Author:

Abstract:

There is concern that the current intensive agricultural practices pose a threat to agricultural soils, leading to degradation of the physical quality. The most important indicator is the soil structure which is defined as the way the different soil particles are arranged in aggregates differing in their shape, size, stability, and degree of adhesion. This soil property is recognised as one important control of many biological and physical soil processes. Soils with good physical quality function well under different circumstances and for several purposes, like crop growth. Plants require a rootzone with water, nutrients and air in the right proportions. The soil should also preserve biodiversity and carbon storage. Building soil health and maintaining good overall soil quality, whilst maintaining productivity for now and into the future, is a major goal for current agricultural research. Carefully selecting cropping systems that consider, apart from the crop sequence, the tillage practices, the management of the crop residues and the addition of organic materials can contribute to improving soil quality. In this dissertation, firstly, we developed a scheme to efficiently monitor, store data and evaluate different cropping systems, and secondly, we evaluated the soil aggregation of agricultural soils under practices that have the potential to improve the soil quality. For the first part, short-term experimental cropping systems within the EU-Horizon 2020 project SoilCare, were used to identify their important characteristics and set up a system to enable their implementation, monitoring, and analysis. A data management system was designed to make the experimental data and supporting information readily available to the user in a way that information is useful, easy to access, and downloadable. A database scheme was developed to store the complete record of the details of the experiments, including diary and details of the management practices, metadata of measurements, field and experimental design information. The database was complex in structure and content but had all the information stored comprehensively. This contributed substantially to the organisation of the project, sharing of information among the scientific community when required, and consistent analysis. For the second part, management practices from seven long-term experiments LTEs in five European countries (Belgium, Czech Republic, Hungary, Italy and UK) were selected. These experiments had been running from 8 to 54 years at the moment of sampling. Topsoil samples (0 -15 cm) were collected and analysed to evaluate the effects of three different management categories, i.e., soil tillage, the addition of exogenous organic materials, the incorporation of crop residues, on different topsoil structural characteristics. Inversion tillage is a commonly applied soil cultivation practice in Europe. It has been blamed for deteriorating topsoil stability and declining organic carbon (OC) contents. We evaluated the potential to reverse these negative effects in the topsoil by alternative practices. Hereby different types of conservation tillage (reduced and no-tillage) and increased organic inputs of different origins (farmyard manure, compost, crop residues) combined with inversion tillage were evaluated in contrast to the local control. The impact on topsoil stability, soil aggregates and within these, OC distribution using wet sieving after slaking was tested. Also, the effects on the two main components of organic matter, i.e., particulate organic matter (POM) and mineral associated organic matter (MAOM), were evaluated using dispersion and size fractionation. We found that reduced and no-tillage practices, as well as the additions of manure or compost, increased the aggregates mean weight diameter (MWD) (up to 49% in the Belgian study-site) and topsoil OC (up to 51% in the Belgian study-site), as well as the OC corresponding to the different aggregate size fractions. The incorporation of crop residues had a positive impact on the MWD but a less profound effect both on total OC and on OC associated with the different aggregates. A negative relationship between the mass and the OC content of the microaggregates (53 - 250 μm) was identified in all experiments. There was no effect on the mass of the macroaggregates and the occluded microaggregates (mM) within these, while the corresponding OC contents increased with less tillage and more organic inputs. Inversion tillage led to less POM within the mM, whereas the different organic inputs did not affect it. In all experiments where the POM increased, the total soil organic carbon content (SOC) was also affected positively. In our study, we also related the effect of the above-mentioned management-induced changes of SOC content on the soil water retention (SWR) which is also linked to the soil structure. The changes in the total SOC up to 10 g C kg-1 soil observed for the different management practices, did not cause statistically significant differences in the SWR characteristics as expected according to some literature. The direct impact of the SOC content on SWR was consistent but negligible. On the other hand, the indirect impact of SOC content in the higher soil water potentials, which are mainly affected by soil structure and aggregate composition, prevails. The different water content responses under the various soil water potentials to SOC content changes for each management group implies that one sole conservation measure on its own has a limited effect on SWR. Only a combination of several practices that lead to better soil structure, such as reduced soil disturbances combined with increased exogenous SOM inputs, can lead to better soil water holding capacity and mitigate the negative effects of inversion tillage on topsoil aggregation. Thus, results strongly suggest that soil structural stability is a crucial soil physical property exerting several direct and indirect impacts on the general soil quality and sustainability. This results among others, in a better and stable infiltration, enhances root growth and soil biology and prevents erosion as a combined positive effect. Future research towards soil sustainability should keep sufficient attention to the complex interactions between soil structural stability and formation and cropping management.