2. Materials and methods
2.1. Experimental sites and soils
Details of the study sites are provided in Table 1, and have been
described in previous publications (Sankaran et al., 2004;
Mendham et al., 2004). In summary, the study sites comprised 2
lowland (E. tereticornis) sites, and 2 upland (E. grandis) sites on
Ferralsol soils in Kerala, India. The plantations were established
on sites where Eucalyptus had been grown for 2 rotations since
1977 (E. tereticornis sites), and 3 rotations since 1968 (Surianelli),
and 1958 (Vattavada). The 4 sites represented contrasting
environmental conditions and soil fertility status, which was useful
for interpreting the response of tree growth and N mineralization
to residue manipulation.
2.2. Harvest residue manipulation in the field
Four treatments were studied at each of the sites, comprising
Zero residues (all residues, leaf litter, twigs and bark, removed from
the plots), Burn (all residues redistributed evenly and burnt in the
plot), Single residues (harvest residues retained and spread evenly)
and Double residues (normal residue load plus all harvest originally
removed from the Zero residues treatment). The treatments were
established in a randomised block design with 4 replications. The
gross treated plots were 20 m 20 m size, with an inner measureplot
of 10 m 10 m. Due to space restrictions at the Kayampoovam
site, the gross plots there were 18 m 18 msize. All treatments had
a starter fertiliser at establishment, added at rate of 100 g/tree N:P:K,
17:7:14. The fertilizer was placed at 10-cm depth. The nutrient content
of the harvest residues was reported in Sankaran et al. (2005),
but briefly, the above-ground biomass in the tree crop across
the sites contained of 94–174 kg N ha1, 8–40 kg P ha1, 83–
266 kg K ha1,166–715 kg Ca ha1, and 21–75 kg Mg ha1. More
details on the nutrient contents and distribution within the tree
and understorey components are presented in Sankaran et al., 2005.
2.3. Soil sampling
Soil samples were collected at 1 and 2 years after establishment
of the plantation, from the inner measure plots (10 m 10 m).
Sampling was carried out in the first half of September of the years
1999 and 2000, during the rainy season in Kerala. At this time of
the year the soil moisture level was at field capacity. A total of 9
soil cores were collected from each plot, from the 0–5 and
5–10 cm depth ranges. The 9 cores from each plot were bulked
within depths to produce a single sample for each depth range
from each of the experimental plots. N mineralization was assessed
on the 1999 samples, while other soil chemistry was assessed on
the samples collected in the year 2000. A sample processing error
meant that we could not use the samples collected in 1999 for soil
chemical analysis other than N mineralization.
2.3.1. N-mineralization studies
To collect soil samples for N mineralization assessment, the
mineral soil was firstly exposed by removing surface litter deposits,
and then steel cores (18 cores per plot, 4 cm diameter) were
driven in pairs into the soil to 20 cm depth and extracted with care
so as to not disturb soil inside the core. The cores with soil were
maintained upright in polyethylene bags and transported to
laboratory under cold conditions in insulated containers. One set
of cores (the ‘initial’ samples) was extracted immediately after
transportation to laboratory. The soil cores were sectioned into
0–5 cm, 5–10 and 10–20 cm intervals, the 9 cores per plot were mixed thoroughly within each of the depth ranges, so that each
plot was represented by 3 composite samples (1 for each depth),
then sieved to <5 mm. The other set of cores (the ‘final’ samples)
were incubated in the laboratory for 14 days at 25 C at field
moisture content (which was at or near field capacity). At the
end of incubation period, soil was extracted from the cores and
processed as described for initial samples. The sieved soil samples
(from which 90% of soil was recovered) were stored for 2 days at
4 C until analysis for mineral N.
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