Recent studies have pointed to rock as an overlooked source of nitrogen in select terrestrial ecosystem sites; however, questions remain over the broader significance of these findings for ecologists. How widespread are nitrogen-rich parent materials? Does nitrogen weather from rocks at rates that could fundamentally alter ecosystem structure and functioning? How does biology access rock nitrogen in Earth's critical zone? Here we examine the ecological significance of rock nitrogen weathering rates using a combination of empirical measurements and computational modeling approaches. Our study sites span desert to forest ecosystems, and an array of parent materials, tectonic uplift regimes and mean annual climates. We present measures of rock, soil, and plant chemistry; biomass; and rates of nitrogen weathering from deep bedrock into the soil. Our model simulates global nitrogen weathering rates at 0.5 x. 0.5 lat./long. via a statistical probability ensemble (Monte Carlo) of such factors as parent material chemistry, tectonic uplift, climate and relief. To appraise the relative importance of rock nitrogen sources to ecosystems, we compare our modeled nitrogen weathering fluxes to nitrogen fixation and deposition inputs to the terrestrial biosphere.
Results/Conclusions
Our diverse combination of empirical measurements and modeling approaches suggest that bedrock is a globally important source of nitrogen to natural ecosystems on biologically relevant timescales. Our findings reveal faster rates of microbial nitrogen cycling; higher above-ground biomass stocks; and plant/soil nitrogen enrichment with increasing bedrock nitrogen contents. Nitrogen weathering rates of parent material greatly exceed those for non-essential elements (sodium) and less limiting elements (potassium) in the soil; rapid chemical depletion of rock nitrogen minerals across sites is consistent with biological demand for this limiting nutrient. At the global scale, our model predicts that 12 to 18 teragrams of nitrogen is released from rock annually, with distinct hotspots at higher latitudes and higher elevations, where parent material, climate and tectonic uplift factors promote rock weathering. In boreal forest, for instance, rock nitrogen inputs are estimated to exceed atmospheric nitrogen deposition inputs. This previously unaccounted for rock nitrogen flux has implications for the global carbon cycle, and suggests that rocks can help to explain the “missing nitrogen” source observed for many terrestrial ecosystem sites.