Urban influences on the nitrogen cycle in Puerto Rico

Ortiz-Zayas, J. R., E. Cuevas, O. L. Mayol-Bracero, L.
Donoso, I. Trebs, D. Figueroa-Nieves, and W. H. Mcdowell.
2006. Urban influences on the nitrogen cycle in Puerto Rico.
Biogeochemistry 79:109–133.

Anthropogenic actions are altering fluxes of nitrogen (N) in the biosphere at unprecedented rates. Efforts to study these impacts have concentrated in the Northern hemisphere, where experimental data are available. In tropical developing countries, however, experimental studies are lacking. This paper summarizes available data and assesses the impacts of human activities on N fluxes in Puerto Rico, a densely populated Caribbean island that has experienced drastic landscape transformations over the last century associated with rapid socioeconomic changes. N yield calculations conducted in several watersheds of different anthropogenic influences revealed that disturbed watersheds export more N per unit area than undisturbed forested watersheds. Export of N from urban watersheds ranged from 4.8 kg ha)1 year)1 in the Rı´o Bayamo´ n watershed to 32.9 kg ha)1 year)1 in the highly urbanized Rı´o Piedras watershed and 33.3 kg ha)1 year)1 in the rural-agricultural Rı´o Grande de An˜ asco watershed. Along with land use, mean annual runoff explained most of the variance in fluvial N yield. Wastewater generated in the San Juan Metropolitan Area receives primary treatment before it is discharged into the Atlantic Ocean. These discharges are N-rich and export large amounts of N to the ocean at a rate of about 140 kg ha)1 year)1. Data on wet deposition of inorganic N (NHþ4 þ NO 3 ) suggest that rates of atmospheric N deposition are increasing in the pristine forests of Puerto Rico. Stationary and mobile sources of NOx (NO+NO2) and N2O generated in the large urban centers may be responsible for this trend. Comprehensive measurements are required in Puerto Rico to quantitatively characterize the local N cycle. More research is required to assess rates of atmospheric N deposition, N fixation in natural and human-dominated landscapes, N-balance associated with food and feed trade, and denitrification.


Urbanization and increases in impervious area are known to increase stream runoff and flashiness and several indices have been developed to quantify flashiness in temperate streams. The effectiveness of these indices in the humid tropics and how urbanization influences the flashiness of tropical streams that are known for their inherent flashiness is poorly understood and documented. This study compares two existing flashiness indices and a new approach that quantifies flashiness by the average time between large events on 13 urban to forested streams in Northeastern Puerto Rico. The average time between events of specific magnitudes, the Richards-Baker Flashiness index, and a Frequency of Stage Change approach were calculated and compared using average daily, hourly, and 15 minute discharge data. All analysis was based on USGS discharge records for the same 10 year period, 2000 through 2009. The results indicate that when flashiness is based on average daily stream flow, there was little to no discernible difference in the flashiness of the urban and forested streams. This results because average daily discharge records miss or underestimates the magnitude, duration, and the frequency of most events. When comparing the time between events of a given magnitude using hourly or 15 minute discharge series, the urban drainages have a shorter time period between peaks of a given magnitude than rural drainages. The Frequency of Stage Change approach also indicates that high density urban drainages are flashier than drainages with forested or mixed land use. For the average days and the stage frequency change method, flashiness indices based on hourly time series are similar to those based on the 15 minute series. The analysis indicates that tropical urban streams are flashier than their rural counterparts; however the difference is less than has been reported in temperate studies and is only statistically apparent when using high resolution discharge records.
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