Changes in soil carbon and nitrogen pools in a Mollisol after long-term fallow or application of chemical fertilizers, straw or manures

• Long-term straw and NPK combination has significantly highest inorganic C at 40–80 cm soil depth in all treatments.
• Long-term manure and NPK combination has significantly highest organic C at 0–20 cm soil depth in all treatments.
• The combination of manure, straw and NPK therefore further improve soil C and N storage.

Appropriate fertilizer management can effectively restrain decreases in soil carbon (C) and nitrogen (N) in agricultural Mollisols in northeast China. A 22-yr-old field experiment was employed to explore the effects of fallow, chemical fertilizer N, phosphorus (P), potassium (K), or straw or manure combined with NPK (SNPK, MNPK), and 1.5 times the N rate of MNPK (1.5MNPK) application on soil C and N pools. Soil organic C (SOC), total N (TN), inorganic C (IC), and mineral N (NO3-N, NH4-N) to 100 cm soil depth and C and N in particulate organic matter (POC, PON), soil microbial biomass (MBC, MBN), and dissolved organic matter (DOC, DON) to 40 cm soil depth were determined. In all treatments, the 1.5MNPK treatment had the significantly highest (P < 0.05) SOC and TN concentrations above 40 cm soil depth and the SNPK treatment had the significantly highest (P < 0.05) IC concentration at 40–60 and 60–80 cm soil depths. Comparing the other treatments to 40 cm soil depth, in most cases 1.5MNPK and MNPK treatments at each depth had significantly higher (P < 0.05) POC, PON concentration, POC/SOC and PON/TN. SNPK treatment had significantly higher (P < 0.05) MBN and MBN/TN at 10–20 and 20–40 cm soil depths and the fallow treatment had significantly higher (P < 0.05) MBC and DOC concentrations and higher DOC/SOC ratio at each soil depth. The optimum combination of manure, straw and NPK therefore favored C and N storage in this agricultural Mollisol.