Compaction effects on CO 2 and N 2O production during drying and rewetting of soil

Abstract

The effects of compaction on soil porosity and soil water relations are likely to influence substrate availability and microbial activity under fluctuating soil moisture conditions. We conducted a short laboratory incubation to investigate the effects of soil compaction on substrate availability and biogenic gas (CO 2 and N 2O) production during the drying and rewetting of a fine-loamy soil. Prior to initiating the drying and wetting treatments, CO 2 production (−10 kPa soil water content) from uncompacted soil was 2.3 times that of compacted soil and corresponded with higher concentrations of microbial biomass C (MBC) and dissolved organic C (DOC). In contrast, N 2O production was 67 times higher in compacted than uncompacted soil at field capacity. Soil aeration rather than substrate availability (e.g. NO 3 − and DOC) appeared to be the most important factor affecting N 2O production during this phase. The drying of compacted soil resulted in an initial increase in CO 2 production and a nearly two-fold higher average rate of C mineralization at maximum dryness (owing to a higher water-filled pore space [WFPS]) compared to uncompacted soil. During the drying phase, N 2O production was markedly reduced (by 93–96%) in both soils, though total N 2O production remained slightly higher in compacted than uncompacted soil. The increase in CO 2 production during the first 24 h following rewetting of dry soil was about 2.5 times higher in uncompacted soil and corresponded with a much greater release of DOC than in compacted soil. MBC appeared to be the source of the DOC released from uncompacted soil but not from compacted soil. The production of N 2O during the first 24 h following rewetting of dry soil was nearly 20 times higher in compacted than uncompacted soil. Our results suggest that N 2O production from compacted soil was primarily the result of denitrification, which was limited by substrates (especially NO 3 −) made available during drying and rewetting and occurred rapidly after the onset of anoxic conditions during the rewetting phase. In contrast, N 2O production from uncompacted soil appeared to be primarily the product of nitrification that was largely associated with an accumulation of NO 3 − following rewetting of dry soil. Irrespective of compaction, the response to drying and rewetting was greater for N 2O production than for CO 2 production.