Agriculture Reference
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extracts were measured as inorganic P in the persulphate oxidized extracts used to determine
TDN (Williams et al. 1995). The difference between TDP and DIP was considered to be
organic P (DOP).
Dissolved organic carbon (DOC) in the 0.5M K 2 SO 4 suspension was measured by the
Walkley and Black wet oxidation method (Nelson and Sommers, 1982).
To measure microbial biomass C and N a set of samples was fumigated overnight with
chloroform and extracted with 0.5 M K 2 SO 4 in parallel with a set of unfumigated samples
(Williams and Sparling 1988). Biomass C and N values were calculated from the flushes of
extractable C and N using the recovery factors of 0.45 for C (Sarathchandra et al. 1989) and
0.54 for N (Brookes et al. 1985). The mineralizable N was determined by the anaerobic
incubation of soil samples in closed containers at 40°C for 10 days, as reported by Waring
and Bremner (1964).
Temperature Measurements
To record soil temperature, thermistors combined with dataloggers (UTL-1) were buried
in pairs in the two soil profiles at a depth of about 10 cm (OH horizon). The loggers recorded
the temperature throughout the period November 2001-May 2002. Mean soil temperature was
calculated in each stratum.
Soil Incubation
At the same time of temperature measurement C and N dynamics in the OH horizon were
evaluated through field incubation experiments (November 2001- May 2002). Inorganic N
variations, microbial uptake of mineralised nutrients and dissolved organic forms variation
were measured in the field by the buried bag technique (Eno, 1960; Pastor et al., 1984;
Adams et al., 1989), which enables measurements that account for the differences and
fluctuations in soil temperature to be made while the water content is kept constant (Schmidt
et al., 1999). The technique prevents plant uptake and leaching of mineralised nutrients but
allows air diffusion and uptake by the microbial biomass.
A 100 g of soil was placed in a polyethylene bag, returned to the soil and buried to a
depth of approximately 20 cm. Three bags have been incubated in each area. An another
aliquot of soil was returned to the laboratory and immediately treated with 0.5M K 2 SO 4 for
NH 4 + and NO 3 - analysis as previously described (Williams et al., 1995; Schmidt et al., 1999).
Net N mineralization was calculated as the difference between the sum of NH 4 + and NO 3 -
concentrations in the incubated and initial samples. Microbial N immobilization was
calculated by differences in the microbial N before and after incubation. This is the potential
amount of nutrients that microbes can immobilize inside the bags when the uptake by plant
roots is eliminated (Schmidt et al., 1999). Furthermore, changes in dissolved organic nitrogen
(DON) and carbon (DOC) during incubation were quantified because nutrients may be
released as dissolved organic compounds during microbial dieback, or microbes may
assimilate dissolved organic compounds during re-growth (Schmidt et al., 2002).
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