Microbes out of water: The effects of drying and rewetting stress on organic farm soils

Background/Question/Methods

With the increasing severity of anthropogenic climate change, California will continue to experience oscillations between extreme rainfall and prolonged drought events. These changes have important consequences for microbial processes in agricultural soils. Excess application of chemical fertilizers in conventional farming can result in nitrogen leaching and runoff into local watersheds and cause adverse downstream effects. Organic farms, in contrast, often grow, mow, and then disc nitrogen-fixing cover crops into the soil to replenish nutrients in preparation for a growing season. However, there is still much to be learned about how increases in precipitation extremes will affect soil microbes’ abilities to cycle nitrogen in cover crop amended soils. Here, we seek to investigate changes in soil microbial activity and soil nutrients at different moisture stress levels. Specifically, we investigate how the integration of cover crop residue affects soluble nitrogen retention in soils sampled from Star Route Farms, the oldest certified organic farm in California. We are conducting a fully factorial microcosm experiment testing the effects of cover crop integration and drying duration on microbial activity and soil nutrient levels.

Results/Conclusions

Preliminary results of extractable nitrogen species show significant differences among samples dried for four weeks and then rewet. The effect of cover crops was significant on nitrite levels (p = 0.04) and nitrate levels (p < 0.01), while the effect of rewetting was marginally significant on nitrite levels (p = 0.08). Interestingly, the interaction of cover crop addition and rewetting was a significant predictor of nitrite (p = 0.04) and marginally significant predictor of ammonia (p = 0.08). The increase of nitrite post-wetting without a concurrent increase in nitrate suggests incomplete denitrification, perhaps due to a lack of Nitrobacter spp. Additional ongoing analyses, including RNA-based assays and microbial respiration measurements, will help us to illuminate potential biological drivers. We hope our results will further our understanding of soil microbial responses to future environmental changes and equip agricultural workers with better information to make more informed and climate resilient decisions in a rapidly changing world.