Trafficking intensity index for soil compaction management in grasslands

Good soil structure provides multiple benefits for society but in grass-based production systems is underpinned by trafficking management regime. For Irish soils, there is no soil trafficking intensity index that considers the effect of geo-climatic variability or differences in drainage classes on soil compaction risk. Grazing and machinery activity data were compiled across 38 managed grasslands along with common soil structural quality indicators in order to develop and validate a ‘soil trafficking intensity index for compaction (STICi)’. Two component indices of STICi were developed: (a) a grazing trafficking index (Gi, kg × year ha−1) and (b) a machinery trafficking index (Mi, kg × year ha−1). The average annual grazing trafficking pressure observed was 213,914 kg × year ha−1, and the average annual machinery trafficking pressure was 4,412 kg × year ha−1. These figures represent thresholds above which soils are at higher risk of compaction. Mi spanned a wider range (−2.1 ≤ Mi ≤ 2.8) compared with Gi (−1.32 ≤ Gi ≤ 1.06). STICi and components, when disaggregated by soil drainage class, were able to detect changes in direct indicators of soil structural quality, such as bulk density, total porosity, water holding capacity, water conductivity and visual soil assessment. STICi (Mi and Gi) were also related to indirect indicators, such as, soil carbon content, earthworms and microbial biomass. In general, poorly drained sites showed higher vulnerability to machinery trafficking intensity compared with grazing trafficking pressure. At national scale, STICi can be utilized to identify soils at risk of compaction and underpin targeted management advice for supporting sustainable grassland production.