Effects of mowing and nitrogen addition on the ecosystem C and N pools in a temperate steppe: A case study from northern China

Grasslands, mostly in arid and semiarid regions, play an important role in regulating the dynamics of global terrestrial carbon (C) exchange. However, findings about how global changes affect ecosystem C sequestration are conflicting. To understand how land use and climate change affect ecosystem C and nitrogen (N) pools, this study investigated the effects of mowing and fertilizer on the C and N pools of plants and soil, and on the aggregate-associated C and N content, by means of a field manipulating experiment established in August 2012 in a typical steppe. The results showed that mowing significantly decreased the mass and the C and N pools of shoots by 14.3%, 17.3% and 12.1%, respectively. Compared to ambient N treatments, N addition stimulated shoot biomass and litter mass by 28.8% and 77.4%, respectively, but had no affect on root biomass. Simultaneously, N addition increased the plant N content and the N pools, and it decreased the ratio of plant C to N in the shoots, litter and roots. However, neither mowing nor N addition affected the proportion of macroaggregate, microaggregate, < 0.053 mm size class, geometric mean diameter or aggregate-associated C and N contents. Nitrogen deposition under conditions of climate change stimulated plant productivity but had no affect on soil C sequestration in this short-term experiment. Our results indicate that mowing may be beneficial for providing food for large mammals, but it did not affect soil C and N pools in the study region. Our findings suggest that the response of plant mass and C pools is more sensitive than that of soil C sequestration and soil texture to mowing and N deposition in the temperate steppes of the Mongolian Plateau.