Soil microbes and enzymes respond rapidly to land management practices, therefore, used widely as sensitive indicators to evaluate the influence of land use changes on soil quality (Kooch et al. 2016; 2019). In the present study, a significant variation of soil microbial parameters and enzyme activities were observed under natural forests and cultivated lands which is similar to the results reported by previous studies (Trasar-Cepeda et al., 2008; Araújo et al. 2014; Xiangmin et al. 2014; Raiesi and Beheshti 2015; Silva et al. 2019).
Microbial Activity
The SMBC is the most active carbon fraction and indicate the size of the SC labile pool (Hanson et al. 2000). The present study estimated higher SMBC from forests as compared to cultivated sites with ~ 70–80% of reduction. Our results agree with the findings reported from previous studies indicating higher SMBC under natural forest than degraded lands (Ananyeva et al. 2008; Zhao et al. 2012; Mganga and Kuzyakov 2014; Kooch et al. 2019; dos Santos et al. 2019). Agriculture management practices including intensive tillage often damage the soil aggregates and expose the SOM for oxidation in cultivated land uses (Raiesi and Gilani 2020). This was evident in the present study, where, the SC was declined by 30–40% in VF and 60–80% in AF, respectively, as compared to natural forest. Additionally, the agriculture practices such as soil tillage, cropping sequences, manuring, or residue incorporation influence the soil structure and properties, leading to a reduction in the SC and SMBC (Insam 1990; Cookson et al. 2008; Kabiri et al. 2016). The SMBC is determined by long term accumulation of SOM input in the soil (Moore et al. 2000) and therefore, in cultivated soils, a reduced quantity and quality of SOM, hence, limit the microbial activity and change the microbial composition (Moscatelli et al. 2007; Yang et al. 2010). The significant difference among SC, SN, and SMBC in different land use suggest that the concentration of the SMBC depends upon the quality (labile pool) of SOM (Chen et al. 2005). The greater SMBC in MFC than PFC could be explained by the presence of associated tree species along with P. juliflora as the dominant species resulted in diversity of microbial substrates in soil and favors plant rhizosphere (dos Santos et al. 2019). The previous studies have well documented that plant diversity significantly increases soil microbial biomass (Royer-Tardif et al. 2010; Mazzetto et al. 2016). Furthermore, a higher SC in MFC than PFC could be explained by slower decomposition of SOM as P. juliflora leaves have higher lignin content which doesn’t degrade rapidly. Therefore, the quality of litter also controls the accumulation of SOM (Nsabimana et al. 2004; Rutigliano et al. 2004; Ravindran and Yang 2015). Among the cultivated lands, a higher SC, SN, and SMBC from VF as compared to AF. Similar results were reported by Maharajan et al. (2017), with a decrease in SMBC in conventional than organic farming system and suggested that agriculture practices including intensive tillage along with crop rotations with fallow periods in conventional farming reduce the SOM and SMBC, respectively. The high aboveground biomass in VF (20 g m− 2) as compared to fallow land in AF at the sampling time could have resulted in lower microbial activity in AF. Additionally, a strong positive correlation of SMBC with SC and SN indicated the influence of land use management practices in determining the SOM quantity, quality, and consequently to soil microbial biomass (Maharajan et al. 2017).
The SMBC/SC ratio or MQ has been widely used as an indicator of soil quality and future changes in SOM (Sparling 1992). It also reflects the contribution of the SMBC to SC and can be used as a sensitive measure of soil health under different land use management system (Anderson and Domsch 1989). The previous studies have reported a high MQ at forest as compared to cultivated land (Dinesh et al. 2004; Kooch et al. 2019). Contrastingly, in our study, the higher MQ in AF could be related to lower SC owing to a larger proportion of the SMBC to total SC (Araújo et al. 2013). Hence, the soils in the degraded lands are more sensitive to the change related to land use and management (Sampaio et al. 2008).
The SBR and SSIR are considered as an index of soil quality and are controlled by SOM, soil nutrients, quality of the substrate, and humic substances (Singh et al. 2012; Mganga et al. 2016; Gorobtsova et al. 2016). The SBR provide an estimate of heterotrophic activity of the soil microbial community and availability of soil carbon to soil microorganisms (Cheng et al. 2013). The SSIR or potential respiration rate is the respiration measured when glucose was added as a substrate to the soil and used as a measure of microbial respiration (Mazzetto et al. 2016). Among the land use, the higher values for both the SBR and SSIR were reported from natural forests as compared to cultivated analogs. Kooch et al. (2019), also reported a higher soil SSIR, SMBC, SBR/SMBC under natural forests and plantations as compared to agriculture site. This could be attributed to greater above-ground biomass, below-ground biomass (Meena et al. 2020), fine root activity (Davidson et al. 2002), and litter content (Chodak and Niklińska 2010) in forests which maintain the SC for a longer period. Further, the decrease in available substrates for microbes in soil reduces the SBR and SSIR in cultivated fields (Zhang et al. 2016). The SSIR increased to ~ 10 times to the SBR under all land use. It has been well documented that, the soil microbial respiration is controlled by abiotic factors mainly SM (Zhang et al. 2013; Bao et al. 2016; Meena et al. 2020). The comparatively higher SM in VF (4.70%) than AF (3.92%) favors greater microbial activity in VF (Cook and Orchard 2008). Further, a high nutrient content (SC, SN, SP) in VF also enhance soil microbial respiration in VF than AF (Giesler et al. 2012; Tardy et al. 2014).
The qCO2 provides an integrated measure of the eco-physiological state of soil microbial community and used widely as a critical parameter to determine changes in the SC levels (Anderson and Domsch 1989). The results showed that the largest qCO2 was under the AF which was parallel with the results reported by previous studies (Anderson and Domsch 1990; Alvarez et al. 1995; Nsambimana et al. 2004; Ananyeva et al. 2008; Cheng et al. 2013; Kooch et al. 2019; Xiangmin et al. 2014). This suggested that intensive tillage leading to land degradation in the AF could have reduced the substrate carbon supply, declines the microbial biomass size, activity, and microbial efficiency for substrate carbon utilization (Wardle and Ghani 1995; Six et al. 2006; Yang et al. 2010). Whereas, a comparatively lower qCO2 from forest clearly indicated the continuous availability of the substrate via aboveground/belowground litter addition (Insam 1990). Under disturbed ecosystem, the strong competition for available carbon substrate may favor microbes which use more carbon energy in maintenance then growth (Islam and Weil 2000). Therefore, in cultivated soils, the microbial communities are more stressed and needed a regular supply of carbon sources in order to maintain their activity (Singh et al. 2018).
Soil Enzyme Activity
Soil enzyme activity is influenced by the soil characteristics related to nutrient availability, soil microbial activity, and land use management processes which modified the potential soil enzyme-mediated substrate catalysis (Kandeler et al. 1996). The present study observed significantly higher activity of selected enzymes in natural forests as compared to cultivated land use. Our results were parallel to the previous studies reporting a reduction in soil enzyme activities following the conversion of natural forests into cultivated lands (Araújo et al. 2013; Raiesi and Beheshti 2015; Vinhal-Freitas et al. 2017; Silva et al. 2019). The greater enzyme activity in natural forests could be explained by increases litter quality and quantity (Araújo et al. 2013), plant biomass, and vegetation cover (Martens et al. 2004). The significant differences among forest sites (PFC, MFC) could be attributed to tree species-specific responses to soil nutrients and enzyme activities (Wang et al. 2013). Whereas, in cultivated lands soil tillage, loss of SC, and application of chemical fertilizers can be related to a decrease in enzyme activities (Baligar et al. 2005). Similarly, Raiesi and Beheshti (2015), reported a decrease in enzymatic activities following the conversion forest to croplands could be related to the reduction in SOM content and microbial biomass.
The dehydrogenase activity in soil serves as an indicator of the microbiological redox system and microbial oxidative activities in soil (Casida et al. 1964). Similar to our results, Bonanomi et al. (2011), reported a reduction by 84% in dehydrogenase activity in a low input management regime as compared to the high input management regime. In the present study, the intensive management practices and low levels of SOM input may have declined the activity in cultivated soils.
The β- glucosidase activity in soil is linked to the release of carbohydrates in soil, which provides a major substrate for soil microorganisms. The positive correlation of the SC and SMBC with β- glucosidase activity indicated that low SC and microbial activity in cultivated lands reduced its activity in AF and VF, respectively (Vinhal-Freitas et al. 2017). Similarly, the acid phosphatases activity was also high under forests as compared to cultivated land use. Acid phosphatases activity is also influenced by soil pH, nutrients, SC, SN, SP, SOM quality and quantity, microbial community structure, SM, and soil temperature (Hendriksen et al. 2016; Maharajan et al. 2017, Moghimian et al. 2017).
The urease regulate the SN transformation is involved in the hydrolysis of urea into ammonia and CO2 (Kong et al. 2008). In this study, a higher activity was evaluated under the MFC which were in accordance with previous findings suggesting greater SOM, above and below-ground biomass, detritus pool, and availability of fresh SOM for microbial decomposition in forest soil increases the enzyme activity (de Medeiros et al. 2017; Vinhal-Freitas et al., 2017). However, in AF and VF, an increased activity despite low values of SOC could be attributed by the regular supply of urea fertilizer to the field. Similar to our study, various studies have shown an increased urease activity under forests than the cultivated or disturbed soils (Acosta-Martínez et al. 2003, 2007; da Silva et al. 2012; Araújo et al. 2013). The urease activity is influenced by other soil properties including pH, soil nutrient supply, SN, microbial biomass N, and N fertilizers (Moghimian et al. 2017). Additionally, SM also controls its activity in different land use (Zeng et al. 2009).
The positive correlation of enzyme activity (β-glucosidase and dehydrogenase) with the SC and SMBC indicated its role as a precursor for enzyme synthesis) as the SMBC represent a fraction of the labile SC content (Acosta-Martínez et al. 2007) and SC is essential for regulating the enzyme activities (Raiesi and Beheshti 2015; Silva et al. 2019). However, a weaker correlation of soil parameters with urease was parallel with an earlier study by Klose and Tabatabai (2000), where, SMBC was weakly correlated with urease activity, but it was strongly correlated with SMBN. As reported in earlier studies, a negative correlation of SC with urease activity, suggest that it is regulated by carbon supply to microbes which resulted in SN limitation and increases enzyme production (Bowles et al. 2014; Silva et al. 2019). Further, a non-significant yet positive correlation of SC and SMBC with acid phosphatase activity indicate their importance in regulating its activity (Hendriksen et al. 2016; Acosta-Martínez et al. 2018). However, no correlation was observed among SP and acid phosphatase (Silva et al. 2019).
The PCA evaluated the dissimilarities in soil parameters among the various land use. The disturbed i.e. AF and VF appeared to be different from the respective undisturbed land use i.e. PFC and MFC. The clustering of SC, SN, SMBC, SBR, SSIR, β-glucosidase, dehydrogenase, and acid phosphatases nearby PFC and MFC indicate the critical role of SOM in controlling the microbial and enzyme activities (Cheng et al. 2013). Additionally, the clustering of urease with SM is similar with various findings, suggesting a significant role of SM in enhancing urease activity in soil (Sahrawat 1983). Further, clustering of qCO2 and SMBC/SC near the AF and VF suggest a reduced efficiency of microbes to utilize substrate carbon in degraded lands (Arau ́jo et al. 2014) as compared to more recalcitrant carbon in forest analogs.