4 DISCUSSION
The plant N:P ratio not only is a link factor among different scales of
molecular, cellular, individual, population, community and ecosystem
(Elser et al., 2000; Sterner & Elser, 2002; Elser & Hamilton, 2007)
but is also an important indicator of nutrient supply (Koerselman &
Meuleman, 1996) and dynamic nutrient limitations of the environment
(Güsewell, 2004; Vitousek et al., 2010; Hu et al., 2018). Since species
richness is always associated with productivity (Grime, 2001), nutrient
availability first affects productivity, then plant density over long
time. Therefore, the vegetation N:P ratio affects plant density. In the
present study of a successional series in a semi-arid area, as the
vegetation N:P ratio increased, plant total density increased
significantly in the A. ordosica community (early successional
stage) but decreased significantly in the S. bungeana community
(late successional stage). This suggests that the vegetation N:P ratio
was the mechanism driving negative density dependence.
In the natural world, no organism is isolated; rather, each organism is
a member of a population consisting of many homologous species.
Individuals of the same species have similar requirements for growth,
reproduction and survival. Thus, when their demand for resources exceeds
the supply, conspecifics compete for resources. Plants cannot move on
their own; thus, nutrient limitation will affect their physiology and
biochemistry and limit plant biomass. Strong nutrient competition
increases nutrient stress and affects individual reproduction and
mortality, thereby affecting plant density. The vegetation N:P ratio is
an important indicator to of plant nutrient limitation (Güsewell, 2004;
Vitousek et al., 2010). When nutrient limitation is severe, the
vegetation N:P ratio can affect not only productivity but also the plant
density of the community, especially in arid and semi-arid areas.
In the present study of a successional series in a semi-arid area, as
the vegetation N:P ratio increased
in the early successional-stage community of A. ordosica , total
density increased linearly and significantly, whereas biomass increased,
but the relationship was not significant. For sand-dune plants in the
early successional stage, soil nitrogen varies to a greater extent that
does soil phosphorus (He, You, & Yu, 2016). This difference exists
because of the deposition of atmospheric nitrogen and the transformation
of organic matter via litter decomposition, which causes soil nitrogen
to increase rapidly. Plant nitrogen then increases accordingly, which
eventually leads to a larger vegetation N:P ratio. However, due to the
extremely low nitrogen content in dune soil for the early successional
stage community, the absolute nitrogen contents of plants were low.
Therefore, the total density of the A. ordosica community
increased with increasing vegetation N:P ratio. In the mid-successional
stage, S. alopecuroides community, soil phosphorus began to
accumulate, and soil nitrogen continued to accumulate. There was no
significant statistical relationship between total density and
vegetation N:P ratio; however, the biomass of the S.
alopecuroides community increased significantly with increasing total
density. In the late successional stage, phosphorus was consumed in
large quantities due to its physiological and biochemical effects on
plant drought resistance, and the plant communities were restricted by
phosphorus. In the S. bungeana community, representing the late
successional stage, total density decreased significantly with
increasing vegetation N:P ratio, which indicated significant negative
density restriction in this stage. At this stage, biomass decreased with
increasing total density of the plant community, reflecting phosphorus
limitation; however, the decrease was not significant.
Koerselam & Meuleman (1996) indicated that when the vegetation N:P is
<14, plant growth at the community level is mainly restricted
by N, whereas when N:P is 14-16, plant growth is restricted by N and P,
and when N:P is >16, plant growth is mainly restricted by
P. In the present study, we demonstrated that the vegetation N:P ratios
of the A. ordosica , S. alopecuroides and S.
bungeana communities were 11.27±0.97, 17.08±0.86 and 20.84±1.01. These
results indicated that the A. ordosica community was limited by
N, whereas the S. alopecuroides and S. bungeanacommunities were limited by P. Interestingly, in the early and middle
successional stages, there was a significant quadratic function
relationship between total density and vegetation N:P ratio in the
communities, and the vertex coordinate was (16.6, 353.3). These findings
revealed that the community would face negative density constraints when
the vegetation N:P ratio was close to 16.6. In the middle and late
successional stages, plant growth was mainly restricted by P, which led
to negative density dependence.
For populations and communities, the vegetation N:P ratio is of core
interest when predicting nutrient limitation, whether N limitation or P
limitation (Vitousek et al., 2010). For instance, in our previous
research we established an 80 × 80 m sample plot in an A.
ordosica community on fixed sand-dunes in the Alxa Desert. We
investigated plant density in 400 quadrats (4×4 m) in the plot and
determined total N and available P in the 20 cm soil layer of each
quadrat. Finally, we regressed the density of A. ordosica against
the soil N:P ratio. We found that A. ordosica density had no
significant regression relationship with soil N or P but had a
significant, negative linear relationship with the soil N:P ratio. In
previous work, regression results of soil nutrients and plant density
were similar between a sampling scale of 40×20 m and a sampling scale of
4×4 m (Wu et al., 2009). The above observations suggest that the N:P
ratio is more informative than N or P content alone.
Negative density dependence is a very common phenomenon in nature. The
factors affecting negative density dependence are water, nutrients and
salt. Regarding nutrients, density is not limited so much by N or P but
by the vegetation N:P ratio. The results of the present study indicate
that the vegetation N:P ratio in a semi-arid area is the factor driving
negative density dependence.
With the development of a plant community, the vegetation N:P ratio
continuously changes. From the early successional stage to the late
successional stage, the vegetation N:P ratio gradually increases. In the
early successional stage A. ordosica community, total plant
density increased with increasing vegetation N:P ratio; however, in the
late successional stage S. bungeana community, total plant
density decreased with increasing vegetation N:P ratio. Thus, the
vegetation N:P ratio is a driving factor of plant community variation.