Sod Culture Treatments Positively Affect Soil Fungal Diversity, Soil Enzyme Activities, and Nutrient Uptake in Navel Orange Orchards

Gannan navel oranges (Citrus sinensis Osb. var. brasliliensis Tanaka) are mainly cultivated in mountain orchards. However, inappropriate long-term management practices such as clean cultivation and excessive use of fertilizers markedly decreased the soil fertility, which in turn aggravated soil and water loss and led to soil sealing and acidification, thereby declining the fruit tree yields. In this study, a 10-year field experiment was used to determine the effects of different treatments (clean culture, biochar addition, straw cover, and acrylamide application used in conjunction with sod culture) on the cultivation of navel oranges. The activity of soil sucrase, acid phosphatase, and urease was assayed with salicylic acid colorimetry, paranitrophenol-sodium orthophosphate colorimetry, and sodium phenylate colorimetry respectively. Moreover, the fungi in soil samples were counted using a traditional plate counting method, fungi DNA was extracted and 18S rRNA genes were PCR-amplified, and subsequently analyzed by the Illumina HiSeq2500 platform. The nutrient uptake of orange and soil chemical properties were determined by inductively coupled plasma mass spectrometer (ICP-MS) and soil agrochemical analysis. Results showed that the soil chemical properties, soil microorganisms, enzyme activities, soil microbial diversity, and leaf nutrition of all sod culture treatments were significantly higher than those for clean culture. Moreover, adding biochar (6t/hm²) could promote the soil organic matter (SOM), the content of soil available nutrients such as phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), and boron (B), the activity of acid phosphatase and sucrose, and the number and diversity of fungi in the soil. Meanwhile, the leaf nutrient concentrations of treatments application of biochar (6t/hm²) and rice straw cover (9t/hm²) was significantly higher than that for the control treatment. These results provide evidence that sod culture simultaneously improves soil chemical properties and increases soil biological properties, and thus can be an effective management approach for maintaining orchard ecosystem stability and soil fertility.