Slurry and urea applications are part of normal nutrient management on
grassland farms utilizing grazed grass and silage for animal production. It was
hypothesized that management history would result in a different carbon response
to slurry and urea applications for the same soil type because of differences
in soil micro-environment, including microbial biomass and activity,
are formed and regulated by long-term management history. An Irish grassland
soil of the Skeagh Series was sampled in three fields, each with a long,
consistent management history: Soil A was associated with extensive grazing
by horses; soil B with medium intensity grazing by sheep and cattle, and grass
silage conservation; and soil C with intensive dairy cow grazing. There were
three slurry treatments (S1, the control of no slurry; S2, slurry mixed with soil;
S3, slurry added on the soil surface) and three urea treatments (N1, the control
of no urea; N2, all urea applied at one time; and N3, three application, 30 days
apart, totaling the same amount of urea as N2) designed to supply 36 g C m−2
and 2 g N m−2 during an 85 day incubation trial. Soil pH, total carbon, cold
water extractable organic carbon, soil respiration and two C-related enzymes
(β-glucosidase and CM-cellulase) were measured. All measured soil properties
showed a significant difference (P < 0.05) by management history, indicating
a strong influence of long-term management on response. β-glucosidase and
CM-cellulase activity showed a strong relationship with soil management history
rather than with slurry or urea additions. It was concluded that management
history was important to C dynamics. Slurry mixed with soil resulted in
a greater soil carbon loss than slurry applied on the soil surface. One large
dose of urea caused greater soil carbon loss than multiple small doses.