The implications of tropical deforestation and agriculture on landscape-scale nitrogen cycling

Background/Question/Methods

Deforestation and land conversion of tropical forests in Latin America and the Caribbean (Neotropics) is expected to increase in the next two decades.  However, the implications of agricultural development to terrestrial nutrient balances and exports to aquatic systems in the Neotropics are unknown.  Specifically, the impacts of land-use change on the nitrogen cycle, an intensively studied question in extratropical regions, may differ dramatically in Neotropical regions. Much of these tropical forests are characterized by nitrogen-rich soils as a result of the prevalence of dinitrogen fixing organisms.  Thus, removal of naturally occurring symbiotic and non-symbiotic N₂-fixers by deforestation, combined with agricultural manipulations of the nitrogen budgets (e.g., fertilizer and manure application) may result in unpredictable shifts in the distribution and dynamics of nitrogen.  The objectives of this work were to: (1) estimate the impacts of forest conversion into agricultural land-uses on the local nitrogen cycle; and (2) provide recommendations for sustainable management.  I used a combination of empirical data and models at different spatial scales, ranging from the scale of agricultural parcels to the resolution of global models, to measure how deforestation and agriculture (pasture, coffee plantations and crops) changes the N-budget within each system and N-flow across systems.

Results/Conclusions

I compiled data from experimental and modeling systems and found that intensive agriculture (using some kind of fertilizer) on soils that are known to be N-saturated in intact forests, had a general negative effect on soil N-budgets and flows. Moreover, in many cases the use of fertilizers did not lead to much increased agricultural yield.  Alternative agricultural practices, in which N₂-fixing trees were used for sylvipastoral or shade purposes, are potentially useful to decrease the use of fertilizer, but do not result in equivalent yields. I used a general theoretical framework that compiles my different findings to understand if there are and what are the tipping-points in which the ecosystem N-budget shifts from poor to saturated state and vice-versa.   My findings provide insight on how different approaches to agricultural development affect the sustainability of agricultural uses in neotropical landscapes as well as the integrity of natural forests and waterways.