Dissolved carbon fluxes in a vegetation restoration area of an eroding landscape

Water Research 152 (2019) 106e116

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Dissolved carbon fluxes in a vegetation restoration area of an eroding landscape Jianye Li a, b, Shuguang Liu c, d, Bojie Fu a, b, *, Jian Wang a, b a State Key Laboratory of Urban and Regional Ecology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, PR China b University of Chinese Academy of Sciences, Beijing, 100049, PR China c National Engineering Laboratory of Applied Technology of Forestry & Ecology in South China, Central South University of Forestry and Technology, Changsha, 410004, PR China d College of Life Sciences and Technology, Central South University of Forestry and Technology, Changsha, 410004, PR China

a r t i c l e i n f o

a b s t r a c t :

Article history: Received 22 August 2018 Received in revised form 21 December 2018 Accepted 25 December 2018 Available online 11 January 2019

Dissolved carbon (DC) is a critical component of the global carbon (C) cycle. DC transport occurs through water-driven erosion and infiltration during rain storms. To explore the specific role of DC flux in topsoil C pool dynamics during rainfall events and predict the trend of ratios of lateral versus vertical DC efflux from topsoil in a vegetation restoration area, we measured the major DC fluxes at four runoff plots, during rainfall events in an eroding soil landscape on the Chinese Loess Plateau. The results show that topsoil vertical DC efflux into deep soil layers accounted for approximately 98.7 (±1.0) % of the total dissolved carbon efflux in plots with vegetation versus 95.3% in a plot without vegetation. The carbon sequestration capacity of the top soil would be underestimated by up to 38 (±5) % if the vertical DC efflux was omitted. The ratios of lateral versus vertical DC efflux tended to increase with rainfall intensity. The results of this study improve understanding of the carbon cycle processes during rainfall events in general and estimation of carbon sequestration rates in vegetation restoration regions such as the Chinese Loess Plateau in particular. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Dissolved carbon flux Rainfall events Soil carbon sequestration Vegetation restoration Water-driven erosion Soil carbon loss

1. Introduction Dissolved carbon (DC) is a critical component of the global carbon (C) cycle; it is a potential source of both CO2 and the stabilized C present in subsoil (Kalbitz and Kaiser, 2008; van Hees et al., 2005). Although a comparatively small fraction of the soil carbon (SC) pool (<10%), DC transport is a good pathway between the terrestrial and components of the aquatic C cycle and operates at scales from the local (Jansen et al., 2014) to the global (Regnier et al., 2013). The DC fluxes induced by rain storms in an eroding soil landscape can be divided into two forms: lateral DC flux (including C flux by runoff) and vertical DC flux (including C fluxes by infiltration and rainfall). Gerke et al. (2016) showed that hydrological paths and fluxes greatly affect the DC fluxes in a catchment suffering water-driven

* Corresponding author. State Key Laboratory of Urban and Regional Ecology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P. O. Box 2871, Beijing, 100085, PR China. E-mail address: [email protected] (B. Fu). https://doi.org/10.1016/j.watres.2018.12.068 0043-1354/© 2019 Elsevier Ltd. All rights reserved.

erosion. Jacinthe et al. (2004) reported 29e35% of C transported in runoff has the potential to be mineralized. The mineralizable C in runoff and deposition sediment are mostly labile carbon, especially for dissolved organic carbon (DOC) (Lal, 2003). Using s simulated rainfall experiment, Jin et al. (2010) have shown that the positive relationship between soil C loss, and surface runoff and sediment yield is mainly due to the solubility characteristics of soil organic carbon (SOC). While a large amount of studies have focused on the dynamics of mineral-associated carbon (MC) and particulate carbon (PC) fluxes during soil redistribution (Lal, 2003; Van Oost et al., 2007), few studies have researched the link between erosion and DC fluxes (Doetterl et al., 2016). In addition, most studies (Brye et al., 2001; Kindler et al., 2011) on DC export from soil have been carried out in forested ecosystems, which are mostly not subject to erosion. The number of studies on lateral DC fluxes in an eroding soil landscape is thus rather limited. The DC flux by infiltration or leaching also leads to topsoil C loss (Dlugob et al., 2012; Rumpel and Kogel-Knabner, 2011). Leaching of DC increases the terrestrial C storage and plays an important role in C sequestration (Dlugob et al., 2012; Rumpel and Kogel-Knabner,

J. Li et al. / Water Research 152 (2019) 106e116

2011). However, there is little existing data on the vertical distribution of DC, or on the vertical DC flux through the soil profile of an eroding soil landscape (Chaplot and Poesen, 2012; Zhang et al., 2006). Distinguishing between lateral and vertical DC fluxes is still a crucial step when considering the effect of soil erosion on DC flux. The influencing factors related to runoff and infiltration, like rainfall characteristics, rainfall duration, soil properties, topography, vegetation cover, soil water content, etc., affect the DC fluxes greatly (Foster and Wischmeier, 1974; Nearing et al., 2005). For example, the DOC concentration in the surface runoff of one study changed with rainfall duration and rainfall intensity (Ma et al., 2014). Polyakov and Lal (2004) found that DOC concentration in surface runoff decreased with increasing rainfall duration. Rainfall intensity had a larger effect on lateral DOC transport than vertical DOC. The lateral DOC export reached a maximum level under high intensity rainfall, but the vertical DOC reached a maximum under low intensity rainfall in the control plot of tillage treatment (Ma et al., 2014). Soil water content affected the DOC concentrations significantly. Higher DOC concentrations at higher water contents found in soil water content curves was consistent with previous studies (Falkengrengrerup and Tyler, 1993). In many cases (Doetterl et al., 2016), anaerobic conditions induced by soil water saturation also accelerated the DOC release from soils, with the soil surface becoming sealed after water-driven erosion (Wischmeier and Mannering, 1969). Fiedler and Kalbitz (2003) analyzed the relationship between DOC concentration and redox conditions; they found DOC concentrations were lowest in aerobic soil and highest in highly anaerobic soil. Fluxes of DC have been the most important biophysical C flux in ecological restoration areas, like the Chinese Loess Plateau. The increased vegetation cover due to ‘‘Grain-for-Green Project’’ has substantially reduced soil erosion in the region (Fu et al., 2017; Zhang et al., 2014). Under the ‘‘Grain-for-Green Project’’, the average fractional vegetation coverage (FVC) on the Chinese Loess Plateau increased from about 56% in 1999 to nearly 60% in 2010 (Fu et al., 2017). The FVC on more than 90% of the total area of the Chinese Loess Plateau increased considerably, especially in areas with initially low coverage (Xin et al., 2008; Zhang et al., 2012). Lü et al. (2012) evaluated the decrease in regional water yield after the implementation of the ‘‘Grain-for-Green Project’’, and an estimate of
10.3 mm yr
1 was found across the whole Loess Plateau between 2002 and 2008 (Feng et al., 2012). This change in the hydrology fluxes affected the DC dynamics during each rainfall event. The division of lateral and vertical DC fluxes during water-driven erosion was expected to have changed as a consequence. However, the lack of observation data on the DC fluxes in the vegetation restoration area limited our study and thus did not provide an understanding of the DC dynamics during rain storms after the changing of an underlying surface induced by water-driven erosion. The purpose of our study was to investigate the relationship between lateral and vertical DC flux from topsoil in an eroding soil landscape in a vegetation restoration area and to predict the trend of division of lateral and vertical DC effluxes from topsoil on the Loess Plateau. We address the research questions: (1) what is the role of DC flux in topsoil C pool dynamics and (2) how much lateral versus vertical DC effluxes occur from topsoil in a vegetation restoration area like the Chinese Loess Plateau? 2. Materials and methods 2.1. Study area The study was carried out in the Yangjuangou catchment (36 420 N and 109 310 E) in Yan'an city of Shaanxi Province, China.

107

The catchment is 2.02 km2 and ranges in elevation ranging from 1050 to 1298 m. The soil is Calcaric Cambisol typically with uniform texture and weak structure (Li et al., 2003) (Table 1). This hill-andgully landscape (gully density is 2.74 km km
2), one of the typical landscapes on the Chinese Loess Plateau, is notorious for soil erosion due to high soil erodibility, widespread vegetation destruction, and intensive cultivation. It has been designated as a typical vegetation restoration area in China particularly with the implementation of the “Grain-for-Green Project” since 1999 (Fu et al., 2017). The regional climate is semiarid with a mean annual air temperature and precipitation of 9.8 (±0.8)  C and 531.0 mm, respectively. The growing season is from May to September, and the average growing season precipitation is 422 mm, accounting for 79% of the annual precipitation (Jiao et al., 2016). 2.2. Measurement and collection of samples To measure the impacts of vegetation restoration on the hillslope hydrology and biogeochemical cycles, four runoff plots were set up on a slope of about 20 . Three of the vegetated plots were respectively with cover (Fig. 1) of Robinia pseudoacacia (2  10 m), Spiraea salicifolia (2  10 m), Stipa bungeana community (3  10 m), and a plot with bare soil was the control (3  10 m) (Table 1). These runoff plots are named: A, S, H and CK, respectively. To separate the sediment yield and surface runoff in and out of the plots, polyvinyl chloride (PVC) boards were inserted into the soil (500 mm deep) as the boundary of the plots. A U-shaped PVC runoff gathering pit was installed to transfer the surface runoff to a collecting bucket at the bottom edge of each runoff plot. A total of 19 rainfall events, numbered from 1 to 19 sequentially by date, were recorded between June through October in 2016 (nine rainfall events) and 2017 (ten rainfall events). After each rainfall event, the surface runoff volume (R) was measured, and the runoff accumulated in buckets was collected in 250 ml plastic bottles; the water samples were then chilled at 4  C in a refrigerator and transported to the laboratory for measuring the dissolved carbon concentration in runoff (DCCR) (Jacinthe et al., 2004; Liu and Sheu, 2003). This DCCR includes both the dissolved organic carbon concentration (DOCCR) and the dissolved inorganic carbon concentration (DICCR). One set of rain gauges was installed in each plot that had no rainfall interception (i.e., H and CK). Throughfall was collected at each of the plots with a woody vegetation cover (A and S) using six sets of rain gauges in total installed under the vegetation on each plot. Rainfall or throughfall volume (P) was measured and collected in a plastic bottle (250 ml) on an event basis. The samples were kept in a refrigerator at 4  C for later measurement of the total dissolved carbon concentration in rainfall (DCCP) in the laboratory. This DCCP includes both the dissolved organic carbon concentration (DOCCP) Table 1 Vegetation community and soil characteristics in different runoff plots. The vegetation types were trees in plot A, shrubs in plot S, and grass in plot H. CK is the control plot with bare soil. Plot name

A

S

Fractional vegetation coverage (%) Height (mm) SHDI SHEI Sand (%) Silt (%) Clay (%) Bulk density (g cm
3) Steady infiltration rate (mm hr
1)

61.2 100 5500 1852 ± 158 / / / / 71.1 24.4 4.5 1.1 ± 0.1 137.4

H

CK

83.9 442.8 ± 301.1 4.2 0.87

/ / / /

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Fig. 1. Location of the study region and study plots in China. The inset shows the location of the study region in China. Schematic diagram of the lateral (dissolved carbon flux by runoff) and vertical (dissolved carbon flux by infiltration and rainfall) dissolved carbon flux components during rainfall events and the setup of the experimental facility is in the bottom left corner. Different colors represent different dissolved carbon fluxes by rainfall, runoff and infiltration. The map was created in ArcGIS 10.1. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

and the dissolved inorganic carbon concentration (DICCP). One set of 1400 External Sensor Station (Portable) (Manufacturer: Spectrum Company in America) was installed in each of the runoff plot for measuring the volumetric soil water content (SWC) before and after rainfall event over the top 0e50 mm soil depth. The schematic diagram of the layout of the plot is shown in Fig. 1. Infiltrated soil water was collected using the Model 1900 Soil Water Samplers (Manufacturer: Soilmoisture Equipment Corp in America) after each rainfall event (Doetterl et al., 2016; Zhao et al., 2013) (Fig. 1). Each sampler, came fully assembled, consists of a 48 mm outside diameter PVC tube, a porous ceramic cup with a 200 kPa air-entry value, and a Santoprene stopper. Neoprene tubing that is attached to a 6.35 mm (1/4-inch) diameter access tube is used for sample extraction and evacuation. Clamping rings slip over the folded Neoprene tubing to seal the sampler. An extraction kit is required for sample retrieval and a vacuum pump is required to evacuate the sampler. The soil water sampler, designed for nearsurface installation, was installed at 50 mm soil depth in our study. Two soil water samplers were installed in each plot, one was 2.5 m and the other 7.5 m from the top edge of the plot. The collected infiltration water samples were bottled and kept in a refrigerator at 4  C for later measurement of the dissolved carbon concentration of the infiltration water (DCCI), including the dissolved organic carbon concentration (DOCCI) and the dissolved inorganic carbon concentration (DICCI). 2.3. Laboratory analysis Laboratory analysis was conducted at the State Key Laboratory

of Urban and Regional Ecology, Research Center for EcoEnvironmental Sciences, Chinese Academy of Sciences. The DCCP, DCCR and DCCI were estimated using a Vario TOC analyzer. Filtered samples through filter membrane of 0.45 mm bore diameter were analyzed immediately after sampling or, in the event of a backlog, refrigerated at 4  C until analysis could proceed. Standard solutions of 0, 10, 50 and 100 ppm of C were made using 0, 1, 5 and 10 ml of stock solution, respectively, prepared by dissolving 2.125 g of the oven-dried reagent ‘potassium hydrogen phthalate’ (C8H5KO4) in 1000 ml of distilled water. The output value of the instrument was dissolved carbon concentration (DCC) and dissolved inorganic carbon concentration (DICC). The dissolved organic carbon concentration (DOCC) was estimated as the difference of DCC and DICC. 2.4. Data analyses Statistical analysis was performed using SPSS 21.0 for Windows. Linear regression analysis were used to analyze the relationship between dissolved carbon input fluxes by rainfall and rainfall/ throughfall amount, the ratios of the lateral versus the vertical DC effluxes from topsoil and rainfall intensity, and the ratios of the lateral versus the vertical DC effluxes from topsoil and antecedent soil water content. Statistical significance tests of the results were evaluated at a ¼ 0.05. The infiltration volume (I) below 50 mm depth in each plot was calculated through the water balance equation in each rainfall event. WR is the water retained (mm) in topsoil layer (0e50 mm). SD is the soil depth, which is 50 mm. SWCa is the soil water content after rainfall event, and SWCb is the soil water content before

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The net topsoil DC flux was calculated as the difference between the C input flux by rainfall and the C flux by runoff and infiltration:

rainfall event: I¼P-R-WR

(1)

WR ¼ SD  (SWCa - SWCb)

(2)

Net DC flux ¼ DCP-DCR-DCI

The topsoil DC fluxes induced by water-driven erosion, including the carbon flux by rainfall (DCP), runoff (DCR), infiltration (DCI), and gas fluxes like carbon dioxide (CO2) and methane (CH4) by mineralization of soil organic matter. However, these small gas fluxes were too difficult to measure during rainfall events and they were not included in our study. The topsoil DC fluxes were calculated as the sum of DOC fluxes and dissolved inorganic carbon (DIC) fluxes: DC ¼ DOC þ DIC

(3)

The total topsoil dissolved carbon flux (TDC) over the study period was calculated as the sum of the fluxes measured in all of the 19 rainfall events: TDC ¼

P

DCi, i ¼ 1,2,3, …,19

(4)

DCP, DCR, and DCI were calculated as the product of different hydrological fluxes and DC concentrations of the corresponding hydrological fluxes (i.e., DCCP is DC concentration in rainfall/ throughfall P, DCCR is DC concentration in surface runoff R, DCCI is DC concentration in infiltration water I): DCP ¼ DCCP  P

(5)

DCR ¼ DCCR  R

(6)

DCI ¼ DCCI  I

(7)

(8)

The total net topsoil DC flux was simply the sum of C fluxes of the rainfall events: Total net DC flux ¼

P

NDCi, i ¼ 1,2,3, …,19

(9)

The topsoil dissolved carbon efflux (DCE) was calculated as the sum of C flux by runoff (lateral DCE) and infiltration (vertical DCE): DCE ¼ DCR (lateral DCE) þDCI (vertical DCE)

(10)

The total topsoil dissolved carbon efflux (TDCE) was the sum of C fluxes in all of the rainfall events: TDCE ¼

P

DCEi, i ¼ 1,2,3, …,19

(11)

The representative reference carbon sequestration rate (CSR) in the region was 29 g C m
2 yr
1, derived from previous studies (Chang et al., 2011; Deng et al., 2014b; Wang et al., 2011b).

3. Results 3.1. The hydrological fluxes during water-driven erosion The hydrological fluxes during water-driven erosion are listed in Fig. 2. The total rainfall during the observation period was 254.1 mm and 331.1 mm in 2016 and 2017, respectively. Rainfall intensity ranged from to 0.9e12.5 mm h
1. The largest rainfall (76.7 mm) occurred in rainfall event 16 (23 August 2017) while the smallest (2.8 mm) occurred in rainfall event 19 (22 September

Fig. 2. Hydrological flux characteristics in different rainfall events. (A), (B), (C), and (D) are the characteristics of hydrological fluxes in plots A, S, H, and CK, respectively. Different colored lines represent the different hydrological fluxes on the left vertical axis. The bar represents the rainfall intensity on the right vertical axis. The different rainfall events are shown on the horizontal axis.

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2017). The rainfall interception was 26 (±13) % in plot A and 45 (±16) % in plot S. Vegetation greatly reduced surface runoff. The reduction was 80.4 (±15.6) %, 88.1 (±14.5) %, and 90.2 (±9.5) % in plots A, S, and H, respectively, compared with that in CK. The surface runoff in each plot was relatively small compared to rainfall/throughfall; it amounted to 1.1 (±1.5) %, 1.0 (±1.7) %, 0.5 (±0.8) %, and 4.3 (±6.0) % of rainfall/throughfall in plots A, S, H, and CK, respectively. Compared to rainfall/throughfall, most of water infiltrated from the topsoil layer (0e50 mm) to the deeper soil layer. The infiltration amounted to 91.6 (±9.0) %, 91.7 (±7.5) %, 90.5 (±6.9) %, and 85.3 (±9.4) % of rainfall/throughfall in plots A, S, H, and CK, respectively. These figures illustrate that the vegetation cover resulted in a considerable decreas in the proportion of surface runoff and increased in the proportion of infiltration.

3.2. Topsoil dissolved carbon fluxes 3.2.1. Topsoil dissolved carbon input flux by rainfall The relationships between the rainfall/throughfall amount and the DC input fluxes are shown in Fig. 3. The dissolved organic carbon input fluxes by rainfall (DOCP) all tended to increase with the rainfall/throughfall amount in each plot. The relationships (Table 2) between DOCP and rainfall/throughfall amount were all extremely significant (p < 0.01). The closest relationship occurred in plot S. However, the relationship between the dissolved inorganic carbon input fluxes by rainfall (DICP) and the rainfall/throughfall amount was not significant for plot S (Table 2). Fig. 3D shows that the total

dissolved carbon input flux by rainfall (TDCP) was the most in plot A (2.45 g m
2 in 2016; 3.09 g m
2 in 2017) while it was the least in plot S (1.65 g m
2 in 2016; 1.16 g m
2 in 2017). The TDCP in 2016 was more than that in 2017 except for plot A. The organic TDCP was much more than inorganic TDCP.

3.2.2. Total dissolved carbon effluxes from topsoil The total dissolved carbon efflux from topsoil (TDCE) can be divided into two forms, lateral TDCE (total dissolved carbon flux by runoff (TDCR)) and vertical TDCE (total dissolved carbon flux by infiltration (TDCI)). The vertical TDCE were mainly in the form of DOC (Fig. 4B and C). The vertical TDCE in plots A and S were less than that in the plot CK in both 2016 and 2017. The reductions compared to plot CK for vertical organic TDCE from topsoil were 19.7% and 27.8% in plots A and S in 2016, and 31.9% and 47.5% in 2017, respectively. For vertical inorganic TDCE, the reductions compared to plot CK were 27.6% and 14.1% in plots A and S in 2016 and 10.1% and 35.3% in 2017, respectively. It can be seen that the reductions in 2016 were less than those in 2017. The main reason for the yearly differences in reduction was the different amounts of the rain falling in the two years. In the different plots with vegetation, the vertical TDCE changed greatly. The least amount of vertical TDCE occurred in plot S during both 2016 and 2017, followed by plots A and H. The lateral organic TDCE were much more than the lateral inorganic TDCE in all of the plots, meaning that lateral organic TDCE was the main form of lateral TDCE. The lateral TDCE in plot CK was much more than that in the plots with vegetation (Fig. 4). The

Fig. 3. Topsoil dissolved carbon input flux. The relationships between the carbon input flux by rainfall and rainfall/throughfall amount for dissolved organic (DOC) and inorganic carbon input flux (DIC) are shown for plot A (A), plot S (B), and plots H and CK that have no rainfall interception (C). The left vertical axis is the organic carbon input flux and the right vertical axis is the inorganic carbon input flux. Each dot represents a rainfall event. (D) Shows the total dissolved carbon input flux by rainfall.

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Table 2 The relationships between dissolved carbon input fluxes by rainfall and rainfall/throughfall amount in different runoff plots. Plot name

Relationship between DOCP and rainfall/throughfall amount

Relationship between DICP and rainfall/throughfall amount

Plot A Plot S Plot H/CK

Y ¼ 0.0084X þ 0.051, R2 ¼ 0.485, F ¼ 16.00, p ¼ 0.0009 < 0.01 Y ¼ 0.0051X þ 0.043, R2 ¼ 0.641, F ¼ 30.41, p < 0.0001 Y ¼ 0.0034X þ 0.072, R2 ¼ 0.433, F ¼ 12.98, p ¼ 0.0022 < 0.01

Y ¼ 0.0008X þ 0.0099, R2 ¼ 0.426, F ¼ 12.64, p ¼ 0.0024 < 0.01 Y ¼ 0.0002X þ 0.0062, R2 ¼ 0.167, F ¼ 3.41, p ¼ 0.082 > 0.05 Y ¼ 0.0003X þ 0.0062 R2 ¼ 0.501, F ¼ 17.05, p ¼ 0.0007 < 0.01

vegetation cover reduced the lateral organic TDCE by 66.5%, 74.4%, and 82.2% in plots A, S, and H in 2016 and by 77.7%, 89.8%, and 90.8% in 2017, respectively. The lateral inorganic TDCE in plots A, S, and H were reduced by 67.2%, 83.7%, and 89.5% in 2016 and by 61.7%, 87.4%, and 88.2% in 2017, respectively, compared to plot CK. The reduction in 2017 was more than that in 2016, perhaps a result of the higher rainfall amount in 2017 than 2016. The differences in reduction were also caused by the different vegetation cover in each plot. The least reduction occurred in plot A and most reduction occurred in plot H. This means that out of all the vegetation types, the herbaceous had the most effect on the lateral TDCE. 3.2.3. Total net dissolved carbon fluxes The total net DC fluxes were carbon losses from topsoil in each of the plots. The total net DOC fluxes were more than the total net DIC fluxes. The total net DOC fluxes in 2017 (
20.3 ± 6.3 g m
2) were more than in 2016 (
16.7 ± 3.1 g m
2). The total net DIC fluxes had a similar trend to the total net DOC fluxes (
6.5 ± 1.2 g m
2 in 2017;
3.5 ± 0.6 g m
2 in 2016). The total net DC fluxes in the plots with vegetation were less than that in the plot without vegetation. In the plots with vegetation, plot H had the most total net DC fluxes, followed by plots A and S. The total net DC fluxes in 2017 were more than in 2016. 3.3. Partitioning of the total dissolved carbon effluxes from topsoil It can be seen that the fraction of lateral TDCE in total TDCE was small compared to the fraction of vertical TDCE (Fig. 4). In the plots with vegetation, the lateral organic TDCE occupied 2.3 (±0.9) % of TDCE in 2016 and 0.8 (±0.4) % in 2017. In addition, lateral inorganic TDCE accounted for 1.0 (±0.7) % of TDCE in 2016 and 0.3 (±0.1) in 2017. The lateral TDCE varied greatly with the different vegetation cover. The percentage of lateral TDCE in TDCE in the plots with vegetation was highest in plot A and least in plot H. The differences in the percentage of lateral TDCE between plots with and without vegetation (organic TDCE: 6.9% in 2016 and 3.5% in 2017; inorganic TDCE: 3.7% in 2016 and 1.0% in 2017) showed that the vegetation cover reduced the fraction of lateral TDCE and increased the fraction of vertical TDCE. The percentage of lateral TDCE in total TDCE in 2016 was more than that in 2017. In addition, the arboreal vegetation had the effect of reducing the contribution of vertical TDCE to TDCE, and the herbaceous vegetation had the effect of reducing the contribution of lateral TDCE to TDCE. 3.4. The ratio of lateral versus vertical dissolved carbon effluxes from topsoil

Fig. 4. Partitioning of total topsoil dissolved carbon efflux. (A) Total dissolved carbon flux, (B) total dissolved organic carbon flux, and (C) total dissolved inorganic carbon flux. The different colors represent the different dissolved carbon effluxes by runoff and infiltration. The different runoff plots during different years are shown on the horizontal axis. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

The ratios of the lateral versus the vertical DC effluxes from topsoil (L/V) during each rainfall event are listed in Fig. 5. It can be seen that the ratios in different plots all tended to increase with rainfall intensity. The ratios in the plot without vegetation were generally higher than those in the plots with vegetation. In the plots with vegetation, the average ratios in plot H (organic carbon: 0.007 ± 0.012; inorganic carbon: 0.003 ± 0.005) were the lowest, and the average ratios in plot A (organic carbon: 0.017 ± 0.026; inorganic carbon: 0.009 ± 0.012) were the highest on average.

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Fig. 5. The relationship between the ratios of lateral versus vertical dissolved carbon efflux from topsoil (L/V) and rainfall intensity. (A) Shows the results for dissolved organic carbon and (B) for dissolved inorganic carbon. The symbol dots in different colors represent the four different runoff plots. Each dot represents a rainfall event. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

However, the ratios of L/V changed greatly under different rainfall events. The ratios of L/V were affected by antecedent soil water content (SWCb in the text), and they tended to increase with antecedent soil water content (Fig. 6). The relationship between the ratios of L/V and the antecedent soil water content was not significant (p > 0.05) in any of the plots.

4. Discussion 4.1. The role of dissolved carbon fluxes in a vegetation restoration area Estimates of the DC fluxes ranged from 1 to 10 g C m
2 y
2 for many ecosystems (Brye et al., 2001; Kindler et al., 2011; Neff and Asner, 2001). Compared to C fluxes associated with primary productivity or heterotrophic respiration in terrestrial systems, DC fluxes is a relatively small fraction and generally not considered important (Kalbitz and Kaiser, 2008; Schimel, 1995). However, this and previous studies have shown that the important role of DC flux played in ecosystems cannot be ignored for several reasons: (1) DC flux is a vector for the loss of C from topsoil (Kindler et al., 2011), and (2) DC flux is an important source of organic matter in subsoils (Rumpel and Kogel-Knabner, 2011), which can be substantial for carbon sequestration at the ecosystem level.

The DC input flux by rainfall in topsoil was relatively small compared to the DCE. The differences in carbon input fluxes were caused by the vegetation cover. The rainfall interception varied with vegetation cover (Cook et al., 2006; Shachnovich et al., 2008). Leaf washing, different volumes of rainfall and leaching resulted in the changes of DCC between throughfall and rainfall (Liu and Sheu, 2003), which causing the associated changes of DCCP in different runoff plot (Fig. 7). According to the overlap and similar trends of carbon fluxes, hydrological fluxes and carbon concentration in Fig. 7, it can be clearly seen that the hydrological flux (rainfall/ throughfall) made a greater contribution than DCCP to affect the DCP. Doetterl et al. (2016) also found that DC fluxes are closely associated with the magnitude of water flow. Different FVC and community function and structure in the different plots (Table 1) changed the rainfall/throughfall. The DCP will also be changed correspondingly. The reduction of DCE in the plots with vegetation compared to that in the plot without vegetation has been widely observed in the study area. Vegetation cover can reduce surface runoff (Li et al., 2015; Marques et al., 2007). In the plots with vegetation, the surface runoff was reduced in comparison to plot CK (Fig. 2). As DC fluxes are closely associated with the magnitude of water flow, the lateral DCE decreased with the corresponding reduced surface runoff (Jacinthe et al., 2004). From the water balance equation applied to rainfall events, the infiltration will increase as the surface runoff decreases (plot H) (Li et al., 2015). However, in plots A and S, the rainfall interception by the canopy reduced the infiltration compared to plot CK (Fig. 2). Community functional composition and structure play an important role in reducing surface runoff in semi-arid restored areas (Zhu et al., 2015). The vertical DCE in plots A and S was reduced with a corresponding reduced infiltration volume. The differences between plots A and S were caused by the different community functional compositions and structures within them (Table 1). However, the vertical DCE was less in plot H than it was in plot CK, although the infiltration in plot H was more than it was in plot CK. The DCCI in plot H (DOCCI: 84.7 ± 12.8 mg l
1; DICCI: 18.7 ± 5.5 mg l
1) was less than that in plot CK (DOCCI: 95.5 ± 16.6 mg l
1; DICCI: 23.8 ± 6.8 mg l
1), and this was the main reason that the vertical TDCE in plot H was less than that in plot CK. The DCCI was affected by soil water content, soil pH, infiltration volume, etc. (Ma et al., 2014; Michalzik et al., 2001). The different conditions caused by vegetation cover may be the reason for the lower DCCI in the plots with vegetation. In a vegetation restoration area, the surface runoff becomes less and less during rainfall events in an eroding soil landscape (Fu et al., 2017). As a consequence, the portion of vertical DCE increases with reduced lateral DCE compared to bare soil. In the study area, the vertical DCE accounted for more and more of the DCE, and its contribution will continue to become more important as vegetation restoration continues in the future. Vertical DCE is not only a C loss from topsoil but also a C input for the deeper soil layers (Rumpel and Kogel-Knabner, 2011). DC transport has mainly been studied in temperate forest soils (Michalzik et al., 2001) and has been thought to be the main source of subsoil organic carbon under such conditions (Kaiser and Guggenberger, 2000). As the percentage of the vertical DCE in the CSR was large (63 ± 13%) in our study, vertical DCE may play a critical role in soil carbon sequestration in a vegetation restoration area. On the other hand, the vegetation cover is good for carbon fixing for the increased portion of vertical DCE from topsoil in vegetation restoration areas. The ecosystem of the Chinese Loess Plateau transformed from a net carbon source in 2000 to a net carbon sink in 2008. This change was mainly a result of the increased net primary productivity (NPP) following the vegetation restoration (Feng et al., 2013). However, the reduction of lateral DCE

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Fig. 6. The relationship between the ratios of lateral versus vertical dissolved carbon efflux from topsoil (L/V) and antecedent soil water content. (A) Is for plot A, (B) for plot S, (C) for plot H, and (D) for plot CK. DOC represents the dissolved organic carbon flux. DIC represents the dissolved inorganic carbon flux.

and increased portion of vertical DCE compared to bare soil may be another important reason. 4.2. A potentially underestimated carbon sequestration capacity induced by the topsoil vertical dissolved carbon efflux Estimates of CSR are typically estimated by measuring changes in the carbon storage of topsoil layers (Deng et al., 2017; Wang et al.,

2011b). The DC fluxes in soil layers, particularly DC percolation from topsoil to deeper soil layers, were not considered in the estimation of carbon sequestration due to challenges in direct measurement (Doetterl et al., 2016; Lal, 2003; Yue et al., 2016). Through the partitioning of the TDCE, our study clearly shows that the topsoil vertical TDCE (accounting for 98.7 (±1.0) % of the TDCE) was the main DCE in our vegetation restoration area. The topsoil vertical TDCE occupied 59.4 (±11.0) % in 2016 and 66.7 (±15.5) % in 2017 of

Fig. 7. The normalized dissolved carbon input flux by rainfall, rainfall/throughfall and carbon concentration in rainfall/throughfall in different runoff plots. (A) Shows dissolved organic carbon flux in plot A, (B) shows dissolved organic carbon flux in plot S, (C) shows dissolved organic carbon flux plots H and CK, (D) shows dissolved inorganic carbon flux in plot A, (E) shows dissolved inorganic carbon flux in plot S, and (F) shows dissolved inorganic carbon flux in plots H and CK.

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the CSR on average. The vertical TDCE from topsoil to the deep soil layers was so large that it cannot be ignored. The fate of infiltrated C from topsoil to deeper soil layers is not certain. It is likely some of the infiltrated C materials would be decomposed by mineralization as they move down the soil profile (Deng et al., 2014a; Neff and Asner, 2001), and some may be drained out by base flow. On the other hand, it is still possible that some of the infiltrated C might be retained and protected from decomposition in the deeper soil layers (Ma et al., 2014; Rumpel and KogelKnabner, 2011). On the basis of the above processes, the actual carbon sequestration rate (CSRa) (46.8 g C m
2 yr
1 in plot A; 43.7 g C m
2 yr
1 in plot S; 51.3 g C m
2 yr
1 in plot H) in the study area could be the sum of the CSR and the vertical TDCE from topsoil, if (1) all the infiltrated C is protected from decomposition in the deep layers, and (2) the dynamic replacement hypothesis of C in the topsoil layer (Harden et al., 1999; Stallard, 1998) holds. As an important source of subsoil C, the vertical DCE should be included in the calculation of the carbon sequestration capacity (Rumpel and Kogel-Knabner, 2011). The C sequestration capacity in the vegetation restoration area might be underestimated by up to 37.1 (±4.2) % in 2016 and 39.7 (±5.6) % in 2017 in the plots with vegetation if the vertical TDCE from topsoil to subsoil was not factored into the estimates. Osher et al. (2003) also observed some of the photosynthesis-generated C3 molecules in the subsoil layers that had been lost from surface horizons; this provides supporting evidence that the carbon flux from topsoil to subsoil exists. Without

measuring and tabulating the C accumulated in the subsoil horizons lost from topsoil, we would have overestimated the loss of C due to microbial degradation and underestimated the C sequestration capacity. 4.3. The trend of dissolved carbon loss from soil on the Chinese Loess Plateau The ratios of L/V can help us predict the amount of DC loss from soils on the Chinese Loess Plateau, an important area undergone vegetation restoration (Fu et al., 2017). As the DC fluxes were closely associated with water flow (Doetterl et al., 2016), the ratios of L/V could also be closely related to the hydrological fluxes (including surface runoff and infiltration) during rainfall events in the eroding soil landscape. It can be verified that these hydrological fluxes were actually the main factor affecting the DC fluxes (see Fig. 8). The carbon concentration in the hydrological fluxes made a smaller contribution to the dynamics of DC fluxes than the hydrological fluxes themselves. With an increase of rainfall intensity, the surface runoff increases, which agrees with previous observations (Chaplot and Le Bissonnais, 2003; Ziadat and Taimeh, 2013), suggesting the existence of the infiltration-limited runoff generation mechanism in the region. The loess is believed to have a rather loose structure with adequate water infiltration capacity that should promote a water storage-limited runoff generation mechanism (Liu, 1985). The appearance of infiltration-limited runoff generation under high

Fig. 8. The normalized dissolved carbon flux by runoff or infiltration, runoff or infiltration and carbon concentration in runoff or infiltration in different runoff plots. (A)e(H) show normalized dissolved carbon flux by runoff, runoff and carbon concentration in runoff. (A), (B), (C), and (D) show dissolved organic carbon flux in plots A, S, H, and CK, respectively. (E), (F), (G), and (H) show dissolved inorganic carbon flux in plots A, S, H, and CK, respectively. (I)e(P) show normalized dissolved carbon flux by infiltration, infiltration and carbon concentration in infiltration. (I), (J), (K), and (L) show dissolved organic carbon flux in plots A, S, H, and CK, respectively. (M), (N), (O), and (P) show dissolved inorganic carbon flux in plots A, S, H, and CK, respectively.

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rainfall intensity has important implications for future DC fluxes. More surface runoff, therefore more lateral DC efflux, would be generated under future climate regimes in some regions of the Loess Plateau where storms are expected to intensify (Dawson and Smith, 2007; Müller-Nedebock and Chaplot, 2015). The differences between the plots with and without vegetation elucidated how the role of vegetation cover affected the ratios of L/ V. The vegetation cover will protect the soil aggregate from break down during soil erosion and prevent soil sealing, which can reduce the surface runoff and increase the infiltration, (Mohammad
bregas, 2005, Víctor Hugo Dura
n Zuazo and Adam, 2010; Puigdefa and Pleguezuelo, 2008). However, the canopy of shrubs and trees can also intercept rainfall (Shachnovich et al., 2008; Zhang et al., 2017), which may reduce the infiltration. These findings are closely associated with hydrological fluxes. For these reasons, the affected ratios of L/V in soil with vegetation cover were and generally lower than those for bare soil. On the other hand, the vegetation type will also influence the ratios of L/V much. The highest average ratios of L/V occurred in arboreal plot (A). It was for the low FVC that induced the most runoff among the plots with vegetation cover. In addition, the rainfall interception in arboreal plot was moderate for its low FVC among the plots with vegetation cover. These combined effects made the highest average ratios of L/ V occur in the arboreal plot. The lowest average ratios of L/V occurred in the herbage plot (H). The main reason was for its highest infiltration. There was no rainfall interception by canopy that making the infiltration in shrub plot (S) much more than that in herbage. That resulted in the lower ratios of L/V in herbage than shrub, which make the lowest ratios of L/V in herbage plot among the plots with vegetation cover. On the Chinese Loess Plateau, the most significant climate change in the Holocene was a progressive drying and more frequently years of drought (Fu et al., 2017). Various climate change patterns have been observed in recent decades across the Loess Plateau. For example, the precipitation decreased from 1961 to 2010 at a rate of 11.03 ± 13 mm per decade with large spatial variations (Sun et al., 2015). Rainfall amount, frequency, and intensity all decreased in about 38% of the whole area, while in another approximately 37% of the area, the rainfall amount and frequency deceased but intensity increased (Fu et al., 2017). The changes in rainfall intensity would induce changes in surface runoff generation, as shown previously, and in ratios of L/V (see Fig. 5). Increasing rainfall intensity in an eroding landscape means higher ratios of L/V or more surface runoff during rainfall events in the future than before. One the contrary, decreasing rainfall intensity in some regions would produce lower ratios of L/V and less surface runoff in the future. Higher ratios of L/V mean higher proportions of DC would be lost from the terrestrial ecosystems via lateral movement, which would decrease carbon sequestration in the whole terrestrial ecosystem; lower ratios of L/V would promote soil carbon sequestration naturally on site. Vegetation restoration efforts, such as the “Grain-for-Green Project” implemented since 1999, have increased and will continue to increase vegetation coverage in the study area. Improving vegetation conditions will alter various hydrological processes including increasing rainfall interception, increasing evapotranspiration, improving the infiltration rate, and reducing surface runoff (Wang et al., 2011a). It was observed that more than half of the vegetation restoration area (northeast to southwest of the Loess Plateau) experienced a decrease of 2e37 mm yr
1 in surface runoff. The ratios of L/V were generally lower under improved vegetation conditions, implying that the corresponding lateral DCE from topsoil would decrease, and more DC would infiltrate into the soil with increased infiltration in the future. Some of the infiltrated carbon that is not decomposed would accumulate in deep soil

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layers (Ma et al., 2014), facilitating soil carbon sequestration. The above analysis reveals the emergence of two opposing forces, at least in some regions of the Loess Plateau. On the one hand, the widespread continuous vegetation improvement in the region would alter the hydrological cycle and reduce future lateral DCE. On the other hand, the projected intensified rain storms, at least in certain subregions, will promote runoff generation and lateral DCE. How these two opposing forces play out in determining the hydrological carbon fluxes is still an open question. Apparently, answering this question properly needs a long-term observation network in the field, particularly in the vegetation restoration areas. As there is a large amount of complexity associated with DC flux dynamics in terrestrial systems, more field observations and integrated analysis are needed to unravel that complexity including nonlinear responses of the DC fluxes to rainfall characteristics, climate, and vegetation restoration. 5. Conclusions We observed various topsoil DC fluxes in the field over two growing seasons to better understand the role of DC fluxes in topsoil carbon pool dynamics and predict the trend in the ratios of L/V in the Loess Plateau, and led to three major findings. First, vertical DCE from topsoil was the most important DCE. It accounted for approximately 98.7 (±1.0) % of the total DC effluxes. Second, it is important to consider the topsoil DC fluxes in the estimation of soil carbon sequestration capacity as it may be underestimated by up to 38 (±5) % otherwise. The vertical DC flux is a C loss from the topsoil layer but at the same time a gain at the ecosystem level as it might not be lost after entering deep layers. Third, the ratios of lateral compared to vertical dissolved carbon efflux from topsoil tend to increase with rainfall intensity. Our research provides information on the magnitude of dissolved carbon fluxes during rainfall events in an eroding soil landscape under various vegetations. This information is valuable for improving understanding the carbon cycle processes in general and estimating carbon sequestration rates in vegetation restoration regions such as the Chinese Loess Plateau in particular. Acknowledgements This work was funded by the National Key Research and Development Program of China (No. 2017YFA0604701), National Natural Science Foundation of China (No. 41390464) and Chinese Academy of Sciences (No. QYZDY-SSW-DQC025). We thank Jianbo Liu, Weiliang Chen, Mengmeng Zhang, Fangli Wei and Weiwei Fang for observational data and suggestions that led to clarification of various aspects in the manuscript. References Brye, K.R., Norman, J.M., Bundy, L.G., Gower, S.T., 2001. Nitrogen and carbon leaching in agroecosystems and their role in denitrification potential. J. Environ. Qual. 30 (1), 58e70. Chang, R., Fu, B., Liu, G., Liu, S., 2011. Soil carbon sequestration potential for "grain for green" project in Loess Plateau, China. Environ. Manag. 48 (6), 1158e1172. Chaplot, V.A.M., Le Bissonnais, Y., 2003. Runoff features for interrill erosion at different rainfall intensities, slope lengths, and gradients in an agricultural loessial hillslope. Soil Sci. Soc. Am. J. 67 (3), 844e851. Chaplot, V., Poesen, J., 2012. Sediment, soil organic carbon and runoff delivery at various spatial scales. Catena 88 (1), 46e56. Cook, H.F., Valdes, G.S.B., Lee, H.C., 2006. Mulch effects on rainfall interception, soil physical characteristics and temperature under Zea mays L. Soil Tillage Res. 91 (1), 227e235. Dawson, J.J., Smith, P., 2007. Carbon losses from soil and its consequences for landuse management. Sci. Total Environ. 382 (2e3), 165e190. Deng, L., Liu, G.B., Shangguan, Z.P., 2014a. Land-use conversion and changing soil carbon stocks in China's 'Grain-for-Green' Program: a synthesis. Glob. Chang. Biol. 20 (11), 3544e3556.

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