Impact of monsoon rains on soil microbial community structure and nitrogen mineralization in an arid grassland

Background/Question/Methods

Arizona and New Mexico receive half of their annual precipitation during the summer monsoon season, making this large-scale rain event critical for ecosystem productivity. Moisture is an important regulator of soil processes such as decomposition and nutrient cycling. Microorganisms in soil regulate these processes, and their controls on nutrient availability are particularly important during the monsoon season when plant growth is fast and requires high nutrient input. Little is known about the impact of monsoon rains on soil microorganisms. To explore the effect of natural moisture pulses on soil microbial community structure and nitrogen mineralization, we collected soil before, during, and after monsoon precipitation in the summer of 2010. Five soil cores were taken on each of 22 collection dates from mid-June to the end of September. Total genomic DNA was isolated from individual cores, and replicate extracts were pooled for pyrosequencing of the bacterial 16S rRNA gene and fungal ITS region. We also measured nitrogen pools and determined soil moisture content for each soil core. 

Results/Conclusions

Summer rainfall in 2010 provided 99.8 mm of precipitation to our high-desert grassland site, resulting in an average soil moisture content that ranged from 1.13% to 23.8% over the course of the season. Soil inorganic nitrogen concentrations ranged from 0.72 to 8.8 ug N/g dry soil and declined steadily after rainfall as plants and microorganisms were utilizing these nutrients. On average, two times more nitrate was found in soil than ammonium. Nitrogen concentrations in dry soil were significantly different from those in wet soil for both nitrate and ammonium (p < p < 0.005). Pyrosequencing of the bacterial 16S rRNA gene and fungal ITS region indicated that the bacterial community was dominated by Actinobacteria, while the fungal community was dominated by Ascomycota and Basidiomycota. Bacterial phyla experienced little change in general, though some abundant groups like Actinobacteria and Firmicutes responded instantly to moisture. Fungal groups were more variable throughout the summer season, and some fungal genera, such as Glomus, became dominant after rain. We hypothesize that primary drivers of bacterial community structure are both soil moisture directly and plant growth, while the structure of the fungal community is mainly governed by plant growth.