Grazing and landscape controls on nitrogen availability across 330 South African savanna sites

The availability of nitrogen (N) is an important determinant of ecosystem and community dynamics for grasslands and savannas, influencing factors such as biomass productivity, plant and herbivore composition, and losses of N to waters and the atmosphere. To better understand the controls over N availability at landscape to regional scales, we quantified a range of plant and soil characteristics at each of 330 sites in three regions of South Africa: Kruger National Park (KNP), private game reserves adjacent to KNP (private protected areas – PPAs) and Hluhluwe-iMfolozi Park (HiP). In comparing regions and sites within regions, grazing appeared to have a strong influence on N availability. Sites in the PPAs adjacent to KNP as well as sodic and alluvial sites in general typically had the highest N availability. The high N availability of these sites was not generally associated with greater potential N mineralization, but instead with less grass biomass and more forb biomass that indicated greater grazing pressure. Whereas sodic sites had a long history of high N availability as evidenced by their high soil δ15N, the greater N availability in the PPAs over the two parks appeared to be relatively recent. Grazer biomass, average potential mineralization rates and grass biomass for HiP were greater than KNP, yet there were no differences in N availability as indexed by soil and foliar δ15N between sites in the two parks. Although the short-term increase in N availability in PPAs is not necessarily deleterious, it is uncertain whether current productivity levels in those ecosystems is sustainable. With differences in management causing herbivore biomass to be 150% greater in the PPAs than the adjacent KNP, changes in plant communities and nitrogen cycling might lead to long-term degradation of these ecosystems, their ability to sustain herbivore populations, and also serve as an economic resource for the region.

[1]  Jason Kaye,et al.  Stable Nitrogen and Carbon Pools in Grassland Soils of Variable Texture and Carbon Content , 2002, Ecosystems.

[2]  D. López-Hernández.  Nutrient dynamics (C, N and P) in termite mounds of Nasutitermes ephratae from savannas of the Orinoco Llanos (Venezuela) , 2001 .

[3]  A. Austin,et al.  Global patterns of the isotopic composition of soil and plant nitrogen , 2003 .

[4]  R. Evans,et al.  Effects of native grazers on grassland N cycling in Yellowstone National Park , 1997 .

[5]  Yosef Cohen,et al.  MOOSE BROWSING AND SOIL FERTILITY IN THE BOREAL FORESTS OF ISLE ROYALE NATIONAL PARK , 1993 .

[6]  F. Dakora,et al.  Influence of mycorrhizal associations on foliar δ15N values of legume and non-legume shrubs and trees in the fynbos of South Africa: Implications for estimating N2 fixation using the 15N natural abundance method , 2003, Plant and Soil.

[7]  N. Owen‐Smith.  Functional heterogeneity in resources within landscapes and herbivore population dynamics , 2004, Landscape Ecology.

[8]  P. J. Happe,et al.  Understory patch dynamics and ungulate herbivory in old-growth forests of Olympic National Park, Washington , 1996 .

[9]  G. Likens,et al.  Technical Report: Human Alteration of the Global Nitrogen Cycle: Sources and Consequences , 1997 .

[10]  N. Hobbs.  Modification of Ecosystems by Ungulates , 1996 .

[11]  Thomas J. Stohlgren,et al.  EXOTIC PLANT SPECIES INVADE HOT SPOTS OF NATIVE PLANT DIVERSITY , 1999 .

[12]  C. C. Grant,et al.  The importance of nutrient hot-spots in the conservation and management of large wild mammalian herbivores in semi-arid savannas , 2006 .

[13]  J. Detling,et al.  MAMMALIAN HERBIVORES : ECOSYSTEM-LEVEL EFFECTS IN TWO GRASSLAND NATIONAL PARKS , 1998 .

[14]  Alison J. Hester,et al.  REVIEW: The management of wild large herbivores to meet economic, conservation and environmental objectives , 2004 .

[15]  R. Scholes,et al.  The distribution of sweetveld and sourveld in South Africa's grassland biome in relation to environmental factors , 1995 .

[16]  G. Shaver,et al.  Vertebrate Herbivores and Northern Plant Communities: Reciprocal Influences and Responses , 1994 .

[17]  A. Gallardo,et al.  Soil nitrogen heterogeneity in a Dehesa ecosystem , 2000, Plant and Soil.

[18]  B. Emmett,et al.  Regional Assessment of N Saturation using Foliar and Root $$\varvec {\delta}^{\bf 15}{\bf N}$$ , 2006 .

[19]  James B Grace,et al.  Forage Nutritive Quality in the Serengeti Ecosystem: The Roles of Fire and Herbivory , 2007, The American Naturalist.

[20]  S. McNaughton,et al.  Temporal Asynchrony in Soil Nutrient Dynamics and Plant Production in a Semiarid Ecosystem , 2004, Ecosystems.

[21]  M. Williams,et al.  Correlations between foliar δ15N and nitrogen concentrations may indicate plant-mycorrhizal interactions , 2000, Oecologia.

[22]  P. Högberg,et al.  Tansley Review No. 95 15 N natural abundance in soil-plant systems. , 1997, The New phytologist.

[23]  R. Ruess,et al.  The determination of microbial biomass. , 1999 .

[24]  D. Tilman,et al.  Selective herbivory on a nitrogen fixing legume (Lathyrus venosus) influences productivity and ecosystem nitrogen pools in an oak savanna , 2000 .

[25]  P. Groffman,et al.  Ungulate vs. landscape control of soil C and N processes in grasslands of Yellowstone National Park , 1998 .

[26]  H. Shugart,et al.  Nutrient cycling responses to fire frequency in the Kruger National Park (South Africa) as indicated by Stable Isotope analysis , 2003, Isotopes in environmental and health studies.

[27]  J. Trappe,et al.  Foliar and fungal 15 N:14 N ratios reflect development of mycorrhizae and nitrogen supply during primary succession: testing analytical models , 2005, Oecologia.

[28]  A. Kooijman,et al.  Grazing as a measure to reduce nutrient availability and plant productivity in acid dune grasslands and pine forests in The Netherlands , 2001 .

[29]  John R. Matchett,et al.  FIRE AND GRAZING REGULATE BELOWGROUND PROCESSES IN TALLGRASS PRAIRIE , 2001 .

[30]  Monica G. Turner,et al.  Spatial Heterogeneity and Soil Nitrogen Dynamics in a Burned Black Spruce Forest Stand: Distinct Controls at Different Scales , 2005 .

[31]  T. Siccama,et al.  Response of the natural abundance of 15N in forest soils and foliage to high nitrate loss following clear-cutting , 2002 .

[32]  G. Robertson,et al.  Soil structural and other physical properties. , 1999 .

[33]  R. Scholes,et al.  Biogenic NO emissions from savanna soils as a function of fire regime, soil type, soil nitrogen, and water status , 1996 .

[34]  D. Sigman,et al.  Isotopic evidence for large gaseous nitrogen losses from tropical rainforests. , 2006, Proceedings of the National Academy of Sciences of the United States of America.

[35]  S. McNaughton,et al.  Promotion of the cycling of diet-enhancing nutrients by african grazers , 1997, Science.

[36]  Z. Rengel,et al.  COMPILATION OF SIMPLE SPECTROPHOTOMETRIC TECHNIQUES FOR THE DETERMINATION OF ELEMENTS IN NUTRIENT SOLUTIONS , 2001 .