Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/10/2005
Publication Date: 4/1/2005
Citation: Steenwerth, K.L., Jackson, L.E., Calderon, F.J., Scow, K.M., Rolston, D.E. 2005. Response of microbial community composition and activity in agricultural and grassland soils after simulated rainfall. Soil Biology and Biochemistry. 37: 2249-2262. Interpretive Summary: Periodic wet-dry cycles in Mediterranean climates affect soil microbial processes, but the dynamics will vary depending on the intensity of the land use. Land use intensification (LUI) is associated with decreases in soil quality and shifts in C and N dynamics with increased tillage frequency and external chemical inputs. We previously determined LUI results in distinct microbial communities compared to less intensively managed sites. Here, our objectives were to understand how LUI affected ecosystem functions (EF) provided by the soil microbial community, and to investigate links between soil microbial community composition and EF. The California ecosystems consisted of annual vegetable crops and annual grassland in Salinas Valley, and perennial grass agriculture and native perennial grassland in Carmel Valley. In the agricultural systems, reductions in both the measures of microbial diversity and resistance of the microbial community composition to change after a perturbation were associated with lower EF, i.e., lower microbial responses to increased moisture availability. This suggests that characteristics associated with grassland soils should be considered during management of agricultural soils.
Technical Abstract: We explored the hypothesis that land use intensification decreases the resistance of microbial community composition (MCC) and activity to perturbation. Soil carbon (C) and nitrogen (N) dynamics and microbial responses to a simulated rainfall were measured in grassland (GE) and agricultural ecosystems (AE). The California ecosystems consisted of annual vegetable crops and annual grassland in Salinas Valley, and perennial grass agriculture and native perennial grassland in Carmel Valley. Intact cores (30 cm deep) were collected in March 1999. After equilibration, dry soil cores (approx. -1 to -2 MPa) were exposed to a simulated rainfall of 2.4 cm, and then were measured at zero, six, 24, and 120 h after rewetting. Microbial biomass C and inorganic N did not respond to rewetting. N2O and CO2 efflux and respiration increased after rewetting in all soils, with larger responses in the grassland than in the agricultural soils. Phospholipid fatty acid (PLFA) profiles indicated that changes in MCC after rewetting were most pronounced in intensive vegetable production, followed by the relict perennial grassland. Consistent changes in specific PLFA markers did not occur across all sites. There were more similarities among microbial groups associated with PLFA markers in AE than GE. Differences in responses of microbial communities may be related to the different plant species composition of the grasslands. Agricultural intensification appeared to decrease microbial diversity, as estimated from numbers of individual PLFA identified for each ecosystem, and reduce resistance to change in MCC after rewetting. In AE, reductions in both the measures of microbial diversity and the resistance of MCC to change after a perturbation were associated with lower ecosystem function, i.e., lower microbial responses to increased moisture availability.