Yunying Fang^a, Bhupinder Pal Singh^a,*, Warwick Badgery^b, Xinhua He^a,c,d**

　　^a NSW Department of Primary Industries, Elizabeth Macarthur Agricultural Institute, Menangle, NSW 2568, Australia

　　^b NSW Department of Primary Industries, Orange Agricultural Institute, Orange, NSW 2800, Australia

　　^c Centre of Excellence for Soil Biology, College of Resources and Environment, Southwest University, Chongqing 400715, China

　　^d School of Plant Biology, University of Western Australia, Crawley, WA 6009, Australia

　　*Contacts to bp.singh@dpi.nsw.gov.au and xinhua.he@uwa.edu.au; ** Presenter

　　The allocation dynamics of newly assimilated carbon (C) and nitrogen (N) and their responses to management practices in dryland cropping systems are poorly understood. We aim to enhance this knowledge with relevance to identify management practices that may increase soil organic C (SOC) stocks and N use efficiency. Using in-situ ¹³CO₂ and urea-¹⁵N pulse labelling at the late wheat heading stage, we investigated allocation of newly assimilated C and N in crop and soil pools as influenced by long-term conventional and reduced tillage mixed farming practices. On the first day after labelling, 91−92% of the added ¹³C (1.49 g ¹³CO₂-C m^-2) and 81−82% of the soil applied ¹⁵N (0.1 g urea-¹⁵N m^-2) were recovered in the crop and soil pools. Over 50 days ( i.e. at grain maturity), only 4−5% of the ¹³CO₂ was allocated belowground, with 60−64% of the belowground ¹³C released via soil respiration, and 72−74% of the ¹⁵N was recovered in the soil to 30-cm depth and only 0.5−0.7% was allocated aboveground. The long-term differences in tillage practices did not influence allocation of new C (¹³C) and N (¹⁵N) in the wheat crop–soil pools, including aggregates-size fractions. This may be one of the factors for the lack of effect of the contrasting practices on SOC and N stocks, structural stability, microbial biomass, crop N uptake and wheat productivity. The results suggest soil and agronomic functionality in drylands may not be enhanced through conservation tillage management only.

　　Keywords: ¹³CO₂, ¹⁵N, tillage, soil respiration, soil aggregation, urea