Effects of storage time and straw content of cattle slurry on the mineralization of nitrogen and carbon in soil

Animal slurries are stored for a variable period of time before application in the field. The effect of cattle slurry storage time and temperature on the subsequent mineralization of C and N in soil was studied under laboratory conditions. Urine and faeces from a dairy cow were sampled separately and mixed to a slurry.

After 4 weeks of storage under anaerobic conditions at 15 degrees C, the NH4+ N content exceeded the original urinary N content of the slurry; the NH4+ content increased only slightly during the following 16 weeks of storage. After 4 weeks of storage, the proportion of slurry C in volatile fatty acids (VFA) amounted to 10% and increased to 15% after 20 weeks.

Straw addition to the slurry caused an increase of VFA-C in stored slurry, but had a negligible influence on the proportion of slurry N in the form of NH4+. Slurries subjected to different storage conditions were added to a sandy and a sandy loam soil. After 1 week, the preceding storage period (0-20 weeks) and temperature (5 degrees C or 15 degrees C) had no significant effect on the net release of inorganic N from the slurry in soil.

Thus, the increased NH4+ content in the slurry after storage was followed by increased net N immobilization in soil. Additional straw in the slurry caused increased net N immobilization only in the sandy loam soil. Following anaerobic storage, 8-14% of slurry C was released in gaseous form, and the net mineralization of slurry C after 12 weeks in soil amounted to 54-63%.

The extra net mineralization of C in soil due to straw in slurry was equivalent to 76% of straw C, suggesting that the straw accelerated the mineralization of C derived from faeces, urine and/or soil.