1. CENTRE
FORESTAL
TECNOLÒGIC
DE CATALUNYA
Pinares secundarios en cultivos abandonados:
Evolución del stock de carbono edáfico
Pere Rovira & Aida Sala
Solsona

2. Broadleaf (18)
Conifer (65)
< 900 mm (8)
900 - 1200 (17)
1200 - 1500 (24)
> 1500 mm (12)
-40 -20 0 20 40
Soil carbon change (%)
Guo L.B. & Gifford R.M. (2002). Global Change Biology 8, 345-360.
From crop abandon to forest: changes in soil C stock

3. Cardona (Barcelona): crop abandonment and secondary pine stands development

4. The replacement of crops by secondary forest, in the studied
area of Cardona (Barcelona, Spain):
1. Does it result in a net carbon sequestration?
2. Where do the changes in the total C stock occur?
3. How the C stock is affected by these changes?
- Biochemical quality (labile / recalcitrant fractions)
- Physical protection (POM / organomineral complexes)
4. How the stability of the accumulated C is affected by these
changes?
Defining challenges

5. Plot location: systematic comparison of aerial photographs
1956 2009

6. Delimitation of plot types
Type Dominant species 1956 2009 Features
Crop Triticum aestivum Crop Crop Terracing
New forest 1 Pinus nigra Crop Forest Terracing
New forest 2 Pinus nigra Forest Forest Terracing
Old forest Pinus nigra Forest Forest No terracing; never
cropped apparently
In all cases:
Marl-type substrate, rich in carbonates, lclay-loam textures.

7. Organic horizons (litter): L, F and H
sampled with aluminum cylinders.
8 cylinders per plot
(forest plots only)
Mineral soil down to 30 cm:
sampled with prismatic core samplers.
5 cores per forest plot,
4 cores per agricultural plot.
Soil sampling

8. 0
1
5
15
30
Soil sampling
Depth, cm

9. Organic horizonsMineral soil (0-30 cm)kgCm-2
0
2
4
6
8
Crops
New forest 1
New forest 2
Old forest
Quantifying total C stock, down to 30 cm depth
-2
Carbon sequestration potential, on the very long term: about 3 kg C m
About 1 kg in the mineral soil, about 2 kg in the litter horizons
Deep soil (> 30 cm depth): much more difficult to predict.

10. H horizon
Kg C m-2
0 1 2 3 4
L horizon
kg C m-2
0.0 0.5 1.0 1.5 2.0
New forests 1
New forests 2
Old forests
F horizon
0.0 0.5 1.0 1.5 2.0
Total organic horizons
0 1 2 3 4
Organic horizons

11. 5 - 15 cm
Kg OC m-2
0 1 2 3 4
0 - 5 cm
0 1 2 3 4
Crops
New forest 1
New forest 2
Old forest
15 - 30 cm
0 1 2 3 4
Mineral horizons

12. Incubation under standard conditions:
25ºC, optimal humidity
C respired after 30 days (mg per g OC)
60 70 80 90 100 110
Depth(cm)
-10
0
10
20
30
Crops
New forests 1
New forests 2
Old forests
Litter
Mineral soil
Differences not significant overall
How does stability of soil C change?

13. - Increases in the biochemical quality
- Decreases in the physical protection
- Both
- ...?
Why?

14. Evaluation of biochemical quality
Soil sample
Recalcitrant
residue
Hydrolysate I Hydrolysate II
1M HCl
3 h
6M HCl
24 h
Recalcitrance index =
Unhydrolyzed C
Total C

15. Old forests
0 2 4 6 8 10
New forests 2
Total organic Carbon (% w/w)
0 2 4 6 8 10
0
20
40
60
80
100
New forests 1Crops
0.0 0.5 1.0 1.5 2.0
RecalcitranceindexforCarbon(%)
0
20
40
60
80
100
Recalcitrance is related to OC content

16. Hydrolysis with hydrochloric acid
Recalcitrance index for Carbon (%)
25 30 35 40 45 50 55 60
Depth(cm)
-10
0
10
20
30
Crops
New forests 1
New forests 2
Old forests
Litter
Mineral soil
Recalcitrance is related to land use
Changes in OC biochemical quality do not explain its lower stability in forests.

17. Mechanical shaking
Ultrasonic dispersion
DISPERSED SOIL
Sieving
Centrifugation
Organic fragments
2000 - 200 µm
Organic fragments
200 - 50 µm
Organic fragments
50 - 20 µm
Organo-mineral
complexes
< 20 µm
Particulate
Organic Matter
(POM > 20 µm)
SOIL SAMPLE
(air-dried, 2 mm-sieved)
Evaluation of physical protection of C: size fractionation

18. OC in the < 20 µm fraction (% of total OC)
30 40 50 60 70 80 90
0 - 5 cm
5 - 15 cm
15 - 30 cm
a
b
b
Overall, physical protection increases with depth

19. Mineral soil (first cm not included)
OCinthe<20µmfraction(%oftotalOC)
20
30
40
50
60
70
80
90
Crops
New forest 1
New forest 2
Old forest
P < 0.001
a b b b
Physical protection as related to land use
Overall, the replacement of crops by forests results in a decrease
in the degree of physical protection of soil organic matter.

20. Organic carbon (% w/w)
0 1 2 3 4 5
Carbonin<20µmfraction(%oftotalOC)
45
50
55
60
65
70
75
80
85 Crops
New forests 1
New forests 2
Old forests
R 2
= 0.2026 **
Physical protection as related to OC content of the horizon

21. Physical protection of soil carbon
OC in the < 20 µm fraction (% of total OC)
20 40 60 80 100
Depth(cm) 0
5
10
15
20
25
30
Crops
New forest 1
New forest 2
Old forest
Physical protection as related to land use and position within the soil profile

22. - Evolution of nitrogen forms.
- Evolution of phosphorus forms and availability.
- Evolution of inorganic C (carbonates).
- Chemical fertility: Cation exchange capacity, exchangeable cations
...
etc.
To be continued!
Work under way...

23. Thanks for your attention!
This research was performed under the framework of the CRONOCARB project,
funded by the Ministerio de Ciencia y Tecnología, Spain.