Effect of deforestation and management on soil carbon stocks in the South American Chaco

Osinaga, Natalia, Álvarez, Carina, Taboada, Miguel

Faculty of Agronomy University of Buenos Aires

Argentina

National Institute of Agricultural Technology

Argentina

Scientific and TechnicalNational Research Council

Argentina

Theme 3.3, Tuesday 21, 16.20-17.30, Nº3

LOCATIONGran Chaco, South American Chaco

Subhumid Chaco

Study area

Chaco means hunting land

Annual rainfall: 700-1000 mmMean annual temperature: 21ºCSoils: Haplustolls and Argiustolls Natural ecosystem: Natural forest(trees, scrubs and grass species)

1986199119992008

→ In the last years….

High increase of deforested areaAdvance of agricultural frontierNo-tillage: soybean-maize-cotton

2016

Quantify:

Soil organic carbon (OC) stock up to 1 m deep

Soil organic carbon fractions in surface soil layers

Relationship OC fractions and physical properties

under different land uses:

Cropped: (< 10 and > 20 yrs under agriculture)

Pasture: warm season grasses

Natural forest (trees and underbrush)

8 sites

OBJ

ECTI

VES

Methodologies

• Soil bulk density: cylinder method

• Soil organic carbon: wet digestion

0-100 cm: 0-5 cm, 5-20 cm and then every 20 cm

• Coarse particulate carbon 2000 µm - 212µm (CPC)

• Fine particulate carbon 212µm - 53µm (FPC)

• Resistant carbon <53µm (RC)

• Aggregate stability: Le Bissonnais method

0-5 cm, 5-20 cm

MET

OD

OLO

GY

Data analysisANOVALSD test (comparison of means)Shapiro Wilks (normality)

Organic carbon stock expresed in equivalent soil mass

(9885 Mg ha-1 of soil)

RESU

LTS

AN

D D

ISCU

SSIO

N

•Losses occurred in the first years of conversion•↓Net primary production → ↓ Soil organic carbon input•↑Mineralization (↑t° y SWC) → ↑ Soil organic carbon output•Erosion•Pasture an intermediate situation

So

il o

rga

nic

ca

rbo

n (

Mg

ha-1

)

39%

26%

45%

Soil organic carbon (Mg ha-1)

Dep

th (

cm)

34-48 % of SOC in first 20 cm layerVertical distribution SOC → root pattern

RESU

LTS

AN

D D

ISCU

SSIO

NSoil organic carbon vertical distribution

RESU

LTS

AN

D D

ISCU

SSIO

N

2000 - 212µm 212-53µm <53µm coarse fine resistance

2000 - 212µm 212-53µm <53µm coarse fine resistance

Soil organic carbon fractions in surface layers

Most sensitive fraction coarse particulate 2000-212 µmNatural Forest 65 % of OC is coarse particulate fractionLosses occurred in the first years of conversion

2000 - 212µm 212-53µm <53µm coarse fine resistant

2000 - 212µm 212-53µm <53µm coarse fine resistant

Ag

gre

gat

e st

abili

ty (

mm

)

RESU

LTS

AN

D D

ISCU

SSIO

NAggregate stability Le Bissonnais method

0-20 cm

Ag

gre

gat

e st

abil

ity

(mm

)Perennial roots

OC (Mg ha-1)

58 a 32 b 35 b 32 b

SOC stock, quality and vertical distribution was highly affected by soil use.

CON

CLU

SIO

NS

Thank you!

Grassed pasture could be an interest tool to increase soil carbon stocks.

Physical properties not only depend on OC but also on the presence of perennial vegetation and stresses applied to soil.

The conversion to agriculture produced substantial reductions of OC up to 1-meter deep, mainly of the labile fractions in surface layers.