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.