Influences of N deposition on soil microbial respiration and its temperature sensitivity depend on N type in a temperate forest

https://doi.org/10.1016/j.agrformet.2018.06.018Get rights and content

Highlights

  • Effects of different types of N on microbial respiration and its Q10 were assessed.

  • Organic N had higher suppression effect on microbial respiration than inorganic N.

  • Averaged Q10 was significantly increased by inorganic N rather than other types.

  • Effects of N deposition on temporal variations on Q10 was dependent on N type.

  • Soil NO3-N and NH4+-N regulate microbial respiration and its Q10, respectively.

Abstract

Knowledge on temperature sensitivity (Q10) of soil microbial respiration is crucial to improving the accuracy in predicting soil organic carbon (C) dynamics in climate–C models. However, the responses of soil microbial respiration and its Q10 to nitrogen (N) deposition, particularly different N types, remain unclear. Therefore, we incubated surface soils collected from a temperate forest receiving simulated N deposition under 15 °C and 25 °C for 150 days to examine the effects of different types of N deposition on soil microbial respiration and its Q10 and reveal their underlying mechanisms. A mixture of inorganic and organic N had the highest suppression on soil microbial respiration, followed by organic N and inorganic N. This suggested that the suppression effect of atmospheric N deposition on microbial respiration was underestimated by previous studies based on single inorganic N. Q10 values in all soils ranged from 1.96 to 2.76 with a mean of 2.41 at the end of incubation. Inorganic N significantly increased the averaged Q10 values, suggesting that inorganic N caused soil microbial respiration to become more sensitive to climate warming than organic N. Across the incubation period, Q10 values exhibited substantial temporal variation, which depended on the N type. Soil microbial respiration was negatively controlled by NO3-N and bacteria:fungi and gram-positive:gram-negative bacteria ratios. However, Q10 was positively controlled by soil NH4+-N. Our results highlighted the effects of inorganic and organic N deposition on microbial respiration and its potential mechanisms and implied the necessity of considering the N type when predicting soil C cycling and dynamics in increasing N deposition scenario.

Introduction

Soil microbial respiration is a key process controlling the size of the soil organic carbon (SOC) storage and C loss from terrestrial ecosystems to the atmosphere (Bond-Lamberty and Thompson, 2010). A small change in the rate of soil microbial respiration may have a large effect on the net C flux and climate dynamics because of the large SOC stored in terrestrial soil systems (Bonan, 2008). Soil microbial respiration is primarily controlled by temperature, and its response to warming is called temperature sensitivity (Q10). Q10 is an important mechanism for the possible feedback between the C cycle in terrestrial ecosystem and climate change (Davidson and Janssens, 2006). Furthermore, a small variation in Q10 can cause a large deviation in estimating CO2 release from the soil into the atmosphere (Xu and Qi, 2001). Therefore, understanding the patterns and controlling the mechanism of soil microbial respiration and its Q10 under global warming is important to improve the accuracy of predicting changes in global C cycles and its feedback to climate change.

In the past several decades, many works on microbial respiration have been conducted to investigate the patterns and controlling factors of soil microbial respiration via laboratory incubation and field measurement (Thirukkumaran and Parkinson, 2000; Weand et al., 2010; Wang et al., 2017a). Previous studies demonstrated that soil microbial respiration is closely affected by environmental factors such as temperature, microbial community composition, and nutrient availability, especially in C-rich, N-limited temperate forest ecosystems (Janssens et al., 2010; Coucheney et al., 2013; Du et al., 2014). These environmental factors may be changed by N deposition (Ramirez et al., 2010; Wang et al., 2017b) and consequently influence soil microbial respiration. These pioneering studies have substantially improved our understanding of soil microbial respiration and its response to increasing N deposition. However, the direction and magnitude of the N deposition effect in different studies is controversial (Conde et al., 2005; Janssens et al., 2010; Tu et al., 2013; Wang et al., 2017a). This inconsistency suggests that the responses of soil microbial respiration to N deposition vary among ecosystems and soil types, which calls for further investigation across wide ecosystems. Furthermore, this inconsistency may be attributed to differences in N types used in various experiments.

In the recent decade, Q10 of soil microbial respiration has gained increasing attention because of its significance in regulating soil C cycling (Davidson and Janssens, 2006; Conant et al., 2008). Some incubation studies illustrated that soils with high substrate quality have low Q10 (Fierer et al., 2005; Conant et al., 2008; Craine et al., 2010; Liu et al., 2017). This supports the C quality–temperature hypothesis that Q10 increases with increasing biochemical recalcitrance of soil organic matter. The shift in soil microbial community composition caused by N deposition also affects Q10 (Thiessen et al., 2013; Karhu et al., 2014). For instance, Karhu et al. (2014) found that the microbial community level response to temperature is more often enhanced than reduced in mid- to long-term (90 days) Q10. These studies to some extent advanced our understanding of the mechanisms that regulate Q10. However, the effects of N deposition on Q10 in various experiments are inconsistent (Coucheney et al., 2013; Liu et al., 2016; Wang et al., 2017a). This inconsistency will increase the uncertainty of predicting the response of soil C cycle to global environmental change. More importantly, most previous studies used inorganic or organic N as lone N resources (e.g., Sinsabaugh et al., 2002; Wang et al., 2017a), which may not actually reflect the effects of atmospheric N deposition on soil microbial respiration and its Q10 because it contains inorganic and organic N components (Cornell, 2011). However, in temperate forest ecosystems, the mechanism on how inorganic and organic N deposition affects soil microbial respiration and its response to global warming are still unclear. Without this knowledge, our understanding of the drivers of soil CO2 emission is incomplete.

Therefore, taking the advantage of a long-term N deposition experiment with different types of N deposition in a temperate forest, we conducted an incubation experiment using soils receiving N for a long period of time to assess how inorganic and organic N affect soil microbial respiration and its Q10. We also investigated the underlying mechanisms regulating responses of soil microbial respiration and its Q10 to N deposition. In this study, we provided the first investigation to explore the response of soil microbial respiration and its Q10 to different types of N deposition in a temperate forest. Here, we aimed to address 1) how N types (organic vs. inorganic) affect soil microbial respiration and its Q10 and whether N has an effect on Q10 in terms of temporal variation; and 2) which soil chemical and microbial variables are the key factors controlling soil microbial respiration and its Q10 under N deposition.

Section snippets

Site description

This work was conducted in a mature temperate forest dominated by Larix gmelinii, locating at Laoshan Forest Research Station of Northeast Forestry University in Heilongjiang Province, northeastern China (127°34ʹE, 45°20 °N). The site has a continental temperate monsoon climate, with a strong monsoon windy spring, a warm and humid summer, and a dry and cold winter. Annual precipitation ranges from 600 to 800 mm, most of which falls in July and August. The mean annual air temperature is 2.7 °C,

Soil chemical and microbial characteristics

N deposition increased soil NH4+-N, NO3-N, and total P contents (P <  0.05) but had no effects on SOC, total N, C:N ratio, available P, and pH (Table 1). Soil microbial composition was altered by mixed N deposition (Table 2). Mixed N deposition significantly decreased the concentrations of gram-negative bacterial and fungal PLFAs (P <  0.05) but increased protozoa PLFAs and the ratios of gram-positive to gram-negative bacteria and total saturated to total monounsaturated PLFAs (P <  0.05).

Soil microbial respiration

In this study, we illustrated the effects of different types of N deposition on soil microbial respiration in a temperate forest ecosystem. As we expected, long-term simulated N deposition significantly suppressed microbial respiration, indicating that soil N nutrient may be saturated after receiving N deposition for a long period of time. This result was generally in line with the previous observation in temperate forests (Janssens et al., 2010; Ramirez et al., 2010; Du et al., 2014) and

Acknowledgements

This work was supported by the National Key Research and Development Program of China (grant no. 2016YFA0600801), the National Natural Science Foundation of China (grant no. 31570466), the Strategic Priority Research Program of the Chinese Academy of Sciences (grant no. XDB15010301) and the Shandong Double Tops Program (grant no. SYL2017XTTD03). We thank the anonymous reviewers for their constructive comments which helped us significantly improve the manuscript. We also appreciate the

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