Effects of experimental warming and nitrogen addition on soil respiration and CH4 fluxes from crop rotations of winter wheat-soybean/fallow

Soil respiration and CH4 emissions play a significant role in the global carbon balance. However, in situ studies in agricultural soils on responses of soil respiration and CH4 fluxes to climate warming are still sparse, especially from long-term studies with year-round heating. A warming experiment was conducted at Luancheng research station in the North China Plain from 2008 to 2013. Two levels of temperature (T: increase on average 1.5 degrees C at 5 cm soil depth by infrared heaters, C: ambient temperature) were combined with two levels of nitrogen (N) treatments (N1: with 315 kg N ha(-1) y(-1), NO: no nitrogen input) in the farmland. Soil was found to be a sink for CH4 with no marked seasonal variations. In the wheat-growing season, warming and N input both decreased cumulative CH4 uptake, probably because warming-induced soil drying in N1 treatment reduced (or limited) methanotroph activity by affecting soil NH4 concentration. Across years, CH4 emissions were negatively correlated with soil temperature in Ni treatment. Soil respiration showed clear seasonal fluctuations, with the largest emissions during summer and smallest in winter. Warming and nitrogen fertilization had no significant effects on total cumulative soil CO2 fluxes. Soil respiration was positively correlated with microbial biomass C, and microbial biomass C was not affected significantly by warming or nitrogen addition. The lack of significant effects of warming on soil respiration may have resulted from: (1) warming-induced soil drying offsetting the effects of soil temperature; or (2) adaption of soil respiration to increased temperature. (C) 2015 Published by Elsevier B.V.