N exist in soil?
Soil solution N in European soils
Citrus
Grasslands
Arable
Heathland
Vegetable
Wetland
Biofibres
Forest
Rank these in order of the type and amount of N present
Citrus
Grasslands
Arable
Heathland
Vegetable
Wetland
Biofibres
Forest
Soil solution N concentrations in European soils
Dissolved organic nitrogen (the forgotten N form in soil)
Where is the bottleneck in soil N cycling?
Protein Peptides Amino acids NH4+ NO3-
Using modelling to determine rates of N cycling in soil Crop N demand (150)
Organic matter N (8000)
Soil water N (10)
N2O, N2
Microbial N (150)
NO3-
Isotope (14N, 15N, 12C, 13C, 14C) tracking in the field
Amino acid tracking in the field
DON quantified Soil solution removed
14C-15N-amino
acid added
Measure 14CO2 over time
Uptake into Plants (15N, 14C)
Soil samples
Inject back into soil
Uptake into microbes
Amino acid depletion from soil 𝑓 = 𝑎1 × exp−k1t + 𝑎2 × exp−k2t + 𝑎3 × exp−k3t Triple first order kinetic equation
14
C remaining in soil (% of total)
100
95
90
85
80
75
k1:T1/2 = 15 – 20 mins
70 0
5
10
15
Time (hours)
20
25
Amount of 14C recovered after 48 h (% of total)
Fate of amino acid-C in soil 60
50
40
30
20
10
0
CO2
Microbial biomass
Roots
Shoots
Amino acids Crop N demand (150)
Organic matter N (8000)
Soil water N
(0.3)
N2O, N2
Microbial N (150)
NO3-
Amino acid concentration =0.3 kg N/ ha Amino acid half life = 20 min Flux = 1300 kg N / ha / y
Turbocharged microbial biomass Soil solution amino acid pool turns over 4300 times per year
Where is the bottleneck in soil N cycling?
Protein Peptides Amino acids NH4+ NO3-
60 14 14
50
C-Protein C-Amino acid mixture
First order kinetics
40
14
CO2 evolution (% of total
14
C substrate added)
Protein turnover is 50x slower than for amino acids
30
20
Non-first order kinetics
10
0 0
24
48
72
96
Time (hours)
120
144
168
Microbial peptide utilization is twice as slow as for amino acids Susbtrate utilization rate (nmol g-1 h-1)
400
Peptide Amino acid 300
200
100
0 0.0
0.5
1.0
1.5
2.0
Peptide or amino acid conc. (mM)
2.5
The main organic nitrogen degradation pathway in soil
50 SOM
Protein
2
Peptide
1 Amino acids
2 NH4+
NO3-
Block in acid soils
The bottleneck lies here (proteases)
How can we use this information? CO2
Soil respiration
0 Amino acids
Peptides
Adenosine Nucleoside
Amino sugar
2
Glucosamine
4 Carbohydrates
6
Sucrose
Glucose
Fructose
Valine
Tyrosine
Phenylalanine
Lysine
Leucine
Isoleucine
Glycine
Glutamine
Aspartic acid
Arginine
Alanine
Peptides
Organic acids
Field 2009 Field 2010
Succinic acid
Shikimic acid
Salicylic acid
Oxalic acid
Malonic acid
Malic acid
Glutamic acid
Formic acid
8
Citric acid
Acetic acid
% substrate C contribution to respiration
Mathematical modelling of the importance of dissolved organic C and N in soil respiration
=
Pool size x Flux = Importance
Plant N uptake
The historical view of plant nitrogen acquisition Protein Soil microbes
Soil particle
Peptides
Amino acids
Deaminase
NH4+
NO3-
NH4+
Peptidase
Most N enters soil as protein, which is modified by soil microorganisms until available to plants as inorganic N
Protease
Root
It is now widely accepted that plants can take up nitrogen earlier in protein decomposition Protein Soil microbes
Soil particle
Peptides
Amino acids
Deaminase NH4+
NO3-
NH4+
Peptidase
In some environments, plants can compete successfully with soil microbes for amino acids
Protease
Root
Both plants and soil microbes possess transporters for small peptides. Peptide uptake is energetically favourable
Peptide R F VK K GV
Extracellular medium
R
R F VK K V G R
Cytoplasm
But is this of any significance to the terrestrial nitrogen cycle?
The maritime Antarctic
Signy Island South Orkneys 60º43’S 45º36’W
Antarctic soils
Bare
Higher plants
Penguin colonies
Lichens
Bryophytes
Algae
Higher plants
Extensive carpets of moss with large stocks of proteinaceous organic matter
Over the last 50 y some areas of the maritime Antarctic have warmed almost an order of magnitude faster than the global mean with a concurrent expansion of vascular plants Guglielmin et al., 2012
Over the last 50 y some areas of the maritime Antarctic have warmed almost an order of magnitude faster than the global mean with a concurrent expansion of vascular plants
2°C change
Guglielmin et al., 2012
D. antarctica is often found growing in competition with mosses especially Sanionia uncinata
Both plants have temperature optima for photosynthesis above current maritime Antarctic summer temperatures.
But nutrients are needed to exploit temperature rises
Unlike mosses D. antarctica has roots which penetrate into the organic matter stored in the soil
ca.20% of the soil soluble N pool under grass and moss is small (
