Influence of soil compaction on carbon and nitrogen mineralization of soilorganic matter and crop residues

We studied the influence of soil compaction in a loamy sand soil on C and N mineralization and nitrification of soil organic matter and added crop res idues. Samples of unamended soil, and soil amended with leek residues, at s ix bulk densities ranging from 1.2 to 1.6 Mg m(-3) and 75% field capacity, were incubated. In the unamended soil, bulk density within the range studie d did not influence any measure of microbial activity significantly. A smal l (but insignificant) decrease in nitrification rate at the highest bulk de nsity was the only evidence for possible effects of compaction on microbial activity. In the amended soil the amounts of mineralized N at the end of t he incubation were equal at all bulk densities, but first-order N mineraliz ation rates tended to increase with increasing compaction, although the inc rease was not significant. Nitrification in the amended soils was more affe cted by compaction, and NO3--N contents after 3 weeks of incubation at bulk densities of 1.5 and 1.6 Mg m(-3) were significantly lower (by about 8% an d 16% of total added N, respectively), than those of the less compacted tre atments. The C mineralization rate was strongly depressed at a bulk density of 1.6 Mg m(-3), compared with the other treatments. The depression of C m ineralization in compacted soils can lead to higher organic matter accumula tion. Since N mineralization was not affected by compaction (within the ran ge used here) the accumulated organic matter would have had higher C:N rati os than in the uncompacted soils, and hence would have been of a lower qual ity. In general, increasing soil compaction in this soil, starting at a bul k density of 1.5 Mg m(-3), will affect some microbially driven processes.