Contrasting effects of nitrogen addition and soil warming on soil respiration in an old-growth subtropical forest

Primary forest occupies 34 % of the global forest area and contributes to the mitigation of atmospheric carbon (C) dioxide through C sequestration in vegetation and soil. Environmental changes, including climate warming and increased nitrogen (N) deposition, have been found to affect soil C dynamics. However, there is a major gap in knowledge regarding the response and potential mechanisms of primary forest C cycling to climate warming and increased N deposition. We conducted a three year (2018–2020) field-based soil warming (+3 °C above ambient soil temperature), N addition (50 kg N ha−1 year−1) and their interaction experiment in an over 300-year-old primary forest in subtropical China, and measured soil respiration monthly. In addition, soil enzyme activities and physico-chemical properties were also analyzed. Three years of W treatment did not significantly increase soil respiration, while N treatment significantly decreased it by 24.29 %. Cumulative soil respiration in N and WN treatments decreased by 2.17 and 2.14 Mg C ha−1 year−1, while W treatment increased annual cumulative soil respiration by 0.95 Mg C ha−1 year−1. W treatment indirectly affected soil respiration through its negative impact on C-degrading enzymatic activities and soil moisture. N treatment directly impacted soil respiration and indirectly by effect on soil organic C. These findings demonstrate that soil respiration in the old-growth subtropical forest would cause a minor soil C-climate feedback to 3 °C warming within the first three years, and N deposition may have not strong interaction with warming.