Soil indicators for comparing medium-term organic and conventional agricultural systems

Evaluation of the sustainability and environmental footprint of different agricultural production systems, such as organic and conventional systems, is an open scientific debate. Presently, several agri-environmental indicators have been proposed; however, their reliability and interpretation are still under discussion. The investigation of biological, chemical, and physical soil health variables has been advocated for understanding the sustainability of agricultural systems, even though they are not univocal indicators. In the present study, both biological (abundances of ammonia-oxidizing archaea, AOA; ammonia-oxidizing bacteria, AOB; and nosZ) and physicochemical (pH; soil organic carbon, SOC; total Kjeldahl nitrogen, TKN; available phosphorus, P; and texture) soil variables were investigated to compare two soils under organic (OS) and conventional (CS) management after 15 years. Among the physicochemical variables analyzed, TKN content was higher whereas P content and pH values were lower in the OS than in the CS. The SOC content was not affected by agricultural management. Biological indicators showed significant differences in the soil nitrogen cycle. After 15 years, the OS showed a higher potential for N2O emission reduction (higher nosZ) and NO3- losses in the environment (higher AOA) than the CS. Therefore, we conclude that biological (AOA and nosZ) and chemical (P, TKN, and pH) variables can be regarded as soil health indicators for comparing different agricultural management systems.