Nitrogen mineralization and microbial biomass as affected by soil compaction

Abstract

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 (N 2O, O 2, CO 2), and colony-forming bacteria after anaerobic and aerobic incubation of dilution plates were determined as selected indicators of anaerobicity in the soil. After 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 O 2 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. Soil compaction 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.