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

<div><p>Soil respiration (<i>R</i>_S) represents a large terrestrial source of CO₂ 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 <i>R</i>_S and its components, autotrophic (<i>R</i>_A) and heterotrophic respiration (<i>R</i>_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 <i>R</i>_S, <i>R</i>_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 <i>R</i>_S by 41% and 12% respectively. These treatment effects were additive under combined warming and nitrogen addition. Warming increased <i>R</i>_H by 37% while nitrogen addition had no effect. Warming and nitrogen addition affected the seasonal temperature response of <i>R</i>_S by increasing the basal rate of respiration (<i>R</i>₁₀) by 14% and 20% respectively. There was no significant interaction between treatments for <i>R</i>₁₀. The treatments had no impact on activation energy (<i>E</i>₀). The seasonal temperature response of <i>R</i>_H was not affected by either warming or nitrogen addition. These results suggest that the additional CO₂ emissions from New Zealand tussock grassland soils as a result of warming-enhanced <i>R</i>_S constitute a potential positive feedback to rising atmospheric CO₂ concentration.</p>