CHAPTER 4: Soil, litter and plant dynamics across a precipitation gradient: the effects of fertilization
4.5 Discussion
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a negative feedback results. At KwaMbonambi, the main encroaching species is Dichrostachys cinerea, a known N2-fixing legume. Contrastingly, this study showed no evidence of P
limitation as a result of encroachment at Bergville, despite the high densities of a known N2- fixing woody plant, Acacia sieberiana (recently renamed Vachellia sieberiana). Although this study examined differences between open grasslands and encroached sites, P limitation may be better studied through the examination of woody gradients. For example, Blaser and others (2014) found that total P and extractable P pools increased along a shrub-cover gradient.
Interestingly, in our study, the change in total soil P between encroached and open grasslands was minimal for low-precipitation sites, indicating that water availability may be the main driver of encroachment for these sites. However, the roles of grazing (Ward and Esler 2011), fire limitation (Throop and others 2017) and/or global climate change (especially increases in CO2
concentrations) cannot be excluded (Ward and others 2014).
Both litter % N (used as an index of quality) and C:N (used as an index of
decomposition) were expected to increase with fertilizer addition and woody plant encroachment.
Zhang and others (2008) found that total nutrient concentration and the C:N ratio of the litter were the two factors that had the greatest effect on litter mass loss across a wide range of plant species and tissues. Interestingly, we found only marginal effects of fertilization addition on litter quality. Here again, for low-precipitation sites, water availability may have limited nutrient uptake into plant tissue. Lü and others (2014) found that water addition rather than N addition benefitted plant species in a semi-arid grassland. We did note increasing litter % N quality in encroached sites with increasing MAP. This result implies that increased litter quality by
encroaching woody plants is only beneficial to surrounding vegetation when precipitation is not limiting.
Lower C:N ratios in encroached areas have been reported by several studies, possibly due to the presence of N2-fixing legumes (Hudak and others 2003; Baer and others 2006; Zhu and
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others 2016). For example, Sugihara and others (2014) found that forest species which were able to fix N effectively contributed to the low C:N ratio of the forest soil. Both the wetter sites studied here, KwaMbonambi and Bergville, are dominated by N2-fixing legumes. Litter C:N ratios were on average higher in the low-precipitation sites, indicating lower decomposition.
Hudak and others (2003) found that severely encroached areas had lower C:N ratios throughout the soil profile than less encroached areas. The C:N ratio of litter decreased across the
precipitation gradient for the encroached sites, highlighting the positive effect of encroachment on litter decomposition when water is not limiting.
We found higher foliar N concentrations in grass leaves in the encroached plots, with only the lowest precipitation site, Middelburg, showing no positive effect of encroachment on grass foliar quality. While increases in soil N, and consequently foliar N, are sometimes observed beneath encroaching woody canopies even in the absence of N2-fixing species (Eldridge and others 2011), the woody plants in Middelburg are not N2-fixing (O'Connor and Roux 1995).
Although the term woody plant encroachment has become synonomous with
desertification and declines in species diversity, Grellier and others (2013) have found positive effects of woody encroachers on plant diversity. Eldridge and Soliveres (2015) have suggested optimising landscape management to maximise the services provided by shrub-encroached areas.
The purportedly positive effect of woody plant encroachment was elucidated by our study finding that forb cover increased in encroached plots. Barbosa da Silva and others (2016), also found that the encroachment of a woody invader was correlated to an increase in species richness and a decrease in important native forage grasses. Again, while not all woody encroachers are N2-fixing, plants growing at high densities create ‘islands of fertility’ (Hibbard and others 2001;
Ravi and others 2010; Ward and others 2018), attributed to both abiotic and biotic processes such as local-scale variations in soil erosion and deposition, plant uptake of water and nutrients by lateral roots spreading into the surrounding soils, and activities of soil micro and macro-
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organisms (Ravi and others 2010). Increased presence of forbs may also contribute to increased diversity, contrary to the widespread notion that encroachment is directly linked to reductions in diversity and ecological function (Eldridge and others 2011).
The effects of encroachment on net primary productivity are related to MAP, with reported decreases in ANPP in xeric sites, but dramatic increases at higher precipitation sites (Knapp and others 2008). Likewise, this study has shown that the lower precipitation sites exhibited minimal fluctuations in above-ground grass productivity, as compared to the high precipitation sites.
Several studies of fertilizer addition effects in South African grasslands have found that the addition of both nitrogen and phosphorus led to a higher yield in grass biomass than
fertilization with one of these nutrients only (Fynn and others 2005; Craine and others 2008;
Tsvuura and Kirkman 2013). Patterns of species richness in this study show that nutrient additions (N and N+P) resulted in declines in species richness, but for higher precipitation sites only. Plant diversity is generally shown to be positively related to precipitation (Adler and Levine 2007). Several N-addition experiments have shown a decline in species richness (Clark and Tilman 2008; Ward and others 2017), and similar trends have been reported for
atmospherically deposited N (Stevens and others 2004; Dingaan and others 2017). A recent study by Ward and others (2017) compared the results of a long-term (65 years) experiment in a South African grassland with the world's longest-running ecological experiment, the Park Grass study at Rothamsted, U.K. The results showed remarkable similarities to those of the Park Grass study, particularly the seemingly universal overall inverse relationship between species richness and ANPP, as well as the negative impact of nitrogen addition on species richness. Improved nutrient availability may be advantageous to grasses in terms of productivity, assuming that grasses are superior competitors for soil resources such as water and nutrients because of access to
precipitation in the upper soil layers (Walter 1939; Knoop and Walker 1985; Ward and others
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2013) and their fine-root morphology (Aerts and others 1991). However, rapid N absorption by C4 grasses could effectively allow a few resource-capturing grasses to dominate, thereby competitively excluding other species and decreasing overall species diversity (Busso 2014).