Combined effects of soil moisture and nitrogen availability variations on grass productivity in African savannas

Abstract

Savannas cover about 20% of the Earth's land area and 50% of Africa. As an indispensable component of savanna, grasses play an important role in these ecosystems. A better understanding of grass productivity and its controlling factors in savanna ecosystems could therefore be a key to understand the functioning of savannas and predict savanna responses to future climatic changes. In this study, a stable isotope fertilization experiment was conducted to determine how factors limiting grass production in savannas differ across regional climate gradients. The study was conducted on the geomorphically homogenous Kalahari Transect (KT), which offers an ideal setting to study nutrient and vegetation dynamics independently of confounding soil effects. The results show that the grasses assimilated the added fertilizer at all the sites but they did not respond to nitrogen fertilization for both dry and wet years, and at both dry and wet ends of the Transect. Although prior studies have proposed a switch between water and nitrogen limitations between arid and mesic savannas, our results suggest that nitrogen availability may not limit grass productivity across the whole KT. Thus, although the traditional classifications as nutrient poor (broad-leaf) and nutrient rich (fine-leaf) savanna ecosystems may still be useful, it does not necessarily imply the existence of nitrogen limitation in the nutrient poor area; in fact, it is more likely that the herbaceous species found in the more humid sites (nutrient poor sites) are already adapted to lower nitrogen availability.

Ecosystem-scale spatial heterogeneity of stable isotopes of soil nitrogen in African savannas

“…The physical parameters such as soil texture (>96% of sand) and bulk density (around 1.4-1.5 g cm -3 along the whole transect) do not have significant variations along the KT (Wang et al 2007b;Thomas et al 2008). (TRMM), and was reported earlier (Wang et al 2010b). Five individual trees and grasses were randomly selected at each site, foliar and root samples (0-30 cm and 30-50 cm for trees, 0-30 cm for grasses) were collected.…”

Nutrient limitations on aboveground grass production in four savanna types along the Kalahari Transect

“…Interannual variability in precipitation is positively correlated with precipitation and decreases from north to south along the KT (Scanlon et al, 2005). For year 1 and year 2 of the experiment, the following rainfall data (3B43 monthly converted to annual) were collected by the Tropical Rainfall Measuring Mission (TRMM) by NASA, from north to south, Year 1: 564 mm/year, 450 mm/year, 362 mm/year, 436 mm/year and Year 2: 914 mm/year, 804 mm/year, 621 mm/year and 597 mm/year (Wang et al, 2009). All sites are at similar elevation between 1084 m and 1115 m, and are characterized by co-existing trees and grasses but support different dominant species (Table 1).…”

African dryland ecosystem changes controlled by soil water

“…Many factors drive ecosystem changes in African drylands. These include climate change, CO 2 fertilization, fire regime, grazing, and agriculture (Andela, Liu, van Dijk, de Jeu, & McVicar, ; Higgins & Scheiter, ; Liu, van Dijk, McCabe, Evans, & de Jeu, ; Muller, Overbeck, Pfadenhauer, & Pillar, ; Wigley, Bond, & Hoffman, ; Yu & D'Odorico, ), but it is generally believed that vegetation changes in these areas are mainly controlled by water availability (Andela, Liu, van Dijk, de Jeu, & McVicar, ; Brandt et al, ; Liu, van Dijk, McCabe, Evans, & de Jeu, ; Wang, D'Odorico, O'Halloran, Caylor, & Macko, ). However, the relationship between vegetation and soil water has been found to be stronger than that between vegetation and rainfall, probably due to precipitation redistribution and its lag effects on vegetation growth (Ibrahim, Balzter, Kaduk, & Tucker, ; Ji & Peters, ; Wei et al, ).…”