This presentation was presented during the 2 Parallel session on Theme 2, Maintaining and/or increasing SOC stocks for climate change mitigation and adaptation and Land Degradation Neutrality, of the Global Symposium on Soil Organic Carbon that took place in Rome 21-23 March 2017. The presentation was made by Mr. Tshering Dorji, from Ministry of Agriculture and Forest - Bhutan, in FAO Hq, Rome

1. Effects of land use/cover on soil
aggregate-associated organic
carbon in a montane ecosystem
GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17 1
Tshering Dorji¹, Inakwu O. A. Odeh² & Damien J. Field²
¹National Soil Services Centre, Ministry of Agriculture & Forests; Bhutan
²Faculty of Agriculture and Environment, University of Sydney; Australia
Correspondence: tsericdoji@gmail.com

2. Background
 SOC forms an integral part of a functional soil
and it varies in space and time.
 SOC serves as a common indicator for soil
quality, soil security, water security and
environment sustainability.
 Information on SOC and its pools in relation to
land use/cover is vital for achieving land
degradation neutrality (LDN), mitigating
climate change, and enhancing ecosystem
services.
GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17 2

3. Background
 There is a huge gap in knowledge and data on
SOC in relation to land use/cover and soil
stability in Bhutan.
 This study investigated the effects of land
use/cover on soil aggregate fractions, aggregate
stability, aggregate-associated organic carbon
and the latter’s role in soil aggregate stability in
a montane ecosystem of Bhutan.
3GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17

4. Materials & Method
4GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17
Fig. 1 Study area (Dorji et al., 2014)

5. Materials & Method
 Soil aggregate samples were collected
from the A horizon using cLHS
(Minasny and McBratney, 2006).
5GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17
 Aggregates (3-5mm) were wet-
sieved into >2mm (large macro-
aggregates), 0.25-2 mm (small
macro-aggregates), 0.053-0.25 mm
(micro-aggregates), and <0.053
mm (mineral fraction) (Yoder,
1936; Kemper & Rosenau, 1986).
Fig. 2 Sampling sites

6. Materials & Method
 Different aggregate fractions were analyzed to
determine their C concentrations using IRMS.
 The Mean Weight Diameter (MWD) was computed
according to Kemper and Rosenau (1986).
6GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17
 One way analysis of variance followed by the post-hoc
Tukey-Kramer HSD test (α = 0.05) was performed.

7. Results
7GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17
LULC n > 2.0 mm 0.25-2.0 mm 0.053-0.25 mm < 0.053 mm
type Soil aggregates (%)
Fir 2 93.01±1.23Aa* 0.28±0.09Bbc 0.07±0.01Bbc 2.17±0.17Bb
Broadleaf 3 86.53±2.96Aa 0.29±0.18Bbc 0.16±0.07Bbc 2.31±0.60Bb
Mixed conifer 13 86.19±2.27Aa 0.51±0.20Bc 0.16±0.05Bbc 2.98±0.66Bb
Grassland 3 89.47±1.40Aa 0.77±0.50Bbc 0.26±0.10Bbc 2.76±0.90Bb
Shrubland 5 90.03±3.04Aa 0.50±0.15Bc 0.21±0.08Bbc 1.88±0.42Bb
Blue pine 11 87.72±2.30Aa 1.68±1.20Bbc 0.31±0.13Bc 1.48±0.19Bb
Orchard 4 88.66±4.50Aa 1.42±0.84Bbc 0.38±0.19Bbc 1.87±0.18Bb
Paddy land 3 34.86±7.15Ac 15.35±3.44Aa 21.07±6.41Aa 11.67±4.07Aa
Dry land 6 63.52±10.13Ab 7.75±2.83Bb 7.96±3.59Bb 4.25±1.33Bb
*Mean value followed by standard error; within rows, values followed by the same capital letter (A-B) are not significantly
different (p < 0.05); within columns, values followed by the same small letter (a-b) are not significantly different (p < 0.05). LULC
land use/land cover; n number of observations
Table 1 The distribution of soil aggregate fractions under different LULC types.

8. Results
8GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17
Fig. 3 Soil aggregate stability under different LULC types. Values with same
letter (a-c) are not significantly different (p < 0.05) from each other. MWD
mean weight diameter.

9. Results
9GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17
LULC > 2.0 mm 0.25-2.0 mm 0.053-0.25 mm <0.053 mm
type AAOC (g kg-1)
Fir 81.44±20.73Aa* 0.20±0.10Bc 0.17±0.10Bbc 11.43±10.50Ba
Broadleaf 77.90±29.53Aa 0.11±0.02Bc 0.08±0.01Bc 14.13±9.79ABa
Mixed conifer 63.35±7.48Aa 0.30±0.12Bc 0.23±0.09Bbc 16.15±5.01Ba
Grassland 51.17±19.43Aab 0.28±0.16Bc 0.23±0.12Bbc 7.53±1.39Ba
Shrubland 38.03±9.84Aab 0.18±0.06Bc 0.16±0.05Bbc 11.79±3.76Ba
Blue pine 33.75±6.27Aab 0.23±0.12Bc 0.23±0.15Bbc 4.96±1.54Ba
Orchard 32.24±7.15Aab 0.32±0.16Bc 0.25±0.12Bbc 2.34±0.98Ba
Dry land 18.38±2.80Ab 3.35±1.22Bb 1.55±0.56Bb 4.69±0.84Ba
Paddy land 11.15±2.19Ab 7.00±1.09Aa 3.54±0.76Aa 8.68±4.55Aa
Table 2 AAOC of different aggregate fractions under different LULC types.
*Mean value followed by standard error; within rows, values followed by the same capital letter (A-B) are not significantly
different (p < 0.05); within columns, values followed by the same small letter (a-b) are not significantly different (p < 0.05).
LULC land use/land cover, AAOC aggregate-associated organic carbon.

10. Results
10GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17
Fig. 4 Mean weight diameter (MWD) plotted against
AAOC of the large macro-aggregates.
 Indicates the
upper threshold
of SOC to enhance
aggregate stability

11. Summary
 Land use/cover has a huge impact on soil
aggregate distribution, aggregate stability and
aggregate-associated organic carbon.
 Macro-aggregates dominate the aggregate
distribution under natural land cover than
under agricultural land.
 Large macro-aggregates have a greater
influence on aggregate stability than other
aggregate fractions.
11GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17

12. Summary
 Soil aggregate stability is relatively high under
natural LULC types than under agricultural land.
 Large macro-aggregates store maximum
amount of SOC than other aggregate fractions.
 Although aggregate stability increases with SOC
concentration, there is an upper threshold
beyond which the aggregate stability does not
increase (≈70 g C/kg).
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13. Take Home Message
 Despite its rugged and fragile environment, soils in
Bhutan are found to be relatively stable than it is
thought.
 Rapid change in land use/cover, due to climate
change and rapid socio-economic development,
might undermine soil stability and lead to severe
land degradation and rapid release of CO₂.
 Appropriate land use plans and policies should be
put in place to achieve LDN, reduce climate
change, and ensure continuous ecosystem services
in the country.
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14. Acknowledgements

15. References
 Dorji, T., Odeh, I. O. A., Field, D. J. & Baillie, I. C. 2014b. Digital
soil mapping of soil organic carbon stocks under different
land use and land cover types in montane ecosystems,
Eastern Himalayas. Forest Ecology and Management, 318,
91-102.
 Kemper, W.D., Rosenau, R.C., 1986. Aggregate stability and
size distribution. In: Klute, A. (Eds), Methods of soil analysis.
Part 1. Physical and mineralogical methods. pp 425-442.
 Minasny, B., McBratney, A.B., 2006. A conditioned Latin
hypercube method for sampling in the presence of ancillary
information. Computers & Geosciences 32, 1378-1388.
 Yoder, R. E. 1936. A direct method of aggregate analysis of
soils and a study of the physical nature of erosion losses.
Agronomy Journal, 28, 337-351.
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