The impact of protozoa on the availability of bacterial nitrogen to plants

Summary Microbial N from ¹⁵N-labelled bacterial biomass was investigated in a microcosm experiment, in order to determine its availability to wheat plants. Sterilized soil was inoculated with either bacteria (Pseudomonas aeruginosa alone or with a suspension of a natural bacterial population from the soil) or bacteria and protozoa to examine the impact of protozoa. Plant biomass, plant N, soil inorganic N and bacterial and protozoan numbers were determined after 14 and 35 days of incubation. The protozoa reduced bacterial numbers in soil by a factor of 8, and higher contents of soil inorganic N were found in their presence. Plant uptake of N increased by 20010 in the presence of protozoa. Even though the total plant biomass production was not affected, the shoot: root ratios increased in the presence of protozoa, which is considered to indicate an improved plant nutrient supply. The presence of protozoa resulted in a 65010 increase in mineralization and uptake of bacterial ¹⁵N by plants. This effect was more pronounced than the protozoan effect on N derived from soil organic matter. It is concluded that grazing by protozoa strongly stimulates the mineralization and turnover of bacterial N. The mineralization of soil organic N was also shown to be promoted by protozoa.Communication No. 9 of the Dutch Programme on Soil Ecology of Arable Farming Systems     

CO2 evolution and denaturing gradient gel electrophoresis profiles of bacterial communities in soil following addition of low molecular weight substrates to simulate root exudation

Simulating the evolution of both ¹⁴C and ¹²C-CO₂ in the rhizoplane was monitored during the diffusion of ¹⁴C-labelled glucose, oxalic acid, or glutamic acid into soil from a filter placed on the surface of a sandy loam. After 3 and 7 d, soil was sampled from four layers (0-2, 2-4, 4-6, and 6-14 mm) to determine residual ¹⁴C in each layer. The mineralisation pattern of oxalic acid was characterised by a lag phase probably due to the presence, in the early stages of exposure, of a few microorganisms able to mineralise this substrate. Glucose and glutamic acid showed a positive priming effect with a CO₂ flush from native organic matter. Oxalic and glutamic acids changed the denaturing gradient gel electrophoresis profiles of soil bacterial communities with the appearance of a few extra-bands in the 0-2 mm soil layer. The addition of the substrates onto the soil surface formed a gradient due to their diffusion in soil. That of oxalic acid was specific probably because almost all of this compound reacted with CaCO₃ and was localised in the 0-2 mm soil layer.     

Protozoan predation and the turnover of soil organic carbon and nitrogen in the presence of plants

Summary The impact of protozoan grazing on the dynamics and mineralization of ¹⁴C- and ¹⁵N-labelled soil organic material was investigated in a microcosm experiment. Sterilized soil was planted with wheat and either inoculated with bacteria alone or with bacteria and protozoa or with bacteria and a 1:10 diluted protozoan inoculum. ¹⁴C–CO₂ formation was continuously monitored. It served as an indicator of microbial activity and the respiration of soil organic C. The activity of protozoa increased the turnover of ¹⁴C-labelled substrates compared to soil without protozoa. The accumulated ¹⁴C–CO₂ evolved from the soils with protozoa was 36% and 53% higher for a 1:10 and for a 1:1 protozoan inoculum, respectively. Protozoa reduced the number of bacteria by a factor of 2. In the presence of protozoa, N uptake by plants increased by 9% and 17% for a 1:10 and a 1:1 protozoan inoculum, respectively. Both plant dry matter production and shoot: root ratios were higher in the presence of protozoa. The constant ratio of ¹⁵N: ¹⁴⁺¹⁵N in the plants for all treatments indicated that in the presence of protozoa more soil organic matter was mineralized. Bacteria and protozoa responded very rapidly to the addition of water to the microcosms. The rewetting response in terms of the ¹⁴C–CO₂ respiration rate was significantly higher for 1 day in the absence and for 2 days in the presence of protozoa after the microcosms had been watered. It was concluded that protozoa improved the mineralization of N from soil organic matter by stimulating the turnover of bacterial biomass. Pulsed events like the addition of water seem to have a significant impact on the dynamics of food-chain reactions in soil in terms of C and N mineralization.Communication No. 19 of the Dutch Programme on Soil Ecology of Arable Farming Systems     

Protease and deaminase activities in wheat rhizosphere and their relation to bacterial and protozoan populations

Protease and deaminase activities and population dynamics of bacteria and protozoa were measured in the rhizosphere of wheat to study their interactions with the mineralization of nitrogen. The experimental design allowed the separation of roots and soil material by means of a gauze. The most pronounced rhizosphere effect was detected for all the measured variables in the soil closest to the gauze. The number of bacteria was significantly higher in the presence than in the absence of plants up to 4 mm away from the soil-root interface and the closer to this interface the higher the number. Protozoan and bacterial population dynamics were positively correlated; generally, populations of flagellates and amoebae were comparable and their sum accounted for the population of total protozoa. For both enzyme activities the rhizosphere effect extended up to 2 mm away from the soil-root interface. The histidinase activity was of bacterial origin, while it is likely that bacteria, protozoa and root hair all contributed to the overall caseinase activity. Decomposition of root exudates and native organic matter in the rhizosphere, reflected by a growing microbial population, is associated with nitrogen mineralization through increases in caseinhydrolysing and L-histidine-deaminating activities. The adopted soil-plant microcosm is suitable for the study of the rhizosphere effect over time of incubation and distance gradient from the soil-root interface.     

Nitrous oxide emission from urine-treated soil as influenced by urine composition and soil physical conditions

Urine patches from cattle and sheep on pastures represent considerable, highly localized N applications. Subsequent nitrification and denitrification of the nitrogenous compounds may result in high nitrous oxide (N₂O) emissions. Not much is known about the extent of these emissions, or about possible mitigation options. The aims of this study were to experimentally quantify the effects of urine composition, dung addition, compaction and soil moisture on N₂O emissions from urine patches. For an incubation study at 16 °C, soil was collected from a typic Endoaquoll, and N₂O production was monitored during a 103-day period. Emissions for the whole period averaged 0.3 and 0.9% of the applied urine-N for dry and moist soil, respectively. When compacted or when dung was added, emissions from moist soils increased to 4.9 and 7.9%, respectively. Both addition of dung and soil compaction resulted in a delay of the peak N₂O emission of approximately 10-15 days. No significant effect of amount of urine-N on emission percentages was detected. Changing the volume of urine with equal amounts of urine-N resulted in highly significant effects, peaking with an emission of 2.3% at a water-filled pore space (WFPS) of 78%. When the soil was water-saturated, N₂O production was delayed until evaporation had decreased moisture contents. We concluded that denitrification was the main N₂O forming process in the incubation study. Emission factors for urine reported in the literature do not generally include the potentially considerable effects of compaction or combination with dung. We conclude that realistic emission factors should take into account such an effect, together with estimates for the occurrence of camping areas in pastures. From our results, the best mitigation strategies appear to be increasing the volume of urine through feed additives, and avoiding compaction and promoting more homogeneous application of N through a lower cattle stocking rate. Also, research efforts may be targeted at management practices to avoid camping areas in pastures.     

Nitrous oxide emission from animal manures applied to soil under controlled conditions

Animal manures may differ strongly in composition and as a result may differ in the emission of N₂O following application to soil. An incubation study was carried out to assess the effects of type of mineral N fertilizer and manure, application technique and application rate on N₂O emission from a sandy soil with low organic matter content. Fluxes of N₂O were measured 30 times over a 98-day period. The total N₂O emission from mineral N fertilizer ranged from 2.1 to 4.0% of the N applied. High emissions were associated with manures with high contents of inorganic N, easily mineralizable N and easily mineralizable C, such as liquid pig manure (7.3-13.9% of the N applied). The emission from cattle slurries ranged from 1.8 to 3.0% and that of poultry manures from 0.5 to 1.9%. The total N₂O emission during the experimental period tended to increase linearly with increasing N application rate of NH₄NO₃ and liquid pig manure. The N₂O emission from surface-applied NH₄NO₃ was significantly smaller than that following the incorporation of NH₄NO₃ in the soil. The N₂O emission from pig manure placed in a row at 5 cm depth was significantly higher than from surface-application and other techniques in which manure was incorporated in the soil. The results show that modification of the composition and application technique may be tools to mitigate emission of N₂O.