Soil carbon and nitrogen dynamics throughout the summer drought in a California annual grassland

- Relatively labile N and C substrates accumulate in soils during the dry season.
- Microbial biomass and moisture control substrate accumulation in surface soils.
- Limited physical accessibility controls substrate accumulation in deeper layers.

Drought is common in soil, yet its intensity and immediate effects vary with the timing of precipitation and the depth in the soil profile. We set out to analyze the patterns of soil C and N dynamics through the dry summer in a California annual grassland, and to analyze the processes that control those dynamics. During the dry season, inorganic N, soluble organic matter, and microbial biomass accumulated in soil. Concentrations were generally greater in the surface soil layer (0-10 cm) but increased in both surface and deeper soil (10-20 cm) over the course of the summer. There was a positive relationship between drought length, microbial biomass C and N, extractable C and N, and microbial activity upon rewetting. Upon rewetting of dry soils there was a pulse in nitrate availability (under field conditions) and, in the top 10 cm, a positive correlation between the initial CO2 pulse released upon rewetting and the drought length. In contrast, below 10 cm there was no correlation between the rewetting CO2 pulse and drought length. Soils from the top 10 cm and below 10 cm dried under controlled laboratory conditions showed the same CO2 pulse response to the length of drying as those in the field. These results suggest that the substrates that accumulate during the dry season are relatively labile, and that, in surface soils, biomass/substrate amounts and water availability control their persistence, while in deeper layers, some other factor is more important.