Effects of sod culture on soil organic carbon and its components in a 'Nanfeng tangerine' orchard

Abstract:【Objective】Citrus is one of the important fruits in China. The area and output rank the first in the world.‘Nanfeng tangerine'is native to Nanfeng county, Jiangxi province. There is a cultivation history of 1 300 years.‘Nanfeng tangerine'is a well-known easy peeler in China. In the past years, ‘Nanfeng tangerine'has been introduced and cultivated in many places due to its popularity in the domestic and foreign markets. In 2016, ‘Nanfeng tangerine'planting area reached 75 000 hm2 with a production of 1.3 million tons.‘Nanfeng tangerine'industry has become a pillar industry for local farmers to increase income. However, traditional orchard management with clear soil tillage has led to a series of ecological and environmental problems such as soil property degradation, soil erosion, environmental pollution, loss of biodiversity and weakening of system resistance, which not only reduces the productivity of the orchard, but also leads to production and quality decline of‘Nanfeng tangerine', and therefore restricts the sustainable development of‘Nanfeng tangerine'industry. Sod culture in orchard is the main model of eco-orchard construction in advanced countries such as European countries, the United States and Japan. It improves soil physical and chemical properties, increases soil organic matter content, maintains soil nutrient balance, ameliorates orchard ecological environment, and effectively solves the potential negative effect due to long-term tillage in orchard. In this study, the changes in soil organiccarbon and its components in orchards under sod culture and traditional clear tillage (control) were studied in order to provide the theoretical basis for the construction of reasonable soil management model and to promote the sustainable development of‘Nanfeng tangerine'industry.【Methods】The experiment was conducted in a‘Nanfeng tangerine'orchard with three treatments: sod culture with Trifolium repens, sod culture with Lolium multiflorum and clear tillage as control. The grasses were planted by inter-row sowing in the spring of 2014. The average sowing volume was 30 kg· hm-2, mowed twice a year, and covered in situ. In the control plots, weeding was regularly implemented. We took the soil samples at different layers from 0 to 40 cm in depth 30 to 40 cm away from the trunk using the five-point method in mid-October 2016. We tried to minimize the disturbance of soil samples during the collection and transport processes, so as not to damage soil aggregates. Field samples were divided into three equal portions: one for the determinations of soil nutrients and total organic carbon (TOC) , microbial biomass carbon (MBC) , water-soluble organic carbon (WSOC) and easily oxidized organic carbon (ROC) ; one for the determinations of organic carbon component library in soil aggregates (where soil samples were gently broken into small pieces, and then screened through the 5 mm sieve to break the large aggregates, and finally dried naturally) ; one for the determinations of light organic carbon (LFOC) and recombinant organic carbon (HFOC) (grinded after naturally dried and screened through 2 mm sieve) . Organic matter and gravel (>2 mm) were discarded during the grinding process.【Results】The contents of TOC, MBC, WSOC, ROC, LFOC and HFOC in sod culture with white clover were 11.71 g·kg^-1, 690.81 mg·kg^-1, 355 mg·kg^-1, 2.62 g·kg^-1, 1.02 g·kg^-1 and 5.03 g·kg^-1, which were increased by 11.74%, 52.69%, 80.20%, 67.95%, 92.45% and 18.08% compared with the control, respectively. The contents of TOC, MBC, WSOC, ROC, LFOC and HFOC in sod culture with ryegrass were 10.40 g·kg^-1, 540.15 mg·kg^-1, 243 mg·kg^-1, 2.07 g·kg^-1, 0.83 g·kg^-1 and 4.58 g·kg^-1, which were 3.72%, 19.38%, 23.35%, 32.69%, 56.60% and 7.51% higher than the control, respectively. The difference between the two treatments was significant. There were some differences in the content of soil aggregates with different particle sizes in different treatments. Compared with the control, the content of micro-aggregates (>2 000 µm) in the treatments with ryegrass and white clover increased by 311.66% and 205.52%, respectively. The content of aggregates (250-2 000 µm) increased by 16.86% and 39.76%, respectively, and the contents of micro-aggregates (53-250 µm) were reduced by 14.59% and 23.04%, respectively, and the differences were significant. There was no significant difference in the content of micro-aggregates (<53 µm). The organic carbon content in large soil aggregates (>2 000 µm and 250-2 000 µm) increased significantly in sod culture treatments, but there was little effect on the organic carbon content in micro-aggregates (53-250 µm and <53 µm).【Conclusion】The contents of soil TOC, MBC, WSOC, ROC, LFOC and HFOC, soil texture and soil quality were improved by sod culture, and the treatment with the legume white clover was better than that with grass rye grass in improving soil organic carbon content. In addition, sod culture significantly raised the amount of soil aggregates (>2 000 µm and 250- 2 000 µm) and the organic carbon contents in the aggregates, and reduced the micro-aggregate (53-250 µm) content and effectively improved the stability of soil structure.