UC Riverside

Changes in precipitation patterns can influence soil nitrogen (N) cycling by altering the timing and extent of moisture availability. Arid conditions, associated with more frequent and severe droughts, can increase soil inorganic N concentrations when diffusion constraints decouple N sources and sinks, making this N susceptible to loss. Further, shifts in precipitation patterns in one season can “carry over” N into subsequent seasons, known as precipitation legacies. However, how these legacies increase soil N availability through changes in inorganic N production (i.e., gross N mineralization and nitrification) remain unclear. We manipulated the amount and timing of precipitation in a pinyon–juniper dryland by either adding or excluding precipitation during the winter or summer months and then measured the effects on inorganic N production using isotope pool dilutions. Excluding precipitation in the winter wet season—the most severe drought imposed—reduced average (± std. error) gross N mineralization by ~ 2.5 times to 0.3 ± 0.08 µg N g−1soil h−1 (p = 0.0463) and gross nitrification by ~ 9.5 times to 0.2 ± 0.05 µg N g−1soil h−1 (p < 0.0001) relative to ambient conditions, yet inorganic N production persisted under dry conditions. Because this mineralized N was not subsequently taken up by N sinks, inorganic N accumulated in the soil; soil N concentrations were ~ 6 times greater than ambient conditions (0.5 ± 0.2 µg NO3– -N g−1 soil) under experimental winter drought. This winter drought also produced a legacy effect the following summer, where gross N mineralization declined by ~ 2 times below ambient (Winter- = 0.4 ± 0.2 µg N g−1soil h−1) even though the drought had ended. Across seasons, precipitation legacy effects reduced gross N mineralization when organic N was depleted, but did not affect gross nitrification, favoring the accumulation of nitrate over ammonium, a more mobile anion susceptible to loss. Precipitation legacies also carried over ammonium from summer to winter, likely due to the buildup of ammonium from dry conditions in the previous summer. Our results highlight how the legacies of past precipitation can increase soil N availability, selecting N species that are vulnerable to loss when ecosystem N sources and sinks decouple. These legacies can favor N loss over retention, contributing to ecosystem N limitation and to overall declining N availability in regions experiencing increasing drought frequency.