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).