Introduction
Global atmospheric nitrogen (N) deposition had increased 5 to 20-fold since the preindustrial period, with average deposition rates of 2 g m-2 yr-1 in recent years, and the deposition of reactive N will continue to increase over the next century (Galloway et al., 2008; Sutton & Bleeker, 2013), which greatly influenced main components of ecosystem N pool (i.e., plant, soil and microbe N pools) of grassland ecosystems (Galloway et al., 2008; Gruber & Galloway, 2008; Lebauer & Treseder, 2008). Most studies simulated N deposition were designed to supply N fertilization once or twice a year (Bustamante et al., 2012; Haugwitz & Michelsen, 2011) which cannot reflect the natural N deposition with dry or wet N deposition. Moreover, grazing or mowing is the main land use way for maintaining the natural grassland ecosystem in northern China. Both N deposition and mowing have significant effects on the N cycling in grassland ecosystem by changing nutrient availability and leaching within them (Collins, Knapp, Briggs, Blair, & Steinauer, 1998; Isbell et al., 2013; Wang et al., 2011).
The experimental results from various individual studies are different which have been conducted to explore how N deposition or fertilization affects N turnover processes in ecosystems, especially in soil N pool (i.e. N ammonification, nitrification, denitrification and immobilization) (Bhatti, Ahmad, Qasim, Riaz, & Cresser, 2018; Lu et al., 2011). There were studies showed that N addition might increase, decrease or have minor effect on soil N turnover (Brenner, Boone, & Ruess, 2005; Kowaljow & Mazzarino, 2007; Riley, 1998). Most of previous studies focused on the effects of N input on soil net N mineralization rates, these researches about the responses of soil gross N turnover to N enrichment are still limited. The growing season of most of the northern natural grassland begins in May, the rapid growth period of plants is from June to July, so fertilization once or twice a year could not reflect the effects of the N deposition pattern under natural conditions.
Previous studies showed that grazing or mowing (Collins et al., 1998; Giese et al., 2013) also strongly affects nutrient cycling processes because mowing removed aboveground biomass and decreased litter biomass, which could alter soil surface temperature and moisture and further affect soil microbial activity and N turnover (Baptist et al., 2013; Ma, Guo, Xin, Yuan, & Wang, 2013; Robson, Lavorel, Clement, & Roux, 2007; Wang et al., 2015). And some researches indicated that mowing reduced root and soil N concentrations (Kitchen, Blair, & Callaham, 2009; Turner, Seastedt, & Dyer, 1993), which likely due to loss of N with aboveground biomass removal, the change of soil N concentrations would directly or indirectly effects on soil N turnover. However, mowing effects in grassland ecosystems on soil N turnover are the response mechanisms of soil gross N turnover are the response mechanisms of soil gross N turnover and the interactions of N deposition remain unclear.
The area of Inner Mongolia steppe is about 1.18 million km2, and it is of great importance for ecological conversation and local farmer livelihood. We performed the experiment of N addition amounts with different N addition frequency and mowing to determine their individual and potential interactive effects on the gross N mineralization rate of the soil using a 15N isotope dilution technique (Müller, Stevens, & Laughlin, 2004) and other N pool components (i.e., plants, soil and microbes). There is a lack of understanding of the interaction between the annual N deposition pattern (i.e. N supply frequency in different months) and mowing affect the N turnover processes limits our ability to predict the effects of future climate change for natural mowed grassland ecosystems. Consequently, our study hypothesis that fertilization during different months of the year can increase soil gross N mineralization and plant productivity, because N addition during different months in N-deficient grassland can stimulate soil microbial activities and meet the N demand of different plants at different stages.
The objectives of this study are: (i) to investigate how gross N turnover responds to levels and frequencies of N addition and mowing; (ii) to examine how N addition level and frequency and mowing impact the N and C pool within plants, soil, and microbes in the temperate steppe; and (iii) to find the relationship between gross N turnover and the size of the plant and soil N pool under different N addition levels.