Nonlinear responses of community diversity, carbon and nitrogen cycles of grassland ecosystems to external nitrogen input

doi:10.17521/cjpe.2022.0107

Abstract

Abstract:

Understanding the response patterns and potential mechanisms of structure and function in grassland ecosystems to nitrogen (N) enrichment is essential to evaluate ecological impacts of external N input. The muti-level N manipulative experiment offers the possibility to explore the nonlinear response patterns and associated mechanisms of structure and function in grassland ecosystems to additional N input. In this review, we summarized the impacts of additional N inputs on community diversity, carbon (C) and N cycling in grassland ecosystems around the world. Numerous studies illustrated that N enrichment induced the decline of plant species diversity, plant functional diversity and soil bacteria richness in grassland ecosystems, yet the change of fungal diversity was not significant. Above- and below-ground plant productivity showed different responses to N input: aboveground plant productivity exhibited initial increasing and subsequent saturation trends, but root productivity and root:shoot ratio showed downward patterns, and root turnover rate appeared a single-peak pattern of first increasing and then decreasing with the continuous increase of N addition rate. Meanwhile, different C decomposition processes responded variously to N enrichment. Specifically, litter decomposition rates exhibited multiple response of “exponential decrease, liner increase or insignificant change with N addition level”. However, the relationship of soil respiration and CH₄ consumption with N addition was dominated by a single peak trend of increasing at low to medium N levels but declining at high N levels. Likewise, different soil C fractions showed multiple response patterns to N input. N addition generally stimulated soil C storage and particulate organic C accumulation, while the mineral-associated organic C exhibited divergent responses of “increase, unaltered, and decrease” along the N addition gradient. In addition, plant N uptake exhibited initial increasing and subsequent situation trends along N addition gradients, while different soil N transformation processes showed differentiated responses along N addition gradients and the relationship between N₂O emission and N addition rate varied among various grassland ecosystems. An exponential increase of N₂O fluxes with N addition rate was observed in temperate grasslands, while the patterns of first increase and then saturation or linear increase of the N-induced changes in N₂O emissions had been discovered in alpine grasslands. Future studies should focus on the nonlinear responses of rhizosphere processes and phosphorus (P) cycle to external N input, and also explore potential mechanisms from the aspect of multi-dimensional biodiversity changes.

Key words: grassland ecosystem, species diversity, functional diversity, ecosystem productivity, litter decomposition, soil respiration, soil nitrogen transformation process, greenhouse gas flux

YANG Yuan-He, ZHANG Dian-Ye, WEI Bin, LIU Yang, FENG Xue-Hui, MAO Chao, XU Wei-Jie, HE Mei, WANG Lu, ZHENG Zhi-Hu, WANG Yuan-Yuan, CHEN Lei-Yi, PENG Yun-Feng. Nonlinear responses of community diversity, carbon and nitrogen cycles of grassland ecosystems to external nitrogen input[J]. Chin J Plant Ecol, 2023, 47(1): 1-24.

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URL: https://www.plant-ecology.com/EN/10.17521/cjpe.2022.0107

https://www.plant-ecology.com/EN/Y2023/V47/I1/1

Figures/Tables 6

Fig. 1 Multi-level nitrogen (N) manipulation experiment in an alpine steppe on the Qingzang Plateau (photo credit: LIU Yang). This experiment was maintained by Dr. YANG Yuan-He’s group in Institute of Botany, Chinese Academy of Sciences, and located in Sanjiaocheng sheep farm, Gangca County, Qinghai Province. It was conducted since May 2013, with eight N levels (0, 1, 2, 4, 8, 16, 24, 32 g·m-2·a-1). The form of N fertilizer is NH4NO3.

Fig. 2 Responses of multidimensional biodiversity to nitrogen input and their primary mechanisms. Orange arrows indicate downward trends, while green arrows indicate non-significant changes.

Fig. 3 Responses of vegetation productivity and its allocation to nitrogen (N) input and their primary mechanisms. ANPP, aboveground net primary productivity; P, phosphorus. The positive sign means the stimulating effects of N input, and vice versa.

Fig. 4 Responses of soil respiration to nitrogen (N) input and their primary mechanisms.

Fig. 5 Responses of various soil carbon fractions to nitrogen input and their primary mechanisms. MAOC, mineral-associated organic carbon; POC, particulate organic carbon; SOC, soil organic carbon. The arrow-up or positive sign means the stimulating effects of nitrogen input, and vice versa.

Fig. 6 Responses of nitrous oxide (N2O) emissions to nitrogen (N) input and their primary mechanisms in tropical and temperate (A) as well as alpine grasslands (B). AOA, ammonia-oxidizing archaea.

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