Deforestation for Pasture Alters Nitrogen and Phosphorus in Small Amazonian Streams

Abstract

The clearing of moist, lowland tropical forest for cattle pasture represents a widespread land use change that has consequences for soil biogeochemical cycles, as well as the links between soil processes and the concentrations 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 nitrogen (N) and phosphorus (P) in soil solution and stream water. We compared two pairs of watersheds containing second-order streams that drained land in either forest or pasture at Fazenda Nova Vida, a cattle ranch in central Rondô nia. Measurements were made during the dry season (low flows) and the wet season (high flows). Forest soil solution had higher NO 3 Ϫ concentrations than pasture, but similar concentrations of NH 4 ϩ and PO 4 3Ϫ . Higher solution NO 3 Ϫ led to higher ratios of dissolved inorganic N:P in forest soils. Pasture streams had higher concentrations of total suspended solids, particulate organic carbon (POC), and particulate organic N (PON) during the dry season, but not during the wet season. Pasture streams also had lower concentrations of NO 3 Ϫ than forest streams. This was consistent with previous studies that found lower extractable NO 3 Ϫ concentrations and lower rates of net N mineralization and net nitrification in the soils of the pasture watersheds compared with forest watersheds. Dissolved organic N (DON) dominated stream water dissolved-N concentrations in forest (53-76%) and pasture (67-84%). Higher dissolved inorganic N (DIN) concentrations in forest streams coupled 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 streams. Periphyton bioassays in forest and pasture streams confirmed that N limited algal growth in pasture streams where light was available. These results suggest that links among deforestation, soil biogeochemistry, and the stoichiometry of N and P reaching streams in small watersheds have the potential to influence the structure of these aquatic ecosystems.

Do secondary compounds inhibit microbial- and insect-mediated leaf breakdown in a tropical rainforest stream, Costa Rica?

“…Changes to riparian plant communities can potentially alter the quality and availability of basal leaf litter resources in tropical stream food webs. These Wndings are important given that lowland tropical riparian forests are becoming severely degraded, often suVering decreases in tree species diversity and abundance (Pringle et al 2000;Neill et al 2001;Benstead et al 2003).…”

Carbon and nitrogen stoichiometry and nitrogen cycling rates in streams

“…In such cases, decreased N retention in streams could be due partially to a net decrease in C:N of the nutrient pools left behind (after export), concomitant increases in N-specific uptake rates in biomass and, ultimately, saturation of biotic demand. Removal of tropical forest increased N limitation by decreasing N inputs from terrestrial vegetation and soils (Neill et al 2001), suggesting that removal of C-rich litter sources does not always decrease N limitation.…”

Effects of land cover on chemical characteristics of streams in the Cerrado region of Brazil

“…Thus, the study of small catchments can integrate many processes in changing landscapes (Alexander et al 2000;Campbell et al 2004). For example, Neill et al (2001) working in the western Amazon demonstrated changes in solute concentrations with lower nitrate concentrations and decrease in N:P ratios in streams draining pastures relative to forests, in addition to higher concentrations of total suspended solids, particulate organic carbon and particulate organic nitrogen in the dry season. The observed decrease in stream NO 3 -in pastures was consistent with previous studies that found lower extractable NO 3 -concentrations and lower rates of net N mineralization and net nitrification in the soils of the pasture watersheds compared with forest watersheds (Neill et al 1995).…”

“…While studies evaluating the effects of changes in land use and land cover on the biogeochemistry of rivers in Brazil are relatively common in the Amazon Neill et al 2001;Ballester et al 2003;Biggs et al 2002Biggs et al , 2004Figueiredo et al 2010), biogeochemical evaluations of the impacts of land use changes in the Cerrado region are still scarce in spite of its hydrological importance on a country-wide scale. Some of the most important hydrological basins in Brazil have their headwaters in the Cerrado: Araguaia-Tocantins, Paraná, and São Francisco basins.…”