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Imprint of oaks on nitrogen availability and δ15N in California grassland-savanna: A case of enhanced N inputs?

Plant Ecology

By:
and
DOI: 10.1007/s11258-006-9238-9

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Abstract

Woody vegetation is distributed patchily in many arid and semi-arid ecosystems, where it is often associated with elevated nitrogen (N) pools and availability in islands of fertility. We measured N availability and δ15N in paired blue-oak versus annual grass dominated patches to characterize the causes and consequences of spatial variation in N dynamics of grassland-savanna in Sequoia-Kings Canyon National Park. We found significantly greater surface soil N pools (0–20 cm) in oak patches compared to adjacent grass areas across a 700 m elevation gradient from foothills to the savanna-forest boundary. N accumulation under oaks was associated with a 0.6‰ depletion in soil δ15N relative to grass patches. Results from a simple δ15N mass balance simulation model, constrained by surface soil N and δ15N measured in the field, suggest that the development of islands of N fertility under oaks can be traced primarily to enhanced N inputs. Net N mineralization and percent nitrification in laboratory incubations were consistently higher under oaks across a range of experimental soil moisture regimes, suggesting a scenario whereby greater N inputs to oak patches result in net N accumulation and enhanced N cycling, with a potential for greater nitrate loss as well. N concentrations of three common herbaceous annual plants were nearly 50% greater under oak than in adjacent grass patches, with community composition shifted towards more N-demanding species under oaks. We find that oaks imprint distinct N-rich islands of fertility that foster local feedback between soil N cycling, plant N uptake, and herbaceous community composition. Such patch-scale differences in N inputs and plant–soil interactions increase biogeochemical heterogeneity in grassland-savanna ecosystems and may shape watershed-level responses to chronic N deposition.

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Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Imprint of oaks on nitrogen availability and δ15N in California grassland-savanna: A case of enhanced N inputs?
Series title:
Plant Ecology
DOI:
10.1007/s11258-006-9238-9
Volume
191
Issue:
2
Year Published:
2007
Language:
English
Contributing office(s):
Forest and Rangeland Ecosystem Science Center
Description:
12 p.
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Plant Ecology
First page:
209
Last page:
220
Country:
United States
State:
California