Li Qiang, Philp Joshua, Denton Matthew D, Huang Yingxin, Wei Jian, Sun Huijuan, Li Yang, Zhao Qian
Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun, China.
Jilin Provincial Key Laboratory of Grassland Farming, Science and Technology Department of Jilin Province, Changchun, China.
Front Plant Sci. 2023 Apr 26;14:1076894. doi: 10.3389/fpls.2023.1076894. eCollection 2023.
Symbiotic N fixation inhibition induced by N supply to legumes is potentially regulated by the relative N and P availability in soil. However, the specific responses of different legume species to changes in N:P availability remain unclear, and must be better understood to optimize symbiotic N fixation inputs under N enrichment. This study investigated mechanisms by which soil N and P supply influence the symbiotic N fixation of eight legume species, to quantify the inter-specific differences, and to demonstrate how these differences can be determined by the stoichiometric homeostasis in N:P ratios (H).
Eight herbaceous legume species were grown separately in outdoor pots and treated with either no fertilizer (control), N fertilizer (14 g N m), P fertilizer (3.5 g P m) or both N and P fertilizer. Plant nutrients, stoichiometric characteristics, root biomass, non-structural carbohydrates (NSC), rhizosphere chemistry, P mobilization, root nodulation and symbiotic N fixation were measured.
N addition enhanced rhizosphere P mobilization but drove a loss of root biomass and root NSC exudation of P mobilization compound (organic acid), especially so in treatments without P addition. N addition also induced a 2-14% or 14-36% decline in symbiotic N fixation per plant biomass by legumes in treatments with or without P addition, as a result of decreasing root biomass and root NSC. The changes in symbiotic N fixation were positively correlated with stoichiometric homeostasis of N:P ratios in intact plants without root nodules, regardless of P additions.
This study indicates that N addition can induce relative P limitations for growth, which can stimulate rhizosphere P mobilization at the expense of root biomass and carbohydrate concentrations, reducing symbiotic N fixation in legumes. Legume species that had less changes in plant N:P ratio, such as maintained symbiotic N fixation to a greater extent under N addition.
向豆科植物供应氮素所诱导的共生固氮抑制作用可能受土壤中氮和磷的相对有效性调控。然而，不同豆科植物物种对氮磷有效性变化的具体响应仍不明确，为了在氮富集条件下优化共生固氮输入，必须更好地理解这一点。本研究调查了土壤氮和磷供应影响8种豆科植物共生固氮的机制，以量化种间差异，并证明这些差异如何由氮磷比（H）的化学计量稳态决定。
8种草本豆科植物分别种植在室外花盆中，分别施以不施肥（对照）、氮肥（14克氮/平方米）、磷肥（3.5克磷/平方米）或氮磷肥。测量了植物养分、化学计量特征、根生物量、非结构性碳水化合物（NSC）、根际化学性质、磷活化、根瘤形成和共生固氮。
添加氮素增强了根际磷的活化，但导致根生物量和根NSC损失以及磷活化化合物（有机酸）的渗出，在不添加磷的处理中尤其如此。添加氮素还导致在添加或不添加磷的处理中，豆科植物每单位植物生物量的共生固氮量下降2 - 14%或14 - 36%，这是由于根生物量和根NSC减少所致。无论是否添加磷，共生固氮的变化与无根瘤完整植物中氮磷比的化学计量稳态呈正相关。
本研究表明，添加氮素会诱导生长的相对磷限制，这会刺激根际磷的活化，但以根生物量和碳水化合物浓度为代价，从而降低豆科植物的共生固氮。植物氮磷比变化较小的豆科植物物种，如在添加氮的情况下能在更大程度上维持共生固氮。
