UC Merced

In pristine montane meadows, soils can sequester more carbon (C) per year than soils from tropical forests. However, many of these meadows have been degraded from overgrazing by domestic livestock, road construction, and other human disturbances, altering their hydrology, and resulting in more frequent soil drying and rewetting (DRW) cycles. Such increased frequency of DRW cycles could lead to reduction in soil C and nitrogen (N) pools via the “Birch Effect,” a frequently observed phenomenon where an increase in soil organic matter decomposition (SOM) occurs immediately following the rewetting of dried soils. Using laboratory incubations, we evaluated the potential impact of increased fluctuations in soil water content on greenhouse gas emissions (CO2, N2O, CH4), microbial C and N, salt-extractable organic C and N, rates of net N transformations, potential exoenzyme activities, organic matter functional composition, and reduction-oxidation active metals across two pairs of meadows (undegraded-degraded and restored-unrestored) in the Sierra Nevada, California, USA, with different degrees of degradation. Replicate air-dry soil samples (0-15 cm depth) were subjected to five different water treatments over a 91-d period (including a seven-day preincubation): no water addition (air-dry control), maintained at field capacity, maintained at saturation, maintained at saturation for one-week pre-incubation, allowed to dry for three weeks and then rewetted to saturation (4 cycles), and maintained at saturation for one-week pre-incubation, allowed to dry for one week and then rewetted to saturation (12 cycles). We found that cumulative net CO2 emission was not increased under the two DRW treatments compared to the continuously saturated condition for any of the four soils assessed. Surprisingly, cumulative net CO2 emissions were either greater or similar under continuously saturated soil moisture conditions compared to soils maintained at field capacity. Only for the degraded soil was cumulative net N2O emissions higher under DRW treatments (4 cycle only) than under continuously saturated conditions. There were no significant differences in net CH4 emissions across all five treatments for any soil. While there was an immediate increase in net CO2 and N2O (but not CH4) emissions following the rewetting of dried soils analogous of the Birch effect (especially in the 4 cycle treatment), there were only relatively minor changes in other soil biochemical processes and pools compared to the consistently saturated soils. Overall, our results were largely consistent with field measurements that suggest the observed decreases in C storage in degraded Sierran meadow soils are due to reductions in C inputs rather than increases in C outputs.