DYNAMICS OF C IN A PASTURE GRASS (PANICUM-MAXIMUM VAR TRICHOGLUME) SOIL SYSTEM

A lack of available nitrogen is the primary mineral constraint to the maintenance of productivity by grass-based pastures in northern Australia. It is hypothesised that under N stress, plants maximize soil exploration by the allocation of a large proportion of their resources below-ground which imposes constraints on yield from tops. A carbon balance of mature plants of the C4 grass green panic (Panicum maximum) was conducted to investigate this hypothesis. Plants were grown for 88 days in a chamber containing atmospheric concentrations of CO2 labelled with C-14. The effects of nitrogenous fertilizer, removal of shoots and shading on the partitioning of dry matter and C-14 between plant parts and to the soil, and respirational losses of C-14 from roots and rhizosphere microorganisms were measured. Additions of fertilizer restored the productivity of rundown plants by increasing the dry matter (DM) and C-14 content of shoots and crowns. Root DM was not affected by fertilizer but the C-14 content was increased. Thus, N fertilizer increased the proportion of DM allocated to shoots relative to roots whether or not shoots were removed. Although the absolute amounts of C-14 lost as CO2 from root-rhizosphere respiration increased as a result of applications of fertilizer N, the proportional losses were not affected. Over all treatments, loss of C-14 by root-rhizosphere respiration was highly correlated with total plant DM (r2 = 0.78). Removal of shoots did not affect shoot DM production but it did decrease the size of the crowns for both N treatments. The amount of C-14 in the shoots of rundown (but not of fertilized) plants was increased by cutting, while for crowns, roots and the microbial biomass, the amount of C-14 was reduced in both rundown and fertilized plants. Shading rundown plants decreased shoot DM and C-14 in the shoots, crowns, roots, biomass and soil organic matter, plus that lost through respiration compared to rundown plants in full sunlight. It is concluded that rundown in green panic pastures is primarily due to the allocation of a large proportion of dry matter to the roots in response to low availability of soil N. Increased yields associated with the alleviation of N deficiency in uncut plants resulted primarily from the direction of recent photosynthate to shoots and to a lesser extent, from mobilization of C-12 from roots.