Long-term manure and fertilizer effects on soil organic matter fractions and microbes under a wheat–maize cropping system in northern China

As an essential indicator of soil quality, soil organic carbon (SOC) and its different labile fractions have an important role in determining soil chemical, physical, and biological properties. The objective of this study was to evaluate the soil carbon (C) and nitrogen (N) contents in different soil organic matter (SOM) pools (light and heavy fractions), the role of light- and heavy-fraction C in SOC sequestration, and culturable microbial counts 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) OM, organic manure; (2) 1/2OMN, half organic manure with mineral fertilizer NPK; (3) NPK, mineral fertilizer NPK; (4) NP, mineral fertilizer NP; (5) PK, mineral fertilizer PK; (6) NK, mineral fertilizer NK; and (7) CK, unfertilized control. Carbon and N contents of the light and heavy fractions were highest in the OM treatment, while the CK treatment showed the lowest value. Application of half organic manure with mineral fertilizer NPK (treatment 2) significantly increased C and N contents of the light and heavy fractions in soil in comparison with application of mineral fertilizer alone (treatments 3, 4, 5, and 6). For the mineral fertilizer treatments, a balanced application of NPK (treatment 3) showed higher C and N contents of the light and heavy fractions than an unbalanced use of fertilizers (treatments 4, 5, and 6). Heavy-fraction C dominated total SOC storage in all treatments. The total SOC increase under fertilization treatments is attributed to an increase in C content of both the light and the heavy fraction. However, the SOC increase for the manure treatments was mainly due to an increase in the C content of the heavy fraction, whereas that for mineral fertilizer treatments was mainly due to an increase in the C content of the light fraction. The total soil culturable microbial counts (including bacteria, fungi, and actinomycetes) was observed to be highest for the OM treatment, while the control plot showed the lowest value. Application of half organic manure with mineral fertilizer NPK was found to produce a higher culturable microbial counts than application of mineral fertilizers alone, and the NPK treatment gave a higher culturable microbial counts than other mineral fertilizer treatments (NP, PK or NK). Light-fraction C is probably the better predictor of microbial abundance, as it correlated more strongly with culturable microbial counts than total SOC did.