Liming offsets the increase in soil greenhouse gas emissions due to non-native earthworms in sugar maple forests.

Abstract

Sugar maple (Acer saccharum Marsh.) forests are the dominant ecosystems in southern Quebec (Canada) and are widely used for maple syrup production, wood products manufacturing, recreation, and sometimes converted to hybrid poplar plantations. Consequently, human footprints on these ecosystems are multifarious, with potential impacts on soil greenhouse gas (GHG) emissions. We studied the principal and interactive effects of three anthropogenic factors (liming, introduction of non-native earthworms, and tree litter quality) on soil CO2 and N2O emissions, and on related soil properties. Thirty-two PVC pipes (1.0 m x 30 cm dia.) were set upright and filled with homogenized soil collected from a sugar maple stand. Each of these mesocosms was assigned one of eight treatments from a 2×2×2 factorial array of three experimental factors (± liming, ± earthworms, maple vs. poplar litter), replicated in four complete blocks. Over the course of a 15-month trial, we measured soil CO2 and N2O emissions from each mesocosm. At the conclusion of the trial, we measured soil pH, % organic matter (SOM), mineralizable nitrogen (Nmin), water-stable aggregate index (WSAI), δ13C, and mineral-associated organic matter (C-MAOM) at each of four soil depths (0, 20, 40 and 60 cm). The effects of earthworms (+EW) and liming on the response variables were generally greater than the effects of litter types. Liming increased pH by 0.6 units in the soil surface layer. Treatments had negligible effects on SOM throughout the soil profile. Nmin increased by factors of ×15 and ×7 in the surface layer of the Liming and EW treatments respectively. In contrast, mineralizable NO3-/NH4+ ratios were 125 and 80 in the EW and EW+Liming treatments respectively, and only 30 in the Liming and Control treatments, suggesting that nitrification was stimulated by soil mixing/aeration rather than by pH. Accordingly, cumulative N2O emissions were higher in the EW and Ew+Liming treatments (500 and 250 mg N2O-N m-2, respectively) compared to the Control and Liming treatments (< 50 mg N2O-N m-2). Likewise, cumulative CO2 emissions increased in the EW treatment and decreased in the Liming treatment relative to the Control; liming offset the positive effect of earthworms when both factors were combined.  Liming increased δ13C by 3‰ in the soil surface layer, hinting that lower CO2 emissions in this treatment could have resulted from higher microbial processing of litter leading to more stable SOM. However, all treatments had no effect on C-MAOM, suggesting instead that higher δ13C in the Liming treatment resulted from higher 13C in the liming material compared to native soil C. Moreover, both Liming and EW treatments increased WSAI, thus refuting the premise that CO2 and aggregate stability were related. We conclude that the spread of non-native earthworms in sugar maple forests of southern Quebec is potentially increasing soil N2O and CO2 emissions by up to one order of magnitude. Increased N2O emissions are likely due to increased nitrification, whereas CO2 emissions cannot be predicted by changes in C-stability. Liming could potentially be used to mitigate the positive effects of earthworms on soil GHG emissions.