Submitted to: Soil Biology and Biochemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/25/1999
Publication Date: N/A
Citation: ETTEMA, C.H., LOWRANCE, R.R., COLEMAN, D.D. RIPARIAN SOIL RESPONSE TO SURFACE NITROGEN INPUTS: TEMPORAL CHANGES IN DENITRIFICATION, LABILE AND MICROBIAL C AND N POOLS AND BACTERIAL AND FUNGAL RESPIRATION. SOIL BIOLOGY AND BIOCHEMISTRY. 31:1609-1624. 1999. Interpretive Summary: Riparian ecosystems are known to be effective at trapping and/or removing nutrients including nitrogen (N). Much of the trapping ability or ability to act as a N sink is due to uptake of N by vegetation and microbial removal of N by denitrification. Unfortunately, long-term enrichment of ecosystems with N may lead to N saturation and a decrease in ability of ecosystem to remove or trap N and thus a decrease in water quality improve ment function of the ecosystem. These changes may be manifested through changes in soil microbiological populations and processes in soil of the affected ecosystem. Adding N to soils of a riparian ecosystem experimental- ly, either in one large pulse or multiple smaller pulses is one way to test effects of long term N additions. N additions to a riparian forest were shown to affect denitrification rate (microbial reduction of nitrate to dinitrogen gas) where the soil was wetter and had higher soil organic matter. There was also an increase in types of nematodes that feed on bacteria and increased fungi levels in the soil. Although N additions were effectively removed from the soil of the riparian ecosystem, removal appears to have been due to denitrification rather than immobilization into biomass of microorganisms. These findings indicate that riparian ecosystem may be well adapted to long-term N additions and that N saturation hypothesized in other forest ecosystems is of less concern in riparian ecosystems adapted to high levels of N input.
Technical Abstract: A nitrogen (N) addition experiment was conducted to investigate potential in soil microbial characteristics of riparian forests subject to chronic N inputs from agriculture. Study was conducted in a mature riparian forest buffer in the Gulf-Atlantic Coastal Plain of Georgia. Temporal changes in denitrification, soil respiration, soil microbial biomass and soil bacterial and fungal activity were measured for 6 months following single and repeated inorganic N additions to riparian zone. Effects of additions were followed in a zone 1 (nearest the stream) and a zone 2 (closer to the adjacent agricultural area) within riparian site, corresponding to two zones of a managed riparian forest buffer. Zone 1 soil had higher soil moisture, larger labile and microbial C and N pools, higher denitrifica- tion, higher respiration and a lower pH than zone 2 soils. Over time, all added N was effectively removed from zone 1 by denitrification, regardless of wheter N was added in a single pulse or multiple small pulses. In contrast, added N did not significantly stimulate dentrification in zone 2 soil, which lacked anaerobic conditions and had lower labile C levels. N addition did not result in net N immobilization in either zone, indicating that microbial immobilization and storage of N 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, observation of increased bacterivorous nematode populations in N amended soils suggest that bacterial production had been temporarily stimulated by N addition.