Fate of photosynthesized carbon as regulated by long–term tillage management in a dryland wheat cropping system

Abstract Tracking photosynthesized carbon (C) allocation into different C pools is crucial for management of C sequestration, especially in agroecosystems. However, the effects of long–term tillage management on allocation and dynamics of recently fixed C in the crop–soil–atmosphere system have been rarely investigated under dryland conditions. Using in–situ 13CO2 repeat labeling, this study quantified the photosynthesized C input to soil, and assessed the responses of allocation dynamics, microbial utilization, and aggregate protection of newly fixed C in three long–term (>10y) tillage practices (no–till, chisel–till, and plow–till) in a dryland wheat cropping system. Regardless of tillage practice, the 13C included into the shoots, roots, soil respiration, rhizosphere, and bulk soils accounted for 46–64%, 4.6–6.0%, 17–38%, 6.4–11.7%, and 2.8–7.2%, respectively, of added 13C over a 35–d chase period. Owing to relatively low plant biomass, compared to plow–till, long–term no–till and chisel–till on average lowered plant 13C fixation and its allocation in rhizosphere soil by 17% and 11%, and 21% and 15%, respectively. Nevertheless, the 13C relocated to bulk soil was significantly higher under both no–till (0.24 g m−2) and chisel–till (0.22 g m−2) than that under plow–till (0.18 g m−2) over the chase period. This could be partially attributed to the decreased allocation of belowground 13C to root–derived CO2 releases and the increased C retention in both macroaggregates and microaggregates in conservation tillage plots. Moreover, the reduced microbial utilization of new C in rhizosphere soil, as indicated by the low 13C amount in microbial biomass, may further facilitate new C accumulation in bulk soil under reduced tillage. Our findings suggest that despite low photosynthetically–fixed C input to belowground pools during the early growth stages, soils under long–term conservation tillage, in particular no–till, can have a distinct efficiency advantage in soil carbon sequestration by enhancing photosynthesized C preservation in soil aggregates and decreasing new C loss from root–derived CO2 release in the dryland wheat–soil system.