Allocation into soil organic matter fractions of 14C captured via photosynthesis by two perennial grass pastures

Perennial grass pastures are being increasingly adopted, but little is known about the flows of carbon (C) from photosynthesis into soil organic matter (SOM) that could be used for calculations in carbon accounting. Repeat-pulse labelling of perennial grass pastures (kikuyu and Rhodes grass) with 14C in the field in Western Australia was used to trace the allocation of C to SOM fractions and to determine the stability of each fraction over an extended period. For kikuyu, >40% of the 14C fed to the plants was allocated belowground within 10 days of labelling, and after 1 year half of this remained. Allocation of 14C belowground under Rhodes grass ranged between 20 and 24% of 14C applied and remained constant for up to 6 months. At least 90% of the 14C belowground was found in the surface 300 mm of soil. The allocation of 14C to the coarse (50 µm–2 mm) and fine (<50 µm) SOM fractions was similar in magnitude for the two grasses and remained stable through time. It was estimated that in 1 year ~1 t C ha–1 was assimilated into the coarse + fine SOM fractions under kikuyu. However, Rhodes grass was not uniformly distributed across the paddock, thereby reducing the estimates of assimilation of C belowground in these systems to one-tenth of that under kikuyu. Data obtained will help validate plant–soil models for assessing rates of C sequestration under perennial pastures.