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Title: Elevated CO2, not defoliation, enhances N cycling and increases short-term soil N immobilization regardless of N addition in a semiarid grassland

Author
item DIJKSTRA, FEIKE - University Of Sydney
item HUTCHINSON, GORDON - Retired ARS Employee
item REEDER, JEAN - Retired ARS Employee
item Lecain, Daniel
item Morgan, Jack

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/24/2011
Publication Date: 8/4/2011
Citation: Dijkstra, F.A., Hutchinson, G.L., Reeder, J.D., Lecain, D.R., Morgan, J.A. 2011. Elevated CO2, not defoliation, enhances N cycling and increases short-term soil N immobilization regardless of N addition in a semiarid grassland. Soil Biology and Biochemistry. 43:2247-2256.

Interpretive Summary: Ecologists and agricultural scientists are keenly interested in the direct effects of rising CO2 in Earth’s atmosphere on plants and agro-ecosystems. Increases in CO2 can enhance plant growth through stimulation of photosynthesis. However, this positive effect on plant growth has been observed to decline over time due to apparent effects of rising CO2 on reducing the availability of soil N, a critical plant nutrient. An experiment was conducted to evaluate how exposure to elevated CO2 and simulated grazing (defoliation) affects soil and plant N in natural soil/plant mesocosms extracted from a native prairie. The results suggest that higher CO2 concentrations may stimulate the transfer of plant available soil N into soil organic matter, where it is less available to plants, but that simulated grazing has no significant effect on this nutrient transfer. These results suggest that the potential positive effect of CO2 on photosynthesis and growth of native grasslands may be limited by the transfer of N into forms less available to support plant growth.

Technical Abstract: Elevated CO2 and defoliation effects on nitrogen (N) cycling in rangeland soils remain poorly understood. Here we tested whether effects of elevated CO2 and defoliation (clipping to 2.5 cm height) on N cycling depended on soil N availability (addition of 1 vs. 11 g N/m2) in intact mesocosms extracted from a semiarid grassland. Mesocosms were kept inside growth chambers for one growing season, and the experiment was repeated the next year. We added 15N (1 g/m2) to all mesocosms at the start of the growing season. We measured total N and N in plant, soil inorganic, microbial and soil organic pools at different times of the growing season. We combined the Plant, soil Inorganic, and Microbial N pools into one pool (PIM-N pool) to separate biotic + inorganic from abiotic N residing in soil organic matter (SOM). With the N measurements we were then able to calculate transfer rates of N from the active PIM-N pool into SOM (soil N immobilization) and vice versa (soil N mobilization) throughout the growing season. We observed significant interactive effects of elevated CO2 with N addition and defoliation with N addition on soil N mobilization and immobilization. However, no interactive effects were observed for net transfer rates. Net N transfer from the PIM-N pool into SOM increased under elevated CO2, but was unaffected by defoliation. Elevated CO2 and defoliation effects on the net transfer of N into SOM may not depend on soil N availability in semiarid grasslands, but may depend on the balance of root litter production affecting soil N immobilization and root exudation affecting soil N mobilization. We observed no interactive effects of elevated CO2 with defoliation. We conclude that elevated CO2, but not defoliation, may limit plant productivity in the long-term through increased soil N immobilization.