Short-term study on 13Carbon discrimination on irrigated tropical pasture

A better understanding of pasture ecosystem can be obtained through the Use Of C-13 discrimination technique. In this context, an experiment, assigned in a randomized complete block design with four replicates, was conducted to evaluate the (DC)-C-13 (96) discrimination, nitrogen (N) yield (kg N ha(-1)), total nitrogen content (g kg dry matter (DM)(-1)) and dry matter yield (kg DM ha(-1)) on an irrigated Tanzania grass pasture (Panicum maximum, Jacq.) receiving increasing rates (0, 30, 60, 90 and 120 kg N ha(-1) cut(-1)) of N fertilizer during the summer. Dry matter yield, N yield and nitrogen concentration increased quadratically with increasing levels of N fertilizer (P < 0,05). On the other hand, D values tended to decrease linearly with increasing levels of N fertilizer (P > 0,05). Besides that, negative and significant correlations (P < 0,05) were evident between either D values and dry matter yield (R = -0,4807) and D values and N yield (R = -0,5245). Overall results allow to establish the following conclusions: 1) at lower N fertilizer inputs tropical pastures tended to show higher discrimination against C-13 though this effect might be associated with lower N concentrations in plant tissue that, in turn, might add inefficiency to the C4 photosynthetic pathway and 2) lower dry matter and N yields were associated with higher C-13 discrimination values. Conversely, higher dry matter and N yields were associated to lower C-13 discrimination values.