Nitrogen mineralization and microbial biomass as affected by soil compaction

Soil compaction may retard decomposition of organic matter and N mineralization and increase gaseous losses of N. We studied the effect of soil compaction on the turnover of N from added organic materials in pots with Italian-ryegrass (Lolium multiflorum Lam.) plants. Solid cattle manure or 15N-labelled white-clover (Trifolium repens L.) material was incubated at controlled temperature (15°C) and moisture (pF 2.4 or pF 1.8) in a sandy loam with a bulk density of 1.1 or 1.4 g cm−3. The distribution of labelled clover N was determined after 22, 42, 64 and 98 days. Also, net N mineralization from manure and clover was determined by subtraction of the values for unamended soil. Hydrogen sulphide, volatile fatty acids, soil acidity, phytotoxicity (bioassay), soil atmosphere composition (N2O, O2, CO2), and colony-forming bacteria after anaerobic and aerobic incubation of dilution plates were determined as selected indicators of anaerobicity in the soil. 98 days at pF 2.4, soil compaction (1.4 g cm−3) had reduced the net mineralization of clover 15N by 18% compared to uncompacted soil, a reduction corresponding to 4% of added 15N. Total 15N recovery was not reduced by compaction, and there was no evidence of anaerobic metabolism. Consequently, increased gaseous N losses or retarded decomposition due to O2 deficiency could not account for the difference. Compaction increased 15N retention in soil organic matter by 8% and in microbial biomass (chloroform fumigation-extraction) by 1% of added 15N. The compaction effects increased successively during the incubation. The negative effect of compaction on N mineralization was stronger at the higher soil moisture content (pF 1.8, sampled on day 64 only), but no evidence of anaerobicity was detected. Compaction effects on N mineralization, bacterial biomass (microscopy) and microbial biomass determined by difference (amended minus unamended soil) agreed with the 15N results. ompaction reduced the volume of pores with neck dia > 30 μm, i.e. pores available to nematodes, from 30.4 to 14.6% of total bulk volume. The volume of pores < 3 μm, i.e. pores that are unavailable to cellular organisms or available only to bacteria and fungi, increased from 12.7 to 15.6%. The results strongly suggest that N mineralization in compacted soil was reduced by increased physical protection of organic materials and microbial biomass against further attack, particularly by nematodes grazing on microorganisms.