Trajectories of Nutrient Limitation Under Nitrogen Enrichment.

Nitrogen typically limits plant growth on developmentally young soils, when other essential nutrients are relatively abundant in freshly weathered bedrock.  Nitrogen limitation leads to strong nitrogen retention in soils, which over thousands of years should promote sufficient nitrogen accumulation to overcome limitation.  Yet, field data routinely show soil nitrogen stocks much lower than long-term accumulation potentials.  This nitrogen deficit has prompted interest in understanding nitrogen loss pathways that are not directly controlled by nutrient limitation processes.  Disturbances represent one class of processes that can cause large nitrogen losses from nitrogen-limited ecosystems, but in some cases severe disturbance favors symbiotic nitrogen fixing plants that increase soil nitrogen stocks.  High nitrogen input from symbiotic fixation, as well as chronic deposition and geologic sources, may overcome nitrogen limitation relatively early in soil development.  Due to couplings between biogeochemical cycles, high nitrogen accumulation can have cascading effects on other potentially growth-limiting nutrients in soils.  Classical nutrient limitation theory emphasizes a switch from nitrogen to phosphorus limitation over time, driven ultimately by soil phosphorus occlusion and loss.  In contrast, rapid nitrogen enrichment on developmentally young soils can accelerate phosphorus cycling and availability, particularly though organic phases. Excessive nitrogen enrichment also accelerates soil acidification and losses of easily weathered base cations. In this way, rapid nitrogen enrichment can lead to a trajectory of nutrient limitation that is marked by transient base cation limitation (or co-limitation) on soils where phosphorus depletion has not yet reached a terminal steady state.  Progress in evaluating how nitrogen enrichment leads to phosphorus or base cation (or other) nutrient limitation can be improved by cross-system comparisons of nitrogen enrichment from biological fixation, atmospheric deposition, and geologic sources, as well as coordinated fertilization experiments on soils of varied nutrient status.