Topsoil vertical gradient in different tillage systems: An analytical review

It is often assumed that a homogeneous distribution of soil properties occurs in the topsoil layer, but the vertical variation of soil properties in cultivated soils would mainly depend on the tillage system. In remote sensing, for example, only information strictly limited to the soil surface is accessible, although the interest of the user communities would concern the topsoil rootzone, i.e. until a depth of 30-40 cm. The present study provides an analysis of data collected from studies in which the vertical variation of the main soil properties was reported for four types of tillage systems: i) deep inversion ploughing (DP); ii) subsoiling (SUB); iii) minimum tillage (MT) and no-tillage (NT). To the best of our knowledge, a review about the effects of different tillage systems on soil vertical variation seems to be lacking in the current literature. A keyword-based search followed by a two-stage manual selection provided a total of 43 papers with data sets suitable for the analysis. The soil data, extracted by these papers, were standardized to avoid different measurement units and different site-specific baselines. This method allowed to obtain the soil data extracted by different papers worldwide, comparable to study the vertical gradient. A gradient of soil organic carbon (SOC) was reported in all the 43 selected papers, whereas total nitrogen (TN) in 20 papers, therefore reliable global models of vertical variation could be obtained for these properties. Linear models of SOC vertical variation were obtained for MT and SUB, whereas second order polynomial functions were calculated for DP and NT. The full cross-validation of the models provided R2 higher than 0.8 and errors (RMSE) from 1.55 to 3.23 g.kg 1, with similar efficiency for SOC and TN. Predictive models of vertical variation of C/N ratio and bulk density (BD) were also carried out, whereas data of pH, CEC, and textural fractions were found only in few papers and modelling was not possible. The models proposed by this analytical review might be employed to extend the estimation of surface soil properties to the whole topsoil layer of cultivated soil, according to the adopted tillage method. In addition, the results of the paper highlighted the impact of tillage system on the vertical gradient of SOC, TN, and BD, allowing a better quantification of the carbon and nitrogen stock in agricultural soils.