Soil Physical Health in Mixed Crop Rotations: Roles of Grassland Proportion, Cattle Manure and Grass‐Mixture Diversity

ABSTRACT Integrating grasslands into cropping systems has benefits for nutrient cycling and weed control. Beyond that, grasslands are also recognized for their ability to improve soil organic carbon (SOC) and structural stability (SS). This study examined how grassland proportion, fertilization and grassland mixture, both during the grassland phase and in the follow‐on crops, affect SOC content, water‐stable aggregates (WSA), and clay dispersibility (CD) in the 0–20 cm plough layer within a long‐term rotation experiment in Denmark. Samples were taken from two crop rotations (with 2 or 4 years of grassland in a 6‐year rotation), including follow‐on crops, with each grassland featuring either 3‐species ( Lolium perenne L. 80%, Trifolium repens L. 16% and Trifolium pratense L. 4%) or 12‐species (3‐species 75% + 5 forbs 14.4% + 4 additional legumes 10.6%), being either unfertilized or manured (cattle slurry; 300 kg total‐N ha −1 ). There were no interactions between grassland proportion, fertilization and grassland mixture on SOC and SS. Increased grassland proportion and fertilization significantly enhanced WSA by ~2.5% and SOC by 0.17 and 0.11 g 100 g −1 , respectively. Only the grassland proportion significantly influenced CD, with the higher grassland proportion being associated with lower CD values (~2.4 g kg −1 ). SOC and CD were negatively correlated, but the correlation declined with sward age. Greater species diversity improved WSA without affecting SOC and showed legacy effects in follow‐on crops even after tillage. These findings indicate that WSA responds faster than SOC and may be a more sensitive indicator of soil health for evaluating management practices in grassland rotations.