Human activities have more than tripled the amount of N and P available to ecosystems, resulting in complex effects on the environment. The objectives of this study are to evaluate the N saturation status of chaparral ecosystems exposed to elevated atmospheric N deposition and identify mechanisms for the destabilization of P in soils and lacustrine sediments that may be contributing to the eutrophication of high-elevation Sierra Nevada lakes. The N saturation status of a chaparral catchment was assessed through an N budget including atmospheric N inputs, hydrologic and gaseous N outputs, belowground N dynamics, and isotopic separation of streamwater nitrate sources. Phosphorus pools in soil and lake sediment were identified and monitored seasonally to understand how changes in physicochemical conditions and hydrology affect the transport of P to Sierran lakes. In chaparral, seasonal transitions, microbial C limitation, and the asynchrony between N availability and plant N demand modulated the kinetic N saturation of the catchment. The transition from dry to wet soil conditions induced rapid nitrification of ammonium leading to large hydrologic and gaseous N losses. However, during the growing season, N losses were minimal highlighting the limitations of applying standard N-saturation theory to xeric landscapes. In high elevation catchments, P is tightly bound by Al and atmospheric P inputs and rock weathering during the Holocene are the primary sources of present day soil and sediment P. Lake sediments represent a strong sink for P in high elevation lakes even during periods of anoxia. During snowmelt, 27% of the soil P cycles between the inorganic and organic pools raising the possibility that changes in snowpack dynamics could potentially explain long-term trends in P supply to lakes. Although intrinsic differences exist between chaparral and subalpine catchments, they respond similarly to the Mediterranean climate of California. The loss of tight biological control of N and P during seasonal transitions in the Sierra Nevada is similar to the response of ecosystems to periodic disturbances like fire and logging. Thus, nutrient limitation of these terrestrial and aquatic ecosystems and their sensitivity to anthropogenic pollution is tightly linked to interannual and long-term changes in climate.

Nitrogen enrichment due to atmospheric nitrogen deposition has affected plant growth and microbial activity globally, leading to an increase in soil organic carbon in many ecosystems. Drylands cover ~45% of the global land area and store ~32% of the global carbon stocks, but the response of dryland carbon storage to atmospheric nitrogen deposition remains unclear and understudied relative to mesic systems. To help identify changes in carbon storage, soil organic carbon can be separated into a plant-derived, particulate organic carbon fraction, and a largely microbially-derived, mineral-associated organic carbon fraction. Observations from mesic systems suggest that nitrogen enrichment should increase the efficiency by which microbes incorporate carbon into mineral-associated forms (carbon stabilization efficiency) if pH stays constant. Under acidification, a common response to nitrogen deposition—microbial biomass and enzymatic organic matter decay should decrease, leading to a build-up in particulate organic carbon. However, in drylands, where organic carbon often associates with mineral surfaces via Ca-bridging, acidification can also abiotically affect mineral-associated organic carbon if Ca is leached. To test how nitrogen deposition affects dryland carbon storage I used four long-running experimental nitrogen deposition experiments in Southern California, where two of the sites showed strong nitrogen-induced acidification. I studied changes in soil organic carbon fractions, soil extracellular enzyme activities, microbial carbon stabilization efficiency and exchangeable Ca. Experimental nitrogen deposition had relatively small effects on soil organic carbon storage, which appeared to be mostly driven by soil physicochemical changes. Particulate organic carbon did not increase despite previously reported increases in plant biomass, and decreases in microbial biomass and extracellular enzyme activities in acidified sites. Furthermore, microbial carbon stabilization efficiency was unaffected by N fertilization in non-acidified sites, but decreased in short-term but not long-term incubations in acidified sites. Importantly, mineral-associated organic carbon decreased significantly at one of the acidified sites, likely as result of pH-induced Ca loss. Our measurements suggest that long-term effects of nitrogen fertilization on dryland carbon storage might be of abiotic nature, such that drylands where Ca-stabilization of soil organic carbon is prevalent and may undergo acidification, may be most at risk for significant loss of mineral-associated organic carbon.

Future climate projections predict a global increase in precipitation variability as well as a trend towards aridity, which can influence how nitrogen (N) is cycled in terrestrial ecosystems. Increasingly arid conditions can alter N cycling by increasing soil inorganic N concentrations and making this N more susceptible to loss. Further, shifts in precipitation patterns in one season can establish effects on the N cycle that can “carry-over” into the following season, known as precipitation legacy effects. However, how current and historic precipitation patterns may alter soil N availability through changes in inorganic N supply—i.e., gross N mineralization and gross nitrification—is not well constrained. We measured the effects of altering precipitation variability on soil N cycling using an isotope pool dilution on soils from a Pinyon–Juniper dryland under a precipitation manipulation experiment. We found that wintertime drought, the most extreme drought we imposed, reduced gross N mineralization by ~2.5x and gross nitrification rates by ~9.5x compared to ambient conditions, establishing a legacy effect that reduced gross N mineralization by ~2x compared to ambient conditions the following summer. As we observed an increase in soil N concentrations (~6x greater than ambient conditions) simultaneous with lower gross rates of inorganic N supply under drought, our results imply that dry conditions must have reduced N immobilization rates below those of inorganic N supply to account for the increased N availability. Our measurements show that extreme drought can have negative consequences on inorganic N supply that persist even after the drought ends, suggesting these responses may become more common as drought is expected to increase in frequency and severity across the globe.