Effects of Experimental Nitrogen Deposition on Dryland Soil Organic Carbon Storage

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