Effects of N addition and clipping on above and belowground plant biomass, soil microbial community structure, and function in an alpine meadow on the Qinghai-Tibetan Plateau

Highlight

• The effects of clipping and N addition on soil microbial properties in an alpine meadow were evaluated.

• The moderate clipping alone increased root exudation rates.

• The effects of N addition were neutral or negative in the heavy clipping treatment.

• N addition significantly increased soil microbial biomass in the moderate clipping.

Abstract

Nitrogen (N) enrichment and grazing are two factors that have profound effects on grassland ecosystem processes and functions. This study evaluated separate and combined effects of clipping and N addition on soil properties, soil microbial community structure, and soil extracellular enzyme activity within our experiment design which included three clipping intensities (i.e., unclipped, moderate, and heavy clipping) and two different N treatments (with and without N addition) in an alpine meadow ecosystem on the Qinghai-Tibetan Plateau, China. Results revealed that N addition alone did not significantly affect total soil organic carbon (TOC), total soil N (TN), dissolved organic carbon (DOC), microbial biomass carbon (MBC), and N (MBN) after 3 years of treatments. The moderate clipping alone increased root exudation rates whereas the heavy clipping decreased root exudation rates, TOC, TN, DOC, MBC and MBN, fungi biomass, and the ratio of fungi to bacteria. The effects of N addition were generally neutral or negative in the heavy clipping treatment. In contrast, N addition significantly increased aboveground biomass, MBC, MBN, DOC, and soil water content but decreased inorganic N in the moderate clipping. RDA analysis showed that N addition could stimulate soil microbial community structure in the moderate clipping treatment by impacting the availability of labile soil C. Our results improve the understanding of the role of N addition in regulating soil C storage among different grazing intensities in alpine meadows.

Introduction

Alpine meadows contribute greatly to the global soil carbon (C) and nitrogen (N) pools while accounting for 48% of the Qinghai-Tibetan Plateau's land area [1].Within this region, the availability of soil N has substantially increased via atmospheric nitrogen deposition [2]. Additional N can have great influences on soil C sequestration and soil microbial community composition through shifts in the composition of the plant community, C/N ratio of substrates, and soil pH [[3], [4], [5]]. Numerous studies have reported the impacts of N enrichment on soil microbe biomass, community composition, and enzyme activities [[4], [5], [6]]. Large variation in the responses of microbial biomass to N addition has been reported including declines [7], no response [8], and increases [9]. These inconsistent results may be due to the difference in ecosystem biomes, N form, N release rates [5,10,11], and/or experimental durations [4]. Previous studies have shown that the ratio of fungi to bacteria increased in N-rich ecosystems [12] because fungi are more adaptable to soil acidification than bacteria [13]. In contrast, other studies found that N enrichment decreased fungal biomass, and the ratio of fungi to bacteria, in response to increased availability of N which favors bacteria rather than fungi in an N-limited ecosystem [14]. These varied experimental results indicate that the influence of N enrichment on soil microbial biomass and community composition could be ecosystem-specific and such studies in alpine grasslands needs further investigation.

Grazing has large cascading effects on soil C storage by influencing plant community composition, litter quality, root exudation, and soil microbial diversity [15,16]. Consistent results on responses of soil C and N to grazing have not yet been observed. Some studies suggest that intermediate grazing intensity could increase C and N pools because moderate intermediate grazing can increase soil microbial community stabilization [17]. Other studies showed that the grazing effect on soil C and N pools depends on the grazing intensity. Light and moderate grazing generally increased soil microbial activity measured via soil enzymes and microbial biomass [18,19], while heavy grazing decreased plant biomass, soil organic carbon, and soil microbial community diversity [[20], [21], [22], [23]]. Furthermore, the effects of grazing on ecosystem C cycles are regulated by other environmental factors such as soil N availability, especially in the N-limited alpine meadow ecosystems. In other words, the interactive effects of N addition and grazing could be absent, synergetic, or antagonistic. However, this remains an open question as to whether N addition could intensify or mitigate the impact of grazing on soil C sequestration and soil microbial characteristics in alpine grasslands.

Atmospheric N deposition ranges from 4 to 13.8 kg N ha⁻¹year⁻¹ on the Qinghai-Tibetan Plateau [24,25]. Meanwhile, alpine grasslands are currently suffering from overgrazing [26]. Due to climate change and overgrazing during the last decades, some alpine meadow pastures on the Tibetan Plateau have been degraded indicated by the reduction of productivity and ecosystem services [27]. N fertilization has been used to restore the productivity of degraded grasslands [28]. Therefore, studies on whether and how N addition moderates the effects of grazing are of great significance for the improvement of degraded alpine grasslands. In this study, we examined the effect of different clipping intensities and N addition on soil properties, soil microbial community structure, and soil extracellular enzyme activity in the 0–20 cm soil depth of an alpine meadow on the Qinghai-Tibet Plateau. Specifically, we test the hypotheses that heavy clipping would have negative effects on soil properties and soil microbial communities, and moderate clipping would have the opposite effect. We also test that N addition would stimulate carbon accumulation and promote microbial growth because grasslands are often N limited and that N addition would alleviate the negative effects of heavy clipping on the soil properties and soil microbial communities.