Does diversity matter? Soil microbial functioning and greenhouse gas fluxes in cover crop mixtures

Sustainable agricultural management is needed to reduce negative effects of current intensive agricultural practices. Intensification of agricultural land use management can lead to reduced soil organic matter (SOM) and microbial functional diversity in the soil. One of the options to increase sustainability in agricultural practices is the diversification of the cropping system via the use of wider crop rotations, intercropping and growing of cover crops. A major reason to grow cover crops is to reduce nutrient losses during the fallow period of the soil. Furthermore, cover crops are used to suppress weeds, plant pathogens, as well as to protect against erosion and stimulate nutrient cycling in the soil. There are indications that cover crops are beneficial by enhancing SOM content, aggregate stability and microbial functioning. Reducing the fallow period of the soil can be beneficial to enhance microbial diversity and main crop yields. It is hypothesized that using cover crop mixtures instead of cover crop monocultures will have an extra positive effect as mixtures will increase complementarity, possibly leading to increased biomass production and chemically more diverse root exudates and plant residues. In this PhD project, I studied if cover crop mixtures are beneficial compared to cover crop monocultures both during cover crop growth and after cover crop incorporation in the soil. Field and pot experiments were executed to determine if cover crop mixtures enhance nutrient cycling in the soil, reduce GHG emissions, stimulate a diverse microbial community and enhance yield of the main crop. To determine long term effects, the field experiment was conducted for four years.

First, we studied in a pot experiment if diversity of cover crop residues will increase microbial functional diversity during decomposition (Chapter 2). Cover crop residues of single species and diverse mixtures of 3 or 15 species were mixed with arable soil. Addition of cover crop residues rapidly increased microbial biomass in the soil during the first two weeks of the experiment. However, microbial biomass increase did not differ between monocultures and mixtures. With the use of Biolog ECO plates, we tested if residue diversity increased microbial metabolic potential as a proxy of functional diversity. Microbial communities extracted from pots amended with residue mixtures were able to use significantly more substrates than the ones extracted from pots with amended with residues derived from a single cover crop species. Microbial communities from pots with residues of Vicia sativa showed high functional diversity as well. However, metabolic activity of the microbes from the V. sativa amended pots was mainly associated with easily degradable substrates while microbial communities from mixed residue amended pots were able to consume a wider set of substrates. This indicates that residues from cover crop mixtures increase the substrate utilization range and therefore the number of niches for microbes compared to residues derived from a single species. Furthermore, we found that the C:N ratio of cover crop residues rather than residue diversity influences greenhouse gas (CO2 and N2O) fluxes. This indicates that composition of the residues is important for nutrient cycling while diversity of cover crop residues influences the soil microbial functional diversity.

To be able to evaluate the potential of organic residues, including cover crops, to mitigate greenhouse gasses (GHGs), pot experiments in controlled climate chambers were set up to determine if residues can minimize emissions. The aim of Chapter 3 was to test differences between organic amendments (compost, sewage sludge, digestate and cover crop mixture) and understand how (combinations of) organic amendments influence the global warming potential (GWP) of an agricultural soil. Organic residues were added to the soil in two different amounts (5 and 20 ton*ha-1) at different soil moisture levels (40% and 65% of soil water holding capacity). GHG fluxes (CO2, CH4 and N2O) were measured over time while abundance of microbial groups involved in nutrient cycling (nosZ, bacterial and archaeal amoA, nifH, mcrA and pmoA) was measured at the beginning and at the end of the incubation. Compost resulted in the lowest GHG balance while the mixture of cover crop residues showed the highest GHG emissions. However, the used compost is poor in mineral nutrients and can reduce yield of the main crop. To overcome this problem, combining compost with nutrient rich organic amendment (e.g. sewage sludge or digestate) can minimize the trade-off between obtaining high yields and minimizing GHG emissions. Additionally, all amendments increased microbial communities involved in nutrient cycling and GHG consumption. Mixed cover crop residues led to the highest increase. However, this is dependent on the amount of the added residues. Applying 20 ton per hectare of residues strongly increased microbial groups, while 5 ton per hectare did not significantly increase these microbial groups compared to un-amended soil.

Apart from studying the effect of plant diversity of cover crop residues, microbial diversity itself can influence processes like decomposition and nutrient cycling. As fungi play a major role in decomposition, the aim of Chapter 4 was to determine if fungal diversity enhance decomposition, which was assessed by executing a meta-analysis. An extensive literature search was performed to find papers that studied decomposition with a fungal diversity gradient both in manipulated and field experiments and in aquatic and terrestrial environments. Increased fungal diversity coincided with increased decomposition rate of plant residues (leaf litter and wood). However, in artificially manipulated experiments, fungal diversity rapidly reached a saturation level at two fungal species and further increase of diversity did not enhance decomposition. In field experiments, however, fungal diversity was positively correlated with decomposition regardless of the diversity level. This suggests that manipulated experiments are not representative for field situations and that it is necessary to study microbial diversity in the field. Furthermore, plant residue quality influences the fungal diversity – decomposition relationship. Increasing the C:N ratio of the residue reduced the positive effect of diversity on decomposition. These results indicate that both microbial diversity and residue quality are important to estimate decomposition rates of plant residues.

In Chapter 5, results of cover crop diversity effects of a multiannual field experiment are described both during cover crop growth and after cover crop incorporation in the soil. We hypothesized that cover crop mixtures reduce greenhouse gas (GHG) fluxes and increase soil microbial diversity compared to cover crop monocultures. Three cover crop species (Avena strigosa, Vicia sativa and Raphanus sativus) were grown in randomized block design with all possible combinations of the three species. GHG fluxes were measured regularly throughout the year. We found increased emissions in the plots with cover crops compared to fallow plots, both during cover crop growth and after cover crop incorporation. Cover crop mixtures did not reduce GHG fluxes compared to cover crop monocultures. N2O emission peaked after cover crop incorporation and was correlated with increased denitrification and nitrification rates. Furthermore, contrasting to our hypothesis, we did not find effects of cover crops on microbial biomass or - diversity in the field. This indicates that, in this field experiment, cultivating mixtures of cover crops for four years did not develop benefits compared to monocultures with respect to GHG fluxes and microbial biomass, - diversity and - activity.

Overall, my thesis research compared performance of mixtures and monocultures of cover crops both in pot experiments during decomposition and in a four-year field experiment. The results show that residues of cover crop mixtures have the potential to increase microbial functional diversity and stimulate microbial groups involved in nutrient cycling. This indicates a promising perspective to use cover crop mixtures in the field. However, in our field experiment, mixtures did not result in reduced GHG emissions or increased microbial biomass or diversity. Continuation of field experiments for a longer period is needed to determine if mixtures are advantageous compared to monocultures to increase sustainability in agricultural systems.