Chapter 4: EFFECTS OF LAND USE AND MANAGEMENT ON SOIL FUNGAL
4.3 RESULTS
4.4.1 Effects of different land uses and management practices on soil fungal
18–43% suggested by Vainio and Hantula (2000). This is discussed further in Chapter 5.
It is thus apparent that many factors, including PCR bias, could have influenced the nature of the DGGE fungal banding profiles obtained. The use of a single primer pair;
the denaturing gradient; similarities in eukaryote G + C content; and limited sequence divergence in fungal 18S rDNA compared to bacterial 16S rDNA, could all have contributed to the observed sparse banding patterns.
4.4.1 Effects of different land uses and management practices on soil
also be expected in the maize soil, which had a slightly higher pH (4.5) but a slightly lower organic C content than the sugarcane soil. In addition, soil physicochemical analyses also indicated, that exchangeable K and, to a lesser extent, extractable P, had influenced microbial communities under these two land uses (Figure 3.1). As previously noted in section 3.4.1, the sugarcane fields are tilled only every 6–7 years (at replanting), whereas maize fields are tilled annually. Repeated primary and particularly secondary tillage is likely to result in mechanical disruption and destruction of filamentous fungal hyphae (Pennanen et al., 2004), thus affecting the fungal biomass.
Under pine at this site, the soil pH was low (~4.1), and the dense layer of pine needles covering the soil surface, together with decaying root tissues and small branches, provided large inputs of organic matter to the soil. In the destruction of forest litter, wood and woody tissues, fungi such as cellulolytic and ligninolytic basidiomycetes are especially prominent (Alexander, 1977). Thus a large, diverse fungal community would have been expected to occur under this land use. However, community richness was lower than that under sugarcane, kikuyu pasture and native grassland, suggesting that the pine soil fungal community possibly consisted of a limited number of highly- specialised fungal species, adapted to the prevailing conditions. During a study of fungal succession on leaf litter, Atlas and Bartha (1987) found that a simultaneous, parallel succession of bacterial populations probably occurred, which may have affected the observed fungal succession. In the present study, the rich, widely diverse bacterial community found at this site, may possibly have had a dampening influence on the fungal population, as a consequence of substrate competition, resulting in a less rich, less genetically diverse fungal community than expected.
At this site, permanent kikuyu pasture and native grassland soil fungal communities were of similar richness, although the soil organic C content was higher under the former than under the latter land use. Exchangeable Mg was also higher under kikuyu than under native grasslands and PCA analysis (Figure 3.1) showed that organic C, Mg and pH were the main factors influencing microbial communities at this site.
Grasslands throughout the world owe their continued existence to fungal activity, because saprotrophic fungi decompose dead organic matter, thereby recycling carbon and other nutrients. In grasslands as in forests, basidiomycetes are the major
decomposers of dead organic matter (Deacon et al., 2006). NMS analysis of fungal community composition, based on the presence or absence of bands (groups of species) in the DGGE gels, clearly separated the kikuyu and native grassland communities. However, ANOVA indicated that differences in fungal community evenness were low; accordingly, the Shannon Weaver index showed a low diversity.
In an earlier study at this site, Graham (2003), using PLFA analysis, found that the fungal/bacterial ratio was lowest under kikuyu pasture and highest under native grassland. The results also indicated that, in comparison with native grassland, improved kikuyu pasture soil had a higher organic matter content and a smaller fungal community of similar richness, but with lower evenness and a lower diversity index.
Therefore, although the number of fungal species in the soil under kikuyu and native grassland was essentially the same, the low evenness and diversity was regarded as indicative of the presence of a small number of dominant species in the kikuyu fungal community. This was interpreted as a reflection of the complexity of the interactions between land use and the size and composition of soil microbial communities. These results support the findings in the current study.
The close relationship between fungal communities under maize (conventional tillage) and wattle (Figure 4.4) at this site, was unexpected and in contrast to the findings in the equivalent bacterial study. The banding patterns, both in terms of band number and bands common to both maize and wattle fungal communities, were very similar in duplicate DGGE gels. This indicated that fungal populations at both sites contained essentially the same groups of species. However, soil physicochemical analyses (Figure 3.1) showed that soils under these two land uses differed considerably. Maize soil had a higher P and K content, whereas wattle soil had a higher pH, organic C and exchangeable Ca and Mg content. Other research has shown that fungal/bacterial ratios are about five times lower in arable than in forest soils, due to the frequent disturbances associated with tillage and the presence of a distinct, thick litter layer under forests (Pennanen et al., 2004). One factor which may possibly have contributed significantly to the similarity in the two communities is that wattle trees drop only small amounts of litter, so the soil surface is essentially bare (Dlamini and Haynes, 2004). Other important factors to be considered, are that wattle soil pH (~5.5), was the highest of all the land uses at this site, and the bacterial community was the most diverse. These conditions would tend to favour bacterial growth over
that of fungi, causing the former to proliferate more rapidly than the latter, and to become the dominant microbial population.
4.4.1.2 Mount Edgecombe (site 2)
At this site there was an overall significant difference in fungal community structure under the various sugarcane trash managements. Analysis of the DGGE profiles clearly separated the different fungal communities from the four crop residue management soils. Those treatments retaining a full trash blanket (T) supported communities with a greater richness than those under pre-harvest burnt sugarcane (Bto). TF had the highest richness and BtoFo the lowest. CCA showed that soil organic C content was one of the main factors involved in differentiating fungal communities under trashed treatments from those under pre-harvest burning. Loss of exchangeable Mg (associated with soil acidification) was the main factor differentiating communities under fertilized conditions, from those under unfertilized conditions. However, addition of fertilizer increased fungal community richness relative to unfertilized treatments. As previously noted, fungi have a selective advantage over bacteria in acid soils, so applications of fertilizer that increase acidity due to nitrification of ammonium sulphate (Graham, 2003) would tend to favour fungal dominance over bacteria. This author had previously reported that for unfertilized treatments (Fo), at this site, trash retention had greatly increased the fungal to bacteria ratio, which supports the findings in the present study. In contrast however, this did not apply to fertilized plots (F).
At this site, both the greater soil fungal richness and the larger diversity index under trash retention compared to burning, were as expected. Surface retention of crop residues stimulates fungal growth more than that of bacteria, causing the fungi to predominate (Holland and Coleman, 1987). In the present study, fungal mycelium was clearly visible, both ramifying through the decaying trash layer and on the soil surface below. This suggests that the decaying crop residues provide a medium which supports a more diverse fungal community than that under the bare soil surface of pre- harvest burnt sugarcane. Normally a few dominant species occur in a trophic level, although the less abundant species largely determine the species diversity of that level
and of the whole community. Diversity generally decreases when one or a few populations attain high densities, with high numbers tending to signify successful competition but also the dominance of a single population (Atlas and Bartha, 1987).