4.1 Effect of closed and open mixed sal forest on different soil properties
The results of the present study revealed that the soil physicochemical (bulk density, moisture, pH, WHC, particle size, SOC, Av. N, Av. P, and Av. K) properties and SOC stocks differ significantly in open and closed mixed sal forest. Soil physicochemical properties vary with land use and changes in soil management practices (George et al., 2013; Gonnety et al., 2013; Malik et al., 2018); space and time because of varied topography, climate, vegetation cover, weathering process, and microbial activities (Paudel and Sah, 2003), and by a group of interactive controls such as disturbance regime and human activities (Jenny, 1994). Thus, Parent rocks, vegetation cover, and land use all influence soil properties within short distances. In our study, the average bulk density (1.31 to 1.40 Mg m− 3) was found in 1.0 m soil profile depth, similar to the value (1.12 to 1.38 Mg m− 3) up to 1.5 m soil depth (Dixit, 2022), close to the value 1.28 Mg m− 3 reported by Kafle (2019) up to 1.0 m soil depth. Higher bulk density was found in open mixed sal forest compared to closed mixed sal forest (Table 1) this is due to the greater organic matter deposition on the floor of closed forest while in open forest less accumulation of organic matter was observed. The organic matter acts as a sponge, increases pore spaces, and reduces the bulk density (Hamza and Anderson 2005; Agbeshie et al., 2020). Furthermore, anthropogenic activities and intensive gazing significantly affect the bulk density (Jhariya & Singh, 2021; Oraon et al., 2018; Shah et al., 2017). Soil moisture % was significantly higher in closed mixed sal forest than open mixed sal forest, which may be due to closed mixed sal forest provide shade with high canopy cover as well as higher accumulation of plant litter that reduces direct sunlight, temperature, and evaporation loss on the soil surface (Muscolo et al., 2021; Naudiyal and Schmerbeck, 2017; Sahani and Behera, 2001). Moreover, open canopy resulted in a significant loss of soil moisture (Bargali et al., 2018). Similar results were reported by Thakrey et al. (2022) and Shankar and Garkoti (2023), they found opening in the forest canopy leads to reduction in soil moisture. The regenerating capacity of sal makes its pH acidic in nature (Bhatnagar, 1965). The pH was moderately acidic (4.93–5.30) within the range reported by Jha et al. (1999) (4.65–5.97), and significantly (p < 0.05) higher in open mixed sal forest. This could be associated with several factors including rapid leaching of the basic cations due to the faster rate of litter decomposition in undisturbed forest and higher accumulation of organic matter, release acids, which decreases the soil pH (Paudel and Sah, 2003; Gairola et al., 2012; De Haan, 1997). Soil particle size distribution affects podzolization, hydraulics, productivity, and soil erosion (Singh et al., 2012; Mohammadi et al., 2019). In our study sand and silt particles were higher in open mixed sal forest, while clay particles were higher (20.45%) in closed mixed sal forest than in open mixed sal forest (12.99%) (Table 1). Soil texture is a static property that remains stable even with management measures, but it can be affected by soil erosion or human activities (Ansari et al., 2022). Soil organic carbon, available (N, P, K), and SOC stocks were found to be higher in closed mixed sal forest as compared to open mixed sal forest which agrees with Burke (1989), he proposed that the presence of N, P, K, soil physical characteristics, and organic matter was higher near the forest canopy and lower in areas with low tree density. Similar finding was also reported by Singh and Singh (2002) he mentioned that nutrient availability in soil was significantly higher under the tree canopy then the open forest area or non-planted sites. The nutrient availability in forest soil basically depends on the balance between nutrient incorporation and decomposition rates in the forest ecosystem (Ahirwal et al., 2017; Tiwari et al., 2019). The amount of litterfall can positively impact the addition of primary limiting nutrients (N, P, and K) and organic carbon into the soil (Das and Mondal, 2016; Singh et al., 2000). However, the removal of vegetation cover and biomass due to anthropogenic activities has a negative impact on litter production and accumulation of organic matter on the forest floor further affecting decomposition and nutrient cycling which accelerates nutrient loss (Mehta et al., 2008; Oraon et al., 2018), as well as vegetation degradation significantly reduces the organic carbon concentration into the soil (Wu et al., 2021)
4.2 Vertical distribution of different soil properties under closed and open mixed sal forest
Anthropogenic activities in mixed sal forest have resulted in various observable and significant (p < 0.05) changes in soil physiochemical properties across the soil profile (up to 1.0m soil depth) (Tables 2 & 3). Open mixed sal forest soils had significantly (p < 0.05) higher (5.8 to 10.3%) bulk density across the soil profile depths (D1:0-20cm to D5:80-100cm), the maximum bulk density (1.53 Mg m− 3) was found in the bottom most soil profile layer (D5) of the open mixed sal forest while the lowest (1.18 Mg m− 3) was found in the surface soil of the closed mixed sal forest (Tables 2 & 3), and it increased with soil depth in both forests. An increase in bulk density down the soil depth has been reported by several authors (Arora and Chaudhry, 2014; Ahirwal and maiti, 2016; Pathak and Reddy, 2021; Dixit, 2022; Raha et al., 2022). Increase in bulk density with soil depth due to less organic matter and overburden of overlying layers (Grüneberg et al. 2014). The maximum soil pH (5.53 ± 0.03) was recorded in the bottommost profile layer (0.8-1.0 m) of open mixed sal forest and the minimum (4.72 ± 0.06) in the surface layer (0-0.2 m) of closed mixed sal forest with a gradual increment in pH with soil depth up to the 1.0 m depth was found in the present study (Tables 2 & 3). A similar trend in pH was also reported by Han, et al. (2020), He suggested that the increase in soil pH with depth may be due to neutralization by humic acids in the upper soil and a decrease in the loss of carbonate minerals in the deeper soil. The studied chemical properties SOC and Available (N, P, and K) of the soil were found to be significantly (p < 0.05) different between different soil layers (Tables 2 & 3). The closed mixed sal forest was significantly higher (p < 0.05), 5.3 to 41.3% in soil organic carbon (SOC), 2 to 8% in available nitrogen (Av. N), 1 to 36.9% in available phosphorus (Av. P) and 6.2 to 16.4% in the available potassium (Av. K) under different soil depths compared to the open mixed sal forest soils (Tables 2 & 3). The gradual reduction in SOC% and plant-available nutrients has been seen throughout the soil profile depth. This might be linked to increased intake and decreased contribution of nitrogen, phosphorus, and potassium nutrients by the litters (Shrestha et al. 2004; Twongyirwe et al. 2013; Manpoong and Tripathi 2019). In forest soil of USA (Jobbagy and Jackson, 2000) reported the 86% decrease in SOC in deeper depth compare to surface layer. A larger amount of soil nutrients such as nitrogen, phosphorus, and potassium in the topsoil profile can be attributed to significant litter decomposition and increased organic matter inputs from litterfall (Manpoong and Tripathi 2019).
4.3 Soil organic carbon stocks, stratification ratio and correlation of different soil properties
In the perennial woody trees, the 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). The higher SOC stocks in closed mixed sal forest and variation between the forest types could be attributed to the different quantities and qualities of organic substances through fresh leaf litter fall, living organisms and root activity (e.g. turnover and exudates) (Vesterdal et al., 2008). However SOC stocks were higher in surface soils under both the land uses. This is in line with the earlier studies of the north eastern and north western Himalayan regions under similar climatic conditions (Shrestha et al. 2004; Jhariya and Singh, 2021; Pathak and Reddy, 2021). In Germany, a study involving more than 2500 sites by Vos et al. (2019) revealed that land use, land-use history, and clay content influenced the carbon content in surface soil and subsurface soils (L1:0-0.15m to L2:0.15-0.30m), whereas stratigraphy, parent material, and relief influenced the carbon content in subsoil to deeper soil profiles (L3:0.30–0.45 to L6:0.80-1.0m). Leaf litter, ripening fruits, twigs, and branches had a major impact on the carbon content (TC, TOC, and SMBC) in the surface soil profile layer, but contributions from plant roots are more significant in the subsurface to deeper soil profile layers (Dhyani and Tripathi, 2000; Lenka et al., 2012). The higher SRs values in closed mixed sal forests may have resulted from higher SOC stocks in the surface soils, whereas lower SRs in open mixed sal forests may have resulted from open sal forests themselves having much lower SOC stocks in the surface soil, with the decline being much more minor than in closed sal forests (Ansari et al., 2022).
3.4 Seasonal effect on vertical distribution of soil microbial biomass carbon (SMBC) under closed and open mixed sal forest
Besides the importance of physicochemical and biological processes to maintain soil health, SMB may be the ‘keystone’ biological driver of ecosystem functioning and an important index for soil health and environmental sustainability (Singh and Gupta, 2018). The closed mixed sal forest had significantly (p < 0.05) higher SMBC content than the open mixed sal forest, range of SMBC reported in the present study (12.0 to 591µg C g− 1) irrespective of forest type and seasons, close to the value (17.08 to 484.52 µg C g − 1) reported by (Tomar and Baishya, 2020). Higher amount of SMBC in closed mixed sal forest could be due to the presence of more organic matter and SOC content while in open mixed sal forest various anthropogenic activities like collection of minor forest produce, surface burning, and grazing reduce the depositions of organic matter on forest floor. Soil microbes require sufficient carbon, nitrogen, and energy sources for their synthesis and metabolism, which can be provided by high levels of soil organic matter and total nitrogen (Li et al., 2014). Soil microbial properties are influenced by soil moisture, temperature, nutrient supply, and human disturbances (Campbell et al., 1999). The distinct seasonal pattern of SMBC was similar in open and closed sal mixed forest, the value being highest during monsoon and lowest during pre-monsoon (Fig. 4). Several authors found higher SMBC content during monsoon (Devi and Yadava, 2006; Patel et al., 2010; Bargali et al.,2018; Tomar and Baishya, 2020; Lepcha and Devi, 2020; Rawat et al., 2021; Manral et al., 2023). This might be due to the high temperature and moisture during the monsoon significantly promote the growth of soil microbes and contribute to the soil microbial biomass (Saratchandra, 1984; Devi and Yadava, 2006; Edwards et al., 2013; Bargali et al., 2018; Rawat et al., 2021; Manral et al., 2023). Also, during the wet period, high relative humidity accelerates fungi growth, which increases microbial biomass carbon (Acea and Carballas 1990). Diaz-Ravina et al. (1995) found that microbial biomass was lower in dry periods due to water limitation rather than temperature. The availability of water has a significant impact on the survival and activity of soil microorganisms (Uhlirova et al., 2023). When water is scarce, it reduces intracellular water potential, leading to dehydration and inhibition of microbial activity (Wall and Heiskanen, 2003). During periods of moisture limitation, microbial communities experience starvation. Therefore, drought stress is considered the most common environmental stress for soil microorganisms. (Wolinska and Stępniewska 2012).
Besides land use type and season, soil depth is another crucial factor that regulates the SMBC. In the present study, SMBC was more in the topmost soil layer and less in the bottommost (Fig. 4). This pattern arises due to lower carbon and nitrogen concentrations in the subsoil and higher organic matter content in the topsoil, which provide the energy source to the microbial community, and hence promote microbial biomass. Similar findings on SMBC across soil depth in various land use types were reported by several authors (Liu et al., 2017; Soleimani et al., 2019; Lepcha and Devi, 2020; Ansari et al., 2022).