Sunstainable silvopastoral solutions can prevent the collapse of the tropical nitrogen cycle

Background/Question/Methods

Warm and moist tropical forests are many times rich in soil nitrogen and can therefore offer ideal habitat for agricultural land-uses such as crops and pastures.  However, this nitrogen-rich state appears to be related to the existence of dinitrogen (N₂) fixing trees in these forests.  Deforestation, and forest conversion to agricultural land-uses, often leads to a gradual decline in the availability of soil nitrogen until agricultural productivity is no longer profitable.  At this stage either anthropogenic nitrogen (fertilizer and manure) is added, or degraded farmland is abandoned, and substituted by clearing additional forest area.  Our work aims at understanding: (1) how forest conversion to agriculture can lead to the collapse of the nitrogen cycle? and (2) are there alternative management schemes, for example silvopastoral cattle pasture systems, that can provide a sustainable solution?  We used a model of the local nitrogen cycle to evaluate how forest conversion to different land-use systems (pasture, silvopasture, coffee plantations and crops) influences both the local nitrogen budget and nitrogen exports from the system.  We compared our simulations to empirical data of the nitrogen budgets in forests, pastures and silvopastoral systems in Amazonian Colombia.

Results/Conclusions

Our results indicate that the nitrogen-rich conditions that develop in mature tropical forests are sensitive to agricultural exploitation.  We show how cycles that consist of deforestation, followed by exploitation, land abandonment and forest regrowth, influence soil nitrogen conditions and can lead to a collapse from a nitrogen-rich tipping point in the forest to a degraded nitrogen-poor state in which forest cannot regenerate.  Silvopastoral practices, whereby scattered N₂-fixing trees grow in the pasture and provide shade and nitrogen inputs, are potentially useful to create a sustainable pastoral system and to prevent the collapse of the nitrogen cycle.  Our empirical results from Macagual, Colombia, show high nodulation (indicative of N₂ fixation rate) in N₂-fixing Inga trees growing in silvopastoral systems.  We show that these N₂-fixing trees increase local nitrogen availability.  Our results indicate that N₂-fixing trees play an important role in maintaining the nitrogen cycle in many tropical forests and can serve to create a sustainable agricultural solution that preserves soil nitrogen without significant compromise in the production.