EFFECTS OF SOIL COMPACTION AND WATER-FILLED PORE-SPACE ON SOIL MICROBIAL ACTIVITY AND N LOSSES

Soil compaction is a significant production problem for agriculture because of its negative impact on plant growth, which in many cases has been attributed to changes in soil N transformations. A laboratory experiment was conducted to study the effect of soil compaction and water-filled pore space on soil microbial activity and N losses. A hydraulic soil compaction device was used to evenly compress a Norfolk loamy sand (fine-loamy, siliceous, thermic Typic Kandiudults) soil into 50 mm diameter by 127 mm long cores. A factorial arrangement of three bulk density levels (1.4, 1.6, and 1.8 Mg/m3) and four water-filled pore space levels (60, 65, 70, 75%) was used. Fertilizer application of 168 kg N/ha was made as 1.0 atom % N-15 as NH4NO3. Soil cores were incubated at 25-degrees-C for 21 d. Microbial activity decreased with both increasing water-filled pore space and soil bulk density as measured by CO2-C entrapment. Nitrogen loss increased with increasing bulk density from 92.8 to 334.4 g N/m3 soil at 60% water-filled pore space, for 1.4 and 1.8 Mg/m3, respectively. These data indicate that N loss and soil microbial activity depends not only on the pore space occupied by water, but also on structure and size of soil pores which are altered by compaction.