Salinity is a serious limiting factor for the growth of rhizobia. Some rhizobia are tolerant to salt stress and promote plant growth, but the mechanisms underlying these effects are poorly characterized. The growth responses and osmoprotectants in four Bradyrhizobium strains were examined under salt stress in this study. Two-dimensional electrophoresis (2-DE) and mass spectrometry were conducted to investigate protein profiles in rhizobia exposed to salt stress. Subsequently, salt tolerance in stylo (Stylosanthes guianensis) inoculated with rhizobia was further detected in hydroponics. Results showed that the Bradyrhizobium strain RJS9-2 exhibited higher salt tolerance than the other three Bradyrhizobium strains. RJS9-2 was able to grow at 0.35 M NaCl treatment, while the other three Bradyrhizobium strains did not grow at 0.1 M NaCl treatment. Salt stress induced IAA production, and accumulation of proline, betaine, ectoine, and trehalose was observed in RJS9-2 but not in PN13-1. Proteomics analysis identified 14 proteins regulated by salt stress in RJS9-2 that were mainly related to the ABC transporter, stress response, and protein metabolism. Furthermore, under saline conditions, the nodule number, plant dry weight, and N concentration in stylo plants inoculated with RJS9-2 were higher than those in plants inoculated with PN13-1. These results suggest that the tolerance of RJS9-2 to salt stress may be achieved by the coordination of indole-3-acetic acid (IAA) production, osmoprotectant accumulation, and protein expression, thus promoting stylo growth.

The application of green manure is a common way to increase the soil’s level of total organic carbon (TOC) and its fractions. However, the amount of green manure to apply and how the combined application of phosphate fertilizer affects the movement of TOC, and of its fractions, is still unclear. We conducted a column experiment with two treatments of phosphate fertilizer (with and without) and green manure (0, low amount level, high amount level). The longitudinal movement distance and accumulation amount of each organic carbon fraction were investigated after 14 days and 28 days. The results indicated that green manure, phosphate fertilizer, and incubation time affected the movement of the soil organic carbon fractions by affecting the initial quality of the green manure (TOC, cellulose, and lignin content), as well as the changes in quality. Green manure significantly increased the accumulation amount of the organic carbon fractions in the soil, and the high-level input of green manure increased the movement distance and accumulation amount of the organic carbon fractions; phosphorus fertilizer did not have a significant effect on the movement distance of the organic carbon fractions, but it did significantly affect accumulation amount. The 28-day incubation period increased the movement distance and the accumulation amount of the organic carbon fractions, with the exception of the particulate organic carbon (POC), compared to the 14-day incubation period. Taken together, these findings suggest that the high-level of input of green manure combined with the application of P fertilizer is beneficial for increasing the movement of the organic carbon fractions to the depth of the soil, and promotes their accumulation, which is an important agronomic management strategy for improving soil acidity in tropical regions.

Intercropping green manure (GM) may be a good solution to the problems of acid soil in tropical plantations. Soil organic nitrogen (No) may change due to the application of GM. A three-year field experiment was conducted to determine the effect of different utilization patterns of Stylosanthes guianensis GM on soil No fractions in a coconut plantation. Three treatments were set: no GM intercropping (CK), intercropping and mulching utilization pattern (MUP), and intercropping and green manuring utilization pattern (GMUP). The content dynamics of soil total N (TN) and soil No fractions including of non-hydrolysable N (NHNo) and hydrolyzable N (HN) in the cultivated soil layer was examined. The results showed that after three years of intercropping, the TN content of the MUP and GMUP treatment was 29.4% and 58.1% respectively higher (P < 0.05) than those of the initial soil, and the No fractions content of GMUP and MUP treatment was 15.1%-60.0% and 32.7%-111.0% higher (P < 0.05) than those of the initial soil. The further results indicated that after three years of intercropping, compared with CK, GMUP and MUP could increase the content of TN by 32.6% and 61.7% respectively, and No fractions content was also increased by 15.2%-67.3% and 32.3%-120.3%% respectively (P < 0.05). The No fractions content of GMUP treatment was 10.3%-36.0% higher than those of MUP treatment (P < 0.05). These results indicated that intercropping Stylosanthes guianensis GM could significantly increase the soil N including of the TN an No fractions content, and the GMUP was more effective than MUP, therefore, GMUP is a better GM utilization pattern to improve the soil fertility and should be popularized in the tropical fruit plantation.

No abstract