Soil pH and earthworms affect herbage nitrogen recovery from solid cattle manure in production grassland

Long term use of inorganic fertilisers and reduced organic matter inputs have contributed to acidification of agricultural soils. This strongly affects the soil dwelling fauna and nutrient mineralisation. Organic fertilisers such as solid cattle manure (SCM) resurge as an option to overcome this acidification problem and to provide the required blend of essential macro- and micronutrients for plant growth. We assessed the effects of earthworm density (400 or 700 m−2) at two levels of soil pH (ambient and increased), with or without application of solid cattle manure (SCM), on herbage nitrogen (N) uptake, and CO2 and N2O emissions over a period of 134 days using undisturbed soil cores from an acid peat grassland in a mesocosm experiment. Liming proved to be beneficial for earthworm performance and grassland productivity. A higher soil pH and earthworm density resulted in a higher soil biological activity measured as soil respiration. The combined application of lime and earthworms increased herbage apparent N recovery from SCM by 83% compared to SCM only. In the manured treatments, herbage N uptake was positively correlated with earthworm density (R2 = 0.92). N2O emissions increased by 37% when SCM was applied compared to the unfertilised control. Following SCM application, the cumulative increase in herbage N uptake was almost ten times greater than the measured total N2O losses. No relationship was observed between earthworm density and level of N2O emission. N mineralisation and herbage N uptake from SCM in acidic peat grasslands were greatly stimulated by the combined increase in soil pH and earthworm density. This stimulated the activity of soil biota, resulting in an increased herbage N recovery from the applied SCM.