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Organic carbon in soil physical fractions under different-aged plantations of Mongolian pine in semi-arid region of Northeast China
Applied Soil Ecology 44 (2010) 42–48
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Organic carbon in soil physical fractions under different-aged plantations of Mongolian pine in semi-arid region of Northeast China Fu-Sheng Chen a,b,*, De-Hui Zeng b, Timothy J. Fahey c, Peng-Fei Liao a a
College of Life Sciences, Nanchang University, Nanchang 330031, China Daqinggou Ecological Station, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China c Department of Natural Resources, Cornell University, Ithaca, NY 14853, USA b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 June 2009 Received in revised form 15 September 2009 Accepted 18 September 2009
In order to understand the changes of surface soil carbon (C) storage following the afforestation of sandy grasslands, we used physical fractionation procedures to quantify C concentrations and sucrase enzyme activity in bulk soil and different particle fractions along two replicate chronosequences of Mongolian pine (Pinus sylvestris var. mongolica Litv.) plantations in the southeastern Keerqin Sand Lands, Northeast China. Carbon concentration in bulk topsoil (0–15 cm) initially decreased following afforestation of grassland and subsequently increased as the forest matured. In general, this pattern of C concentration changes was associated with all particle-size fractions (except clays) and both macroand microaggregates. The patterns of topsoil C were also influenced by wind erosion and deposition, with marked increases in the relative mass of silt and fine sand fractions occurring during forest development. The loss of aggregates immediately following afforestation was counteracted by formation of aggregates as the forests developed, contributing to the stabilization of carbon. To enhance soil C storage during afforestation of sandy soils in such semi-arid regions it is recommended to minimize disruption of grassland vegetation during the planting stage. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Ecological restoration Extracellular enzyme Keerqin Sand Lands Pinus sylvestris var. mongolica Litv. Soil carbon storage Soil physical fractionation
1. Introduction Land use and land cover changes have been shown to have significant impacts on soil physical structure that often result in changes in soil organic carbon (C) storage and turnover (Jastrow, 1996; Six et al., 2002). The restoration of degraded arid and semiarid lands by the introduction of woody species has become a worldwide method for protecting soils, combating desertification, supplying timber, and increasing C sequestration (Kumar et al., 2001; Gru¨nzweig et al., 2003; Maestre and Cortina, 2004; Nosetto et al., 2006; Lal, 2009). Although it is certain that afforestation will contribute to C sequestration in forest biomass, the impact that forest planting will have on soil C storage is much less certain, and may vary with other factors such as precipitation, soil texture, stand age, and forest type (Archer et al., 2001; Davis and Condron, 2002; Guo and Gifford, 2002; Paul et al., 2002). Soil C sequestration may be limited if rates of C mineralization increase during afforestation; this effect can be facilitated by coarse soil texture and low-activity clay mineralogy (Richter et al., 1999; Paul et al.,
* Corresponding author at: No. 999, Xuefuda Road, Honggutan New District, Nanchang 330031, China. Tel.: +86 791 3969534; fax: +86 791 3969533. E-mail address: [email protected] (F.-S. Chen). 0929-1393/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsoil.2009.09.003
2002). Knowledge of the changes in soil organic C following grassland afforestation is incomplete, and more data are needed (Nilsson et al., 1995; Jackson et al., 2002; Gru¨nzweig et al., 2003; Nosetto et al., 2006), particularly for the extensive semi-arid sandy soils of northern China (Hu et al., 2008). In recent decades, afforestation has become an increasingly important method of land-cover change in the arid and semi-arid regions of northern China, including extensive sandy areas (1.53 106 km2). These plantations have been established in the interest of desertification control and timber production in sandy areas (Chen et al., 2006; Zeng et al., 2009). The Mongolian pine (Pinus sylvestris var. mongolica Litv.) is one of the most commonly planted trees. By 1998, the area of Mongolian pine plantations had reached 1.78 million ha in Inner Mongolia (State Forestry Administration of China, 2000). The planting of Mongolian pine began in the 1950s in Zhanggutai, Zhangwu County, Liaoning Province, located in the southeastern Keerqin Sand Lands (Fig. 1). It seems likely that this extensive change in land use could profoundly affect soil C dynamics, yet little is known about the effects of this afforestation activity on soil C stocks (Hu et al., 2008). Several processes could influence net C storage following afforestation of the Sand Lands with pine. First, increased rates of C mineralization or decreased in situ supply of detrital C could reduce soil C storage immediately following afforestation (Richter et al.,
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2.2. Study plots
Fig. 1. Map of research region (southeastern Keerqin Sand Lands), and two Mongolian pine afforestation chronosequences (Zhanggutai and Daqinggou) in Northeast China.
1999; Guo and Gifford, 2002). Second, as the forest develops net C accumulation could occur as a result of increased detrital production and protection of soil organic matter by physical or biotic mechanisms (Kuzyakov et al., 2000). In particular, soil organic matter dynamics have been linked to changes in soil physical structure, especially aggregate formation (Six et al., 2000, 2002; Allison and Jastrow, 2006). The objective of the present study was to evaluate the response of soil C pools in topsoil to afforestation of degraded grassland, with pine in the Keerqin Sand Lands, Northeast China. We conducted a study on two replicate chronosequences of pine plantation in this region. Based in part on previous studies of stand development on other sites in this region (Zhao et al., 2007; Hu et al., 2008; Chen et al., 2009), we hypothesized that topsoil C storage would decline initially following plantation establishment, as a result of reduced detrital inputs and possibly accelerated losses by mineralization and wind erosion. Thereafter, we expected accumulation of C in topsoil accompanied by increasing protection of C in micro- and macroaggregates. 2. Method 2.1. Study area The study area is located in the southeast of the Keerqin Sand Lands (Fig. 1), a sub-humid arid area in the temperate climatic zone. It has average annual precipitation of 450 mm, with more than 60% occurring in June–August; annual potential evaporation of 1300– 1800 mm; and average annual temperature of 6.3 8C, with the lowest monthly mean temperature occurring in January (12.5 8C) and the highest in July (23.8 8C). The mean annual frost-free period is 145–150 days. The native vegetation in the study area is grassland (dominated by Artemisia scoparia, Pennisetum flaccidum, Erodium stephanianum, and Phragmites communis), and grass-elm savanna (Ulmus pumila). The soil is an aeolian sand (Typic Ustipsamment), developed from sandy parent material through the action of wind, with low organic C, nitrogen, phosphorus contents (Chen et al., 2002) and 10–12% water holding capacity.
Two chronosequences of pine afforestation were established in the study area, one in Zhanggutai Town of Zhangwu County, Liaoning Province (428430 N, 1228220 E) and another in Daqinggou National Nature Reserve (428450 N–428480 N, 1228130 E–1228150 E). Both areas have a long history of livestock grazing, but grazing intensity has been lower in the latter area where it was eliminated on establishment of the reserve in the mid-1990s. Each chronosequence consisted of a young (12–14 years), mid-aged (20–25 years) and old (32–40 years) pine plantation as well as adjacent grassland. Each of the plantations was established by planting nursery-raised seedlings in pits of 40 cm 40 cm 40 cm size, with a spacing of 3 m 1 m or 2 m 2 m. Pruning of about 25% of the crown was conducted after 15–17 years of afforestation (Hu et al., 2008). All the chosen forest and grassland stands have southerly aspects, and are on slopes of <58. All stands are characterized by similar soil and climatic conditions. We established study plots of 20 m by 20 m in each of the two study areas. The basic stand characteristics of the experimental plots were measured in July 2007 by taking the diameter at breast height of each tree and estimating canopy height with a clinometer (Table 1). 2.3. Sample collection Each plot was further divided into four subplots (10 m 10 m). In each subplot, we removed surface litter and randomly collected five soil cores in the 0–15 cm layer with an auger (inner diameter of 5 cm), and then the five soil cores were composited into one sample. Therefore, we obtained four samples for each plot. Soils were transported on ice to the laboratory and stored at 4 8C. Field-moist soils were processed within 3 days of collection by gently breaking apart cores along natural break points, then passing the soil through an 8-mm sieve. Root pieces and organic debris (not incorporated into aggregates and longer than 8 mm) that passed through the sieve were removed. After thorough mixing, a subsample was air-dried for soil organic C analysis of each bulk sample. Another subsample of the bulk soil was frozen at 20 8C for enzyme analysis. The remaining soil was stored at 4 8C until it was needed in one of three separate fractionation procedures (below) within another 20 days. Like the bulk soil, part of soil fractions after fractionation were air-dried for soil organic C analysis, and part of these were frozen at 20 8C for enzyme analysis. 2.4. Soil fractionation procedure Particle-size distributions of soil samples were determined by wet sieving following a modified version of the Cambardella and Elliott (1993) procedure. Particle fractions were retained for measurement of C concentration and sucrase activity.
Table 1 Vegetation characteristics in two Mongolian pine afforestation chronosequences in the Keerqin Sand Lands, Northeast China measured in July 2007. Stand symbol
Ecosystem type
Stages
Age (years)
Density (trees ha1)
Basal area of trees (m2 ha1)
Height (m)
Location
G1 P1 P3 P5 G2 P2 P4 P6
Grass Mongolian Mongolian Mongolian Grass Mongolian Mongolian Mongolian
Grassland Young Middle-aged Old Grassland Young Middle-aged Old
0 12 20 32 0 14 25 40
0 1000 1000 850 0 825 1000 750
0 5.02 11.30 26.69 0 4.15 25.43 36.80
0 4 6 12 0 4 10 16
Daqinggou Daqinggou Daqinggou Daqinggou Zhanggutai Zhanggutai Zhanggutai Zhanggutai
Mean 1 SE (n = 4).
pine pine pine pine pine pine
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Fig. 2. The fractionation steps of macroaggregate (a) and microaggregates (b). Flow diagram was created based on a combination of methods given in the text of Six et al. (2000) and Allison and Jastrow (2006).
Water-stable macroaggregates without the free and released interaggregates were separated from bulk soil by using a procedure modified from Allison and Jastrow (2006) (Fig. 2). Field-moist samples were wet sieved by hand and fractionated using Ludox AS40 solution (Aldrich; density 1.3 g cm3, adjusted to pH 7 with HCl), which separates free and released interaggregates from the macroaggregates (Allison and Jastrow, 2006). Stable microaggregates were separated from bulk soil by using a microaggregate isolator following a wet sieving procedure modified from Six et al. (2000) (Fig. 2). 2.5. Soil analysis A subsample was dried at 105 8C for gravimetric determination of the water content of each sample. Soil mass values for each fraction and bulk soil were transformed into dry mass according to their water contents. The soil organic C (mg per g) for each sample (four samples per site for each fraction) was determined by the H2SO4–K2Cr2O7 oxidation method (Nelson and Sommers, 1982). The ratios of the mass and organic C concentrations in each fraction to those in bulk soil were defined as relative mass and relative C contents, respectively. Sucrase is an extracellular enzyme that catalyzes the hydrolysis of sucrose into glucose and fructose, and has been generally used to assess soil C turnover rate from complex compounds to smaller products (Ratledge, 1994; Ko¨gel-Knabner, 2002; Nannipieri et al., 2002). Sucrase activity for each site (one sample per site for each fraction due to limiting soil mass) was measured by Hoffmann– Seegerer method (Zhou, 1988). In a 100 ml flask, 10 g soil was combined with 10 ml 20% sucrose solution in pH 5.5 buffer and shaken vigorously at 37 8C for 24 h. The content of sucrose was measured using starch indicator following the addition of 0.2 mol L1 Na2SO4. Reagent blanks were produced for each sample by adding toluene to soil subsamples prior to reagent additions to inhibit sucrase activity. The difference in content of sucrose between reagent blanks and samples was taken as the sucrase activity (ml (10 g soil)1 (24 h)1). 2.6. Statistical analyses All data were analyzed using the SPSS 11.5 for Windows software package (2002). The data were tested for homogeneity of
variances (Brown and Forsythe’s variation of Levene’s test) before further testing. No transformation was done since data met the assumption of homogeneity of variances. A two-way ANOVA was carried out for testing the effects of site and stand age on fraction masses and C stocks. Post hoc mean separation was evaluated using Tukey’s multiple comparisons and one-way ANOVA was used to identify the differences of fraction masses, C stocks and sucrase activities in bulk soil and each soil fraction among grassland and different-aged plantations of Mongolian pine. A Spearman correlation (n = 24) was used to test the relationship among relative soil mass, C concentration, relative C content and sucrase activities in the six soil fractions under four different-aged stands. For all data, all differences reported in the text were tested and considered significant at a = 0.05. 3. Results 3.1. Particle-size distributions Topsoils (0–15 cm depth) in all the plots were classified as sands with low contents of clay (0.7–1.0%) and silt (9–13%) particles. Two-way ANOVA revealed a strong stand age effect on the content of coarse sand, fine sand, silt and clay, but no effect of site or site by stand age interaction (Table 2). In particular, the relative amount of fine sand and silt increased significantly in the old plantations and coarse sand decreased (Table 3). 3.2. C concentrations in particle-size classes The overall C concentration in the bulk topsoil (0–15 cm) varied among stand ages (F3,9 = 43.29, P < 0.001) and between the two chronosequence sites (Fig. 3). In general, the highest bulk soil C concentration was observed in the grassland and the old forest with significantly lower values in the young and mid-aged stands. The C concentrations in the four different particle-size fractions also varied significantly with stand age (Table 2). In contrast with particle-size mass distributions, however, C concentration differed significantly between the two chronosequences, with generally higher values in the Zhanggutai than in Daqinggou site. Significant site by stand age interactions were observed for the C concentration in the sand fractions so that the pattern of change across ages was also not the same in both chronosequences (Table 2).
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Table 2 F values from ANOVA of stage and site effects on relative mass, carbon concentration, relative carbon content for each soil fraction in Mongolian pine afforestation chronosequences, Northeast China. Source of variation
Variables
df
F values Coarse sand
Fine sand
Silt-sized
Clay-sized
Macroaggregate
Microaggregate
Stage
Relative mass Carbon concentration Relative carbon content
3
107.99*** 47.25*** 10.79***
73.06*** 79.73*** 97.67***
37.83*** 60.55*** 94.74***
3.35* 71.93*** 4.70*
22.05*** 38.83*** 29.72***
22.14*** 25.89*** 15.73***
Site
Relative mass Carbon concentration Relative carbon content
1
2.56NS 20.80** 10.26**
2.30NS 32.26*** 38.57***
0.00NS 6.98* 1.57NS
2.78NS 311.04*** 0.11NS
2.44NS 52.69*** 32.91***
3.01NS 50.43*** 25.37***
Stage site
Relative mass Carbon concentration Relative carbon content
3
2.71NS 8.11* 4.33*
1.18NS 4.73* 2.07NS
3.50* 2.92NS 3.30*
1.95NS 136.54*** 3.84*
5.54** 12.56* 2.42NS
2.68NS 16.62*** 12.93***
NS
Not significant. P < 0.05. ** P < 0.01. *** P < 0.001. *
Table 3 Relative soil mass in different fractions of Mongolian pine afforestation chronosequences in semi-arid region of Northeast China. Stages
Coarse sand [g (g bulk)1]
Fine sand [g (g bulk)1]
Silt-sized [g (g bulk)1]
Clay-sized [mg (g bulk)1]
Macroaggregate [g (g bulk)1]
Microaggregate [g (g bulk)1]
Grassland Young Middle-aged Old
0.54 0.01a 0.52 0.02a 0.35 0.01b 0.32 0.01b
0.34 0.01b 0.39 0.02b 0.53 0.01a 0.55 0.01a
0.11 0.00b 0.09 0.00c 0.10 0.00bc 0.13 0.00a
10.48 0.66ab 7.44 0.74d 8.38 0.84cd 9.19 0.80bc
0.45 0.01a 0.38 0.01b 0.38 0.02b 0.50 0.02a
0.53 0.01c 0.60 0.01b 0.58 0.02bc 0.68 0.02a
Note that in the aggregate fractionation procedure the two aggregate-size classes are not mutually exclusive and may not sum to 100% (Allison and Jastrow, 2006). Different letters within each column indicate the significant difference of mean values (P < 0.05) among grassland and different age plantations of Mongolian pine.
fine sand fractions (old-stage, Table 4). Not surprisingly, C concentrations were highest in the clay fraction, intermediate in the silt and lowest in the sand fractions. The relative contents of C (mg C (g bulk soil)1) in the various particle-size fractions in the topsoil also differed markedly across ages, reflecting the combination of relative mass changes and C concentration changes, described above (Table 2). The temporal patterns in relative C content generally followed those for C concentration with initial decreases following afforestation followed by increases as the plantations matured (Table 4). Fig. 3. The topsoil (0–15 cm) organic carbon concentrations in two Mongolian pine afforestation chronosequences in the Keerqin Sand Lands, Northeast China. The lines a top the bars indicate standard error. Different lowercases and capital letters indicate the significant differences among stand stages in Daqinggou and Zhanggutai, respectively.
However, in general, C concentrations decreased markedly in all particle-size fractions in the initial stage of grassland afforestation (10–12 years) and thereafter C concentrations increased in the old plantations, attaining values comparable to the grasslands (Table 4). In both chronosequences, the C concentration of the coarse sand fraction increased earlier (mid-stage) than that in the
3.3. Aggregates For both aggregate-size classes (Fig. 2), we observed highly significant stand age effects on the mass of aggregates, but no significant site effects (Table 2). The content of macroaggregates was significantly higher in the grassland and old forest plantations than in the young and mid-aged forests. In contrast, microaggregates were highest in the old forest, lowest in the grassland and intermediate in the young and mid-aged stands (Table 3). In contrast to the observation for aggregate mass, C concentrations in both aggregate sizes varied significantly both among stand
Table 4 Soil carbon concentration and relative carbon content in different fractions of Mongolian pine afforestation chronosequences in semi-arid region of Northeast China. Indices
Stages
Coarse sand
Fine sand
Silt-sized
Clay-sized
Macroaggregate
Microaggregate
Soil carbon concentrations [mg (g fraction)1]
Grassland Young Middle-aged Old
32.82 3.73b 23.53 1.36c 45.34 0.73a 37.04 0.96b
44.34 0.86a 33.07 1.63b 28.40 1.29b 43.98 1.73a
108.80 1.81a 85.39 2.24c 95.42 0.89b 108.98 1.78a
367.92 16.10a 335.62 4.34a 368.86 7.18a 349.17 7.00a
39.70 2.61a 23.72 1.78b 31.49 1.24b 30.50 2.55b
38.07 4.23a 25.54 1.31b 24.41 1.81b 31.24 1.99ab
Relative soil carbon contents [mg (g bulk)1]
Grassland Young Middle-aged Old
17.59 2.06a 12.03 0.57b 15.98 0.43a 11.98 0.53b
15.18 0.43b 12.91 0.97c 14.87 0.66bc 24.30 1.03a
11.70 0.26b 7.60 0.45d 9.43 0.25c 13.59 0.17a
3.87 0.33a 2.51 0.26b 3.06 0.27ab 3.21 0.30ab
17.71 1.28a 9.14 0. 94c 11.85 0.77bc 15.20 1.12ab
20.33 2.43a 15.31 0.90ab 14.14 1.15bc 21.28 1.47a
Different letters within each column indicate the significant difference of mean values (P < 0.05) among stand stages.
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Fig. 4. Soil sucrase activities among soil fractions of Mongolian pine afforestation chronosequences in semi-arid region of Northeast China. Because of limited sample sizes, it was necessary to pool replicate samples within particle-size and aggregate classes for sucrase assays; hence, only for the bulk soil fraction was the multiple comparisons feasible. The lines a top the bars indicate standard error. Different letters indicate the significant difference of mean values for bulk soil (P < 0.05) among stand stages.
ages and between sites, and strong site by age interactions were also observed (Table 2). Hence, the two chronosequences did not exhibit exactly the same patterns of change in C concentrations in the aggregates. The only pattern that was consistent between the two chronosequences was the trend towards the highest C concentration in both aggregate sizes in the grassland compared with the forest (Table 4). 3.4. Sucrase activity Because of limited sample sizes, it was necessary to pool replicate samples within particle-size and aggregate classes for sucrase assays; hence, only for the bulk soil fraction was the twoway ANOVA feasible. Again, a very strong stand age effect was observed (F3,28 = 20.84, P < 0.001) with the highest sucrase activities in the grassland and old forest, and lower values in the young and middle-aged forests (Fig. 4). Sucrase activity was clearly higher in the small-size particle fractions (silt and clay, Fig. 4) than other fractions. Additionally, sucrase activity was positively correlated with C concentration and negatively correlated with soil relative mass and C contents (Table 5). 4. Discussion The C concentration in the topsoil decreased significantly in the first decade following afforestation in the Keerqin Sand Lands with P. sylvestris var. mongolica. This observation matches some previous observations from this semi-arid area (Zhao et al., 2007; Hu et al., 2008) and some other afforested sites (Jackson et al., 2002). This initial loss of soil C following afforestation has been commonly attributed to the net effect of decreased organic matter inputs and losses through decomposition (Amundson, 2001). Our data would support this mechanism as the C Table 5 Spearman’s correlation coefficients among relative soil mass, carbon concentration, relative carbon content and sucrase activities of Mongolian pine afforestation chronosequences in semi-arid region of Northeast China (n = 24, four stand ages and six soil fractions). Indicators
Relative soil mass
Carbon concentration
Relative carbon content
Carbon concentration Relative carbon content Sucrase activity
0.808*** 0.831*** 0.782***
0.457* 0.898***
0.560**
* ** ***
P < 0.05. P < 0.01. P < 0.001.
concentration in all the particle-size fractions and in aggregates decreased significantly in the young forest plantations. These results need to be qualified with the observation that changes in soil bulk density could influence the interpretation, and bulk density was not measured in this study. However, in the same study areas, Chen et al. (2009) measured bulk density in 0–15 cm topsoil of grassland and pine afforestation sites aged 15–47 years; differences in bulk density among those plots were relatively minor (1.52–1.67 cm g3), with slightly lower values in young forests and highest in grassland and old forests. In addition, our results suggest the likely role of wind erosion of soil particles (Dong et al., 2000), because we observed significant decreases in the mass of the C-rich silt (29%) and clay (19%) fractions in the young plantations compared with grassland (Tables 3 and 4). Hence, the human disturbance associated with reclaiming grassland into forest plantation and fostering seedling survival apparently promotes wind erosion probably by decreasing herb cover and increasing soil bare time. These Typic Ustipsamment soils appear to be particularly susceptible to wind erosion because of the limited development of soil structure (Buschiazzo et al., 1999). This decline in soil C stock might be ameliorated by adoption of improved afforestation practices. In particular, efforts should be made to retain as much herbaceous cover as possible, and planting time arranged to coincide with the beginning of the wet season in late spring to early summer. During the course of pine stand development, we observed an increase in the C concentration of the bulk soil, particularly in the old stands, where topsoil C had returned to levels similar to the grassland (Fig. 3). The increase in topsoil C was associated with marked increase in the C concentration in both silt and sand particle-size fractions. In contrast, the C concentration in microaggregates and macroaggregates did not increase significantly with stand development, but the relative C storage in aggregates did increase significantly because the mass abundance of both classes of aggregates increased significantly with forest development (Table 4). As noted earlier this result is subject to some uncertainty because changes in soil bulk density were not measured in the present study (but see Chen et al., 2009). The effect of afforestation of agricultural soils on aggregate formation has been described previously (e.g. Six et al., 2002), but to our knowledge, no studies of the sequence of coincident temporal changes in soil C and aggregates during stand development have been reported. Six et al. (2002) indicated that the stabilization of soil C in forest soil was associated both with formation of aggregates and especially with the physical protection of microaggregates within macroaggregates. Our results suggest further that it is primarily the accumulation of aggregate mass rather than
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increasing C concentration in the aggregates that contributed to C accumulation during pine afforestation on degraded grassland in the Keerqin Sand Lands. The patterns of topsoil C that we observed during pine stand development were influenced by the deposition of aeolian sediment (part of loose ground surface material, Dong et al., 2000). A marked increase in the relative mass of the fine sand and silt particle-size fractions was observed in old forests of both chronosequences, and these increases were coincident with a decrease in the coarse sand fraction (Table 4). Small-size particles (fine sand and silt) are the most susceptible to wind erosion (Chen and Fryrear, 1996), and the landscape surface roughness of the pine plantations presumably encouraged deposition of small-size particles (Nickling, 1994) from the regional airstream where wind erosion is pervasive (Dong et al., 2000). The contribution of these particles to the soil C changes remains uncertain because the C concentration of the deposited particles has not been measured. Future studies of soil response to afforestation in this and other arid regions should focus attention on the quantitative role of wind erosion and deposition in influencing topsoil properties. Previous studies of soil C dynamics and aggregates have emphasized the role of physical protection from microbes and enzymes within aggregates (Tisdall and Oades, 1982; Sollins et al., 1996; Udawatta et al., 2008). We observed that sucrase activity decreased following afforestation of grassland and then increased significantly as the forest developed (Fig. 4). We further observed that C concentrations across all particle-size fractions and stand ages were highly correlated with sucrase activity (Table 5), matching the results of other studies of hydrolase (Marx et al., 2005; Allison and Jastrow, 2006). The highest sucrase activities were observed in the clay fraction where C concentrations were also highest. It is uncertain whether this observation reflects rapid turnover of this pool or enzyme coexistence with C substrates owing to stabilization on clay surfaces (Zimmerman et al., 2004). In general, the coincident changes in soil C concentrations, aggregates and sucrase activity across the two chronosequences in the present study suggest complex interactions among the initial loss and subsequent redevelopment of the soil C pool in afforested sandy sites. In addition, some trends with stand development in sucrase activity in the various particle fractions were notable and deserve further study: (1) increasing sucrase activity with forest development in all particle-size fractions and (2) decrease in sucrase activity in the fine fractions and in microaggregates immediately following afforestation of degraded grassland (Fig. 4). Ideally, studies of this kind should include several replicate chronosequences, but we were only able to analyze two replicate chronosequences and the generality of our conclusions needs additional study. In particular, results of the two-way ANOVA (Table 2) indicated some site effects on soil C dynamics in the study area. Although vegetation and soils were similar at both sites, bulk soil C concentrations were consistently higher in the more heavily grazed Zhanggutai site than in the Daqinggou Reserve (Table 2). Previous studies in this region have indicated that heavy grazing can result in increased soil nutrient availability (Chen et al., 2006, 2009) and grazing typically increases soil bulk density, water content and root turnover (Evans and Belnap, 1999; Giese et al., 2009). We suggest that this difference in grazing history may have contributed to the lack of a consistent temporal pattern of change in C concentration in the two aggregate-size classes in the present study. Hence, although initially afforestation consistently results in some loss of C from aggregates in this region, subsequent accumulation of C in aggregates depends in part on site history and possibly also subtle differences in soil properties. Future studies to disentangle these complex interactions could be designed with these observations in mind and thereby allow a better understanding of soil C dynamics and nutrient supply in infertile soils.
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Acknowledgements This study was supported by grants from the National Natural Science Foundation of China (Nos. 30872011 and 30600473) and National Key Basic Research Program of China (No. 2007CB106803). We thank Yi Gan, Haijun Hu and Shigen Wu for their field work; Zhexia Zhao, Li Li and Na Deng for soil analyses. The comments from several anonymous reviewers and the editor have greatly improved the quality of this paper. References Allison, S.D., Jastrow, J.D., 2006. Activities of extracellular enzymes in physically isolated fractions of restored grassland soils. Soil Biol. Biochem. 38, 3245–3256. Amundson, R., 2001. The carbon budget in soils. Annu. Rev. Earth Pl. Sci. 29, 535– 562. Archer, S., Boutton, T.W., Hibbard, K.A., 2001. Trees in grasslands: biogeochemical consequences woody plant expansions. In: Schulze, E.D., Heimann, M., Harrison, S., Holland, E., Lloyd, J., Prentice, I.C., Schimel, D. (Eds.), Global Biogeochemical Cycles in the Climate System. 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Applied Soil Ecology journal homepage: www.elsevier.com/locate/apsoil
Organic carbon in soil physical fractions under different-aged plantations of Mongolian pine in semi-arid region of Northeast China Fu-Sheng Chen a,b,*, De-Hui Zeng b, Timothy J. Fahey c, Peng-Fei Liao a a
College of Life Sciences, Nanchang University, Nanchang 330031, China Daqinggou Ecological Station, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China c Department of Natural Resources, Cornell University, Ithaca, NY 14853, USA b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 June 2009 Received in revised form 15 September 2009 Accepted 18 September 2009
In order to understand the changes of surface soil carbon (C) storage following the afforestation of sandy grasslands, we used physical fractionation procedures to quantify C concentrations and sucrase enzyme activity in bulk soil and different particle fractions along two replicate chronosequences of Mongolian pine (Pinus sylvestris var. mongolica Litv.) plantations in the southeastern Keerqin Sand Lands, Northeast China. Carbon concentration in bulk topsoil (0–15 cm) initially decreased following afforestation of grassland and subsequently increased as the forest matured. In general, this pattern of C concentration changes was associated with all particle-size fractions (except clays) and both macroand microaggregates. The patterns of topsoil C were also influenced by wind erosion and deposition, with marked increases in the relative mass of silt and fine sand fractions occurring during forest development. The loss of aggregates immediately following afforestation was counteracted by formation of aggregates as the forests developed, contributing to the stabilization of carbon. To enhance soil C storage during afforestation of sandy soils in such semi-arid regions it is recommended to minimize disruption of grassland vegetation during the planting stage. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Ecological restoration Extracellular enzyme Keerqin Sand Lands Pinus sylvestris var. mongolica Litv. Soil carbon storage Soil physical fractionation
1. Introduction Land use and land cover changes have been shown to have significant impacts on soil physical structure that often result in changes in soil organic carbon (C) storage and turnover (Jastrow, 1996; Six et al., 2002). The restoration of degraded arid and semiarid lands by the introduction of woody species has become a worldwide method for protecting soils, combating desertification, supplying timber, and increasing C sequestration (Kumar et al., 2001; Gru¨nzweig et al., 2003; Maestre and Cortina, 2004; Nosetto et al., 2006; Lal, 2009). Although it is certain that afforestation will contribute to C sequestration in forest biomass, the impact that forest planting will have on soil C storage is much less certain, and may vary with other factors such as precipitation, soil texture, stand age, and forest type (Archer et al., 2001; Davis and Condron, 2002; Guo and Gifford, 2002; Paul et al., 2002). Soil C sequestration may be limited if rates of C mineralization increase during afforestation; this effect can be facilitated by coarse soil texture and low-activity clay mineralogy (Richter et al., 1999; Paul et al.,
* Corresponding author at: No. 999, Xuefuda Road, Honggutan New District, Nanchang 330031, China. Tel.: +86 791 3969534; fax: +86 791 3969533. E-mail address: [email protected] (F.-S. Chen). 0929-1393/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsoil.2009.09.003
2002). Knowledge of the changes in soil organic C following grassland afforestation is incomplete, and more data are needed (Nilsson et al., 1995; Jackson et al., 2002; Gru¨nzweig et al., 2003; Nosetto et al., 2006), particularly for the extensive semi-arid sandy soils of northern China (Hu et al., 2008). In recent decades, afforestation has become an increasingly important method of land-cover change in the arid and semi-arid regions of northern China, including extensive sandy areas (1.53 106 km2). These plantations have been established in the interest of desertification control and timber production in sandy areas (Chen et al., 2006; Zeng et al., 2009). The Mongolian pine (Pinus sylvestris var. mongolica Litv.) is one of the most commonly planted trees. By 1998, the area of Mongolian pine plantations had reached 1.78 million ha in Inner Mongolia (State Forestry Administration of China, 2000). The planting of Mongolian pine began in the 1950s in Zhanggutai, Zhangwu County, Liaoning Province, located in the southeastern Keerqin Sand Lands (Fig. 1). It seems likely that this extensive change in land use could profoundly affect soil C dynamics, yet little is known about the effects of this afforestation activity on soil C stocks (Hu et al., 2008). Several processes could influence net C storage following afforestation of the Sand Lands with pine. First, increased rates of C mineralization or decreased in situ supply of detrital C could reduce soil C storage immediately following afforestation (Richter et al.,
F.-S. Chen et al. / Applied Soil Ecology 44 (2010) 42–48
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2.2. Study plots
Fig. 1. Map of research region (southeastern Keerqin Sand Lands), and two Mongolian pine afforestation chronosequences (Zhanggutai and Daqinggou) in Northeast China.
1999; Guo and Gifford, 2002). Second, as the forest develops net C accumulation could occur as a result of increased detrital production and protection of soil organic matter by physical or biotic mechanisms (Kuzyakov et al., 2000). In particular, soil organic matter dynamics have been linked to changes in soil physical structure, especially aggregate formation (Six et al., 2000, 2002; Allison and Jastrow, 2006). The objective of the present study was to evaluate the response of soil C pools in topsoil to afforestation of degraded grassland, with pine in the Keerqin Sand Lands, Northeast China. We conducted a study on two replicate chronosequences of pine plantation in this region. Based in part on previous studies of stand development on other sites in this region (Zhao et al., 2007; Hu et al., 2008; Chen et al., 2009), we hypothesized that topsoil C storage would decline initially following plantation establishment, as a result of reduced detrital inputs and possibly accelerated losses by mineralization and wind erosion. Thereafter, we expected accumulation of C in topsoil accompanied by increasing protection of C in micro- and macroaggregates. 2. Method 2.1. Study area The study area is located in the southeast of the Keerqin Sand Lands (Fig. 1), a sub-humid arid area in the temperate climatic zone. It has average annual precipitation of 450 mm, with more than 60% occurring in June–August; annual potential evaporation of 1300– 1800 mm; and average annual temperature of 6.3 8C, with the lowest monthly mean temperature occurring in January (12.5 8C) and the highest in July (23.8 8C). The mean annual frost-free period is 145–150 days. The native vegetation in the study area is grassland (dominated by Artemisia scoparia, Pennisetum flaccidum, Erodium stephanianum, and Phragmites communis), and grass-elm savanna (Ulmus pumila). The soil is an aeolian sand (Typic Ustipsamment), developed from sandy parent material through the action of wind, with low organic C, nitrogen, phosphorus contents (Chen et al., 2002) and 10–12% water holding capacity.
Two chronosequences of pine afforestation were established in the study area, one in Zhanggutai Town of Zhangwu County, Liaoning Province (428430 N, 1228220 E) and another in Daqinggou National Nature Reserve (428450 N–428480 N, 1228130 E–1228150 E). Both areas have a long history of livestock grazing, but grazing intensity has been lower in the latter area where it was eliminated on establishment of the reserve in the mid-1990s. Each chronosequence consisted of a young (12–14 years), mid-aged (20–25 years) and old (32–40 years) pine plantation as well as adjacent grassland. Each of the plantations was established by planting nursery-raised seedlings in pits of 40 cm 40 cm 40 cm size, with a spacing of 3 m 1 m or 2 m 2 m. Pruning of about 25% of the crown was conducted after 15–17 years of afforestation (Hu et al., 2008). All the chosen forest and grassland stands have southerly aspects, and are on slopes of <58. All stands are characterized by similar soil and climatic conditions. We established study plots of 20 m by 20 m in each of the two study areas. The basic stand characteristics of the experimental plots were measured in July 2007 by taking the diameter at breast height of each tree and estimating canopy height with a clinometer (Table 1). 2.3. Sample collection Each plot was further divided into four subplots (10 m 10 m). In each subplot, we removed surface litter and randomly collected five soil cores in the 0–15 cm layer with an auger (inner diameter of 5 cm), and then the five soil cores were composited into one sample. Therefore, we obtained four samples for each plot. Soils were transported on ice to the laboratory and stored at 4 8C. Field-moist soils were processed within 3 days of collection by gently breaking apart cores along natural break points, then passing the soil through an 8-mm sieve. Root pieces and organic debris (not incorporated into aggregates and longer than 8 mm) that passed through the sieve were removed. After thorough mixing, a subsample was air-dried for soil organic C analysis of each bulk sample. Another subsample of the bulk soil was frozen at 20 8C for enzyme analysis. The remaining soil was stored at 4 8C until it was needed in one of three separate fractionation procedures (below) within another 20 days. Like the bulk soil, part of soil fractions after fractionation were air-dried for soil organic C analysis, and part of these were frozen at 20 8C for enzyme analysis. 2.4. Soil fractionation procedure Particle-size distributions of soil samples were determined by wet sieving following a modified version of the Cambardella and Elliott (1993) procedure. Particle fractions were retained for measurement of C concentration and sucrase activity.
Table 1 Vegetation characteristics in two Mongolian pine afforestation chronosequences in the Keerqin Sand Lands, Northeast China measured in July 2007. Stand symbol
Ecosystem type
Stages
Age (years)
Density (trees ha1)
Basal area of trees (m2 ha1)
Height (m)
Location
G1 P1 P3 P5 G2 P2 P4 P6
Grass Mongolian Mongolian Mongolian Grass Mongolian Mongolian Mongolian
Grassland Young Middle-aged Old Grassland Young Middle-aged Old
0 12 20 32 0 14 25 40
0 1000 1000 850 0 825 1000 750
0 5.02 11.30 26.69 0 4.15 25.43 36.80
0 4 6 12 0 4 10 16
Daqinggou Daqinggou Daqinggou Daqinggou Zhanggutai Zhanggutai Zhanggutai Zhanggutai
Mean 1 SE (n = 4).
pine pine pine pine pine pine
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Fig. 2. The fractionation steps of macroaggregate (a) and microaggregates (b). Flow diagram was created based on a combination of methods given in the text of Six et al. (2000) and Allison and Jastrow (2006).
Water-stable macroaggregates without the free and released interaggregates were separated from bulk soil by using a procedure modified from Allison and Jastrow (2006) (Fig. 2). Field-moist samples were wet sieved by hand and fractionated using Ludox AS40 solution (Aldrich; density 1.3 g cm3, adjusted to pH 7 with HCl), which separates free and released interaggregates from the macroaggregates (Allison and Jastrow, 2006). Stable microaggregates were separated from bulk soil by using a microaggregate isolator following a wet sieving procedure modified from Six et al. (2000) (Fig. 2). 2.5. Soil analysis A subsample was dried at 105 8C for gravimetric determination of the water content of each sample. Soil mass values for each fraction and bulk soil were transformed into dry mass according to their water contents. The soil organic C (mg per g) for each sample (four samples per site for each fraction) was determined by the H2SO4–K2Cr2O7 oxidation method (Nelson and Sommers, 1982). The ratios of the mass and organic C concentrations in each fraction to those in bulk soil were defined as relative mass and relative C contents, respectively. Sucrase is an extracellular enzyme that catalyzes the hydrolysis of sucrose into glucose and fructose, and has been generally used to assess soil C turnover rate from complex compounds to smaller products (Ratledge, 1994; Ko¨gel-Knabner, 2002; Nannipieri et al., 2002). Sucrase activity for each site (one sample per site for each fraction due to limiting soil mass) was measured by Hoffmann– Seegerer method (Zhou, 1988). In a 100 ml flask, 10 g soil was combined with 10 ml 20% sucrose solution in pH 5.5 buffer and shaken vigorously at 37 8C for 24 h. The content of sucrose was measured using starch indicator following the addition of 0.2 mol L1 Na2SO4. Reagent blanks were produced for each sample by adding toluene to soil subsamples prior to reagent additions to inhibit sucrase activity. The difference in content of sucrose between reagent blanks and samples was taken as the sucrase activity (ml (10 g soil)1 (24 h)1). 2.6. Statistical analyses All data were analyzed using the SPSS 11.5 for Windows software package (2002). The data were tested for homogeneity of
variances (Brown and Forsythe’s variation of Levene’s test) before further testing. No transformation was done since data met the assumption of homogeneity of variances. A two-way ANOVA was carried out for testing the effects of site and stand age on fraction masses and C stocks. Post hoc mean separation was evaluated using Tukey’s multiple comparisons and one-way ANOVA was used to identify the differences of fraction masses, C stocks and sucrase activities in bulk soil and each soil fraction among grassland and different-aged plantations of Mongolian pine. A Spearman correlation (n = 24) was used to test the relationship among relative soil mass, C concentration, relative C content and sucrase activities in the six soil fractions under four different-aged stands. For all data, all differences reported in the text were tested and considered significant at a = 0.05. 3. Results 3.1. Particle-size distributions Topsoils (0–15 cm depth) in all the plots were classified as sands with low contents of clay (0.7–1.0%) and silt (9–13%) particles. Two-way ANOVA revealed a strong stand age effect on the content of coarse sand, fine sand, silt and clay, but no effect of site or site by stand age interaction (Table 2). In particular, the relative amount of fine sand and silt increased significantly in the old plantations and coarse sand decreased (Table 3). 3.2. C concentrations in particle-size classes The overall C concentration in the bulk topsoil (0–15 cm) varied among stand ages (F3,9 = 43.29, P < 0.001) and between the two chronosequence sites (Fig. 3). In general, the highest bulk soil C concentration was observed in the grassland and the old forest with significantly lower values in the young and mid-aged stands. The C concentrations in the four different particle-size fractions also varied significantly with stand age (Table 2). In contrast with particle-size mass distributions, however, C concentration differed significantly between the two chronosequences, with generally higher values in the Zhanggutai than in Daqinggou site. Significant site by stand age interactions were observed for the C concentration in the sand fractions so that the pattern of change across ages was also not the same in both chronosequences (Table 2).
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Table 2 F values from ANOVA of stage and site effects on relative mass, carbon concentration, relative carbon content for each soil fraction in Mongolian pine afforestation chronosequences, Northeast China. Source of variation
Variables
df
F values Coarse sand
Fine sand
Silt-sized
Clay-sized
Macroaggregate
Microaggregate
Stage
Relative mass Carbon concentration Relative carbon content
3
107.99*** 47.25*** 10.79***
73.06*** 79.73*** 97.67***
37.83*** 60.55*** 94.74***
3.35* 71.93*** 4.70*
22.05*** 38.83*** 29.72***
22.14*** 25.89*** 15.73***
Site
Relative mass Carbon concentration Relative carbon content
1
2.56NS 20.80** 10.26**
2.30NS 32.26*** 38.57***
0.00NS 6.98* 1.57NS
2.78NS 311.04*** 0.11NS
2.44NS 52.69*** 32.91***
3.01NS 50.43*** 25.37***
Stage site
Relative mass Carbon concentration Relative carbon content
3
2.71NS 8.11* 4.33*
1.18NS 4.73* 2.07NS
3.50* 2.92NS 3.30*
1.95NS 136.54*** 3.84*
5.54** 12.56* 2.42NS
2.68NS 16.62*** 12.93***
NS
Not significant. P < 0.05. ** P < 0.01. *** P < 0.001. *
Table 3 Relative soil mass in different fractions of Mongolian pine afforestation chronosequences in semi-arid region of Northeast China. Stages
Coarse sand [g (g bulk)1]
Fine sand [g (g bulk)1]
Silt-sized [g (g bulk)1]
Clay-sized [mg (g bulk)1]
Macroaggregate [g (g bulk)1]
Microaggregate [g (g bulk)1]
Grassland Young Middle-aged Old
0.54 0.01a 0.52 0.02a 0.35 0.01b 0.32 0.01b
0.34 0.01b 0.39 0.02b 0.53 0.01a 0.55 0.01a
0.11 0.00b 0.09 0.00c 0.10 0.00bc 0.13 0.00a
10.48 0.66ab 7.44 0.74d 8.38 0.84cd 9.19 0.80bc
0.45 0.01a 0.38 0.01b 0.38 0.02b 0.50 0.02a
0.53 0.01c 0.60 0.01b 0.58 0.02bc 0.68 0.02a
Note that in the aggregate fractionation procedure the two aggregate-size classes are not mutually exclusive and may not sum to 100% (Allison and Jastrow, 2006). Different letters within each column indicate the significant difference of mean values (P < 0.05) among grassland and different age plantations of Mongolian pine.
fine sand fractions (old-stage, Table 4). Not surprisingly, C concentrations were highest in the clay fraction, intermediate in the silt and lowest in the sand fractions. The relative contents of C (mg C (g bulk soil)1) in the various particle-size fractions in the topsoil also differed markedly across ages, reflecting the combination of relative mass changes and C concentration changes, described above (Table 2). The temporal patterns in relative C content generally followed those for C concentration with initial decreases following afforestation followed by increases as the plantations matured (Table 4). Fig. 3. The topsoil (0–15 cm) organic carbon concentrations in two Mongolian pine afforestation chronosequences in the Keerqin Sand Lands, Northeast China. The lines a top the bars indicate standard error. Different lowercases and capital letters indicate the significant differences among stand stages in Daqinggou and Zhanggutai, respectively.
However, in general, C concentrations decreased markedly in all particle-size fractions in the initial stage of grassland afforestation (10–12 years) and thereafter C concentrations increased in the old plantations, attaining values comparable to the grasslands (Table 4). In both chronosequences, the C concentration of the coarse sand fraction increased earlier (mid-stage) than that in the
3.3. Aggregates For both aggregate-size classes (Fig. 2), we observed highly significant stand age effects on the mass of aggregates, but no significant site effects (Table 2). The content of macroaggregates was significantly higher in the grassland and old forest plantations than in the young and mid-aged forests. In contrast, microaggregates were highest in the old forest, lowest in the grassland and intermediate in the young and mid-aged stands (Table 3). In contrast to the observation for aggregate mass, C concentrations in both aggregate sizes varied significantly both among stand
Table 4 Soil carbon concentration and relative carbon content in different fractions of Mongolian pine afforestation chronosequences in semi-arid region of Northeast China. Indices
Stages
Coarse sand
Fine sand
Silt-sized
Clay-sized
Macroaggregate
Microaggregate
Soil carbon concentrations [mg (g fraction)1]
Grassland Young Middle-aged Old
32.82 3.73b 23.53 1.36c 45.34 0.73a 37.04 0.96b
44.34 0.86a 33.07 1.63b 28.40 1.29b 43.98 1.73a
108.80 1.81a 85.39 2.24c 95.42 0.89b 108.98 1.78a
367.92 16.10a 335.62 4.34a 368.86 7.18a 349.17 7.00a
39.70 2.61a 23.72 1.78b 31.49 1.24b 30.50 2.55b
38.07 4.23a 25.54 1.31b 24.41 1.81b 31.24 1.99ab
Relative soil carbon contents [mg (g bulk)1]
Grassland Young Middle-aged Old
17.59 2.06a 12.03 0.57b 15.98 0.43a 11.98 0.53b
15.18 0.43b 12.91 0.97c 14.87 0.66bc 24.30 1.03a
11.70 0.26b 7.60 0.45d 9.43 0.25c 13.59 0.17a
3.87 0.33a 2.51 0.26b 3.06 0.27ab 3.21 0.30ab
17.71 1.28a 9.14 0. 94c 11.85 0.77bc 15.20 1.12ab
20.33 2.43a 15.31 0.90ab 14.14 1.15bc 21.28 1.47a
Different letters within each column indicate the significant difference of mean values (P < 0.05) among stand stages.
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Fig. 4. Soil sucrase activities among soil fractions of Mongolian pine afforestation chronosequences in semi-arid region of Northeast China. Because of limited sample sizes, it was necessary to pool replicate samples within particle-size and aggregate classes for sucrase assays; hence, only for the bulk soil fraction was the multiple comparisons feasible. The lines a top the bars indicate standard error. Different letters indicate the significant difference of mean values for bulk soil (P < 0.05) among stand stages.
ages and between sites, and strong site by age interactions were also observed (Table 2). Hence, the two chronosequences did not exhibit exactly the same patterns of change in C concentrations in the aggregates. The only pattern that was consistent between the two chronosequences was the trend towards the highest C concentration in both aggregate sizes in the grassland compared with the forest (Table 4). 3.4. Sucrase activity Because of limited sample sizes, it was necessary to pool replicate samples within particle-size and aggregate classes for sucrase assays; hence, only for the bulk soil fraction was the twoway ANOVA feasible. Again, a very strong stand age effect was observed (F3,28 = 20.84, P < 0.001) with the highest sucrase activities in the grassland and old forest, and lower values in the young and middle-aged forests (Fig. 4). Sucrase activity was clearly higher in the small-size particle fractions (silt and clay, Fig. 4) than other fractions. Additionally, sucrase activity was positively correlated with C concentration and negatively correlated with soil relative mass and C contents (Table 5). 4. Discussion The C concentration in the topsoil decreased significantly in the first decade following afforestation in the Keerqin Sand Lands with P. sylvestris var. mongolica. This observation matches some previous observations from this semi-arid area (Zhao et al., 2007; Hu et al., 2008) and some other afforested sites (Jackson et al., 2002). This initial loss of soil C following afforestation has been commonly attributed to the net effect of decreased organic matter inputs and losses through decomposition (Amundson, 2001). Our data would support this mechanism as the C Table 5 Spearman’s correlation coefficients among relative soil mass, carbon concentration, relative carbon content and sucrase activities of Mongolian pine afforestation chronosequences in semi-arid region of Northeast China (n = 24, four stand ages and six soil fractions). Indicators
Relative soil mass
Carbon concentration
Relative carbon content
Carbon concentration Relative carbon content Sucrase activity
0.808*** 0.831*** 0.782***
0.457* 0.898***
0.560**
* ** ***
P < 0.05. P < 0.01. P < 0.001.
concentration in all the particle-size fractions and in aggregates decreased significantly in the young forest plantations. These results need to be qualified with the observation that changes in soil bulk density could influence the interpretation, and bulk density was not measured in this study. However, in the same study areas, Chen et al. (2009) measured bulk density in 0–15 cm topsoil of grassland and pine afforestation sites aged 15–47 years; differences in bulk density among those plots were relatively minor (1.52–1.67 cm g3), with slightly lower values in young forests and highest in grassland and old forests. In addition, our results suggest the likely role of wind erosion of soil particles (Dong et al., 2000), because we observed significant decreases in the mass of the C-rich silt (29%) and clay (19%) fractions in the young plantations compared with grassland (Tables 3 and 4). Hence, the human disturbance associated with reclaiming grassland into forest plantation and fostering seedling survival apparently promotes wind erosion probably by decreasing herb cover and increasing soil bare time. These Typic Ustipsamment soils appear to be particularly susceptible to wind erosion because of the limited development of soil structure (Buschiazzo et al., 1999). This decline in soil C stock might be ameliorated by adoption of improved afforestation practices. In particular, efforts should be made to retain as much herbaceous cover as possible, and planting time arranged to coincide with the beginning of the wet season in late spring to early summer. During the course of pine stand development, we observed an increase in the C concentration of the bulk soil, particularly in the old stands, where topsoil C had returned to levels similar to the grassland (Fig. 3). The increase in topsoil C was associated with marked increase in the C concentration in both silt and sand particle-size fractions. In contrast, the C concentration in microaggregates and macroaggregates did not increase significantly with stand development, but the relative C storage in aggregates did increase significantly because the mass abundance of both classes of aggregates increased significantly with forest development (Table 4). As noted earlier this result is subject to some uncertainty because changes in soil bulk density were not measured in the present study (but see Chen et al., 2009). The effect of afforestation of agricultural soils on aggregate formation has been described previously (e.g. Six et al., 2002), but to our knowledge, no studies of the sequence of coincident temporal changes in soil C and aggregates during stand development have been reported. Six et al. (2002) indicated that the stabilization of soil C in forest soil was associated both with formation of aggregates and especially with the physical protection of microaggregates within macroaggregates. Our results suggest further that it is primarily the accumulation of aggregate mass rather than
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increasing C concentration in the aggregates that contributed to C accumulation during pine afforestation on degraded grassland in the Keerqin Sand Lands. The patterns of topsoil C that we observed during pine stand development were influenced by the deposition of aeolian sediment (part of loose ground surface material, Dong et al., 2000). A marked increase in the relative mass of the fine sand and silt particle-size fractions was observed in old forests of both chronosequences, and these increases were coincident with a decrease in the coarse sand fraction (Table 4). Small-size particles (fine sand and silt) are the most susceptible to wind erosion (Chen and Fryrear, 1996), and the landscape surface roughness of the pine plantations presumably encouraged deposition of small-size particles (Nickling, 1994) from the regional airstream where wind erosion is pervasive (Dong et al., 2000). The contribution of these particles to the soil C changes remains uncertain because the C concentration of the deposited particles has not been measured. Future studies of soil response to afforestation in this and other arid regions should focus attention on the quantitative role of wind erosion and deposition in influencing topsoil properties. Previous studies of soil C dynamics and aggregates have emphasized the role of physical protection from microbes and enzymes within aggregates (Tisdall and Oades, 1982; Sollins et al., 1996; Udawatta et al., 2008). We observed that sucrase activity decreased following afforestation of grassland and then increased significantly as the forest developed (Fig. 4). We further observed that C concentrations across all particle-size fractions and stand ages were highly correlated with sucrase activity (Table 5), matching the results of other studies of hydrolase (Marx et al., 2005; Allison and Jastrow, 2006). The highest sucrase activities were observed in the clay fraction where C concentrations were also highest. It is uncertain whether this observation reflects rapid turnover of this pool or enzyme coexistence with C substrates owing to stabilization on clay surfaces (Zimmerman et al., 2004). In general, the coincident changes in soil C concentrations, aggregates and sucrase activity across the two chronosequences in the present study suggest complex interactions among the initial loss and subsequent redevelopment of the soil C pool in afforested sandy sites. In addition, some trends with stand development in sucrase activity in the various particle fractions were notable and deserve further study: (1) increasing sucrase activity with forest development in all particle-size fractions and (2) decrease in sucrase activity in the fine fractions and in microaggregates immediately following afforestation of degraded grassland (Fig. 4). Ideally, studies of this kind should include several replicate chronosequences, but we were only able to analyze two replicate chronosequences and the generality of our conclusions needs additional study. In particular, results of the two-way ANOVA (Table 2) indicated some site effects on soil C dynamics in the study area. Although vegetation and soils were similar at both sites, bulk soil C concentrations were consistently higher in the more heavily grazed Zhanggutai site than in the Daqinggou Reserve (Table 2). Previous studies in this region have indicated that heavy grazing can result in increased soil nutrient availability (Chen et al., 2006, 2009) and grazing typically increases soil bulk density, water content and root turnover (Evans and Belnap, 1999; Giese et al., 2009). We suggest that this difference in grazing history may have contributed to the lack of a consistent temporal pattern of change in C concentration in the two aggregate-size classes in the present study. Hence, although initially afforestation consistently results in some loss of C from aggregates in this region, subsequent accumulation of C in aggregates depends in part on site history and possibly also subtle differences in soil properties. Future studies to disentangle these complex interactions could be designed with these observations in mind and thereby allow a better understanding of soil C dynamics and nutrient supply in infertile soils.
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Acknowledgements This study was supported by grants from the National Natural Science Foundation of China (Nos. 30872011 and 30600473) and National Key Basic Research Program of China (No. 2007CB106803). We thank Yi Gan, Haijun Hu and Shigen Wu for their field work; Zhexia Zhao, Li Li and Na Deng for soil analyses. The comments from several anonymous reviewers and the editor have greatly improved the quality of this paper. References Allison, S.D., Jastrow, J.D., 2006. Activities of extracellular enzymes in physically isolated fractions of restored grassland soils. Soil Biol. Biochem. 38, 3245–3256. Amundson, R., 2001. The carbon budget in soils. Annu. Rev. Earth Pl. Sci. 29, 535– 562. Archer, S., Boutton, T.W., Hibbard, K.A., 2001. Trees in grasslands: biogeochemical consequences woody plant expansions. In: Schulze, E.D., Heimann, M., Harrison, S., Holland, E., Lloyd, J., Prentice, I.C., Schimel, D. (Eds.), Global Biogeochemical Cycles in the Climate System. 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