Abstract
The N fertilizer addition induced higher N2O emission in various soils including the lawn soil, however, a key gap in knowledge lies in identifying the ecological consequences of the N2O emission and potentially associated microbial mechanisms. This study evaluated the effect of N fertilizer on N2O emissions and associated microbial mechanism in lawn soil through incubated experiment under different N application rates. In addition, the contribution of different microbial communities to N2O emissions was quantified by combining biological inhibitors with high-throughput sequencing. The results indicated that N fertilizer addition induced higher N2O emissions in lawn soil and the contribution of fungi to N2O emissions was significantly higher than that of bacteria. We found a positive correlation between N2O emission and Ascomycota through RDA analysis. The growth trend of Ascomycota during the four nitrogen fertilizer treatments was consistent with the N2O emission trend in lawn soil. The relative abundance ofPyrenochaetopsis , Myrothecium , and Humicola was positively correlated with N2O emission. Thus,Pyrenochaetopsis , Myrothecium , and Humicola were found to be the main functional microorganisms leading to N2O production in lawn soil. Our findings can deepen the understanding on N2O emission and associated microbial mechanism in lawn soil with N fertilization.
Keywords: lawn soil, N fertilizer, N2O emission, biological inhibitors, fungi

1. Introduction

Lawn, including urban green space, parks, athletic fields, roadsides and home lawns, is one of the most popular landscapes worldwide and provides numerous ecological, environmental and economic benefits[1]. However, the rapid increase of lawn area has raised concerns about turfgrass breeding, resistance of lawn, and pollution associated with fertilizers and pesticides, which are used for lawn establishment and maintenance. Still, the N2O emission from urban lawn soil has received less attention. It is unclear how the N2O emission from lawn soil change along lawn management.
Nitrous oxide (N2O) is one of the three major greenhouse gases [2]. N2O can generate 300 times stronger effect on global warming than carbon dioxide in the troposphere and is a stable greenhouse gas. It has the capacity to reacts with ozone and became the dominant substance in ozone depletion[3]. N2O is also an important part of the nitrogen cycle in terrestrial ecosystems. Not always are high. It depends on the ecosystem and management practice. The impact of those practices (tillage, fertilization, irrigation) is studied aiming to reduce their contribution to N2O emissions. More attention that the addition of exogenous nitrogen, such as atmospheric nitrogen deposition and fertilization, alters the pathway of N2O production [4–6]. Meanwhile, previous studies have shown that the addition of a nitrogen fertilizer could increase N2O emissions[7-10] For instance, compared with unfertilized lawn soil, nitrogen fertilization significantly increased N2O emission in Baltimore, USA[11]. Further, a fertilization addition experiment in lawn in Phoenix, Arizona, USA also reported urbanization increased N2O emissions compared to native landscapes, primarily due to large amount of nitrogen fertilizer applied to lawn[12]. However, other studies found that the addition of nitrogen fertilizer did not increase N2O emission, for instance, studies on sandy loam and grasslands in the UK found that the addition of nitrogen fertilizer did not increase N2O emissions [13]. Similar results were also found in a farmland soil study in Shandong Province, China [14]. These results indicate that the effect of exogenous nitrogen addition on N2O emission has not been elucidated yet, and emissions could be either inhibited or promoted, which likely was related to uncertain factors such as nitrogen fertilizer form, vegetation type, soil properties, and microbial action. Thus, it is important to understand the effect of fertilization on soil N2O emission when formulating the scientific rational for nitrogen regulation.
N2O is generally produced in soils through microbiological nitrification and denitrification in soils. Therefore, Microorganisms play a key role in soil nitrogen transformation. Previous studies suggested that N2O emission in soil was caused by bacterial nitrification and denitrification, because the conventional nitrogen transformation is thought to be dominated primarily by bacteria[15]. However, recent studies have shown that fungi are the main players in N2O emission. In the arid grassland of the southwestern United States, N2O was mainly produced by the nitrification and denitrification processes of fungi, not bacterial ammonia oxidation [16]. N2O in the Qinghai-Tibet Plateau within China was also found to be largely produced by fungi [17]. These studies debunked the conventional idea that soil N2O was produced primarily by bacteria. With the mature application of 15N labelling technology, the contribution of heterotrophic nitrification to N2O emissions has attracted increasing attention, and fungi are considered the largest functional microorganisms[18,19]. Some studies also suggested that fungi played an even greater role in N2O emission. In the temperate grassland, fungi contributed to 86-89% of the N2O emissions [20]. Fungi contributed to 54% of N2O emissions during the nitrification process and 63% in the denitrification process in an alpine grassland [17]. Moreover, some studies have showed that bacteria are the main contributors to N2O emissions. For instance, bacterial denitrification was dominant in farmland soils in northern China (winter wheat and summer maize rotation farmland) [21]. Therefore, the microbiological mechanism involved in N2O emission from different vegetation types and ecological regions varies considerably. Clarifying the microbial mechanism of N2O emission is a prerequisite o for managing N2O emissions.
Lawn, which has the characteristics of high coverage, more roots, and more fertilization, is the main vegetation of sports fields and urban park green spaces and is quite different from natural grassland and farmland soil. In 2001, China’s urban lawn area was 0.94 million ha, while in 2010, it grew to 2.13 million ha, an increase of 1.3 times[22]. With the policy about greenbelt issued, lawn area will continue to increase in China. Therefore, proper management of this ground cover is important for improving urban environmental quality and formulating more accurate emission reduction measures. In a previous study, we found that fertilization promoted N2O emission of lawn soil, and heterotrophic nitrification contributed to N2O emission by 63% [23]. Yet, we still know very little on the associated microbial mechanism of N2O emission from lawn soils. In this study, we investigated N2O emission from lawn soil and the associated microbial mechanism under three fertilization gradients (N300, N255, and N150). Our study provides insights into the impacts of N fertilizer addition on N2O emission in lawn soil and the improvement of fertilizer utilize strategy. We hypothesized that N2O emission increased with the increase of nitrogen application, and fungi contributed more to N2O emissions than bacteria in lawn soil.