EFFECTS OF SOIL COMPACTION ON N-MINERALIZATION AND MICROBIAL-C AND MICROBIAL-N .1. FIELD-MEASUREMENTS

Soil biological parameters, such as soil respiration or N-mineralizati on, may be more sensitive to soil compaction than physical parameters. Therefore we studied the effects of soil compaction on net N-minerali zation and microbial biomass dynamics in the field. The soils were sil ty clay loams (Typic Endoaquepts) in either a well-structured permanen t pasture with high organic-C content (46 mg g(-1)) or a site which ha d been continuously cropped with cereals for 28 years with low organic -C content (21 mg g(-1)) and a very poor structure. Compaction treatme nts were applied by five passes of a tractor (total weight 4880 kg, sp eed 2.2 m s(-1)). An energy flux of either 2712 J m(-2) (assuming defl ecting tyres) or 6056 J m(-2) (assuming rigid tyres) per pass of the r ear tyres was estimated. Soil dry bulk densities were initially 1.00 a nd 1.30 Mg m(-3) in the pasture and cropped sites, respectively, and i ncreased significantly only in the less dense pasture site, However, s oil surface CO2-fluxes decreased substantially after compaction on bot h sites (57-69%) because of the highly reduced air permeability of the topsoil, At the cropped site this was also accompanied by a significa nt decrease in oxygen-diffusion rate (45%), Using the in situ core tec hnique with covered cores the apparent net N-mineralization rate was l ess in compacted than in non-compacted areas of the pasture ((0.27 and 0.38 mu g N g(-1) day(-1), respectively), but did not differ at the c ropped site (average 0.15 mu g N g(-1) day(-1)). However, N-mineraliza tion measurements by the in situ core technique were found to be probl ematic as denitrification possibly occurred and concealed actual net N -mineralization. Microbial biomass did not change significantly as a r esult of the compaction treatment, but was shown to either decrease or increase over time depending on the methodology used to estimate micr obial biomass.