Exotic grass litter modulates seasonal pulse dynamics of CO2 and N2O, but not NO, in Mediterranean-type coastal sage scrub at the wildland-urban interface

Abstract

Mediterranean shrublands adjacent to urbanization experience nitrogen deposition and exotic grass invasions which likely have downstream consequences for carbon and nitrogen emissions from soils. We tested a hypothesis that soil wetting produces trace gas pulses modified by seasonal wetting history, grass litter availability, and cover type in these systems. Over two seasons, we conducted 48-h wetting experiments and measured CO2, N2O, and NO pulses at an invaded, polluted California shrubland following grass litter addition to sites dominated by either an invasive grass (Schismus barbatus) or a native shrub (Erioginum fasciculatum). CO2 and N2O pulses consistently appeared 15 min post-wetting and diminished within 12 h; NO peaked later and remained elevated at 24 h. All pulses were stronger in the dry season than wet season. Grass litter amendments increased CO2 and dry-season N2O pulses without significantly modifying NO pulses. Grass cover reduced CO2 pulses compared to shrub cover. Our results support the hypothesis that shrubland soils produce stronger pulses of CO2, N2O, and NO during the dry season when wetting is less frequent. We show that invasive grass litter can provide a labile C source that stimulates CO2 and N2O, but not NO, emissions from shrubland soils.