Estimated soil particles delivery to the swamp derived from TOC concentrations and ERT sediment volumes.
Estimated soil particles delivery to the swamp derived from TOC concentrations and ERT sediment volumes.
Unit Time (yrs) TOC stock (kgC m −2 ) Volume a (m 3 ) Surface b (m 2 ) TOC c (kgC yr −1 ) Soil TOC d (MgC yr −1 )
IV 29 (1976–2005) 28.3 650 722 705 24–31 III
a Median values in Table 5 .
b Median volumes divided by average thickness in Table 5 .
c Soil-derived TOC contribution calculated by multiplying TOC stock by surface and dividing by time.
d TOC content divided by average TOC concentration (26.1 ± 3.1 MgC g −1 ) of sediment load at W 1 monitored between 2001 and 2005.
and 32–40 Mg yr −1 for the 1951–1975 period. Although these measurements are downward 137 Cs bound soil particles redistribution by erosion dur-
associated with important uncertainties linked to high spatial variability, the esti- ing the cultivation years ( Valentin et al., 2008 ).
mated delivery is consistent with sediment input–output monitored between 2001
and 2005 with upstream (W 1 , Fig. 1 ) and downstream (W 2 , Fig. 1 ) gauging weirs
(28.6 Mg yr −1 , Table 7 ). Using 137 data in Table 6 , soil TOC accumulation in the swamp can be estimated to 5.2. Linking soil Cs and TOC inventories in the catchment
33 MgC for 42 years (since the 137 Cs fallout maximum of 1963). With an additional
10% of the total as Napier grass derived organic matter (ca. 3.6 MgC since 1963, Fig. 9 ),
Along steep slopes, correlative 137 Cs and TOC contents are usu-
ally interpreted as reflecting the impact of intense tillage erosion only partly counterbalanced by organic matter accumulation
TOC storage in the swamp reached ca. 36 MgC. Accordingly, soil depletion along
catchment slopes is
and vegetation growth in the swamp.
With an initial TOC content of 6.3 kgC m −2
on soil organic matter ( Li et al., 2006 ). Because Cs is less bound to
for the topsoil 30 cm horizon in 1963 and an average “present-day” content of soil organic matter than to clay minerals ( Livens and Baxter, 1988;
4.67 kgC m −2 , conversion of high forest to arable land and cultivation induced a loss Staunton and Roubaud, 1997 ), the positive correlation between soil
(32.5 ha) provides ca. 530 MgC. Therefore, a TOC and
of nearly 1.63 kgC m −2 (ca. 26%, 0.039 kgC m −2 yr −1 ). Scaling this value to the upper 137
Cs inventories is not a direct link. This relationship
catchment surface
TOC accumulation of
must rather be interpreted as reflecting ca. 36 MgC in swamp sediments only represented ca. 7% of soil carbon loss in the a common redistribution catchment since 1963. of 137 Cs- and TOC-bound soil particles ( Garcia-Oliva et al., 1995;
Ritchie and McCarty, 2003; Martinez et al., 2010 ). It is known that
the breakdown of organic matter-rich topsoil aggregates reduces
5. Discussion the TOC content of soils by releasing fine size mineral-bound
organic matter ( Feller and Beare, 1998 ) and light charcoal com-
5.1. Causes for heterogeneous redistribution of 137 Cs along ponents ( Rumpel et al., 2006 ) that are translocated down slope catchment’s slopes or exported by overland flow. However, as our 137 Cs measure- ments fingerprint soil TOC content, the correlation is based, for
Soil particles redistribution rates can be estimated for cultivated each site, on the remaining soil TOC content since radionuclide
soils from mass balance models that link the residual soil 137 Cs
fallout. Therefore, besides soil erosion, other processes such as
activity to the reference inventory, tillage dilution, soil particle size biomass burning after clearing, reduced input by fallow vegeta-
and time-variant 137 Cs fallout (e.g., Walling and He, 2001 ). These tion with respect to the former high forest, export of labile carbon
models do not apply for our study. Slash and burn is essentially a no- fractions with dissolved loads and mineralization of soil organic till cultivation system as soil disturbance is limited to patchy hoeing matter also contributed to soil TOC depletion, either in situ or fol-
and hand pulling for weed control that mainly concern the top first lowing soil particle’s displacements along slopes. The depletion
3 cm of the soil. In Houay Pano such tillage spread since 1996 ( Dupin
rate derived from our study, 39 gC m −2 yr −1 during 42 years, is high
et al., 2009 ). Short-distance relocation of 137 Cs and 210 Pb xs bound
when compared catchment specific deliveries of 1.2 gC m −2 yr −1 in soil particles along slopes and within the soil is the combined result 2002 ( Chaplot et al., 2005 ) or 0.85 gC m −2 yr −1 in 2003 ( Chaplot and
of faunal activity, overland flow, local rill erosion (e.g., Chaplot et al., Poesen, 2012 ) previously measured for Houay Pano. During these
2007 ) and spot tillage. Hence, clear identification of accumulation two years only 2–3% of soil TOC annual loss was exported. Because
and erosion gradients over short distances remains difficult. More- of a limited storage area in the catchment, sediment delivery ratios
over, rainfall simulation experiments showed that soil surfaces in were obviously much higher in the past, in particular during the
the Houay Pano catchment can be rapidly transformed to crusted critical period of forest clearing in 1965–1975 ( Fig. 10 ). Shortening
surfaces with micro-terraces on steep slopes, much more perme- of the shifting cultivation cycle after 1984 apparently took place in
able and less erodible than for low slopes ( Ribolzi et al., 2011 ). Such a more steady state way and involved the removal of soil particles
heterogeneity working at fine scales is also reflected in the high derived from cultivated soil horizons depleted in TOC with respect
variability of soil surface features and runoffs measured within 1- to soils formerly under forest cover. This reduced storage is con-
m 2 neighbouring plots under natural rainfall ( Patin et al., 2012 )
firmed by lower soil TOC input in the swamp (ca. 2 kgC m −2 , Fig. 9 B) and by the high scatter of average 137 Cs inventories with increasing although Napier grass covered most of the area, thereby increasing
distance of this study. At catchment scale; however, two trends in as much soil-derived particles trapping and retention. Other in situ
137 Cs activity redistribution were statistically highlighted ( Table 3 ):
processes, in particular mineralization, should have also played a
an increase from crest to valley bottom and a decrease with the key role in regard to soil TOC depletion. Gregorich et al. (1998) number of shifting cultivation cycles. They can be explained by reported 20–30% TOC losses by mineralization in North American
Table 7
Estimated soil derived particles delivery to the swamp deduced from measured sediment yields in weir W 1 upstream and weir W 2 downstream of swamp.
Weir of sub-catchment Sediment delivery (Mg ha −1 yr −1 ) Catchment area (ha) Sediment input (Mg yr −1 ) Sediment output (Mg yr −1 ) W 1 1.56 a 19.6 30.6 –
W 1 –W 2 1.56 a 12.7 b 19.8 – W 2 0.67 a 32.5 – 21.8
Total sediment delivery to the swamp (Mg yr −1 ): 30.6 + 19.8 − 21.8 = 28.6
a Valentin et al. (2008) .
b 32.5 − 19.6 − 0.19 = 12.7 ha with swamp area = 0.19 ha.
54 S. Huon et al. / Agriculture, Ecosystems and Environment 169 (2013) 43– 57
soils, mainly during the first years of cultivation. This proportion In Vietnam, finding lower slope soils more depleted in TOC should be enhanced under tropical conditions due to higher soil (10.5 mgC g −1 ) than upper slope soils (12.2 mgC g −1 ), Wezel et al.
temperature and biological activity. Our results are consistent with (2002) speculated that low TOC contents at lower slope posi-
experiments carried out in Luang Prabang by Roder et al. (1997) in tions were due to enhanced mineralization, crop export and slash and burn fields with 6–8 year fallow prior to upland rice culti- frequent cultivation. Direct links between soil erosion and topo-
vation. In their study soil TOC depletion reached ca. 230 gC m −2 yr −1
graphic position are confounded with historic farmer’s preferences
for the top 25 cm layer after 4 years and only about 10–20% was but erosion should still be considered as an important trig-
attributed to burning and soil erosion ( Roder et al., 1995a ). Min- ger. eralization thus represented the major soil TOC depletion process,
initiated under rice cropping and continued over the following two
years of fallowing. 5.4. Linking soil TOC accumulation rates in the swamp with land
use change
5.3. Soil TOC depletion in the shifting cultivation system
Our study outlines three successive stages of soil TOC trans-
Plots in the Houay Pano catchment were cultivated on average fer from slopes to the swamp. A high and steady input of TOC of
5.3 times over 42 years that is one year out of eight. Deforesta- 8.5 ± 1.8 kgC ha −1 yr −1 is generated in the period during which the
tion and cropping caused a loss of ca. 21.5% of the initial TOC catchment is still under forest ( Fig. 9 ). A bamboo dominated forest
content in the 0–10 cm topsoil ( Fig. 4 ). TOC depletion down to typically consists of densely packed culms with little understory
22.7 mgC g −1 in 2005 is consistent with shifting cultivation fields protecting the soil. Vigiak et al. (2008) demonstrated in Houay
located near Houay Pano: 28.4 mgC g −1 in 1991 ( Roder et al., 1995b )
Pano that not only bamboo poorly trapped sediments originat- and 22.3 mgC g −1 in 1998 ( MSEC, 1999 ). Historic records in Houay ing from uphill positions, but also generated in situ soil erosion
Pano demonstrated that sites deforested early were also the most contrarily to grass strips and banana stands. Similarly, Bamboo pro-
frequently cultivated since. Cultivation without external inputs
vided poor soil protection in Japan where stands were linked to
universally decreases TOC pools, however, in Houay Pano, higher landslides and gullies ( Hiromasa et al., 2004 ) and in China where
TOC values after 7–8 rotations were measured compared to 3–5 weeding and tillage of Phyllostachus sp. had a detrimental effect
rotations and to sites still under forest. This can only be explained on soil TOC content ( Xu et al., 2008 ). Moreover, bamboo in Houay
by a higher initial TOC content. The effect of shifting cultivation Pano was subject to synchronous flowering followed by massive
on soil TOC depends on the initial organic matter level of the die-off, apparently once every 10–15 years causing disturbance
soil, i.e., its equilibrium level ( Nye and Greenland, 1960 ). When and likely erosion. With deforestation Phyllostachus sp. largely dis-
this level is high, the TOC recovery during fallowing could also be appeared. Soil TOC delivery increased to 19.6 ± 5.5 kgC ha −1 yr −1 elevated. This would explain why, in Houay Pano, the frequently in the period of high forest clearing and the cultivation of a cultivated sites contain more carbon ( Fig. 3 ). However, TOC lev- single rice crop ( Fig. 9 ). Few field data document such a pro- els in topsoil decrease after 8 rotations or more. In the Nam Khan cess in tropical environments. Kotto-Same et al. (1997) reported,
hinterland the soil TOC content under primary forest was system- in Cameroon, that greatest losses in soil carbon stocks resulted
atically below (27.2 mgC g −1 ) that of cultivated sites (34.9 mgC g −1 )
from land preparation prior to first cultivation; thereafter soil TOC
and the first soil survey in Houay Pano ( MSEC, 1999 ) provided pools remained relatively constant throughout shifting cultiva- higher soil TOC for cultivated soils (averaging 22.3 mgC g −1 ) than tion as shown in our study. Evidence is shown by the “low” soil
in the remaining forested sites (averaging 19.4 mgC g −1 ). Khamou TOC transfer, averaging 6.5 ± 4.8 kgC ha −1 yr −1 , for the period fol-
farmers thus made preferential choices for cultivation that include lowing 1984 when the slopes were covered by alternating crops
the initial TOC level of the forest. Saito et al. (2006) found that and fallows ( Fig. 9 ). Monitoring suspended loads at the outlet
shifting cultivators in northern Laos first all seek black soils (“din of cultivated plots and sub-catchments showed that erosion was
dam” in Lao) for rice cultivation. Sites in Northern Thailand, sub- related to the cultivation years with very low or even no out- ject to shifting cultivation for about 100 years, had all similar puts during fallow periods ( Valentin et al., 2008 ). Thus, once the
low TOC but, in forest, soil TOC was still lower ( Zinke et al.,
initial forest has been largely removed, and averaging over cul-
1978 ). Several studies in SE Asia assessed the influence of shif- tivation and fallow years, annual soil particle supply remained
ting cultivation on TOC by comparing cultivated soils with nearby rather low, only slightly above the values obtained before defor-
uncultivated forest soils. The latter, called undisturbed, natural,
estation.
high, mature, sacred or primary forest, served as control. Con-
Wetlands, such as the swamp area in Houay Pano, are usually
clusions were that soil TOC is not or weakly affected by shifting considered as capable of sequestrating high amounts of carbon due
cultivation because of similar TOC storage in cultivated and forested to the slow decay of organic matter under anaerobic conditions
soils ( Forsyth, 1994; Bech Bruun et al., 2006; Ziegler et al., 2007; ( Whiting and Chanton, 1993 ). Such areas play a key role for carbon
de Neergaard et al., 2008; Aumtong et al., 2009 ). However, the
storage within continental hydrosystems ( Stallard, 1998; Van Oost
assumption that the forest near the cultivated sites represents et al., 2012 ). The extent of storage depends on the surface covered
an earlier stage is unfounded when farmers prefer more fer-
by wetlands, rather limited in the Houay Pano catchment. A frac-
tile soils and leave forest on poorer sites. The TOC loss due to
tion of soil and plant TOC accumulated in the swamp was likely lost
shifting cultivation is underestimated when the initial level of a by post-deposition mineralization. However, because the stream is
cultivated site is unknown and a nearby forest site is chosen as perennial and the swamp is fed by a permanent water table, highly
control. reducing conditions with O 2 -depleted waters prevail all year long
In this study both 137 Cs and TOC inventories increase with ( Huon et al., 2008 ) and only slow mineralization rates could have
lower topographic positions indicating an uphill supply. In con- taken place. Furthermore, the distribution of TOC content with sed-
trast, de Neergaard et al. (2008) in Sarawak and Aumtong
iment depth ( Fig. 8 B) does not correspond to a regular time-related
et al. (2009) in Thailand did not find a significant effect
decaying profile (e.g., Gälman et al., 2008 ) but rather reflects the
of slope position on TOC in cultivated catchments, and con-
extent of soil and vegetation inputs. Association with iron hydrox-
cluded that erosion was not the overriding force. An alternative ides, abundant in soils of the catchment, may also enhance the
explanation is that farmers have depleted the richer lower
preservation of TOC as shown in other sedimentary environments
slope sites until all slope positions reached the same level.
( Lalonde et al., 2012 ).
S. Huon et al. / Agriculture, Ecosystems and Environment 169 (2013) 43– 57 55
6. Conclusions
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