3.1 Changes in plant biomass and N-mineralization potential
Growth of S. hamata and P. pedicellatum showed significant changes in rooting depth, shoot and root biomass, and mulch stock between the developmental ages of revegetation. The results showed that legumes biomass (26.82 Mg ha-1) was higher than the grass biomass (19.02 Mg ha-1) after 5 years of revegetation (Table 1). Biomass accumulation with the age since revegetation and their subsequent conversion to dead biomass increase mulch density from 11.4 to 28.0 Mg ha-1 (Figure 4). Continuous increase in mulch density resulted in greater mulch accumulation on the dump surface that has a significant effect on soil temperature and moisture content. The biomass residues showed a greater amount of C in grasses while N concentration was higher in legumes (Table 2). Inorganic N concentration in legume residue ranged from 2.18 – 2.55 g kg-1compared to 0.32-1.44 g kg-1 in the grass with shoot biomass contributing the highest concentration. The biomass residues used for incubation showed differences in C:N ratio for a shoot (12.4) and root (22.6) of S. hamata and shoot (20.3) and root (31.1) of P. pedicellatum . Changes in labile N pool from soil extract amended with biomass residues were greater by legume than grass with incubation time (Figure 5). The mineralization rate showed greater values (25-27 mg N kg-1 wk-1) during the first 3-weeks of incubation and declined to 15 mg N kg-1wk-1 toward the 14th week by the legume. Similarly, shoot biomass of grass showed the lowest N mineralization ranging from 7-8 mg N kg-1wk-1 in first 3-weeks and 2 mg N kg-1 wk-1 by the end of the incubation period. The cumulative N mineralization rate was greater by legume shoot biomass (166 mg N kg-1soil) compared to grass (58 mg N kg1 soil). Linear regression analysis showed that the percentage of mineralized N was highly dependent on TN pool of decomposing biomass and negative curvilinear relationship with biomass C:N ratio in both shoot and root biomass of legume and grass (Figure 6).