Multi-criteria indices to evaluate the effects of repeated organic amendment applications on soil and crop quality

The objective was to develop a multi-criteria tool to compare fertilizing practices either based on mineral fertilizer (CONT + N) or repeated applications of exogenous organic matter (EOM) and considering the positive but also the negative impacts of these practices. Three urban composts (a municipal solid waste or MSW, a co-compost of sewage sludge and green waste (GWS), and biowaste (BIO)) and a farmyard manure (FYM) have been applied biennially over 14 years. Soils and crops were sampled repeatedly and >100 parameters measured. The development of different quality indices (QI) was used to provide a quantitative tool for assessing the overall effects of recycling different types of EOM. A minimum data set was determined and 7 indices of soil and crop quality were calculated using linear scoring functions: soil fertility, soil biodiversity, soil biological activities, soil physical properties, soil contamination (⿿available⿿ and ⿿total⿿) and crop productivity. All QI varied between 0 and 1, 1 being the best score. EOM amendments significantly increased soil biodiversity, biological activities and physical properties with intensity generally depending on their characteristics. FYM was the most efficient EOM to improve soil biological properties. EOM application lead to similar yields as mineral fertilizers but grain quality was slightly decreased. Thus, mineral fertilizers remained more efficient at improving crop productivity index (QI = 0.88) than EOM although BIO was not significantly different than CONT + N. All EOM improved soil fertility but only BIO was significantly higher (QI = 0.86). EOM added a range of nutrients but an excess of P (e.g. GWS) can negatively impact the soil fertility index. EOM negatively affected the soil contamination index when considering total concentrations but decreased available fractions and consequently the risks of transfer. BIO was the most efficient EOM for most indices including improving the index of ⿿available⿿ soil contamination. This study demonstrated the positive impact of repeated EOM applications on soil and crop quality in a loamy soil.