Soil carbon pools and carbon management index under different land use systems in the Central Himalayan region

Abstract

We investigated different land use and cropping systems in the Almora region of Central Himalaya to assess total organic carbon (TOC), microbial biomass carbon (MBC), particulate organic carbon (POC), labile carbon (LC), microbial quotient (MQ) (i.e., ratio of MBC to TOC) and measured the carbon management index (CMI). The TOC content recorded the highest value in undisturbed forest (45.4 g kg⁻¹ soil) and lowest in barren land (18.4 g kg⁻¹ soil). The MBC values varied from 146 mg kg⁻¹ in barren land to 783 mg kg⁻¹ in undisturbed oak forest. Land under organic farming showed higher LC values (4.0 g kg⁻¹) than soya bean–wheat and fodder crops. The average POC values ranged from 0.9 g kg⁻¹ in barren land to 11.0 g kg⁻¹ in undisturbed oak forest. Variation of these parameters with season and depth was also observed. The CMI was highest under the forest ecosystem and lowest in barren land. Our study thus revealed that cultivation of Himalayan soils has significantly reduced the soil organic carbon pools and thus maintenance of natural forest or eco-friendly practices such as inclusion of legumes and application of organics is urgently needed for sustainable use of these ecosystems.

Keywords:

• Land use systems
• microbial biomass carbon
• particulate organic carbon
• labile carbon
• microbial quotient
• carbon management index
• Himalayan region

Introduction

Increasing anthropogenic disturbances of the Himalayan mountains have become a major cause of soil degradation and depletion of carbon stocks. Therefore, assessing soil quality of the region is of paramount importance in developing potential strategies for restoration and improving agricultural sustainability in the region. Soil organic matter (SOM) is a major determinant of sustainability of agricultural systems and changes in the quality of SOM can occur in both total and active, or labile C pools (Blair et al., 1995).

Labile carbon (LC) is the fraction of soil organic carbon with most rapid turnover times and its oxidation drives the flux of CO₂ between soils and atmosphere. Labile organic matter fractions include microbial biomass C, particulate organic matter, readily mineralizable C, easily extractable C and carbohydrates (Haynes, 2005). Among different management practices conservation tillage and agro forestry system are believed to offer high potential for increasing soil organic carbon (SOC) storage in agricultural soils (Lal, 2002, Luo et al., 2010). In this region, most of the previous studies focused on fertility evaluation and crop yields in relation to soil, water and fertilizer management, but how the land use and cropping systems affect the pools of SOC in the Himalayan region is not well studied. The objectives of the present investigation were to assess the depth wise distribution of various soil C fractions under different land use and cropping systems and the effect of season on them. We have also worked out the carbon management indices at the ecosystem level. The information generated are expected to be useful in developing strategies for better management of soil C in the Himalayan and similar mountain ecosystems.

Materials and methods

Study site

Five different and contrasting land use systems were selected for the study (Table I). Of this, four systems (organic farming, soya bean–wheat, fodder crops and barren land) are located at the experimental farm of Vivekananda Institute of Hill Agriculture, located in the Indian Himalayan region of Uttarakhand, India. The climate is sub-temperate, characterized by moderate summer (May–June), extreme winter (December–January) and general dryness, except during the southwest monsoon season (June–September). The undisturbed Oak (Quercus incana) forest system is a part of Binsar wildlife sanctuary in the state of Uttarakhand.

Table I. Some soil properties at the time of sampling in July 2009.

Land use and cropping systems	Location	Description	pH	EC (dS m^−1)	Sand (%)	Silt (%)	Clay (%)	N (%)	Available K (mg kg^−1)	TOC (g kg^−1 soil)
Oak (Quercus incana) forest	Corbet National Park, Binsar 29°39′N 79°07′E. Altitude 2400 m above sea level	Natural vegetation predominantly under oak trees	5.7	0.11	55	29	16	0.24	377.8	43.5
Fodder crops	Hawalbagh 29°36′ N longitude 70°40′ E latitude. Altitude 1250 m above sea level	Fodder crops with maize and cowpea for more than 10 years	5.9	0.05	49	27	24	0.13	112	28.8
Soya bean–wheat	Hawalbagh 29°36′ N longitude 70°40′ E latitude. Altitude 1250 m above sea level	Soya bean–wheat systems for more than 10 years	6.0	0.20	44	37	19	0.27	324	21.7
Organic farming	Hawalbagh 29°36′ N longitude 70°40′ E latitude. Altitude 1250 m above sea level	Field with organic manures under vegetables for more than 10 years	6.2	0.25	58	31	11	0.27	298	27.5
Barren land	Hawalbagh 29°36′ N longitude 70°40′ E latitude. Altitude 1250 m above sea level	Barren land mostly denuded with sparse vegetation	6.1	0.13	52	32	16	0.12	189	17.4

Soil and weather characteristics

Some of the soil properties at the time of first sampling (July 2009) are presented in Table I. During the two sampling periods (summer and winter), the temperature and rainfall conditions were found to show contrasting values. During July, the temperature was found to be 30.1°C (maximum), 20.9°C (minimum) and in January, 20.3°C (maximum) and −0.3°C (minimum). Rainfall also varied between the two sampling months: 137.5 mm in July (summer) and 16.5 mm in January (winter).

Soil sampling

The soil samples of three different depths (0–5, 5–10 and 10–15 cm) were collected from the five different land use and cropping systems in July 2009 and January 2010. Three composite surface soil samples were collected from each site for each depth. For making one composite sample, at least five soil cores were collected and pooled. Pseudo-replication approach of sampling for ecosystem studies has also been adopted by other workers (Patra et al., 2005). All chemical results are means of triplicate analysis and are expressed on the oven-dry basis. Soil moisture was determined after drying at 105°C for 24 h.

Soil analyses

The total organic carbon (TOC) was determined by wet oxidation diffusion method (Snyder & Trofymow, 1984) and microbial biomass carbon (MBC) by fumigation extraction method given by Horwath and Paul (1994). The particulate organic carbon (POC) was determined following the procedure outlined by Cambardella and Elliot (1992). The amount of carbon in soil which is oxidizable by 333 mM KMnO₄ is considered as the LC which was determined by following the procedure of Blair et al. (1995). Microbial quotient (MQ) was measured as the ratio of biomass C to total soil organic C (Chhonkar et al., 2007). Carbon Management Index (CMI) was calculated using the procedure given by Blair et al. (1995).

Soil in the barren land was taken as the reference sample.

Statistical analysis

Analysis of variance (ANOVA) and Duncan's multiple range test (DMRT) for comparison of means were performed using software SAS 9.1.3.

Results and discussion

Total organic C

Highest accumulation of TOC was observed in case of undisturbed oak forest while barren land showed the lowest value (Table II). During summer season, the average TOC content in the undisturbed natural forest was 43.5 g kg⁻¹ soil, while that of barren land was 17.4 g kg⁻¹ soil. All the land use systems showed higher accumulation of soil organic C in the 0–5 cm depth and then decreased with increasing depth. Higher TOC values were observed in winter season. In winter, the highest average TOC content was found in the undisturbed forest (47.7 g kg⁻¹ soil), while barren land showed the lowest value (19.4g kg⁻¹ soil) (Table II). In both seasons, fodder system contained higher amount of TOC than the organic farming and soya bean–wheat systems. TOC content of soya bean–wheat system in the 0–15 cm was found to be significantly lower than all the other systems except barren land. After considering the factors of season and depth, the undisturbed forest was found to maintain the highest TOC content irrespective of the season and depth differences.

Table II. Seasonal changes on total organic C (TOC), microbial biomass C (MBC), particulate organic C (POC), and KMnO₄ oxidizable C (i.e., labile) at different depths under different land use systems in the Central Himalayan region. The means followed by different letters are significantly different at p < 0.05, according to Duncan's multiple range test (DMRT) of means.

	TOC (g kg^−1)			MBC (mg kg^−1)			POC (g kg^−1)			KMnO4 – C (g kg^−1)
	Depth (cm)			Depth (cm)			Depth (cm)			Depth (cm)
Land use and cropping systems	0–5	5–10	10–15	0–5	5–10	10–15	0–5	5–10	10–15	0–5	5–10	10–15
Summer sampling
Organic farming	30.8^b,c	26.8^c,d,e	24.8^c,d,e,f	694^c	683^c	666^c	6.7^d	6.3^e	6.3^e	4.8^c,d	3.8^c,d,e	3.6^c,d,e
Soya bean–wheat	25.5^c,d,e	21.7^d,e,f,g	18.1^f,g,h	483^f	454^f,g	433^g	2.7^g	2.7^g	2.7^g	3.5^c,d,e	3.6^c,d,e	2.9^c,d,e
Fodder crop	34.1^b	28.2^b,c,d	24.3^c,d,e,f	624^d	577^e	598^d,e	5.5^f	5.2^f	5.2^f	3.7^c,d,e	2.8^c,d,e	3.0^c,d,e
Oak forest	45.8^a	43.2^a	41.6^a	923^a	871^b	848^b	11.0^a	10.4^b	10.1^c	23.2^a	10.3^b	5.3^c
Barren land	20.9^e,f,g,h	16.7^g,h	14.6^h	188^h	146^i	130^i	1.0^h	0.8^h	0.7^h	2.2^d,e	1.7^e	1.5^e
Winter sampling
Organic farming	32.0^c,d	27.4^d,e,f	25.4^e,f	507^d	477^e	456^e,f	7.2^c	7.0^c,d	7.0^c,d,e	4.5^c	3.7^c	3.5^c
Soya bean–wheat	27.0^d,e,f	22.5^f,g	19.5^g,h	376^h	352^h	316^i	3.0^f	2.7^f	2.7^f	3.5^c	3.5^c	2.8^c
Fodder crop	36.7^c	32.1^c,d	30.0^d,e	475^e	421^g	433^f,g	6.4^c,d,e	5.9^e	6.2^d,e	3.7^c	3.2^c	3.4^c
Oak forest	49.5^a	47.8^a,b	44.1^b	722^a	681^b	653^c	12.1^a	11.7^a	10.6^b	21.0^a	9.2^b	7.9^b
Barren land	23.1^g	19.9^g,h	15.2^h	171^j	128^k	111^k	1.1^g	1.0^g	0.7^g	2.0^c	1.7^c	1.4^c

The higher TOC content in the forest soil is due to large annual addition of organic matter in the form of leaf litter, which remains in the soil due to the absence of any disturbance. The low temperature condition due to the high altitude, prevalent in the ecosystem may result in reduced or slower rate of residue decomposition, which adds to higher carbon values (Haynes, 2005). A slower rate of SOM decomposition in these systems and the absence of annual tillage contribute to their accumulation (Haynes, 2005). The higher carbon content in the fodder system in comparison to other systems may be due to the higher clay content, which preserves the organic matter within the aggregates. The higher TOC contents observed in winter may also be due to the very low temperature, nearly freezing conditions prevailing in the area continuously for four months. This low temperature condition reduces the microbial activity in the soil and mineralization or decomposition of organic matter, and thus the organic matter present in soil is much more preserved when compared to the summer conditions.

Microbial biomass carbon

Like TOC, greatest accumulation of MBC was observed in undisturbed natural forest while barren land showed the lowest value (Table II). During summer season, the MBC content in the undisturbed natural forest was 881 mg kg⁻¹ soil, while that of barren land was 155 mg kg⁻¹ soil. Higher accumulation of MBC was observed in the 0–5 cm depth. In case of organic farming, the MBC values were found to be statistically at par in all the depths during summer. MBC recorded lower values in the winter season, being highest in the undisturbed forest (685 mg kg⁻¹ soil), while barren land showed the lowest value (137 mg kg⁻¹ soil). In both seasons, organic farming system was found to contain significantly higher amount of MBC than the fodder and soya bean–wheat systems. The soya bean–wheat system showed similar MBC values in 0–5 and 5–10 cm depths, in winter as well as summer. The MBC values under different land use systems in the 0–15 cm soil depth can be arranged in the order: forest > organic farming > fodder > soya bean–wheat > barren land.

Soil management practices and land use patterns strongly affect the size of the microbial biomass pool. Our results are in agreement with earlier studies, where agricultural practices promoting SOM accumulation have increased amounts of labile organic matter (Haynes, 2000; Mandal et al., 2007). Such increases are probably the result of organic matter added by roots, root exudation/secretion, biomass production in the rhizosphere in the rotations including pastures; and to a lesser extent to less soil tillage management. The readily metabolizable carbon and N in organic manure in addition to increasing root biomass and root exudates due to greater crop growth are the most influential factors contributing to the biomass increase (Masto et al., 2006). The higher MBC values observed in the undisturbed forest can be attributed to the relatively continuous and more organic matter deposition via leaf litter. The surface soil exhibited higher MBC compared to lower soil depths primarily because of addition of leftover crop residues and root biomass into the topsoil. The arid, low moisture, nearly freezing low temperature conditions prevailing in the area during winter may reduce the proliferation and activity of microbes in soil, thus results in lower MBC values.

Particulate organic carbon

The POC fraction also varied significantly among different systems. In summer, the average POC values ranged from 0.84 g kg⁻¹ in barren land to 10.48 g kg⁻¹ in case of undisturbed forest (Table II). All the land use systems showed higher values of POC in the 0–5 cm depth. In undisturbed forest, POC values increased from 10.48 g kg⁻¹ in summer to a higher value of 11.46 g kg⁻¹ in winter season. In both seasons, organic farming system was found to contain a significantly higher amount of POC than the fodder and soya bean–wheat systems. POC values of soya bean–wheat system in the 0–15 cm were significantly lower than all the other systems except barren land. Thus, the POC values under different land use systems in the 0–15 cm soil depth followed the order: forest > organic farming > fodder > soya bean–wheat > barren land.

The high POC values under the undisturbed forest were in accordance with the observations of Wander et al. (1998) that land management systems with minimum soil disturbance can lead to an accumulation of POC. Also the tannins and lignin formed from the decomposition of oak forest residues may protect the carbon from rapid decomposition and thus preserve it in the aggregates. As POC is mainly a physically protected organic fraction, it was exposed to soil disturbance in the surface layers. This was supported by Six et al. (1998) that the soil disturbances like tillage could decrease POC. In all the five land use systems, POC accounted for 4% (barren land) to 28% (forest) of the TOC at 0–15 cm depth, in both the seasons. The smaller proportions of POC to SOC in our study were probably related to the warm and wet subtropical climate (July) which is highly favored for biological decomposition of recent organic material inputs, leading to less accumulation of POC in summer (Chen et al., 2004).

Labile carbon

The different systems have revealed significant variations in LC fractions. In summer, it ranged from 1.8 g kg⁻¹ in barren land to 12.9 g kg⁻¹ in undisturbed forest (Table II). Organic farming system contained significantly higher LC values than fodder system, whereas, values of organic farming and soya bean–wheat systems were at par. Only forest system recorded a significant decrease in the LC values with successive depths. LC values of all the systems were found to be higher in the summer. During winter season, the highest average LC content was found in the undisturbed forest (12.7 g kg⁻¹ soil), while barren land showed the lowest value (1.7 g kg⁻¹ soil). Organic farming system was found to contain a higher amount of LC than the soya bean–wheat system and fodder system. The seasonal fluctuations of LC values were not significant. If season and depths are compared, LC values under different systems can be arranged as: forest > organic farming > fodder ≈ soya bean–wheat > barren land.

Labile pools of organic C are more readily influenced by management practices than the recalcitrant pools (Biederbeck et al., 1994). The LC values ranged from 10.4 to 51.9% of total organic C in the various land use systems under study. High organic matter additions and fertilizer treatments in multiple cropping systems significantly enhanced the root biomass yield which might be responsible for increased LC in soil. Nevertheless, roots are also reported to exude carbon compounds that are labile in nature (Contech et al., 1997). The high values of LC in forest can be attributed to the constant supply of easily decomposable leaf litter throughout the year and high MBC values. The seasonal effect on the LC values was found to be very negligible. The lower values of labile C in the cropping systems can be associated with aggregate disruption and greater organic matter oxidation in conventional agricultural systems based on ploughing and harrowing (Bayer et al., 2006).

Microbial quotient

The MQ varied significantly between different systems investigated (Figure 1). In summer, it varied from 0.89% in barren land to 2.52% in organic farming system, which is 183% higher over barren land. Seasonal effect on MQ was found to be significant in all the systems and recorded much lower values during winter. In both the seasons, organic farming was found to maintain the highest values, over rest of the land uses. The MQ values under different land use systems in the 0–15 cm soil depth followed the order: organic farming > soya bean–wheat ≈ fodder ≈ forest > barren land

Figure 1.  Effect of different land use and cropping systems on microbial quotient in summer and winter seasons. The bars with different letters are significantly different at p < 0.05, according to DMRT.

The values pertaining to MQ (MBC/SOC) were within the range of 1–4% as reported earlier (Brookes et al., 1984), except in case of barren land (Figure 1). The higher conversion to microbial biomass suggests better stability of organic C in organic farming system. MBC generally comprises 1–5% of the SOC (Masto et al., 2006); in the present study, the amount of MBC was within this range, and the lowest and the highest values of MQ being associated with barren land and organic farming plots, respectively.

Carbon management index

The CMI did not vary significantly among all the systems, even though the total C contents varied significantly in the soils under different land use and cropping systems (Figure 2). CMI showed similar trend in both the seasons i.e., summer and winter. In summer and winter, forest system was found to have the highest value followed by organic farming, soya bean–wheat system, and fodder system. In winter also CMI values followed the same trend i.e., forest > organic farming > soya bean–wheat ≈ fodder > barren land. Unlike other parameters in the study, the CMI values did not fluctuate conspicuously among the systems (Figure 2).

Figure 2.  Effect of different land use and cropping systems on the CMI.

The CMI was designed to give an indication of the C dynamics of the system. Although total C varied significantly among all the different land use systems, CMI showed a dissimilar trend in the present study (Figure 2) except in case of forest ecosystem. The value itself is not important but the differences reflect how different land uses are affecting the systems (Blair et al., 1995). The regular addition of organic matter in case of forest and organic farming systems proved increased potential to increase the CMI by increased inputs and lower losses (Blair, 2000). Carbon compounds particularly the more labile fractions provide energy for soil organisms and stimulate their activity, which contributes to nutrient release from plant and animal residues and the synthesis of humic substances that affect both soil physical and chemical fertility.

Acknowledgements

The first author is thankful to IARI, New Delhi, and Government of India, for financial support for carrying out this work.