Due to low rates of NPP and small soil organic matter pools, microbial activity in semiarid ecosystems is expected to be C- rather than N-limited. However, seasonal patterns of environmental conditions and plant phenology may exert strong effects on soil C and N availability.
Our objective was to examine whether soil microbial C and N dynamics vary with seasonal variations in soil moisture, and determine if these semiarid soils were primarily C- or N-limited. We collected soils from intact mesocosms planted with the invasive annual cheatgrass on five sampling dates over 17 months in a semiarid rangeland. On each sampling date, we measured C mineralization and gross N cycling rates, and microbial biomass pools during short-term laboratory incubations. We assessed microbial N limitation by comparing gross NH₄⁺ assimilation and nitrification rates under ¹⁵N-stimulated and unstimulated conditions. We hypothesized that soil C availability and gross N immobilization rates would increase with seasonal patterns of soil moisture, such that soil microbial activity would be N-limited in autumn and spring.

Results/Conclusions

Our results indicate that soil microbial activity was C-limited during the summer and autumn, but N-limited in the spring. C mineralization rates and microbial biomass pools increased with soil moisture from summer to autumn, with greatest C mineralization rates in the spring. In addition, gross rates of NH₄⁺ cycling were significantly greater in spring compared to summer and autumn, consistent with greater C availability. While NH₄⁺ assimilation and nitrification rates accounted for similar proportions of gross N mineralization, addition of ¹⁵N had a significant seasonal effect on the fate of available NH₄⁺ consumed by microbes. ¹⁵NH₄⁺ addition significantly stimulated autotrophic nitrification rates in summer and autumn, but stimulated NH₄⁺ assimilation in spring. While microbial activity in many soils has previously been considered to be C-limited, we conclude that there is an important temporal component to the relative degree of C versus N availability in these semiarid soils.