A model analysis of N and P limitation on carbon accumulation in Amazonian secondary forest after alternate land-use abandonment

Productivity and carbon (C) storage in many mature tropical forests are considered phosphorus (P) limited because of advanced soil weathering. However, disturbance can shift limitation away from P and toward nitrogen (N) because of disproportionately large N losses associated with its mobility relative to P in ecosystems. This shift was illustrated by model analyses in which large disturbances including timber extraction and slash-burn were simulated in a P-limited tropical forest. Re-accumulation of ecosystem C during secondary forest growth was initially N-limited, but long term limitation reverted to P. Mechanisms controlling shifts between N and P limitation included: (1) N volatility during slash combustion produced ash that increased soil solution P more than N, (2) a wide N: P ratio in residual fuel and belowground necromass relative to soil organic matter (SOM) N: P produced a simultaneous P sink and N source during decomposition, (3) a supplemental ( to aerosol deposition) external N source via biological N fixation. Redistribution of N and P from low C: nutrient SOM to high C: nutrient vegetation was the most important factor contributing to the resilience of ecosystem C accumulation during secondary growth. Resilience was diminished when multiple harvest and re-growth cycles depleted SOM. Phosphorus losses in particular resulted in long-term reductions of C storage capacity because of slow re-supply rates via deposition and the absence of other external sources. Sensitivity analyses limiting the depth of microbially active SOM in soil profiles further illustrated the importance of elements stored in SOM to ecosystem resilience, pointing to a need for better knowledge on the functioning of deeply buried SOM.