GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17 1

Effects of land use/cover on soil aggregate-associated organic

carbon in a montane ecosystem

Tshering Dorji¹, Inakwu O. A. Odeh² & Damien J. Field²

¹National Soil Services Centre, Ministry of Agriculture & Forests; Bhutan²Faculty of Agriculture and Environment, University of Sydney; Australia

Correspondence: tsericdoji@gmail.com

2GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17

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.

3GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17

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.

4GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17

Materials & Method

Fig. 1 Study area (Dorji et al., 2014)

5GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17

Materials & Method

Soil aggregate samples were collected from the A horizon using cLHS (Minasny and McBratney, 2006).

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

6GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17

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).

One way analysis of variance followed by the post-hoc Tukey-Kramer HSD test (α = 0.05) was performed.

7GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17

Results

LULC n > 2.0 mm 0.25-2.0 mm 0.053-0.25 mm < 0.053 mmtype 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.

8GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17

Results

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.

9GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17

Results

LULC > 2.0 mm 0.25-2.0 mm 0.053-0.25 mm <0.053 mmtype 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.

10GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17

Results

1.2

1.6

2

2.4

2.8

3.2

3.6

MW

D (m

m)

0 20 40 60 80 100 120AAOC (g/kg) (>2.0 mm)

R² = 0.32p < 0.0001

Fig. 4 Mean weight diameter (MWD) plotted against AAOC of the large macro-aggregates.

Indicates the upper threshold of SOC to enhance aggregate stability

11GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17

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.

12GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17

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).

13GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17

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.

Acknowledgements

15GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON | 21 – 23 MARCH 2017 | FAO-ROME, ITALY | # GSOC17

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.