3. Result
3.1. Alfalfa C, N, P concentrations and stoichiometric ratio
in differently age
groups
The C, N and P concentrations of alfalfa were significantly affected by
stand age (Fig. 2I, II, and III). The variation of the C content in the
alfalfa fluctuated slightly from different age groups, with an average
value of 453.0 g·kg-1, and the fluctuations did not
exceed 4.7%. The change trend of C content was to increased first and
then decreased, with significant maximum and minimum values in 7-year
stand and 1-year stand, respectively. The N content in the plant varies
from 28.6-45.7 g·kg-1, but there was no obvious trend
in the change of alfalfa N concentration among in various years, and the
minimum value was in 7-year stand, the maximum was in 10-year. P content
decreased from 4.0 g·kg-1 to 1.9
g·kg-1 between 1-year to 7-year stand, but increased
to 2.5 g·kg-1 in 9-year and 10-year stands.
The changes of alfalfa C:N, C:P and N:P were significantly related to
stand age (Fig. 2IV, V, and VI). Alfalfa C:N and C:P had similar trend,
and they all increased and then decreased with stand age. C:N had a
significant highest value in 7-year stand and the lowest in 10-year
stand. The 7-year stand also had the highest value of C:P, but the
lowest C:P in the 1-year stand. The N:P ratios of alfalfa was positively
correlated with the stand age (P <0.01), and there was
over double increase in 10-year stand.
3.2. Soil OC, TN and TP concentrations and stoichiometric
ratio in differently age
groups
Soil OC was affected significantly by alfalfa stand age (Fig. 3I). The OC
in the first two years was lower than 2.4 g·kg-1 of
the FL, but it was increased with stand age. The significant maximum
value of OC was 9.62 g·kg-1, appeared in 9-year stand.
Soil TN was also significantly affected by the stand age (Fig. 3II), but
the TN content were 0.12-0.23 g·kg-1 in 1-3 year
stands, there was no obvious difference compared with the FL. There was
a significant increase in 7-10 years, an increase of 4.5-7 times
relative to the FL. The maximum TN appeared in 10-year stand and was
significantly higher than other age groups. The fluctuation of soil TP
was not as obvious as the SOC and soil TN (Fig. 3III), only in the
1-year, 9-year and 10-year stands had a significant increase, other age
groups had no significant increase compared with the FL of 0.5
g·kg-1, and the maximum TP content appeared in 9-year
stand, higher by 50.0% then FL.
Soil C:N was significantly reduced by the cultivation of alfalfa compared
to the FL (Fig. 3IV). The first year of alfalfa planting fell by 45.1%,
which was to the greatest extent. Then C:N started to rise, but it was
always 16.7-34.0% lower than FL, and the maximum C:N appeared in 3-year
stand in all alfalfa plantation. Soil C:P was slightly lower than the FL
in the first two age groups, and then increased with stand age (Fig.
3V), The maximum C:P appeared in 10-year stand, which was 2.8 times of
FL. Soil N:P ratio was significantly correlated with alfalfa stand age
(Fig. 3VI). The soil N:P of all year stands were higher than FL, but it
did not increase significantly except 5-10 year stands. The change trend
of N:P was similar to the C:P, increased with stand age. The maximum
value of C:P appeared in 10-year stand, was 4.9 times higher compared to
the FL.
3.3. Relationships between alfalfa growth and soil
stoichiometry
Alfalfa biomass was significant affected by growth age (Fig. 4I). It
increased first and then decreases, with the largest biomass in the
2-year stand, the smallest in the 10-year stand, and the maximum value
is 287.3% higher than the minimum value. Underground biomass generally
increased with growth age (Fig. 4II), but the relatively higher in
1-year stand may be due to more weed roots in the newly established
plantation of alfalfa. Therefore, it led to a root-shoot ratio of
similar trends (Fig. 4III).
The biomass of alfalfa was negatively correlated to alfalfa N
(R2=0.19, P<0.05) and positively related to
alfalfa P (R2=0.26, P=0.02) (Fig. S1V and VI), so
there were negatively related between alfalfa biomass and alfalfa C:P
and N:P (Fig. 5). Underground biomass was negatively related to alfalfa
P (R2=0.43, P<0.01) (Fig. S2VI) and
positively related to alfalfa C:P and N:P (Fig. 5). The root-shoot ratio
was positively related to alfalfa N (R2=0.27, P=0.02)
(Fig. S3V), while negatively related to alfalfa P
(R2=0.25, P=0.02) (Fig. S3VI), thus there was a
positively correlation between root-shoot ratio and alfalfa N:P (Fig.
5).
The biomass of alfalfa was negatively related to soil OC
(R2=0.70, P<0.01), TN
(R2=0.81, P<0.01), TP
(R2=0.42, P<0.01) (Fig. S1I, II and III),
soil C:P ratio and soil N:P (Fig. 5). While underground biomass was
positively related to soil OC (R2=0.65,
P<0.01), TN (R2=0.69, P<0.01), TP
(R2=0.34, P<0.01) (Fig. S2I, II and III),
soil C:P ratio and soil N:P (Fig. 5). So it can be easily inferred that
the root-shoot ratio was also significantly positively correlated with
soil OC (R2=0.75 , P<0.01), TN
(R2=0.82, P<0.01), TP
(R2=0.43, P<0.01) (Fig. S3I, II and III),
soil C:P ratio and soil N:P (Fig. 5).
The redundancy analysis in our article revealed the C, N, P,
stoichiometric relationship between alfalfa and soil. The results showed
that during the growth of alfalfa, the content of nutrient elements in
the soil would be significantly affected by the C, N, P and
stoichiometric ratio of alfalfa (the together explain value of total
variation was 50.86%), alfalfa TP and N:P have the most significant
influence (Fig. 6). Soil OC and alfalfa C content have a significant
positive correlation, but N and P had no significant correlation between
soil and alfalfa. The C:P and N:P ratios have significant positive
correlation between soil and alfalfa (Fig. 6).