Competitive interactions in two different plant species: Do grassland mycorrhizal communities and nitrogen addition play the same game?

Abstract

In the Tibetan Plateau grassland ecosystems, nitrogen (N) availability is rising dramatically; however, the influence of higher N on the arbuscular mycorrhizal fungi (AMF) might impact on plant competitive interactions. Therefore, understanding the part played by AMF in the competition between Vicia faba and Brassica napus and its dependence on the N-addition status is necessary. To address this, a glasshouse experiment was conducted to examine whether the grassland AMF community's inocula (AMF and NAMF) and N-addition levels (N-0 and N-15) alter plant competition between V. faba and B. napus. Two harvests took day 45 (1^st harvest) and day 90 (2^nd harvest), respectively. The findings showed that compared to B. napus, AMF inoculation significantly improved the competitive potential of the V. faba. In the occurrence of AMF, V. faba was the strongest competitor being facilitated by B. napus in both harvests. While under N-15, AMF significantly enhanced tissue N:P ratio in B. napus mixed-culture at 1^st harvest, the opposite trend was observed in 2^nd harvest. The mycorrhizal growth dependency slightly negatively affected mixed-culture compared to monoculture under both N-addition treatments. The aggressivity index of AMF plants was higher than NAMF plants with both N-addition and harvests. Our observation highlights that mycorrhizal associations might facilitate host plant species in mixed-culture with non-host plant species. Additionally, interacting with N-addition, AMF could impact the competitive ability of the host plant not only directly but also indirectly, thereby changing the growth and nutrient uptake of competing plant species.

Keywords: grassland AMF inoculum; host plant; nitrogen deposition; non-host plant; plant competition.

Figure 1

In monoculture and mixed-culture V. faba and (B) napus under AMF inoculation and N-addition conditions of N-0 and N-15 were imposed for both harvests (Day 45 1^st harvest and Day 90 2^nd harvest).

Figure 2

Root and shoot biomass production per plant (mean ± SE) in monoculture and mixed-culture V. faba and B. napus under AMF inoculation and N-addition conditions of N-0 and N-15. (A, B) show 1st harvest, (C, D) show 2nd harvest. Different letters denote significantly different means (Duncan test: P < 0.05).

Figure 3

Tissue N:P ratio per plant (mean ± SE) in monoculture and mixed-culture V. faba and B. napus under AMF inoculation and N-addition conditions of N-0 and N-15. (A) shows 1st harvest, (B) shows 2nd harvest. Different letters denote significantly different means (Duncan test: P < 0.05). For statistical details, see Table 2 and Supplementary Table 1 .

Figure 4

Mycorrhizal growth dependency (MGD) of V. faba and B. napus under N-addition conditions of N-0 and N-15. (A) shows 1st harvest, (B) shows 2nd harvest. * denotes significant difference.

Figure 5

Effects of AMF inoculation on V. faba and B. napus nder N-addition conditions of N-0 and N-15, evaluated using the aggressivity index (AGR) and relative interaction intensity index (RII). (A, C) show 1st harvest, (B, D) show 2nd harvest. For statistical details, see Table 3 . * denotes significant difference.