Changes in water-holding capacity of fine slate waste during decomposition of added

plant litters.

Mark Nason, Farrar JF, Healey JH, Jones DL, Williamson JC, Rowe EC.

University of Wales, Bangor

m.a.nason@bangor.ac.uk

Background

• In natural systems, soil organic matter accumulation depends on the input of carbon as plant litter

• Soil water holding capacity is determined by Soil Organic Matter(SOM) content

• Establishment of plants in old hard-rock quarries is likely to belimited by availability of water

• Chemical characteristics of litters from different plants vary

• Rate of soil organic matter accumulation is dependent on litterdecomposition rate

• Litter decomposition rate is dependent on litter chemical composition

Feedback mechanisms between plant growth and soil development

CENTURY Soil C

Soil Water

Rain

Plant C

Photosynthesis

Litterfall

Drainage

Litter

Metabolic litter Structural litter

Active SOM

Slow SOM

Passive SOM

CENTURY SOM Sub-model (Parton et. al., 1987)

1) Addition of plant litter to slate sand will increase its water holding capacity

3) The increase in soil water holding capacity caused by addition of low C:N leaf litter will be of shorter duration than the increase in water holding capacity caused by addition of high C:N leaf litter

2) Leaf litter decomposition rate can be predicted from litter characteristics (C:N, %lignin)

4) The amount of C in one CENTURY soil organic matter pool will be more closely correlated with soil water holding capacity than total soil C.

Hypotheses

Experimental Design

• Senesced leaf litter collected in litter-traps during autumn 2000• Chopped to pass an 8mm sieve• Thoroughly mixed with slate sand • 5g woodland soil inoculum added• Incubated outside

4 treatments (litter amendments)

Amendment Rate (dry g per 500g slate sand pot)

NoneAlder litter 10Birch litter 10Alder and birch litter 5 + 5

3 replicates x 6 retrieval times (0, 2, 4, 8, 16, 32, 64 weeks)Completely randomised

Collecting litter

• Litter-trap under Betula pendula at Penrhyn Quarry

Methods 1: Measuring CENTURY equivalent pools

Air dry soil

>2000 m LITTER

<2000 m 250-2000 m

150-250 m

<150 mSLOW

PASSIVE

Fresh soil

Soluble C Microbial C

ACTIVE

Methods 2: Measuring soil water holding capacity

• Field capacity moisture content

• Available water content

Soil soaked for 24 hours, covered and allowed to drain for 48 hours.

Moisture content of saturated soil = field capacity

• Moisture content at permanent wilting point

Water potential determined by Dewpoint psychrometer

Moisture content when water potential is -1.5Mpa = moisture contentat permanent wilting point

Available water content = moisture content at field capacity -moisture content at permanent wilting point

Alder Alnus glutinosa%C = 48.8%N = 2.7C:N = 18.1

Birch Betula pubescens/pendula%C = 53.1%N = 0.7C:N = 75.9

50:50 mix Alder:Birch %C = 50.9 %N = 1.7 C:N = 29.9

Initial litter characteristics

Field capacity moisture content

0

50

100

150

200

250

300

350

400

0 2 4 6 8 10 12 14 16 18time (wks)

mg

wat

er /

g dr

y so

il

Control

Alder

Birch

Alder and birch

Moisture content at permanent wilting point

0

20

40

60

80

100

0 2 4 6 8 10 12 14 16 18time (wks)

mg

wat

er /

g dr

y so

il

Birch

Alder and birch

AlderControl

Available water content

0

50

100

150

200

250

300

350

400

0 2 4 6 8 10 12 14 16 18time (wks)

mg

wat

er /

g dr

y so

il Alder

Alder and birch

Birch

Control

Mean amount of carbon (g) in analytical pools

g ca

rbon

per

pot

0

1

2

3

4

5

6

7

8

>2mm Floated OM 250um-2mm Floated OM 150-250umMineral bound OM 250um-2mm Mineral bound OM 150-250um Mineral bound OM 63-150um Soluble and microbial

A AB B N A AB B N A AB B N

A = AlderAB = Alder and birchB = BirchN = None

Time 0 2 weeks 16 weeks

Mean amount of carbon (g) in CENTURY equivalent pools

g ca

rbon

per

pot

0

1

2

3

4

5

6

7

8

Structural litter CMetabolic litter CActive CSlow CPassive C

A AB B N A AB B N A AB B N

A = AlderAB = Alder and birchB = BirchN = None

Time 0 2 weeks 16 weeks

Amount of >2mm litter C

0

1

2

3

4

5

6

0 2 4 6 8 10 12 14 16 18time (wks)

g >

2m

m C Birch

Alder and birch

Control

Alder

Amount of >250um litter C

0

1

2

3

4

5

6

0 2 4 6 8 10 12 14 16 18

time (wks)

g >

25

0u

m C

Birch

Control

Alder

Alder and birch

1) All litter amendments have raised soil water holding capacity

Conclusions

5) Rapid transfer of C from CENTURY Litter to CENTURY Passive pool… …but can we be sure it is passive?

2) Duration of soil water holding capacity increase is positivelycorrelated with litter C:N (in the short term). Litter C:N indicates litter decomposition rate and the litter fraction holds the most water

3) Initial litter characteristics indicate decomposition rate

4) Soil water holding capacity is more tightly correlated withCENTURY Litter C than CENTURY Slow, Passive or Active C

6) Decomposition of high C:N litter stimulated by proximity to litterwith low C:N

>2mm litter fraction

Litter Initial C (g) 16 weeks C (g) C lost (g) C lost (%)

Alder 4.88 0.57 4.31 88Birch 5.39 2.69 2.62 49Alder and birch 5.01 1.15 3.93 77

Expected C lost from alder and birch litter = 69%

Observed C lost from alder and birch litter = 77%

= 8% higher than expected

Mechanism: decomposers use extra N (or labile C) of alder litter to decompose birch litter.

Decomposition of high C:N litter (birch) is positively stimulated by mixing with litter of low C:N (alder)

Measuring soil formation at Penrhyn Quarry

above ground biomass

stem basal area

litter input

decomposition

litter blow-out

What does this mean for restoration practitioners?

Soil water holding capacity and rates of soil organic matteraccumulation are determined by chemical characteristics of plant litter

Slowly decomposing litter of high C:N will provide water to surrounding plants for longer

Shrubs and understorey plants help retain litter on exposed sites

When planting trees, think about litter!