Decomposition in a non-Leibig universe

Background/Question/Methods

Liebig’s law of the minimum was derived to predict the fertilizer most likely to limit crop yield in a given field. It has become a pervasive meme in ecology given its simplicity and its evocation of parsimony. Applied to more complex ecological systems and processes, Leibig’s law remains a useful rule of thumb for identifying candidate elements for nutrient limitation. In tropical ecology, for example, it has spawned valuable work documenting the geography of N versus P limitation of both NPP and decomposition. However, the notion of a single limiting nutrient for processes that sum the interactions of thousands of species at multiple spatial scales is increasingly untenable.

Results/Conclusions

On the Gigante plots in Panama, 40x40 m plots have been fertilized with N, P, K and a micronutrient cocktail for over 10 years. These experiments have yielded evidence of at least three nutrients--P, K, and one micronutrient--limiting decomposition.  Moreover, different nutrients enhance the decomposition rates of different kinds of detritus.

Sodium is an element unnecessary to most plants, but vital to fungi and animals. We have shown that consumers increasingly crave sodium as one samples forests from coastlines to the continental interior. In two such continental forests in Peru and Ecuador, we added NaCl to plots from 0.25 m² to 16 m², mimicking natural inputs of urine and rainfall respectively. Both enhanced the activity of animal and fungal decomposers; both increase decomposition rates up to 50%. Is Na special? Or does the distribution of S, Ca, Mg, Mn, Fe, I, Mo, and other elements, at grains from soil particles to hectares, also play significant roles driving ecosystem respiration?

The way ahead is in many ways simple, as the lack of evidence for a non-Leibig world derives from a lack of evidence. Ecology needs more maps of biogeochemistry at scales relevant to the organisms and processes studied; a deeper understanding of the way that biochemistry mediates population interactions; and more lab and field experiments exploring the biogeography of the periodic table.