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Supporting information legends
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Supplementary Methods
3
Supplementary Table 1 Respective pool and budget of each base cation in the lowland
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
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tropical forest of Southern China
Supplementary references
23
Supplementary Methods
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To explore the respective pool and budget of each base cation in this low-land tropical forest,
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we collected stream water, bulk precipitation, and soil from the respective watershed near to
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our study site. Soil samples were collected at 10 cm depth intervals down to the deepest soil
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layer at three random sites each time. Altogether there were three sampling times (July 2005,
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July 2006 and July 2007. Exchangeable base cations (i.e., K+, Na+, Ca2+, Mg2+) were
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extracted with 1 mol l-1 CH3COONH4 (100 ml per 5 g soil).
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Ratios of base cations to silicate during weathering: Base cations in stream water could be
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derived from not only atmospheric input but also the weathering of soil minerals. To estimate
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the contribution of weathering to the calculation of base cation budget, a laboratory
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experiment was conducted to evaluate the ratio of base cations to silicate (BC/Si ratio) for
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subtropical site A. Soil samples collected from all layers at this site were dried and mixed
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evenly. Four hundred ml of deionized water was added to a Teflon flask containing 40 g of
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composite soil, and two replicates were conducted. Sufficient CO2 gas was injected into the
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flask approximately twice a week to promote weathering. Thirty ml of soil suspension was
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collected and the same volume of deionized water was added into the flask once a week for 2
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months. After filtering the soil suspension, the filtrate was analyzed for dissolved base cations
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and silicate using the same methods as above. During the experiment, silicate and base cation
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contents increased with time, suggesting solubilization of these components through the
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weathering process (data not shown). Based on these data, BC/Si ratios were evaluated to be
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0.10, 0.89, 0.17, and 0.86 for Na+/Si, K+/Si, Mg2+/Si, and Ca2+/Si, respectively.
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Bulk precipitation and stream water (β-stream in Yoshimura, et al., Tropical Zoology,
2
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2012, 25(1): 16-30) were collected at least once a month during the period Aug. 2006 – Jul.
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2007. Samples were filtered through 0.45 μm membrane filter and analyzed for dissolved
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ions. The amounts of precipitation and stream runoff during the sampling period were 1529
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and 950 mm, respectively. Atmospheric deposition (dry + wet) was estimated as those in bulk
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precipitation and output as those in stream water. The supply of each base cation by the
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weathering process was calculated based on the following equation, assuming that silicate
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concentration was constant in both soil solution and stream water (the weathering rate of base
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cations may be underestimated, because of the possible Si immobilization):
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Weathering supply flux of base cations (mol m-2 yr-1)
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= SiO2 in stream water (mol L-1) × Stream water flux (L m-2 yr-1) × BC/Si ratio
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BC/Si ratios were evaluated to be 0.10, 0.89, 0.17, and 0.86 for Na+/Si, K+/Si, Mg2+/Si, and
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Ca2+/Si, respectively, according to our weathering experiment. Base cation pool in soil is
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assumed to be affected solely by these input and output fluxes, as follows:
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⊿pool (m mol m-2 yr-1) = (Atmospheric input + Weathering supply) – Stream runoff
(1).
(2).
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Net plant uptake of base cations in this mature forest is considered to be negligible, because
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uptake rate is assumed to be similar to the decomposition rate of soil organic matter.
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3
Supplementary Table 1 Respective pool and budget of each base cation in the lowland tropical
forest of Southern China
Na+
K+
Mg2+
Ca2+
Total BC
210
2950
600
870
4630
16
16
3
28
63
Weathering supply / m mol m-2 yr-1 ** 4
32
6
31
72
Stream runoff / m mol m-2 yr-1 **
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23
39
78
209
⊿pool
-49
25
-30
-19
-74
20
46
63
Pool
/ m mol m-2 (0-100 cm depth) *
Atmospheric input
/ m mol m-2 yr-1 **
/ m mol m-2 yr-1
Predicted lifetime / yr
4
*Averaged 05’-07’ data of base cation content for all soil layers were used. **Fluxes of base cations were
calculated from water budget during the period Aug. 06-Jul. 07. Weathering supply was estimated from
silicate concentration in stream water using BC/Si ratios determined from weathering experiments.
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73
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77
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