3.2 Changes in soil properties
Changes in soil physicochemical and biological properties after 1 and 5-years of grass-legume revegetation were significantly different from natural forest soils (Table 3). The disturbed topsoil prior to grass-legume seeding had a pH (4.7), BD (1.26 g cm-3), Av-N (46.68 mg kg-1), TN (389.60 mg kg-1), SOM (0.81%), SOC (0.34%), Av-P (0.74 mg kg-1), UA (1.26 µg NH4+ g-1h-1), DHA (1.57 µg TPF g-1h-1) and CEC (5.23 cmol (p+) kg-1). Changes in soil pH (5.8-5.6) with increasing age of revegetation were not significant (p>0.05). BD showed an increasing trend with soil depth and found highest in 1-year revegetated soil. A significant increase in the mulch density over the years resulted in favorable soil conditions which help increase the topsoil moisture content of the reclaimed waste dump from 3.8 to 11.8% compared to a natural forest (7.20%) and decrease soil temperature (26-230C) at the top layer (0-10 cm) (Table 3; Figure 4). SOC concentrations doubled in 5-years and significantly different (p<0.05) in both the soil depth. SOM concentration improved due to enrichment of grass-legume mulch which decomposed over the years but significantly lower compared to forest soil. Soil Av-N and Total-N concentrations were significantly greater after 5-years (132 and 1165 mg kg-1) compared to initial concentrations (54 and 579 mg kg-1) in the top layer. Decrease soil C:N ratio justifies greater accumulation of N over C due to an abundance of legume biomass residue. Both DHA and UA reduced with soil depth as maximum interaction of soil microbes was found to occur in the top surface (0-10 cm). Highest DHA was observed in forest soil (10.14 µg TPF g-1 h-1) whereas DHA in reclaimed soil increased significantly (p<0.05) from 3.7 to 5.32 µg TPF g-1 h-1 (Table 3). Similarly, UA also showed a significant increase from 4.57 – 7.11 µg NH4+ g-1h-1 among the revegetated sites during 1 to 5-years but non-significant when compared to forest soil. Soil C and N stocks increased significantly with revegetation age and decreased with the soil depth (Table 4). After 5-years of grass-legume revegetation SOC stock increase by 66% in the top layer. Soil N stock was also significantly higher (p<0.05) after 5-years of revegetation (1.47 ± 0.21 Mg ha-1) compared to forest N stock (1.00 ± 0.32 Mg ha-1).
Additionally, an increase in mulch thickness (0, 3, 6, 9, and 12 cm) on the dump surface accelerated the soil microbial biomass C and N even in the early stages of revegetation promoting soil fertility. The level of improvement was highest with 12 cm mulch thickness for all the assessed soil parameters (Figure 7). The amount of potentially mineralizable biomass residues in the soil correlated positively with N pool, SOM, microbial, and enzyme activity (Table 5). Soil CO2 flux rates differ significantly under different land covers and showed peak values for NF (Figure 8). The mean soil CO2 flux was found in the order of natural forest (3.17 ± 0.55µmol m-2 s-1) > 5-years (1.21 ± 0.15µmol m-2 s-1) >1-year (1.01 ± 0.62µmol m-2s-1) reclaimed waste dump over the experimental periods of revegetation.