Chapter 2: LITERATURE REVIEW
2.3 FACTORS AFFECTING SOIL MICROBIAL DIVERSITY
2.3.2 Effects of land use
2.3.2.2 Grassland
Galdos et al. (2009), also working with sugarcane, evaluated the effects of trash management on the carbon dynamics of a sugarcane crop, but on land previously converted from native forest. They studied two chronosequences in plots which had been replanted to sugarcane, with or without pre-harvest burning, 2, 6 and 8 years prior to sampling. They concluded that soils from the area converted to the unburnt management for 8 years, had higher levels of total C, microbial biomass C and particulate organic matter C than those from plots under residue burning.
inputs for 50 years reduced soil bacterial community abundance but not diversity. The bare-fallow soils supported a species-rich, metabolically active bacterial community similar in diversity to that under permanent grassland. This suggested that bacterial diversity was independent of plant inputs.
One of the most widespread changes in land use and land cover is that of woody plant encroachment on grassland ecosystems, which has contributed to extensive changes in the productivity and biogeochemistry of the affected areas (Hollister et al., 2010). The authors characterised the structure and functional capacity of soil bacterial and fungal communities occurring in four vegetation types of a mesquite- (Prosopis glandulosa) encroached mixed grass prairie. Woody plant encroachment caused substantial changes to the structure and function of grasslands and grassland-like ecosystems, and a linkage between above- and below-ground communities was shown. They concluded that soil microbial communities, particularly the fungal component, might be altered by woody plant encroachment of grasslands.
In some parts of the world, agricultural overproduction has resulted in the release of land from agricultural management, and attempts are now being made to restore the former species-rich vegetation. The effect of grassland succession on bacterial populations was monitored by Felske et al. (2000) in experimental plots, which showed a constant decline in nutrient levels and changes in vegetation as soon as fertilization and agricultural production stopped. From this time, the grasslands showed specific shifts in bacterial community composition, where total bacterial numbers increased. This correlated with the collapse of the dominant Lolium perenne grass population and with an increase in the rate of organic matter mineralization.
Although the vegetation clearly changed, no evidence of strong competition or major species replacement in the soil microbial community was found.
Fungi and bacteria are important degraders of chitin, thus contributing to the recycling of vital carbon and nitrogen resources in soil. Metcalfe et al. (2002) assessed a bacterial chitinolytic population in upland pasture, to establish how lime and sewage sludge applications affected microbial chitinolytic activity. Bacterial enrichment occurred in response to burying chitin-containing litter at the field site. Sludge application significantly reduced chitinase diversity but increased chitinolytic activity.
Levels of actinobacteria increased with sludge treatment and also with lime-plus- sludge treatment. Readily available C and N sources did not repress chitinases but promoted chitinolytic activity of specific groups of actinobacteria. The authors concluded that sludge amendment might adversely affect the presence and diversity of chitinase genes in soil so should be used with caution.
Pasture-based production systems are complex and subject to a wide range of management practices. Wakelin et al. (2009) determined the response of the soil microbial community to, and the effects of, various pasture-based land managements such as pasture type, grazing, liming, P fertilization and sampling date on the soil bacterial and fungal community structure. Results showed that liming of acidic soil was the strongest factor affecting soil microbial community structure, increasing fungal phylotype richness but not bacterial. In addition, microbial nitrogen fixing and nitrification capacity was increased. The effects of pasture type (annual or perennial) were minimal and denitrification capacity was not affected by pasture management.
Additions of P fertilizer increased the intensity of pasture production and affected fungal and bacterial community structure, as did an increased stocking rate.
Significant shifts in the soil biota occurred during the growing season. Overall results showed that soil biota, particularly soil fungi under pastures, were highly responsive to agricultural land use and management, which changed the dominant soil fungi and bacteria, and influenced the microbial groups involved in key processes.
Maharning et al. (2009) reviewed soil community changes during secondary succession to naturalised grasslands. Succession from former agricultural land and pastures to naturalised grassland was associated with changes in plant biodiversity and in the soil microbial communities. These changes resulted from a reduction in, or elimination of, management practices such as grazing, or applications of fertilizer.
Naturalised grasslands differed from intensively-managed pasture, in that plant succession progressed in the former but was suppressed in the latter. Two factors influencing the soil community were nutrient availability and soil microhabitat diversity. Different plant species affected soil microbes differently as they varied in their chemistry and physiology, in the quality and quantity of their litter, and in rhizodeposition. Long-term effects of continuous, selective grazing led to a shift in the plant community to a sward dominated by plants of lower litter quality, which
negatively affected the soil biota. Nutrient returns in the form of urine and manure, also altered the quality of resources entering the system. Nutrients released into the rhizosphere by plants positively influenced microbial activity, with bacteria being more directly associated with root exudates than fungi, the latter mainly being associated with root turnover. Applications of fertilizer directly affected the quality and productivity of plants, and had both a positive and negative effect on the soil microbiota. During succession, fungal biomass was greater in older systems whereas bacterial biomass remained constant.
Potthast et al. (2010) examined the effect of a slash and burn approach for converting vast areas of rain forest into pastures. The converted pastures however, frequently became overgrown with tropical bracken fern, which led to abandonment of these sites, as bracken-dominated areas were extremely difficult to recultivate. The authors investigated the implications of invasive bracken on soil biogeochemical properties. A comparison of active and abandoned pasture sites, showed that displacement of grass by bracken in abandoned sites lowered the soil pH, reduced the amounts of readily available carbon and nitrogen, reduced microbial biomass and activity, and resulted in a higher relative abundance of actinomycetes. Readily available organic C from grass litter was preferentially utilized. They concluded that differences in litter quality between grass and bracken initiated changes in soil biogeochemical and microbial properties after pasture conversion.