No abstract

To improve sustainability in agricultural systems, winter cover crops are increasingly replacing fallow to stimulate soil functions that reduce nutrient losses and greenhouse gas production, reduce pests for the next cash crops, increase soil organic matter pools and reduce erosion. Several of these functions are highly dependent on soil microbes decomposing cover crop residues. Since cover crop species differ in their traits it is hypothesized that plant species residue mixtures with complementary characteristics perform better by stimulating soil microbial functional diversity. To test this, residues of cover crop monocultures and mixtures were mixed with agricultural soil in a microcosm experiment, and fungal and bacterial biomass, microbial metabolic potential, greenhouse gas emissions and soil nutrients were measured during 50 days. Fungal biomass increased for all treatments compared to the control (no additions). However, there were no significant differences between cover crop mixtures and monocultures. Biolog ECO plates were used as a proxy for the metabolic potential of the microbial community. The number of substrates used was significantly higher in soil amended with residues of cover crop mixtures indicating an increased number of substrate niches for microbes. C:N ratio of cover crop residues was shown to be an important variable in explaining dynamics of CO2 and N2O emissions. Mixtures of cover crops showed reduced N2O and CO2 emissions compared to monocultures at the start 2 of the experiment, but did not reduce greenhouse gas emissions over the whole incubation period. Adding nitrogen to the cover crop treatment with the highest C:N ratio (oat) did increase N2O emissions, but not CO2 emissions suggesting that decomposition rate of oat residues is not limited by nitrogen availability. Overall, mixtures of cover crops stimulated microbial functional diversity in soil incubations. Although this may have positive implications for soil quality and functioning in agricultural fields, further studies are needed to verify if these results hold under field conditions.

Organic fertilizers have been shown to stimulate CH4 uptake from agricultural soils. Managing fertilizer application to maximize this effect and to minimize emission of other greenhouse gasses offers possibilities to increase sustainability of agriculture. To tackle this challenge, we incubated an agricultural soil with different organic amendments (compost, sewage sludge, digestate, cover crop residues mixture), either as single application or in a mixture and subjected it to different soil moisture concentrations using different amounts of organic amendments. GHG fluxes and in vitro CH4 oxidation rates were measured repeatedly, while changes in organic matter and abundance of GHG relevant microbial groups (nitrifiers, denitrifiers, methanotrophs, methanogens) were measured at the end of the incubation. Overall the dynamics of the analyzed GHGs differed significantly. While CO2 and N2O differed considerably between the treatments, CH4 fluxes remained stable. In contrast, in vitro CH4 oxidation showed a clear increase for all amendments over time. CO2 fluxes were mostly dependent on the amount of organic residue that was used, while N2O fluxes were affected more by soil moisture. Several combinations of amendments led to reductions of CO2, CH4, and/or N2O emissions compared to un-amended soil. Most optimal GHG balance was obtained by compost amendments, which resulted in a similar overall GHG balance as compared to the un-amended soil. However, compost is not very nutrient rich potentially leading to lower crop yield when applied as single fertilizer. Hence, the combination of compost with one of the more nutrient rich organic amendments (sewage sludge, digestate) provides a trade-off between maintaining crop yield and minimizing GHG emissions. Additionally, we could observe a strong increase in microbial communities involved in GHG consumption in all amendments, with the strongest increase associated with cover crop residue mixtures. Future research should focus on the interrelation of plants, soil, and microbes and their impact on the global warming potential in relation to applied organic amendments.

No abstract

Fungi play an important role in carbon and nutrient cycling. It is, however, unclear if diversity of fungi is essential to fulfill this role. With this meta-analysis, we aim to understand the relationship between fungal diversity and decomposition of plant materials (leaf litter and wood) in terrestrial and aquatic environments. The selection criteria for papers were the presence of a fungal diversity gradient and quantification of decomposition as mass loss. In total 40 papers met the selection criteria. We hypothesized that increase of fungal species will result in stronger decomposition, especially in species poor communities. Both artificial inoculated and naturally assembled fungal communities were included in the analysis in order to assess whether manipulated experiments are representative for field situations. We found a significant positive effect of increased fungal diversity on decomposition. However, in manipulated experiments this relationship was only positive when a control treatment of one fungus was compared with multispecies communities. This relationship became negative when comparisons of higher initial richness (at least two fungal species as control) were included. In contrast, under natural field conditions increased fungal diversity coincided with increased decomposition. This suggests that manipulated experiments are not representative for field situations. Possible reasons for this are discussed. Yet, both in manipulated and field experiments, environmental factors can influence diversity decomposition relationships as indicated by a negative relationship of increasing C:N ratio on the effect of fungal diversity on decomposition. Overall, our results show that fungal diversity can have an important role in decomposition, but that design of experiments (manipulated or field) and quality of the plant material should be taken into account for interpretation of this diversity-functioning relationship.