Plant-microbe competition for nitrogen and phosphorus in tropical and temperate forests

Background/Question/Methods

Nitrogen (N) and phosphorus (P) limit plant growth and carbon (C) storage in terrestrial ecosystems. Soil microorganisms obtain nutrients from decomposition and asymbiotic N fixation. These strategies contrast with plant nutrient acquisition: uptake from soil, resorption of nutrients from leaves before litterfall, and symbiotic N fixation. Comparing the dynamics of nutrient mineralization during decomposition with plant nutrient acquisition strategies will improve understanding of nutrient limitation. Here we ask: are plant-microbe interactions facilitative, competitive, or neutral with regard to N and P; and are there differences between tropical and temperate forests? We performed a meta-analysis of mineralization N/P and compiled N/P of ecosystem pools and fluxes from other meta-analyses. Comparing the stoichiometry of plants and microbes can be used to infer patterns of nutrient interactions. Additionally, we developed a model with four compartments (plant, litter, microbe, and inorganic), and tracked N and P of each. In this model, plants acquire nutrients via resorption, root uptake, and N fixation. Before leaves fall, plants lose nutrients that are not resorbed. Litter nutrients are available for microbial use or mineralization. The model was parameterized with tropical and temperate forest data, and used to compare nutrient acquisition strategies of primary producers and decomposers. 

Results/Conclusions

Our results show differences between tropical and temperate ecosystems, consistent with ecological theory that suggests tropical forests are P limited and temperate forests are N limited. In the tropics, the N/P of litter (48/1) and leaves (43/1; McGroddy et al. 2004) and are higher than in temperate forests (litter: 17/1; leaves: 25/1 McGroddy et al. 2004). Litter N/P is higher than leaf N/P in the tropics, but vice-versa in temperate forests. Mineralization N/P is lower than litter N/P in both tropical (by 8/1) and temperate forests (by 9/1), showing that microbial decomposers preferentially mineralize P relative to N, globally. Mineralization N/P in temperate forests is within the range of microbial stoichiometry (6/1 – 15/1; Xu et al. 2012, Cleveland and Liptzin 2007), but in the tropics mineralization N/P is higher than microbial stoichiometry. These results provide evidence for facilitation in tropical forests: plants preferentially resorb P and microbes preferentially mineralize P. On the contrary, in temperate forests, plants and microbes compete for N. These results suggest that P is cycled more quickly than N by microbes in both tropical and temperate ecosystems, likely due to phosphorus-mineralizing enzymes. Plant-microbe facilitation may also contribute to high productivity and C sequestration in tropical ecosystems.