4.1 Effect of grass-legume mixture on N-mineralization
The growth of grass-legume mixture after 5 years of revegetation on the waste dump has substantially increased the biomass by 41% and mulch stock by 93% (Table 1). An increase in grass-legume mulch thickness up to 12 cm in 5-years showed a positive effect on moisture and temperature in the upper layer (0-10 cm) of soil. Due to an increase in mulch accumulation maximum soil temperature was dropped by 9°C which increased moisture content of the reclaimed dump surface by 7.5% compared to bare soil (Figure 2). These changes are attributable to thick surface mulch formed over the dump surface which creates a cooler soil environment, reduces evapotranspiration, and holds soil moisture by ameliorating the surface temperature. Our results are consistent with the findings of Wang, Liu, Wu, Li, & Wang, (2017).
Microbial decomposition of biomass residue releases potential nutrients available to plant depends on the rate of mineralization which regulates the conversion of organic to inorganic N pool (Abdelhafez et al., 2018; Ansong Omari et al., 2018; Marzi, Shahbazi, kharazi, & Rezaei, 2020). Inorganic N concentration in legume shoot and root biomass accounted for 0.91% and 0.42% respectively of the biomass total N pool whereas cumulative N-mineralization by legume residues were three-folds greater than the grass (Table 2; Figure 5). The study of C and N mineralization reported that net N mineralization was greater by legumes (33%) than non-legumes (20%) species and differs across shoot and root biomass of both the species (Li et al., 2020; Li et al. 2019). The increase in labile N pool in the reclaimed dump soil indicates the presence of mineralizable organic N contributed by legume and grass biomass in the mixture after one to five years of revegetation.
After 5-years of revegetation, increase in soil moisture and the enrichment of mulch stock constituting high-quality N rich legume biomass with low C:N ratio enhances the N-mineralization process (Figure 6; Figure 7) and makes it desirable for the functioning of microbial populations (Jilkova, Strakova, & Frouz, 2020; Lei & McDonald, 2019; Li et al., 2019). Legume biomass with low C:N ratio can be readily decomposed by soil microbes leaving excess N in soil that can be used by associated plant species whereas, higher C:N ratio of grass biomass contributes to soil C but in long term, the accumulation of SOC can be controlled by N concentration. These difference in the chemical characteristics of the biomass residues of grass-legume mixture incorporated in soil shifts the nutrient cycling via mineralization which stimulated the soil microbial activities (Amorim et al.,2020; Bhandari, West, & Acosta-Martinez, 2020; Prommer et al., 2020) and enzymatic activities (Joniec, 2018; Sekaran et al., 2020; Tian et al., 2017). Therefore, understanding the N mineralization patterns of grass and legume biomass returned to the soil during revegetation is important for the management of soil N dynamics in nutrient cycling.