Agricultural productivity relies on a wide range of ecosystem services provided by the soil biota. Plowing is a fundamental component of conventional farming, but long-term detrimental effects such as soil erosion and loss of soil organic matter have been recognized. Moving towards more sustainable management practices such as reduced tillage or crop residue retention can reduce these detrimental effects, but will also influence structure and function of the soil microbiota with direct consequences for the associated ecosystem services. Although there is increasing evidence that different tillage regimes alter the soil microbiome, we have a limited understanding of the temporal dynamics of these effects. Here, we used high-throughput sequencing of bacterial and fungal ribosomal markers to explore changes in soil microbial community structure under two contrasting tillage regimes (conventional and reduced tillage) either with or without crop residue retention. Soil samples were collected over the growing season of two crops (Vicia faba and Triticum aestivum) below the seedbed (15 to 20 cm). Tillage, crop and growing stage were significant determinants of microbial community structure, but the impact of tillage showed only moderate temporal dependency. Whereas the tillage effect on soil bacteria showed some temporal dependency and became less strong at later growing stages, the tillage effect on soil fungi was more consistent over time. Crop residue retention had only a minor influence on the community. Six years after the conversion from conventional to reduced tillage, soil moisture contents and nutrient levels were significantly lower under reduced than under conventional tillage. These changes in edaphic properties were related to specific shifts in microbial community structure. Notably, bacterial groups featuring copiotrophic lifestyles or potentially carrying the ability to degrade more recalcitrant compounds were favored under conventional tillage, whereas taxa featuring more oligotrophic lifestyles were more abundant under reduced tillage. Our study found that, under the specific edaphic and climatic context of central Belgium, different tillage regimes created different ecological niches that select for different microbial lifestyles with potential consequences for the ecosystem services provided to the plants and their environment.

Soil fertility diagnostics rely not only upon measurement of available nutrients but also upon the ability of the soil to retain these nutrients. Near infrared (NIR) reflectance spectroscopy is a rapid and non-destructive analytical technique which allows the simultaneous estimation of standard soil characteristics and does not require the use of chemicals. Previous studies showed that NIR spectroscopy could be used in local contexts to predict soil properties. The main goal of our research was to build a methodological framework for the use of NIR spectroscopy on a more global scale. The specific goals of this study were (i) to identify the best spectral treatment and processing—LOCAL versus GLOBAL—regression methods, Nil to compare the performance of NIR to standard chemical protocols and (iii) to evaluate the ability of NIR spectroscopy to predict soil total organic carbon (TOC), total nitrogen (TN), clay content and cationic exchange capacity (CEC) for a wide range of soil conditions. We scanned 1300 samples representative of the main soil types of Wallonia under crop, grassland or forest. Various sample preparations were tested prior to NIR measurement. The most appropriate options were selected according to analysis of variance and multiple means comparisons of the spectra principal components. Fifteen pre-treatments were applied to a calibration set and the prediction accuracy was evaluated for GLOBAL and LOCAL modified partial least square (MPLS) regression models. The LOCAL MPLS calibrations showed very encouraging results for all the characteristics investigated. On average, for crop soil samples, the prediction coefficient of variation ( CV_p) was close to 15% for TOC content, 7% for TN content and 10% for clay content and CEC. The comparisons of repeatability and reproducibility of both NIR and standard methods showed that NIR spectroscopy is as reliable as reference methods. Prediction accuracy and technique repeatability will allow the use of NIR spectroscopy within the framework of the soil fertility evaluation and its replacement of standard protocols. LOCAL MPLS can be applied within global datasets, such as the International global soil spectral library. However, the performance of LOCAL MPLS is linked to the number of similar spectra in the dataset and more standard measurements are needed to characterise the least widespread soils.