Responses of a South African mesic grassland to long-term nutrient enrichment and cessation of nutrient enrichment.
Loading...
Date
2023
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Nitrogen and phosphorus are two of the most important limiting nutrients required for plant
growth and production within grasslands. South African ecosystems are generally considered
to be nutrient poor with acidic soils. Increased soil acidity exacerbated by nutrient enrichment
are expected to transform South African grasslands over a long period. This PhD research
project investigated the cumulative effect of 70 years of nitrogen (N) – in the form of limestone
ammonium nitrate (LAN) and ammonium sulphate (ASU), phosphorus (P) – in the form of
superphosphate and dolomitic lime application on the Ukulinga Grassland Nutrient Experiment
(UGNE). The UGNE has been in operation since 1951 and is located on the plateau at the
Ukulinga Research Farm, KwaZulu-Natal, South Africa. Here, the purpose was to evaluate
potential changes in above-ground net primary productivity (ANPP), species composition, soil
variables and species diversity. In the first experiment (Chapter 2), nutrient-enriched plots were
abundantly dominated by taller grass species like Megathyrsus maximus rather than shorter
species like Themeda triandra and Tristachya leucothrix. Limestone ammonium nitrate and P
did not affect species richness as strongly as ASU, suggesting nutrient identity to be an
important factor to consider. Ammonium sulphate enrichment was also associated with low
soil pH, enhancing these plots' elevated aluminium (Al) concentration. Considering this key
result, careful monitoring of soil pH and Al concentration is required moving forward as Al
toxicity may cause a threat to sensitive plant species. To further emphasize the negative effects
of soil acidification induced by N on the UGNE, the greenhouse pot-trial experiment (Chapter
5) revealed that the N-enriched soils are extremely acidic and P- deficient, further stunting
nodulation development in Vachellia sieberiana saplings. Vachellia sieberiana is a common
nitrogen-fixing species that encroaches mesic environments in South Africa, but its competitive
ability may be reduced within grasslands under extremely acidic conditions. Phosphorus
enrichment can replenish important soil nutrients and further improve soil fertility. The effects
of long-term P enrichment (70 years) and short-term cessation of P enrichment (3 years) has
received little to no attention in South African grasslands, therefore this was investigated in the
second experiment (Chapter 3). Phosphorus is an important macro-nutrient and essential for
biological nitrogen fixation (BNF). Therefore, it is expected that long-term P enrichment would
modify soil properties and indirectly influence plant cover percentage and composition. What
was unknown was if the cessation of the P enrichment would revert plant cover and
composition towards untreated/control conditions. In chapter 3, the results revealed that
changes in plant cover among the three treatments of control, P enrichment and cessation of P enrichment were consistent. Furthermore, no difference was observed among the treatments
for the ANPP, species diversity indices and richness. However, the Indicator Species Analysis
proved that Setaria nigrirostris and Bidens pilosa were strongly associated with the cessation
of P enrichment treatment. The third experiment (Chapter 4) provided the opportunity to assess
the best models that predict species compositional changes and species richness declines. Here,
it was clear that the enrichment of more nutrients shifted both grass and forb species
composition. For grass species composition, the shift in cover-abundance was from the shorter
Tristachya leucothrix (no nutrients added) species to Aristida junciformis (only LAN added)
and Megathyrsus maximus (LAN + P added). For forb species composition, Cephalaria
pungens occurred in a high cover-abundance in control plots and Bidens pilosa occurred in
LAN + P enriched plots. Interestingly, LAN-only and LAN + P enriched plots showed the
greatest shift in both grass and forbs when compared to ASU-only, ASU + P and P-only
enriched plots. In terms of overall species declines, the models presented identified the number
of nutrients added and ANPP as the best predictors. Light was not identified as a significant
predictor. The results provided partial evidence for the nitrogen detriment hypothesis and
biomass-driven hypothesis over the niche dimension hypothesis.
The key findings from all the experiments highlight that the following factors: 1) nutrient-poor
soils, 2) type of nitrogen used for enrichment, 3) soil acidification and 4) high soil aluminium
concentrations are important in the observed changes in the soil chemical properties, species
composition, species diversity, species richness and species growth dynamics on the UGNE.
The results of this research project also emphasize the importance of long-term studies in
assessing if the cessation of nutrient enrichment is a strategy for ecosystem rehabilitation. It
was identified here that long-term nutrient enrichment heavily modifies a mesic grassland
community and alternative rehabilitation methods may need to be implemented.
Description
Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.