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MICROBIAL IMMOBILIZATION AND THE RETENTION OF ANTHROPOGENIC NITRATE IN A NORTHERN HARDWOOD FOREST
Author(s) -
Zogg Gregory P.,
Zak Donald R.,
Pregitzer Kurt S.,
Burton Andrew J.
Publication year - 2000
Publication title -
ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.144
H-Index - 294
eISSN - 1939-9170
pISSN - 0012-9658
DOI - 10.1890/0012-9658(2000)081[1858:miatro]2.0.co;2
Subject(s) - forest floor , environmental science , biomass (ecology) , organic matter , soil organic matter , soil biology , ecology , agronomy , environmental chemistry , chemistry , soil water , biology , soil science
To determine the importance of microorganisms in regulating the retention of anthropogenic NO 3 − , we followed the belowground fate and flow of 15 NO 3 − in a mature northern hardwood forest, dominated by Acer saccharum Marsh. Total recovery of added 15 N (29.5 mg 15 N/m 2 as NaNO 3 ) in inorganic N, microbial immobilization in forest floor and soil microbial biomass, soil organic matter, and root biomass pools (0–10 cm depth) was 93% two hours following application of the 15 NO 3 − but rapidly dropped to ∼29% within one month, presumably due to movement of the isotope into other plant tissues or deeper into soil. Microbial immobilization was initially (i.e., at 2 h) the largest sink for 15 NO 3 − (21% in forest floor; 16% in soil microbial biomass). After one month, total 15 N recovery varied little (24–18%) throughout the remainder of the growing season, suggesting that the major N transfers among pools occurred relatively rapidly. At the end of the four‐month experiment, the main fates of the 15 N label were in soil organic matter (7%), root biomass (6%), and N immobilized in forest floor and soil microbial biomass (6%). Temporal changes in the 15 N enrichment (atom % excess 15 N) of plant and soil pools during the first month of the experiment indicated the dynamic nature of NO 3 − cycling in this forest. The 15 N enrichment of soil microbial biomass and the forest floor significantly increased two hours after isotope additions, suggesting rapid microbial immobilization of NO 3 − . In contrast, the 15 N enrichment of soil organic matter did not peak until day 1, presumably because much of the added 15 N cycled through microorganisms before becoming stabilized in soil organic matter, or it directly entered soil organic matter via physical processes. Furthermore, the 15 N enrichment of root biomass (<0.5‐mm diameter and 0.5–2.0 mm diameter) was greatest between day 7 and day 28, following significant increases in the 15 N enrichment of soil organic matter (day 1) and, more importantly, NH 4 + (day 2). From these data we conclude that microorganisms are immediate, short‐term sinks for anthropogenic NO 3 − . Although the long‐term fate of NO 3 − additions to this forest is likely in soil organic matter and plants, the cycling of N through microorganisms appears to be the major short‐term factor influencing patterns of NO 3 − retention in this ecosystem.