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The spring application of fertiliser N often fails to stimulate grass growth in improved grass pastures on peaty soils. Fertiliser utilisation efficiencies under these conditions have been found to be low, suggesting that available N is not taken up by the plant. Previous work has suggested that in this type of system, the soil microbial biomass may function as a strong sink for fertiliser N and therefore limit plant growth in the Spring. A series of laboratory based experiments utilising reconstituted and intact cores, and homogenised peat, was set up to identify the factors controlling the competition between N uptake by plants and N immobilisation by soil microorganisms following the addition of fertiliser N to peat. Microbial biomass N concentrations were determined in order to quantify the amount of N present in the microbial pool. The use of 15N labelled fertilisers and selective biocides provides a powerful tool with which to characterise the microbial population responsible for the immobilisation of N under these conditions. Improvement of a grass pasture at Sletill Hill has resulted in the formation of a distinct layer comprised of partially decomposed roots, underneath the surface vegetation and it was within this layer, that microbial immobilisation of fertiliser N was found to occur. Approximately 30% of applied N (equivalent to ca 50 kgN ha-1) was found within the microbial biomass in this layer, 30 days after the addition of fertiliser N. Intact cores were removed from Sletill Hill and maintained under controlled abiotic conditions. Water table level and temperature were found to be important in controlling the extent of microbial immobilisation of applied N. Lowering the water table level increased the quantity of N present in plant and microbial N pools, particularly at lower temperatures (8°C). At higher temperatures (20°C), plant uptake of N tended to be less due to a restriction on plant growth caused by 'droughty' soil conditions.

The importance of microbial biomass of low concentration, regular additions of soluble organic substrate to carbon-limited soils, such as might occur with throughfall, has been studied. The conclusions depended upon the method used to measure C concentrations. Using a TOCsin automated analyser, the high level treatment enhanced biomass C, whereas the lower level treatment had small and variable effects, but using dichromate oxidation led to the conclusion that the low level treatment had a negative effect, and even the high level treatment had no beneficial effect. A further evaluation showed that the TOCsin procedure gave low recoveries of biomass C for bacterial monoculture suspension, an effect attributed to transport-related phenomena, and could not cope with continuous SO₄^2- inputs. However, use of Cl^- to extract C before and after fumigation altered the amount of C obtained. Therefore dichromate oxidation was used subsequently. The changes in microbial biomass, ATP concentration and respired CO₂ for glucose-amended soil have been measured over 9 days, and the dynamic responses shown to differ. The large changes in ATP-to-biomass C ratio show that ATP should not be used as a surrogate for biomass C determination after fresh substrate addition. Subsequently, ¹⁴C-labelled pea plant residues have been used to study differences in response of the three determinants to incorporation of water-soluble (WS), water-insoluble (WI), and unwashed (UW) plant residue materials. For plant materials too, ATP-to-biomass C ratios changed in response to substrate additions, increasing substantially (as for glucose) for WS additions, but decreasing after W1 residue incorporation. In the shorter term, higher biomass C was favoured more by incorporation of WI than of WS plant residues.

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The structure and function of microbial communities are critical to the maintenance and sustainability of terrestrial ecosystem processes. Consequently, there is substantial interest in assessing how microbial communities respond to various land management practices, and if alterations to the characteristics of microbial communities has the potential to disrupt ecosystem processes. This thesis was conducted to identify the long term effects of fertilisation and different levels of post-harvest organic matter removal on the characteristics of the FH litter and soil microbial communities in six, second rotation Pinus radiata plantation forests located around New Zealand. The six sites, established between 1986 and 1994, were sampled in 2002 and 2003. Various physical and chemical properties of the sites were measured, and litterfall production was determined. The microbial biomass in the FH litter layer and soil was determined by chloroform fumigation-extraction, and Biolog plates were used to assess the relative differences in microbial community diversity, based on patterns of substrate utilisation. Fertilisation substantially altered the physical and chemical properties of the forest floor, including FH litter moisture content, mass, carbon content, nitrogen content and carbon: nitrogen ratio and soil pH, nitrogen content and carbon: nitrogen ratio. The same range of FH litter and soil properties were also significantly changed by different levels of organic matter removal. The biomass and diversity of the FH litter and soil microbial communities were significantly altered by fertilisation and organic matter removal, and the differences in the microbial community characteristics were significantly correlated to the effects of the fertilisation and organic matter removal treatments on the physical and chemical environment in the majority of cases. The physical and chemical properties of the sites were significantly correlated to estimates of wood production, and it was also found that the characteristics of the microbial community were strongly related to productivity at several sites. The results demonstrated that fertilisation and organic matter removal regimes have had long term effects on the microbial communities at the sites. The persistence of the effects of the organic matter removal treatments were particularly noteworthy, as these treatments were applied at site establishment, and despite no subsequent reinforcement over the life of the trials, were still substantially influencing the physical, chemical and microbiological properties of the FH litter and soil up to 17 years later. The results of this thesis also emphasised the value of long-term experiments in assessing the effects of disturbance on the physical, chemical and microbiological characteristics of forest ecosystems. Further research into the specific nature of the relationship between site productivity and microbial community characteristics was suggested as an important focus for future studies.

Soil is the world’s largest terrestrial carbon (C) sink, and is estimated to contain approximately 1600 Pg of carbon to a depth of one metre. The distribution of soil organic C (SOC) largely follows gradients similar to biomass accumulation, increasing with increasing precipitation and decreasing temperature. As a result, SOC levels are a function of inputs, dominated by plant litter contributions and rhizodeposition, and losses such as leaching, erosion and heterotrophic respiration. Therefore, changes in biomass inputs, or organic matter accumulation, will most likely also alter these levels in soils. Although the soil microbial biomass (SMB) only comprises 1-5% of soil organic matter (SOM), it is critical in organic matter decomposition and can provide an early indicator of SOM dynamics as a whole due to its faster turnover time, and hence, can be used to determine soil C dynamics under changing environmental conditions.¶ Approximately 932 million ha of land worldwide are degraded due to salinity and sodicity, usually coinciding with land available for agriculture, with salinity affecting 23% of arable land while saline-sodic soils affect a further 10%. Soils affected by salinity, that is, those soils high in soluble salts, are characterised by rising watertables and waterlogging of lower-lying areas in the landscape. Sodic soils are high in exchangeable sodium, and slake and disperse upon wetting to form massive hardsetting structures. Upon drying, sodic soils suffer from poor soil-water relations largely related to decreased permeability, low infiltration capacity and the formation of surface crusts. In these degraded areas, SOC levels are likely to be affected by declining vegetation health and hence, decreasing biomass inputs and concomitant lower levels of organic matter accumulation. Moreover, potential SOC losses can also be affected from dispersed aggregates due to sodicity and solubilisation of SOM due to salinity. However, few studies are available that unambiguously demonstrate the effect of increasing salinity and sodicity on C dynamics. This thesis describes a range of laboratory and field investigations on the effects of salinity and sodicity on SOC dynamics.¶ In this research, the effects of a range of salinity and sodicity levels on C dynamics were determined by subjecting a vegetated soil from Bevendale, New South Wales (NSW) to one of six treatments. A low, mid or high salinity solution (EC 0.5, 10 or 30 dS/m) combined with a low or high sodicity solution (SAR 1 or 30) in a factorial design was leached through a non-degraded soil in a controlled environment. Soil respiration and the SMB were measured over a 12-week experimental period. The greatest increases in SMB occurred in treatments of high-salinity high-sodicity, and high-salinity low-sodicity. This was attributed to solubilisation of SOM which provided additional substrate for decomposition for the microbial population. Thus, as salinity and sodicity increase in the field, soil C is likely to be rapidly lost as a result of increased mineralisation.¶ Gypsum is the most commonly-used ameliorant in the rehabilitation of sodic and saline-sodic soils affected by adverse soil environmental conditions. When soils were sampled from two sodic profiles in salt-scalded areas at Bevendale and Young, SMB levels and soil respiration rates measured in the laboratory were found to be low in the sodic soil compared to normal non-degraded soils. When the sodic soils were treated with gypsum, there was no change in the SMB and respiration rates. The low levels of SMB and respiration rates were due to low SOC levels as a result of little or no C input into the soils of these highly degraded landscapes, as the high salinity and high sodicity levels have resulted in vegetation death. However, following the addition of organic material to the scalded soils, in the form of coarsely-ground kangaroo grass, SMB levels and respiration rates increased to levels greater than those found in the non-degraded soil. The addition of gypsum (with organic material) gave no additional increases in the SMB.¶ The level of SOC stocks in salt-scalded, vegetated and revegetated profiles was also determined, so that the amount of SOC lost due to salinisation and sodication, and the increase in SOC following revegetation relative to the amount of SOC in a vegetated profile could be ascertained. Results showed up to three times less SOC in salt-scalded profiles compared to vegetated profiles under native pasture, while revegetation of formerly scalded areas with introduced pasture displayed SOC levels comparable to those under native pasture to a depth of 30 cm. However, SOC stocks can be underestimated in saline and sodic landscapes by setting the lower boundary at 30 cm due to the presence of waterlogging, which commonly occurs at a depth greater than 30 cm in saline and sodic landscapes as a result of the presence of high or perched watertables. These results indicate that successful revegetation of scalded areas has the potential to accumulate SOC stocks similar to those found prior to degradation.¶ The experimental results from this project indicate that in salt-affected landscapes, initial increases in salinity and sodicity result in rapid C mineralisation. Biomass inputs also decrease due to declining vegetation health, followed by further losses as a result of leaching and erosion. The remaining native SOM is then mineralised, until very low SOC stocks remain. However, the C sequestration potential in these degraded areas is high, particularly if rehabilitation efforts are successful in reducing salinity and sodicity. Soil ecosystem functions can then be restored if organic material is available as C stock and for decomposition in the form of either added organic material or inputs from vegetation when these salt-affected landscapes are revegetated.

The pursuit of agricultural sustainability is necessary to ensure global food security into the future. To achieve sustainability, production systems around the world use different approaches. Utilizing several biological and physical indicators, this study investigates two agricultural production systems and assesses how management has affected the long-term health and sustainability of the soils. The first study assessed the effect of variable intensities of tillage on three Saskatchewan soils under low-disturbance (LD) management for the ten years prior to tillage. The soils represented were in the Grey, Black and Brown soils zones at sites located near Tisdale, Rosthern and Central Butte, Saskatchewan, respectively. A completely randomized block design utilized four treatments of varying tillage intensity. Samples were taken in spring before planting and after harvest at all sites. The soils were analyzed for microbial indicators of health by assessing dehydrogenase, urease, protease, and alkaline phosphatase activities. Microbial biomass carbon (MBC), microbial biomass nitrogen (MBN) and microbial quotient nitrogen (MQN) also were analyzed. Traditional soil nutrient and physical parameters were measured. The tillage intensities affected each parameter differently likely due to the differences in litter quality at each site. The high intensity tillage treatment decreased dehydrogenase activity at Tisdale at May, while in Rosthern dehydrogenase activity was increased in the moderate intensity tillage treatment and decreased by the high intensity tillage treatment. At Central Butte no effect was detected until October when dehydrogenase activity was increased by the low and moderate tillage intensity treatments. Protease and urease activities were affected at Rosthern only where the moderate intensity tillage treatment decreased activity relative to the control treatment. Soil physical parameters were not affected by tillage intensity; however nutrient levels were impacted by the increasing tillage intensity. Specifically, NO3- was reduced at Tisdale and was increased at Rosthern. Phosphate levels were reduced by the high tillage intensity in Rosthern whereas, with increasing tillage, the opposite occurred at Tisdale and Central Butte. The responses were strongly influenced by site characteristics, especially soil zone, organic matter content and surface litter abundance and quality. These effects were short-term, having no long-term impact on the agricultural sustainability or health of the soil, although knowledge of litter condition and quality is agronomically beneficial in order to predict soil responses to intense tillage events. iii The second part of the study was to assess the success of grass terraces on preserving the soil health of hillslope farm plots with Oxisolic soils in southern Ethiopia. Soil erosion has a devastating impact on hillslope agriculture in Ethiopia causing severe land degradation. An adjacent terraced and unterraced hillslope was chosen and sampled, along with a second unterraced slope for comparison. These soils were analyzed for dehydrogenase, alkaline phosphatase, and urease activities, as well as total C and total N. The plots above the terraces [terraced upper and unterraced upper] had higher urease activities than the plots below [terraced lower and unterrraced lower]. The impact of a vegetative strip that had formed a terrace 20 years ago was still evident in consistently higher alkaline phosphatase, urease, and dehydrogenase activities than the other plots. Simple methods of erosion prevention on erosion prone hill-slopes indicated that vegetative strips leading to terracing have a positive effect on soil health and functionality, promoting the long-term agricultural productivity and sustainability of these landscapes.