Deforestation for pasture alters nitrogen and phosphorus in small Amazonian Streams

The clearing of moist, lowland tropical forest for cattle pasture represent s a widespread land use change that has consequences for soil biogeochemica l cycles, as well as the links between soil processes and the concentration s of dissolved and particulate materials in rivers and streams. We examined how conversion of tropical forest to actively grazed cattle pasture in the Brazilian Amazon influenced the concentrations of different forms of nitro gen (N) and phosphorus (P) in soil solution and stream water. We compared t wo pairs of watersheds containing second-order streams that drained land in either forest or pasture at Fazenda Nova Vida, a cattle ranch in central R ondonia. Measurements were made during the dry season (low flows) and the w et season (high flows). Forest soil solution had higher NO3- concentrations than pasture, but similar concentrations of NH4+ and PO43- Higher solution NO3- led to higher ratios of dissolved inorganic N:P in forest soils. Past ure streams had higher concentrations of total suspended solids, particulat e organic carbon (POC), and particulate organic N (PON) during the dry seas on, but not during the wet season. Pasture streams also had lower concentra tions of NO3- than forest streams. This was consistent with previous studie s that found lower extractable NO3- concentrations and lower rates of net N mineralization and net nitrification in the soils of the pasture watershed s compared with forest watersheds. Dissolved organic N (DON) dominated stre am water dissolved-N concentrations in forest (53-76%) and pasture (67-84%) . Higher dissolved inorganic N (DIN) concentrations in forest streams coupl ed with a trend toward higher DON and PON concentrations in pastures led to small differences in total N (TN) concentrations between land uses. Lower ratios of inorganic and total dissolved N:P in pasture streams suggested a switch from P limitation in forest streams to N limitation in pasture strea ms. Periphyton bioassays in forest and pasture streams confirmed that N lim ited algal growth in pasture streams where light was available. These resul ts suggest that links among deforestation, soil biogeochemistry, and the st oichiometry of N and P reaching streams in small watersheds have the potent ial to influence the structure of these aquatic ecosystems.