Measuring Subsidence in a Degraded Peat Soil: Nitrogen Mineralization

under Rice Cultivation

Emilie Kirk Bruce Linquist

Chris van Kessel University of California, Davis

Aragon, 2013

Sacramento - San Joaquin River Delta

Cross-section of Twitchell Island

Deverel et al., 2007

Average soil loss of 1-3 cm per year!!

>6m below mean sea level

160 levee failures in the last century in California DWR, 2007

Why grow rice in the Delta?

• Slow or reverse soil subsidence

• Maintain flooded conditions most of the year, mimicking seasonal wetlands

Anaerobic soil should reduce microbial activity

• Agriculture continues in Delta

N Fertilizer Study

• 180 kg N/ha average uptake with no N fertilizer!

• N uptake in well-fertilized CA rice ~140 kg N/ha Linquist et al., 2009

Nitrogen Budget Objectives

Quantify the environmental sources of N under rice production on Twitchell Island

Use N mineralization as a proxy to estimate subsidence

Nitrogen Budget Hypotheses

1. Majority of plant-available N from peat mineralization

2. A small amount of N from previous year’s crop residue

3. N from water sources are negligible

Materials & Methods: Site Setup

- RCBD experiments with 4 blocks

- Replicated in 2 fields

- Soils are Typic Haplosaprists

13-15% C in the upper 30cm

- Conventionally managed paddy rice fields

Materials & Methods

Residue • In situ mesocosms

+ H2O

- H2O

• Surface water sampled for growing season

Water

• 15N-tracer method for growing season

• Field residue sampled over-winter

30 c

m

60 cm

Annual N Budget Components

Growing Season Over-winter

Peat +

Residue +

Subsurface Water +

Irrigation Water

Peat +

Residue

Growing Season N Budget: Surface water

Irrigation water 3 kg N/ha

Methods:

[Inlet N] * ET

Unfertilized N Uptake = 167 kg N/ha

Growing Season N Budget: Crop residue

Irrigation water 3 kg N/ha

Rice residue 4 kg N/ha

Methods: 15N-labeled rice residue tracer study

Unfertilized N Uptake = 167 kg N/ha

Subsurface N 40 kg N/ha

Growing Season N Budget: Subsurface N

Irrigation water 3 kg N/ha

N assumed from peat mineralization! • Peat layer extends several meters • NH4

+ enriched groundwater •No difference in d15N between treatments

Unfertilized N Uptake = 167 kg N/ha

Rice residue 4 kg N/ha

Subsurface N 40 kg N/ha

Growing Season N Budget: Peat

Irrigation water 3 kg N/ha

Peat mineralization 120 kg N/ha

Methods: By difference – remaining N from surface peat mineralization

Unfertilized N Uptake = 167 kg N/ha

Rice residue 4 kg N/ha

Growing Season Mineralization Section 1: Plant N derived from source Field 1

N uptake from peat (kg N ha-1) 160 (12)

N uptake efficiency (NUE) (%) 0.50

Total N mineralized from peat (kg N ha-1) 320 (25)

0

300

600

900

1200

1500

1800

0 10 20 30 40 50 60 70 80 90 100

N m

iner

aliz

ed f

rom

pea

t

(kg

N h

a-1)

NUE (%)

Over-winter Mineralization

Spring soil NO3 – rice straw mineralization N

= over-winter peat mineralization N

Section 2: Overwinter mineralization Field 1

Spring soil NO3-N (kg N ha-1) 42 (0.5)

N mineralized from straw over-winter (kg N ha-1) 21 (2)

Over-winter peat N mineralization (kg N ha-1) 21 (2)

Mineralization Subsidence Growing season

mineralization

320 kg N ha-1

+

Over-winter mineralization

21 kg N ha-1

=

Total N mineralized from peat

341 (44) kg N ha-1

Factors for calculations:

- C:N

- Carbon input from straw

- Soil C%

- Bulk density

Mineralization Subsidence

Field 1

Mass soil lost

-24300 (5103) kg soil ha-1

↓ Bulk density 0-30cm

78200 (2000) (kg cm-1 ha-1)

Estimated Subsidence

- 0.31 (0.03) cm

Field 2

Mass soil lost

-22440 (4264) kg soil ha-1

↓ Bulk density 0-30cm

55500 (2500) (kg cm-1 ha-1)

Estimated Subsidence

- 0.40 (0.05) cm

The Big Picture

Corn

Soil loss

- 2.5cm/year (Hatala et al. 2012)

Rice

Soil loss

- 0.3 - 0.4cm/year

Wetlands

Soil gain

Special thanks to

Funding: NIFA - Ag. and Food Research Initiative UCD Dept. Plant Sciences

C. Abernilla J. Casey B. Gornto

C. Mikita M. Lundy M. Simmonds

G. Pearson And many others!

Emilie Kirk MSc. Candidate, Soils & Biogeochemistry erkirk@ucdavis.edu 530 – 204 – 9030

Threat to fresh water supply

RMA, in CALFED 2007

NUE Assumptions and Subsidence

y = 110.71x-1.528

R² = 0.9693

0

0.5

1

1.5

2

2.5

3

3.5

4

0 10 20 30 40 50 60 70 80 90 100

Est

ima

ted

Su

bsi

den

ce (

cm y

r -1

)

Nitrogen Uptake Efficiency (NUE) (%)

Range I: Mineralization pulses, poor synchrony

Range II: Mineralization continuous, good synchrony

15N recovery – only a small proportion was accumulated in the

AGB

0

10

20

30

40

50

60

70

80

90

100

Straw Straw+Fert Straw Straw+Fert

Field 3 Field 4

Per

cen

t 15N

rec

ov

ery

(%

)

Soil Aboveground Biomass Losses

Subsurface N Discussion

0

4

8

12

16

20

0 5 10 15 20

Wel

l dep

th b

elow

soil

surf

ace

(ft)

NH4-N (ppm)

Twitchell Island Groundwater NH4-N by Well Depth, 7-8 Aug.

2012

Field 1 NH4-N Field 10 NH4-N

Response to N fertilization: grain yield and N uptake across 4 siteyears

ab ab a

ab b

a ab

bc c c

R² = 0.8937

R² = 0.9841

0

100

200

300

400

500

0

2000

4000

6000

8000

10000

12000

0 40 80 120 160

N U

pta

ke

(kg N

ha

-1)

Gra

in Y

ield

(k

g h

a-1

)

N Fertilizer Applied (kg N ha-1)

Grain yield Aboveground Biomass N uptake