Nitrogen cycling in tropical and temperate savannas

Abstract

Savannas are the most common vegetation type in the tropics and subtropics, ranging in physiognomy from grasslands with scattered woody plants to woodlands with heterogeneous grass cover. Productivity and organic matter turnover in savannas are controlled by interactions between water and nutrient availability, and this basic environmental structure is modified by fire frequency and land management practices. We compared temperate and tropical savannas in order to understand the strength of nitrogen (N) limitation of productivity. American tropical and temperate savannas are N limited systems, and the N cycle differs according to the woody plant density, fire frequency, land use change, N deposition and N fixation. Grazing and conversion to pasture have been the predominant land-use changes in most savannas. In the Cerrado and the Llanos tropical savannas, intensified use of fire for pasture management is leading to decreased woody plant density. Oppositely, in the Chaco and North American temperate savannas, fire suppression and grazing are leading to increases in woody density. In addition, the higher soil P availability in the Gran Chaco and the higher N deposition in North American savannas may be contributing to increases of N cycling and net productivity rates. Some aspects of the N budget for savannas of the American continent are still unclear and require further analysis to determine rates of N fixation, and to understand how spatial and temporal soil heterogeneity control N fluxes through soil solution and into streams.

Attack frequency and the tolerance to herbivory of Neotropical savanna trees

“…4). Although the mechanisms underlying these variable responses in plant chemistry are unclear, they could be related to the very low concentration of soil N in the Cerrado (Bustamante et al 2006) and to C. hydrangeaefolia 0.29 (0.14-0.40) a 0.10 ( differences in how species store or allocate these elements (Goetz and Prince 1999).…”

Watershed responses to Amazon soya bean cropland expansion and intensification

“…Nitrate concentrations at depth increase in soya bean fields (figure 5), indicating that soils could have significant capacity to retain N that is fixed by the soya bean crop. Soya bean rates of N fixation have not been quantified directly at Tanguro Ranch but have been estimated at up to 170 kg N ha 21 under similar cropping systems [48].…”

Ecophysiology of forest and savanna vegetation

“…Overall, there does not seem to be any great difference between eastern Amazonian cerrado and forest soils, both often being rather infertile but with good water-holding characteristics [Motta et al, 2002;Quesada et al, 2009b]. Nevertheless, as is outlined by Bustamante et al [2006] and Nardoto et al [2006], slow rates of nitrogen mineralization during the dry season and significant fire-associated losses of nitrogen could both be contributing factors to the apparent low nitrogen availability of cerrado as opposed to forest ecosystems in Amazonia.…”