Introduction
Root system, as an
important functional organ of plants, provides material basis for plant growth
and regeneration, and directly affects plant performance (Jha and
Mohapatra 2010). Fine roots generally refer to roots less
than 2 mm in diameter (Finér et al. 2011; Liu et al. 2014). These are the most active part of the
underground part of the crop, which absorb nutrients and water. It has a huge
surface area, strong physiological activity, and strong response to the
availability of soil resources (Bennett et al. 2002). Although,
the proportion of fine roots in total root system is not high, fine roots can
acquire nutrient resources and release organic matter into the soil. Meanwhile,
fine roots play an important role in energy and material flow in the biosphere.
These can not only transfer carbon into underground carbon pools, but also
promote plant uptake of water and nutrients (Pregitzer et al. 2002). Moreover, fine roots can return nutrients fixed by
photosynthesis to the soil, which can make plants grow sustain ably. Fine root
turnover is an important component of material cycle and energy flow in
ecosystem (Majdi 2001). Nutrients and organic matter enter soil through fine
root turnover are important sources of soil fertility maintenance (Lee and Jose
2003). It plays an important role in restoring and increasing soil fertility,
improving plant nutrition and primary productivity. Therefore, the study on
fine root turnover of alfalfa is of great significance to clarify the
above-ground yield formation of alfalfa.
Alfalfa (Medicago sativa L.) is perennial high-quality
leguminous forage, known as the "King of Forage" (Yacoubi et al. 2011). The growth, death and turnover of fine roots of alfalfa are
closely related to the external climatic and soil environmental conditions.
Soil nutrient directly affect the physiological activity of fine roots and the
distribution of carbohydrates, thus affecting the production and turnover of
fine roots of alfalfa (Zhu et al. 2013). Thus, had great significance to the growth and
development of alfalfa, productivity and the flow of matter and energy in
ecosystems (Chang et al. 2012).
When the availability of soil resources increased, it could promote the
growth and biomass accumulation of fine roots, and the ability to absorb water
and nutrients of fine roots would also be enhanced (Zhang et al. 2019). The results showed that the improvement of soil water
and nutrient availability could stimulate the growth, increase the biomass of
fine roots, branching of lateral roots, and then increase account of new roots,
meanwhile, it can prolong or shorten the life span of fine roots, and increase
the respiration rate (Williamson et al. 2001). Phosphorus is a large number of elements needed
for alfalfa growth and its application can accelerate cell division, promote
root and above ground growth, thereby promoting the transport of photosynthate
in plant leaves, and then increase the above ground yield (Singh and Reddy
2014). Other studies found that fine root C/P ratio was positively correlated
with phosphorus use efficiency of above ground litter (Okada et al. 2017). It has become an important nutrient limiting the growth of
plants and soil organisms in terrestrial ecosystems (Vitousek et al. 2010). Studies have shown that the application of
appropriate phosphorus can improve the extracellular enzymes involved in
decomposition, which is conducive to the degradation of cellulose and lignin in
fine roots of crops, thus promoting the decomposition of fine roots of crops (Cormier et al. 2015). Fertilization can increase soil enzyme activity, which is
beneficial to the decomposition of fine roots and soil organic matter (Jiang et al. 2014).
Minirhizotron technology is a method of recording and studying plant
roots by scanning images. Its greatest advantage is to dynamically monitor the
underground parts of the same sample at different times in a fixed point,
continuous, periodic and non-destructive manner. This technique has been widely
used to study the growth and death dynamics of fine roots and estimate their
turnover. It has been successfully applied in alfalfa fine root turnover
research (Ren et al. 2015).
There are continuous material circulation and energy flow between root
system and soil, which fundamentally affect the process of nutrient utilization
by crops. At present, the research on fine roots of alfalfa mainly focused on
the biomass, spatial distribution and dynamic change in soil. Production and
death cycle of alfalfa fine root under phosphorus treatment was relatively few,
especially the study on the characteristics of fine root turnover under drip
irrigation with different phosphorus application rates are rarely reported, and
the relationship between the indexes of fine root of alfalfa under drip
irrigation was still unclear. Therefore, this experiment studied the growth and
turnover of fine roots of alfalfa under different P treatment and drip
irrigation conditions, to understand the growth and death dynamics and spatial
distribution characteristics of fine roots of alfalfa, and clarified the
dynamic characteristics of fine roots rotation of alfalfa. To provide
theoretical basis for the study of nutrient uptake by drip alfalfa under
irrigation fine root rotation, the high quality and efficient production of
alfalfa, the relationship between phosphorus application and fine root turnover
was clarified in Xinjiang oasis region of China.
Materials and Methods
Site description
Field experiments were conducted in 2017 and 2018 at the agricultural
demonstration park of the Tianye Group Agricultural Research Institute,
Shihezi, Xinjiang, China (44°26'N, 85°95'E). The maximum temperature occurs in July, and the highest precipitation in May
in 2017 and 2018 (Fig. 1). The annual number of sunshine hours ranged from 2721~2818
h, and the soil type was grey desert soil. The physical and chemical properties
of the 0~20 cm plough layer soil are shown in Table 1.
Experimental design
This experiment adopted single factor randomized block
design with phosphorus fertilizer rates: 0, 50, 100 and 150 kg·ha-1.
The phosphorus fertilizer was mono ammonium phosphate (total P2O5
52%) with three repetitions. Fertilizer was dripped with water at the branching
stage, first cut, sec cut and third cut 3~5 days after cutting.
In this experiment, uninoculated
WL354HQ alfalfa seeds (Beijing Zhengdao Ecology Technology Co., Ltd.) were sown
on April 19, 2015 with spacing of 20 cm,
the sowing depth of 1.5~2.0 cm using 18 kg·ha-1 seed rate in a plot with
area of 5 m x 8 m. 1 m wide walkway was set up between the districts to prevent water infiltration between residential
areas. The drip irrigation belt was
buried at the shallow 8~10 cm surface
soil with a spacing of 60 cm. The working pressure of the drip irrigation belt
was 0.1 MPa with the diameter of 12.5 mm, the dripper flow of 1.1 L·h-1, and the dripper spacing of 20 cm. An internal-embedded drip irrigation
belt was used and the distance of the drop head was 20 cm. In addition to water factors, other weeding management
was carried out in accordance with the local farmland according to the field
growth.
Minirhizotron
installation and data acquisition
The growth and
death of alfalfa roots were continuously observed in 2017 and 2018 by CI-600
root monitoring system (CID BIO-Science, United States), and the minirhizotron
were installed in the experimental field in April 10, 2016. A total of 12
minirhizotrons were buried in each plot. According to Johnson et al.
(2001) and other methods, minirhizotrons were installed in the center of the
experimental plot (Fig. 2). The length of
minirhizotrons was 1 m, and the installation angle was 45° with the ground. The
exposed part of the soil surface was about 12～15 cm, and
the vertical depth was about 60 cm. The matching plastic cap of the
minirhizotron was applied to the mouth of the minirhizotron. The exposed part
of the minirhizotron was wrapped in two layers with black plastic bags and
fastened with rubber band and tape to prevent exposure of the scanner during
root measurement, dust and water entering the inner wall of the minirhizotron
after the minirhizotron cover slipped or damaged. Then a wooden stick was
inserted about 1 m long into the position 10 cm away from the minirhizotron in
order to find the location of the minirhizotron intuitively during observation,
and prevent the human inadvertent destruction of the minirhizotron body during
alfalfa cutting.
Table 2: Fine root crop of
alfalfa in different soil layers (cm·cm-3)
|
Soil depth (cm) |
Phosphorus treatments |
2017 |
2018 |
||||||||
|
22/5 |
21/6 |
21/7 |
22/8 |
23/9 |
23/5 |
24/6 |
24/7 |
23/8 |
24/9 |
||
|
0～20 |
0 kg· ha–1 |
0.1835c |
0.2359d |
0.1955d |
0.2495c |
0.2431d |
0.1952d |
0.2266d |
0.202d |
0.2414c |
0.2191d |
|
50 kg· ha–1 |
0.196b |
0.2623c |
0.2391c |
0.2778b |
0.2809c |
0.2313c |
0.2808c |
0.2262c |
0.2942b |
0.2597c |
|
|
100 kg· ha–1 |
0.2411a |
0.3992a |
0.3318a |
0.3413a |
0.3327a |
0.2770b |
0.3356a |
0.2938a |
0.3335a |
0.3251a |
|
|
150 kg· ha–1 |
0.2461a |
0.3484b |
0.2832b |
0.3335a |
0.3058b |
0.2942a |
0.3146b |
0.2812b |
0.3230a |
0.2807b |
|
|
20～40 |
0 kg· ha–1 |
0.0891c |
0.1746c |
0.1982d |
0.1855d |
0.1642c |
0.0886d |
0.1555d |
0.1786d |
0.1723d |
0.1499c |
|
50 kg· ha–1 |
0.0933c |
0.1928b |
0.2064c |
0.1999c |
0.1688c |
0.1351c |
0.1865c |
0.2022b |
0.1894c |
0.1645b |
|
|
100 kg· ha–1 |
0.1960a |
0.2817a |
0.2628a |
0.2693a |
0.2076a |
0.1975a |
0.2565a |
0.2142a |
0.2483a |
0.1996a |
|
|
150 kg· ha–1 |
0.1768b |
0.1940b |
0.2221b |
0.2295b |
0.1807b |
0.1792b |
0.2142b |
0.1933c |
0.2309b |
0.1954a |
|
|
40～60 |
0 kg· ha–1 |
0.0678c |
0.0905c |
0.0884b |
0.0944b |
0.0604b |
0.0712b |
0.0924d |
0.0857c |
0.092c |
0.0671b |
|
50 kg· ha–1 |
0.0771b |
0.100ab |
0.0935b |
0.1219a |
0.0631b |
0.0974a |
0.1099b |
0.1006b |
0.1204b |
0.0927a |
|
|
100 kg· ha–1 |
0.0807b |
0.1061a |
0.1122a |
0.1262a |
0.0866a |
0.0973a |
0.1313a |
0.1626a |
0.1297a |
0.0954a |
|
|
150 kg· ha–1 |
0.1126a |
0.0952bc |
0.0926b |
0.1267a |
0.0800a |
0.0920a |
0.1007c |
0.094b |
0.1148b |
0.0992a |
|
The different small letter in column is
significant (P ＜0.05). The same
From May to
September in 2017 and 2018, CI-600 was used every 30 days to scan and collect
root growth images in minirhizotron, totally 10 times. And the stage of crop at
the time of measurement was the early flowering stage. The specific image
scanning dates were May 22, June 21, July 21, August 22, September 23, in 2017,
May 23, June 24, July 24 and August 23, September 24, in 2018, respectively. The
collected pictures were Table 1: Soil physicochemical properties (0–20 cm depth) at the
experimental station
|
Years |
Bulk density (g cm−3) |
Available P (mg kg−1) |
Organic matter (g
kg−1) |
Alkaline N (mg kg−1) |
Total P (g kg−1) |
Available K (mg kg−1) |
|
2017 |
1.47 |
16.5 |
25.6 |
73.1 |
0.21 |
330 |
|
2018 |
1.46 |
15.2 |
24.1 |
71.3 |
0.19 |
324 |
%2068-76_files/image002.png)
Fig. 1: Precipitation and
average temperature at the experimental site during the growing season of
alfalfa in 2017 and 2018
%2068-76_files/image004.png)
Fig. 2: Minirhizotrons
installation
brought back to the laboratory for processing with the root image
analysis by WinRHIZO TRON MF 2014b. Roots appearing in observation windows were
recorded as living roots in white and old roots in brown, while those appearing
in observation windows are recorded as dead roots when fine roots completely
turn black, cortex shedding or obvious folds and disappearance occur. The
vertical depth of the minirhizotron was about 60 cm which divided into 0~20, 20~40
and 40~60 cm from top to bottom. Roots less than 2 mm in the observation window
were recorded only.
Measurement index
and method
Fine root length
density: Taking the root
length density as the basic parameter, taking the existing length of the fine
root in the whole minirhizotron as a whole, the root length density of the
whole root canal is the total standing crop. The root length density of the
fine root in different diameter classifications is the standing crop of the
fine root in different diameter classifications. And the root length density of
the fine root in different soil layers is the standing crop of the fine root in
different soil layers. The concrete formulas are as follows:
RLD = RL × sinθ / (A × 4 × DOF)
In the formula, RL (cm) is the root length of the whole minirhizotron, A
(cm) is the area of the scanned image (422.3 cm2), A×4 (cm) is the
area of the whole minirhizotron, RLD (cm.cm-3) is the root length
density production, DOF is the thickness of the soil layer, generally 0.2～0.3 cm. In
this study, DOF (cm) is 0.2 cm. Because the minirhizotron is 45° from the
ground, it is necessary to multiply the fine root length density by sin 45° to
obtain the unit volume root length density at the vertical height. The vertical
depth of micro-root canals is about 60 cm, and the standing crop of fine roots
in 0~20 cm, 20~40 cm and 40~60 cm soil layers are expressed every 20 cm from
top to bottom.
Fine root
production and mortality of alfalfa: Fine root production of alfalfa refers to the root length of new roots
and the increase of elongation growth of old roots during the last sampling
period and the previous sampling period in the same treatment during the
growing season. Fine root mortality includes the reduction of original root
length caused by the death of original roots and the feeding of root-feeding
animals.
Turnover rate: Turnover rate (yr-1) was = Annual fine root
production (cm.cm-3·yr-1)/maximum annual fine root standing
crop (cm.cm-3).
Statistical analysis
WPS 2016 was used
to collate the data of fine root standing crop, production, mortality. DPS 7.05
(Data Processing System, China) was used to analyze the data. Duncan method was used to analyze the
difference significance of the data (P < 0.05). Pearson correlation
analysis in SPSS 19.0 (SPSS Inc., Chicago, IL, USA) was used to analyze the
relationship between fine root standing crop and each factor. Drawing with
Origin 8.0 (OriginLab OriginPro, USA).
Results
Total hay yield of
alfalfa
The total hay yield
of alfalfa increased first and then decreased with the increase of phosphorus
application, and reached the highest at 100 kg P ha-1, which was 20.74 t·ha-1 and 19.83 t·ha-1 in 2017 and 2018, respectively (Fig. 3). The total hay yield in 2017 was higher than 2018 under P
application treatments. The total hay yield of alfalfa at 100 kg P ha-1 was
significantly higher than 0 kg P ha-1 and 50 kg P ha-1 treatments (P
< 0.05). The total hay yield of alfalfa at 50 kg P ha-1 was not significantly
different from 150 kg P ha-1.
Total fine root in
standing crops
The fine root in standing
crop of alfalfa under different phosphorus treatments had two peaks in June and
August in both years, respectively (Fig. 4). Fine root standing crop declined
from August to September. With the increase of phosphorus application, the fine
root standing crop of alfalfa increased first and then decreased, and reached
the maximum value at 100 kg P ha-1. The fine root in standing crop of alfalfa at 100 kg P ha-1 and 150 kg P ha-1 treatments was larger than 50 kg P ha-1 treatment. The fine root in standing crop of alfalfa at
100 kg P ha-1 treatment was 34.00 to 61.05% higher than 0 kg P ha-1 treatment. The fine root in standing crop of alfalfa at
100 kg P ha-1 was 17.80 to 41.78% higher than 50 kg P ha-1 treatment (P < 0.05).
In all soil layers, there were two peaks of fine root in standing
crop of alfalfa during June and August of each year (Table 2). Fine root in standing
crop of alfalfa was the lowest in May and mainly concentrated in 0~20 cm under
different soil layers. With the increase of soil depth, the fine root standing
crop of alfalfa decreased gradually.
%2068-76_files/image006.jpg)
Fig. 3: Total hay yield of alfalfa under drip irrigation in different
fertilization treatments in 2017 and 2018
Note: The small
letters in the picture indicate that different treatments have significant
differences at the 0.05 level
P0, P1, P2, and P3 mean 0
kg· ha–1, 50 kg· ha–1, 100 kg· ha–1 and 150
kg· ha–1, respectively
%2068-76_files/image008.png)
Fig. 4: Total fine
root standing crop of alfalfa under different treatments (cm·cm-3)
In 0–20 cm soil layer, the highest fine root standing crop of alfalfa
was 100 kg P ha-1 than 0 kg P ha-1 and 50 kg P ha-1 treatments (P < 0.05). Except September 24,
2018, the fine root standing crop of alfalfa at 100 kg P ha-1 treatment was significantly higher than other treatments
in 20~40 cm soil layer (P < 0.05). In 40~60 cm soil layer, the fine
root standing crop of alfalfa applied with 100 kg P ha-1 treatment was significantly larger than 0 kg P ha-1 treatments (P < 0.05).
Fine root
production and mortality
There were two
peaks in fine root production of alfalfa during May-June and July–August, of
both years respectively (Fig. 5). The fine root production of 100 kg P ha-1 treatment was significantly higher than 0 kg P ha-1 and 50 kg P ha-1 treatment in each period (P < 0.05). Except
for 22 August to 23 September in 2017 and 24 June to 24 July in 2018, the fine
root production of phosphorus application was significantly higher than 0 kg P ha-1
(P < 0.05).
The fine root production of 100 kg P ha-1 treatment was 27.24 to 76.91% higher than 50 kg P ha-1 treatment. Fine root mortality reached the highest
level in August to September in two years, and the fine root mortality of 100 kg P ha-1 reached the maximum, 0.1903 and 0.1640 cm-3,
respectively. The mortality of 100 kg P ha-1 and 150 kg P ha-1 treatments
was significantly higher than 0 kg P ha-1 and 50 kg P ha-1 treatments
(P < 0.05). Fine root mortality of 100 kg P ha-1 treatment reached the lowest level in May to June,
0.0601 and 0.0620 cm-3, respectively.
Fine root turnover
rate
The annual
production of fine root, annual mortality of fine root, maximum annual standing
crop of fine root and turnover rate all reached the maximum under 100 kg P ha-1 treatment. The turnover rates were 0.2134 and 0.1814 at
100 kg P ha-1 treatment, respectively, which were higher 10.0 and
5.8% than 0 kg P ha-1 treatment
(Table 3). Turnover rate at 100 kg P ha-1 treatment was significantly higher than other
treatments (P < 0.05), but there was no significant difference
between 150 kg P ha-1 treatment and 0 kg P ha-1 treatment in 2018 (P > 0.05). The annual
production of fine root, maximum annual standing crop of fine root of 100 kg·P ha-1 treatment was significantly larger than 0 kg P ha-1 treatment, and the annual fine root production and the
maximum annual fine root standing crop at 100 kg P ha-1 treatment were significantly larger than other
treatments (P < 0.05). The annual fine root mortality at 100 kg P ha-1 and 150 kg P ha-1 treatments was significantly higher than 0 kg P ha-1 and 50 kg P ha-1 treatments
(P < 0.05).
Correlation
between annual total hay yield and annual standing crop, production, mortality
of fine root and turnover rate
Pearson
correlation analysis showed (Table 4) that annual total hay yield of alfalfa
and annual production of fine root had significant correlation (P < 0.01),
between annual total hay yield of alfalfa and annual fine root standing crop
had significant correlation (P < 0.05) and annual production of fine
root and the annual fine root standing crop (P < 0.01). Fine root
mortality and annual fine root standing crop and annual fine root production
had significant correlation (P < 0.05). There was no significant
correlation between turnover rate and other indicators (P > 0.05).
Discussion
The growth of
forage roots will affect the accumulation of dry matter in the aerial part;
fine roots had a great impact on the growth and mortality of alfalfa roots.
Fine root production and mortality directly affected the net primary
productivity of alfalfa (Norby et al. 2004). Studies have shown that when a certain amount of
fertilizer is applied to the soil, the availability of nutrients can be
increased, the distribution of C to the underground increases, the growth of
fine roots and the accumulation of biomass can be promoted, and the ability of
fine roots to absorb nutrients and water can be enhanced (Meinen et al. 2009). In present study, the total hay yield of alfalfa
reached its maximum when the application of phosphorus was 100 kg·ha-1. The main reason was
that proper application of phosphorus fertilizer could significantly increase
the chlorophyll content of alfalfa leaves, thereby increasing the
photosynthesis rate of alfalfa, promote the growth of alfalfa plants, and then
increasing the hay yield of alfalfa (Aranjuelo et al. 2007). When phosphorus fertilizer was applied, alfalfa
had higher fine root production, which indicated that the application of
phosphorus fertilizer could increase the absorption surface area, facilitate
alfalfa to absorb nutrients in soil, and further promote the growth of above
ground in alfalfa plants.
Fertilization under drip irrigation had a significant effect on fine
root growth of alfalfa. The fine root standing crop and production of alfalfa
under different phosphorus application treatments increased first and then
decreased with the increase of fertilizer application. This indicated that root
growth had a strong fertilizer-tropism. To meet the demand of alfalfa for
phosphorus, alfalfa enlarges its root development by expanding the root volume
and increasing the root length to promote the absorption of limited nutrients
in the soil (Akhtar and Siddiqui 2009). It indicated that proper application of phosphorus fertilizer
could effectively promote the growth and accumulation of fine roots, but
excessive application of phosphorus fertilizer would restrict root growth,
reduce root length and surface area, thus slowing down the growth process of
alfalfa.
Table 3: Fine root turnover rate
of alfalfa in different treatments (yr-1)
|
Years |
Phosphorus treatments |
Annual production of fine root (cm·cm-3·yr-1) |
Annual mortality of fine root (cm·cm-3·yr-1) |
Maximum annual standing crop of fine root
(cm·cm-3) |
Turnover
rate (yr-1) |
|
2017 |
0 kg· ha–1 |
0.1027d |
0.0709d |
0.5295d |
0.1940d |
|
50 kg· ha–1 |
0.1245c |
0.0879c |
0.5996c |
0.2076b |
|
|
100 kg· ha–1 |
0.1679a |
0.1406a |
0.7870a |
0.2134a |
|
|
150 kg· ha–1 |
0.1421b |
0.1343b |
0.6896b |
0.2060c |
|
|
2018 |
0 kg· ha–1 |
0.0867d |
0.0664c |
0.5057d |
0.1714c |
|
50 kg· ha–1 |
0.1078c |
0.0945b |
0.6040c |
0.1785ab |
|
|
100 kg· ha–1 |
0.1312a |
0.1191a |
0.7235a |
0.1814a |
|
|
150 kg· ha–1 |
0.1173b |
0.1148a |
0.6687b |
0.1755bc |
Table 4: The correlation
analysis between annual standing crop of fine root and annual production of
fine root, annual mortality of fine root, turnover rate
|
Index |
Annual total
hay yield |
Annual
standing crop of fine root |
Annual
production of fine root |
Annual
mortality of fine root |
|
Annual
standing crop of fine root |
0.9860* |
|
|
|
|
Annual
production of fine root |
0.9980** |
0.9940** |
|
|
|
Annual
mortality of fine root |
0.9350 |
0.9780* |
0.9560* |
|
|
Turnover
rate |
0.9240 |
0.8660 |
0.9120 |
0.8000 |
Note: * Significant correlation was found at
the 0.05 level (bilateral), ** significant correlation was found at the 0.01
level (bilateral)
%2068-76_files/image010.png)
Fig. 5: Fine root production and mortality of
alfalfa in different treatments (cm·cm-3)
Note: Ⅰ: 22/5 - 21/6, 2017; 23/5 - 24/6, 2018 Ⅱ: 21/6 - 21/7, 2017; 24/6 - 24/7, 2018
III: 21/7 - 22/8, 2017; 24/7 - 23/8, 2018 Ⅳ: 22/8 -23/9, 2017; 23/8 -24/9, 2018
The lowercase letters in the picture indicate
that different treatments have significant differences at the 0.05 level
P0, P1,
P2, and P3 mean 0 kg· ha–1,
50 kg· ha–1, 100 kg· ha–1 and 150 kg· ha–1,
respectively
It was found that the seasonal variation of fine root nutrients of
alfalfa was different (Table 1), which reflected the response of plant fine
root growth to nutrients. From May to June, temperature gradually increased,
nutrient availability and mobility in soil were higher, fine root growth
accelerated, plants would allocate more C to fine root, root nutrient uptake
increased (Son and Hwang 2003). In mid-summer, the precipitation in Xinjiang
was less, and the soil water content is often reduced due to evaporation and
strong transpiration of plants will result in the decrease of fine root biomass
of alfalfa and accelerates fine root death under drought conditions (Yuan and Chen
2010). Under drought conditions, the amount of water absorbed by fine roots
decreased, but the respiration of roots to maintain cell membranes and enzyme
activities still consumed a large amount of carbon. Therefore, the amount of
fine root death increased in order to reduce energy consumption (Bai and Li 2003).
The death of fine roots is related to the distribution of
photosynthates to roots, which is a complex physiological and ecological
process. Studies
have shown that the more water and nutrients the fine roots absorb, the more carbon they
distribute to the fine roots, and the longer their life span
(Luo and Zhao, 2019). Once the absorption capacity of fine roots decreases, the
distribution of carbon to fine roots decreases immediately, and fine roots grow
old and die (Bai and Li 2003). In this study, fine root death of alfalfa changed
little from the end of June to August. From August to September, fine root death
was the highest in growing season, because with the advent of autumn, the
above-ground parts of alfalfa began to wither, photosynthesis of leaves
weakened, temperature began to decrease (Brassard et al. 2009), fine root
began to stop growing gradually, carbohydrates allocated to roots decreased (López
et al. 2001), and soil temperature continued to decrease. Fine
roots died a lot during this period (Jha and Mohapatra 2010). Moreover,
alfalfa had the last cuts after September, further aggravating the death of
fine roots. The fine root mortality of alfalfa exceeded the growth rate after
September. This may be due to the quickening of fine root death at the end of
the growing season with the reduced supply of irrigation and growth after the
above ground part harvested. With the rapid end of the growing season and the
arrival of winter, underground heat conditions will become more uncomfortable.
The availability of soil nutrients after fertilization
was affected by many factors, such as rhizosphere, root exudates and
microorganisms. The response of fine roots was different in different soil
layers. After fertilization, surface nutrient branches infiltrate downward
after irrigation, which will increase the nutrient content in deep soil and
increase the existing fine roots (Zhou and Wang 2015). Moreover, in present study, the fertilizer was
applied with water drip. Drip irrigation can change the conditions of water and
temperature in surface soil, which was beneficial to root growth. At the same
time, suitable soil temperature and water conditions can also contribute to the
decomposition of litter on the ground, thus improving the soil conditions in
surface soil, enriching the nutrients in surface soil and promoting the growth
and accumulation of fine roots (Amin et al. 2014). Therefore, the distribution of fine roots in surface soil is the
largest. Suitable application of phosphorus could promote root growth,
strengthen rhizosphere process and increase the proportion of fine roots, and
promote alfalfa growth and nutrient uptake in early stage (Wang et al. 2013). Under suitable environmental conditions, the
activities of soil animals and microorganisms increased, which was more
conducive to the decomposition of fine roots. Phosphorus application directly
affected the metabolic activities of plants, soil animals and microorganisms,
the quantity of soil enzyme secretion, the activity of litter decomposition
enzymes (Qualls and Richardson 2000), and the increase of enzyme activity can
also contribute to the decomposition, transformation of organic matter and the
release of nutrient elements in litter. In this study, a small amount of
phosphorus application significantly reduced the fine root standing crop and
production of alfalfa (Fig. 4), resulting in the whole absorption capacity of
root system to decline, and ultimately inhibit the growth of root system.
The root distribution of alfalfa has obvious vertical characteristics.
The total fine root standing crop of 0~20 cm and 20~40 cm soil are
significantly higher than 40~60 cm soil, mainly due to the differences in
spatial distribution of soil resources availability and environmental
conditions (Kalliokoski et al. 2010). The vertical distribution of fine roots was
mainly attributed to the higher temperature in the surface soil and the higher
content of available nutrients in the soil is conducive to the growth and
absorption of fine roots, and plays a greater role in nutrient acquisition and
carbon cycle (Ibrahima et al. 2010). Secondly, the bulk density and texture of the
surface soil were good, while the lower soil temperature and poor soil are not
conducive to the growth of fine roots (Makita et al. 2011). Lower
nutrient content in deep soil also affects root distribution in deep soil. With
the increase of soil depth, soil compactness gradually increases, and
permeability was poor, which was harmful to fine root growth (Zhou and Shangguan
2007). Phosphorus application treatments was significantly greater than no
phosphorous application, probably because no application of fertilizer would
inhibit root growth, reduce root density and lower biomass.
Fine root turnover was the main way to return carbon and nutrients to
soil and was determined by fine root life, fine root senescence affects fine
root life (Xiong et al. 2017). The faster fine root senescence, the shorter
fine root life, the faster fine root turnover, the greater carbon consumption,
the more nutrients returned to the soil, and the faster nutrient cycle. The
highest fine root turnover was achieved at 100 kg·ha-1 phosphorus application. This relatively high fine
root turnover rate indicated that plants had vigorous life activities, and
could continuously produce new fine roots to replace old fine roots to absorb
water and nutrients, thus increasing the use efficiency of water and nutrients
in roots (Cormier et al. 2015). This
experiment showed that the fine root turnover rate in 2018 was lower than 2017
(Table 3). It may be that the growth of alfalfa for many years affected the
secretion behavior of alfalfa roots, and the HPO42-
secretion significantly reduced. Because of the relative stability of
phosphorus in soil, the decrease of HPO42- secretion in
root system changed the micro-domain cycle balance of phosphorus at the
root-soil interface, and in fact reduced the chance of phosphorus reuse by root
system. Studies on crop roots show that changes in phosphorus availability can
change the morphology and configuration of plant roots, increase the length,
density of root hairs and the length and quantity of lateral roots, increase
the distribution of available nutrients in higher areas of soil, and thus
improve the effective absorption of phosphorus (Jing et al. 2010).
Conclusion
There were two
peaks of fine root standing crop and fine root production of alfalfa in June
and August under different phosphorus application treatments, respectively. Fine
root mortality reached the highest from August to September. With the increase
of phosphorus application, the fine root standing crop of alfalfa increased
first and then decreased, and reached the maximum value at 100 kg P ha-1 treatment. Alfalfa fine root standing crop mainly
concentrated in 0~20 cm, with the increase of soil depth, alfalfa fine root
standing crop decreased gradually. The turnover rates of the two-year at 100 kg P ha-1 treatment
higher than 0 kg P ha-1 treatment were 10 and 5.8%. The annual fine root
production and maximum standing crop at 100 kg P ha-1 treatment was significantly larger than other P
application treatments. When the amount of P2O5 was 100
kg·ha-1, the turnover
rate of fine roots of alfalfa was improved, and fine root standing crop and
fine root production of alfalfa significantly affected the dry matter yield
above ground.
Acknowledgements
The research was supported by the National Natural Science Foundation of
China (31660693),
the Fok Ying Tung Education Foundation (171099), the the China Postdoctoral Science
Foundation (2018T111120, 2017M613252), the Youth Innovation Talent Cultivation
Program of Shihezi University (CXRC201605) and the China Agriculture Research
System (CARS-34).
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