Reply on RC1

Abstract

We evaluated the performance of a new, simple test to evaluate soil structural stability. The QuantiSlakeTest (QST) consists in a quantitative approach of the slake test, a dynamic weighting of a dried structured soil sample once immersed in water. The objective of this work was threefold: we aimed to (i) derive indicators from QST curves to evaluate soil structural stability regarding the underlying mechanisms of soil disaggregation; (ii) establish the relationship between soil properties and QST indicators; and (iii) assess how QST indicators respond to contrasting soil management practices. To meet these goals, we sampled the soil of 35 plots from three long-term field trials in the silt loam region of Belgium dealing with contrasting organic matter inputs, tillage treatments and P-K fertilisation, respectively. For each plot, QST curves were compared to the three tests of Le Bissonnais, targeting specific mechanisms of soil disaggregation. Shortly after immersion in water, soil mass increases due to the rapid replacement of air by water in soil porosity. Then soil mass reaches a maximum before decreasing, once mass loss by disaggregation exceeds mass gain by air loss. Our results confirmed that the early mass loss under water is mainly related to slaking, whereas after a longer time period, clay dispersion becomes the dominant process of soil disaggregation. The overall soil structural stability was positively correlated to the soil organic carbon (SOC) content and negatively correlated to the clay content of soil. Accordingly, the SOC:clay ratio was closely related to QST indicators. Nevertheless, for a similar carbon (C) input, green manure and crop residues were more efficient in decreasing clay dispersivity whereas farmyard manure promoted SOC storage and was more efficient against slaking. QST curves had a strong discriminating power between reduced tillage and ploughing regardless of the indicator, as reduced tillage increases both total SOC content and root biomass in the topsoil. The QST has several advantages. It is rapid to run, doesn't require expensive equipment or consumables and provides a high density of information on both specific mechanisms of soil disaggregation and the overall soil structural stability. As an open access program for QST data management is currently under development, the test has a strong potential for adoption by a widespread community of end-users.