Long-term applications of chemical and organic fertilizers on plant-available nitrogen pools and nitrogen management index

The objective of this study was to evaluate plant-available N pools and the role of N management index (NMI) in the surface (0–20 cm) of a fluvo-aquic soil after 18 years of fertilization treatments under a wheat–maize cropping system in the North China Plain. The experiment included seven treatments: (1) NPK, balanced application of chemical fertilizer NPK; (2) OM, application of organic manure; (3) 1/2OMN, application of half organic manure plus chemical fertilizer NPK; (4) NP, application of chemical fertilizer NP; (5) PK, application of chemical fertilizer PK; (6) NK, application of chemical fertilizer NK; and (7) CK, unfertilized control. Total organic N (TON), microbial biomass N (MBN), labile N (LN), inorganic N (ION, including ammonium (NH4+)–N and nitrate (NO3−)–N) contents, net ammonification rate (NAR), net nitrification rate (NNR), net N mineralization rate (NNMR), and NMI in the fertilized treatments were higher than in the unfertilized treatment. Application of chemical fertilizer N (NPK, NP, and NK) increased ION in soils, compared with application of organic N or control. Nitrate N prevailed over exchangeable NH4+–N in all treatments. Nitrogen storage of the OM- and 1/2OMN-treated soils increased by 50.0% and 24.3%, respectively, over the NPK-treated soil, which had 5.4–22.5% more N than NP-, PK-, and NK-treated soils. The MBN, LN, and ION accounted for 1.7–2.4%, 25.7–34.2%, and 1.4–2.9% of TON, respectively, in different fertilization treatments. The surface soils (0–20-cm layer) in all treatments mineralized 43.6–152.9 kg N ha–1 year–1 for crop growth. Microbial biomass N was probably the better predictor of N mineralization, as it was correlated significantly (P < 0.01) with NNMR. The OM and 1/2OMN treatments were not an optimal option for farmers when the crop yield and labor cost were taken into consideration but an optimal option for increasing soil N supply capacity and N sequestration in soil. The NPK treatment showed the highest crop yields and increased soil N fractions through crop residues and exudates input, and thus, it may be considered as a sustainable system in the North China Plain.