In situ assessment of new carbon and nitrogen assimilation and allocation in contrastingly managed dryland wheat crop-soil systems

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 identifying management practices that may increase soil organic C (SOC) stocks and N use efficiency. Using in-situ13CO2 and urea-15N pulse labelling of wheat (Triticum aestivum L.) at the late heading stage, we investigated allocation of newly assimilated C and N in crop and soil pools as influenced by long-term conventional tillage (CT) and reduced tillage (RT) 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 this 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 aggregate-size fractions. This may be one of the factors in 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.