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.