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Influence of long-term successive rotations and stand age of Chinese fir (Cunninghamia lanceolata) plantations on soil properties
Geoderma 306 (2017) 127–134
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Influence of long-term successive rotations and stand age of Chinese fir (Cunninghamia lanceolata) plantations on soil properties
MARK
Selvalakshmi Selvaraja,b, Vasu Duraisamyc, Zhijun Huangb, Futao Guoa, Xiangqing Maa,b,⁎ a b c
College of Forestry, Fujian Agriculture and Forestry University, Shangxiadian Road, Cangshan District, Fuzhou, 350002, China Chinese Fir Engineering Technology Research Center of the State Forestry Administration, Fuzhou 350002, China ICAR-National Bureau of Soil Survey and Land Use Planning, Nagpur, Maharashtra 440 033, India
A R T I C L E I N F O
A B S T R A C T
Handling Editor: Junhong Bai
Tree plantations contribute towards balancing global carbon (C) and nitrogen (N) cycles, with the C:N ratio being a key factor determining soil fertility in plantations. In the present study, we investigated how the management practices of Chinese fir (Cunninghamia lanceolata) plantations affect soil organic carbon (SOC), C:N ratio and soil quality. We assessed how these soil properties vary for stands of (1) different ages (up to 97 years) within the same rotation and (2) similar ages but in different rotations (up to four). Soil samples were collected and analysed from incremental depths (0–20, 20–40, 40–60, 60–80, and 80–100 cm). Continuous replanting of Chinese fir at the same site caused SOC stock and C:N ratio to decline after the second rotation. SOC stock (0–100 cm) decreased by 3, 3.6, and 14.3% between the first and second, second and third, and third and fourth rotations, respectively. The SOC concentration and C:N ratio declined from 21- to 40-year-old stands, and then increased in the 97-year-old stand throughout all soil depths. The stratification ratio (SR) index of SOC stock showed that continuous cultivation causes soil quality to decrease with increasing rotation cycle. Approximately 35–45% of equivalent soil mass SOC stocks were distributed in the upper soil layer (0–20 cm) in stands of all ages, indicating more organic C accumulation in the surface layer compared to subsurface layers (> 20 cm). In conclusion, we recommend that (1) cutting cycles of the stands should be increased from 20 to 25 years (current practice) to ~30 years of age and (2) plantations should only be cultivated to the second rotation to maintain site productivity, which would maximise both the ecological and economic value of this practice to the environment.
Keywords: Chinese fir plantations C:N ratio Rotation Soil organic carbon Stratification ratio
1. Introduction Soil organic carbon (SOC) and nitrogen (N) influence plant growth and productivity in terrestrial ecosystems. On global scale, 383 Pg of soil C (Pan et al., 2011) and 133–140 Pg of soil N (Batjes, 1996) are stored within top 1 m of the soil depth. In China, SOC and N accumulates in the surface layer (0–20 cm) at an average rate of 366.7 and 21.1 kg ha− 1 year−1 respectively (Zhang et al., 2010; Liu et al., 2013). Several indicators have been proposed to evaluate soil quality (Doran and Jones, 1996; Qi et al., 2009; Armenise et al., 2013; Vasu et al., 2016). Quiroga et al. (2005) suggested that SOC is the best indicator of soil quality and soil productivity. In comparison, Franzluebbers (2002) suggested using the stratification ratio (SR) to evaluate soil quality, because of the importance of understanding the depth distribution of soil properties. SR is defined as “the ratio of a soil property at the surface layer to that at a deeper layer”. It is a good indicator of SOC stock because it can be used to infer the rate and
⁎
amount of SOC accumulation (Moreno et al., 2006; Fernandez-Romero et al., 2016). Higher SR indicates undisturbed soil and high soil quality (Lozano-Garcia et al., 2016). Increased SR can be related to the rate and amount of SOC sequestration. Conversely, SR < 2 is common in degraded soils (Franzluebbers, 2002). The vertical distribution of the C:N ratio influences how soils respond to changes in their use and management (Callesen et al., 2007). Marty et al. (2017) reported that the C:N ratio decreases with soil depth in the forests of eastern Canada. However, there is limited knowledge about how C:N ratios change after natural forests are converted to plantations and subjected to many rotation cycles. Thus, it is necessary to investigate how SOC and the C:N ratio change across successive rotation cycles of commercial tree plantations to elucidate potential economic and environmental impacts of this practice. In particular, this study stresses the importance of studying SOC and the C:N ratio in soil control sections (1 m depth) to understand how plantations influence the dynamics of large SOC stocks present in the subsoil (Lorenz and Lal,
Corresponding author at: College of Forestry, Fujian Agriculture and Forestry University, Shangxiadian Road, Cangshan District, Fuzhou 350002, China. E-mail address: [email protected] (X. Ma).
http://dx.doi.org/10.1016/j.geoderma.2017.07.014 Received 4 March 2017; Received in revised form 25 May 2017; Accepted 13 July 2017 0016-7061/ © 2017 Elsevier B.V. All rights reserved.
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Staff, 2014). The soil texture of the sample site ranged from sandy clay to clay loam. The soil profile was well developed with charcoal deposition in the organic layer, due to slash burn management practices. The thickness of the A horizon ranged from ~10 to 20 cm, whereas that of the B horizon was characterised by accumulation of clay and iron oxides. The main characteristics and topsoil properties of the studied sites are presented in Table 1. Mature timber of Chinese fir plantations is harvested by clear-cutting, followed by burning of the bark and residues. This practice influences soil CO2 flux and alters the stability of organic carbon (Song et al., 2013). This practice also modifies C cycling by incorporating pyrogenic matter, like charcoal, in the surface and subsurface soil horizons. Traditionally, coarse roots were excavated and burned after tree cutting, and the abandoned site is used for intercropping with annual crops before replanting. Plantations are primarily restricted to slopes and, after one or two rotation, allowed to regenerate naturally by stump sprouting or natural seeding. This regeneration phase serves as a fallow period, helping to restore soil fertility at the site (Yu, 1997). Weeding is conducted twice a year in the first three years after planting, followed by standard thinning practices (removal of alternate rows of branches and cutting of the crowns) at 10–13 years of age. The ideal rotation period for Chinese fir is 25–30 years, but currently it has been shortened to ~20–25 years (Tian et al., 2011).
2005; Hobley and Wilson, 2016; Vasu et al., 2016). Furthermore, the importance of sampling deep in the soil layer (i.e. 1 m) to evaluate soil C accurately has been highlighted (Harper and Tibbett, 2013; Rumpel and Kogel-Knabner, 2010). However, changes in soil quality related to equivalent soil mass (esm) and SOC stock in the forest plantations of China remain poorly documented. For instance, a few studies have shown that SOC content declines up to a depth of 60 cm with increase in the number of rotations in subtropical regions of China (Fang, 1987; Shao, 1992; Zhang et al., 2004; Jian et al., 2009). Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) is an endemic, evergreen coniferous species that is cultivated as a commercial tree. In southern China, only a few old stands of Chinese fir are protected, with plantations being dominated by young (~ 6 years) and mid-rotation (~ 12 years) age classes. A first-generation stand planted in 1919 in Nanping (the study area) is the oldest stand of Chinese fir in China. Here, we aimed to investigate how different rotations of Chinese fir plantations affect SOC and the C:N ratio. Specifically, we examined the dynamics of soil quality with respect to SOC stock of a Chinese fir plantation containing stands of (1) different stand ages (up to 97 years) within the same rotation and (2) similar stand ages but in different rotations (up to four). Our results are expected to provide information on the optimal number of rotations to maintain soil health, providing both economic benefits to plantation management and environmental benefits.
2.2. Experimental techniques 2. Materials and methods 2.2.1. Chronosequence approach In 2015, 11 Chinese fir stands of different ages (12-, 21-, 40-, and 97-year-old stands in the first rotation; 1-, 12-, 21-, and 31-year-old stands in the second rotation; 13- and 21-year-old stands in the third rotation; 10-year-old stand in the fourth rotation) were selected for the study. The stand age in each rotation was carefully selected to be as similar as possible across rotations to compare the soil carbon storage of similarly aged stands. For instance, we used 12- (first and second rotation), 13- (third rotation), and 10- (fourth rotation) year-old stands as similarly aged stands (~ 12-year-old stands) across four rotations. To minimise variation among sites, plantations which close each other were selected.
2.1. Study area The study area is located in a small watershed in Wangtai Town, Nanping City, Fujian Province, China (Fig. 1). This region has a subtropical monsoon climate, with a mean annual temperature of 19.3 °C and a relative humidity of 83% (Guo et al., 2014). The mean annual precipitation is 1699 mm, with most precipitation occurring from March to August. The mean annual evapotranspiration is 1413 mm. The altitude of the study area ranges from 150 to 250 m above mean sea level and the slope is ~ 30–40°. The soil is red earth derived from granite (Guo et al., 2005), equivalent to Hapludult based on United States Department of Agriculture (USDA) soil taxonomy (Soil Survey
Fig. 1. Map of sampling locations of Chinese fir (Cunninghamia lanceolata) plantations of varying rotations and stand ages.
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Table 1 Site characteristics and properties of nitrogen (N), phosphorus (P) and potassium (K) (1 m depth) across the Chinese fir plantations. Site characteristics
First rotation
Mean elevation (m) Aspect Slope (°) Tree density (ha− 1) Mean DBH (cm) Mean height (m) Total N (g kg− 1) Total P (g kg− 1) Total K (g kg− 1) NO3-N (mg kg− 1) NH4-N (mg kg− 1) Available P (mg kg− 1) Available K (mg kg− 1)
Second rotation
Third rotation
Fourth rotation
12-Year
21-Year
40-Year
97-Year
1-Year
12-Year
21-Year
31-Year
13-Year
21-Year
10-Year
206–248 SE 22–35 4225 9.75 9.73 1.10 0.24 48.06 1.36 4.08 8.06 50.40
219–235 SE 21–34 3350 11.92 11.61 1.07 0.21 52.12 2.45 3.70 7.64 57.84
216–238 SE 24–34 1542 20.69 25.10 0.80 0.20 45.09 1.47 3.65 7.17 56.63
212–240 SE 23–37 825 34.39 34.11 0.91 0.24 42.64 4.99 4.43 6.18 60.82
215–245 SE 22–37 – – – 0.90 0.24 54.33 4.43 4.00 5.27 71.43
216–238 SE 25–38 2392 15.16 13.86 1.12 0.22 48.63 4.00 3.04 6.21 68.42
215–243 SE 24–38 2158 17.28 14.10 1.05 0.20 44.58 3.32 4.76 6.99 66.59
218–237 SE 21–32 1818 15.7 20.8 0.83 0.21 51.86 3.96 4.11 7.65 64.89
216–239 SE 25–35 3775 10.52 10.80 0.99 0.19 37.87 1.31 2.69 4.16 58.91
214–244 SE 26–34 2516 12.84 16.37 1.06 0.16 34.08 3.20 4.45 8.20 58.30
218–242 SE 21–32 2775 10.45 9.66 0.99 0.15 29.61 1.29 2.82 4.50 54.03
SE-Southeast; DBH diameter at breast height (1.3 m above ground).
year-old stand (second rotation). Mi is the soil mass of horizon i and BD is the bulk density corresponding to horizon i. The SOC stocks were calculated as the products of soil bulk density, SOC concentration, and the depth correction as follows:
2.2.2. Soil sampling Five plots (20 × 20 m) were randomly selected in each of the 11 stands. In each plot, three pits were dug diagonally. From each pit, soil samples were collected at five depth intervals (0–20 cm, 20–40 cm, 40–60 cm, 60–80 cm, and 80–100 cm). The soils from the three sampling pits were mixed thoroughly to form a single composite sample for each soil layer and were sealed in air-tight bags. Two core soil samples were also collected to determine soil bulk density (BD) for every 20-cm sample interval using metal soil cores (200 cm3) to estimate esm. In total, 275 mineral soil samples [five quadrats at the five specified depths] in 11 different stands of different ages were collected.
Mg SOC ha −1 =
Mass of soil at 105°C Volume of core − volume of coarse fragments
(3)
(1)
The fresh weight of the soil was measured in the field. Field-moist soils were dried at room temperature and sieved through a 2-mm mesh to remove roots and gravel. The samples were sieved through a 0.074mm sieve to determine SOC. Total carbon (TC) and total inorganic carbon (TIC) were measured using a Shimadzu total organic carbon analyser, TOC-5000A (Shimadzu, Kyoto, Japan). The value of SOC was derived using the formula SOC = TC − TIC. The C:N ratio was determined by dry combustion using a C/N analyser (Elementar Vario EL III Analyser, Germany).
SRAL
Tadd (cm) =
∑i
Mest (Mg ha −1) − Mi (Mg ha −1)] ∙0.1 ha m−2 BDi
esm =
SOC (Mg ha−1) at L1 Average of SOC (Mg ha−1) at L1 + L2 + L3 + L4 + L5
(4)
where L1 is 0–20 cm, L2 is 20–40 cm, L3 is 40–60 cm, L4 is 60–80, and L5 is 80–100 cm. 2.3. Statistical analysis Two-way factorial ANOVA was performed to test for significant differences in BD, SOC, and C:N ratio among stand ages and soil sampling depths. The data were analysed after testing for homogeneity of variance using Levene's test. When constant variance was not satisfied, a log or square transformation was used. Multiple comparisons of the means of BD, SOC, and C:N ratio among stand age, rotations, and soil sampling depths were performed using Tukey's HSD test (P = 0.05). All statistical analyses were performed with SPSS 17.0 software.
2.2.4. Variables analysed The variables analysed were soil organic carbon concentration (g kg− 1), C:N ratio, and soil bulk density (BD) (g cm− 3). Equivalent soil mass (esm) correction was adopted to determine the SOC stock, rather than volume-based methods (Ellert and Bettany, 1995). In the present study, esm was the mass of the heaviest soil layer in the 21-yearold stand (second rotation). The mean soil mass of the 21-year-old stand was calculated for each layer down to a depth of 1 m. The difference between the soil mass of each layer of esm and lighter soil (i.e. in other stands) was used to obtain depth corrections based on the following equation: hz
[SOCi (kg Mg−1) ∙BDi ∙Ti corr (m) ∙10−3 Mg kg−1∙10 4 ha −1 m2]
where SOCi is the concentration of SOC of the horizon i, BD is the bulk density corresponding to horizon i and Ti corr is the depth correction of horizon i, calculated as the fixed sample thickness (20 cm) plus the additional thickness (Tadd). The stratification ratio was calculated for SOC stock, following the procedure of Franzluebbers (2002). The stratification ratio of the average layer (SRAL) and the percent distribution of SOC in each layer were also calculated. SRALs reflect the percentage of a given soil property in the surface layer relative to the cumulative value of the same property at the total soil depth (Hernanz et al., 2009). It was calculated as follows:
2.2.3. Soil analysis Bulk density cores were dried at 105 °C until a constant dry weight was reached. Coarse fragments (stones, rocks, and roots) were removed manually, and their mass was subtracted from the total weight. Bulk density was calculated using the following equation:
BD =
hz
∑i
3. Results 3.1. Concentration and stock of SOC and C:N in successive rotations SOC concentration and C:N ratio for the four rotations (~ 12-yearold stand) are presented in Table 2. Continuous plantation of Chinese fir caused the SOC concentration to decline by 21.5% by the fourth rotation. The concentration of C:N ratio also decreased by 4.5% (first to second rotation), 0.44% (second to third rotation), 1.14% (third to fourth rotation), and 6.0% (first to fourth rotation) at a soil depth of
(2)
where Tadd is the additional thickness of all soil horizons hz down to a depth of 1 m. Mest is the equivalent soil mass of each layer in the 21129
130
First rotation 12-Year-old 1.14 ± 0.15a 1.24 ± 0.13a 1.26 ± 0.25a 1.29 ± 0.05a 1.32 ± 0.06a First rotation 21-Year-old 1.13 ± 0.06a 1.14 ± 0.14a 1.21 ± 0.04a 1.27 ± 0.04a 1.27 ± 0.09a First rotation 40-Year-old 1.18 ± 0.18a 1.24 ± 0.11a 1.25 ± 0.10a 1.28 ± 0.10a 1.37 ± 0.15a First rotation 97-Year-old 1.21 ± 0.07a 1.21 ± 0.15a 1.22 ± 0.10a 1.27 ± 0.09a 1.35 ± 0.10a
BD (g cm− 3)
12.3 ± 1.28Aa 10.8 ± 1.5Aa 8.7 ± 1.0Ba 6.8 ± 0.7Cab 5.9 ± 0.3Cab
10.4 ± 1.5Aa 8.8 ± 1.8ABa 6.5 ± 1.3BCa 5.5 ± 1.0Cab 4.8 ± 1.5Cb
11.1 ± 0.4Aa 8.9 ± 0.8Ba 7.4 ± 0.5Ca 6.6 ± 0.2Db 5.7 ± 0.2Eab
19.6 ± 2.5Aab 14.7 ± 3.0Ba 7.8 ± 1.5Ca 5.5 ± 1.1CDa 4.2 ± 0.7Dab
12.2 ± 4.6Ab 7.6 ± 2.1Bb 4.7 ± 1.5BCb 3.5 ± 1.0BCb 2.9 ± 0.9Cb
16.4 ± 3.1Aab 8.1 ± 1.3Bb 5.7 ± 0.3BCab 4.8 ± 0.4Cab 3.8 ± 0.5Cb
Aa
12.2 ± 1.6 9.6 ± 1.2Ba 8.0 ± 0.7BCa 7.1 ± 0.6Ca 6.5 ± 0.9Ca
C:N
22.6 ± 8.5 10.6 ± 3.5Bab 7.5 ± 1.6Ba 5.6 ± 0.9Ba 5.4 ± 1.2Ba
Aa
SOC (g kg− 1) Second rotation 1-Year-old 1.07 ± 0.08a 1.15 ± 0.13a 1.18 ± 0.13a 1.21 ± 0.09a 1.27 ± 0.08a Second rotation 12-Year-old 1.12 ± 0.10a 1.17 ± 0.19a 1.19 ± 0.09a 1.20 ± 0.12a 1.22 ± 0.08a Second rotation 21-Year-old 1.26 ± 0.18a 1.27 ± 0.19a 1.39 ± 0.20a 1.39 ± 0.11a 1.44 ± 0.16a Second rotation 31-Year-old 1.19 ± 0.04b 1.21 ± 0.11b 1.29 ± 0.06ab 1.32 ± 0.05ab 1.35 ± 0.07a
BD (g cm− 3)
15.9 ± 2.3Ab 8.4 ± 1.0Ba 4.7 ± 0.5Ba 4.0 ± 1.1Ca 3.6 ± 0.3Cab
20.2 ± 2.6Aab 10.8 ± 1.2Ba 7.1 ± 0.6Cab 5.6 ± 1.1Ca 4.8 ± 0.8Ca
24.0 ± 4.3 10.0 ± 2.6Ba 7.4 ± 2.3BCa 5.3 ± 1.7BCa 4.6 ± 1.6Cab
Aa
20.3 ± 5.4Aab 8.3 ± 1.1Ba 5.8 ± 1.2BCab 3.9 ± 0.5BCa 3.0 ± 0.6Cb
SOC (g kg− 1)
11.6 ± 0.7Aa 10.1 ± 0.8Ba 7.1 ± 0.3Ba 6.1 ± 0.8CDa 5.7 ± 0.3Dab
11.9 ± 0.7Aa 9.7 ± 0.7Ba 8.2 ± 0.5Ca 6.9 ± 0.7Da 6.2 ± 0.6Da
12.1 ± 0.8 9.0 ± 0.6Ba 7.8 ± 1.3BCa 6.5 ± 1.0Ca 6.1 ± 1.2Ca
Aa
12.8 ± 1.5Aa 9.0 ± 0.6Ba 7.5 ± 0.7Ca 6.1 ± 0.2Da 4.8 ± 0.5Db
C:N
Third rotation 13-Year-old 1.19 ± 0.06b 1.21 ± 0.09b 1.32 ± 0.08ab 1.34 ± 0.10ab 1.38 ± 0.10a Third rotation 21-Year-old 1.04 ± 0.09c 1.09 ± 0.06bc 1.21 ± 0.09abc 1.26 ± 0.11ab 1.29 ± 0.15a
BD (g cm− 3)
18.3 ± 2.9Ab 10.7 ± 1.1Bb 7.4 ± 1.4Cb 5.8 ± 1.6Cb 4.8 ± 1.6Cb
19.1 ± 2.3 9.8 ± 1.7Ba 7.2 ± 0.4Ca 4.6 ± 0.8Da 4.1 ± 0.7Da
Aa
SOC (g kg− 1)
Aa
11.1 ± 0.8Ab 9.7 ± 0.5Bb 7.9 ± 0.6Cb 7.0 ± 0.9CDb 6.2 ± 0.9Db
12.7 ± 0.9 9.1 ± 0.4Ba 8.0 ± 0.2Ca 6.3 ± 0.7Da 6.0 ± 0.6Da
C:N
Fourth rotation 10-Year-old 1.10 ± 0.07a 1.13 ± 0.06a 1.14 ± 0.09a 1.18 ± 0.07a 1.20 ± 0.07av
BD (g cm− 3)
15.8 ± 2.8A 9.8 ± 0.6B 6.7 ± 0.3C 4.4 ± 0.8C 3.9 ± 0.6C
SOC (g kg− 1)
Data are mean ± standard deviation in each column followed by the different upper case letter indicating significant difference in the soil depths in the given stand age (P < 0.05). Data are mean ± standard deviation in each column followed by the different lower case letter indicating significant difference among different stand age in the same soil layer in the given rotation (P < 0.05).
0–20 20–40 40–60 60–80 80–100
0–20 20–40 40–60 60–80 80–100
0–20 20–40 40–60 60–80 80–100
0–20 20–40 40–60 60–80 80–100
0–20 20–40 40–60 60–80 80–100
Depth (cm)
Table 2 Bulk density (BD), soil organic carbon (SOC) and C:N ratio at given rotation and stand ages of Chinese fir plantations.
10.6 ± 1.0A 8.5 ± 0.7B 7.0 ± 0.5C 5.9 ± 0.8CD 5.4 ± 0.8D
C:N
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0-20
Soil organic carbon (Mg ha-1)
140
a
20-40
40-60
a
60-80
80-100
layers (Table 4). Soil layers with SRAL values > 2 contained > 40% of the total esm SOC (Fig. 3a–c). The SRAL value of esm SOC was higher (> 40%) in the top soil (SRAL [1]) in all stands (regardless of age) in the second and third rotations (Fig. 3b, c). This fact implies that the SOC accumulated in the top soil contributes 40.26% to the total SOC accumulation in 1 m depth. Similarly, approximately 36% and 39% of SOC accumulated in the top layer in 21- and 40-year-old stands (first rotation), respectively, whereas 37% accumulated in 10-year-old stands (fourth rotation), which was considered low compared to the level in the other stands (Fig. 3a, d).
a
120
a
100 80 60 40 20 0 First rotation
Second rotation
Third rotation
4. Discussion
Fourth rotation
Rotation
4.1. Changes in SOC concentration across successive rotations
Fig. 2. Cumulative stock values (Mg ha− 1) of a) soil organic carbon (SOC) at ~ 12-yearold stand of four rotations. Error bars indicate standard error of the mean. The legend represents the soil depth in cm. Lowercase letters show significant differences among rotations at total 1 m depth (P = 0.05).
Our results clearly show that continuous replanting of Chinese fir at the same site causes the SOC and C:N ratio to decline after the second rotation. The concentration of SOC marginally (but not significantly) increased from the first rotation to second rotation, similarly to previous studies (Shao, 1992; Jian et al., 2009). However, these studies only used three rotations, and thus missed the significant decline in SOC concentration and C:N ratio between the third and fourth rotation in the subsurface soil horizons (60–80 and 80–100 cm) (Table 2). This phenomenon causes reduction in the density of the understory vegetation with each additional rotation (Zhang et al., 2004; Wen et al., 2010). Moreover, the reduction in the C:N ratio is also enhanced by the uptake of N by the plantation at each rotation. The present study indicates that SOC stock is increasingly depleted with each rotation. For example, SOC stock (0–100 cm) decreased by 3.01, 3.58, and 14.31% between the first and second, second and third, and third and fourth rotations, respectively (Fig. 2). Zhang et al. (2004) recorded a 10% and 15% decline in SOC stock between the first and second and second and third rotations, respectively. These values were greater than those observed in the current study due to intensive site preparation practices of Chinese fir plantations. The esm SOC stock declined in the subsoil layers (> 20 cm) (Fig. 2), supporting the results of Chen et al. (2013), which suggested that this phenomenon negatively affects the contribution of SOC to roots in the subsurface horizons. Diochon and Kellman (2009) recorded that the rate of SOC decomposition increased in subsurface horizons of mineral soils after harvesting. Thus, esm SOC stocks in the subsurface layers appear to be more susceptible to disturbance by anthropogenic activity, such as harvesting practices. Our results confirm that the deeper soil layer (i.e. not just the surface layer) must be analysed to understand the dynamics of SOC in the management of plantations (Harper and Tibbett, 2013).
1 m (Table 2). Rotation cycles (first to fourth) had no significant influence on total SOC stock, even though the total concentration (0–100 cm) ranged from 101.95 Mg ha− 1 to 127.12 Mg ha− 1 (Fig. 2). Moreover, when comparing the soil depths, SOC stock was highest in the surface layer (0–20 cm), with the second rotation containing 59% of the entire stock out of all rotations (Fig. 2). 3.2. Characteristics of SOC and C:N ratio among stands of different ages SOC concentration and C:N ratio decreased from 21- to 40-year-old stands, and increased again in the 97-year-old stand (first rotation). SOC concentration varied among the 21- and 40-year-old stands at soil depths of 20–40, 40–60, and 60–80 cm (P < 0.05); however, variation was not observed at any soil depth in the 40- and 97-year-old stands (Table 2). Concentration of SOC and C:N ratio increased from 1- to 12year-old stands. Although the SOC concentration increased, there was no significant difference in the soil depth (below 20 cm) between these two stands (Table 2). In the stands of the second rotation, SOC and C:N ratio were lowest in the 31-year-old stand at all soil layers. The 12-yearold stand (second rotation) accumulated higher concentrations of SOC (24.0 g kg− 1) than stands of any other age class in the top soil (0–20 cm). 3.3. Distribution of soil properties at different soil depths SOC accumulation and C:N ratio decreased with increasing soil depth (Table 2). In stands of all ages, the SOC concentration and C:N ratio were significantly higher in the top soil (0–20 cm) compared to all other soil layers (P < 0.05). The C:N ratio significantly decreased (P < 0.05) with soil depth in the 97-year-old stand. BD also increased with increasing depth, but no significant difference was observed between surface and subsurface layers (P > 0.05) (Table 2). The interaction of SOC, C:N ratio, and BD was significantly related to soil depth in the first three rotations. SOC concentration in the first rotation and the C:N ratio in the second rotation were influenced by the interactive effects of soil depth and the stand age of plantations (Table 3).
4.2. Changes in SOC concentration in stands of different ages The SOC concentration and C:N ratio decreased in the surface soils with increasing stand age (12-, 21-, and 31-year-old stands) in the first rotation. The oldest stand (97-years-old) contained higher SOC concentrations than the 40-year-old stand at all soil depths, due to the accumulation of organic matter through the decomposition of litter and underground vegetation. Earlier studies have also indicated that older stands accumulate higher SOC than other stands (Zhou et al., 2006; Luyssaert et al., 2008; Chen et al., 2013). SOC declined in the 1-year-old plantation (second rotation), resulting in loss of surface soil C. This could be attributed to the removal of burnt material and erosion caused by high intensity rainfall after slash and burn practices (Zhang et al., 2004). Previous studies on Chinese fir have also shown that the efficiency of nutrient uptake significantly increases with increasing stand age, causing soil fertility to decline (Ma et al., 2007; Zhou et al., 2015). In our study, 40- and 31-year-old stands had lower concentrations of SOC than the 21-year-old stand in the corresponding rotation. Thus, the SOC concentrations of Chinese fir plantations could be sustained by
3.4. Effects of stand age on stratification ratio of SOC In Hapludult soils of Chinese fir plantations, the SR of the SOC stock increased with depth for all stand ages studied (Table 4). This could be attributed to the low SOC content in the subsurface soil layer (80–100 cm). With respect to soil quality, the SR of the SOC stock was > 2 in all stand ages in the subsoil layers of > 40 cm. These results indicated high soil quality, whereas the ratios < 2 frequently found in the subsoil layer (0–20:20–40) represented degraded soils. SR decreased between the third and fourth rotation in the corresponding soil 131
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Table 3 Comparison of effects of stand age and soil depth on Chinese fir plantations including all stand ages at given rotation using two-way ANOVA. Soil characteristics
Stand age
Soil depth
Soil depth × stand age
F
P
F
P
F
P
R2
I rotation SOC (g kg− 1) C:N ratio BD (g cm− 3)
14.00 12.06 1.38
< 0.05 < 0.05 0.254
81.59 90.64 4.20
< 0.05 < 0.05 < 0.05
2.20 0.57 0.54
< 0.05 0.85 0.87
0.832 0.835 0.256
II rotation SOC (g kg− 1) C:N ratio BD (g cm− 3)
10.27 1.87 10.94
< 0.05 0.141 < 0.05
194.19 187.34 2.447
< 0.05 < 0.05 < 0.05
1.78 1.82 1.04
0.06 < 0.05 0.41
0.912 0.907 0.408
III rotation SOC (g kg− 1) C:N ratio BD (g cm− 3)
1.97 0.31 16.35
0.16 0.57 < 0.05
141.77 92.38 9.82
< 0.05 < 0.05 < 0.05
0.27 2.09 0.44
0.89 0.09 0.77
0.934 0.904 0.589
Results of the fourth generation cannot be compared due to the absence of multiple stand ages. Bold values indicate significant differences among stand age or soil depth (P < 0.05).
However, SR is not widely used in plantations and forest lands. Soils with an SR value < 2 are considered degraded (Franzluebbers, 2002; Melero et al., 2012; Lou et al., 2012). The SR index of esm SOC for the surface soil (0–20 cm) showed poor soil quality (Table 4); however, the quality increased with soil depth in stands of all ages, supporting the observations made by Lozano-Garcia et al. (2016) in the natural areas of the Mediterranean as well as studies investigating the influence of land use and tillage management practices on SOC (Spargo et al., 2008; Papini et al., 2011; Corral-Fernández et al., 2013). We also found that the percentage distribution of esm SOC was greater in the top soil (Fig. 3a–d), similarly to Hernanz et al. (2009), who recorded higher SRAL values in the surface layer (indicating higher nutrient stocks) under different tillage systems.
Table 4 Stratification ratios for soil organic carbon (SOC) stock in different stand ages. The stratification ratios for each of the three soil properties in 0–20 cm layer to 20–40, 40–60, 60–80 and 80–100 cm layers. Stand age
Depth (cm) 0–20:20–40
0–20:40–60
0–20:60–80
0–20:80–100
First rotation 12-Year-old 21-Year-old 40-Year-old 97-Year-old
1.89 1.25 1.60 2.04
2.61 2.50 2.68 2.85
3.39 3.15 3.11 3.25
3.59 4.09 4.09 3.86
Second rotation 1-Year-old 12-Year-old 21-Year-old 31-Year-old
2.23 2.25 1.94 2.09
2.92 2.93 2.61 3.63
4.45 4.19 3.26 4.01
5.99 4.74 3.85 4.26
Third rotation 13-Year-old 21-Year-old
2.21 2.19
3.21 2.98
4.19 3.39
4.41 4.01
Fourth rotation 10-Year-old
1.65
2.36
3.10
3.38
4.4. Implications for forest management It is important to understand how forest management practices influence SOC dynamics to establish the relationship between timber yield and soil quality. Previous studies suggest that continuous cultivation of Chinese fir on the same site causes timber yield and soil quality to decline (Wei et al., 2012; Ma et al., 2007). Inappropriate management practices affect SOC concentrations during the early stages of plant development. For example, slash and burn causes a fivefold increase in the rate of nitrogen loss in the first six years after planting (Ma et al., 1997). In the present study, SOC concentrations were higher during the second rotation for a 12-year-old stand compared to a 1-year-old stand, which was located in a site that had been clear-harvested and slash-burned. Stem and wood removal during clearharvest and controlled slash burning significantly reduces C and N content and soil respiration in the soil surface due to higher temperatures (Davis et al., 2003; Guo et al., 2010). This could be managed by the cultivation of suitable annual inter-crops that enhance soil fertility by improving soil properties and microbial activity (Wang et al., 2005). Gutrich and Howarth (2007) indicated that the storage of carbon in forest soils is independent of stand age and timber management practices. In southern China, where most forest lands have been converted to plantations, it is essential to balance both ecological and economic benefits by defining the optimal number of rotations and age at which stands are harvested. However, more studies are required to confirm this relationship to facilitate comparisons among the number and length of rotations. For instance, previous studies on Chinese fir were limited to only three rotations. The current study filled this knowledge gap, allowing us to provide two evidence-based suggestions to improve current management practices. First, continuous rotations of Chinese fir at the same site causes
increasing the stand age from 25 years but not > 30 years. Tian et al. (2011) also suggested that stands should be grown for approximately five more years at sites that are currently managed by clear-harvesting at ~20–25 years. 4.3. Variation in SOC at different soil depths In general, soil organic matter transformations are affected by a number of factors, such as decomposition, immobilization, mineralization, and nutrient cycling. These factors are expected to be more active near the soil surface. The surface horizon typically contains higher SOC concentrations than the subsurface layers, irrespective of plantation species, land-use type, and vegetation type (Groppo et al., 2015). We found that SOC concentrations and the C:N ratio declined with increasing soil depth (Table 2). Compared to the subsurface layers, the higher SOC content in the surface layer (0–20 cm) might be attributed to the higher density of plant roots and soil fauna, as well as the deposition of large amounts of plant litter. Thus, our results were consistent with the declining SOC content as a function of depth (Marinho et al., 2017). The stratification ratio (SR) was calculated for esm stock SOC to characterise soil quality at different depth intervals. SR is used to measure how the soil profile responds to anthropogenic disturbance. 132
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Fig. 3. Stratification ratio of average layers and distribution of a) SOC (soil organic carbon) in the different stands of first rotation b) SOC in the different stands of second rotation c) SOC in the different stands of third rotation d) SOC in the stand of fourth rotation.
biomass of carbon and nitrogen) of soil to elucidate how these properties influence timber yields after different numbers of rotations. In conclusion, we suggest that current management practices be revised to only use two rotations and to fell trees between 25 and 30 years of age only.
reduction in SOC stock after the second rotation, whereas SR significantly declines between the third and fourth rotation, suggesting decrease in soil quality; thus, a maximum of two rotations only should be practiced at a given site. The same site may be used for intercropping with annual plants or first should be replanted along with broad-leaved species, which might help to sustain the yield. Second, total SOC concentration and the C:N ratio decreased from 21- to > 30-year-old stands in the first and second rotations; thus, the current practice of harvesting trees at < 20 years is inappropriate for cultivation. We recommend that the clear-harvest of stands should be increased by approximately five years, but should not exceed 30 years.
Acknowledgements This work was supported by the National Natural Science Foundation of China (31370619) and Twelfth Five Year National Science and Technology Support Program (2015BAD09B010102). We also express our gratitude to Dr. J.L. Hernanz, Universidad Politecnica de Madrid, Spain, for his assistance in evaluation of SRAL indices. Our appreciation is extended to the field assistants of Wangtai town, for providing details about the stand ages and rotation periods to collect soil samples and the forest managers for providing us with management history.
5. Conclusions Our study showed that the number of rotations and the age of stands within each rotation influenced soil properties that might reduce the quality of timber being produced. Our findings highlight the dynamic nature of SOC over time, showing that more SOC accumulates in mature stands (97-year-old stand), whereas it is depleted after the second rotation. Our results validate the importance of sampling deeper soils (to 1 m) to understand how stand age and the number of rotations affect soil properties, including SOC stock. Future studies should incorporate estimates of physical, chemical and biological properties (microbial
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Influence of long-term successive rotations and stand age of Chinese fir (Cunninghamia lanceolata) plantations on soil properties
MARK
Selvalakshmi Selvaraja,b, Vasu Duraisamyc, Zhijun Huangb, Futao Guoa, Xiangqing Maa,b,⁎ a b c
College of Forestry, Fujian Agriculture and Forestry University, Shangxiadian Road, Cangshan District, Fuzhou, 350002, China Chinese Fir Engineering Technology Research Center of the State Forestry Administration, Fuzhou 350002, China ICAR-National Bureau of Soil Survey and Land Use Planning, Nagpur, Maharashtra 440 033, India
A R T I C L E I N F O
A B S T R A C T
Handling Editor: Junhong Bai
Tree plantations contribute towards balancing global carbon (C) and nitrogen (N) cycles, with the C:N ratio being a key factor determining soil fertility in plantations. In the present study, we investigated how the management practices of Chinese fir (Cunninghamia lanceolata) plantations affect soil organic carbon (SOC), C:N ratio and soil quality. We assessed how these soil properties vary for stands of (1) different ages (up to 97 years) within the same rotation and (2) similar ages but in different rotations (up to four). Soil samples were collected and analysed from incremental depths (0–20, 20–40, 40–60, 60–80, and 80–100 cm). Continuous replanting of Chinese fir at the same site caused SOC stock and C:N ratio to decline after the second rotation. SOC stock (0–100 cm) decreased by 3, 3.6, and 14.3% between the first and second, second and third, and third and fourth rotations, respectively. The SOC concentration and C:N ratio declined from 21- to 40-year-old stands, and then increased in the 97-year-old stand throughout all soil depths. The stratification ratio (SR) index of SOC stock showed that continuous cultivation causes soil quality to decrease with increasing rotation cycle. Approximately 35–45% of equivalent soil mass SOC stocks were distributed in the upper soil layer (0–20 cm) in stands of all ages, indicating more organic C accumulation in the surface layer compared to subsurface layers (> 20 cm). In conclusion, we recommend that (1) cutting cycles of the stands should be increased from 20 to 25 years (current practice) to ~30 years of age and (2) plantations should only be cultivated to the second rotation to maintain site productivity, which would maximise both the ecological and economic value of this practice to the environment.
Keywords: Chinese fir plantations C:N ratio Rotation Soil organic carbon Stratification ratio
1. Introduction Soil organic carbon (SOC) and nitrogen (N) influence plant growth and productivity in terrestrial ecosystems. On global scale, 383 Pg of soil C (Pan et al., 2011) and 133–140 Pg of soil N (Batjes, 1996) are stored within top 1 m of the soil depth. In China, SOC and N accumulates in the surface layer (0–20 cm) at an average rate of 366.7 and 21.1 kg ha− 1 year−1 respectively (Zhang et al., 2010; Liu et al., 2013). Several indicators have been proposed to evaluate soil quality (Doran and Jones, 1996; Qi et al., 2009; Armenise et al., 2013; Vasu et al., 2016). Quiroga et al. (2005) suggested that SOC is the best indicator of soil quality and soil productivity. In comparison, Franzluebbers (2002) suggested using the stratification ratio (SR) to evaluate soil quality, because of the importance of understanding the depth distribution of soil properties. SR is defined as “the ratio of a soil property at the surface layer to that at a deeper layer”. It is a good indicator of SOC stock because it can be used to infer the rate and
⁎
amount of SOC accumulation (Moreno et al., 2006; Fernandez-Romero et al., 2016). Higher SR indicates undisturbed soil and high soil quality (Lozano-Garcia et al., 2016). Increased SR can be related to the rate and amount of SOC sequestration. Conversely, SR < 2 is common in degraded soils (Franzluebbers, 2002). The vertical distribution of the C:N ratio influences how soils respond to changes in their use and management (Callesen et al., 2007). Marty et al. (2017) reported that the C:N ratio decreases with soil depth in the forests of eastern Canada. However, there is limited knowledge about how C:N ratios change after natural forests are converted to plantations and subjected to many rotation cycles. Thus, it is necessary to investigate how SOC and the C:N ratio change across successive rotation cycles of commercial tree plantations to elucidate potential economic and environmental impacts of this practice. In particular, this study stresses the importance of studying SOC and the C:N ratio in soil control sections (1 m depth) to understand how plantations influence the dynamics of large SOC stocks present in the subsoil (Lorenz and Lal,
Corresponding author at: College of Forestry, Fujian Agriculture and Forestry University, Shangxiadian Road, Cangshan District, Fuzhou 350002, China. E-mail address: [email protected] (X. Ma).
http://dx.doi.org/10.1016/j.geoderma.2017.07.014 Received 4 March 2017; Received in revised form 25 May 2017; Accepted 13 July 2017 0016-7061/ © 2017 Elsevier B.V. All rights reserved.
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Staff, 2014). The soil texture of the sample site ranged from sandy clay to clay loam. The soil profile was well developed with charcoal deposition in the organic layer, due to slash burn management practices. The thickness of the A horizon ranged from ~10 to 20 cm, whereas that of the B horizon was characterised by accumulation of clay and iron oxides. The main characteristics and topsoil properties of the studied sites are presented in Table 1. Mature timber of Chinese fir plantations is harvested by clear-cutting, followed by burning of the bark and residues. This practice influences soil CO2 flux and alters the stability of organic carbon (Song et al., 2013). This practice also modifies C cycling by incorporating pyrogenic matter, like charcoal, in the surface and subsurface soil horizons. Traditionally, coarse roots were excavated and burned after tree cutting, and the abandoned site is used for intercropping with annual crops before replanting. Plantations are primarily restricted to slopes and, after one or two rotation, allowed to regenerate naturally by stump sprouting or natural seeding. This regeneration phase serves as a fallow period, helping to restore soil fertility at the site (Yu, 1997). Weeding is conducted twice a year in the first three years after planting, followed by standard thinning practices (removal of alternate rows of branches and cutting of the crowns) at 10–13 years of age. The ideal rotation period for Chinese fir is 25–30 years, but currently it has been shortened to ~20–25 years (Tian et al., 2011).
2005; Hobley and Wilson, 2016; Vasu et al., 2016). Furthermore, the importance of sampling deep in the soil layer (i.e. 1 m) to evaluate soil C accurately has been highlighted (Harper and Tibbett, 2013; Rumpel and Kogel-Knabner, 2010). However, changes in soil quality related to equivalent soil mass (esm) and SOC stock in the forest plantations of China remain poorly documented. For instance, a few studies have shown that SOC content declines up to a depth of 60 cm with increase in the number of rotations in subtropical regions of China (Fang, 1987; Shao, 1992; Zhang et al., 2004; Jian et al., 2009). Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) is an endemic, evergreen coniferous species that is cultivated as a commercial tree. In southern China, only a few old stands of Chinese fir are protected, with plantations being dominated by young (~ 6 years) and mid-rotation (~ 12 years) age classes. A first-generation stand planted in 1919 in Nanping (the study area) is the oldest stand of Chinese fir in China. Here, we aimed to investigate how different rotations of Chinese fir plantations affect SOC and the C:N ratio. Specifically, we examined the dynamics of soil quality with respect to SOC stock of a Chinese fir plantation containing stands of (1) different stand ages (up to 97 years) within the same rotation and (2) similar stand ages but in different rotations (up to four). Our results are expected to provide information on the optimal number of rotations to maintain soil health, providing both economic benefits to plantation management and environmental benefits.
2.2. Experimental techniques 2. Materials and methods 2.2.1. Chronosequence approach In 2015, 11 Chinese fir stands of different ages (12-, 21-, 40-, and 97-year-old stands in the first rotation; 1-, 12-, 21-, and 31-year-old stands in the second rotation; 13- and 21-year-old stands in the third rotation; 10-year-old stand in the fourth rotation) were selected for the study. The stand age in each rotation was carefully selected to be as similar as possible across rotations to compare the soil carbon storage of similarly aged stands. For instance, we used 12- (first and second rotation), 13- (third rotation), and 10- (fourth rotation) year-old stands as similarly aged stands (~ 12-year-old stands) across four rotations. To minimise variation among sites, plantations which close each other were selected.
2.1. Study area The study area is located in a small watershed in Wangtai Town, Nanping City, Fujian Province, China (Fig. 1). This region has a subtropical monsoon climate, with a mean annual temperature of 19.3 °C and a relative humidity of 83% (Guo et al., 2014). The mean annual precipitation is 1699 mm, with most precipitation occurring from March to August. The mean annual evapotranspiration is 1413 mm. The altitude of the study area ranges from 150 to 250 m above mean sea level and the slope is ~ 30–40°. The soil is red earth derived from granite (Guo et al., 2005), equivalent to Hapludult based on United States Department of Agriculture (USDA) soil taxonomy (Soil Survey
Fig. 1. Map of sampling locations of Chinese fir (Cunninghamia lanceolata) plantations of varying rotations and stand ages.
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Table 1 Site characteristics and properties of nitrogen (N), phosphorus (P) and potassium (K) (1 m depth) across the Chinese fir plantations. Site characteristics
First rotation
Mean elevation (m) Aspect Slope (°) Tree density (ha− 1) Mean DBH (cm) Mean height (m) Total N (g kg− 1) Total P (g kg− 1) Total K (g kg− 1) NO3-N (mg kg− 1) NH4-N (mg kg− 1) Available P (mg kg− 1) Available K (mg kg− 1)
Second rotation
Third rotation
Fourth rotation
12-Year
21-Year
40-Year
97-Year
1-Year
12-Year
21-Year
31-Year
13-Year
21-Year
10-Year
206–248 SE 22–35 4225 9.75 9.73 1.10 0.24 48.06 1.36 4.08 8.06 50.40
219–235 SE 21–34 3350 11.92 11.61 1.07 0.21 52.12 2.45 3.70 7.64 57.84
216–238 SE 24–34 1542 20.69 25.10 0.80 0.20 45.09 1.47 3.65 7.17 56.63
212–240 SE 23–37 825 34.39 34.11 0.91 0.24 42.64 4.99 4.43 6.18 60.82
215–245 SE 22–37 – – – 0.90 0.24 54.33 4.43 4.00 5.27 71.43
216–238 SE 25–38 2392 15.16 13.86 1.12 0.22 48.63 4.00 3.04 6.21 68.42
215–243 SE 24–38 2158 17.28 14.10 1.05 0.20 44.58 3.32 4.76 6.99 66.59
218–237 SE 21–32 1818 15.7 20.8 0.83 0.21 51.86 3.96 4.11 7.65 64.89
216–239 SE 25–35 3775 10.52 10.80 0.99 0.19 37.87 1.31 2.69 4.16 58.91
214–244 SE 26–34 2516 12.84 16.37 1.06 0.16 34.08 3.20 4.45 8.20 58.30
218–242 SE 21–32 2775 10.45 9.66 0.99 0.15 29.61 1.29 2.82 4.50 54.03
SE-Southeast; DBH diameter at breast height (1.3 m above ground).
year-old stand (second rotation). Mi is the soil mass of horizon i and BD is the bulk density corresponding to horizon i. The SOC stocks were calculated as the products of soil bulk density, SOC concentration, and the depth correction as follows:
2.2.2. Soil sampling Five plots (20 × 20 m) were randomly selected in each of the 11 stands. In each plot, three pits were dug diagonally. From each pit, soil samples were collected at five depth intervals (0–20 cm, 20–40 cm, 40–60 cm, 60–80 cm, and 80–100 cm). The soils from the three sampling pits were mixed thoroughly to form a single composite sample for each soil layer and were sealed in air-tight bags. Two core soil samples were also collected to determine soil bulk density (BD) for every 20-cm sample interval using metal soil cores (200 cm3) to estimate esm. In total, 275 mineral soil samples [five quadrats at the five specified depths] in 11 different stands of different ages were collected.
Mg SOC ha −1 =
Mass of soil at 105°C Volume of core − volume of coarse fragments
(3)
(1)
The fresh weight of the soil was measured in the field. Field-moist soils were dried at room temperature and sieved through a 2-mm mesh to remove roots and gravel. The samples were sieved through a 0.074mm sieve to determine SOC. Total carbon (TC) and total inorganic carbon (TIC) were measured using a Shimadzu total organic carbon analyser, TOC-5000A (Shimadzu, Kyoto, Japan). The value of SOC was derived using the formula SOC = TC − TIC. The C:N ratio was determined by dry combustion using a C/N analyser (Elementar Vario EL III Analyser, Germany).
SRAL
Tadd (cm) =
∑i
Mest (Mg ha −1) − Mi (Mg ha −1)] ∙0.1 ha m−2 BDi
esm =
SOC (Mg ha−1) at L1 Average of SOC (Mg ha−1) at L1 + L2 + L3 + L4 + L5
(4)
where L1 is 0–20 cm, L2 is 20–40 cm, L3 is 40–60 cm, L4 is 60–80, and L5 is 80–100 cm. 2.3. Statistical analysis Two-way factorial ANOVA was performed to test for significant differences in BD, SOC, and C:N ratio among stand ages and soil sampling depths. The data were analysed after testing for homogeneity of variance using Levene's test. When constant variance was not satisfied, a log or square transformation was used. Multiple comparisons of the means of BD, SOC, and C:N ratio among stand age, rotations, and soil sampling depths were performed using Tukey's HSD test (P = 0.05). All statistical analyses were performed with SPSS 17.0 software.
2.2.4. Variables analysed The variables analysed were soil organic carbon concentration (g kg− 1), C:N ratio, and soil bulk density (BD) (g cm− 3). Equivalent soil mass (esm) correction was adopted to determine the SOC stock, rather than volume-based methods (Ellert and Bettany, 1995). In the present study, esm was the mass of the heaviest soil layer in the 21-yearold stand (second rotation). The mean soil mass of the 21-year-old stand was calculated for each layer down to a depth of 1 m. The difference between the soil mass of each layer of esm and lighter soil (i.e. in other stands) was used to obtain depth corrections based on the following equation: hz
[SOCi (kg Mg−1) ∙BDi ∙Ti corr (m) ∙10−3 Mg kg−1∙10 4 ha −1 m2]
where SOCi is the concentration of SOC of the horizon i, BD is the bulk density corresponding to horizon i and Ti corr is the depth correction of horizon i, calculated as the fixed sample thickness (20 cm) plus the additional thickness (Tadd). The stratification ratio was calculated for SOC stock, following the procedure of Franzluebbers (2002). The stratification ratio of the average layer (SRAL) and the percent distribution of SOC in each layer were also calculated. SRALs reflect the percentage of a given soil property in the surface layer relative to the cumulative value of the same property at the total soil depth (Hernanz et al., 2009). It was calculated as follows:
2.2.3. Soil analysis Bulk density cores were dried at 105 °C until a constant dry weight was reached. Coarse fragments (stones, rocks, and roots) were removed manually, and their mass was subtracted from the total weight. Bulk density was calculated using the following equation:
BD =
hz
∑i
3. Results 3.1. Concentration and stock of SOC and C:N in successive rotations SOC concentration and C:N ratio for the four rotations (~ 12-yearold stand) are presented in Table 2. Continuous plantation of Chinese fir caused the SOC concentration to decline by 21.5% by the fourth rotation. The concentration of C:N ratio also decreased by 4.5% (first to second rotation), 0.44% (second to third rotation), 1.14% (third to fourth rotation), and 6.0% (first to fourth rotation) at a soil depth of
(2)
where Tadd is the additional thickness of all soil horizons hz down to a depth of 1 m. Mest is the equivalent soil mass of each layer in the 21129
130
First rotation 12-Year-old 1.14 ± 0.15a 1.24 ± 0.13a 1.26 ± 0.25a 1.29 ± 0.05a 1.32 ± 0.06a First rotation 21-Year-old 1.13 ± 0.06a 1.14 ± 0.14a 1.21 ± 0.04a 1.27 ± 0.04a 1.27 ± 0.09a First rotation 40-Year-old 1.18 ± 0.18a 1.24 ± 0.11a 1.25 ± 0.10a 1.28 ± 0.10a 1.37 ± 0.15a First rotation 97-Year-old 1.21 ± 0.07a 1.21 ± 0.15a 1.22 ± 0.10a 1.27 ± 0.09a 1.35 ± 0.10a
BD (g cm− 3)
12.3 ± 1.28Aa 10.8 ± 1.5Aa 8.7 ± 1.0Ba 6.8 ± 0.7Cab 5.9 ± 0.3Cab
10.4 ± 1.5Aa 8.8 ± 1.8ABa 6.5 ± 1.3BCa 5.5 ± 1.0Cab 4.8 ± 1.5Cb
11.1 ± 0.4Aa 8.9 ± 0.8Ba 7.4 ± 0.5Ca 6.6 ± 0.2Db 5.7 ± 0.2Eab
19.6 ± 2.5Aab 14.7 ± 3.0Ba 7.8 ± 1.5Ca 5.5 ± 1.1CDa 4.2 ± 0.7Dab
12.2 ± 4.6Ab 7.6 ± 2.1Bb 4.7 ± 1.5BCb 3.5 ± 1.0BCb 2.9 ± 0.9Cb
16.4 ± 3.1Aab 8.1 ± 1.3Bb 5.7 ± 0.3BCab 4.8 ± 0.4Cab 3.8 ± 0.5Cb
Aa
12.2 ± 1.6 9.6 ± 1.2Ba 8.0 ± 0.7BCa 7.1 ± 0.6Ca 6.5 ± 0.9Ca
C:N
22.6 ± 8.5 10.6 ± 3.5Bab 7.5 ± 1.6Ba 5.6 ± 0.9Ba 5.4 ± 1.2Ba
Aa
SOC (g kg− 1) Second rotation 1-Year-old 1.07 ± 0.08a 1.15 ± 0.13a 1.18 ± 0.13a 1.21 ± 0.09a 1.27 ± 0.08a Second rotation 12-Year-old 1.12 ± 0.10a 1.17 ± 0.19a 1.19 ± 0.09a 1.20 ± 0.12a 1.22 ± 0.08a Second rotation 21-Year-old 1.26 ± 0.18a 1.27 ± 0.19a 1.39 ± 0.20a 1.39 ± 0.11a 1.44 ± 0.16a Second rotation 31-Year-old 1.19 ± 0.04b 1.21 ± 0.11b 1.29 ± 0.06ab 1.32 ± 0.05ab 1.35 ± 0.07a
BD (g cm− 3)
15.9 ± 2.3Ab 8.4 ± 1.0Ba 4.7 ± 0.5Ba 4.0 ± 1.1Ca 3.6 ± 0.3Cab
20.2 ± 2.6Aab 10.8 ± 1.2Ba 7.1 ± 0.6Cab 5.6 ± 1.1Ca 4.8 ± 0.8Ca
24.0 ± 4.3 10.0 ± 2.6Ba 7.4 ± 2.3BCa 5.3 ± 1.7BCa 4.6 ± 1.6Cab
Aa
20.3 ± 5.4Aab 8.3 ± 1.1Ba 5.8 ± 1.2BCab 3.9 ± 0.5BCa 3.0 ± 0.6Cb
SOC (g kg− 1)
11.6 ± 0.7Aa 10.1 ± 0.8Ba 7.1 ± 0.3Ba 6.1 ± 0.8CDa 5.7 ± 0.3Dab
11.9 ± 0.7Aa 9.7 ± 0.7Ba 8.2 ± 0.5Ca 6.9 ± 0.7Da 6.2 ± 0.6Da
12.1 ± 0.8 9.0 ± 0.6Ba 7.8 ± 1.3BCa 6.5 ± 1.0Ca 6.1 ± 1.2Ca
Aa
12.8 ± 1.5Aa 9.0 ± 0.6Ba 7.5 ± 0.7Ca 6.1 ± 0.2Da 4.8 ± 0.5Db
C:N
Third rotation 13-Year-old 1.19 ± 0.06b 1.21 ± 0.09b 1.32 ± 0.08ab 1.34 ± 0.10ab 1.38 ± 0.10a Third rotation 21-Year-old 1.04 ± 0.09c 1.09 ± 0.06bc 1.21 ± 0.09abc 1.26 ± 0.11ab 1.29 ± 0.15a
BD (g cm− 3)
18.3 ± 2.9Ab 10.7 ± 1.1Bb 7.4 ± 1.4Cb 5.8 ± 1.6Cb 4.8 ± 1.6Cb
19.1 ± 2.3 9.8 ± 1.7Ba 7.2 ± 0.4Ca 4.6 ± 0.8Da 4.1 ± 0.7Da
Aa
SOC (g kg− 1)
Aa
11.1 ± 0.8Ab 9.7 ± 0.5Bb 7.9 ± 0.6Cb 7.0 ± 0.9CDb 6.2 ± 0.9Db
12.7 ± 0.9 9.1 ± 0.4Ba 8.0 ± 0.2Ca 6.3 ± 0.7Da 6.0 ± 0.6Da
C:N
Fourth rotation 10-Year-old 1.10 ± 0.07a 1.13 ± 0.06a 1.14 ± 0.09a 1.18 ± 0.07a 1.20 ± 0.07av
BD (g cm− 3)
15.8 ± 2.8A 9.8 ± 0.6B 6.7 ± 0.3C 4.4 ± 0.8C 3.9 ± 0.6C
SOC (g kg− 1)
Data are mean ± standard deviation in each column followed by the different upper case letter indicating significant difference in the soil depths in the given stand age (P < 0.05). Data are mean ± standard deviation in each column followed by the different lower case letter indicating significant difference among different stand age in the same soil layer in the given rotation (P < 0.05).
0–20 20–40 40–60 60–80 80–100
0–20 20–40 40–60 60–80 80–100
0–20 20–40 40–60 60–80 80–100
0–20 20–40 40–60 60–80 80–100
0–20 20–40 40–60 60–80 80–100
Depth (cm)
Table 2 Bulk density (BD), soil organic carbon (SOC) and C:N ratio at given rotation and stand ages of Chinese fir plantations.
10.6 ± 1.0A 8.5 ± 0.7B 7.0 ± 0.5C 5.9 ± 0.8CD 5.4 ± 0.8D
C:N
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0-20
Soil organic carbon (Mg ha-1)
140
a
20-40
40-60
a
60-80
80-100
layers (Table 4). Soil layers with SRAL values > 2 contained > 40% of the total esm SOC (Fig. 3a–c). The SRAL value of esm SOC was higher (> 40%) in the top soil (SRAL [1]) in all stands (regardless of age) in the second and third rotations (Fig. 3b, c). This fact implies that the SOC accumulated in the top soil contributes 40.26% to the total SOC accumulation in 1 m depth. Similarly, approximately 36% and 39% of SOC accumulated in the top layer in 21- and 40-year-old stands (first rotation), respectively, whereas 37% accumulated in 10-year-old stands (fourth rotation), which was considered low compared to the level in the other stands (Fig. 3a, d).
a
120
a
100 80 60 40 20 0 First rotation
Second rotation
Third rotation
4. Discussion
Fourth rotation
Rotation
4.1. Changes in SOC concentration across successive rotations
Fig. 2. Cumulative stock values (Mg ha− 1) of a) soil organic carbon (SOC) at ~ 12-yearold stand of four rotations. Error bars indicate standard error of the mean. The legend represents the soil depth in cm. Lowercase letters show significant differences among rotations at total 1 m depth (P = 0.05).
Our results clearly show that continuous replanting of Chinese fir at the same site causes the SOC and C:N ratio to decline after the second rotation. The concentration of SOC marginally (but not significantly) increased from the first rotation to second rotation, similarly to previous studies (Shao, 1992; Jian et al., 2009). However, these studies only used three rotations, and thus missed the significant decline in SOC concentration and C:N ratio between the third and fourth rotation in the subsurface soil horizons (60–80 and 80–100 cm) (Table 2). This phenomenon causes reduction in the density of the understory vegetation with each additional rotation (Zhang et al., 2004; Wen et al., 2010). Moreover, the reduction in the C:N ratio is also enhanced by the uptake of N by the plantation at each rotation. The present study indicates that SOC stock is increasingly depleted with each rotation. For example, SOC stock (0–100 cm) decreased by 3.01, 3.58, and 14.31% between the first and second, second and third, and third and fourth rotations, respectively (Fig. 2). Zhang et al. (2004) recorded a 10% and 15% decline in SOC stock between the first and second and second and third rotations, respectively. These values were greater than those observed in the current study due to intensive site preparation practices of Chinese fir plantations. The esm SOC stock declined in the subsoil layers (> 20 cm) (Fig. 2), supporting the results of Chen et al. (2013), which suggested that this phenomenon negatively affects the contribution of SOC to roots in the subsurface horizons. Diochon and Kellman (2009) recorded that the rate of SOC decomposition increased in subsurface horizons of mineral soils after harvesting. Thus, esm SOC stocks in the subsurface layers appear to be more susceptible to disturbance by anthropogenic activity, such as harvesting practices. Our results confirm that the deeper soil layer (i.e. not just the surface layer) must be analysed to understand the dynamics of SOC in the management of plantations (Harper and Tibbett, 2013).
1 m (Table 2). Rotation cycles (first to fourth) had no significant influence on total SOC stock, even though the total concentration (0–100 cm) ranged from 101.95 Mg ha− 1 to 127.12 Mg ha− 1 (Fig. 2). Moreover, when comparing the soil depths, SOC stock was highest in the surface layer (0–20 cm), with the second rotation containing 59% of the entire stock out of all rotations (Fig. 2). 3.2. Characteristics of SOC and C:N ratio among stands of different ages SOC concentration and C:N ratio decreased from 21- to 40-year-old stands, and increased again in the 97-year-old stand (first rotation). SOC concentration varied among the 21- and 40-year-old stands at soil depths of 20–40, 40–60, and 60–80 cm (P < 0.05); however, variation was not observed at any soil depth in the 40- and 97-year-old stands (Table 2). Concentration of SOC and C:N ratio increased from 1- to 12year-old stands. Although the SOC concentration increased, there was no significant difference in the soil depth (below 20 cm) between these two stands (Table 2). In the stands of the second rotation, SOC and C:N ratio were lowest in the 31-year-old stand at all soil layers. The 12-yearold stand (second rotation) accumulated higher concentrations of SOC (24.0 g kg− 1) than stands of any other age class in the top soil (0–20 cm). 3.3. Distribution of soil properties at different soil depths SOC accumulation and C:N ratio decreased with increasing soil depth (Table 2). In stands of all ages, the SOC concentration and C:N ratio were significantly higher in the top soil (0–20 cm) compared to all other soil layers (P < 0.05). The C:N ratio significantly decreased (P < 0.05) with soil depth in the 97-year-old stand. BD also increased with increasing depth, but no significant difference was observed between surface and subsurface layers (P > 0.05) (Table 2). The interaction of SOC, C:N ratio, and BD was significantly related to soil depth in the first three rotations. SOC concentration in the first rotation and the C:N ratio in the second rotation were influenced by the interactive effects of soil depth and the stand age of plantations (Table 3).
4.2. Changes in SOC concentration in stands of different ages The SOC concentration and C:N ratio decreased in the surface soils with increasing stand age (12-, 21-, and 31-year-old stands) in the first rotation. The oldest stand (97-years-old) contained higher SOC concentrations than the 40-year-old stand at all soil depths, due to the accumulation of organic matter through the decomposition of litter and underground vegetation. Earlier studies have also indicated that older stands accumulate higher SOC than other stands (Zhou et al., 2006; Luyssaert et al., 2008; Chen et al., 2013). SOC declined in the 1-year-old plantation (second rotation), resulting in loss of surface soil C. This could be attributed to the removal of burnt material and erosion caused by high intensity rainfall after slash and burn practices (Zhang et al., 2004). Previous studies on Chinese fir have also shown that the efficiency of nutrient uptake significantly increases with increasing stand age, causing soil fertility to decline (Ma et al., 2007; Zhou et al., 2015). In our study, 40- and 31-year-old stands had lower concentrations of SOC than the 21-year-old stand in the corresponding rotation. Thus, the SOC concentrations of Chinese fir plantations could be sustained by
3.4. Effects of stand age on stratification ratio of SOC In Hapludult soils of Chinese fir plantations, the SR of the SOC stock increased with depth for all stand ages studied (Table 4). This could be attributed to the low SOC content in the subsurface soil layer (80–100 cm). With respect to soil quality, the SR of the SOC stock was > 2 in all stand ages in the subsoil layers of > 40 cm. These results indicated high soil quality, whereas the ratios < 2 frequently found in the subsoil layer (0–20:20–40) represented degraded soils. SR decreased between the third and fourth rotation in the corresponding soil 131
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Table 3 Comparison of effects of stand age and soil depth on Chinese fir plantations including all stand ages at given rotation using two-way ANOVA. Soil characteristics
Stand age
Soil depth
Soil depth × stand age
F
P
F
P
F
P
R2
I rotation SOC (g kg− 1) C:N ratio BD (g cm− 3)
14.00 12.06 1.38
< 0.05 < 0.05 0.254
81.59 90.64 4.20
< 0.05 < 0.05 < 0.05
2.20 0.57 0.54
< 0.05 0.85 0.87
0.832 0.835 0.256
II rotation SOC (g kg− 1) C:N ratio BD (g cm− 3)
10.27 1.87 10.94
< 0.05 0.141 < 0.05
194.19 187.34 2.447
< 0.05 < 0.05 < 0.05
1.78 1.82 1.04
0.06 < 0.05 0.41
0.912 0.907 0.408
III rotation SOC (g kg− 1) C:N ratio BD (g cm− 3)
1.97 0.31 16.35
0.16 0.57 < 0.05
141.77 92.38 9.82
< 0.05 < 0.05 < 0.05
0.27 2.09 0.44
0.89 0.09 0.77
0.934 0.904 0.589
Results of the fourth generation cannot be compared due to the absence of multiple stand ages. Bold values indicate significant differences among stand age or soil depth (P < 0.05).
However, SR is not widely used in plantations and forest lands. Soils with an SR value < 2 are considered degraded (Franzluebbers, 2002; Melero et al., 2012; Lou et al., 2012). The SR index of esm SOC for the surface soil (0–20 cm) showed poor soil quality (Table 4); however, the quality increased with soil depth in stands of all ages, supporting the observations made by Lozano-Garcia et al. (2016) in the natural areas of the Mediterranean as well as studies investigating the influence of land use and tillage management practices on SOC (Spargo et al., 2008; Papini et al., 2011; Corral-Fernández et al., 2013). We also found that the percentage distribution of esm SOC was greater in the top soil (Fig. 3a–d), similarly to Hernanz et al. (2009), who recorded higher SRAL values in the surface layer (indicating higher nutrient stocks) under different tillage systems.
Table 4 Stratification ratios for soil organic carbon (SOC) stock in different stand ages. The stratification ratios for each of the three soil properties in 0–20 cm layer to 20–40, 40–60, 60–80 and 80–100 cm layers. Stand age
Depth (cm) 0–20:20–40
0–20:40–60
0–20:60–80
0–20:80–100
First rotation 12-Year-old 21-Year-old 40-Year-old 97-Year-old
1.89 1.25 1.60 2.04
2.61 2.50 2.68 2.85
3.39 3.15 3.11 3.25
3.59 4.09 4.09 3.86
Second rotation 1-Year-old 12-Year-old 21-Year-old 31-Year-old
2.23 2.25 1.94 2.09
2.92 2.93 2.61 3.63
4.45 4.19 3.26 4.01
5.99 4.74 3.85 4.26
Third rotation 13-Year-old 21-Year-old
2.21 2.19
3.21 2.98
4.19 3.39
4.41 4.01
Fourth rotation 10-Year-old
1.65
2.36
3.10
3.38
4.4. Implications for forest management It is important to understand how forest management practices influence SOC dynamics to establish the relationship between timber yield and soil quality. Previous studies suggest that continuous cultivation of Chinese fir on the same site causes timber yield and soil quality to decline (Wei et al., 2012; Ma et al., 2007). Inappropriate management practices affect SOC concentrations during the early stages of plant development. For example, slash and burn causes a fivefold increase in the rate of nitrogen loss in the first six years after planting (Ma et al., 1997). In the present study, SOC concentrations were higher during the second rotation for a 12-year-old stand compared to a 1-year-old stand, which was located in a site that had been clear-harvested and slash-burned. Stem and wood removal during clearharvest and controlled slash burning significantly reduces C and N content and soil respiration in the soil surface due to higher temperatures (Davis et al., 2003; Guo et al., 2010). This could be managed by the cultivation of suitable annual inter-crops that enhance soil fertility by improving soil properties and microbial activity (Wang et al., 2005). Gutrich and Howarth (2007) indicated that the storage of carbon in forest soils is independent of stand age and timber management practices. In southern China, where most forest lands have been converted to plantations, it is essential to balance both ecological and economic benefits by defining the optimal number of rotations and age at which stands are harvested. However, more studies are required to confirm this relationship to facilitate comparisons among the number and length of rotations. For instance, previous studies on Chinese fir were limited to only three rotations. The current study filled this knowledge gap, allowing us to provide two evidence-based suggestions to improve current management practices. First, continuous rotations of Chinese fir at the same site causes
increasing the stand age from 25 years but not > 30 years. Tian et al. (2011) also suggested that stands should be grown for approximately five more years at sites that are currently managed by clear-harvesting at ~20–25 years. 4.3. Variation in SOC at different soil depths In general, soil organic matter transformations are affected by a number of factors, such as decomposition, immobilization, mineralization, and nutrient cycling. These factors are expected to be more active near the soil surface. The surface horizon typically contains higher SOC concentrations than the subsurface layers, irrespective of plantation species, land-use type, and vegetation type (Groppo et al., 2015). We found that SOC concentrations and the C:N ratio declined with increasing soil depth (Table 2). Compared to the subsurface layers, the higher SOC content in the surface layer (0–20 cm) might be attributed to the higher density of plant roots and soil fauna, as well as the deposition of large amounts of plant litter. Thus, our results were consistent with the declining SOC content as a function of depth (Marinho et al., 2017). The stratification ratio (SR) was calculated for esm stock SOC to characterise soil quality at different depth intervals. SR is used to measure how the soil profile responds to anthropogenic disturbance. 132
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Fig. 3. Stratification ratio of average layers and distribution of a) SOC (soil organic carbon) in the different stands of first rotation b) SOC in the different stands of second rotation c) SOC in the different stands of third rotation d) SOC in the stand of fourth rotation.
biomass of carbon and nitrogen) of soil to elucidate how these properties influence timber yields after different numbers of rotations. In conclusion, we suggest that current management practices be revised to only use two rotations and to fell trees between 25 and 30 years of age only.
reduction in SOC stock after the second rotation, whereas SR significantly declines between the third and fourth rotation, suggesting decrease in soil quality; thus, a maximum of two rotations only should be practiced at a given site. The same site may be used for intercropping with annual plants or first should be replanted along with broad-leaved species, which might help to sustain the yield. Second, total SOC concentration and the C:N ratio decreased from 21- to > 30-year-old stands in the first and second rotations; thus, the current practice of harvesting trees at < 20 years is inappropriate for cultivation. We recommend that the clear-harvest of stands should be increased by approximately five years, but should not exceed 30 years.
Acknowledgements This work was supported by the National Natural Science Foundation of China (31370619) and Twelfth Five Year National Science and Technology Support Program (2015BAD09B010102). We also express our gratitude to Dr. J.L. Hernanz, Universidad Politecnica de Madrid, Spain, for his assistance in evaluation of SRAL indices. Our appreciation is extended to the field assistants of Wangtai town, for providing details about the stand ages and rotation periods to collect soil samples and the forest managers for providing us with management history.
5. Conclusions Our study showed that the number of rotations and the age of stands within each rotation influenced soil properties that might reduce the quality of timber being produced. Our findings highlight the dynamic nature of SOC over time, showing that more SOC accumulates in mature stands (97-year-old stand), whereas it is depleted after the second rotation. Our results validate the importance of sampling deeper soils (to 1 m) to understand how stand age and the number of rotations affect soil properties, including SOC stock. Future studies should incorporate estimates of physical, chemical and biological properties (microbial
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