Effects of long-term fertilization on soil humus carbon and nitrogen fractions in a wheat-maize cropping system

that applying organic manure or its combination with fertilizer NPK and the balanced application of fertilizer NPK could be the keys for the increase of soil CHA, CFA, CHM, CLHA, CLFA, NHA, NFA, NHM, NLHA and NLFA contents; soil labile humus C and N fractions were more sensitive to fertilization practices than soil humus C and N fractions, respectively. The contents of soil humus (including humic acid, fulvic acid, and humin) and labile humus (including labile humic acid and fulvic acid) carbon and nitrogen fractions in 0–20 cm soil layer were studied on a long-term fertilization field under wheat-maize cropping rotation in North China Plain. The long-term fertilization experiment was designed to include 7 treatments: fertilizer NPK (NPK), organic manure (OM), half organic manure with fertilizer NPK (1/2OMN), fertilizer NP (NP), fertilizer PK (PK), fertilizer NK (NK) and control (CK). After 18 years fertilization, all the manure and fertilizer treatments showed higher contents of humic acid carbon (CHA) and nitrogen (NHA), fulvic acid carbon (CFA) and nitrogen (NFA), humin carbon (CHM) and nitrogen (NHM), labile humic acid carbon (CLHA) and nitrogen (NLHA), and labile fulvic acid carbon (CLFA) and nitrogen (NLFA), and higher proportions of CHA, CFA, CLHA and CLFA in total organic carbon (CT) and of NHA, NFA, NLHA and NLFA in total organic nitrogen (NT). The increased rate of contents of soil labile humus C and N fractions were higher than that of soil humus C and N fractions, respectively. Among the fertilization treatments, treatment OM has the highest contents of soil humus and labile humus carbon and nitrogen fractions, followed by treatment 1/2OMN, and the treatments of applying chemical fertilizer alone. Balanced application of fertilizers (treatment NPK) showed higher contents of soil humus and labile humus carbon and nitrogen fractions than imbalanced use of fertilizers (treatments NP, PK, and NK). It was suggested that applying organic manure or its combination with fertilizer NPK and the balanced application of fertilizer NPK could be the keys for the increase of soil CHA, CFA, CHM, CLHA, CLFA, NHA, NFA, NHM, NLHA and NLFA contents; soil labile humus C and N fractions were more sensitive to fertilization practices than soil humus C and N fractions, respectively.