Purpose

There have been many studies about the effects of long-term fertilizations on crop yields and soil organic carbon. However, little attention has been paid to the response of soil dissolved organic carbon (DOC) to fertilization practices. The aims of this work were to investigate the variations of the content and the chemical composition of soil DOC under long-term different fertilizations and to evaluate their correlations with maize yields.

Materials and methods

A long-term fertilization experiment under a double maize cropping system was established in 1986 in acidic Ultisols of southern China. Soil samples were collected in April 2014 from the following ten treatments: (1) CK (non-fertilization), (2) N (only N fertilization), (3) P (only P fertilization), (4) K (only K fertilization), (5) NP (N and P fertilization), (6) NK (N and K fertilization), (7) NPK (N, P, and K fertilization), (8) HNPK (double doses of NPK fertilization amount), (9) OM (only organic manure), and (10) NPKM (NPK fertilization plus organic manure). The soil basic chemical properties including the content of DOC were determined. Then, the chemical composition of soil DOC was assessed through three-dimensional excitation-emission matrix (3DEEM) and parallel factor analysis (PARAFAC).

Results and discussion

The results showed that long-term application of organic manure significantly increased the content of soil DOC and obviously changed the chemical composition of soil DOC. Based on the 3DEEM and PARAFAC, organic manure treatments (OM, NPKM) had higher proportions of tryptophan-like components and lower proportions of tyrosine-like components as compared with other treatments (CK and inorganic fertilizer treatments). For the fluorescence indices, organic manure treatments showed significantly higher humification index (HIX) and lower biological index (BIX) and McKnight index than other treatments. Aggregated boosted trees analysis revealed that the DOC content explained 24.6% of the total variance in maize yields and ranked first among all tested soil chemical properties. Furthermore, all fluorescence properties of DOC were correlated with maize yields, in which the HIX index was the most important factor.

Conclusions

This research demonstrated that not only the content of DOC but also the chemical composition of DOC in acidic Ultisols were significantly correlated to maize yields. It may be an effective way to improve soil quality and productivity by increasing DOC content and HIX index. From the perspective of soil DOC, our results proved again that application of organic manure could be the most suitable fertilization practice in acidic Ultisols of southern China.