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Modeling the flow of 15 N after a 15 N pulse to study long‐term N dynamics in a semiarid grassland
Author(s) -
Dijkstra Feike A.
Publication year - 2009
Publication title -
ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.144
H-Index - 294
eISSN - 1939-9170
pISSN - 0012-9658
DOI - 10.1890/08-1172.1
Subject(s) - cycling , grassland , nitrogen cycle , biomass (ecology) , mineralization (soil science) , ecosystem , nitrogen , agronomy , competition (biology) , biogeochemical cycle , ecology , environmental science , chemistry , environmental chemistry , biology , soil water , forestry , organic chemistry , geography
Many aspects of nitrogen (N) cycling in terrestrial ecosystems remain poorly understood. Progress in studying N cycling has been hindered by a lack of effective measurements that integrate processes such as denitrification, competition for N between plants and microbes, and soil organic matter (SOM) decomposition over large time scales (years rather than hours or days). Here I show how long‐term measurements of 15 N in plants, microbes, and soil after a one‐time addition of 15 N (“labeled” N) can provide powerful information about long‐term N dynamics in a semiarid grassland. I develop a simple dynamic model and show that labeled‐N fractions in plant and microbial‐N pools (expressed as a fraction of total N in each pool) can change long after 15 N application (≥5 years). These 15 N dynamics are closely tied to the turnover times of the different N pools. The model accurately simulated the labeled‐N fractions in aboveground biomass measured annually during five years after addition of 15 N to a semiarid grassland. I also tested the sensitivity of five different processes on labeled‐N fractions in aboveground plant biomass. Changing plant/microbial competition for N had very little effect on the labeled‐N fraction in aboveground biomass in the short and long term. Changing microbial activity (N mineralization and immobilization), N loss, or N resorption/re‐translocation by plants affected the labeled‐N fraction in the short term, but not in the long term. Large long‐term effects on the labeled‐N fraction in aboveground biomass could only be established by changing the size of the active soil‐N pool. Therefore, the significantly greater long‐term decline in the labeled‐N fraction in aboveground biomass observed under elevated CO 2 in this grassland system could have resulted from an increased active soil‐N pool under elevated CO 2 (i.e., destabilization of soil organic matter that was relatively recalcitrant under ambient CO 2 conditions). I conclude that short‐ and long‐term labeled‐N fractions in plant biomass after a 15 N pulse are sensitive to processes such as N mineralization and immobilization, N loss, and soil organic matter (de‐)stabilization. Modeling these fractions provides a useful tool to better understand N cycling in terrestrial ecosystems.