PARENT SESSION
Oral Session #93: Forest Carbon Dynamics: Soils and litter.
Presiding: J. Ward
Thursday, August 8. 1:00 PM to 3:45 PM. Grand Ballroom Central, Radisson.

The influence of a N-fixing shrub on soil C dynamics in a California forest soil.

Winsome, Thais^*,1, Horwath, William, Scow, Kate¹, Powers, Robert², ¹ University of California, Davis, CA² USDA Forest Service Pacific Southwest Research Station, Redding, CA

ABSTRACT- Field studies of forest ecosystems have shown a positive relationship between the presence of N-fixing shrubs or trees and soil C stocks, but explanations for this remain unresolved. To identify potential mechanisms, we conducted a laboratory study of soils from a research plantation of Ponderosa pine (Pinus ponderosa Doug. ex Laws.) containing stands with (UP) and without (UA) understory vegetation. Soils were incubated for 57 weeks both unamended and with litter amendments applied as Ponderosa pine (P) and mixtures of pine with the N-fixing shrub Ceanothus integerrimus (PC), uniformly labeled with ¹³C and ¹⁵N in a reciprocal-label design that allowed us to partition all litter- and soil-derived C and N pools. There were significant interactions between vegetation history and litter amendment in C mineralization patterns, with a higher (p<0.01) net turnover of both soil- and pine-derived C in the P treatments compared to the PC treatments, especially in the UA soil. Two-pool models of C-mineralization rates showed a decrease (p<0.05) in both the size and mean residence time of stable soil C in the P treatments. Associated with these patterns were significantly lower (p<0.05) microbial substrate-use efficiencies and greater (p<0.01) mineralization of soil-derived N in the P treatments, suggesting that both residue biochemistry and N-limitation may be factors contributing to the observed higher net C loss. Thus, N-fixers may play a dual role in forest C dynamics by promoting both the retention of stable soil C and the accrual of new, labile C.

KEY WORDS: forest, carbon, nitrogen, N-fixer