Initial analysis
Average values of the physico-chemical properties of feldspar mine spoil were shown in (Fig 1). The texture of the investigated mine spoil was dominated by gravel (72%), with varying shares of sand (11.5%), silt (10.8) and clay (5.5%). The pH of the mine spoil was found highly alkaline in nature and seasonally provide cohesive environment for water holding and microbial mineral solubilization. Organic C and EC of feldspar mine spoil were also found to be low. Extremely low contents of macro and micronutrients (N, P, K, Zn, Cu and Mn) were recorded. Whereas, the values of K, Ca and Fe elements were recorded slightly high as feldspar is a major source of these elements
(Vapur et al., 2017). The spoil had a very low dehydrogenase activity. Thus biological activity of the studied mine spoil was also found to be low.
Growth parameters
Feldspar mine spoil has given good response in all the five plant species upon inoculation with various microorganisms. Improvement in plant height, shoot diameter, canopy and dry weight were observed, shown in (Table 1). In the present study microorganisms contributed maximum in the growth of
Acacia nilotica with the treatment of
Azotobacter+ OM+ NPK 50%. In
Cyamopsis tetragonoloba and
Cassia angustifolia, treatment of
Azospirillum+ OM+ NPK 50% gave the maximum increment in growth. Plant wise maximum height is attended by
A. nilotica (130 cm) and
C. tetragonoloba (121 cm), which is higher in comparison with
C. angustifolia (67),
A. senegal (55 cm) and
P. cineraria (92 cm) as in Fig 2.
Microorganisms have enhanced 30-35% of the plant growth over the control and observed equivalent/higher, when it gives in combination with 50% NPK.
Azotobacter releases growth promoting hormones, which are responsible for the enhancement in plant growth
(Jnawali et al., 2015) this is evident from the results by
Azotobacter + NPK 50% than chemical fertilizer alone.
Wu et al., (2006) supports our findings. Whereas, in
Acacia senegal and
Prosopis cineraria treatment of AMF + Om + NPK 50% have shown highest growth stimulatory effect, followed by
Azotobacter + OM+ NPK 50%.
Muthukumar and Udaiyan, (2006), found similar results in significant growth of
Prosopis cineraria. Organic manure has been commonly used in the ecological restoration of waste mine land
(Peng et al., 2016) and in our study also it is responsible for improving the quality of soil. Also organic manure acts as a slow-release fertilizer, along with microbial inoculums in different combinations
(Xu et al., 2016). Added to the our findings
Mere et al., (2013) observed that organic manures provide nutrients, substrate for the growth of microorganisms, reduce the soluble and exchangeable aluminum (one of the main components of feldspar) temporarily by forming complexes or chelates in acidic soils and providing favorable environment for plant growth. Thus, in the present study too, OM might have supported the bio-inoculants in establishment of various plant species in feldspar spoil.
Enzymatic activities
Dehydrogenase (DHA) and alkaline phosphatase in rhizosphere were significantly increased after the introduction of bio-inoculants in comparison with uninoculated control shown in (Table 2). Feldspar mine spoil inoculated with
Azospirillum showed enhanced DHA activity followed by
Azotobacter and AMF, but maximum DHA activities were observed in the treatment of
Azospirillum + OM+ NPK 50%. Highest dehydrogenase was may be due to the presence higher organic matter that supports microbial biomass and consequently affects the concentration of dehydrogenase. Our findings are supported by
Prasanthi et al., (2019). DHA is well responded with AMF also as,
Qian et al., (2012) found in maize plant on coal mine spoil. Whereas, alkaline phosphatase was found maximum in treatment with AMF+ OM+ NPK 50% and similar trend was seen in all plant species. This finding is supported by
Gucwa-Przepiora et al., (2016). In our study along with the amendments legumes are also playing a critical role in the recovery of microbial activity and N accumulation that results in the improvement and enhancement of enzymatic activity in the mine spoil.
Olsen P and organic carbon
Availability of Olsen P and organic-C was found maximum with AMF+ Om+ NPK 50% which is higher than NPK 100% alone, as AMF have been found to mineralise maximum soil phosphorous in this treatment as shown in (Table 3). The enhancement is may be due to the accumulation of organic matter in the mine soil, which has accelerated organic carbon production as stated by
Ahirwal et al., (2017). We also found that organic carbon is directly proportional to the soil organic matter therefore organic matter in the treatments also played equally important role along with the bioinoculants in enhancing organic carbon. Our findings are confirmed by
Rath et al., (2010). Legumes are also known to increase soil organic carbon and we have also seen the same increment of organic carbon in our plants grown on feldspar spoil. This is supported by
Kumar et al., (2018), who stated that legumes have the capacity to store 30% higher soil organic carbon than other species, due to nitrogen fixing capability.
Metabolites
Total chlorophyll and reducing sugar were significantly enhanced in all the plant species upon microbial inoculation in various combinations, in the range of 40-45 % and 45-51% respectively over the control as shown in (Fig 3 and 4). The chlorophyll content in the leaves of
Acacia nilotica increased by 44.82% and 41.08% after inoculation in
Azotobacter+ OM+ NPK 50% treatment and AMF+ OM+ NPK 50% respectively over the control, whereas the corresponding increase in the reducing sugar by 50.6% and 49.3%, respectively for both the treatments. Chlorophyll content (41.8%) and reducing sugar (49.1%) also increased in the leaves of
Cyamopsis tetragonoloba after inoculation with
Azotobacter+ OM+ NPK 50%. These results agree with
Dhawi et al., (2015). The increase in chlorophyll content may be due to microbial solubilization and mobilization of macro and micronutrients in the rhizosphere by the organic acids produced by microbial inoculants as supported by
Subba-Rao, (1981).
Kaur et al., (2015) in a study on chickpea stated that Rhizobium inoculation significantly increased chlorophyll content as compared to control thus supported the role of AMF in our study.
Jin et al., (2015) reported that the level of sugar content increased in the leaves of maize plant as level of nitrogen increased. Similarly, in our studies percentage of reducing sugar enhanced significantly with the increase of nitrogen, may be due to the nitrogen fixation by the microbial inoculants and legumes.
Macronutrients
Microorganisms are getting acquainted in new environment and significantly contributing in nutrient solubilization, as evident by the enhanced uptake of nitrogen (N), phosphorous (P), potassium (K), calcium (Ca) and magnesium (Mg) in the range of 30-60% over the uninoculated control shown in (Table 4).
Azospirillum+ OM+ NPK 50% (T-12) recorded maximum nitrogen uptake in
Acacia nilotica, Acacia senegal and
Prosopis cineraria, followed by
Azotobacter + OM+ NPK 50% (T-7) and AMF+ OM+ NPK 50% (T-16) while
Azotobacter + OM+ NPK 50% has been observed to support high nitrogen uptake in
Cyamopsis tetragonoloba and
Cassia angustifolia. Nitrogen fixing microorganisms along with 50% NPK have shown at par nitrogen uptake as of with 100% NPK inoculation. In phosphorus uptake, AMF showed maximum phosphorus influx in all five plants with feldspar mine spoil and have given better results in the presence of 50% NPK and higher in AMF+ OM+ NPK 50%. AMF easily absorb elements such as phosphorus and nitrogen which are essential for plant growth as stated by
Maiti and Ahirwal, (2019). This result is in the line with
Yang et al., (2016), who observed that legume- rhizobia have always provided a synergistic approach on nitrogen management.
Ahirwal et al., (2017) has also confirmed the results and found increment in nitrogen upon introducing leguminous trees with mine spoils. We have observed highest N uptake with
Azospirillum and significant intake with AMF. Similarly T-7 is responsible for the highest uptake of K. Comparatively crops had maximum uptake of K than trees. Feldspar has insoluble form of potassium (8-10%) and therefore, this K could be effectively utilized by native potassium solubilizing microorganisms
(Kasana et al., 2017). This could be the reason for the highest accumulation of K in the plants.
Overall both bacteria and fungi have increased the uptake of Ca and Mg in all the five plants. Comparatively Ca uptake has been higher than Mg as feldspar already contains Ca in excess
(Vapur et al., 2017) as shown in (Fig 5). Crops showed best results with T-12 and T-16 whereas, trees were best with T-7 in the uptake of Ca and Mg. Other than bioinoculants legumes also contributed in the uptake of these nutrients. As supported by
Nyoki and Ndakidemi, (2014) that rhizobial inoculation of legume crops supplemented with P fertilizer improves the uptake of Ca and Mg.
Micronutrients
The uptake of micronutrients copper (Cu), manganese (Mn), zinc (Zn) and iron (Fe) increased over the control by 80% maximum in (Table 5). There is no uniformity in the absorption of micronutrients among all the introduced bio-inoculants along with various combinations, but AMF have shown consistently highest uptake in all the plants with feldspar spoil. Since feldspar and mica are most likely responsible for the release of these metal elements (
Singh and Schulze, 2015) may be therefore, significant higher availability of micronutrients in feldspar mine spoil, is the reason for the high concentration of these elements in the plants, as
Rao et al., (1996) found the same results with gypsum mine spoil. The mycorrhiza-assisted plants have also reported for the uptake of nutrients such as phosphorus (P) and zinc (Zn) that normally have low diffusion rates in the soil
(Lendenmann et al., 2011).
In our studies bio-inoculants might have developed microclimate and initiate nutrient solubilization which has readily taken up by the plants and used in the metabolism and resulted in higher growth over or among the treatments. The microbial inoculants performed well in disturbed feldspar soil in the presence of organic manure and inorganic NPK. Legume species have also supported vice-versa. Application of organic manure and bioinoculants supported legumes to increase yield and soil fertility. Therefore, plantation of legume species for improving the fertility status of feldspar mine spoil could be a good option because of their N-fixing ability and consequently for restoring the soil-nutrient cycling in the degraded spoil. Thus the combination of legumes under agrosilviculture technique and used treatments proved to be a satisfactory trial for the future rehabilitation processes.