Soil organic carbon stabilization changes with an altitude gradient of land cover types in central Himalaya, India

Catena 170 (2018) 374–385

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Soil organic carbon stabilization changes with an altitude gradient of land cover types in central Himalaya, India

T

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J. Dinakarana, , Abhishek Chandraa, K.P. Chamolib, Jyotishman Dekac, K.S. Raoa a

Natural Resource Management Laboratory, Department of Botany, University of Delhi (North Campus), Delhi 110007, India Department of Botany, A.P.B. Govt. Post Graduate College, Agastyamuni district, Rudraprayag, Uttarakhand 246421, India c Faculty of Science, University of Science & Technology, Meghalaya 793101, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: Soil organic carbon HF-soluble carbon FeDCB/AlDCB oxides Inceptisols Central Himalaya

Soil organic carbon (SOC) stabilization in a mountain forest ecosystem is a key component of the global carbon cycle to mitigate the rising level of atmospheric carbon dioxide. We investigated the effects of different types of land cover at different altitudes on SOC stabilization in the Himalayan region, India. We chose four different land covers at different altitudes, viz., pasture land (~2500 m), mixed cover (~2200 m), chirpine (~950 m) and agricultural land (~950 m). Organic carbon (OC), total nitrogen (N), and total microbial activity (MAt) in soils up to depths of 100 cm and under different types of land cover were assessed to study the impact of vegetation cover on the storage of SOC and N. Hydrofluoric acid (HF) soluble carbon, FeDCB/AlDCB oxides and stable carbon isotopes (δ13C) were determined in the soils under different types of land cover to evaluate the amounts of mineral-associated carbon and understand the organic carbon stabilization process. The maximum SOC stock was observed under a mixed land cover (281.86 t ha−1), followed by pasture land cover (229.61 t ha−1), chirpine (182.16 t ha−1) and agricultural land cover (70.20 t ha−1) up to a depth of 100 cm. Higher SOC contents, N contents and MAt were closely linked to the surface layers (0–30 cm), while HF-soluble carbon, FeDCB and AlDCB oxides were linked to sub-surface layers (30–100 cm) of soils under different types of land cover. HF-soluble carbon was more abundant in mixed and pasture land covers compared with chirpine and agricultural land covers. The linear relationship between δ13C values and HF-soluble carbon, FeDCB/OC, δ13C, AlDCB/OC and δ13C in soils under mixed and pasture land covers confirmed that the subsurface soil constituted more decomposed carbon and its associated FeDCB/AlDCB minerals. Nevertheless, this trend was not observed in other types of land cover located at lower altitudes and might be due to the root exudates and/or mixing of young and old carbon. These results suggest that the land cover types at higher altitudes in central Himalaya may be considered as a potential sink for the sequestration of atmospheric carbon and as potential sites for the stabilization of sequestered carbon in soils. However, land cover types at lower altitudes in central Himalaya must be managed by better soil management practices to sequester and stabilize more carbon in soils.

1. Introduction A mechanistic understanding of soil organic carbon (SOC) stabilization in any terrestrial ecosystem is very crucial for regulating the carbon cycle between terrestrial ecosystems and the atmosphere. Forest ecosystems contain more carbon per unit area as biomass and organic carbon (OC) in soils than any other land use types, and their soils are a potential sink for atmospheric carbon dioxide (Lal, 2005). Soils are the largest reservoir of OC in the terrestrial ecosystem, storing more carbon than vegetation and the atmosphere combined (Eswaran et al., 1993; Lal, 2004). Global SOC storage in the top 3 m was shown to of 2344 Pg of carbon (Jobbagy and Jackson, 2000). SOC is heterogeneous in

⁎

Corresponding author. E-mail address: [email protected] (J. Dinakaran).

https://doi.org/10.1016/j.catena.2018.06.039 Received 6 January 2018; Received in revised form 22 June 2018; Accepted 30 June 2018

0341-8162/ © 2018 Elsevier B.V. All rights reserved.

nature, and it consists of various functional pools that are stabilized by specific mechanisms and have certain turnover rates (Six et al., 2002). There are three different conceptual pools of SOC in the available literature: 1) the labile pool, 2) the intermediate pool and 3) the stable pool of SOC (Six et al., 2002; Lorenz et al., 2007; Lutzow et al., 2007). The labile pool is chemically degradable and physically accessible to microorganisms. If organic carbon is chemically degradable and physically inaccessible to microbes, it is called the stable pool of carbon. Various factors determine the inaccessibility of the SOC to microbial communities: 1) the selective preservation of biochemically resistant molecules such as lignin and tannins, 2) inclusion into aggregates and 3) attachment to clay mineral surfaces (Eusterhues et al., 2005; Mikutta

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a

b

Fig. 1. The digital elevation map (DEM) of the study region (a) and locations (b). SITE 1: Chirpine (~950 m); SITE 2: Agricultural land (~950 m); SITE 3: Mixed cover (~2200 m); SITE 4: Pasture land (~2500 m).

organic carbon from soils using hydrofluoric acid (HF). They also observed a positive relationship between HF-soluble carbon and old organic carbon (indicated by low 14C activity) in deeper soils. Furthermore, they found a significant relationship between HF-soluble carbon and Fe/Al oxides in soils. The Fe/Al oxides on clay minerals have the capacity to adsorb organic matter, and the relationship between stable carbon content and Fe/Al content suggests the importance of these oxides in stabilization of carbon in soils (Baldock and Skjemstad, 2000;

et al., 2006; Kleber et al., 2011; Adhikari and Yang, 2015; Mu et al., 2016). The organic carbon adsorbed on soil mineral surfaces (stable pool) is considered inaccessible to microbial communities and in turn persists for a longer time in soil (Eusterhues et al., 2005; Kleber and Johnson, 2010). The proportion of stable carbon (mineral bound carbon) is greater in deeper than in surface soil horizons (Eusterhues et al., 2005; Jagadamma et al., 2010). Eusterhues et al. (2003) successfully extracted the mineral-bound 375

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Table 1 Site numbers, latitude, longitude, elevation, type of vegetation cover and soil bulk density values of the selected sites in central Himalaya. Site no

Latitude N

Longitude E

Elevation (m asl)

Type of vegetation cover

Soil bulk density (g cm−3)a

1 2 3

30°25′39.34″ 30°25′35.96″ 30°38′01.06″

79°05′09.06″ 79°05′20.65″ 78°58′33.13″

~950 ~950 ~2200

1.60–1.71 g cm−3 1.61–1.69 g cm−3 0.99–1.10 g cm−3

4

30°38′12.46″

78°58′06.81″

~2500

Chirpine (Pinus roxburghii) Agricultural land (wheat) Mixed cover - predominantly composed of Alnus nepalensis, Quercus leucotrichophora, Rhododendron arboretum, Pinus roxburgii, Bauhinia variegate, Quercus glauca, Quercus floribunda, Berberis aristata and Lyonia ovalifolia Pasture - predominantly occupied by seasonal herbs and grasses.

a

1.35–1.69 g cm−3

Ranges up to a depth of 100 cm.

different villages, i.e., Chandrapuri and Trijuginarayan, which are located at different elevation levels in the central Himalayan region, Uttarakhand, India (Fig. 1). The selected study sites are located in the lesser Himalayan segment. Geologically, the parent material of the study sites is dominated by crystalline rocks, including phyllite, feldspar, mica and quartz (Valdiya, 1980). The study sites soils are dominated by Inceptisols and Entisols (National Bureau of Soil Survey, 1985). The mean annual minimum and maximum temperature of the Chandrapuri and Trijuginaryan villages are 7 °C and 34 °C, and −2 °C and 25 °C, respectively. The mean annual precipitation of the Chandrapuri and Trijuginaryan village is 1416 mm and 2500 mm, respectively (Kaur and Purohit, 2015). There is moderate snowfall (1–3 ft) during the winter months (December to February) at the Trijuginarayan village sites. Forests in the central Himalayan region are dominated by chirpine (Pinus roxburghii Sarg), oak (Quercus leucotrichophora A Camus) and mixed types. The chirpine often develops as a pure stand and is also mixed with broad-leaved species such as Quercus leucotrichophora A Camus, Quercus glauca Thumb and Rhododendron arboretum Sm (Zobel and Singh, 1997). Four different study sites, two sites from each village with an elevation range from ~950 m to ~2500 m, were selected from the central Himalayan region (Fig. 1 and Table 1): 1) chirpine cover, 2) agricultural land (> 25 years of cultivation), 3) mixed cover and 4) pastureland. In our study, the selected agricultural land was created by the activities of local farming communities on the slope of the mountain via clearing the forest, moving the topsoil and tilling the land to form terraces over several generations.

Wiseman and Püttmann, 2006; Wagai and Mayer, 2007). It is evident that Fe/Al oxides play a major role in the stabilization of carbon in deeper soils. The elevated carbon dioxide concentrations in the atmosphere are expected to increase the quantity of fresh carbon inputs from root (as exudates) into the soil (Hogberg et al., 2001; Fransson, 2012), and they can stimulate the microbial population and in turn accelerate the decomposition of stored carbon in soils (Fontaine et al., 2003, 2007). Moreover, Keiluweit et al. (2015) reported that a common root exudate (oxalic acid) induced mineralization of mineral-bound organic carbon in soil. Thus, assessing the stable or mineral associated organic carbon in various forest ecosystem soils is very crucial when we speak about the effects of global climate change on SOC stabilization on a regional to a global scale. In mountain ecosystems, biotic and abiotic factors viz., microclimate, soil types, slope, and vegetation cover, impact the amount of soil carbon storage (Jiménez and Villar, 2017). The central Himalayan mountain ecosystems are very fragile due to the recent increase in developmental activities, population and the rising temperature. In addition to global climate change, other factors, such as elevated carbon dioxide in the atmosphere, net primary productivity, erratic rainfall and soil erosion, are crucial in the destabilization of SOC dynamics in the Himalayan forest ecosystem (Longbottom et al., 2014). The Himalayan mountain forest ecosystems have the highest soil carbon densities compared with other forest ecosystems in India (Chhabra et al., 2003; Martin et al., 2010; Singh et al., 2011). The SOC stock is greatly influenced by the land use type and climatic conditions of the Himalayan ecosystem (Singh et al., 2011). Conversion of the natural forest area into cultivated and/or other land use types of Himalayan mountain ecosystems significantly reduces organic carbon accumulation in the soil (Martin et al., 2010). However, no study has provided information about the proportion of minerals associated with carbon and their stabilization process in central Himalayan soils with respect to different land use/cover types. Additionally, there is a paucity of information on the role of Fe/Al oxides in the stabilization of SOC in the central Himalayan mountain land use/cover types. We hypothesized that different land cover types at different altitudes in central Himalaya would influence the accumulation of mineral-associated organic carbon and its stabilization process. The objectives of this study were i) to determine the vertical distribution of total organic carbon (SOC), nitrogen (N), total microbial activity (MAt), HF-soluble carbon, FeDCB, AlDCB and δ13C in soils under different types of land cover at different altitudes in central Himalaya, ii) to investigate the HF-soluble carbon correlation with FeDCB, AlDCB, and δ13C in soils, and iii) to understand the role of FeDCB and AlDCB oxides on SOC stabilization under different types of land cover in central Himalaya.

2.2. Soil sampling and processing

2. Materials and methods

Soil samples were collected from the selected land cover sites using an AMS professional soil sampling kit. Based on our primary survey in the selected region (unpublished results) with respect to rooting depth, soil carbon and nutrient concentrations, and to present precise data for mineral-associated organic carbon, Fe/Al oxide contents and other soil properties, we chose the depth intervals of 0–5 cm, 5–10 cm, 10–15 cm, 15–20 cm, 20–25 cm, 25–30 cm, 30–40 cm, 40–50 cm, 50–60 cm, 60–80 cm and 80–100 cm. At each site, five sampling points were chosen within the 500 × 500 m area, excluding the agricultural land where 100 × 150 m was used to collect the soil samples. A total of 220 soil samples (5 points × 11 depth × 4 sites) were collected from the study region. At each site, five soil samples were collected independently at different depth intervals. The collected soil samples were pooled together to obtain a composite sample for each depth. The collected soil samples were air-dried, and the visible roots and gravel were removed by hand picking. For the chemical analyses, all the soil samples were gently ground with a mortar and pestle and passed through a 2-mm sieve prior to analysis.

2.1. Study area

2.3. Soil carbon, nitrogen and stable carbon isotope analysis

The Himalayan forests exhibit a range of rich biodiversity and vegetation, from alpine meadows to tropical dry deciduous forests in the foothills (Zobel and Singh, 1997). This study was conducted in two

The sieved soil samples were used for determination of the soil carbon and nitrogen concentration using a CHNS analyser (Elementar GmbH, Germany). The SOC stock was calculated using the following 376

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using the method developed by Adam and Duncan (2001). Briefly, 2 g of soil was added to a 250-ml conical flask containing 15 ml of 60 mM phosphate buffer at pH 7.6. Next, 2 ml of 1 mg of fluorescein diacetate (FDA)/ml solution was added to the flask, and it was placed in a BOD incubator shaker at 100 RPM for 30 min at 30 °C. Then, 15 ml of a chloroform:methanol (2:1) mixture was added to terminate the reaction. The suspension was subsequently centrifuged at 5000 RPM for 5 min. Finally, the supernatant was used to measure the absorbance at λmax 490 nm using a spectrophotometer (RIGOL, USA).

formula (Xu et al., 2016): K

SOC stock (kg C m−2) =

∑ SOCi × BDi × Di × (1 − Ci)/100. i=1

where SOCi, BDi, Di and Ci represent the SOC content (g kg−1), bulk density (g cm−3), soil thickness (cm) of the layer and > 2-mm fractions (%) of layer I, respectively. Subsequently, the SOC values (kg m−2) were converted (10,000 m2 = 1 ha) into t ha−1 (1000 kg = 1 t) for each depth. The SOC mitigation potential (carbon equivalent to CO2) of each land cover was calculated by multiplying by a factor 3.67 (IPCC, 2001; Wani et al., 2014). The fine and ground soil samples (carbonate free) were used for stable carbon isotope analysis. The stable carbon isotopes (13C/12C) in the soil samples were analysed in the stable isotope core laboratory, School of Biological sciences, Washington State University, Pullman, WA, USA, using an Isotope Ratio Mass spectrometer (Delta plusXP, Thermofinnigan, Bremen). The values are expressed as δ13C relative to the VPDB standard (Vienna Pee Dee Belemnite). The sample calibration and correction details are described elsewhere (Santrock et al., 1985; Coplen, 1994; Coplen et al., 2006). The carbon isotopic fractionation factor (α) was calculated indirectly as the slope of the linear relationship between the lnC/C0 (lnC and C0 represents the log-transformed SOC of the particular layer and SOC of the surface layer, respectively) and δ13C values (Accoe et al., 2002; Diochon et al., 2009; Wang et al., 2015).

2.7. Statistical analysis SPSS 16.0 software for windows was used to perform the statistical analyses. Analysis of variance (ANOVA) was conducted to determine whether the SOC, N, MAt, soil δ13C, HF-soluble carbon, FeDCB and AlDCB contents were significantly different between different depths and different land covers at P < 0.05. The post hoc Tukey's honest significant difference (HSD) test was conducted to determine the mean differences in different soil parameters among soil depths for each land cover at P < 0.05. Principal component analysis (PCA) was conducted for different soil parameters to appraise the interrelationship between different soil parameters for each land cover using XLSTAT for windows. 3. Results 3.1. SOC stock, N content, C:N ratio and MAt in surface and sub-surface soils under different types of land cover

2.4. HF treatment

The SOC stock results are shown in Table 2 and Fig. 2. The ANOVA results showed that the SOC stock values were significantly different (P < 0.05) across different types of land cover and different soil depths (Table S1). The post hoc (Tukey's HSD) results revealed significant differences in the variations in SOC stock at different depths at P < 0.05, except in a few cases of chirpine and agricultural land cover (Table 2). In all sites, SOC stock was more abundant in the surface layers (0–30 cm) than the deeper layers (30–100 cm) of soils under different types of land cover (Table 2 and Fig. 2). The maximum SOC stock was observed under the mixed land cover (281.86 t ha−1), followed by pasture land cover (229.61 t ha−1), chirpine (182.16 t ha−1) and agricultural land cover (70.20 t ha−1). Similarly, the SOC mitigation potential (carbon equivalent to CO2) was highest under mixed land cover (1034.43 t ha−1), followed by pasture land cover (842.66 t ha−1), chirpine (668.51 t ha−1) and agriculture land cover (257.65 ha−1). The soil N content displayed a similar pattern, decreasing with an increasing depth of SOC at different depths under different types of land cover (Table 3). The maximum N content was detected in soils under a mixed land cover (0.40 ± 0.21 to 11.34 ± 1.02 g kg−1), followed by pasture land cover (0.35 ± 0.09 to 6.16 ± 0.15 g kg−1), chirpine (0.46 ± 0.03 to 2.63 ± 0.42 g kg−1) and agricultural land cover (0.15 ± 0.04 to 0.70 ± 0.23 g kg−1) (Table 3). The ANOVA results revealed significant differences in the variations in N contents across different types of land cover at P < 0.05. The post hoc (Tukey's HSD) results for N content in soils at different depths were significantly different at P < 0.05, except in deeper soil layers (30–100 cm). In total, approximately 84, 70, 64 and 71% of the SOC stock and 87, 78, 72, and 71% of the N content occurred in the top 30 cm of soil under pasture, mixed, chirpine and agricultural land covers, respectively. In contrast, the SOC and N contents and soil C:N ratio under different types of land cover did not exhibit any decreasing and/or increasing trends at different depths (Table 4). The maximum C:N ratio was observed in soils with an agricultural land cover (14.77 to 33.70), followed by mixed land cover (11.83 to 18.76), chirpine (7.48 to 18.81) and pasture land cover (6.76 to 11.69) (Table 4). The microbial activity (Mat) in soils under different types of land cover is shown in Table 5, revealing a similar pattern of SOC and N contents at different depths, decreasing with increasing depth, under different types of land cover. The MAt

The mineral-associated carbon from soils was removed using HF (Eusterhues et al., 2003). Briefly, 10 ml of HF (10%) was added to the ground bulk soil sample in a 50-ml conical flask. The soil suspension was shaken for 2 h at room temperature. Next, the soil suspension was centrifuged vigorously at 3000 rpm for 15 min, and then the supernatant was carefully removed. This protocol was repeated five times, and the soil residue was washed with distilled water to remove excess salt and HF. The samples were then dried and weighed. The HF-soluble carbon (% of SOC) was calculated using the following equation (Spielvogel et al., 2008):

HF − soluble carbon (%of SOC) = [(Mass ut × SOC ut /MassHFres × SOCHFres)/Mass ut × SOC ut )] × 100 where Massut is the initial mass (in grams) of the untreated sample, MassHFres is the mass (in grams) of the HF-treated sample, SOCut is the carbon concentration of the untreated sample and SOCHF res is the carbon concentration in the soil after HF treatment. 2.5. Fe and Al extraction Fe (iron) and Al (aluminium) oxides were estimated by the dithionite–citrate–bicarbonate (DCB) method (Mehra and Jackson, 1960). Briefly, 4 g soil was placed in a 100-ml conical flask containing 40 ml of 0.3 M Na-citrate solution and 5 ml of 1 M NaHCO3 solution. One gram of Na2S2O4 was added to the solution, and then the conical flask was kept in a water bath (80 °C) for 15 min. The mixture was stirred constantly during the 15-minute time period. After digestion, 10 ml of saturated NaCl solution was added to the mixture for flocculation. The solution was mixed, warmed in a water bath (80 °C) and centrifuged at 1600–2200 rpm for 5 min. The clear supernatant was used for determination of FeDCB and AlDCB oxides. An atomic absorption spectrophotometer (GBC-AAS) was used to measure Fe and Al oxides in the samples. 2.6. Total microbial activity (MAt) Total microbial activity (MAt) was determined in the soil samples 377

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Table 2 SOC stock (t ha−1) under different types of land cover at different altitude gradients of central Himalaya. Vegetation cover

Pasture

Mixed cover

Chirpine

Agriculture

(~2500 m)

(~2200 m)

(~950 m)

(~950 m)

65.95 35.33 27.84 24.86 23.35 20.47 20.36 17.73 17.36 16.79 11.81

26.32 ± 4.18a 18.94 ± 1.74b 18.83 ± 0.71b 18.06 ± 0.50c 17.41 ± 1.04d 16.81 ± 0.86e 15.25 ± 0.36f 14.64 ± 0.20f 14.21 ± 0.40g 13.57 ± 0.27h 8.12 ± 0.27i

11.35 ± 2.26a 11.20 ± 1.69b 7.73 ± 0.88c 7.85 ± 0.15c 5.94 ± 0.68d 5.62 ± 0.27e 4.65 ± 0.61f 4.23 ± 0.60g 4.07 ± 0.10g 3.90 ± 0.10h 3.67 ± 0.10h

Soil depth (cm)

SOC (t ha−1)

0–5 5–10 10–15 15–20 20–25 25–30 30–40 40–50 50–60 60–80 80–100

59.86 ± 5.10a 46.88 ± 1.61b 33.93 ± 0.32c 24.72 ± 1.20d 16.17 ± 0.64e 10.49 ± 0.61f 9.95 ± 0.1g 9.64 ± 1.09h 6.71 ± 1.23i 6.41 ± 0.43j 4.85 ± 0.10k

± ± ± ± ± ± ± ± ± ± ±

5.18a 2.98b 5.52c 0.17d 2.0e 0.28f 0.78f 1.73g 1.63h 1.02i 1.12j

± values indicates the standard deviation, n = 3; means that do not share a small alphabet letter (after SD) within the same column indicate significant different (Tukey's HSD) at P < 0.05 level.

relationship was found between FeDCB/OC and δ13C values of pasture land cover (R2 = 0.68), while it was very weak under mixed (R2 = 0.28), chirpine (R2 = −0.11) and agricultural land cover (R2 = 0.39) (Fig. 4d). The relationship between AlDCB/OC and δ13C values under pasture land cover (R2 = 0.92) and mixed cover (R2 = 0.58) was good (Fig. 4e, f) while it was weak under chirpine (R2 = 0.18) and agricultural land cover (R2 = 0.49).

activity varied significantly at different depths and under different types of land cover at P < 0.05. MAt was highest under pasture land cover (0.15 ± 0.01 to 6.54 ± 0.04 fluorescein μg g−1), followed by mixed land cover (0.33 ± 0.07 to 3.64 ± 0.04 fluorescein μg g−1), chirpine (0.18 ± 0.01 to 3.94 ± 0.05 fluorescein μg g−1) and agriculture land cover (0.37 ± 0.01 to 1.13 ± 0.02 fluorescein μg g−1). 3.2. Stable carbon isotopes (δ13C) and their relationship with HF-soluble carbon, FeDCB/OC and AlDCB/OC

3.3. HF-soluble carbon, dithionite extractable Fe and Al oxides, FeDCB/OC and AlDCB/OC in relation to land cover types

The δ13C values of soils at five different depths under pasture land cover, mixed cover, chirpine and agricultural land cover ranged from −23.86‰ to −20.47‰, −27.24‰ to −24.60‰, −25.51‰ to −22.31‰, and − 25.41‰ to −22.61‰, respectively (Fig. 3). Enrichment of δ13C values (with increasing depth) was observed under mixed and chirpine land covers. However, this study did not note any enrichment of δ13C values in the soils of pasture and agricultural land cover. The isotopic fractionation factor (α), as the slope of the linear relationship between lnC/C0 and δ13C values of soils, was 0.99 (R2 = 0.98) and 1.08 (R2 = 0.73) under pasture land cover and mixed land cover, respectively (Fig. 4a, b). However, we did not identify a strong linear relationship between lnC/C0 and δ13C values of chirpine and agricultural land cover. This study revealed a linear relationship (R2 = 0.50) between δ13C and HF-soluble carbon values of mixed land cover (Fig. 4c), but relationship between δ13C and HF-soluble carbon values of pasture land cover (R2 = −0.23), chirpine (R2 = −0.32) and agricultural land cover (R2 = −0.30) was observed. A strong linear

250.00

0-30 cm 30-60 cm 60-100 cm

200.00

SOC Stock (t ha-1)

The HF-soluble carbon (% of SOC) is shown in Table 6. The ANOVA results showed the variations in HF-soluble carbon (% of SOC) across different types of land cover were significantly different at P < 0.05 (Table S1). This study did not detect an increasing trend of HF-soluble carbon with increasing soil depths under different types of land cover. However, the maximum amount of HF-soluble carbon was detected in the deeper soil layers under different types of land cover. The HF-soluble carbon was higher under mixed land cover (32 to 81%), followed by pasture land cover (11 to 57%), chirpine (18 to 51%) and agricultural land cover (10 to 48%). The FeDCB and AlDCB oxides at different depths of soils under different types of land cover (Fig. 5a, b) varied significantly at P < 0.05. FeDCB occurred more under mixed land cover (26.33 ± 0.35 to 37.15 ± 0.54 g kg−1), followed by pasture land cover (24.44 ± 0.16 to 27.77 ± 0.16 g kg−1), chirpine (18.30 ± 0.16 to 24.98 ± 0.14 g kg−1) and agricultural land cover (15.58 ± 0.32 to 22.45 ± 0.56 g kg−1). AlDCB oxides occurred more under pasture land cover (4.02 ± 0.01 to 12.06 ± 0.03 g kg−1), followed by mixed land cover (2.01 ± 0.02 to 7.52 ± 0.01 g kg−1), chirpine (0.20 ± 0.02 to 4.02 ± 0.01 g kg−1) and agricultural land (0.10 ± 0.01 to 0.19 ± 0.02 g kg−1). The ratio of FeDCB/OC and AlDCB/OC, indicating the freely available oxides against the available carbon in soils, is shown in Fig. 6a, b. The ratio of FeDCB/OC and AlDCB/ OC in soils increased with increasing depth under different types of land cover.

150.00

100.00

4. Discussion 50.00

4.1. Impacts of different types of land cover at different altitudes on SOC stock, N content, MAt and C:N ratio

0.00 Pasture

Mixed

Chir pine

Agricultural land

The SOC stock differed significantly (P < 0.05) from mixed land cover to agricultural land cover, supporting the hypothesis that different types of vegetation cover have a significant impact on SOC storage. Earlier global studies on SOC dynamics have agreed that the relative distribution of carbon in soils is mainly dependent on the type of

Land over

Fig. 2. SOC stock (t ha−1) under different types of land cover at different depths. Values are significant different at P < 0.05 across the land cover types and different depths. 378

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Table 3 Soil nitrogen content (g kg−1) under different types of land cover at different altitude gradients of central Himalaya. Vegetation cover

Pasture (~2500 m)

Soil depth (cm)

Nitrogen (g kg−1)

0–5 5–10 10–15 15–20 20–25 25–30 30–40 40–50 50–60 60–80 80–100

6.16 5.41 4.33 3.13 2.16 1.44 1.01 0.80 0.80 0.35 0.38

± ± ± ± ± ± ± ± ± ± ±

0.15a 0.16b 0.03c 0.12d 0.06e 0.06f 0.00g 0.11h 0.12h 0.09i 0.01i

Mixed cover (~2200 m)

Chirpine (~950 m)

Agriculture (~950 m)

11.34 ± 1.02a 6.34 ± 0.30b 5.14 ± 1.55c 3.30 ± 0.02d 2.72 ± 0.20e 2.25 ± 0.03f 2.13 ± 0.08f 2.24 ± 0.27f 1.95 ± 0.27g 0.74 ± 0.01h 0.41 ± 0.21h

2.63 2.05 1.88 1.40 1.77 1.95 1.34 0.95 0.76 0.95 0.46

0.70 0.74 0.28 0.29 0.26 0.26 0.25 0.24 0.21 0.18 0.15

± ± ± ± ± ± ± ± ± ± ±

0.42a 0.17b 0.07c 0.01c 0.10d 0.09e 0.04f 0.03g 0.04g 0.03h 0.03i

± ± ± ± ± ± ± ± ± ± ±

0.23a 0.17a 0.09bc 0.06b 0.04bc 0.05bc 0.06bc 0.04bc 0.04cd 0.05de 0.04e

± values indicates the standard deviation, n = 3; means that do not share a small alphabet letter (after SD) within the same column indicate significant different (Tukey's HSD) at P < 0.05 level. Table 4 C:N ratio under different types of land cover at different altitude gradients of central Himalaya. Pasture (~2500 m)

Soil depth (cm)

C:N

0–5 5–10 10–15 15–20 20–25 25–30 30–40 40–50 50–60 60–80 80–100

11.69 10.50 10.32 10.28 10.73 10.27 10.25 10.23 8.48 8.94 6.73

Mixed cover (~2200 m)

Chirpine (~950 m)

Agriculture (~950 m)

11.96 11.87 13.00 16.38 17.49 18.76 18.66 16.84 16.95 15.75 11.83

12.09 11.11 12.20 11.34 11.71 11.00 13.47 16.88 18.81 12.23 7.48

19.57 19.21 33.70 31.90 28.08 25.73 19.12 17.39 20.04 14.77 16.77

15-20

Depth (cm)

Vegetation cover

0-5

40-50

-27

Soil depth (cm)

Pasture (~2500 m)

Mixed cover (~2200 m)

Chirpine (~950 m)

Agriculture (~950 m)

0–5 5–10 10–15 15–20 20–25 25–30 30–40 40–50 50–60 60–80 80–100

6.54 5.29 3.79 2.95 2.35 1.31 1.77 0.33 0.86 0.21 0.15

3.64 2.81 2.43 2.05 1.97 1.41 1.51 1.12 0.62 0.61 0.33

3.94 2.96 2.53 1.41 1.55 1.33 0.81 0.36 0.30 0.28 0.18

1.13 1.11 1.08 0.84 0.59 0.50 0.43 0.43 0.43 0.40 0.37

0.04a 0.05b 0.04c 0.06d 0.41e 0.15f 0.23g 0.02h 0.06i 0.02j 0.01j

± ± ± ± ± ± ± ± ± ± ±

0.04a 0.03b 0.05c 0.02d 0.01e 0.01g 0.05f 0.01h 0.08i 0.07i 0.07j

± ± ± ± ± ± ± ± ± ± ±

0.05a 0.05b 0.09c 0.14d 0.30e 0.07f 0.04g 0.14h 0.03i 0.01ij 0.01j

± ± ± ± ± ± ± ± ± ± ±

Pasture land (~2500 m) Mixed (~2200 m)) Chirpine (~950 m) Agricultural Land (~950 m)

80-100

Table 5 Total microbial activity (fluorescein μg g−1) in soils at different depths under different types of land cover.

± ± ± ± ± ± ± ± ± ± ±

25-30

-26

-25

-24

-23

-22

-21

-20

Fig. 3. Soil δ13C values of different land cover types in central Himalaya.

organic carbon in soils (Kirschbaum, 1995; Griffiths et al., 2009). The reduced temperature and slow SOM decomposition rate at sites of higher elevation are probably the main factors responsible for the accumulation of more SOC under a mixed land cover than other types of land cover situated at lower altitudes. Therefore, the mixed land cover at higher altitudes sequesters a sufficient amount of carbon in the soil and acts as a potential sink for carbon in the central Himalayan region. This study supports the general hypothesis that land with different types of tree species, i.e., mixed land cover, generally stores large amounts of SOC with favourable climatic conditions (Jobbagy and Jackson, 2000; Dinakaran and Krishnayya, 2010) because they receive a greater and more diverse nature of litter inputs from above as well as below the ground compared with land dominated by a single tree species. The lower SOC content in chirpine and agricultural land (Table 2) compared with the mixed and pasture land cover may be attributed to the increase in temperature at lower altitudes, accelerated mineralization rate of soil organic matter and tilling practices of the land (Post et al., 2001; Wang et al., 2010). The SOC values of agricultural land reported in the present study were reduced at depths from 0 to 30 cm compared with the natural land cover types (Table 2), confirming that anthropogenic factors such as ploughing of the land and agricultural management activities could reduce ~50% of the stored organic carbon in soils (Post et al., 2001). The SOC stock in surface soil layers (0–30 cm) under different types of land cover was significantly (P < 0.05) higher than in deeper soil

0.02a 0.02ab 0.01b 0.04c 0.03d 0.06e 0.02fg 0.01f 0.04fg 0.06gh 0.01h

± values indicates the standard deviation, n = 3; means that do not share a small alphabet letter (after SD) within the same column indicate significant different (Tukey's HSD) at P < 0.05 level.

vegetation cover and the climatic conditions (Jobbagy and Jackson, 2000; Dinakaran and Krishnayya, 2008; Kumar and Sharma, 2015). The SOC values in the present study were comparable to those of other studies conducted in the central Himalayan region (Sheikh et al., 2009; Kumar and Sharma, 2015). The SOC stock was significantly elevated under a mixed land cover at higher altitudes than other types of land cover situated at lower altitudes (Table 2). The present study results corroborate earlier studies indicating that vegetation and climate are intrinsically linked and play a crucial role in the accumulation of 379

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Fig. 4. Relationship between ln(C/CO) and the δ13C values of Pasture (a): R2 = 0.98, P < 0.0001,Y = −20.66 + 0.99 × X; and Mixed (b): R2 = 0.73, P < 0.05, Y = −26.70 + −1.08 × X; δ13C and HF soluble carbon values of Mixed cover (c): R2 = 0.50, P = 0.11, Y = 248.42 + 7.99 × X; FeDCB/OC and δ13C values of Pasture land cover (d): R2 = 0.67, P = 0.05; Y = −21.29 + −0.288 × X; AlDCB/OC and δ13C values of Pasture land cover(e): R2 = 0.92, P < 0.01; Y = −20.67 + −2.10 × X; and Mixed land cover (f): R2 = 0.58, P = 0.08; Y = −26.61 + 22.24 × X, in central Himalaya.

are available for microbes in comparison to deeper soil layers. The C:N ratio provides additional information on the degree of soil organic matter decomposition, quality of organic matter and N limitations in soils (Batjes, 1996; Martins et al., 2011). The range of C:N ratios in soils at different depths under pasture (6.73–11.69), mixed (11.83–18.76) and chirpine (7.48–18.81) land covers were within the range of the C:N ratios reported for different soil types worldwide (Batjes, 1996). In the present study, the soil C:N ratios (14.77–33.70) of agricultural land cover were slightly higher than the global pattern of C:N ratios, which could be attributed to the continuous addition of farmyard manure (composed of leaf litter, wheat/rice straw and cow

layers (30–100 cm) (Table 2). However, the very low organic carbon content in deeper soil profiles under different types of land cover has been attributed to the reduced levels of litter inputs and lower decomposition rate (Jobbagy and Jackson, 2000; Dinakaran and Krishnayya, 2010). Generally, MAt in soils is mainly resource-dependent and linked to the SOC and nitrogen concentration (Wardle, 1992). The higher MAt in soils under a mixed land cover (Table 5) could be due to the diverse nature of the litter types and/or greater substrate availability in the soils. The PCA results also confirmed the close relationship of SOC, N and MAt (Fig. 7a, b, c, d) with surface soil layers (i.e., up to 30 cm), where the maximum amount of litter and substrates 380

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(Sollins et al., 1984; Whalen et al., 2000). Thus, the higher soil C:N ratios at different depths of agricultural land could be due to faster depletion of nitrogen, the accumulation of large amounts of non-humified organic matter and increased microbial nitrogen immobilization (Whalen et al., 2000; Brady and Weil, 2002; Balieiro et al., 2012). The low range of C:N values in the surface layers of soils under pasture, mixed and chirpine land covers (Table 4) clearly revealed the loss of labile and/or easily bioavailable components, such as carbohydrates and proteins (Baldock and Skjemstad, 2000; Krull et al., 2003), due to decomposition. Moreover, previous studies have described linear increases in the C:N ratio of soils with the altitude of mountainous ecosystems (Tanner et al., 1998; He et al., 2016). In contrast, linear increases of the C: N ratio were not observed in soils under different types of land cover at different altitudes (Table 4). Thus, the C:N ratios of soils at different depths under various types of land cover appeared to be strongly controlled by the microclimatic condition, vegetation cover type, litter production, agricultural management practices and rate of soil organic matter decomposition (Cools et al., 2014; Wang et al., 2014; Marty et al., 2017). The SOC mitigation potential (carbon equivalent to CO2) reached a maximum under higher altitude land cover types, which clearly suggested that the higher altitude land cover types sequestered the

Table 6 HF-soluble (% of TOC) or mineral associated carbon (stable pool) at different depths of soils under different types of land cover. Depth (cm)

Pasture (~2500 m)

HF-soluble (% of TOC) 0–5 24.10 5–10 27.20 10–15 10.87 15–20 14.52 20–25 39.51 25–30 19.48 30–40 24.80 40–50 21.98 50–60 14.67 60–80 57.47 80–100 51.71

Mixed cover (~2200 m)

Chirpine (~950 m)

Agriculture (~950 m)

32.21 66.02 43.73 35.31 80.76 55.48 74.72 48.91 53.39 66.12 52.86

49.65 44.37 46.64 17.97 50.84 45.55 43.75 41.40 45.30 28.55 26.12

30.69 42.52 29.11 30.37 10.12 26.16 21.57 22.95 12.44 37.41 48.16

dung) to the land, as a traditional practice in the study region, during each cropping season by the local people. Few studies have reported that the wide range of C:N ratios in soil is due to the accumulation of more microbial-produced carbohydrates, proteins and amino acids

40.00

a

35.00

Total FeDCB (g kg-1)

30.00 25.00 20.00 15.00 10.00 5.00 0.00

14

Pature land (~2500 m) 12

b

Mixed cover (~2200 m)

Total AlDCB (g kg-1)

Chirpine (~950 m) 10

Agricultural land (~950 m) 8 6 4 2 0 0-5

5-10

10-15

15-20

20-25

25-30

30-40

40-50

50-60

60-80

80-100

Depth (cm) Fig. 5. Total FeDCB (g kg

−1

) (a) and AlDCB (g kg

−1

) (b) content in the soils at different depths under different types of land cover. 381

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0-5

Agricultural land (~950 m) Chirpine (~950 m) Mixed cover (~2200 m) Pasture land (~2500 m)

5-10 10-15

Depth (cm)

15-20

a

20-25 25-30 30-40 40-50 50-60 60-80 80-100 0

1

2

3

4

5

6

7

8

9

10

FeDCB/OC

0-5

b

5-10 10-15

Depth (cm)

15-20 20-25 25-30 30-40 40-50 50-60 60-80 80-100 0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

AlDCB/OC Fig. 6. FeDCB/OC (a) and AlDCB/OC ratio (b) of soils at different depths under different types of vegetation cover.

soils under different types of land cover (Fig. 3) are comparable to the δ13C values of the C3 (−35‰ to −20‰) plant species groups (Ehleringer et al., 2000; Wang and Hsieh, 2002). The enrichment of δ13C values with increasing soil depths under mixed, chirpine and agricultural land covers (Fig. 3) is due to the isotopic fractionation associated with organic matter decomposition (Ehleringer et al., 2000; Wynn et al., 2006). The correlation between δ13C and ln(C/CO) of mixed land cover (Fig. 4b) confirmed that the 13C enrichment in soils was due to soil organic matter decomposition. Under pastureland land cover, depletion of the δ13C value was observed with increasing soil depth (Figs. 3 and 4a). The depletion of δ13C values in soils under pasture land cover may be due to the selective utilization of organic compounds in the litter by microbial decomposition, plant community (C3 or C4) shifts and mixing of young and old carbon in the soil profiles (Ehleringer et al., 2000; Wynn et al., 2006).

maximum amount of carbon in the soils compared with the lower altitude land cover types in the central Himalayan region. According to Minasny et al. (2017), soils with a lower carbon density will have higher carbon sequestration potential in the near future if they are properly managed. In the present study, soils under chirpine and agricultural land cover stored very low amounts of carbon compared with the higher altitude land cover types (Table 2). They represent potential sites for the sequestration of carbon in soils in the near future if they are properly managed by the forest and local administrators.

4.2. Influence of land cover types on stable carbon isotopes (δ13C) in soils Stable carbon isotopes (δ13C) are useful tracers in the study of the soil carbon cycle in any ecosystem (Ehleringer et al., 2000; Wang and Hsieh, 2002). Terrestrial plant communities are divided into C3, C4 and CAM (Crassulacean acid metabolism) plant photosynthetic communities (Boutton et al., 1998; Ehleringer et al., 2000). The variation in δ13C values of litter and different depths of soils under different types of land cover is due to changes in 13C values of the atmosphere, litter decomposition by microbial communities, selective utilization of organic compounds in the litter by microbial decomposition, plant community (C3 or C4) shifts and mixing of young and old carbon in the soil profiles (Ehleringer et al., 2000; Mehta et al., 2013). The δ13C values of

4.3. HF-soluble carbon and dithionite-extractable Fe/Al oxide concentrations and their relationships with soil δ13C values, FeDCB/OC and AlDCB/OC The dissolution of minerals by HF treatment will release the amounts of organic carbon compounds that are old as well as adsorbed organic carbon on mineral surfaces (Eusterhues et al., 2003; Schöning 382

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a

FeDCB

6 4

0 -2

40-50 25-30 50-60 30-40 15-20 C:N 10-15 20-25 FeDCB/OC

AlDCB/OC

5-10 0-5

80-100 60-80

MAt N SOC

-4

HF-Soluble carbon

-6 -8

-6

-4

-2

0

2

4

6

HF-Soluble carbon

F3 (11.31 %)

F2 (17.61 %)

AlDCB

2

b 8 7 6 5 4 3 2 1 0 -1 -2 -3 -4

8

C:N 30-40 20-25 60-80 25-30 80-100 FeDCB/O 40-50 C 50-60 AlDCB/O CAlDCB FeDCB

3 2

AlDCB/OC C:N AlDCB HF-Soluble 20-25 carbon 25-30

50-60

1 0

10-15

30-40

-1

FeDCB/OC 60-80

-2

F2 (19.14 %)

F2 (29.15 %)

40-50

SOC

15-20

N MAt0-5

5-10

80-100

-3 -5

-4

-3

-2

-1

SOC0-5 N

F1 (61.81 %)

c FeDCB

10-15 15-20

MAt

-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8

F1 (73.81 %)

4

5-10

0

1

2

3

5 4 3 2 1 0 -1 -2 -3 -4

N 5-10 MAt SOC 0-5 15-20 1…

25-30 20-25

AlDCB/OC AlDCB 80-100 60-80 FeDCB/OC 40-50 FeDCB 30-40 50-60

C:N

-5 -4 -3 -2 -1 0

4

d

HF-Soluble carbon

1

2

3

4

5

6

F1 (63.10 %)

F1 (45.09 %)

Fig. 7. Principal component analysis (PCA) shows the soil properties related to different soil depths of a) pasture b) mixed c) chirpine and d) agricultural land cover.

have great potential for the stabilization of carbon in subsurface soils. The higher concentration of AlDCB oxides in the soils of pasture land (Fig. 5b) and the relationship between AlDCB/OC and δ13C values clearly suggest that AlDCB over FeDCB oxide minerals may control the stabilization of organic carbon in subsurface layers. Thus, FeDCB and AlDCB oxides play a pivotal role in stabilization of organic carbon under mixed and pasture land covers situated at higher altitudes in the central Himalayan region. With other types of land cover, we found evidence of higher contents of HF-soluble carbon and FeDCB/AlDCB oxides in deeper soil layers (> 30 cm) compared with surface soil layers (Table 6 and Fig. 5a, b). The weak relationship between HF-soluble carbon and FeDCB/AlDCB oxides under pasture, chirpine and agricultural land covers clearly indicated the adsorption of organic carbon to mineral surfaces mainly depended on the addition of fresh carbon via dissolved organic carbon, root exudates and the interaction mechanism between organic compounds and other minerals in the soils (Kaiser and Guggenberger, 2000; Guggenberger and Kaiser, 2003). In an earlier study, Wiseman and Püttmann (2006) reported that irrespective of the presence of Fe/Al oxides in soils, the interaction with organic matter may determine the stabilization of carbon in the soils. Gu et al. (1994) reported that ligand exchange of the carboxyl/hydroxyl group of organic matter with iron oxide surfaces under a slightly acidic pH is the dominant interaction mechanism. Eusterhues et al. (2003) have proposed that the high levels of organic matter in surface layers in comparison to Fe oxides may not show any relationship due to the high organic carbon loading on Fe oxides and/or other minerals for organic matter stabilization. The increasing trend of FeDCB/OC and AlDCB/OC while increasing the soil depth under different types of land cover (Fig. 6a, b) confirmed the above findings. Additionally, the PCA results clearly showed that FeDCB/AlDCB, FeDCB/OC, AlDCB/OC and HF-soluble carbon ere closely associated with deeper soil layers under different types of land cover (Fig. 7a, b, c, d). The results of our study clearly suggest that the

and Kögel-Knabner, 2006; Kogel-Knabner et al., 2008), which are considered the stable pool of carbon in soils. In the present study, carbon loss after HF treatment ranged from 10 to 81% of the total carbon in the surface and sub-surface layers of soils (Table 6). The present study results are in accordance with other studies showing greater carbon loss in subsurface soil layers due to HF treatment (Eusterhues et al., 2003, 2005; Schöning and Kögel-Knabner, 2006). Previous studies have demonstrated a correlation between HF-soluble carbon and 14C activity and δ13C values of soil profiles (Trumbore, 2000; Eusterhues et al., 2003; Schöning and Kögel-Knabner, 2006), and they have also confirmed the old carbon associated with minerals in deeper soil profiles. However, some studies have observed HF-soluble carbon fraction in soils consisting of young and old organic components (Swanston et al., 2005; Eusterhues et al., 2005; Tan et al., 2007). The strong relationship between δ13C values and HF-soluble carbon under a mixed land cover (Fig. 4c) confirmed that the subsurface soil layers contained more old carbon that was probably attached to minerals. However, this trend was not noticed with other types of vegetation cover. The weak relationship between δ13C values and HF-soluble carbon with other soil land cover profiles could be attributed to the continuous addition of new carbon via dissolved organic carbon (DOC), root exudates and/or soil carbon mixing (Kaiser and Guggenberger, 2000; Guggenberger and Kaiser, 2003; Eusterhues et al., 2003). Fe and Al oxides play a major role in stabilization of carbon in subsurface soils (Kiem and Kogel-Knabner, 2002; Kaiser et al., 2002; Eusterhues et al., 2003; Zhao et al., 2016). The values determined for FeDCB and AlDCB oxides in the present study were comparable to those reported in previous studies (Eusterhues et al., 2003, 2005; Wiseman and Püttmann, 2006). The higher amounts of FeDCB and AlDCB in soils under mixed and pasture land covers at higher altitudes (Fig. 5a, b) would provide a significant surface area for organic carbon sorption and lead to carbon stabilization. Hence, higher altitude land cover soils 383

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mechanism with organic matter may control the stabilization of organic carbon in the central Himalayan region. Overall, our results suggest that the vegetation cover at lower and higher altitudes may be considered as a potential sink for the sequestration of atmospheric carbon and as potential sites for the stabilization of sequestered carbon in the soils of central Himalaya. However, the soil erosion, excess levels of root exudate release into the soil due to climate change and land use management activities in the Himalaya might affect the sequestration of carbon in the soils and in turn affect the mineral-protected carbon. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.catena.2018.06.039.

stabilization of organic carbon at different depth of soils under different types of vegetation cover in the central Himalayan region may depend on the fresh carbon supply from belowground roots, the amount of Fe/ Al oxide minerals and their interaction mechanisms (Gu et al., 1994; Kaiser and Guggenberger, 2000; Guggenberger and Kaiser, 2003). Nevertheless, further research examining the interaction mechanism between FeDCB/AlDCB oxides and organic carbon in soils under different types of land cover at lower altitudes of Himalayan ecosystems should be pursued to acquire a better understanding. 4.4. Importance of HF-soluble or mineral-protected SOC and their potential feedback in response to climate change

Acknowledgements Multiple findings from earlier studies suggest that the highly humified organic carbon attached to mineral oxides, especially in deeper soil layers, is not accessible to microbes for further mineralization and is stabilized for a longer time (Eusterhues et al., 2005; Jiménez and Villar, 2017). In the present study, the average proportion of mineralprotected organic carbon (HF soluble) in the total SOC stock (up to 100 cm) was 28, 55, 40 and 28% for pasture, mixed, chirpine and agricultural land covers, respectively. The elevated atmospheric CO2 is projected to increase the quantity and composition of root exudates into soils and in turn impact the mineral-protected organic carbon via an alternative mechanism of the priming effect (Keiluweit et al., 2015). Here, we showed that a large amount of SOC (stable carbon) accumulated under higher altitude land cover types in central Himalaya and that the mineral-protected SOC may be vulnerable to severe loss if higher amounts of the fresh carbon supply (root exudates) are released into soils due to climate change (Keiluweit et al., 2015). An earlier study (Singh et al., 2011) has shown that the SOC stock in western Himalaya is significantly affected by land use changes and climate. Furthermore, soil erosion is a significant degradation process, reducing the SOC pool via soil runoff and deposition during the monsoon season in the Himalaya (Sitaula et al., 2004; Longbottom et al., 2014). Therefore, degradation of the sequestered SOC pool potentially occurs in the Himalayan region via multiple factors associated with the climate change phenomenon. Thus, quantification of mineral-protected organic carbon and the role of Fe/Al oxide minerals under different types of land cover at different altitudes and in different ecosystems should be a high priority to prepare a relevant soil management practice to maintain and/or enhance the mineral-protected organic carbon in soils to combat climate change. Since assessing the mineral protected carbon (HF-soluble carbon) and the role of Fe/AlDCB oxides on the stabilization of organic carbon in soils under different types of land cover at different altitudes of the Himalayan region is poorly understood, the findings of this study open a new vista for understanding the role of altitude-based land cover types and Fe/AlDCB oxides on SOC stabilization.

Authors are thankful to SERB, DST, New Delhi for financial assistance through the DST fast track scheme for young scientist (SR/FT/LS59/2012), UGC-DSK-PDF (BSR/BL/16-17/0146) and University of Delhi under strengthened R&D program of Faculty at University of Delhi. Authors are thankful to Dr. R.K. Maikhuri for helping us to select the study sites. Authors are thankful to Mr. Benjamin Harlow at the stable isotope core laboratory, School of Biological sciences, Washington State University, Pullman, WA, USA for stable isotope analysis. Logistic support from Head, Department of Botany, University of Delhi is thankfully acknowledged. References Accoe, F., Boeckx, P., Cleemput, O.V., Hofman, G., Zhang, Y., Li, R.H., Guanxiong, C., 2002. Evolution of δ13C signature related to total carbon contents and carbon decomposition rate constants in a soil profile under grassland. Rapid Commun. Mass Spectrom. 16, 2184–2189. Adam, G., Duncan, H., 2001. Development of sensitive and rapid method for the measurement of total microbial activity using fluroscein diacetate (FDA) in a range of soils. Soil Biol. Biochem. 33, 943–951. Adhikari, D., Yang, Y., 2015. Selective stabilization of aliphatic organic carbon by iron oxide. Sci. Rep. 5, 11,214. http://dx.doi.org/10.1038/srep11214. Baldock, J.A., Skjemstad, J.O., 2000. Role of the soil matrix and minerals in protecting natural organic materials against biological attack. Org. Geochem. 3 (1), 697–710. Balieiro, F.C., Benites, V.M., Caiafa, A.N., Alves, B.J.R., Fontana, A., Canellas, L.P., 2012. Vegetation influence on organic matter source of black soils from high altitude rocky complexes traced by 13C and 15N isotopic techniques. Catena 99, 97–101. Batjes, N.H., 1996. Total carbon and nitrogen in the soils of the world. Eur. J. Soil Sci. 47 (2), 151–163. Boutton, T.W., Archer, S.R., Midwood, A.J., Zitzer, S.F., Bol, R., 1998. δ13C values of soil organic carbon and their use in documenting vegetation change in a subtropical savanna ecosystem. Geoderma 82, 5–41. Brady, N., Weil, R.R., 2002. The Nature and Properties of Soils, 13th ed. Prentice-Hall, Upper Saddle River. NJ, pp. 960. Chhabra, A., Palria, S., Dadhwal, V.K., 2003. Soil organic carbon pool in Indian forests. For. Ecol. Manag. 173, 187–199. Cools, N., Vesterdal, L., De Vos, B., Vanguelova, E., Hansen, K., 2014. Tree species is the major factor explaining C:N ratios in European forest soils. For. Ecol. Manag. 311, 3–16. Coplen, T.B., 1994. Reporting of stable hydrogen, carbon, and oxygen isotopic abundances. Pure Appl. Chem. 66, 273–276. Coplen, T.B., Brand, W.A., Gehre, M., Gröning, M., Meijer, H.J., Toman, B., Verkouteren, R.M., 2006. New guidelines for δ13C measurements. Anal. Chem. 78, 2439–2441. Dinakaran, J., Krishnayya, N.S.R., 2008. Variations in type of vegetal cover and heterogeneity of soil organic carbon in affecting sink capacity of tropical soils. Curr. Sci. 94, 1144–1150. Dinakaran, J., Krishnayya, N.S.R., 2010. Variations in soil organic carbon and litter decomposition across different tropical vegetal cover. Curr. Sci. 99 (8), 1051–1060. Diochon, A., Kellman, L., Beltrami, H., 2009. Looking deeper: an investigation of soil carbon losses following harvesting froms managed northeastern red spruce (Picea rubens Sarg.) forest chronosequence. For. Ecol. Manag. 257, 413–420. Ehleringer, J.R., Buchmann, N., Flanagan, L.B., 2000. Carbon isotope ratios in belowground carbon cycle processes. Ecology 10 (2), 412–422. Eswaran, H., Berg, E.V.D., Reich, P., 1993. Organic carbon in soils of the world. Soil Sci. Soc. Am. J. 57, 192–194. Eusterhues, K., Rumpel, C., Kleber, M., Kogel-Knabner, I., 2003. Stabilization of soil organic matter by interactions with minerals as revealted by mineral dissolution and oxidative degradation. Org. Geochem. 34, 1591–1600. Eusterhues, K., Rumpel, C., Kogel-Knabner, I., 2005. Stabilization of soil organic-matter isolated via oxidative degradation. Org. Geochem. 36, 1567–1575. Fontaine, S., Mariotti, A., Abbadie, L., 2003. The priming effect of organic matter: a question of microbial competition? Soil Biol. Biochem. 35, 837–843. Fontaine, S., Barot, S., Barre, P., Bdioui, N., Mary, B., Rumpel, C., 2007. Stability of organic carbon in deep soil layers controlled by fresh carbon supply. Nature 450, 277–280. Fransson, P., 2012. Elevated CO2 impacts ectomycorrhiza-mediated forest soil carbon

5. Conclusions The variations in altitude and types of land cover have a significant effect on the soil carbon stock, nitrogen content, HF-soluble carbon concentration, and FeDCB and AlDCB contents in the central Himalayan region. The present study confirms the hypothesis that soils under higher altitude land cover store larger amounts of stable and/or mineral-associated carbon than those under lower altitude land cover types. The greater amounts of FeDCB and AlDCB oxides in soils under mixed and pasture land cover at higher altitudes may provide a significant surface area for organic carbon sorption and have great potential for the stabilization of carbon in subsurface soils. In addition, the relationship between δ13C values and HF-soluble carbon, FeDCB/OC and δ13C, AlDCB/OC and δ13C in soils under mixed and pasture land covers confirms that the subsurface soil constitutes more decomposed carbon attached to Fe/AlDCB minerals. Variations in HF-soluble carbon and FeDCB and AlDCB oxides at different soil depths under different types of land cover clearly suggest that the pedon-level specific interaction 384

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