Conversion of forest to agriculture in A
Agriculture, Ecosystems and Environment 184 (2014) 101– 114
Contents lists available at ScienceDirect
Agriculture, Ecosystems and Environment
jo ur nal ho me page: www.elsevier.com/locate/agee
Conversion of forest to agriculture in Amazonia with the
chop-and-mulch method: Does it improve the soil carbon stock?
Anne-Sophie Perrin a , ∗ , Kenji Fujisaki a , b , Caroline Petitjean c , Max Sarrazin d , Mathieu Godet a , Bernard Garric a , Jean-Claude Horth a , e , Luiz Carlos Balbino f ,
Austrelino Silveira Filho g , Pedro Luiz Oliveira de Almeida Machado h , Michel Brossard b
a Centre Technique Interprofessionnel des Oléagineux et du Chanvre (CETIOM), Etablissement Public Local d’Enseignement et de Formation Professionnelle
b Agricole (EPLEFPA) de la Guyane, Savane Matiti, BP 53, 97355 Macouria, Guyane franc¸ aise, France IRD – UMR 210 Eco&Sols (INRA, SupAgro, CIRAD, IRD), bâtiment 12, 2 place Viala, F-34060 Montpellier
Cedex 02, France
CNRS – Université des Antilles et de la Guyane, UMR EcoFoG, Campus agronomique, 97310 Kourou, Guyane d franc¸ aise, France
IRD – US122, Laboratoire des Moyens Analytiques (LAMA), route de Montabo, F-97323 Cayenne Cedex, Guyane franc¸ aise, France
Cedex, Guyane franc¸ aise, France
e Chambre d’Agriculture de Guyane, 8 avenue du Général de Gaulle, BP 544, F-97333 Cayenne
g EMBRAPA Cerrados, Cx Postal 08223, CEP 73310-970 Planaltina, DF, Brazil
h EMBRAPA Amazonia Oriental, Cx Postal 48, CEP 6691 7-900 Belém, PA, Brazil
EMBRAPA Arroz e Feijao, Cx Postal 179, CEP 75375-000 Santo Antonio de Goias, GO, Brazil
Article history: Fire-free forest conversion with organic inputs as an alternative to slash-and-burn could improve agro-
Received 15 April 2013
ecosystem
sustainability. We
assessed soil carbon mass changes in a sandy–clayey and well-drained soil
Accepted November Guiana after forest clearing by the chop-and-mulch method and crop establishment. At the
Received in revised form 1 November 2013
8 2013 in French
experimental site of Combi, native forest was cut down in October 2008; woody biomass was chopped
and incorporated into the top 20 cm of soil. After about one year of legume and grass cover, three forms
Keywords:
land
French Guiana of management were compared: grassland (Urochloa ruziziensis ), maize/soybean crop rotation with
disk tillage and in direct seeding without tillage. There were four replicates. We measured 14.16 kg Fire-free m − 2 Deforestation
of carbon in 2 mm-sieved soil down to 2 m depth for the initial forest. Forest clearing did not induce
Annual crops significant soil compaction; neither did any specific agricultural practice. In converted soils, C stocks
Brachiaria were measured in the 0–30 cm layer after each crop for three years. Carbon mass changes for soil frac-
No-tillage tions <2 mm (soil C stock) and >2 mm (soil C pool) in the 0–5, 5–10, 10–20 and 20–30 cm soil layers
were assessed on an equivalent soil mass basis. One year and 1.5 years after deforestation, higher C
stocks (+0.64 to 1.16 kg C m − 2 yr − 1 ) and C pools (+0.52 to 0.90 kg C m − 2 yr − 1 ) were measured in converted
soils, compared to those of the forest into the top 30 cm of soil. However, the masses of carbon in these
converted soils declined later. The highest rates of carbon decrease were measured between 1.5 and 2
years after forest conversion in the <2 mm soil fraction, from 0.46 kg C m − 2 yr − 1 (in grassland soils) to
0.71 kg C m − 2 yr − 1 (in cropland under no tillage). The carbon pool declined during the third year at rates
of 0.41 kg C m − 2 yr − 1 (cropland under disk tillage) to 0.76 kg C m − 2 yr − 1 (grassland soils). Three years after
forest conversion, C masses in the top 30 cm of soils for grassland showed similar values than for forest.
In comparison, the carbon stock in cropped soils managed under no tillage in direct seeding (without
mulch) was significantly 17% and 16% lower than in forest and grassland soils, respectively. None of the
studied agricultural practices succeeded in accumulating carbon from the chopped forest biomass.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction sustainability, are still not given sufficient consideration, espe-
cially in tropical humid and equatorial climates (e.g. Powlson Despite their importance, the dynamics of soil organic mat- et al., 2011; Lal, 2012 ). Between 2000 and 2007, gross tropical
ter in relation to changes in land use, as well as agro-system
deforestation is estimated to − have resulted in CO 2 –C emissions
of 2.82 ± 0.45 Pg C yr 1 (including 1.37 in Latin America) that has
to be compared to the C sink due to tropical forest regrowth ∗
of 1.72 ± 0.54 Pg C yr − 1 (0.86 in Latin America) ( Pan et al., Corresponding author. 2011 ). E-mail addresses: perrin@cetiom.fr , anne-sophie.perrin@laposte.net Thus, emission from tropical land-use change is estimated to
(A.-S. Perrin). 1.10 ± 0.70 Pg yr − 1 of equivalent CO 2 –C (0.51 in Latin America)
0167-8809/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.agee.2013.11.009
A.-S. Perrin et al. / Agriculture, Ecosystems and Environment 184 (2014) 101– 114
and represents approximately the emission from global land-use successive cropping cycles lead to increased soil C and N of micro-
change. During the 2000–2007 period, land use change in the trop- bial origin. Treatments with the highest levels of microbial C and
ics contributed to about 12% of global greenhouse gases emissions N were those where the residues were cut, shred and distributed
(∼6% for Latin America) ( GCP, 2012 ).
over the soil surface ( Lopes et al., 2011 ). From another experiment
In these regions, slash-and-burn practices are widely used to in the same region, Comte et al. (2012) argue that chop-and-mulch
convert land for animal and human food production as well as of enriched fallows during the conversion of secondary forest into
for urban development. To achieve a sustainable agriculture, slash- cultivated land could contribute to the accumulation and conser-
and-burn must be coupled with suitable fallow management (e.g. vation of large quantities of organic matter and thus represent
Denich et al., 2005 ). Although appropriate slash-and-burn practices an important nutrient supplier. CO 2 -equivalent emissions related
can contribute to maintain swidden with very high level of bio- to global warming potential (GWP) analysis were estimated by
diversity in the tropics (e.g. Padoch and Pinedo-Vasquez, 2010 ),
Davidson et al. (2008) from measurements of CH 4 , N 2 O and NO
much harm is attributable to current slash-and-burn practices. emissions from soil to assess the fire-free alternative system used
Soils in the humid tropics particularly those developed on highly in this Brazilian region. Results showed that GWP over 100 years
weathered materials present low activity and variable-charge clays was at least five times lower in chop-and-mulch compared with
such as kaolinite and oxy-hydroxide minerals with very low cation slash-and-burn treatments, mainly due to the lack of CH 4 and N 2 O
exchange capacity. Thus, improvements to the cation exchange
emissions during burning phase (67% and 27%, respectively of the
capacity of these soils are closely related to their organic matter total CO 2 -eq calculated for chop-and-mulch). Due to an unsuitable
content (e.g. Boyer, 1982 ). Burning and installation of crops and sampling method, the analysis did not include an assessment of
pasture have been shown to decrease long-term biological (e.g. changes in the soil organic carbon stock, so it might have underes-
Luizão et al., 1992; Decaëns et al., 2004 ), chemical (e.g. Lal et al., timated the advantage of the chop-and-mulch method. Thus, the
1986; Sarrailh, 1990; Cerri et al., 1991; Desjardins et al., 1994;
effect of this fire-free clearing method with forest residue inputs
McGrath et al., 2001; Palm et al., 2005; Vågen et al., 2006; Okore on soil carbon stocks in Amazonia remains unknown.
et al., 2007 ) and physical (e.g. Grimaldi and Boulet, 1989; Chauvel The present study focuses on an experiment with neotropical
et al., 1991; Müller et al., 2004; Desjardins et al., 1994; Koutika forest conversion. This experiment was conducted on a dedicated
et al., 1997 ) properties relevant to provide adequate soil fertility site in French-Guiana using a chop-and-mulch method with incor-
for crop production. In addition, damage to health can result from poration of forest biomass in the top 20 cm of the soil. Crops (a
burning such as pulmonary illness due to smoke or intoxication maize/soybean rotation, with and without soil tillage) and grass-
by mercury leaching into the aquatic environment (e.g. Carmouze land systems were established on site one year after forest clearing
et al., 2001; Farella et al., 2006 ). Less obviously, large scale burning and were cultivated for 2 years.
of forest is also thought to increase the size of the refractory pool of The objective of this study was to assess the impact of forest
dissolved organic carbon in the deep ocean ( Dittmar et al., 2012 ).
conversion into agriculture using the chop-and-mulch system on
Furthermore, when compared to the producing area, the deforested short-term soil organic carbon stocks. Carbon stock changes were
area is much greater in slash-and-burn based agrosystems, which evaluated before deforestation and then at 4 dates during the first
require land rotation and regular burning ( Kato et al., 1999; Sanchez three years following forest conversion. Soil organic carbon (C)
et al., 2005; Denich et al., 2005 ). Consequently, while demographic mass changes were measured in fine and coarser soil fractions and
pressure expands, these practices lead to increasing primary forest the influence of soil management practices on these changes was
deforestation.
assessed.
In French Guiana, forest covers more than 96% of the emerged
surface ( FAO and ITTO, 2011 ). Demographic and economic growth
is rapid ( INSEE, 2010; CEROM, 2008 ) and the area of forest con- 2. Materials and methods
verted into cropped areas for subsistence or family/small scale
farming has almost doubled between 1990–2006 and 2006–2008 2.1. Site description and experimental design
( IFN, 2009a,b ). Alternative agricultural practices should aim to limit soil organic In French Guiana the wet tropical climate (AMi type accord- carbon losses and particulate emissions from burning (e.g. Brady, ing to Köppen–Geiger system, in Peel et al., 2007 adapted from
1996 ). Forest conversion with high organic inputs, as an alternative Köppen, 1936 ) is directly influenced by the seasonal north/south
to slash-and-burn, could improve agrosystem sustainability. The movements of the Inter-Tropical Convergence Zone, with a dry effects of adding woody residues on soil organic carbon stocks and season from mid-August to mid-November, and a rainy season crop productions require more investigation, especially in tropical the rest of the year (usually interrupted by a short dry sea- climates (e.g. Barthes et al., 2010 ). Slash-and-mulch or chop-and- son in February/March).
◦ The ′ ′′ experimental site of this 55 study, N/52 55 01 W), is located 12 km south of steep slopes and in areas of monsoonal climates were traditionally Sinnamary. Mean annual precipitation and mean annual air
mulch production practices that are specifically adapted for use on “Combi” (5 ◦ 17 ′
used, for example in Costa Rica (locally named frijo tapado ) where temperature are 2771.2 ± 628.8 mm and 27.3 ± 0.5 ◦ C; minimum
the rainfall is so continuous as to preclude burning of vegetation. rainfall is recorded in September (46.8 ± 55.0 mm) and maximum in
In this context, Bellows et al. (1996) reported that natural fallows May (477.7 ± 190.1 mm) (Météo-France data 1970–2009). Monthly
and mulching practices provide nutrient recycling, reduce pest and rainfall does not exceed 110 mm from August to November with
disease infestations and inhibit weed regrowth. The mulch layer only 11% of the mean annual precipitation falling during this 4-
is also reported to reduce or eliminate the need for crop mainte- month period.
nance labor inputs and to control nutrient losses due to erosion (e.g. Slopes are less than 10% and do not exceed 7% over 90% of land
Bellows et al., 1996 ). More recently, in the Brazilian state of Pará area. The sandy–clayey, nutrient-poor soils are classified as “fer-
in the eastern Amazon region, in the context of small-scale farms rallitisol meuble kaolinitique jaune” ( AFES, 2009 ) or Hyperferralic
and swidden-fallow agriculture, the use of chop-and-mulch prac- Ferralsol ( FAO, 2006 ).
tices has been expanded to conserve good crop production with The clearing of the 2 ha of native forest occurred at the begin-
shortened fallow regeneration time ( Kato et al., 1999; Denich et al., ning of October 2008 (dry season) ( Figs. 1 and 2 ). Trees and
2004, 2005; Borner et al., 2007 ). Higher organic carbon contents stems with a diameter of less than 15 cm were chopped using a
were measured in the system with one cropping cycle, and two hydraulic vertical axis mulcher with chains mounted on a 20 ton
A.-S. Perrin et al. / Agriculture, Ecosystems and Environment 184 (2014) 101– 114
Fig. 1. Forest clearing at the Combi experimental site: (A) chopping of trees and stems in primary forest with a hydraulic vertical axis mulcher equipped with chains, mounted
on a 20 ton wheeled excavator in October 2008, (B) large woodchips in cover crops (April 2009), (C) chopping of large wood chips and cover crops with a forestry mulcher
mounted on a self-propelled wheeled machine in October 2009, and (D) soybean plots in July 2010.
wheeled excavator ( Fig. 1 A). Timber with no commercial value
grasses and legumes mixture) and NPK fertilisers were applied on
and residual trunks were piled every 40 m in windrows and were soils ( Fig. 2 ). On the 26th of October 2009, the cover crop and large
carefully removed from the site during the 2009 dry season.
wood chips of forest trees lying on the soil surface ( Fig. 1 B) were
Track-type tractors were used to minimize impacts (compaction chopped into smaller pieces (up to about 5–7 cm long chips) by a
and depletion) on the surface soil layer. After soil liming and
forestry mulcher mounted on a self-propelled wheeled machine
incorporation with discs to 20 cm depth, cover crops were sown (a and incorporated into the 0–10 cm depth soil layer ( Fig. 1 C).
FOREST
Chop & mulch
GRASSLAND
Tillage 0-20cm
Urochloa ruziziensis DISK TILLAGE
Cover crops (Grasses & legumes)
Maize Soybean
Maize Legumes cover
NO TILLAGE
Maize + Uroch.
Soybean
Maize Legumes cover
T0
T1
T1.5
T2
T3
(08-10-01)
(10-04-25)
(10-10-04)
1 1-18)
1 1-07)
Fig. 2. History of land use for the four treatments compared. November 2008: neotropical forest clearing followed by manual spreading of crushed limestone (1000 kg ha − 1 : 50% CaO) and dolomite (450 kg ha − 1 : 30% CaO and 20% MgO) on soil which was mixed into the 0–20 cm layer with heavy disk harrow. January 2009: manual broadcast
seeding of paddy rice (Oryza sativa L.), Urochloa ruziziensis cv. ruzi (often referred to the literature as B. ruziziensis ), Stylosanthes capitata and Stylosanthes macrocephala cv. BRS
Campo Grande, Calopogonium mucunoïdes Desv. was carried out, which received 40.5 kg ha − 1 of P 2 O 5 (super phosphate granules containing 46% of Ca 2+ ) and 25.5 kg ha − 1 of N, P 2 O 5 and K 2 O. In November 2009: setting up of cropping systems, four treatments: (1) neotropical forest, (2) grassland based on Urochloa ruziziensis cv. ruzi mowed 2–3
times annually, (3) conventional tillage based on surface plowing with disks twice a year and maize (Zea mays L.)/soybean (Glycine max L. Merr) annual crop rotation, and (4)
direct seeding of maize/soybean − 1 annual crop rotation (no-tillage). G and CT plots were previously tilled with an offset disk harrow. In November each year, CT and DS plots
received 450 and 300 kg ha of CaO and MgO, respectively. In grassland parcels, the same doses of CaO and MgO were applied beginning of November 2010 only. Grassland
51–51–51 of N–P 2 O 5 –K 2 O respectively on December 2009 and June 2011. Grassland biomass was cut and either removed at the beginning of September 2010
developed from sowing of Urochloa ruziziensis cv. Ruzi. and of small quantity of regrowth plants of the previous cover plants. Grassland plots received respectively
and 50–60–60 kg ha − 1
and April 2011, and at the end of August 2011 or mowed and left on soils at the beginning of May 2010 and middle of January 2011. In December 2009: maize was sown as
well as Urochloa ruziziensis cv. ruzi in each inter-row of DS plot and received 136–90–90 kg ha − 1 of N–P 2 O 5 –K 2 O, respectively. In May and June 2010: soybean was sown for
CT and DS plots, respectively (seeds were − previously inoculated 1 with peat). The planting of Urochloa ruziziensis cv. ruzi and Stylosanthes guianensis cv. Campo grande failed
in DS. Soybean received 114–120 kg ha of P 2 O 5 –K 2 O. In December 2010, maize was sown and received 169–111–132 kg ha − 1 of N–P 2 O 5 –K 2 O, respectively. In June 2011,
Crotalaria juncea and Stylosanthes capitata and Stylosanthes macrocephala cv. BRS Campo Grande were manually sown, fertilized with 60–60 kg ha − 1 of P 2 O 5 –K 2 O and were
ground and distributed on the soil surface in October 2011.
A.-S. Perrin et al. / Agriculture, Ecosystems and Environment 184 (2014) 101– 114
Three management systems ( Fig. 2 ) were set up (outside the Switzerland). Total C and N contents in soil and plant residue sam-
windrow areas) with a randomized complete block design of
ples were measured by dry combustion using a ThermoQuest NA
10 m × 20 m plots distributed over the area taking into account of 2100 analyzer on crushed samples (<200 m) (NF ISO 10694 and NF
declivity (four replicates for each treatment): ISO 13878). The analyzer was previously calibrated with acetanilide
C 8 H 9 NO (CE instruments 338 36700). The standard ranges were
- G: Grassland of Urochloa ruziziensis cv. ruzi (syn. Brachiaria ruz- checked using Soil Reference Material for N and C (det n ◦ 338 40025
iziensis ) not grazed but mowed 2–3 times per year lot N12A) and the precisions of C and N content measurements were
- NT: maize (Zea mays L.)/soybean (Glycine max L. Merr) rotation 1.29% and 0.5%, respectively. In this experiment, soil organic car-
under no tillage and managed with direct seeding bon content was assumed to be equal to total carbon content since
- DT: maize (Z. mays L.)/soybean (G. max L. Merr) rotation under inorganic carbon can be ignored.
disk tillage using two passes of a heavy disk harrow.
2.3. Sampling and analyses of biomass inputs and outputs
The adjacent forest site was used as a reference site ( Fig. 2 ).
A few days after forest clearing, the woody biomass lying on
2.2. Soil sampling, physical and chemical analyses the soil surface (excluding the litter biomass) was quantified in 14
randomly distributed quadrants of 0.65 m 2 over the experimen-
During the dry season (end of September 2008) and just before tal site. Samples were air-dried for 7 months in an air-conditioned
site deforestation, soil cores were carefully collected in 17 pits of room at 25 ◦ C before being weighed and crushed into smaller pieces
dimensions 0.60 m × 0.60 m × 0.60 m distributed over the area. The (<10 cm). Just before harvest, the fresh biomass of crops was esti-
litter layer was carefully removed before samplings. Deeper soil mated in each plot using samples of 2 adjacent areas (1linear meter
samples were collected down to 2 m depth in two soil pits. Cores for each) i.e. 3 and 2 m 2 in a location chosen randomly in maize
(0.05 m thickness × 0.10 m diameter) were collected continuously and soybean crops, respectively. For each sample, the stover was
down to 0.75 m depth (cores were collected one next to the other separated from the grain, dried and weighed to determine total
to avoid soil compaction). dry matter and grain yield. The biomass of mulch cover (mainly U.
In agricultural plots, soil sample cores (0.05 m thick-
ruziziensis ) and weed contributions were measured from 3 m 2 and
ness × 0.08 m diameter) were collected following a regular
2 m 2 samples. Total grass biomass returned to soils or exported was
grid sampling method with a hand auger at six points in each measured just before cutting (harvest or regeneration cutting) from
10 m × 20 m plots (evenly distributed over the plot, at least 3 m two 1 m 2 quadrants per plot. The quantities of biomass (crops or
from the edges, in the inter-row). Collected soil layers were 0–5, grass residues) returned to soils correspond to the total dry matter
5–10, 10–20 and 20–30 cm. Two cores of 0.05 m thickness were of the samples (minus the exported grain for maize and soybean).
mixed to form the 10–20 and 20–30 cm layers. These samples
All sub-samples were dried at 65 ◦ C until constant weight, then
were taken after the crop harvests during dry period in November weighed and ground to form a powder suitable for chemical anal-
2009 (T1) and then at the beginning of April 2010, and in October yses. C inputs from biomass were quantified by multiplying their
2010 and November 2011. dry matter by the corresponding mean C content, measured by dry
Soil samples were air-dried and sieved (2 mm) before analy- combustion using a ThermoQuest NA 2100 analyzer on crushed
ses. For all samples, plant residues and mineral soil fractions larger samples (<200 m). Standard ranges were checked using standard
than 2 mm were carefully isolated and weighed. Special care was reference material 1573a (tomato leaves) attested by the National
taken to separate the fine plant residues from the fine earth; plant Institute of Standards and Technology (USA) and the precisions
residues that passed through the 2 mm sieve were isolated and
were 2.0% and 1.08%, respectively for N and C analyses. The mass of
added to plant debris of the soil fraction >2 mm. The samples were carbon contained in a sample is obtained by multiplying the plant
weighed and their moisture content was determined on a sub- mass by 42.7% for forest (mean carbon content measured in forest
sample (ca. 30 g) after oven drying at 105 ◦ C for 48 h to obtain the roots in Bréchet, 2009 ), and by 47% for agricultural plant residues
dry mass in order to calculate the bulk density. The plant residues (this study, see Table 3 ). fraction was oven dried at 60 ◦ C for 72 h and weighed prior to
analysis. Soil bulk density (D b ) was determined on all soil samples 2.4. Masses of carbon in soil fractions and calculating rates of
collected with cylinders of 392.5 cm 3 (forest soils) and 251.2 cm 3 change
(agricultural soils).
Particle size distributions were determined for the <2 mm soil For each soil − sample, the carbon stock in the <2 mm soil fraction
fractions on each forest soil sample after hexametaphosphate dis- (C < 2mm in kg m 2 ) was calculated from the measured soil organic
persion and sedimentation (manual pipette method). Soil pH in carbon content (g C kg − 1 ) in the <2 mm soil fraction for the layer
water (1:2.5 M:M) and 1 N KCl were determined. The exchangeable thickness concerned (l in dm):
cations Ca 2+ , Mg 2+ , Na + and K + were extracted from all individ- = × × ×
ual samples in a solution of ammonium acetate (1 N, pH7.0) and C < 2mm
SF (<2mm)
C (<2mm)
D b 1,
analyzed using atomic absorption and emission spectrometry Var- where SF (<2mm) is the proportion (%, w/w) of <2 mm soil fraction
ian AA1275 (NF-X31-108, AFNOR, 1996 ). P was extracted in a in the whole dry soil sample, C (<2mm) is the organic carbon con-
sodium bicarbonate and ammonium fluoride solution (pH 8.5)
centration (g C kg − 1 ) of this fraction and D − b is soil bulk density
(Olsen modified, Dabin et al., 1967 ) and measured by colorime- (Mg m 3 ).
try after complexation of phosphates with ammonium molybdate The
− mass of
carbon contained in the >2 mm soil fraction (C > 2mm
in the presence of antimony (III) and reduction with ascorbic
in kg m 2 ) is calculated as:
acid ( Murphy and Riley, 1962 ). Cation exchangeable capacity was
determined in accordance with the norm AFNOR (1996) NF-X31- C > 2mm = SF (>2mm) × C (>2mm) × D b × 1
310 standard. Exchangeable acidity and potential acidity were
where SF
(>2mm) is the proportion (%, w/w) of the >2 mm soil fraction
determined after an extraction with 1 N KCl and sodium acetate
in the whole dry soil sample and C (>2mm) is the C content (g C kg − 1 )
0.5 N (pH 7.0) solutions, respectively ( EMBRAPA, 1997 ). Al 3+ and
of this fraction.
H + were analyzed by titration using a Metrohm E 536 potentio-
graph equipped with a 665 Dosimat Metrohm (Metrohm, Herisau, C tot = C < 2mm + C > 2mm
A.-S. Perrin et al. / Agriculture, Ecosystems and Environment 184 (2014) 101– 114
1 The ) masses of C < 2mm and C > 2mm for the agricultural treatments −
were calculated on an equivalent mass to account for differences kg extr.
in bulk densities as recommended by Ellert and Bettany (1995) and P (mg
Ellert and Gregorich (1996) . Forest soil was used as a reference i.e.
carbon stocks in agricultural treatments were corrected to refer to C:N
the same soil mass as in the forest soil layer concerned. C stocks N )
and pools were calculated by subtracting the total C concentration kg
of the extra-weighted of soil in the deepest layer (either 0–5, 5–10, Total (g
10–20 or 20–30 cm for the stocks or pools to 5, 10, 20 and 30 cm, (%) respectively).
V 19.05 (3.27)
Changes ) in C
< 2mm or C > 2mm masses over time at successive time 1 −
periods of the 3-year study were calculated as: kg CEC
tn−tn− 1 C = C n − C n− 1
Total
(cmol(+)
where tn−tn− 1 C is the rate of − − change of soil C mass
(kg C m 2 year 1 ); C n and C n− − 1 are the soil C masses (kg C m 2 )
kg
measured at
(n−1)
sampling date t n and previous sampling date t ,
acidity
respectively. Exch. (cmol(+) 1.24 (0.11) 1.25 (0.05) 1.11 (0.06) 0.82 (0.08) 0.51 0.18 0.08
In this study, the C contained in the <2 mm fractions is gen- 1 ) erally referred to as the “carbon stock” in the literature since it − kg
is considered to be the more stable soil carbon fraction. C > 2mm is
here referred
to as the carbon pool, which acts as a supply to the 3+ Al (cmol(+)
<2 n.m. mm
soil fraction. The consideration of pools is essential in this )
0.59 n.m.
study dealing with short-term land-use change with high inputs of − 1
organic matter and non-stabilized soils.
kg
2.5. Statistical analysis K (cmol(+)
For all variables, treatment means, sampling dates or time
kg
periods
were compared using one-way analysis of variance
(ANOVA),
after verification of the normal distribution of data.
Na
(cmol(+)
ANOVA was followed by the Tukey or Dunnett (for comparison with − ) 1 reference forest) post hoc test at a significance level of 0.05 (if not kg
specified). In these comparisons, we considered that C masses in 2+
forest soil layers did not change during the time of the experiment. Mg (cmol(+)
These statistical analyses were conducted using XLSTAT soft-
ware version 7.5.2 (Addinsoft ®
− kg
3. Results
2+ Ca (cmol(+) 0.96 (0.22) 0.26 (0.04) 0.16 (0.02) 0.11 (0.01) 0.12 0.11 0.07
3.1. Physical and chemical properties and carbon stock of forest soils clearing. KCl
the 4.24 (0.07) 4.11 (0.02) 4.45 0–0.2 4.68 m layer (clay con-
The soils of the site have a sandy–clayey texture in pH
tent 259 ± 28 g kg − 1 ) to a clayey–sandy texture in the 0.2–2 m layers (clay content forest O 2
362 ± 22 g kg − 1 ). Gravels represent about 78, 119, 91 and 66 g kg − 1 in the 0–20, H
4.54 4.69 20–30, 4.68 30–60 and >60 cm of the forest bulk soil. Coarse sand in the <2 mm soil frac- before tion represent
pH
Soil bulk densities are increasing from 1.02 in surface 5 cm to 1.51 on site average 1 below −
more than 70% of total sand ( Table 1 ). Silt represents only 2.9–4.6%. of
( Table 2 ). kg (g The
45 cm soil
low values measured throughout the soil profile for pH, CEC, and
Combi mm
base saturation ( v ) are characteristic of Amazonian ferralsols. − 1 Exchangeable alu- Clay <2 fraction)
minum concentrations range between 1.2 cmol(+) kg in − 1 the 0–10 cm layer and the of
0.6 cmol(+) − 1 kg
in the 30–60 cm layer. These soils are very poor in total nitrogen at
(<2 g kg soil ) and extractable phosphate (<8 mg kg − 1 ). These low values are compara- soils kg
ble to those previously recorded in French Guiana (e.g. Lévêque, 1967 ).
(g
mm In the forest soils, mean bulk density is 1.02 Mg m − 3 in the 0–5 cm layer and Silt <2 fraction)
increases progressively up to 1.47 Mg m − 3 in the 20–30 cm layer ( Table 2 and Fig. 3 ).
forest layers.
<20 g measured.
Mean carbon concentrations ( Table 2 ) are 26.80 g kg − 1 in the first 5 cm of soil,
kg − 1 from 10 to 25 cm, <10 g kg − 1 from 25 to 55 cm and then <5 kg − 1
− 2 g down
native
other
of 1 to of 200 cm. C stocks ± for
− 2 in forest soils are 5.50 0.68 − kg 2 C m
in the first 30 cm of soil, −
(6.4) 436.9 432.9 456.5 not kg
Coarse
m the first
6.55 ± 0.71 kg C m down to 40 cm, 9.67 kg C m and 14.16 kg C − 2 in (g soil
n.m., n =
meter and in the 0–2 m soil profile, respectively. mm Sand <2 fraction)
Fine
(2.0) 171.5 154.5 120.4 brackets. and cm
3.2. Biomass and carbon inputs to soils during the chop-and-mulch experiment parameters mm 30
Aerial biomass inputs to soils in the converted plots are shown in Table 3 . A − 1 Extr.,
extractable; between to
biomass (chopped vegetation) of 24.05 ± 14.28 Mg of dry matter per hectare was
kg
given down
90.6 63.0 lying 68.4 on the soil after forest clearing ( Table 3 ). The large standard deviation on this
chemical Gravels (g
are layer
measurement is the result of the spatial variability of biomass inputs due to natural and errors per
variation in the ages and species of trees in the initial forest. However, this variation 1 depth a a = 17
was decreased by the use of a forestry mulcher in November 2009 prior to crop a exchangeable;
establishment, a which incorporated smaller woodchips into the surface 0–10 cm of
Physical
Soil
(cm)
Table
Exch., Standard
A.-S. Perrin et al. / Agriculture, Ecosystems and Environment 184 (2014) 101– 114
Table 2
Soil bulk density (Mg m − 3 ), organic carbon content (g kg − 1 ) and carbon stock (kg C m − 2 ) in soil fraction <2 mm at Combi site before forest clearing.
Soil layer (cm) Bulk density (Mg m − 3 )
Carbon (g kg − 1 of soil) Carbon stock (kg C m − 2 ) Mean ± SE Mean ± SE Mean ± SE
45–50 n.m. n.m. n.m. 50–55 1.52 ± 0.00 5.10 ± 0.42 0.36 ± 0.02
n.m., not measured.
Mean ± standard error, n = 17 per layer except for 20–25 cm, 30–35 cm and 40–45 cm (n = 3), >55 cm (n = 2) and 180–200 cm (n = 1).
the soil ( Fig. 1 ). During forest conversion to farmland, all of the forest litter and part from crops/cover plants residues represent 49.5–51.2%, 15.8–16.4% and 34.6–32.4%
of the roots were also incorporated into the soil surface 20 cm at the same time as of total inputs, respectively.
the wood chips and agricultural lime in December 2008. During the studied period, After the first harvest of maize, the quantities of aerial biomass which were
total aerial biomass inputs to the soil were 67.5 ± 15.78 and 65.3 ± 15.65 Mg of dry returned to the soil in NT plots (8.5 Mg DM ha − 1 ) were significantly higher than
matter per hectare in treatments NT and DT, respectively. Inputs from chopped in DT (4.9 Mg DM ha − 1 ). Restitutions after the other crops were not significantly
vegetation residues and litterfall, from year one cover crops (legume and grass) and different for DT and NT treatments because the establishment of the cover plant (U.
Table 3
Dry matter (Mg ha − 1 ) and C and N contents (g kg − 1 ) of above-ground biomass returned to soils or removed from plots, during the three years after the clearing of the forested
site.
Date Type of above-ground biomass n Dry matter (Mg ha − 1 )
Total N% Total C C/N Mean ± SD Mean ± SD Mean ± SD Mean ± SD
T0 (October 08) Forest chopped-biomass 15 24.1 ± 14.3 0.5 ± 0.1 46.9 ± 2.5 108 ± 31
Forest litter 15 9.4 ± 5.2 *
49.2 ± 1.9 ** T1 (November 09) Cover plants 9 10.7 ± 3.4 0.9 ± 0.1 45.5 ± 0.3 53 ± 5
T1.5 (April 10) Maize NT (Stovers, leaves and spathes) 4 2.0 a ± 3.4 0.5 ± 0.1 45.3 ± 0.4 85 ± 10
Urochloa ruziziensis mulch NT 4 6.5 ± 2.0 1.1 ± 0.2 44.2 ± 0.5 42 ± 6
Residual coverplant and wood chips n.m. a n.m. n.m. n.m. Maize grains harvested NT 4 6.5 ± 2.0 1.3 ± 0.3 43.9 ± b 0.4 34 ± 7
Maize DT (Stovers, leaves and spathes) 4 3.9 ± 0.6 0.4 ± 0.1 45.6 ± 0.4 117 ± 23
Urochloa ruziziensis − regrowth DT 4 1.0 ± 0.7 1.5 ± 0.4 43.3 ± 1.0 30 ± 7
Maize grains harvested DT 4 5.7 b ± 2.4 1.1 ± 0.1 44.0 ± 0.4 40 ± 4
Grass restitution (on May 2010) n.m. n.m. n.m. n.m. Hay harvested 8 5.5 ± 2.1 1.6 ± 0.4 44.7 ± 0.5 29 ± 6
T2 (October 10) Soybean stems NT 4 0.3 a ± 0.1 0.5 ± 0.1 46.3 ± 0.5 96 ± 15
Soybean leaves + residual mulch NT 4 2.8 ± 0.1 0.7 ± 0.1 46.2 ± a 0.3 71 ± 12
Soybean grains harvested NT 4 2.8 ± 0.4 6.5 ± 0.2 53.3 ± 0.4 8 ± 0
Soybean stems DT 4 0.7 b ± 0.1 0.5 ± 0.1 45.6 ± 0.3 104 ± 23
Soybean leaves DT 4 1.4 ± 0.3 0.6 ± 0.0 b 44.3 ± 0.3 70 ± 2
Soybean grains harvested DT 4 2.0 ± 0.1 6.4 ± 0.3 53.0 ± 0.3 8 ± 0
Grass restitution No
Hay harvested 8 11.0 ± 1.7 0.5 ± 0.1 45.6 ± 0.2 104 ± 26
T2.5 (April 11) Maize NT (Stovers, leaves and spathes) 4 7.5 ± 0.9 0.8 ± 0.1 45.6 ± 0.2 61 ± 7
Urochloa ruzi + residual mulch NT 4 0.9 ± 0.3 2.2 ± 0.1 43.0 ± 0.7 19 ± 1
Maize grains harvested NT 4 5.1 ± 0.9 1.5 ± 0.1 43.7 ± 0.8 30 ± 2
Maize DT (Stovers, leaves and spathes) 4 7.9 ± 1.3 0.7 ± 0.1 45.8 ± 0.3 64 ± 12
Maize grains harvested DT 4 5.0 ± 1.0 1.6 ± 0.1 43.9 ± 0.2 28 ± 2
Grass restitution (on January 11) 8 2.6 ± 0.3 1.2 ± 0.1 44.8 ± 0.3 38 ± 5
Hay harvested 8 2.2 ± 0.4 1.2 ± 0.1 44.9 ± 0.7 37 ± 4
T3 (November 11) Crotalaria + Stylo restitution NT 4 5.4 ± 1.2 1.5 ± 0.1 45.6 ± 0.1 31 ± 3
Maize residues NT n.m.n.s. n.m. n.m. n.m. Crotalaria + Stylo restitution DT 4 6.2 ± 0.2 1.4 ± 0.2 45.6 ± 0.2 32 ± 3
Grass restitution 8 No
Hay harvested 3.8 ± 0.5 1.0 ± 0.1 45.9 ± 0.3 46 ± 4
n.m., not measured. T1.5: the quantities of aerial biomass (maize stovers, leaves and spathes + Urochloa ruziziensis ) and the quantities of maize grains harvested were signifi-
cantly different in NT and DT treatments as indicated by different lower case letters (Tukey test, p < 0.05). T2: the quantities of aerial biomass (soybean stems + leaves + residual
mulch for NT) and the quantities of soybean grains harvested were significantly different in NT and DT treatments as indicated by lower case letters (Tukey test, p < 0.05).
** Janssens et
al. (1998) . Hättenschwiler et al. (2011) .
A.-S. Perrin et al. / Agriculture, Ecosystems and Environment 184 (2014) 101– 114
Fig. 3. Soil bulk density (Mg m − 3 ) and C content (g kg − 1 ) of the soil fraction <2 mm in the 0–5, 5–10, 10–20 and 20–30 cm layers in Combi site for forest, grassland (G),
maize/soybean crop rotation plots with disk tillage (DT) and with no tillage in direct seeding (NT). Mean and standard error. Mean values followed by the same lower case
letter for the same layer and the same sampling date did not differ significantly by the Tukey test. Mean values followed by the same upper case letter for the same layer did
not differ significantly from reference forest (Tukey and Dunnett tests), p < 0.05.
ruziziensis ) was unsuccessful due to failure of the seeds to germinate. Hence this parameters ( http://www.gip-ecofor.org/f-ore-t/paracou.php ), and the same typical
treatment is here considered as a simple direct seeding management which differs forest species composition − 2 ( Petitjean, 2013 ). Janssens et al. (1998) reported
from mulch-based direct seeding. 0.94 ± 0.52 kg m of dry litterfall. A total forest litterfall of 8.3 Mg ha − 1 yr − 1 Although litter was not sampled in our forested site, various data can was estimated by Chave et al. − 1 (2010) . Hättenschwiler et al. (2011) measured
be found in previous ± ◦ ′ ′′ ◦ ′ ′′ studies at the Guyaflux experimental site in Paracou 492 19 g kg of C (dry matter) and 11 ± 3 g N kg − 1 dry matter in leaf litter from
(5 16 54 N, 52 54 44 W), especially in forested areas showing the same soil 45 tree species at the Paracou site.
A.-S. Perrin et al. / Agriculture, Ecosystems and Environment 184 (2014) 101– 114
Carbon inputs from forest litter, forest chopped-biomass and year one cover
crops amounted to 0.462 ± 0.256, 1.128 ± 0.670, and 0.487 ± 0.153 kg C m − 2 , respec-
tively. These three input types induced by land use conversion method accounted
for 65.4% and 67.6% of total aerial C inputs to soils between T0 and T3 for NT and DT,
respectively.
Chopped forest-biomass had high C:N ratios (108.5 ± 31.0) similar to maize and
soybean stems (85–117) which would indicate low mineralization rates. U. ruz-
iziensis in grassland or as mulch in NT had C:N values of less than half these values
( Table 3 ).
3.3. Soil bulk density and carbon content after forest conversion
At sampling date T1, bulk densities range between 1.04 and 1.22 Mg m − 3 in the
0–5 cm layer and increase progressively to 1.50–1.56 Mg m − 3 in the 20–30 cm layer
for all the agricultural soils ( Fig. 3 ). Bulk densities did not differ significantly between
plots of all the treatments within a given block (data not shown), or considering the
whole site in the 0–30 cm layer. These values increase with time for each soil layer.
Compared to forest soils, NT plots were significantly more compacted in the 0–5 and
10–20 cm soil layers at all sampling dates, in the 20–30 cm at T2 and in the 5–10 cm
layer at T3. Disk tillage (DT) soils had a significantly higher mean bulk density than
forest in the 10–20, 20–30 and 0–5 cm soil layers at T1.5, T2 and T3, respectively.
From T1 to T3, for a given soil layer, differences in mean bulk densities remain
small between treatments. In no-tillage plots, mean bulk densities were significantly
higher in the 0–5 cm layer at T1 than in the other treatments. Significantly lower
values were found for grassland soils (G) at T1.5 below 10 cm compared to other
treatments. However at T2 soil bulk densities were not significantly different for
any treatments in each layer. At T3, data ranges between 1.12 and 1.30 Mg m − 3 in the 0–5 cm and between 1.47 and 1.57 Mg m − 3 in the deepest layer (20–30 cm). DT
was significantly less compacted than other treatments in the 5–10 cm layer. In
grassland plots in T3, Db were significantly higher than in NT soils in the 5–10 cm
layer and lower than for other treatments between 10 and 20 cm. No-tillage plots
showed the highest mean values at T3 for all soil layers (although not significantly
so).
Soil carbon contents (C < 2mm ) ( Fig. 3 ) increased significantly for all treatments
in T1 and T1.5 compared to forest between 20 and 30 cm (except for DT in T1.5)
and in T1.5 between 5 and 10 cm. In T2, only grassland showed significantly higher
values than in forest below 20 cm. In contrast, C contents were lower than in forest
in the 0–5 cm layer for all treatments in T2 and T3, for grassland in the 20–30 cm
layer in T2 and for NT in the 10–20 cm layer in T3. Significant differences between Fig. 4. Change rate of carbon masses tn−t (n−1) , in kg C m − 2 yr − 1 ) in the soil fractions
treatments can be observed after T2 with lower values for NT in T2 below 10 cm and <2 mm and >2 mm in surface 30 cm for grassland (G), maize/soybean crop rotation
in T3 above 20 cm. DT and NT plots present significantly lower values in the 0–5 cm under disk tillage (DT) and under no tillage with direct seeding (NT) during the 3
layer in T3 compared to grassland and forest. years following forest conversion at Combi site. Mean ± standard error. T0 is samp-
ling date just before land conversion. Tn: sampling date where n corresponds to
3.4. Carbon masses in soil fractions <2 mm and >2 mm year(s) after land conversion. Period duration (year): T0 to T1 1.13 years; T1 to T1.5
(maize cycle) 0.43 year; T1.5 to T2 (soybean cycle) 0.44 year and T2 to T3 1.09 years.
For the studied treatments, carbon concentrations in soil layers followed sim- For a given soil fraction, significant difference of soil carbon masses between two
ilar trends as C stocks in soils because bulk densities show only small variations sampling dates are reported by asterisks (*p < 0.05; ***p < 0.0001). For each period,
(see Section 3.3 ). In forest soils at T0 (October 2008), C < 2mm mean stocks were 1.24, differences between treatments were not significant as assessed by Tukey test.
1.05, 1.76, 1.44 and 5.50 kg C m − 2 in the 0–5, 5–10, 10–20, 20–30 and 0–30 cm soils
layers, respectively. More than a year later (414 days, at T1), C < 2mm mean stocks
( Table 4 ) did not differ significantly between treatments when considering each respectively. The mass of carbon in the surface 30 cm of the soils did not exceed
layer separately. Values lay between 6.14 and 6.40 kg C m − 2 in the 0–30 cm layer. 0.83 kg C m − 2 .
Higher C < 2mm mean stocks (1.69 to 1.76 kg C m − 2 ) found in the − were 2 20–30 cm layer Overall, the highest total soil carbon values (C tot ) (in kg C m ) which were
of agricultural plots than in forest soils (significant for G and NT treatments). After significantly higher than in forest soils (6.12±0.21) were measured at T1 for DT
forest conversion, the highest C < 2mm stocks were found at T1.5, with mean values (7.65 ± 0.43) or NT (7.72 ± 0.42), and at T1.5 for G (8.19 ± 0.37) ( Table 4 ). Later, C tot of 1.24–1.40, 1.33–1.42, 2.04–2.15 and 1.56–1.86 kg C m − 2 in the 0–5, 5–10, 10–20, fell in every layer for all converted soils. At T2 and T3, values in NT soils were lower
and 20–30 cm layers, respectively, for agricultural soils. At this sampling date, sig- than those of other agricultural plots (significant except in the 20–30 cm layer). At
nificantly more carbon was contained in the <2 mm fractions of 5–10 cm layers for T3, C tot was significantly lower for NT soils than in forest soils.
all agricultural plots (which did not differ) than in forest soils. When considering For surface 5 cm in the forest soil, 20% and 5% of total C mass (0–30 cm) were
the surface 30 cm of soils, converted plots contained higher C < 2mm mean stocks contained in <2 mm and >2 mm fractions, respectively ( Table 5 ). After land use con-
than initial forest soils but this difference is significant for G soils only ( Table 4 ).
version, the percentage carbon ( Table 5 ) in the <2 mm fraction of the 0–5 cm soil
After one year of maize/soybean rotation (at T2), C < 2mm was lower for each soil layer fell to 16% of the C tot contained in the 0–30 cm soil layer whilst that of the
layer than at T1.5 although not significantly so. At T2 and T3, C < 2mm mean stocks >2 mm fractions rose to 8%. The distribution of carbon in soil surface 30 cm is also
were significantly lower in the surface 5 cm for all converted soils (between 0.86 slightly modified in the 5–10 cm layer, with a higher proportion of carbon in the
and 1.01 kg C m − 2 ) than in forest soils ( Table 4 ). NT plots contained less carbon in >2 mm fraction in cultivated soils (maximum 6%) than in forest soil (2%). Soil lay-
the <2 mm soil fraction in each layer than grasslands (significant except in 0–5 cm ers 10–20 cm and 20–30 cm contained 26–29% and 21–25% of total carbon in the
for T2 and in 20–30 cm for T2 and T3). DT also had slightly lower values than G but <2 mm, respectively, and 2–6% and 1–3% in fraction >2 mm, respectively.
not significantly so.