Soil ecologists have long sought to understand why novel organic inputs lead to inconsistent effects on soil organic matter (SOM). Organic inputs are known to stimulate SOM decay, especially where microbial decomposers are nitrogen (N)-limited and soil organic N is unprotected from microbial degradation. However, where inorganic N is available or organic N is protected, microbial decomposers may have little incentive to degrade SOM and organic inputs may therefore build up SOM. Invasive plants represent an ideal system for investigating the context-dependent effects of novel organic inputs because they inhabit a wide range of soils. To determine whether the effects of novel organic inputs on SOM depend on soil properties, we quantified the effects of an invasive grass, Microstegium vimineum, on soil carbon (C) and N pools in arbuscular mycorrhizal (AM)-dominated and ectomycorrhizal (ECM)-dominated plots within three temperate forests (in Georgia, Indiana, and North Carolina). M. vimineum’s unique 13C signature allowed us to partition M. vimineum-derived and native SOM dynamics. Given evidence that ECM-dominated soils have low N availability and a high proportion of unprotected SOM compared to AM-dominated soils, we hypothesized that M. vimineum inputs would drive SOM losses in ECM-dominated plots and SOM accrual in AM-dominated plots.
In general, ECM soils experienced greater invasion-induced losses of particulate organic matter (POM) than AM-dominated soils. The stronger effects in ECM soils likely resulted from lower inorganic N availability and greater unprotected N in these soils than in AM-dominated soils. In addition, we observed greater M. vimineum-derived C in AM-dominated soils (ca. 7%) compared to ECM-dominated soils (ca. 2%), supporting our hypothesis that organic inputs should accumulate in soils with high inorganic N availability and low SOM accessibility. However, there was no evidence that this accumulation of M. vimineum-derived C led to a net change in SOM stocks in AM-dominated areas. Rather we observed an invasion-associated increase in the more-protected mineral associated organic matter (MAOM) pool in ECM-, but not AM-dominated plots. Our results suggest that organic inputs to low inorganic N soils fuel microbes to process unprotected SOM, leading to declines in the POM pool, but a buildup of the MAOM pool. The differential response of SOM pools with different residence times suggests that short-term effects of novel organic inputs likely do not reflect long-term changes in SOM.