Allocation of photosynthetic carbon to nodules of soybean in three geographically different Mollisols

• The nodulation of soybean geographically varied in Mollisol regions.
• Nodule density increased as latitude increases.
• Nodules of soybean grown in high-latitude Mollisols have stronger C-sink.

The development of legume nodules and N2-fixation demand fair proportion of photosynthetic carbon (C). Soil as a determinant of nodulation may influence C allocation to nodule and subsequent underground C fates. However, there is little information on the photosynthetic C allocation to the nodule of soybean grown in Mollisols in Northeast China. Such research is crucial for optimizing the strategies of C/N management to improve nodulation and productivity in soybean farming systems. With a pot experiment, soybean plants were grown in three Mollisols sampled from low- to high-latitude in Northeast China and labeled with 13CO2 at the R4 (full pod) and R5 (initial pod filling) stages. The nodule characteristics and underground 13C distribution were investigated. The nodule number and nodule density were in the order of Mid- > High- > Low-latitude Mollisol, resulting in 980, 578 and 252 nodules per plant, and 11.1, 8.2 and 2.7 nodules per m of root length, respectively. The 13CO2 pulse-chase labeling showed that higher proportion of 13C was recovered in Mid-latitude Mollisol at R4, and in both Mid- and High-latitude at R5. Moreover, a 53.8% of increase on C-growth rate of nodule was also found in the two soils compared to Low-latitude. This suggests that nodules in high-latitude Mollisols have stronger C-sink activity than low-latitude ones, which attribute to nodule number and C-growth rate of nodule. The nodules accounted for 15.7%, 28.0% and 11.5% of net underground 13C in Low-, Mid- and High-latitude Mollisols, respectively. Therefore, the extent of nodule-driving underground C allocation varied with Mollisols from geographically different regions.