Soil carbon stocks and pools in forest soils manual felling ( Ghuman et al., 1991; Desjardins et al., 2004 ). This

4.1. Soil carbon stocks and pools in forest soils manual felling ( Ghuman et al., 1991; Desjardins et al., 2004 ). This

result is in accord with another study on chop-and-mulch land The mean soil carbon stock (C < 2mm only) in the surface 0–20 cm clearing method of a 12–15 year-old fallow forest in the eastern

layer of neotropical forest at Combi site (4.05 kg C m − 2 ) was

Amazon on soils with similar textural parameters ( Comte et al.,

much higher than that reported by Desjardins et al. (2004) in

2012 ). In our study, the mulch formed by the chopped vegetation

eastern Amazonia (2.92 kg C m − 2 , in the Brazilian state of Pará, may have protected soil from compaction during the clearing phase

of tractor (track-type). Soil tillage with incor- Combi (5.50 kg C m 2 ) is similar to values measured in Rondônia poration of chopped vegetation into the surface layer (0–20 cm)

sandy–clayey acrisols). C < 2mm stock in the 0–30 cm soil layer in as well as the type

by Carvalho et al. (2010) on Juliana farm on a Rhodic Kandiudox of soils just after the clearing undoubtedly modified soil porosity

soil (5.63 kg C m − 2 ) and Maia et al. (2010) on oxisols (Santa Luzia and could have stimulated soil

biological activity due to increased

D’Oeste, 5.30–5.57 kg C m 2 ). Considering the pedon, the C < 2mm

substrate availability. As shown in Fig. 1 B, woodchips still covered

stock in the 0–2 m soil depth at Combi, 14.16 kg C m − 2 , corresponds much of the soil surface even after their incorporation into

0–20 cm

to the more frequent range of values (10–20 kg C m 2 ) estimated for layer and could have helped to reduce physical constrains caused by

67 sites across Amazonia by Quesada (2010) . In our study, about 39% rainfall during the first year. Furthermore, cover crops roots might

of the carbon estimated for the 0–2 m forest profile is contained in have helped to restore soil porosity.

the surface 0–30 cm. Nevertheless, compaction increased slightly during the 3 years

Estimates by Bréchet (2009) for roots biomass in the upper following land use change, especially in NT soils. Compaction

30 cm of forest soils at Paracou site are in the same range as our increased between T2 and T3 in the 0–10 cm layer ( Fig. 3 ), prob-

values ( Table 4 ), with 0.547 and 0.640 kg C m − 2 contained in roots ably due to more intense climatic events recorded in April–May

of diameters <2 mm and >2 mm, respectively for the 0–30 cm soil 2011 after maize and grass harvests and at sowing of soybean. Soil

layer. Boulet and Humbel (1980) measured a total root biomass of surface compaction might have been caused by machinery being

1.63 kg m − 2 (i.e. 0.69 kg C m − 2 assuming 42.7% of C in roots) for sim- used in wetter soil conditions than during previous crops.

ilar well-drained ferralsols in the 0–2 m layer, of which 75% and 83%

were located in the surface 20 and 40 cm of the soil, respectively 4.3. Short term relative depletion of soil surface carbon

and only 6% in the 1–2 m layer.

4.3.1. First year after clearing

4.2. Forest conversion into grassland and cropland During this experiment, land clearing with the chop-and- mulch method led − to the incorporation of 1.59 kg C m − 2 (i.e. Total above-ground biomass in forests of French Guiana has 0.462 ± 1.128 kg C m 2 ) from the aboveground forest biomass into

been estimated in previous studies on similar soils to 318 ± 17 mg the 0–30 cm soil layer, in addition to the initial 0.63 kg C m − 2 mainly of dry matter per hectare ( Sarrailh, 1984; Puig et al., 1990 ), what contained in forest roots. The grinding of the aerial parts of cover

corresponds to a high value for Amazonia as reported by Anderson crops incorporated 0.487 kg C m − 2 more in the 0–5 cm and litter

et al. (2009) . The quantity of chopped biomass that was incor-

layer. Death of and exudation by cover crop roots also increased

porated into the surface 20 cm of soils represents 3–13% of the soil carbon masses. Thus, during the first year, at least 2.71 kg C m − 2 estimated total above-ground biomass of native forest. As reported was added to the soils, increasing C masses in the whole soil pro-

by Denich et al. (2004) , the efficiency of the chopping process file by about 0.61–2.23 kg C m − 2 (72–98% of the C tot in the <2 mm

depends on plant morphological parameters such as stem diame- fraction) compared to forest soils. We can assume that litter and

ter, height and biomass of the predominant trees and shrubs. In the a major part of forest roots were decomposed rapidly enough to

case of neotropical forest, the use of a more powerful wood crusher significantly supply the >2 mm soil fraction. This is supported by

A.-S. Perrin et al. / Agriculture, Ecosystems and Environment 184 (2014) 101– 114 111 the decomposition rates measured in nearby Guianese neotropical mismanagement. In a meta-analysis based on results from Mato-

forest. Indeed, Sarrailh (1990) measured that forest litterfall was grosso and Rondônia states, Maia et al. (2009) concluded that the

completely decomposed after 195 days. Once incorporated into soil, effect of forest conversion to pasture on soil C stocks of Amazonia

litter might show a slower or faster decomposition/mineralization and Cerrado biomes shows contrasting results, depending on the

rate but we can hypothesize that its decomposition was largely

management applied to the pasture.

completed in T1. Moreover, Bréchet (2009) measured a turnover In contrast to grassland, the decrease in soil carbon stocks in

rate of 0.59–0.84 per year for fine forest roots (diameter < 2 mm) at our experiment is significant for croplands. Compared to the disk-

Paracou. In addition, the low mean C:N ratio of cover crops indi- tillage treatment, no-tillage practices induced the highest depletion

cates probable rapid decomposition ( Table 3 ). These three input rates in both soil fractions. This difference cannot be explained by

types (litterfall, roots, covercrops), which have fast decomposition smaller C inputs in NT plots. Between T1 and T3, slightly larger

potentials given the temperature/climate regimes in French Guiana C quantities were indeed returned to soils with above-ground

and soil fauna activity under tropical humid conditions (e.g. Tian biomass under no-tillage plots (1.15 for NT cf. 0.96 kg C m − 2 for et al., 1992, 1993; Tian, 1998 ), totalize 1.58 kg C m − 2 and are likely DT). Moreover, successful U. ruziziensis association between T1 to have supplied the <2 mm soil fraction during the first year. and T1.5 in NT certainly increased the amount of C inputs into

soils from roots compared to DT. We can then suppose that faster

4.3.2. Grassland and cropland with and without tillage practice decomposition and mineralization processes occurred in no-till Literature data about carbon stock changes during the period plots compared to tillage plots contrarily to what is usually reported

inferior to 3 years following land clearing in Amazonia are very in the literature for tillage versus no-tillage practice (e.g. Balesdent

scarce. Published data concern soils after 2–3 years of grassland or et al., 2000 ).

cropland installation following slash-and-burn and are based on In Amazonia, studies on crop systems and tillage effects on chronosequence approaches that increase uncertainty on carbon soils after clearing are scarce. In the humid zone of Nigeria, on

stocks measures because of spatial heterogeneity of soils. Ultisols, after 4 and 5 years of cropping (maize/cassava with- In our study, for all converted soils, the most rapidly decreasing out fertilization), the quantity of most of the labile carbon pool

rates of C occurred between T1.5 and T2 in the fine fraction (<2 mm) was not found significantly different after slash-and-burn or bull-

and during the 3rd year after land conversion in the coarse fraction dozed non-windrowed forest clearing ( Okore et al., 2007 ). After

(>2 mm). However, for the three types of land use, rates of change forest conversion with burning, crop successions of 2 and 6 years

of carbon masses differed for both soil fractions ( Fig. 4 ). As far as under no-tillage caused the depletion of 0.29 and 0.71 kg C m − 2 we know, this has not been observed until now. in the C stock of the 0–30 cm soil layer ( Carvalho et al., 2010 , In our study, the grassland system is characterized by

Rondônia). After 1 and 5 years of conversion to an Integrated Crop-

large amounts of above-ground biomass export, equivalent Livestock system under NT, soil carbon stocks decreased by 0.62 and