Carbon and nitrogen mineralization of semi-arid shrubland soils exposed to chronic nitrogen inputs and pulses of labile carbon and nitrogen

•We tested if C and N pulses affected microbial activity in soil exposed to N input.•We hypothesized that C pulses would increase microbial activity but not N pulses.•C pulses stimulated microbial respiration and net N immobilization.•N input increased N accumulation but not microbial respiration.•Microbial N retention in chaparral soil was high with sufficient labile C.

Semi-arid shrublands experience chronic anthropogenic nitrogen (N) inputs and episodic pulses of N and carbon (C). Chronic N deposition can cause N saturation, leading to soil acidification and N loss; however, some ecosystems experience acidification and N loss before N saturation while others exhibit an increase in N retention with prolonged N deposition. These divergent responses may be due to variations in soil N and C availability and microbial activity. We tested whether labile C and/or N pulses affected microbial activity (respiration and net N mineralization) in soil exposed to chronic N input. We hypothesized that C pulses would increase microbial activity, but that pulsed and chronic N enrichment would decrease activity. We found that a C pulse stimulated microbial respiration and net N immobilization, while pulsed and/or long-term N enrichment significantly increased N accumulation but not microbial respiration. Nitrate retention was highest in soil exposed to a C pulse and chronic N enrichment, indicating substantial capacity for microbial N retention with sufficient labile C availability. Our data suggest that N retention in semi-arid shrublands is driven more by spatial and temporal variations in labile C availability than exceedance of N storage capacity.