Riparian soil response to surface nitrogen input: temporal changes in denitrification, labile and microbial C and N pools, and bacterial and fungal respiration

To investigate potential changes in soil microbial characteristics of riparian zones subject to chronic nitrogen influx, we conducted an N-addition experiment in a riparian forest in the Coastal Plain of Georgia, USA. During 6 months, we quantified temporal changes in denitrification and respiration rates, microbial biomass, and bacterial and fungal activity, following single and repeated inorganic N additions to the near-stream zone (‘zone 1’) and the upslope area (‘zone 2’) within the riparian site. Zone 1 soil had significantly higher soil moisture, larger labile and microbial C and N pools, higher denitrification, bacterial and fungal respiration rates, and a lower pH than zone 2 soil. Over time, all added N was effectively removed from zone 1 soil by denitrification, regardless of whether N was added in a single large pulse or multiple small pulses. In contrast, added N did not significantly stimulate denitrification in zone 2 soil, which lacked sufficient anaerobicity and had lower labile C levels. In neither zone did N addition result in net N immobilization into microbial biomass, suggesting that microbial storage is not a major N-removal mechanism in this riparian forest. In both zones N additions slightly reduced microbial C and basal respiration, and increased fungal activity. Although bacterial activity was not significantly affected, the observation of increased bacterivorous nematode populations in N-amended soils suggests that bacterial production had been temporarily stimulated by N addition. However, it may be expected that long-term effects of chronic N influx mostly negatively affect microbial biomass and activity, which could threaten the seemingly indefinite N-removal potential of denitrifiers in N-loaded riparian systems.