Reactive nitrogen (N) losses from agricultural systems contribute to climate change and regional air and water pollution. These losses are not well quantified in the tropics, but estimates of crop N uptake are often less than 50% N fertilizer applied, suggesting that N losses could be high. Our expectations of N losses from N fertilizer use are largely based on agroecosystems in well-studied temperate zones. Far less is known about N losses in tropical regions where highly-weathered soils differ from many temperate soils. For example, highly-weathered tropical Ultisol and Oxisol soils have high anion exchange capacity, which aids in nitrate retention. Estimating the magnitude of N losses in tropical agroecosystems is important because most of the land available for agricultural expansion is in tropics. Our meta-analysis of nitrate leaching, nitrous oxide, and nitric oxide losses from fertilized tropical agricultural systems addresses the following questions:
1) How large are nitrate leaching, nitrous oxide emissions, and nitric oxide emissions from fertilized tropical agricultural systems, as fluxes and as proportions of N inputs?
2) How do these losses compare to temperate agricultural systems?
Nitrate leaching was very variable and much higher than expected—often exceeding 10% of N inputs. In fact, measured nitrate N leaching was often greater than annual fertilizer N input. Median nitrate losses were 18.6 kg N ha-1 y-1, a mean of 22% of N inputs which is not significantly different leaching in temperate and subtropical sites found in the literature. Nitrous oxide losses had a small range: 0-3% of N inputs with mean losses of 6.8 kg N ha-1 y-1. Average nitric oxide N losses were 5.17 kg N ha-1 y-1, approximately 0-6% of fertilizer N input. Nitrate leaching losses are a larger source of N losses from the tropical sites in this study compared to gaseous nitric oxide and nitrous oxide losses. Nitrate leaching is not significantly different from our temperate and subtropical comparisons. This suggests that nitrate leaching losses are one of the largest sources of N losses in the tropics, and could be a management target for reducing N losses from tropical agroecosystems.