4.2 Effect of grass-legume mixture on SOM and enzymes activity
Soil fertility in terms of SOM, C and N concentration readily declines in topsoil during initial use at the waste dumpsite, which can be recovered through revegetation and subsequent nutrient cycling via decomposition and mineralization (Campos, Etchevers, Oleschko, & Hidalgo, 2014; de Oliveira et al., 2015; Semenov et al., 2019). Analysis of variance (ANOVA) showed that the growth of grass-legume mixture on dump surface bought significant changes in both above and below-ground soil systems (Table 3). Enrichment of grass-legume mulch over 5-years of revegetation increased the SOC and SOM concentration by 94% and 35% respectively in 0-10 cm depth. However, when compared with forest soil, both SOC and SOM concentration was still 23% and 111% lower in reclaimed soil. Similarly, SOC and N stock in the blanketed soil also showed increment by 66% and 122%, respectively, after 5-years compared to initial concentration (Table 4). These differences are mainly attributed to the presence of rapidly decomposing legume residue, impedes the recovery of SOM, and potentially available forms of C and N pool governed by soil microbes and plant rooting depth will absorb a large amount of the nutrient, leading to increased microbial activity. Previous studies have also reported that the incorporation of legume biomass helps in the recovery of soil C (Guan et al., 2016; Wu, Liu, Tian, & Shi, 2017) and N stock in the early stage of biological reclamation (Ahirwal, Maiti, & Reddy, 2017a; Elgersma & Soegaard, 2016).
The changes in root and shoot biomass and mulch accumulation in mixture modifies the composition of mulch residue added to the soil, which resulted from an increase in N mineralization, promotes C and N recovery link to increase soil microbial biomass C and N (Figure 7). At the same time, enzymes related to soil C and N decomposition processes i.e. dehydrogenase and urease enzyme activity demonstrate 56 and 44% increase, respectively after 5 years of revegetation compared to 1st year which probably reflected the general build-up of the microbial biomass. These results show the positive correlation and linkage between nutrient parameters, microbial and enzyme activities in a high decomposition driven soil environment (Table 5) that are in agreements with the similar trends explained in other studies (Hou et al., 2018; Luo et al., 2018; Zuber & Villamil, 2016). During land use change, plantation of legume (M. sativa ) significantly improved the soil N concentration and urease enzyme activity after 3-years of the plantation (Yu, Lin, Liu, & Wang, 2020). Our finding illustrates the fact that reclaimed soils under grass-legume revegetation have higher N mineralization potential and beneficial for the growth of soil microbial biomass that is related to enzyme activities that play a crucial role in effective recovery of soil nutrients during revegetation.