KEYWORDS
groundwater depth, soil
CO2 concentration, soil CH4concentration, root biomass, soil nutrient content
1 INTRODUCION
Soil carbon pool is the largest natural terrestrial carbon resource and
is closely related to soil fertility and environmental quality (Buysse
et al., 2016; Qu et al., 2018; Sun et al., 2018; Zhang et al., 2018a).
Soil respiration is the primary output of the soil carbon pool (Lou et
al., 2017; Tong et al., 2017), and it embodies root respiration and
microbial respiration (root and mycorrhizosphere respiration,
surface-litter, organic matter decomposition, and so on) (Acosta et al.,
2018; Li et al., 2018).
Microbial
respiration represents microbial decomposition and transformation rate
(Lou, Gu and Zhou, 2017; Qu, Kitaoka and Koike, 2018), whereas root
respiration represents the root metabolism rate that is affected by
photosynthesis,
plant
phenology,
root
biomass, carbohydrate content in plant shoots, and net primary
production, etc. (Savage et al., 2013; Li et al., 2018). The C
accumulation and C recycling in the soil are directly dependent on
photosynthesis as photosynthate are the primary source of carbohydrates
for root respiration or microbial respiration in root exudates (Savage,
Davidson and Tang, 2013). Root biomass is a crucial C source in the
soil. High root biomass is often associated with high root respiratory
activity (Tomotsune et al., 2013), and it provides a high amount of
substrate for microbial decomposition (Wertha and Kuzyakov, 2008).
Briefly, soil respiration is influenced by factors, such as crop growth
(root growth, photosynthetic efficiency, dry matter accumulation, etc.)
and soil environmental factors (microbial activity, enzyme activity and
nutrient content, etc.), and these factors, in turn, are significantly
affected by soil water status and fertilizer application (Zhong et al.,
2016; Li et al., 2018; Hu et al., 2019).
Soil moisture tends to be high in areas at the shallow groundwater level
(Zhang et al., 2018b), and it directly or indirectly affects soil
respiration. In heterotrophic respiration, CO2 and
CH4 are the primary metabolites produced due to aerobic
and anaerobic microbial decomposition, respectively (Cai et al., 2009).
A high level of soil water content leads to waterlogging and reduced
oxygen supply in the soil (Buysse, Flechard, Hamon and Viaud, 2016),
which inhibits aerobic microbial activity and enhances anaerobic
microbial activity (Wang et al., 1996). Thus, CH4 is
replaced by CO2 as the primary metabolite in soil. In
autotrophic respiration, reduced O2 supply to roots
hampers root respiration and root growth by decreasing ATP production,
energy-dependent nutrient uptake and nutrient transport (Aguilar et al.,
2003). Indirect effects of soil moisture are due to anaerobic conditions
resulting from high groundwater level. As demonstrated in the previous
report, it retarded secretion of soil enzymes, such as urease and
phosphatase, etc. by soil microbes and root cells (Pulford and
Tabatabai, 1988). Regardless of soil temperature, decreased secretion of
enzymes led to decreased enzymatic activity, which results in slower
cycling of soil nutrients and CO2 production rate than
aerated soil (Cao et al., 2017; Zhang, Zhu, Zhou and Li, 2018b).
Overall, shallow groundwater level profoundly impacted the soil
respiration.
Fertilizer application regulates soil respiration by altering the soil
nutrient content and enzymatic activities. Chemical fertilizer
application influences soil respiration by increasing NPK availability,
growth of crop roots, and microbial population in the soil (Creze and
Madramootoo, 2019). Xue et al. (2014) showed that N (urea) and P
(chemical fertilizer) increased the density, surface area, and dry
biomass weight of crop roots as N and P play crucial roles in the
allocation of photosynthates in shoots and roots. Subsequently, a higher
root abundance and activity led to higher soil CO2emission (Jurasinski et al., 2012; Sun et al., 2018). The application of
urea and ammonium fertilizers increased the microbial activity leading
to an increased SOC decomposition as ammonium is the preferred nitrogen
source for soil microbes (Gong et al., 2011; Geisseler and Scow, 2014).
Also, chemical fertilizer induced increase in root and microbial biomass
increased the secretion of soil enzymes (Guan, 1986; Dick, 1997).
Increased soil enzymatic activity effectively improves the recycling of
soil nutrients and promotes the growth of soil root and microbes (Guan,
1986; Dick, 1997; Iovieno et al., 2009), improving the intensity of soil
respiration. However, the correlation between soil enzymatic activity
and soil respiration has been rarely explored.
In general, the fertilizer application could increase soil respiration
by increasing soil fertility. However, contrasting outcomes were also
reported. Zhu et al. (2016) reported that low concentration of N
supplementation increased soil respiration, underground plant biomass
content, and soil microbial biomass carbon, whereas high concentration
of N supplementation mitigated these factors in the grassland. An
appropriate fertilizer application rate is crucial in regulating soil
environmental conditions for crop root and microbial growth, which are
closely related to soil respiration.
Shallow groundwater level exists widely in Central China in which is one
of the main regions for winter wheat production in China. In the winter
wheat growing season, due to the high intensity of rainfall and poor
drainage conditions (Ren et al., 2016), crop productivity is frequently
affected due to abiotic stress.
Shallow groundwater level alters
the soil environmental conditions, such as physical, biological, and
chemical properties, associated with the crop roots and microbial
growth, and soil biochemical reactions, resulting in the differences of
soil nutrient cycle and crop growth characteristics in the regions
between shallow and normal groundwater level (Grimaldi et al., 2015). It
is a well-known fact that appropriate drainage and fertilizer
application decreases waterlogging and improves soil environmental
conditions. However, the complex correlation between groundwater level,
fertilization level, root growth, enzymatic activity, and soil
respiration and their interaction mechanisms in shallow groundwater
level soil, remains ambiguous.
In this study, we aimed to 1) determine the effects of groundwater depth
and fertilizer level on soil CO2 and CH4concentrations in different stages of winter wheat growth in shallow
groundwater level and 2) investigate the correlation of soil
CO2 and CH4 concentrations with soil
nutrient content, enzymatic activity, and root biomass, to better
understand the influence of shallow groundwater level on soil
environmental conditions.
2 MATERIALS AND MEHTODS
2.1 Experimental site and treatments
The experiment was performed at the Experimental Station of Yangtze
University (Latitude, 30°21’N; Longitude, 112°09’E; Elevation, 31.8 m
above sea level) in Jingzhou, Hubei, China. It is a subtropical humid
monsoon region with a rainy spring and summer with mean annual
precipitation and air temperature of 1100 mm and
16.7oC, respectively. The mean monthly rainfall
increases from 29.6 mm in January to 159.9 mm in June. Groundwater depth
in the experimental region is around 50 cm on average, total salinity in
the groundwater is less than 1 g L-1, and pH is
6.7–8.9. The soil is yellow brown paddy and loamy containing 22% clay
(0–2 µm), 75% silt (2–50 µm), and 3% sand (50–2000 µm),
respectively.
The experiments were conducted in micro-lysimeters that were 112 cm deep
and 70 cm in diameter. The micro-lysimeters were evenly filled layer by
layer with soil collected from a local farm field at the bulk density of
1.27 g cm-3. The micro-lysimeter’s groundwater level
at the depths of 20, 40, 50, 60, and 80 cm in the soil surface was
automatically controlled by using water inlet and outlet apparatus
(Figure 1). Soil filled micro-lysimeters were employed to estimate the
initial contents of organic matter, total N, available P, available K,
and soil pH value (soil: water ratio of 1: 2.5), which were found to be
8.63 g kg-1, 1.29 g kg-1, 16.90 mg
kg-1, 153.76 mg kg-1, and 7.8,
respectively.
Winter wheat (Triticum aestivum L.) was sown on October 28, 2015,
at a density of 210 plants m-2, and harvested on May
4, 2016. During the wheat growth period, compound fertilizer containing
N: P2O5: K2O in the
ratio of 14: 16: 15 and urea with 46% N content were applied thrice
before sowing, at the seedling stage and the jointing stage, and the
mass
proportion
of chemical fertilizer applied at the three stages of wheat growth was
7: 1: 2. In line with the local practices, the application rates of N,
P, and K in the whole growth period of winter wheat were 180, 65, and 60
kg ha-1, respectively, which were designated as the
normal fertilization level treatment (NF) in this study. The rainfall
and mean daily air temperature were 388 mm and 12.2°C, respectively, in
the winter wheat growth period during 2015–2016. As per the weather
records (1952–2016), this region received normal rainfall throughout
the year with a drought index of 0.34 during the winter wheat growth
period (Zhang, Zhu, Zhou and Li, 2018b). No supplementary irrigation was
provided during the whole growth period of winter wheat.
Experimental treatments included five groundwater depths (20, 40, 50,
60, and 80 cm) and three fertilization application rates (low, normal,
and high). The fertilizer application rates for the low fertilization
level and the high fertilization level were 75% (75% NF) and 125%
(125% NF) of the standard fertilizer application rate (NF),
respectively. All experimental treatments in the study were replicated
three times.
2.2 Observation indexes and measurement methods
Geothermometers set to16:00 were used for measuring soil temperature at
the 5, 10, 15, and 20 cm depth per day (Figure 1). The soil was sampled
at the depths of 0–10 and 10–20
cm
during jointing (March), heading (April), and grain filling (May) stages
of a wheat growth three times. Soil water content, organic matter
content, pH value, soil nutrient content (N, P, and K), and the soil
enzymatic activities (urease, alkaline phosphatase, sucrase) were
estimated. To calculate soil water content, an electric oven set to
105oC was used, for calculating soil organic matter
content, titration based wet combustion method was used, and for
calculating soil pH value in 1: 2.5 soil-water extract, a pH meter was
used
(FG3-ELK,
Mettler-Toledo International Trading Co., Ltd, Shanghai, China).
Available soil P content was assayed spectrophotometrically (UV-5500PC
Spectrophotometer, Shanghai Metash Instrument Co., Ltd., China), total
soil N content was determined using an automatic Kjeldahl apparatus
(K9840, Hanon Instrument, Jinan, China), and available K content was
determined using a flame photometer (FP640, Shanghai INESA Scientific
Instrument Co., Ltd., China). Urease, alkaline phosphatase, and sucrase
activities were assayed as per Guan’s methods (Guan, 1986). The released
NH4+ was determined using 10% aqueous
urea as substrate, incubated at 37°C for 24 h, and absorbance was
measured spectrophotometrically at 578 nm wavelength. The urease
activity was expressed as mg NH4+-N (g
soil 24 h)-1. For the determination of alkaline
phosphatase activity, the disodium phenyl phosphate solution was used as
a substrate and incubated at 37°C for 24 h. The resulting phenol
formation was determined spectrophotometrically at 600 nm wavelength,
and alkaline phosphatase activity was expressed as mg phenol (g soil 24
h)-1. For determination of sucrase activity, sucrose
solution was used as the substrate and incubated at 37°C for 24 h. After
incubation, this solution was filtered, and the filtrate was boiled with
3 mL of 3, 5-dinitrosalicylic acid (DNS) in a water bath for 5 min. The
absorbance of the reducing sugars was measured at 508 nm wavelength, and
sucrase activity was expressed as mg of glucose (g soil 24
h)-1.
A syringe suction device was used to sample soil at the 0–5, 5–10,
10–20, and 20–40 cm depth to collect soil gas. A rubber hose with
small pinholes was inserted into the above-mentioned soil depth with the
soil-filled lysimeter (Figure 1). Hose with 4 mm inner radius and 1 mm
thick wall and upper port sealed with rubber plug to maintain tension
after need puncture was used. The soil gas was sucked from the hose
using a needle and injected into a special vacuum bottle.
CO2 and CH4 concentrations were measured
with a gas chromatograph (7890A, Agilent Technologies, Inc., Wilmington,
USA) and expressed as mL/L and μL L-1, respectively.
Plant height, tillering rate, and the number of leaves were calculated
at various winter wheat growth stages. After harvesting, root traits
were measured for the entire intact root system extracted from the
micro-lysimeter and treated individually for each experimental
replication. Cleaned fresh roots were scanned using EPSON perfection
V700 Photo (Epson America, Inc. Long Beach, USA) and analyzed with
WinRHIZO 2009 (Regent Instruments Inc. Quebec, CA), and root diameter
and root length density were averaged and expressed as mm and cm
cm-3, respectively.
2.3 Statistical analysis
One-way analysis of variance (ANOVA) was used to calculate significant
differences between different treatments using SPSS version 21.0 (SPSS
Inc. Chicago, USA). A statistically significant ANOVA F -value was
used to perform the least significant difference test (significance
level of P = 0.05) for the separation of the means. Simple linear
regression and curve estimation analyzed the correlation between the
soil CO2 and CH4 concentrations and soil
water content, root biomass, soil nutrient contents, soil enzymatic
activities. For Pearson correlation analysis, P = 0.05 was
considered as statistically significant.
The average values of soil CO2 and
CH\sout4 concentrations and soil water content,
organic matter content, soil nutrient content, soil enzymatic activities
at 0–20 cm soil depth were used to analyze the correlations between
soil CO2 and CH4 concentrations and the
rest of the factors. The CO2 and CH4concentrations at 0–40 cm soil depth were averaged and employed when
wheat root biomass was involved in the establishment of correlations.
3 RESULTS
3.1 Root parameters
As depicted in Figure 2, total root biomass, mean root diameter, and
root length density first increased and later decreased with increasing
groundwater depth. The maximum values of root biomass and root length
density were recorded at the groundwater depth of 60 cm, and maximum
values of mean root diameter were recorded at the groundwater depth of
about 50 cm (Figure 2). It indicated that root diameter expansion was
impacted more by the water shortage than root biomass accumulation and
root extension. The root biomass and mean root diameter
were
positively correlated to the fertilization level (Figure 2A–B). Out of
all the three fertilization levels tested, root length density in the
normal fertilization level was 11.8–25.4% and 9.2–12.5% higher than
that in the high and the low fertilization levels, respectively.
3.2 Shoot biomass and root-shoot ratio
Identical variation trends were observed in shoot and root biomass
values pertaining to groundwater depth and fertilization levels (Figure
3A). Shoot biomass increased with the declining groundwater level until
the groundwater depth of 60 cm, and
then it decreased with the further increase of groundwater depth (Figure
3A). A higher fertilization level led to higher shoot biomass; however,
it was not statistically significant (Figure 3A).
The root-shoot biomass ratio under low fertilization level was
significantly correlated to the groundwater depth (Figure 3B). A linear
correlation was observed under normal and high fertilization levels
between root-shoot biomass ratio and groundwater depth till groundwater
depth reached 60 cm; however, root-shoot biomass ratio and groundwater
depth exhibited an inverse correlation at groundwater depth
> 60 cm (Figure 3B). It suggested that the groundwater
level (> 60 cm) impacted root growth more than shoot growth
in wheat plants (Figure 3B). Root-shoot biomass ratio decreased with
increasing soil fertilization level (Figure 3B).
3.3 Soil CO2 concentration
Variation in soil CO2 concentration with soil depth
during three winter wheat stages at the different groundwater depths and
fertilization levels is depicted in Table 1. With the increasing
duration of the wheat growth, average soil CO2concentration for all sampling depths first increased and later
decreased, and maximum values appeared
during the heading stage of
vigorous wheat growth (Table 1). It suggested an enhanced effect of root
and microbial respiration on CO2 emission during the
vigorous wheat growth stage. Soil CO2 concentration
increased with soil depth due to atmospheric gas exchange (Table 1).
The CO2 concentration in shallow (≤ 10 cm) soil depth
first decreased and later increased with the increasing groundwater
depth, and the minimum CO2 concentration value appeared
at the groundwater depth of 50–60 cm (Table 1). When the sampling depth
was > 10 cm, soil CO2 concentration
decreased with increasing groundwater depth (Table 1). The
fertilization level did not show any
significant effect on soil CO2 concentration. Average
values of soil CO2 concentration at three different
fertilization levels were 1.26–1.33, 2.55–2.63, and 1.48–1.59 mL/L
during the jointing, heading and filling stages of the wheat growth
period, respectively, under experimental conditions (Table 1).
3.4 Soil CH4 concentration
Soil
CH4 concentration exhibited slight seasonal variations
(Table 2) indicated by the lower values during the heading stage
compared to other wheat growth stages. Soil CH4concentration decreased with
decreasing groundwater depth (Table 2). The significantly higher
CH4 concentration was observed at the sampling soil
depth closer to the groundwater surface. However, soil
CH4 concentration
did not change significantly with soil depth and fertilization levels.
The differences of CH4 concentration values at soil
depths and fertilization levels were less than 7.3% and 13.7%,
respectively.
4 DISCUSSION
4.1 Seasonal variations
Soil CO2 concentration was highest at the heading stage
of wheat growth (Table 1). The seasonal soil respiration variation is
primarily caused by the soil temperature (Hu et al., 2019). Soil
CO2 is primarily derived from soil respiration that
includes heterotrophic respiration and autotrophic respiration. Soil
temperature exerts a significant effect on heterotrophic respiration as
the microbes that contribute to heterotrophic respiration are highly
sensitive to soil temperature (Zhong, Yan, Zong and Shangguan, 2016; Qu,
Kitaoka and Koike, 2018). Besides, microorganisms led nutrient
transformation, and biochemical processes are also dependent on the soil
temperature (Guan, 1986; Qu, Kitaoka and Koike, 2018). Root exudation
and root respiration rate, which serve as crucial drivers of autotrophic
respiration, depends on plant growth and thus are closely associated
with temperature (Liu et al., 2015; Tong, Li, Nolan and Yu, 2017). The
correlations of heterotrophic and autotrophic respirations with
temperature during the winter wheat growth period are
linear and unimodal curve,
respectively (Zhang et al., 2013). Autotrophic respiration was found to
be more sensitive to change in temperature than heterotrophic
respiration (Zhang, Lei and Yang, 2013). The synergistic effect of
autotrophic and heterotrophic respiration resulted in the higher soil
CO2 concentration at 20oC of soil
temperature, which was close to the average temperature during the
heading growth stage of winter wheat (Zhang, Lei and Yang, 2013; Liu et
al., 2015; Zhong, Yan, Zong and Shangguan, 2016; Tong, Li, Nolan and Yu,
2017).
CH4 concentration did not change obviously at different
growth stages of winter wheat (Table 2). It might be due to the
insignificant correlation between CH4 concentration and
soil temperature. As shown in previous studies, high-temperature
stimulated the activity of methane-producing microbes, and
CH4 emission increased with
temperature until the temperature
reached 34.5oC (Cai, Xu and Ma, 2009). However,
CH4 oxidation also increased with soil temperature, and
the optimum temperature for maximum CH4 oxidation was
found to be 20–30oC (Cai, Xu and Ma, 2009; Jassal et
al., 2011). Furthermore, oxidation and transport of CH4,
which are primarily influenced by soil gas diffusivity, serve as the
crucial factors that influence CH4 concentration (Cai,
Xu and Ma, 2009)(Cai, 2009 #161;Cai, 2009 #2120). As per the outcomes
of the current study, altered soil CH4 concentration
might be more dependent on gas diffusivity than soil temperature, in
line with the findings by Jassal, Black, Roy and Ethier (2011).
4.2 Effects of groundwater depth
In this study, the statistical analysis of experimental data indicated
that groundwater depth and root parameters were significantly correlated
(P ≤ 0.001) (Table 3). Higher root parameter values appeared at
50–60 cm of groundwater depth (Figure 2). The root system of winter
wheat was mainly distributed at 10–35 cm soil depth (Hodgkinson et al.,
2017). A shallow groundwater level creates a waterlogging environment,
which affects wheat root growth and shoot biomass adversely (Celedonio
et al., 2017), and inhibits wheat root respiration due to insufficient
oxygen supply (Hodgkinson et al., 2017; Chen et al., 2018). On the other
hand, deep groundwater level mitigates the absorption and utilization of
groundwater by crops. Development of root system plasticity promoted
root extension at higher soil depth and increased the water access from
deeper soil layer with higher water content (Becker et al., 2015; Ali et
al., 2018). Shoot biomass was insignificantly affected by the
groundwater depth (Table 3). However, the root-shoot biomass ratio,
which is an index for differential investment between above-ground and
underground biomass, was significantly affected by groundwater depth
(Table 3). A higher root-shoot biomass ratio was observed at the
groundwater depth of 60 cm. It demonstrated that the adverse effect of
the groundwater level on wheat root growth was higher than shoot growth
at the groundwater level that was deeper than 60 cm.
The concentrations of soil CO2 and CH4were significantly correlated with groundwater depth (P ≤ 0.001)
(Table 3). Groundwater depth was correlated to soil water status and
soil aeration conditions, and it substantially influenced production,
emission, and accumulation of CO2 and
CH4 by affecting soil microbial activity, enzymatic
activity, nutrient cycling, and so on (Buysse, Flechard, Hamon and
Viaud, 2016; Zhang et al., 2018a; Hu et al., 2019). Higher shallow
groundwater levels increase soil water content significantly by reducing
pore space for soil gas and creating anaerobic conditions (Wang and Lu,
2006). It adversely affected the growth of aerobic microorganisms, and
enhanced anaerobic microorganisms’ activity. However, a deeper
groundwater level also inhibits the growth of aerobic microbes that
relies on water for metabolism (Liu et al., 2015). Consequently, soil
CO2 concentration had a quadratic function relation
(P ≤ 0.001) with soil water content (Figure 4A), and soil
CH4 concentration (Figure 5A) were found to be linearly
correlated to soil water content (P ≤ 0.001).
Soil CO2 production depends on crop root growth. Root
respiration represents the metabolism of root cells and respiratory
activity, and higher root biomass showed a higher potential in
increasing autotrophic root respiration (Tomotsune, Yoshitake, Watanabe
and Koizumi, 2013). Meanwhile, greater root residual input provided more
C to rhizospheric microbes, which enhanced C decomposition and
heterotrophic microbial respiration (Wertha and Kuzyakov, 2008). These
findings were validated by the significant positive correlation
(P ≤ 0.001) between the soil CO2 concentration
and root biomass (Figure 6). However, CH4 concentration
did not show a significant correlation with root biomass as
CH4 is not the product of root cell metabolism.
Therefore, CH4 concentration was not significantly
correlated to the growth status of crop root system.
Organic matter and total N in soil were found to be closely correlated
with CO2 and CH4 production. It was
further validated by the positive correlations between
CO2 (Figure 4B–C), CH4 (Figure 5B–C),
and organic matter, total N. Increased organic matter content increased
CO2 and CH4 emissions. Soil
CO2 and CH4 from soil respiration are
derived from microbial decomposition of soil organic matter (Illeris et
al., 2003; Wertha and Kuzyakov, 2008; Li et al., 2013). A higher soil
organic matter resulted in a higher microbial population in the soil.
However, in this study, the correlations of CO2 and
CH4 concentrations with organic matter and total N
contents might be due to the effect of groundwater depth. Previous
studies have demonstrated that the levels of organic matter and total N,
the crucial soil nutrient, were lowest at groundwater depth of 60 cm,
and the enzymatic activities were highest due to the suitable soil
moisture (Zhang, Zhu, Zhou and Li, 2018b). The CO2 and
CH4 concentrations in soil should increase due to the
fast nutrient cycling but decreased near groundwater depth of 60 cm (Liu
et al., 2015; Zhang, Zhu, Zhou and Li, 2018b). It might be due to the
improved soil structure and the enhanced soil aeration conditions (Wang
and Lu, 2006). High level of soil water content can deteriorate soil
structure and make the soil denser to trap CO2 and
CH4, resulting in a high CO2 and
CH4 concentrations in the soil (Yang et al., 2013).
Soil P and K, crucial soil elements involved in protein synthesis,
cation-anion balance, enzyme activation, and so on, are influenced by
the groundwater depth (Kering et al., 2012). However, soil P and K did
not show an apparent effect on soil CO2 concentration
under the experimental conditions (data not shown). Nevertheless,
available K content showed a positive linear correlation (P ≤
0.001) with CH4 concentration (Figure 5D). It might be
due to K+ led inhibition of CH4absorption in soil. The K+ concentration in soil
solution increased osmotic pressure in methane-oxidizing microbial
cells, inhibiting CH4 oxidation, and increasing the
CH4 concentration in soil (Cai, Xu and Ma, 2009).
Soil enzyme activity involves in soil nutrient cycling. However, only
phosphatase was affected by the groundwater depth, while all the three
enzymes (urease, phosphatase and sucrase) were significantly affected by
the fertilization level (Zhang, Zhu, Zhou and Li, 2018b). The
correlation between the CO2 and CH4concentrations with soil enzymatic activities is discussed below.
4.3 Effects of fertilization level
Currently, chemical fertilizer plays a vital role in meeting the
increasing demand for staple grain. Appropriate nitrogen fertilizer
application promotes photosynthesis, a strong root system for higher
nutrient absorption (Olmo et al., 2015; Hirte et al., 2018), thus
increasing dry matter accumulation (Jiang et al., 2008). In this study,
fertilizer application resulted in increased wheat root biomass
(P ≤ 0.001), mean root diameter (P ≤ 0.001), and shoot
biomass (P ≤ 0.01) due to higher nutrients in soil (Table 3) (Ali
et al., 2018; Hirte, Leifeld, Abiven and Mayer, 2018). However,
excessive N application not only increased the resource wastage and
non-point source pollution, but reduced crop root length density,
adversely impacting plant biomass and grain yield (Chen et al., 2018).
Also, fertility deficit certainly decreased photosynthetic activity and
crop efficiency, hindering crop and root system growth (Chen et al.,
2016). These two aspects might explain the increased root length density
in treatment involving normal fertilization application as compared to
the other two fertilization treatments under the experimental conditions
(Figure 2C).
Fertilizers, especially N fertilizers, significantly affected soil
respiration (Zhu et al., 2016; Creze and Madramootoo, 2019). However, as
per the current study, the fertilizer application rate affected the soil
CO2 and CH4 concentrations
insignificantly (Table 3). Nitrogen supplementation inhibits microbial
heterotrophic respiration in soil by suppressing soil microbial biomass
but stimulate root respiration (Wang et al., 2016). However, as per the
previous report, insignificant changes in heterotrophic and soil
respiration after N fertilization application did not affect microbial
biomass significantly (Liu et al., 2015; Zhong, Yan, Zong and Shangguan,
2016). Thus, the precise mechanism for the effect of N fertilizer
application on soil respiration needs further investigation.
Increased total N content led to a decreased C/N ratio and improved soil
CO2 flux (Bellingrath-Kimura et al., 2015; Pires et al.,
2017). Furthermore, soil N/P ratio also influenced autotrophic
respiration and microbial activity. A balanced N/P ratio increased root
biomass accumulation and soil CO2 concentration
(Bellingrath-Kimura et al., 2015; Pires et al., 2017; Sun et al., 2018).
Nitrogen fertilizer application enhanced the root respiration rate by
increasing the availability of soil nutrients, the N content in root,
and photosynthate allocation below the ground (root biomass) (Sun et
al., 2018). This maybe partially resulted in the significant correlation
(P ≤ 0.001) of soil CO2 concentration with total
N content (Figure 4B).
In addition to soil nutrients, the activities of soil
enzymes, i.e., urease (P ≤
0.001), phosphatase (P ≤ 0.01), and sucrase (P ≤ 0.001),
were significantly influenced by fertilization levels (Zhang, Zhu, Zhou
and Li, 2018b) and linearly correlated to soil CO2concentration (Figure 7). Urease, phosphatase, and sucrase in the soil
are mainly secreted by aerobic microbes in the soil and root cells
(Guan, 1986; Wang and Lu, 2006). It might be the reason that
CH4 concentration was not correlated with the soil
enzymatic activities. Soil enzymatic activity can be used as an index of
microbial activity for expressing the soil respiration intensity
(Iovieno, Morra, Leone, Pagano and Alfani, 2009; You et al., 2018).
Catalysis of soil enzymes could accelerate the microbial decomposition
of soil organic matter (Iovieno, Morra, Leone, Pagano and Alfani, 2009;
Xiao et al., 2016). Also, high soil nutrient cycling rate increased the
plant organic matter accumulation and root growth, which in turn
increased the soil root respiration. Thus, the close correlations
between soil CO2 concentration and soil enzymatic
activities might be the outcome of the synergistic effects of fertilizer
level on soil nutrition content, microbial activity, and crop root
growth. However, the contribution of soil enzymatic activities to the
CO2 concentration could be so small that
CO2 concentration was not affected by the fertilization
level (Table 3).
5 CONCLUSIONS
Groundwater depth altered soil moisture and thus significantly affected
root parameters, root-shoot biomass ratio, and the concentrations of
soil CO2 and CH4. The highest root
parameters and root-shoot biomass ratio were observed at the groundwater
depth of 50–60 cm. For CO2 and CH4concentrations, the critical values of gas concentration appeared at the
groundwater depth of 50–60 cm. The significant correlations of nutrient
contents with CO2 and CH4 concentrations
and the positive correlation between CO2 concentration
with root biomass validated the effect of groundwater depth mediated by
soil moisture content and aeration condition.
Fertilization level significantly affected the root parameters and shoot
biomass, and the appropriate application of fertilizer promoted the
growth of crop roots and biomass matter accumulation. Fertilization
levels affected root parameters, soil nutrient, and enzymatic activity,
which were closely related to soil CO2 concentration.
The outcomes of the study showed that CO2 and
CH4 concentrations were independent of fertilization
levels.
In conclusion, crop growth was significantly affected by fertilization
and groundwater depth, and soil respiration was significantly affected
only by groundwater depth. It suggested 50–60 cm as the optimal
groundwater depth for better soil respiration, root growth, matter
accumulation, and distribution in winter wheat crop.
ACKNOWLEDGMENTS
Project supported by the National Key Research and Development Program
of China (2018YFC0406604) and the Special Fund for Agro-scientific
Research in the Public Interest
(201203077).
CONFLICTS OF INTERESTS
The authors have no financial or personal conflicts of interest to
declare.
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