Seasonal allocation of photosynthetically fixed carbon to the soybean-grown Mollisols in Northeast China

Abstract

The knowledge of the contribution of carbon (C) released by growing roots to soil is essential to better understand the terrestrial C cycling and optimally manage soil organic matter in ecosystems. However, little information has been gained on quantifying the distribution of photosynthetically fixed C in the plant–soil system and its contribution to soil C over a growing season in soybean-grown Mollisols, the main soil type in Northeast China. In a pulse-chase labelling experiment, soybean plants grown in Mollisols were labelled with 13CO2 at various growth stages. More than 3/4 of fixed 13C was observed in shoots at Day 0 after labelling, and then the fixed 13C was continually exported from shoots, showing that 7.5% of 13C fixed at V4 (fourth node) and 71.1% at R6 (full seed stage) remained in shoots by the end of the growing season. The 13C recovery in roots decreased over the same period, while soil 13C was significantly increased. The allocation of 13C fixed at different growth stages to underground (roots and soil) varied at the end of the growing season, showing that 13C retained in roots and soil was 6.0 and 12.4% of the net assimilation at V4, compared with 1.4 and 2.1% of that at R6, respectively. Nodules, however, had the highest demand for C at R4 (full pod stage). The contribution of shoot C assimilation to the soil C pool was similar at the growth stages up to R5 before a sharp decrease at R6, and the cumulative contribution reached 93% at R5. Over the whole growing season, it was estimated that ~210 kg of photosynthetically fixed C per ha was accumulated in soil. This indicates that the C flow from soybean plants to soil during growth is a non-negligible source of C pool in Mollisols, and the majority of the C efflux occurs during V4–R5.

Tillage and nitrogen fertilization enhanced belowground carbon allocation and plant nitrogen uptake in a semi-arid canola crop–soil system

“…3 ). These results are consistent with other studies that also reported low belowground C allocation at reproductive stages, while allocating a maximum amount of assimilates to aboveground reproductive organs 21 , 34 , 55 , 56 . Additionally, low root-to-shoot C ratio (0.11–0.15) (Table S2 ) may have caused a net decrease in belowground allocation of 13 C in our study.…”

Allocation of photosynthesized carbon in an intensively farmed winter wheat–soil system as revealed by 14CO2 pulse labelling

“…Assimilated C is translocated to plant–soil compartments such as shoots and roots, exuded into the soil and lost due to respiration, and this allocation pattern changes with crop growth 15 , 20 – 22 . In the present study, the assimilated 14 C in the winter wheat shoots at the labelling event of tillering stage was only 36.4%, but increased at the later labelling events: i.e., the shoots retained the majority (60–74.7%) of their assimilated 14 C at DAL 5 during the elongation, anthesis and grain-filling stages (Fig.…”

“…The majority of labelling studies have been conducted at the early growth stages of wheat, e.g., 60 days after emergence by continuous labelling 31 , 32 and <150 days after emergence by pulse labelling 33 , 34 . Hence, extrapolating the values from early wheat stages to the whole lifetime of wheat plants (approximately 240 days after emergence in northern China) substantially overestimated the amount of C allocated to the belowground pool, as young plants exhibit faster root growth and greater sink strength of C allocated to the roots and soil than do older plants 15 , 16 .…”

“…This decrease might be attributed to differences in photosynthetic capacity and C requirements for root growth at different growth stages 3 , 11 . Extrapolating the values from short periods to the whole lifetime of wheat plants (approximately 240 days after emergence in northern China) might overestimate the amount of C allocated to the belowground pool, as young plants exhibit faster root growth and greater sink strength of C allocated to the roots and soil than do older plants 15 , 16 .…”

“…In current study, we undertook the pulse labelling at different growth stages of the whole winter wheat season and harvested the winter wheat at 5 days after 14 C-labelling (DAL) at each growth stage and at the end of the wheat growth season 10 , 15 , 20 , 21 . The 14 C distribution in the wheat biomass and soil pools at the end of the growing season represents the net contribution of photosynthates formed at various stages of wheat 15 , 20 , 21 . We aimed to quantify the temporal 14 C distribution in the winter wheat biomass and soil pools throughout the whole wheat season.…”