Short-Term Study on 13Carbon Discrimination on Irrigated Tropical Pasture

A better understanding of pasture ecosystem can be obtained through the use of 13C discrimination technique. In this context, an experiment, assigned in a randomized complete block design with four replicates, was conducted to evaluate the Δ 13C (‰) 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, Δ 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 Δ 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 13C 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 13C discrimination values. Conversely, higher dry matter and N yields were associated to lower 13C discrimination values.