Long-term cropping system effects on soil organic matter fractions and enzyme activities in a cool oceanic climate

Information about soil organic matter fraction and enzyme activity responses to agricultural management may help guide decisions that sustain crop productivity and soil health. We measured carbon and nitrogen in bulk soil, mineral-associated organic matter, particulate organic matter, water-extractable organic matter, and the potential activity of β-glucosidase, N-acetyl-β-d-glucosaminadase, acid phosphomonoesterase and arylsulfatase in three cropping systems after: (1) 21 years of conventional- or no-tillage silage corn monoculture (0–20 cm); and (2) 6 years of nitrogen fertilization with or without nitrification inhibitors, and (3) 9 years of 100% or 200% the recommended broadcast or fertigation nitrogen rate in two distinct mature highbush blueberry systems (0–15 cm). Soil organic carbon, particulate organic carbon, particulate organic nitrogen, water-extractable nitrogen, and arylsulfatase activity were 17%, 38%, 50%, 25%, and 68% greater, respectively, with no-tillage than conventional tillage. Particulate organic carbon accumulated with two decades of no-tillage, which increased soil organic carbon without altering mineral-associated organic carbon. Nitrification inhibitors did not impact any soil organic matter fraction or enzyme activity after 6 years in a mature highbush blueberry system. Broadcasted nitrogen led to higher soil organic carbon than fertigation, but excessive application (200% vs 100% rate) depleted soil organic carbon, accumulated reactive nitrogen and reduced potential activity of N-acetyl-β-d-glucosaminadase, acid phosphomonoesterase and arylsulfatase after 9 years in a mature highbush blueberry system. Excessive nitrogen may deplete organic carbon and cause reactive nitrogen accumulation in soil of mature highbush blueberry systems. Intensive agricultural management has tradeoffs for carbon and nitrogen cycling that should be balanced with sustainable crop production.