氮肥和种植密度对达乌里胡枝子的生长与生物固氮的影响

Effects of nitrogen addition and planting density on the growth and biological nitrogen fixation of Lespedeza davurica

氮供给和种植密度是影响植物生长的两个重要因素。豆科植物因其生物固氮能力而在受到氮限制的生态系统中具有重要作用,氮含量增加促进植物生长的同时也会抑制豆科植物的生物固氮能力,种植密度会通过种内竞争影响豆科植物的生长和生物固氮能力,然而少有研究关注氮肥添加和种植密度对豆科植物生长和生物固氮能力的影响。该研究以达乌里胡枝子(Lespedeza davurica)为研究对象,通过温室盆栽实验,探究氮肥和种植密度对其生长和生物固氮的影响。实验设置4个氮添加水平(0、5、10、20 g·m^(-2)·a^(-1))和3种种植密度(1、3、6 Ind.·pot^(-1),约32、96、192 Ind.·m^(-2))。结果发现:1)施肥和密度增加均影响了达乌里胡枝子的生长。叶片碳(C)、氮(N)含量、净光合速率随施氮量增加而增加,氮添加也促进了植物的生长,当施氮量为10 g·m^(-2)·a^(-1)时植物产量达到最大。叶片C、N含量、净光合速率随种植密度增加而下降,密度增加可以促进每盆的总生物量,但对单个植株的生长有负效应。2)氮肥对根瘤形成有抑制作用,但种植密度增加会缓解氮肥对生物固氮能力带来的"氮阻遏"。该实验条件下,当施氮量为10 g·m^(-2)·a^(-1),种植密度为3 Ind.·pot^(-1),或施氮量为5 g·m^(-2)·a^(-1),种植密度为6 Ind.·pot^(-1)时,能最大程度发挥"施氮增产"和种植密度缓解"氮阻遏"的作用。氮添加降低了达乌里胡枝子的根瘤生物量和对根瘤形成的投资(根瘤生物量占总生物量的比例),从而抑制达乌里胡枝子的生物固氮。种植密度增加导致达乌里胡枝子因种内竞争增加而使资源获取受限,从而增加对根瘤的投资和根瘤生物量来获得更多来自大气中的氮。3)结构方程结果显示,氮肥和种植密度通过直接或间接作用,解释了64%的达乌里胡枝子生物量变化和42%的根瘤生物量变化。上述结果表明合理优化豆科植物的施肥量和种植密度可能对人工草地种植以及退化草地恢复管理具有重要意义。

Aims Nitrogen(N) supply and planting density are two important factors influencing plant growth. Legumes are important to ecosystem N input because of their capacity of biological N2 fixation(BNF). Increasing atmospheric N deposition may promote the growth of leguminous plants, but it may also inhibit strongly their BNF capacity. Planting density can also influence the growth and BNF capacity of legumes due to intraspecific competition. However, few studies up to date have focused on the effects of N fertilization and planting density on the growth and BNF capacity of legumes. In this study, we aimed to explore the potentially interactive effects of N fertilization and planting density on the growth and BNF capacity of Lespedeza davurica, a leguminous plant species which is widely distributed throughout the northern China grasslands. Methods A pot experiment was conducted in a greenhouse. The experiment contained four levels of N addition(0, 5, 10, 20 g·m^(-2)·a^(-1)) by NH4NO3 fertilizer and three levels of planting densities(1, 3, 6 Ind.·pot^(-1), i.e. 32, 96, 192 Ind.·m^(-2)). Important findings 1) Our results showed that both N addition and planting density could impact the biomass production of L. davurica. N addition increased plant leaf carbon(C) and N contents and leaf-level net photosynthetic rate. Besides, N addition also stimulated the plant growth at both pot and individual levels, and the yield reached maximum at N addition of 10 g·m^(-2)·a^(-1). Increasing planting density decreased leaf C and N contents, leaf-level net photosynthetic rate, and individual growth, but increased total biomass in each pot. 2) Nitrogen addition reduced the capacity of BNF of L. davurica, while increasing planting density could weaken this suppression effect to some extent. The combination of N addition of 10 g·m^(-2)·a^(-1) and planting density of 3 Ind.·pot^(-1) or N addition of 10 g·m^(-2)·a^(-1) and planting density of 6 Ind.·pot^(-1) could maximize the effects of N application on individual yield and the effects of increasing planting density on the alleviation of BNF suppression. Nitrogen addition suppressed the BNF of L. davurica through reducing plant investment to nodulation and nodule biomass production. The intraspecific competition and resource limitation caused by increasing planting density led to improvement in the investment to nodulation and nodule growth. 3) Structural equation model analyses showed that N addition and planting density combined explained variations in the plant biomass and nodule production either directly or indirectly by 64% and 42%, respectively. The results indicate that it is important to optimize the amount of fertilizer application and appropriate planting density when considering plantation and management of artificial and degraded grasslands.