Effects of large-grained activated humic acid fertilizer on soil aggregates and organic carbon in apple orchard soil

Abstract

The new large-grained activated humic acid fertilizer (LAF) can significantly reduce the amount of chemical fertilizer application and stable fruit yield. Understanding its impacts on soil aggregates and organic carbon is an important basis for revealing its role in driving soil structure of apple orchard. There were four LAF treatments: LAF1 (full fertilization, fertilization period and mass ratio (the same below), germination stage: fruit expansion stage: maturity stage=3:4:3), LAF2 (full fertilization, germination stage: fruit expansion stage: maturity stage=2:3:5), LAF3 (fertilizer application reduction by 1/4, germination stage: fruit expansion stage: maturity stage=2:3:5), LAF4 (fertilizer application reduction by 1/3, germination stage: fruit expansion stage: maturity stage=2:3:5); with no fertilization as control (CK). In a four-year pot experiment, we examined the composition, stabi-lity and organic carbon content of soil aggregates under different fertilization treatments. The results showed that: 1) compared with CK, each treatment of LAF increased the content of >2 mm and 2-0.25 mm aggregate by 53.4%-77.5% and 12.3%-17.0%, respectively. The application of LAF significantly increased the content of soil water stable aggregates, and such effects were positively related with application amount. The content of soil water stable aggregate was the highest in the LAF1 treatment. 2) There was no significant difference in aggregate content of each particle size among LAF treatments, with the proportion of aggregate content of 2-0.25 mm particle size being the highest. 3) Compared with CK, all LAF treatments significantly increased the average weight diameter (MWD) and geometric mean diameter (GMD), and reduced the fractal dimension (D). LAF1 treatment had the highest MWD and GMD values, and had the strongest effect on the stability of soil aggregates. 4) Except for LAF4 treatment, the content of soil organic carbon in other LAF treatments was significantly higher than that in CK, and the content of soil organic carbon in LAF2 treatment was the highest. All LAF treatments increased the organic carbon content of soil aggregates with each particle size. LAF1, LAF2, and LAF3 treatments significantly increased the organic carbon of aggregates with particle size >2 mm. Particle size >2 mm had the highest contribution to the total organic carbon. The contribution rate of water stable large aggregate organic carbon to total organic carbon of LAF treatment was significantly higher than that of CK, which was all higher than 66.0%, and that of LAF1 treatment was the highest. In conclusion, the application of LAF enhanced the formation and stability of water stable aggregates and increased organic carbon content of aggregates in apple orchard soil, with the best performance of the full application. The application of LAF could be used as an effective measure to improve soil structure and fertility in apple orchard.