The fate of carbon in pulse-labelled crops of barley and wheat

Wheat (cv. Gutha) and barley (cv. O'Connor) were grown as field crops on a shallow duplex soil (sand over clay) in Western Australia with their root systems contained within pvc columns. At four stages during growth, the shoots were pulse-labelled for 1.5h with¹⁴CO₂; immediately prior to labelling, the soil was isolated from the shoot atmosphere by pvc sheets. After labelling, the soil atmosphere was pumped through NaOH to trap respired CO₂ and after 2.5, 5, 7.5 and 24 h from the start of labelling, columns were destructively sampled to recover¹⁴C from the roots, soil and shoot. Both species showed similar patterns of¹⁴C distribution and changes in distribution through the growing season. During early tillering, 15-25% of the¹⁴C recovered after 24 h had been respired by the roots and rhizosphere, 17-27% was retained in the roots, 0.4-1.8% was recovered as water-soluble¹⁴C in the soil and the remainder (45-67%) was present in the shoot. These percentages changed during growth so that during grain filling only 2-3% of the¹⁴C recovered after 24 h was as respired CO₂, 2-6% was in the roots, 0.2% was in the soil and over 90% was in the shoot. The distribution of¹⁴C in components of the soil-plant system changed during the 24 h after labelling with the most rapid changes occurring generally during the first 7.5 h after labelling. Using growth measurements from adjacent plots, the amounts of C added to the soil were estimated for the whole season. Carbon input to the soil was about 48 gC m^-2 for wheat and 58 gC m^-2 for barley; the crops produced total shoot dry matter of 494 (wheat) and 735 g m^-2 (barley). Of the C input to the soil, 27.8% (wheat) and 40.3% (barley) was as respired C and only 3.3 (wheat) and 4.1% (barley) was collected as exudate (water-soluble material).