How does soil compaction alter nitrous oxide fluxes? A meta-analysis

Abstract

Nitrous oxide (N2O) is a potent greenhouse gas and precursor of ozone layer depletion. Managed terrestrial ecosystems are major anthropogenic sources of N2O, primarily generated in the soil. The physical makeup of the soil interplays with the underlying biochemistry that produces N2O. Therefore, it has been conceptualized that either compacting or loosening the soil will alter N2O emissions; however, a unified framework of these relationships is yet to be established. Here, we compiled, reviewed and analyzed available field studies that have evaluated how applied compaction or alleviation of compacted soils impacts N2O emissions. Of the 108 available pairwise comparisons, 82 % of the cases showed detrimental increases in N2O emissions caused by increased compaction. Overall, N2O emissions nearly doubled because of soil compaction effects (P < 0.05). This doubling of N2O emissions was linked to a relative reduction of 9 % in soil porosity (i.e., 0.05 cm3 cm−3) caused by applied compaction. This linkage to soil porosity was clearly evident in both croplands and pastures. Across the managed terrestrial ecosystems evaluated, the strongest relative effects of compaction on N2O emissions were found in forest and pasture soils. Overall, N2O emissions in forest soils showed a fivefold increase caused by compaction (P < 0.05; n = 11). This massive response in forest soils can be attributed to: i) their low N2O emission baseline; ii) their acidic pH, which intrinsically favors N2O emissions; iii) having some of the heaviest traffic loads found across our meta-analysis (e.g., 16 Mg); and iv) an increase in soil microporosity. Pasture soils also displayed substantial compaction effects on N2O emissions, with a threefold increase (P < 0.05; n = 25). This was explained in part by the elevated nitrogen input rates in pastures (e.g., 600–1000 kg N ha-1). Conversely, alleviation of soil compaction by implementing controlled traffic farming (CTF) in vegetable and grain croplands reduced N2O emissions consistently in all available pairwise comparisons (P < 0.05; n = 19). At the field scale, CTF greatly decreased N2O emissions by one-third compared with farmlands managed with conventional random traffic (4.48 versus 2.86 kg N2O-N ha-1). However, after reviewing the available literature, we witnessed a paucity of information about how long it takes for soils to self-alleviate from the detrimental effects of compaction on N2O emissions. Additionally, insights gained through meta-analysis revealed the interplay of soil compaction with underlying heavy textures, acidic pH, and reduced porosities, which collectively exacerbate N2O emissions. Nevertheless, these complex interactions operate differentially across contrasting land use options.