Agriculture Reference
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soil N in autumn. The results were compared with simulation of C and
N dynamics using the MOTOR model (Hassink and Whitmore, 1997;
Whitmore et al ., 1997). Laboratory studies were run in parallel to provide
additional information on N immobilization/mineralization kinetics related
to the physical state of PMS (finely mixed materials are expected to be
more accessible to decomposers than large pieces) and the soil biological
condition. Hypothetically, if initial bacteria growth following PMS addition
occurs without grazing by protozoa or nematodes, N mineralization will
occur less rapidly (Bouwman et al ., 1994), and the net N immobilization
phase will last longer.
Materials and Methods
Field experiment
There were three replicate blocks of the following treatments: W 95 , soil
receiving receiving 40 t DM ha −1 PMS with C:N=25 on 6 December
1994 (Vinten et al ., 1998); W 96 , soil receiving 40 t DM ha −1 PMS with
C:N=18on14June 1996; and W 0 , control soil with no PMS applica-
tion. In 1996, the PMS was ploughed in after spreading, and broccoli was
transplanted on 4 July. There were sub-plots fertilized with 125 kg N ha −1
after planting (N1) and unfertilized sub-plots (N0). Marketable yield and
total N and C content were determined. After harvest, the crop residues
were left on the surface. Soil samples (0-20 cm) were taken 14, 35, 53, 68,
91, 124, 152, 185 and 213 days after the PMS application. Inorganic N
content and microbial biomass C and N (Voroney and Paul, 1984) were
determined. Soil was sampled to 2 m on 14 November 1996 and water
content, NH 4 + -N and NO 3 -N content were determined. Emissions of
N 2 O were measured on the N1 treatments using automated cover boxes
(Scott et al ., 1999) from 2 July to 20 August 1996.
Laboratory studies
The role of the physical state of the PMS on decomposition processes
was investigated in the laboratory, focusing on respiration kinetics. Soils
samples were collected from Balmalcolm (Hexpath series, loamy sand), and
from a nearby organic horticultural farm (Fourmerk series, sandy loam).
Duplicate treatments were established in 1 l kilner jars with a vial
containing 10 ml of 0.5 M NaOH. Treatments consisted of 35 g of fresh
soils thoroughly mixed with 5 mg C g −1
soil added in the form of: (i)
0.5
0.5 cm filter paper squares; (ii) 1 cm diameter spheres of filter paper;
(iii) 1 cm diameter spheres of filter paper inoculated with dilute soil
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