Effects of Carbon Dioxide Enrichment and Nitrogen Addition on Inorganic Carbon Leaching in Subtropical Model Forest Ecosystems
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- Liu, J., Xu, Z., Zhang, D. et al. Ecosystems (2011) 14: 683. doi:10.1007/s10021-011-9438-6
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Soil mineral weathering may serve as a sink for atmospheric carbon dioxide (CO2). Increased weathering of soil minerals induced by elevated CO2 concentration has been reported previously in temperate areas. However, this has not been well documented for the tropics and subtropics. We used model forest ecosystems in open-top chambers to study the effects of CO2 enrichment alone and together with nitrogen (N) addition on inorganic carbon (C) losses in the leachates. Three years of exposure to an atmospheric CO2 concentration of 700 ppm resulted in increased annual inorganic C export through leaching below the 70 cm soil profile. Compared to the control without any CO2 and N treatments, net biocarbonate C (HCO3−-C) loss increased by 42%, 74%, and 81% in the high CO2 concentration treatment in 2006, 2007, and 2008, respectively. Increased inorganic C export following the exposure to the elevated CO2 was related to both increased inorganic C concentrations in the leaching water and the greater amount of leaching water. Net annual inorganic C (HCO3−-C and carbonate C: CO32−-C) loss via the leaching water in the high CO2 concentration chambers reached 48.0, 49.5, and 114.0 kg ha−1 y−1 in 2006, 2007, and 2008, respectively, compared with 33.8, 28.4, and 62.8 kg ha−1 y−1 in the control chambers in the corresponding years. The N addition showed a negative effect on the mineral weathering. The decreased inorganic C concentration in the leaching water and the decreased leaching water amount induced by the high N treatment were the results of the adverse effect. Our results suggest that tropical forest soil systems may be able to compensate for a small part of the atmospheric CO2 increase through the accelerated processing of CO2 into HCO3−-C during soil mineral weathering, which might be transported in part into ground water or oceans on geological timescales.
Keywordscarbon dioxideinorganic carbon lossmineral weatheringN additionsoil CO2 fluxsubtropical area
It is widely assumed that elevated atmospheric CO2 leads to reduced stomatal conductance (Saxe and others 1998) and diffusive conductance (Pearson and others 1995; Niklaus and others 1998). Reductions of diffusive and stomatal conductance translate usually into decreased rates of canopy transpiration and increased soil moisture in CO2-enriched plots (Bunce 2004). Higher soil moisture usually causes a higher fraction of precipitation to flow away through streamflow and deep seepage. If CO2 enrichment can also increase the weathering of carbonate minerals in forest soils of tropical and subtropical regions, together with the high precipitation and streamflow in these regions, inorganic C leaching could be an important C sink under the global CO2 enrichment background.
Atmospheric N deposition is a serious problem in southern China. Currently, this leads to high N deposition in precipitation in some forests (30–73 kg N ha−1 y−1) (Ren and others 2000; Mo and others 2006). The N deposition in Guangzhou City of southern China increased from 46 kg N ha−1 y−1 in 1988 to 73 kg N ha−1 y−1 in 1990 (Ren and others 2000). It was reported that increased plant growth under a CO2 enrichment environment depends on N availability (Luo and others 2006). What would be then the effects of elevated atmospheric CO2 with the increased N deposition on the soil mineral weathering in tropical and subtropical forests?
In this study, we used open-top chambers to study the effect of elevated atmospheric CO2 alone and together with N addition in southern China on the dynamics of inorganic C in the soil leaching water over 3 years after the treatments started. We hypothesized: (1) elevated atmospheric CO2 would increase soil moisture, and then increased soil water leaching in the tropical area; (2) elevated atmospheric CO2 would increase soil mineral weathering, which would lead to increased inorganic C leaching; and (3) inorganic C leaching under elevated CO2 would be altered by the N supply.
Open-Top Chamber Design
In March 2005, we collected the soil from a nearby ever-green broad-leaved forest after harvesting. The soil type was ultisol overlying sandstone and shale bedrocks, with a pH value below 4.5 in all the soil layers. Soil organic C is low in this sandy soil. The primary chemical properties were shown in Liu and others (2008). The soil was collected as three different layers (0–20-, 20–40-, and 40–70-cm depth) that were homogenized separately and used to fill the belowground part of the chambers correspondingly. One- to two-year-old seedlings grown in a nursery were transplanted in the chambers without damaging the roots. All the chambers were planted with 48 randomly located seedlings with 8 seedlings for each of the following six species: Castanopsis hystrix Hook.f. & Thomson ex A.DC, Syzygium hancei Merr. et Perry, Pinus massoniana Lambert, Schima superba Gardn. and Champ., Acmena acuminatissima (Blume) Merr. et Perry, and Ormosia pinnata (Lour.) Merr. These species were selected because they are native and the most widely distributed tree species in southern China. As trees were growing fast, one tree per species was harvested at the end of each year to avoid excessively crowded conditions in each chamber.
The open-top chambers used in the experiment were located in an open area where they were exposed to full light and rain. From April 2005, the chambers were exposed to the different treatments. Three chambers received a high CO2 and high N treatment (CN), three chambers received a high CO2 treatment (CC), two chambers a high N treatment (NN), and finally two chambers were used as a control (CK) and did not receive high CO2 or high N treatment. The high CO2 treatments were achieved by supplying additional CO2 from a tank until a CO2 concentration of 700 ppm was reached in the chambers. The high N addition treatments were achieved by spraying seedlings one time a week for a total amount of NH4NO3 at 100 kg N ha−1 y−1. No other fertilizer was used. Because the walls of the chambers in the belowground parts blocked lateral and vertical water fluxes, the seedlings were watered with tap water. All other chambers received the same amount of water as the CK chambers.
Sample Collection and Measurement
To determine soil C content, soil samples were collected for each chamber in July 2005 and November 2008 (the end of soil solution collection). Soil samples were randomly collected from 0–20, 20–40, and 40–60 cm soil layers in each chamber. Each sample consisted of three cores (2.5-cm diameter) by the same depth in each chamber. The composite samples were air dried, gently mixed, and sieved (<2 mm). Dead roots and plant residues were picked out. Soil organic C was determined following the Walkley Black’s wet digestion method (Nelson and Sommers 1982) and inorganic C was measured using colorimetric titration, in which samples were acidified by HCl to dissolve carbonate minerals and release CO2, and then using NaOH to titrate the excess HCl. Weighted percent inorganic C was then obtained by calculation.
Leaching water sample collection started in 2006 in the second year of the treatment applications and continued for three years. Soil leachates were collected at the bottom of the chamber belowground walls in stainless steel boxes. During the dry season, they were collected after each rainfall. During the wet season, they were collected one time a week. Each time, the exact volume of total leachates was measured and 100 ml per box was collected for chemical analysis. The Ca2+ and Mg2+ concentrations in the leachates were measured using inductively coupled plasma atomic emission spectroscopy. Both CO32−-C and HCO3−-C were measured by titration with HCl after the addition of BCP indicator (Vuai and Tokuyama 2007). To determine C input from rainfall, rain water was collected in an open area near the chambers and sent for the same analyses as leachates.
To measure soil CO2 flux (including autotrophic root respiration and heterotrophic microbial respiration), in April 2006, four PVC circular collars (10-cm diameter) were permanently installed in each chamber; each was inserted about 5 cm into the mineral soil between the growing plants. From 26 May 2006, soil CO2 flux measurements were made once a week using an infrared gas analysis system (IRGA, LICOR 6400, LiCor Inc, Lincoln, Nebraska, USA). To avoid extremely high temperatures at noon, soil CO2 fluxes were determined in the morning (09:00–12:00). The LICOR 6400 chamber (with a foam gasket) was placed on the PVC collars making an air-tight seal. Soil CO2 flux of a collar was determined three times repeatedly by measuring the rate of CO2 increase in the LICOR 6400 respiration chamber. The soil CO2 flux was based on the average of the data from the four locations in each chamber (they differed by less than 5% at any measurement period). Soil moisture was recorded at the same time when measuring the soil CO2 flux. It was determined on several random points around each collar using a PMKit (Tang and others 2006).
The growth of five species (Castanopsis hystrix Hook.f. & Thomson ex A.DC, Syzygium hancei Merr. et Perry, Schima superba Gardn. and Champ., Acmena acuminatissima (Blume) Merr. et Perry, and Ormosia pinnata (Lour.) Merr.) was measured as the increment in plant dimensions over time to determine how the species responded to the various CO2 and N treatments. Plant height and basal diameter were measured at the time of planting in early March 2005. They were assessed seven times later in August 2005, November 2005, May 2006, September 2007, January 2008, August 2008 and January 2009, respectively. Plant height was measured from the soil-stem surface to the tip of the apical bud and the diameter was assessed at the soil surface.
To evaluate the effect of treatments on plant growth at different measurement occasions, repeated measures of ANOVA were used. When the effects were significant, they were further analyzed using a Tukey multiple comparison test.
Soil C Content
Comparison of Inorganic and Organic Carbon (C) Contents (Means ± Standard Deviations) of Open-Top Chamber Soils at the Beginning (Initial, July 2005) and at the End of the Experiment (Final, November 2008)
Soil organic C (% means ± SD)
Soil inorganic C (% means ± SD)
0.450 ± 0.035
1.555 ± 0.070
0.019 ± 0.003
0.014 ± 0.005
0.360 ± 0.006
0.992 ± 0.036
0.016 ± 0.003
0.014 ± 0.007
0.220 ± 0.025
0.680 ± 0.041
0.019 ± 0.006
0.018 ± 0.006
0.427 ± 0.033
1.259 ± 0.153
0.019 ± 0.003
0.013 ± 0.002
0.297 ± 0.032
0.812 ± 0.067
0.016 ± 0.003
0.010 ± 0.003
0.140 ± 0.005
0.451 ± 0.030
0.019 ± 0.006
0.011 ± 0.004
0.500 ± 0.090
1.498 ± 0.181
0.019 ± 0.003
0.018 ± 0.002
0.225 ± 0.015
0.851 ± 0.011
0.016 ± 0.003
0.017 ± 0.003
0.180 ± 0.030
0.595 ± 0.051
0.019 ± 0.006
0.018 ± 0.004
0.425 ± 0.003
1.283 ± 0.097
0.019 ± 0.003
0.017 ± 0.04
0.240 ± 0.020
0.853 ± 0.126
0.016 ± 0.003
0.020 ± 0.05
0.180 ± 0.010
0.462 ± 0.106
0.019 ± 0.006
0.026 ± 0.08
Soil Moisture and Soil CO2 Flux
Effects of Treatments on Soil Moisture and Soil CO2 Flux (Means ± Standard Deviations)
Soil moisture (%)
Soil CO2 flux (μmol CO2 m−2 s−1)
Leaching Water Volume and pH
Effects of Treatments on Annual Volumes of Leaching Water and Annual Amounts of DIC (Dissolved Inorganic C) in Leachates
Annual amounts (L ha−1 y−1)
Water × 10−4
Annual amounts(kg ha−1 y−1)
% Increase over the control
Effects of CO2 Treatment (C), N Treatment (N), Season and Month (season) and their Interactions on Concentrations and Monthly Amounts of Water and DIC (Dissolved Inorganic C, CO32−-C and HCO3−-C) in Leachates
C * N
C * season
N * season
C * month (season)
N * month (season)
In 2006, pH values of leaching water increased significantly, from 4.48 in February to 7.79 in December (Figure 3). In both 2007 and 2008, the pH values in the leachates were relatively stable. However, there were no significant differences between the treatments in both 2006 and 2007. The chambers exposed to the elevated CO2 had higher pH values in the leachates (P = 0.008) than the chambers under the ambient CO2 in 2008. The pH values in the leachates were positively related to both leaching water volume and HCO3−-C concentrations.
Inorganic C in the Leachates
The monthly amounts of HCO3−-C in the leachates also varied throughout the whole experimental period. There were more leaching amounts of HCO3−-C in 2008 than the other sampling years. The CO2 and N treatments affected significantly the monthly amounts of HCO3−-C in the leachates (P < 0.001 for both in 2007 and 2008, Table 4). The combined effect of the high CO2 and N treatments on the HCO3−-C concentrations was the apparent interaction (P < 0.001 in 2007 and P < 0.05 in 2008). Compared to the other treatments, the CC treatment showed the greatest monthly amounts of HCO3−-C in the leachates, and the NN treatment had the lowest monthly amounts. The annual net HCO3−-C leaching losses were 47.7, 49.5, and 113.2 kg ha−1 y−1 in the CC treatment in 2006, 2007, and 2008, respectively, which were 42%, 74%, and 81% higher than those of the control (Table 3). The annual net HCO3−-C leaching losses were 38.7, 40.9, and 68.5 kg ha−1 y−1 in the CN treatment in 2006, 2007, and 2008, respectively (Table 3). The N treatment reduced the positive effect of high CO2 treatment on the HCO3−-C loss in the leachates.
Compared to the HCO3−-C, CO32−-C concentrations and amounts in the leachates were low. They could not been detected in 2007. Both concentrations and monthly amounts of CO32−-C in the leachates were affected by the CO2 treatment and the sampling time. The N treatment had no effects on CO32−-C (Table 4). The CC and CN treatments exhibited higher CO32−-C concentrations and monthly leaching amounts than CK and NN treatments. The annual net CO32−-C leaching losses were 0.34 and 0.79 kg ha−1 y−1 in the CC treatment in 2006 and 2008, respectively, which were 72% and 367% higher than those of the control (Table 3). The annual net CO3−-C leaching losses were 0.43 and 0.55 kg ha−1 y−1 in the CN treatment in 2006 and 2008, respectively (Table 3). The CO32−-C concentrations were positively correlated to the Ca and Mg concentrations (R2 = 0.31 and R2 = 0.37, P < 0.05, respectively).
Annual net inorganic C leaching losses were calculated by summing up the C losses from HCO3−-C and CO32−-C. Annual inorganic C leaching losses were highest in the CC treatments with 48.0, 49.5, and 114.0 kg ha−1 y−1 in 2006, 2007, and 2008, respectively, compared with 33.8, 28.4, and 62.8 kg ha−1 y−1 in the CK treatment in the corresponding years. There were 86.5 and 24.1 kg ha−1 increases of inorganic C loss in the CC and CN treatments compared to the CK treatment during the 3 years. However, there was a decrease of 11.75 kg ha−1 in inorganic C loss in the NN treatment when compared to the CK treatment.
Ca and Mg in the Leachates
Among the treatments, greater soil organic C was observed in the CN treatment when compared to the other treatments. This result is in accordance with the previous studies (Luo and others 2006), which showed additional N supply enhanced the effects of CO2 on C accumulations in the soil pools. The high CO2 treatment alone did not affect soil organic C content, and this supports the findings of Schlesinger and Lichter (2001) and Williams and others (2003), and adds to the uncertainty about whether the soil C pool would act as a net sink for atmospheric CO2 under future increasing atmospheric CO2 concentrations (Cox and others 2000). Neither treatments nor sampling times affected soil inorganic C contents; this is in accordance with the finding of Williams and others (2003), where the higher dissolved inorganic C leaching induced by the elevated CO2 than control did not change the soil inorganic C contents which were low, like those in our soils.
Soil Moisture and Soil CO2 Flux
Under elevated atmospheric CO2, higher soil moisture has been reported previously (Niklaus and others 1998; Morgan and others 2004). Increased soil moisture was also shown in the high CO2 treatment chambers in our experiment. The decreased soil moisture in the N treatment was due to the increased plant growth under the N fertilizer condition (Figure 6; Duan and others 2009).
Increased soil CO2 fluxes induced by the CO2 treatment have been found in many studies (King and others 2001; Pendall and others 2001; Gill and others 2002; Suwa and others 2004; Bernhardt and others 2006). Increased root biomass and soil organic matter resulted in increased root and microbial respiration (de Graaff and others 2006). This is the reason fpr the higher soil CO2 fluxes under elevated CO2 (de Graaff and others 2006). Greater soil organic matter was also shown in the CN treatments in our experiment. We also found increased tree growth in this treatment’s chambers (Figure 6). The N treatment increased N availability in the soils for faster plant growth, which led to greater soil organic matter in the CN treatment than in the CC treatment. This would also translate to greater soil CO2 flux rates in the CN treatment than in the CC treatment. Increased soil moisture in the CN and CC chambers also led to higher soil CO2 fluxes in these treatments.
Variations of Leaching Water Volumes and pH in Leachates
In our study, the volume of leachates varied greatly. There were three main reasons for these variations. The main and most obvious one was the rainfall. The volume of leachates showed a monthly pattern that closely matched the monthly rainfall (R2 = 0.49, P < 0.05). The second factor for the variations was the high CO2 treatments. High CO2 concentrations in the CC and CN treatments resulted in a shift of the leaching pattern imposed by the rainfall toward greater leaching rates. This led to annual leaching volumes 69% and 30% greater in the CC and CN treatments, respectively, in 2006 and 18% and 20% in the CC treatment in 2007 and 2008, respectively, compared with those for the control. Increased leaching volumes in these treatments were probably related to the decrease of other water outputs such as lower evaporation and plant transpiration. Indeed, these two treatments also showed the highest soil moisture contents. Nelson and others (2004) also found that elevated CO2 increased soil moisture in a long-term field study and might lead to increased water drainage. They suggested that higher soil moisture under elevated CO2 could cause a greater fraction of precipitation to be lost by run off or deep seepage, which is in accordance with our findings. Finally, the high N treatment was the third factor for the leachate volume variations. High N addition caused a decrease in leachate volume and with time, this decrease became more obvious. This is the consequence of better tree growth following N fertilization (Duan and others 2009). During the low rainfall months, the volume of leachates showed reduced differences between the treatments because less water was available for leaching due to increased plant water use.
In 2006, pH values of leaching water increased significantly. There are three reasons which would lead to the increase in soil water pH values. First, the rise in soil water pH was probably linked to the reduction in NO3 leaching (Liu and others 2008). Secondly, high precipitation amounts and temperatures since April led to higher rates of weathering of soil minerals and soil organic matter decomposition, which produced more cations and accelerated the exchange of H+ with cations. Generally, the seedlings would grow faster during the pioneer stage, which would also facilitate the weathering of soil minerals and soil organic matter decomposition. The chambers exposed to elevated CO2 had higher pH values in the leachates than the chambers under ambient CO2 in 2008. This is due to the higher weathering of carbonate minerals in these chambers.
Variations of Inorganic C Leaching Losses
In our study, the significant increases in CO32−-C and HCO3−-C concentrations in the leachates that occurred in the high CO2 treatment chambers were due to the higher soil CO2 fluxes in the treatment. This result is consistent with the findings in the studies of Andrews and Schlesinger (2001) and Williams and others (2003). Increased concentrations of Ca and Mg were found in the high CO2 treatments, which also indicated that more chemical weathering of carbonate minerals happened in the high CO2 treatment chambers. The increased pH values in the leachate of the CC treatment in 2008 also support this result.
We noted that there are a few studies showing higher HCO3−-C concentrations in the soil water induced by elevated CO2 in forest ecosystems (Andrews and Schlesinger 2001; Williams and others 2003). But none of these could report the exact total amount of inorganic C (CO32−-C and HCO3−-C) losses induced by the elevated CO2 due to experiment design limitations. In addition to the fact that there was a higher inorganic C concentration in the leachates in the high CO2 treatment chambers, we also found that there were greater leaching water amounts in the CC treatment chambers. This led to increased inorganic C leaching exports over the 3 years in these chambers. Compared to the CK chambers, higher soil CO2 together with the increased soil moisture evidenced in our study, created favorable conditions for chemical weathering (Vuai and Tokuyama 2007). Bicarbonate and free base cations are byproducts of carbonate mineral weathering and an increase of HCO3−-C loss is the sign of accelerated weathering.
Increasing atmospheric CO2 and N deposition are two primary and concurrent changes in subtropical China. Elevated CO2 and N addition affected each other in stimulating plant growth (Duan and others 2009), which resulted in complex interactive effects on soil CO2 flux in our study shown in Deng and others (2010). High CO2 treatment increased soil CO2 fluxes and soil moisture contents, and then increased the reactions shown in equations (1) and (2), which led to more inorganic C loss. High N treatment decreased soil moisture and then the leaching water amount, which resulted in lower inorganic C loss. Thus, the positive effect of the high CO2 treatment on inorganic C loss was offset by the high N treatment, indicating a strong interactive effect of these two factors on inorganic C loss. They should be evaluated in combination in subtropical forest ecosystems in China where atmospheric CO2 and N deposition have been increasing simultaneously and remarkably.
Importance of Inorganic C Losses for Mitigating Atmospheric CO2 in Tropical Areas
Recently, the uncertainty about whether the soil C pool will act as a net sink for atmospheric CO2 under elevated CO2 has been reported by some studies (Cox and others 2000; Schlesinger and Lichter 2001; Hagedorn and others 2003; Lichter and others 2005). New sinks for anthropogenic atmospheric CO2 should be sought. Chemical weathering through the conversion of CO2 into its dissolved form which can then be sequestered in water has been considered as a net sink of atmospheric CO2 (Smedberg and others 2006; Oh and Raymond 2006; Gilfillan and others 2009).
After 2 years of observation in the Duke FACE forest, Andrews and Schlesinger (2001) reported that an increase of 55% in atmospheric CO2 over 2 years resulted in a 33% increase in the flux of dissolved inorganic C to groundwater. However, after another 3-year study at the same research site, Oh and others (2007) showed that carbonic acid weathering was increased by less than that suggested by Andrews and Schlesinger (2001) as the results of linear mixed-effects models and soil heterogeneity studies showed. There was no significant interaction effect of elevated CO2 concentration treatment and time detected in soil water chemistry over the 5 years of sample collection. This result is contrary to the findings in our study. Using the same models (linear mixed models), we found that there was a significant interaction effect of elevated CO2 concentration treatment and sampling time on inorganic C loss via leaching water (Table 4). This could be due to the climate and soil type differences between the two study sites. Our results suggest that tropical forest soil systems may be able to compensate for a small part of the atmospheric CO2 increase through accelerated processing of CO2 into HCO3−-C.
Unlike the previous studies, we found that the greater inorganic C loss induced by elevated CO2 was related to both the increased concentration of inorganic C and the increased leaching water amount. When studying the effects of elevated CO2 on soil weathering in the future, the change of soil water amount induced by the elevated CO2 concentration treatment should be considered. In subtropical areas, where rainfall is not distributed evenly throughout the year, with extreme amounts of precipitation in wet seasons, this would increase the weathering of carbonate minerals in the soils in these areas.
There was about a 3 g C m−2 y−1 increase of inorganic C loss at elevated compared to ambient CO2 conditions in our experiment. Using meta-analytic methods, Luo and others (2006) have summarized the results of 104 studies on plant biomass production from free air CO2 enrichment (FACE) and open-top chamber experiments. They concluded that the averaged rate of C accumulation in land ecosystems is approximately 100 g C m−2 y−1 more at elevated than ambient CO2. Compared with the increased C size in plant, litter, and soil pools at elevated than ambient CO2, the C sink via weathering of soil carbonate is small, however, C stored in plants would finally enter into the ground and decompose at a relatively short temporal scale, and soil C sequestration would not be unlimited (Paustian and others 1997; Six and others 2002). Atmospheric C that has been consumed in the chemical weathering process is exported as bicarbonate in part to the sea, where C is stored as CaCO3 at the geological time scale (Smedberg and others 2006). Scaling the inorganic C (HCO3−-C and CO32−-C) loss estimates for the model forest ecosystems under elevated CO2 in our open-top chambers to the global tropical forests suggests a net increased sink of 0.26 × 1014 g C y−1. Therefore, changes in soil water leaching of inorganic C loss are likely to buffer the changes in geologic processes that alter atmospheric CO2. However, as there is an artificial restriction of the chambers that we used in the study to the plant rooting zone, it is possible that we overestimate the effect of high CO2 treatment on soil weathering.
Elevated atmospheric CO2 concentration resulted in increased soil weathering processes as indicated by the increased HCO3−-C and mineral cations measured in the soil leaching water collected at the 70-cm depth. Elevated CO2 concentration also increased soil moisture contents and resulted in greater volumes of leaching water during the high rainfall events. As a consequence, the elevated CO2 concentration treatment caused higher inorganic C (HCO3−-C and CO32−-C) losses by leaching. The annual net HCO3−-C leaching losses were 47.7, 49.5, and 113.2 kg ha−1 y−1 in the chambers exposed to the elevated CO2 concentration treatment in 2006, 2007, and 2008, respectively. The N treatment reduced the positive effect of the elevated CO2 concentration treatment on the inorganic C loss in the leachates. The annual net HCO3−-C leaching losses were 38.7, 40.9, and 68.5 kg ha−1 y−1 in the chambers exposed to both elevated CO2 concentration and N treatments in 2006, 2007, and 2008, respectively. Increased inorganic C loss with the leaching water was indirectly due to the higher soil CO2 fluxes and increased soil moisture contents under the elevated CO2 concentration. The extreme amount of precipitation in wet seasons under the monsoon climate in subtropical areas would increase the inorganic C losses. The consumption of CO2 to produce HCO3−-C through soil chemical weathering may act as a net sink for CO2 in subtropical areas. However, high N deposition in these areas would decrease the positive effect of elevated CO2 concentrations on the consumption of atmospheric CO2/losses of HCO3−-C via belowground water. There was a strong interactive effect of the high CO2 and N treatment on inorganic C loss.
This study was jointly funded by National Natural Science Foundation of China (Grant No. 31070439), the National Key Technology R&D Program (Grant No. 2009BADC6B02), the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. KSCX2-EW-Q-8) and the Natural Science Foundation of Guangdong Province,China (Grant No. 8351065005000001). Meteorological Bureau of Guangdong Province is acknowledged for providing rainfall and temperature data of Guangzhou City from their on-site weather station.