Long-term manure applications to increase carbon sequestration and macroaggregate-stabilized carbon

Long-term manuring supplies organic substrates containing carbon and nitrogen, which are expected to contribute to the stabilization of soil organic carbon (SOC) according to microbial stoichiometry decomposition and nitrogen mining theories. This possibility was evaluated after 33 years of manure application (i.e., farmyard manure at agronomic and elevated rates) compared to an unfertilized control and chemical fertilizer, at an agricultural field site in a Calcic Cambisol on the Loess Plateau. Soil was collected from 0-20 cm (plow layer), 20–40 cm and 40–60 cm depths at the end of the winter fallow season to assess the SOC content in whole soil or in macroaggregate, microaggregate, silt and clay fractions (plow layer only). Long-term management of this site increased the SOC stock (0–60 cm) by 44% in the unfertilized control, up to 68% with chemical fertilizer and from 140% (agronomic rate of farmyard manure) to 189% (elevated rate of farmyard manure). Carbon sequestration efficiency was greater in the unfertilized control (35%) and with chemical fertilizer (up to 31%) than in manure-amended soils (18–23%). Manuring increased the mass of macroaggregates and the specific activity of N-acetyl-glucosaminidase on a per unit SOC basis, which was negatively ( P < 0.001) correlated with the specific respiration rate (on a per unit SOC basis) in the whole soil (three depths) and aggregates (plow layer only). Stable macroaggregates were associated with larger SOC stocks and greater organic nitrogen acquisition by microorganisms in the manure-amended soil. This appears to support the microbial nitrogen mining theory. In conclusion, long-term manure application increased soil aggregation as well as stability of the macroaggregate-associated organic carbon, which contributes to SOC sequestration in the Calcic Cambisols on the Loess Plateau. • Manure application increased carbon sequestration rate but decreased its efficiency. • Organic carbon in macroaggregates was relatively stable. • Microbial mining of organic nitrogen determined soil carbon stability. • Manure application strengthened macroaggregate-stabilized carbon.