Ansa Palojärvi

Evaluation of the fumigation-extraction method for the determination of microbial C and N in a range of forest soils

The release of extractable C and N by chloroform fumigation of 10 forest and two arable soils was compared with microbial C measured by direct microscopic counting (membrane filter technique). The soils varied with respect of pH (3.9–6.8) and content of organic C (2.6–36%). The amount of microbial C based on microscopic counts ranged from 332 to 1342 μ g cm−3. The release of C caused by fumigation correlated well with the results obtained from direct microscopic counting (R2 = 0.87). The regression model calculated for microbial C is Cm = (1.30 Cf + 309) μg cm−3 soil, where subscript m refers to the values for microscopic counting and subscript f to the release caused by fumigation. Assuming that fumigation causes a release of microbial N in the same proportion as for microbial C, the following regression model for microbial N was obtained: Nm (1.38 Nf + 45.3) fig cm−3 soil. The average amount of microbial C determined with the fumigation-extraction method for coniferous forest soils, deciduous forest soils and arable soils was 1.19, 1.13 and 1.40% of total C, respectively. The avarage amount of microbial N in these soils was 5.9, 3.4 and 2.5% of total N. The results suggest that the fumigation-extraction method can be used for the determination of microbial C and N in different types of forest soil.

Influence of decomposer food web structure and nitrogen availability on plant growth

We studied the sensitivity of soil microbial communities and ecosystem processes to variation in the vertical and horizontal structure of decomposer food web under nitrogen poor and N-enriched conditions. Microcosms with humus and litter layer of boreal forest floor, birch seedlings infected with mycorrhizal fungi, and decomposer food webs with differing trophic group and species composition of soil fauna were constructed. During the second growing period for the birch, we irrigated half of the microcosms with urea solution, and the other half with de-ionised water to create two levels of N concentration in the substrate. During the experiment night time respirations of the microcosms were measured, and the water leached through the microcosms was analysed for concentration of mineral N, and nematode numbers. The microcosms were destructively sampled after 37 weeks for plant biomass and N uptake, structure of soil animal and microbial community (indicated by PLFA profiles), and physical and chemical properties of the humus and litter materials. Predatory mites and nematodes had a negative influence on the biomass of their microbivorous and microbi-detritivorous prey, and microbi-detritivores affected the biomass and community structure of microbes (indicated by PLFA-analysis). Moreover, predatory mites and nematodes increased microbial biomass and changed the microbial community structure. The decomposer food web structure affected also N uptake and growth of plants. Microbi-detritivorous fauna had a positive effect, whereas predators of microbial and detritus feeding fauna exerted a negative influence on plant N uptake and biomass production. The impact of a trophic group on the microbes and plant was also strongly dependent on species composition within the group. Nitrogen addition magnified the influence of food web structure on microbial biomass and plant N uptake. We suggest that addition of urea-N to the soil modified the animal-microbe interaction by increasing microbial growth and altering community structure of microbes. The presence of microbi-detritivores and predators reduced loss of carbon from the microcosms, and the food web structure influenced also water holding capacity of the materials. The changes in plant growth, nutrient cycling, size of N and C pools, and in the physical properties of the soil emphasize the importance and diversity of indirect consequences of decomposer food web structure.

Tillage and crop residue management methods had minor effects on the stock and stabilization of topsoil carbon in a 30-year field experiment

We studied the effects of tillage and straw management on soil aggregation and soil carbon sequestration in a 30-year split-plot experiment on clay soil in southern Finland. The experimental plots were under conventional or reduced tillage with straw retained, removed or burnt. Wet sieving was done to study organic carbon and soil composition divided in four fractions: 1) large macroaggregates, 2) small macroaggregates, 3) microaggregates and 4) silt and clay. To further estimate the stability of carbon in the soil, coarse particulate organic matter, microaggregates and silt and clay were isolated from the macroaggregates. Total carbon stock in the topsoil (equivalent to 200 kg m(-2)) was slightly lower under reduced tillage (5.0 kg m(-2)) than under conventional tillage (5.2 kg m(-2)). Reduced tillage changed the soil composition by increasing the percentage of macroaggregates and decreasing the percentage of microaggregates. There was no evidence of differences in the composition of the macroaggregates or carbon content in the macroaggregate-occluded fractions. However, due to the higher total amount of macroaggregates in the soil, more carbon was bound to the macroaggregate-occluded microaggregates in reduced tillage. Compared with plowed soil, the density of deep burrowing earthworms (Lumbricus terrestris) was considerably higher under reduced tillage and positively associated with the percentage of large macroaggregates. The total amount of microbial biomass carbon did not differ between the treatments. Straw management did not have discernible effects either on soil aggregation or soil carbon stock. We conclude that although reduced tillage can improve clay soil structure, generally the chances to increase topsoil carbon sequestration by reduced tillage or straw management practices appear limited in cereal monoculture systems of the boreal region. This may be related to the already high C content of soils, the precipitation level favoring decomposition and aggregate turnover in the winter with topsoil frost.

Cross-Site Soil Microbial Communities under Tillage Regimes: Fungistasis and Microbial Biomarkers

ABSTRACT The exploitation of soil ecosystem services by agricultural management strategies requires knowledge of microbial communities in different management regimes. Crop cover by no-till management protects the soil surface, reducing the risk of erosion and nutrient leaching, but might increase straw residue-borne and soilborne plant-pathogenic fungi. A cross-site study of soil microbial communities and Fusarium fungistasis was conducted on six long-term agricultural fields with no-till and moldboard-plowed treatments. Microbial communities were studied at the topsoil surface (0 to 5 cm) and bottom (10 to 20 cm) by general bacterial and actinobacterial terminal restriction fragment length polymorphism (T-RFLP) and phospholipid fatty acid (PLFA) analyses. Fusarium culmorum soil fungistasis describing soil receptivity to plant-pathogenic fungi was explored by using the surface layer method. Soil depth had a significant impact on general bacterial as well as actinobacterial communities and PLFA profiles in no-till treatment, with a clear spatial distinction of communities (P < 0.05), whereas the depth-related separation of microbial communities was not observed in plowed fields. The fungal biomass was higher in no-till surface soil than in plowed soil (P < 0.07). Soil total microbial biomass and fungal biomass correlated with fungistasis (P < 0.02 for the sum of PLFAs; P < 0.001 for PLFA 18:2ω6). Our cross-site study demonstrated that agricultural management strategies can have a major impact on soil microbial community structures, indicating that it is possible to influence the soil processes with management decisions. The interactions between plant-pathogenic fungi and soil microbial communities are multifaceted, and a high level of fungistasis could be linked to the high microbial biomass in soil but not to the specific management strategy.

Continuous redox potential measurements in zero tilled and ploughed clay soils

Zero tillage with a crop covered soil surface change topsoil structure affecting soil physical conditions. In the present study, tTwo long-term tillage field experiments were utilized to investigate the effects of tillage intensity on soil properties. Our aim was to investigate the effects of tillage intensity change on the living conditions of soil microbes. Four identical measurement stations to determine soil conditions all year round were prepared on ploughed and zero tilled plots. In the present paper, the results of continuous soil redox potential and temperature determinations in the layer of 10–15 cm during one year (June 2009 to May 2010) are introduced. The variation of redox potential results was high as was expected. When the data of one year was examined, clear positive linear dependence between soil redox potential and temperature was observed. Based on the mean values of one year experimental period, no marked differences in topsoil redox potential between autumn ploughed and zero tilled soil were observed. Both during the growing season and outside the growing season after heavy showers, there were periods during which topsoil redox potential decreased rapidly and the change was longer than usually in connection with rapid changes. During these periods, there were differences in soil redox potential between tillage treatments caused probably by the differences in soil structure, temperature and moisture conditions, and the interaction between soil conditions and crop development stage.