4.1 Effect of different AFS on total soil carbon (TOC), TOC Stocks, carbon fractions and carbon pools
The different AFS were having significant (p < 0.05) importance in determining the soil carbon across the soil profile (up to 0.30cm).In a study of more than 2500 sites in Germany, showed that the carbon content in surface soil and subsurface soils were determined by land use, land-use history and clay content, while carbon content in the deeper soil profiles are controlled by stratigraphy, parent material and relief (Vos et al. 2019). A variation in TOC and TOC stocks under different AFS was strongly linked with the species characteristics and their litter input patterns (Ramesh et al. 2015). In the perennial woody systems, different carbon forms are controlled by a complex interaction between the carbon inputs (leaf litter, twig branches etc.), carbon stabilization processes and carbon loss (decomposition) in the soil (Dhyani and Tripathi 2000; Lenka et al. 2012). Carbon dynamics in any system can also be related to the adoption of different management practices along with their past cropping patterns (Lal 2008). In the current study, Poplar based AFS had 10.8 to 23.6% higher (p < 0.05) TOC content and 7.4 to 18.8% higher TOC stock compared to other AFSs, which is in line with the previous study where Singh et al. (2023) reported poplar based AFS had 7 to 17.9% higher TOC content compared to other land uses. This could be due to the annual litter fall accumulation by poplar trees under poplar based AFS in Samastipur, Bihar, ranged from 2.46 (3 years) to 10.63 (9 years) Mg ha–1 yr–1 (Das and Chaturvedi 2005). Similarly, Pande et al. (2002) reported annual litter fall of Teak ranged 3.28 to 4.53 Mg ha–1 yr–1 however Rathore et al. (2013) reported a 10 years old Mango stand produced 1.46 Mg ha–1 yr–1leaf litter biomass. These studies supported that having higher amount of annual leaf litter fall in poplar compared to other AFS significantly improves the TOC content and TOC stocks in the soil across the surface and subsurface soils.
Among the different soil carbon fractions analysed, very labile carbon (F1), labile carbon (F2), less labile carbon (F3) and non-labile carbon (F4) constituted 38.5, 15.3, 20.3 and 25.9% of the total organic carbon (TOC) in the surface soils (0-15cm) and 34.1, 14.3, 22.1 and 29.4% of TOC in the sub surface soils (15–30), respectively. The contribution followed the trend F1 > F4 > F3 > F2, irrespective of different AFS across the soil profile (up to 30cm). The very labile carbon (F1) followed by non-labile carbon (F4) were higher across the different AFSs due to the changes in the quantities and qualities of carbon input through litter biomass, living organisms and root derivatives under the trees (Vesterdal et al. 2008). The dynamics of the different tree species also affect the litter production and responsible for carbon form variation. These organic inputs might enter the very labile carbon, act as a substrate for microorganisms and increase the quantity of labile carbon under trees (Wang et al. 2017). while the less labile carbon (F3) fraction was slightly higher compared to the labile carbon (F2) fraction due to the coarse root biomass or availability of material resistant to the easily decomposition (Cardinael et al. 2015). While the active pool (AP, labile carbon pool) contributed the slightly higher portion (51.1%) and passive pool (PP, non-labile carbon pool) contributed slightly lesser portion (48.9%) of the soil total organic carbon. It has been observed in several studies that the active pools are more susceptible to the management practices (i.e. tilling, ploughing, fertilization, irrigation, organic manuring etc) than the passive carbon pools and because of their rapid responses to environmental changes, they are identified as early indicators of soil quality (Thangavel et al. 2018; Sahoo et al. 2019; Ansari et al. 2022).
4.2 Effect of different AFS on basal respiration (BR), soil microbial biomass carbon (SMBC) and enzymatic activities
The soil MBC (SMBC), which typically makes up between 1to 4% of the TOC, might serve as a precursor to potential soil health degradation and/or aggravation caused by various management practises (Ramesh et al. 2015). Microbial biomass is very important in soils of the tropical and subtropical region as a pool of available nutrient for plant uptake (Santos et al. 2012). The SMBC content decreased with depth across the different AFSs due to the less availability of the leaf litter, nutrient status, ripened fruits, twigs, branches which improves nutrient status and SMBC contents in soils (Ahirwal et al. 2021; Santos et al. 2012) The SMBC content increased with TOC content but contrary Teak based AFS were marginally higher in the both soil depths (up to 0.30cm) and were comparable to the Poplar based AFS which have higher TOC content across the surface and sub-surface soils. This could be due to a better availability of nutrients due to the addition of higher plant quality litters and higher and more diverse soil MBC under tree species may be attributed directly to the quantity and quality of the litter as well as indirectly through changes in the physical and chemical properties of the soil (Dhyani and Tripathi 2000; Lenka et al. 2012). The poplar based AFS were 13.4 to 25.2% higher in the surface and 11.7 to 21.8% in the subsurface in basal respiration (BR) compared to different AFS. This suggests greater biomass in Poplar based AFS, which was followed by greater mineralization of litter (including dry biomass) and root biomass, resulting in higher CO2 flux rates (Ramesh et al. 2015). Because of increased inputs of plant wastes and litter, followed by a superior mineralization process, surface soil will have a higher rate of CO2 evolution than subsurface soil (Kaur et al. 2008). The enzyme activities (FDA, ALP and DHA) also were higher in the Teak based AFS and comparable to the Poplar based AFS while remaining AFS were significantly lesser in the enzyme activities.
The increased SMBC and soil enzyme activity result from the soil's higher C content, which gives microbe’s energy source (Singh et al. 2019) and indicated that the SMBC and soil enzymes are the good indicators for the soil quality (Bergstrom et al. 1998). Higher SMBC and soil enzymes are also related to increased addition of C-substrate and soil microorganisms through increased input of leaf litter and root biomass into the soils of the Poplar and Teak based AFSs (Heintz-Buschart et al. 2020; Singh et al. 2019). The surface and subsurface distribution of SMBC, BR and enzyme activities were also affected due to the decline in the leaf litter fall, plant residues, and fine root biomass of different agroforestry systems (Dhyani and Tripathi 2000). However, a persistent decline in soil C forms (TC, TOC, and C-pools) in deeper depths may be the cause of the decline in soil microbial biomass and enzymatic activities along with soil depths (Singh et al. 2019). As a result, similar to other places, the microbiological characteristics of the region may be taken into account in environmental risk assessments and models as indicators of ecosystem disruption brought on by land use change and management practises.
4.3 Effect of different AFS on microbial quotient (MQ), metabolic quotient (qCO 2 ) and carbon management index (CMI)
The microbial quotient (MQ) represents microbial activity. The values pertaining to MQ (MBC/SOC) were within the range of 1 to 4% as reported earlier (BRooKEs et al. 1984). MQ were comparable among different AFSs except the Semal based AFS. The Semal based AFS's soils' lowest MQ values show that the soil's ability to retain carbon has been compromised, indicating a decline in the soil's quality (Mandal et al. 2020). Due to the higher quantity and quality of plant residue that makes up the deposited organic material on surface soil, microbial activity in terms of basal respiration tends to be higher on the surface soils (0-15cm) compared to subsurface soils (15-30cm) irrespective of land uses which is in line with the Fiahlo et al. (2006) and Arevalo et al. (2010). The metabolic quotient (qCO2) was comparable among different AFSs. The soil microbial community's ability to utilise substrate is often measured using the qCO2 index. Low substrate utilisation of the soil microbial population is indicated by a high qCO2 level (Singh et al. 2023). The land use systems did not show significant variation in microbial quotients and metabolic quotients.
An integrated measurement of SOC's quantity and quality is offered by carbon management index (CMI). Whitbread et al. (1998) proposed CMI as a suitable method for describing soil fertility. CMI can be utilised as a more sensitive indication of the rate of change of SOC in response to soil management changes than a single measure like total SOC concentration. According to Blair et al. (1995), the ability of management practises to advance soil quality has been evaluated using the CMI. Poplar based AFS were significantly higher in the CMI compared to the other AFSs across the surface and sub-surface soils, this could be due to the amount of root breakdown and leaf litter exhibited by the different tree species of the agroforestry systems (Singh et al. 2023). Agroforestry practises encouraged higher CMI values, possibly as a result of changes in the quality of organic matter, such as the C/N ratio, contents of lignin, cellulose, hemicellulose, proteins, and carbohydrates, which altered the lability of C to KMnO4 oxidation. This improvement in organic matter formation may have been a result of the increase in annual C addition as well (Ramesh et al. 2015).
4.4 Principal Component Analysis (PCA)
The first and second main components comprised 64.89 and 17.80% of the total variation, respectively, according to the principal component analysis (PCA) of various soil characteristics (up to 30 cm depth). This is in line with the similar studies of the region (Singh et al. 2023). The most sensitive factors in PC 1 were determined to be TOC and the active carbon pool, whereas the most influential factors in PC 2 were the microbial metabolic quotient (qCO2) and F4 (recalcitrant fraction). In PC1, TOC had the greatest impact. Because it contains both amenable and recalcitrant carbon forms, TOC is heterogeneous. As a result, variations in the various organic carbon fractions will affect the soil's TOC. More specifically, modifications to land management methods have the ability to easily affect the liable form of carbon (Six et al. 2002b). The higher qCO2 values in the subsurface layers suggest that as soil depth increases, bacteria are less efficient at utilizing their substrate.