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

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 (CO2)-C-13 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 C-13 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 C-13 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 C-13 fixation and its allocation in rhizosphere soil by 17% and 11%, and 21% and 15%, respectively. Nevertheless, the C-13 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 C-13 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 C-13 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.