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The Effect of Grassland Management History on Soil Carbon Response to Slurry and Urea

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.