Effects of Soil Warming and Nitrogen Addition on Soil Respiration in a New Zealand Tussock Grassland

Soil respiration (R (S)) represents a large terrestrial source of CO(2) to the atmosphere. Global change drivers such as climate warming and nitrogen deposition are expected to alter the terrestrial carbon cycle with likely consequences for R (S) and its components, autotrophic (R (A)) and heterotrophic respiration (R (H)). Here we investigate the impacts of a 3°C soil warming treatment and a 50 kg ha(−1) y(−1) nitrogen addition treatment on R (S), R (H) and their respective seasonal temperature responses in an experimental tussock grassland. Average respiration in untreated soils was 0.96±0.09 μmol m(−2) s(−1) over the course of the experiment. Soil warming and nitrogen addition increased R (S) by 41% and 12% respectively. These treatment effects were additive under combined warming and nitrogen addition. Warming increased R (H) by 37% while nitrogen addition had no effect. Warming and nitrogen addition affected the seasonal temperature response of R (S) by increasing the basal rate of respiration (R (10)) by 14% and 20% respectively. There was no significant interaction between treatments for R (10). The treatments had no impact on activation energy (E (0)). The seasonal temperature response of R (H) was not affected by either warming or nitrogen addition. These results suggest that the additional CO(2) emissions from New Zealand tussock grassland soils as a result of warming-enhanced R (S) constitute a potential positive feedback to rising atmospheric CO(2) concentration.