Effects of soil compaction on N-mineralization and microbial-C and -N. I. Field measurements

Abstract Soil biological parameters, such as soil respiration or N-mineralization, may be more sensitive to soil compaction than physical parameters. Therefore we studied the effects of soil compaction on net N-mineralization and microbial biomass dynamics in the field. The soils were silty clay loams (Typic Endoaquepts) in either a well-structured permanent pasture with high organic-C content (46 mg g −1 ) or a site which had been continuously cropped with cereals for 28 years with low organic-C content (21 mg g −1 ) and a very poor structure. Compaction treatments were applied by five passes of a tractor (total weight 4880 kg, speed 2.2 m s −1 ). An energy flux of either 2712 J m −2 (assuming deflecting tyres) or 6056 J m −2 (assuming rigid tyres) per pass of the rear tyres was estimated. Soil dry bulk densities were initially 1.00 and 1.30 Mg m −3 in the pasture and cropped sites, respectively, and increased significantly only in the less dense pasture site. However, soil surface CO 2 -fluxes decreased substantially after compaction on both sites (57–69%) because of the highly reduced air permeability of the topsoil. At the cropped site this was also accompanied by a significant decrease in oxygen-diffusion rate (45%). Using the in situ core technique with covered cores the apparent net N-mineralization rate was less in compacted than in non-compacted areas of the pasture ((0.27 and 0.38 μg N g −1 day −1 , respectively), but did not differ at the cropped site (average 0.15 μg N g −1 day −1 ). However, N-mineralization measurements by the in situ core technique were found to be problematic as denitrification possibly occurred and concealed actual net N-mineralization. Microbial biomass did not change significantly as a result of the compaction treatment, but was shown to either decrease or increase over time depending on the methodology used to estimate microbial biomass.