Finding the optimum tillage and cover crop system for optimizing microbial soil health and nutrient cycling

Soil ecosystem services and soil health criteria, such as nutrient cycling, carbon (C) sequestration and water regulation, needs to be maintained and improved to meet our environmental, social and economic demands for the future within the frameworks of global change, population growth and economic independency.

This study investigated variations of soil physical, nutritional and microbial properties due to differing cover cropping systems within the first year. The experiment was located on an ongoing long-term tillage field trial in Hollabrunn, Austria. For 18 years, soils were cultivated either conventional (plough), reduced (grubber), minimized (disk-harrow) or by direct-seeding. Beside the fallow treatment, two mixtures of cover crops were selected. The 23 kg ha^-1 standard mixture contained 65 % buckwheat (Fagopyrum esculentum), 22 % scorpion-weed (Phacelia tanacetifolia) and 13 % mustard (Brassica juncea). The 35 kg ha^-1 advanced mixture contained 14 different species with 19 % Fabaceae and 15 % Brassicaceae. Soils were sampled after sowing of the cover crop in september and in the following june after sowing soybean.

Intensive tillage systems increase mineralization, aeration, redox potential and rooting within the upper most ploughing (Ap) horizon, while decreasing water holding capacity and soil aggregation. However, the higher frequency of machinery passes favors soil compaction in the layer below and limits deep rooting. Reduced tillage systems might provide a less disturbed soil structure, supporting the soil microbiology. In contrast, the reduced aeration and minimized mechanical breakdown should increase water availability, which often is the most critical factor for soil life.

The current results indicated, that soil water contents and aggregate stability was enhanced in the two least intensive treatments. The microbial C and nitrogen (N), dissolved C and N, ergosterol, extracellular enzyme activities (EEA) and the organic C pool were significantly higher with minimized and no-till. This indicated a larger amount of substrates, generating a more active and larger microbial community in the latter.

The vegetative soil coverage between the main crops is another driving factor controlling water and nutrient availability by withdrawal, fixation, mobilization and by feeding the soil rhizosphere.

After one period, cover crop-related effects were found, as the standard mixture had higher EEA, indicating a lower nutrient availability. Larger organic, inorganic and total C pools were found in the advanced mixture. Only the microbial phosphorus (P) was higher in the advanced mixture and no effect was found for microbial C and N. Higher availability of N and P in the advanced mixture due to the higher proportion of Fabaceae and Brassicaceae and their strong mobilizing effect was indicated by the higher ratio of C- to N-allocating EEA and a lower ratio of N- to P-allocating EEA. The fallow treatment was in between both cover crops and seemed to provide considerable amounts of nutrients and water to the main crop, in the first spring.

A combination of minimized or no-tillage and a diverse cover crop seemed to by promising for improving soil health, but characterizing the microbial community by 16S rRNA and ITS is still in progress.