Keywords Nitrogen and phosphorus leaching - Erhai Lake - Irrigation and fertilization - DFT
calculation
Nutrient loss from arable land, especially in the fragile ecological zone of plateau
lakes, resulting in agricultural surface pollution has become a serious threat to
the ecological environment and sustainable human development. Therefore, elucidating
the mechanism of nutrient loss from arable land in plateau lakes and formulating preventive
and control measures are particularly important for reducing agricultural surface
pollution and promoting the sustainable development of agriculture in plateau lakes
and their ecological security.
Highlights
Optimizing irrigation and fertilization reduced N and P losses of 72.4 and 67.6%,
respectively.
Drip irrigation reduces N and P losses by decreasing the intensity of interflow.
Periodical fertilization contributes to improving its efficiency and crop yield.
Interactions of colloids and H2 O with nutrients contribute to their leaching.
1
Introduction
Nitrogen (N) and phosphorus (P) leaching from croplands has received considerable
attention for its potential impact on ecosystem balance and sustainability, especially
in ecologically sensitive and fragile plateau lakes such as Erhai Lake [1 ], [2 ]. Encouragingly, previous studies have shown that changing the fertilization and
irrigation management of cropland in the Erhai Lake basin may be an effective approach
to mitigating its agricultural non-point source pollution [3 ], [4 ]. However, the differences in optimizing fertilization and irrigation management
measures lead to uncertainty about their effectiveness in reducing nutrient leaching
from cropland [3 ], [4 ]. Additionally, the strong interactions between soil leachate and shallow groundwater
in the Erhai Lake basin may significantly affect the effectiveness of cropland optimization
measures [5 ], [6 ]. Therefore, continuous optimization of irrigation and fertilizer management in the
Erhai Lake basin is crucial to mitigating the risk of nutrient losses from croplands
contributing to non-point source pollution in plateau lakes with low ecological sensitivity.
Additionally, understanding the soil–water interaction during nutrient migration is
essential for refining agricultural water and fertilizer management practices.
Improving the efficiency of water and fertilizer use can lead to a significant improvement
in their use, while reducing losses through leaching and ultimately increasing crop
yield. Previous studies have shown that drip irrigation can not only significantly
reduce irrigation water use but also increase yield while reducing nitrogen and phosphorus
leaching [7 ], [8 ]. In Addition, previous reports have shown that precise optimization of fertilization
and topdressing according to crop growth cycles can significantly reduce N leaching
and improve the efficiency of fertilizer to increase crop yield [9 ], [10 ]. However, the spatial variability in soil properties and climatic conditions leads
to uncertainty in the effectiveness of agricultural management practices. Therefore,
it is essential to conduct field experiments in the Erhai Lake basin to investigate
whether optimized irrigation and precision fertilization can be effective measures
to mitigate nutrient loss in croplands and reduce agricultural non-point source pollution
in Erhai Lake. Additionally, improving the mechanism by which water transport in irrigation
affects nutrient migration will further help us to optimize farmland management measures.
Density functional theory (DFT) calculations have been extensively used to investigate
the interactions and adsorption behaviors of soil substances using various exchange-correlation
functions, including the generalized gradient approximation (GGA) functional [11 ]. Previous reports have shown that DFT can clearly demonstrate the interaction mechanism
between soil and nutrients, as well as the electron transfer, chelation, and complexation
mechanisms of soil amendments on soil material transport [12 ], [13 ], [14 ]. Therefore, the application of DFT calculations is poised to enhance our understanding
of the soil–water interactions during irrigation and their impact on the leaching
and loss of dissolved nutrients. More importantly, it will enable us to continually
optimize cropland management measures on this basis to reduce nutrient leaching from
cropland.
This work seeks to examine the effects of alterations in irrigation on the leaching
characteristics of N and P through field experiments on the eastern shore of the Erhai
Lake basin. Furthermore, DFT calculations are utilized to elucidate the interactions
between soil and water migration, as well as their influence on the movement of dissolved
N and P within water. These findings contribute to a better understanding of the impact
of irrigation water on the leaching losses of soil N and P and aid in optimizing irrigation
and fertilization management practices in the Erhai Lake basins. Importantly, this
research holds significant implications for mitigating the potential threat of agricultural
non-point source pollution to the ecologically sensitive plateau lakes.
2
Results and Discussion
2.1
Optimization of Irrigation and Fertilization Contributes to the Reduction of Nutrient
Losses from Cropland
Optimization of irrigation and fertilization practices significantly reduces nutrient
concentrations in soil leachate ([Figs. 1 ], [S1 ], and [S2 ]). Total N (TN) concentrations decreased significantly with increasing soil depth.
TN concentrations in W2 are significantly lower than in W1 , and they decrease with increasing fertilization frequency (T2 –T4 , [Fig. 1a ]). It is worth noting that the TN concentration (67.74 mg L−1 ) in W2 T4 in the 20-cm soil layer is 206.72 mg L−1 lower than that in the control (CK) (274.46 mg L−1 ). Changes in TDN (total dissolvable nitrogen, accounting for over 90% of TN) with
changing irrigation and fertilization practices are similar to those in TN ([Fig. 1b ]). TDN concentration in W2 T4 (64.34 mg L−1 ) in the 20 cm soil layer is 195.29 mg L−1 lower than in CK (259.63 mg L−1 ). Correspondingly, changes in nitrate nitrogen (NO3
− –N), nitrite nitrogen (NO2
− –N), and ammonium nitrogen (NH4
+ –N) are similar to those in TN and TDN ([Figs. 1c ] and [S1d ],[e ]).
Figure 1 Concentrations of (a) TN, (b) TDN, and (c) NO3
− -N in leachates at varying depths (0–20, 20–40, and 40–60 cm) within fields. The letters
“a ”, “b ”, and “c ” denote significant differences in concentration at different depths. The symbols
“**” and “***” indicate significant differences at levels of “p < 0.01” and “p < 0.001” between CK and the experimental treatments, respectively. These findings
are derived from the eighth leachate sample collected in the fields, with a sampling
interval of 15 days.
Total P (TP) concentration decreases with increasing soil depth and reaches 0.91 mg
L−1 in CK in the 20-cm soil layer, which is three times greater than in W2 T4 ([Fig. 2a ]). TDP concentrations decrease significantly with increasing fertilization frequency
and decreasing irrigation. The concentration of TDP in W2 T4 (0.21 mg L−1 ) in the 20-cm soil layer is 0.52 mg L−1 lower than that of CK (0.73 mg L−1 , [Fig. 2b ]). PO4
3− –P concentration in leachates in the 20-cm soil layer showed a similar distribution
with TDP and TP ([Fig. 2c ]), but there is a higher concentration of PO4
3− –P in the 60-cm soil layer than the one in the 40-cm soil layer in CK and W1 . The soil in cropland in the Erhai Lake basin has a texture of sandy silt, with a
high soil porosity [15 ], [16 ]. Implementation of farmland management techniques such as returning straw to the
field and applying organic fertilizers can lead to a notable increase in soil organic
matter (SOM), subsequently enhancing soil porosity and reducing soil bulk density
([Table S1 ]) [17 ], [18 ]. Although high-porosity soils promote gas exchange and nutrient cycling to enhance
crop uptake of soil nitrogen and phosphorus [19 ], [20 ], they also promote water loss during furrow and flood irrigation [21 ], [22 ]. In addition, shallow groundwater in the Erhai basin is capable of frequent material
exchange with soil leachate ([Fig. S6 ]) [15 ], [23 ], which further leads to nutrient leaching from deeper soils [23 ], [24 ]. On the contrary, the reduction of 12.66% to 23.92% in leachate in drip irrigation
indicates its ability to weaken the intensity of soil leaching ([Table S12 ]).
Figure 2 Concentrations of (a) TP, (b) TDP, and (c) PO4
3− –P in leachates at varying depths (0–20, 20–40, and 40–60 cm) within fields. The letters
“a ”, “b ”, and “c ” denote significant differences in concentration at different depths. The symbols
“**” and “***” indicate significant differences at levels of “p < 0.01” and “p < 0.001” between CK and the experimental treatments, respectively. The mark of “ns ” means there is no significant difference between CK and the experimental treatments.
These findings are derived from the eighth leachate sample collected in the fields,
with a sampling interval of 15 days.
Dissolved organic carbon (DOC) concentrations at different soil depths after optimization
of irrigation and fertilization showed significant differences compared to CK ([Figs. 3 ] and [S4 ]). There was a noticeable decrease in DOC concentration with increasing soil depth.
Furthermore, the leachate at a 20 cm soil depth showed a decrease in DOC concentration
with increasing fertilization frequency at the same irrigation ([Fig. 3a ]). The DOC concentration in the 20-cm leachate in the W2 T4 (1.40 mg L−1 ) field was only 46.5% of that in CK (3.01 mg L−1 ). Colloids can hold soluble nitrogen and phosphorus hostage by electrostatic adsorption
and ion exchange of surface charges to influence the transport of soluble nutrients
[25 ], [26 ]. In contrast, drip irrigation avoids colloid migration during diffuse irrigation
to reduce soil nutrient leaching [27 ], [28 ].
Figure 3 Concentrations of (a) DOC and (b) colloid in leachates at varying depths (0–20, 20–40,
and 40–60 cm) within fields. The letters “a ”, “b ”, and “c ” denote significant differences in concentration at different depths. The symbols
“**” and “***” indicate significant differences at levels of “p < 0.01” and “p < 0.001” between CK and the experimental treatments, respectively. These findings
are derived from the eighth leachate sample collected in the fields, with a sampling
interval of 15 days.
To further investigate the effects of alterations in irrigation and fertilization
on the loss of nutrients from croplands, we calculated the total loss of N, P, and
DOC in leachates collected using ceramic suction cups ([Fig. 4 ]). TN losses in W2 T4 (800.43 mg) have a reduction of 72.4% compared to CK (2904.58 mg, [Fig. 4a ]). Furthermore, the cumulative total dissolvable nitrogen (TDN) and NO3
− –N losses in W2 T4 were 764.02 and 677.00 mg, accounting for 72.2 and 69.9% of CK, respectively. The
TP losses after optimization of irrigation and fertilization are ranging from 24.2
to 67.6% of CK (11.16 mg) ([Fig. 4b ]). The reductions of TDP losses in treatments ranged from 29.2 to 66.5% of CK. Notably,
irrigation W1 exhibited a comparable reduction in PO4
3− –P loss, ranging from 76.7 to 77.8% compared to CK. The reduction in DOC loss decreases
with increasing fertilization frequency and decreasing irrigation ([Fig. 4c ]). There was a reduction of 72.6% of DOC loss in W2 T4 (67.73 mg) compared to CK (247.24 mg).
Figure 4 Accumulated losses of (a) N, (b) P, and (c) DOC and colloid of leachates in fields.
The symbol of “***” indicates the significant differences at levels “p < 0.001” between CK and the experimental treatments.
DFT calculations were employed to explore the mechanisms by which irrigation affects
nutrient loss. Adsorption energy can serve as a direct indicator of the strength of
interactions between substances, as determined through the computation of molecular
models ([Fig. 5 ]). The optimized structure of SiO2 –H2 O–NH4
+ ([Fig. 5a ]) exhibits a higher concentration of NH4
+ ions compared to the optimized structure of SiO2 –H2 O–PO4
3− ([Fig. 5b ]), but a lower concentration compared to the optimized structure of SiO2 –H2 O–NO3
− ([Fig. 5c ]). The adsorption energy between SiO2 and NO3
− (−25.06 eV) is 12.83 eV higher than that between SiO2 and NH4
+ (−12.23 eV), but notably lower than that between SiO2 and PO4
3− (−44.07 eV, [Fig. 5d ]). The adsorption energies of H2 O adsorbed on SiO2 surpass those of solutes adsorbed on SiO2 (i.e., NH4
+ , NO3
− , and PO4
3− ). According to DFT calculation, the adsorption energies of colloids with NO3
− , NH4
+ , and PO4
3− are −34.31, −37.64, and −219.21 eV, respectively, which are 2.8, 1.3, and 5.0 times
greater than the adsorption energies of soil and NO3
− , NH4
+ , and PO4
3− ([Table S13 ]). This is attributed to the strong interaction between colloids and dissolved N
and P through mechanisms such as hydrogen bonding, ion exchange, and electrostatic
adsorption [25 ], [26 ]. Additionally, H2 O molecules enhance their interaction with solutes through electrostatic interactions
and electron transfer, thereby increasing their ability to hold nutrient ions adsorbed
on the SiO2 surface. In contrast, drip irrigation contributes to the reduction of soil–water
content and leaching intensity to mitigate the downward force relay of H2 O on solutes during downward solute migration and subsequently avoids soil nutrient
leaching [29 ], [30 ].
Figure 5 Molecular models of (a) crystal face (011) of SiO2 and H2 O–NO3
− solution, (b) crystal face (011) of SiO2 and H2 O–NH4
+ solution, and (c) crystal face (011) of SiO2 and H2 O–PO4
3− solution. (d) Adsorption energy of H2 O adsorbed by SiO2 and solutes adsorbed by SiO2 , respectively. The solutes include NO3
− , NH4
+ , and PO4
3− .
EDDs of the molecular models were used to further investigate the influence of irrigation
water on the movement of nutrients (i.e., NH4
+ , NO3
− , and PO4
3− , [Fig. 6 ]). The blue region indicates a decrease in electrons, while the red region indicates
an increase in electrons. The prominent red region around O in NO3
− and the blue region around H in H2 O indicate electronic migration from H to O ([Fig. 6a ]). In the NH4
+ solution, there is a red region around N and a blue region around H, indicating electron
transfer from N in NH4
+ to H in H2 O ([Fig. 6b ]). Similarly, there is electron transfer from H in H2 O to O in PO4
3− ([Fig. 6c ]). Furthermore, the adsorption energy between H2 O and NH4
+ (−4.91 eV) is comparable to that between H2 O and NO3
− (−4.97 eV), but significantly lower than that of PO4
3− (−15.70 eV, [Fig. S5 ]).
Figure 6 Patterns of EDDs in the solutions of (a) H2 O and NO3
− , (b) H2 O and NH4
+ , and (c) H2 O and PO4
3− .
2.2
Changes in Crop Yield Following the Optimization of Irrigation and Fertilization
To investigate the effects of alterations in irrigation and fertilization on crop
growth, we conducted measurements on crop height and 100-grain weight ([Fig. 7 ]). The results indicate a clear decrease in crop height as the volume of drip irrigation
is reduced and fertilization frequency is increased ([Fig. 7a ]). The tallest crop was observed in W1 T2 , reaching a height of 172.15 cm, which is 4.8 cm taller than CK. Conversely, the
shortest crop height was recorded in W2 T4 (159.32 cm), which is 8.0 cm shorter than the control group. The highest 100-grain
weight was observed in the W2 T4 treatment group (34.94 g), closely followed by the 100-grain weight of W1 T4 (33.98 g, [Fig. 7b ]), they are 11.3 and 8.2% heavier than those of the control group, respectively.
Notably, the 100-grain weight generally increases with a higher fertilization frequency.
Drip irrigation, in comparison to ditch irrigation, can reduce the speed and extent
of nutrient transport during the irrigation process, resulting in a greater retention
of soil nutrients in the surface soil [31 ], [32 ]. The hydrophilic and fertilizer-orientated nature of the plant root system promotes
its lateral development during drip irrigation to enhance nutrient uptake, and the
high-velocity flow of water in furrow irrigation disrupts the soil structure, leading
to stripping and loss of topsoil to reduce soil fertility and crop growth [33 ], [34 ], [35 ]. However, it is difficult for the crop’s laterally growing root system to effectively
utilize the nutrients in the deep soil to meet the growth and development of maize
at the elongation stage, which subsequently limits the height of the maize plant ([Fig. S6 ]) [36 ], [37 ]. Reducing irrigation intensity contributes to avoiding leaching of the soil by the
leaching solution to enhance the retention of N-containing ions in the soil [38 ], [39 ]. It will help to stimulate soil nutrient cycling and crop root development to increase
crop yields [40 ], [41 ]. There are significant differences in the soil nutrient requirements of maize at
different periods [42 ], [43 ], and multiple fertilizer applications can avoid the problems of the uncoordinated
release of fertilizer with the growth period and excess nutrients in a single application
[42 ], [43 ]. At the same time, proper application of fertilizer can effectively stimulate soil
microbial activity to improve nutrient uptake by the crop [40 ], [41 ], [44 ].
Figure 7 (a) Crop height and (b) 100-grain fresh weight in different experiment plots. The
letters “a”, “ab”, and “b” indicate the significant differences between CK and the
experimental treatments.
3
Conclusions
A field experiment to optimize irrigation and fertilization was conducted to study
their effects on N and P losses in farmland, and DFT calculation was employed to reveal
the effects of irrigation-induced interflow on N and P losses. Optimized irrigation
and fertilization have a significant impact on nutrient retention and crop productivity
in the agricultural areas of the Erhai Lake basin. Quantitative drip irrigation (84 m3 ha−1 per 6 days) and staged fertilizer application (elongation and 11-leaf stage) were
effective in reducing N and P losses by 25.2–72.4% and 24.2–67.6%, respectively, while
increasing yields by 11.3%. Furthermore, the DFT calculation clearly showed that the
switching from ditch irrigation to drip irrigation significantly reduced the movement
of nitrogen and phosphorus nutrients carried by irrigation water, thereby increasing
the soil’s nutrient retention capacity. These findings may further expand the current
understanding of the effects of irrigation on nutrient movement and have significant
implications for refining agricultural management practices in the Erhai Lake basin
and increasing crop yield. Importantly, these measures are critical to mitigating
nutrient loss from farmland at its source and addressing the threat of non-point source
pollution from agriculture in plateau lakes.
4
Materials and Methods
4.1
Site Description and Experimental Design
The field experiment was conducted from May 2023 to July 2023 in an agricultural area
in the Erhai Basin of Yunnan Province in southwestern China, which has a meso-subtropical
southwestern monsoon climate. The area experiences an average annual temperature of
15.5°C, with an average maximum temperature of 22.2°C and an average minimum temperature
of 10.2°C. The annual rainfall ranges from 1000 to 1100 mm, with 95% of the rainfall
occurring during the rainy season from May to October. The topsoil typically exhibits
higher nutrient concentrations compared to the subsoil, as indicated in [Table S1 ] detailing the soil properties. The soil in field plots has a texture of sandy silt,
with a bulk density of 1.09, 1.23, and 1.34 g kg−1 at depths of 0–20, 20–40, and 40–60 cm, respectively.
The amount of fertilizer applied during the maize season was 150 kg N ha−1 , 90 kg ha−1 for P2 O5 , and 36 kg ha−1 for K2 O. The application rates for the basic fertilizers N, P2 O5 , and K2 O were 6, 6, and 8 kg ha−1 , respectively. The timing and quantity of topdressing were adjusted according to
corn growth cycle while maintaining a constant total fertilizer rate. This included
CK (T1 , topdressing at the 11-leaf stage), T2 (topdressings at the elongation and 11-leaf stages, respectively), T3 (topdressings at the elongation, 11-leaf, and heading stages), and T4 (topdressings at the elongation, 11-leaf, heading, and maturity stages). The topdressing
method used a pressure differential fertilizer tank for integrated fertilization of
water and fertilizer. The irrigation treatments consisted of CK (furrow irrigation
every 6 days), W1 (drip irrigation every 6 days at 105 m3 ha−1 ), and W2 (drip irrigation every 6 days with 84 m3 ha−1 ). Consequently, the experimental treatments included CK, W1 T2 , W1 T3 , W1 T4 , W2 T2 , W2 T3 , and W2 T4 , with three parallel plots (each with an area of 25 m2 per plot) for each treatment. Other field management measures were carried out according
to local conventional management practices.
4.2
Sampling and Test
A soil profile was excavated in the experimental area to investigate the physicochemical
properties of the soil at different depths, as detailed in [Table S1 ]. After collection, samples from different soil layers were immediately sealed in
plastic bags and stored at 4°C until extraction of ammonium and nitrate. Nitrogen
and phosphorus concentrations in the soil samples were determined using an Auto Analyzer-AA3
(SEAL, Germany). Additionally, periodic soil samples were taken from the different
depths of the profile for soil bulk density measurements [6 ].
Leachate from different soil layers (0–20, 20–40, and 40–60 cm) within each experimental
plot was collected using ceramic suction cups [45 ], [46 ]. The leachate in the ceramic suction cup was monitored and measured every 15 days.
The leachates were then stored at 4°C and analyzed within 48 h. An alkaline potassium
persulfate solution was used to digest the leachate, followed by filtration of samples
through a 0.45-μm membrane for the analysis of total N (TN) using an Auto Analyzer-AA3
(SEAL, Germany) [6 ]. Similarly, concentrations of other nutrients, including total P (TP), total dissolvable
phosphorus (TDP), PO4
3− –P, NO3
− –N, NH4
+ –N, and DOC, were determined using an Auto Analyzer-AA3 (SEAL, Germany) after the
pretreatment of the leachates [47 ], [48 ]. Additionally, five maize plants were randomly selected from each experimental plot,
and their plant height and 100-grain weight were measured to assess the effects of
the different treatments on maize growth and yield.
4.3
Data Analysis
The hydraulic conductivity of soil refers to the volume of water that will move through
a given area over a given time period and under a specific water potential gradient
when the soil is fully saturated with water [49 ]. This property is influenced by soil texture, bulk density, and the distribution
of pores within the soil. The spatial distribution of soil texture, bulk density,
pore distribution, and organic matter content plays an important role in determining
the spatial variability of saturated hydraulic conductivity (Ks
) [50 ]. The formula for calculating hydraulic conductivity is as follows:
K
s
=
10
QL
ΔhAt
where Q is the stable infiltration volume (cm3 ), and L is the length of the cutting ring (cm). The letters h , A , and t indicate the height of the water head (cm), the cross-sectional area of the cutting
ring (cm2 ), and the time interval, respectively. Furthermore, due to the predetermined suction
range of the ceramic suction cup within the soil layer, it is possible to calculate
the reduction rate (Rt
) of nutrient losses per hectare (ha) in the experimental treatments compared to the
control group (CK):
R
t
=
1000
h
V
c
V
ck
C
ck
−
1000
h
V
c
V
t
C
t
1000
h
V
c
V
ck
C
ck
where Vc
is the suction range of the ceramic suction cup. Vck
and Cck
are the leachate volume and its nutrient concentration in the ceramic suction cup
in CK, respectively. Vt
and Ct
mark the leachate volume and its nutrient concentration in the ceramic suction cup
in treatments, respectively. The h means the soil depth. Therefore, the above equation can be simplified as follows:
R
t
=
T
ck
−
T
t
T
ck
where Tck
and Tt
indicate the accumulated N and P losses from leachates in CK and treatments.
Analysis of variance (ANOVA) was used to illustrate the notable differences between
treatments using SPSS software (version 27.0, Inc., USA). In addition, Origin software
(version 2018, USA) was utilized to compute the mean and standard deviation ([Tables S2–S11 ]) and to generate graphical representations.
4.4
DFT Computation
A study conducted by Song et al. suggests that the primary constituent of soil in
the Erhai Lake basin is alpha quartz (α-SiO2 ) with a dominant crystal face (011), as determined by X-ray diffraction (XRD) analysis
[51 ]. Consequently, we constructed molecular models of soil based on SiO2 with a crystal face (011) and studied the soil–water interactions and their effects
on the levels of dissolved N and P. To investigate the variances in the adsorption
of H2 O and solutes by soil, we used molecular dynamics calculations using the GGA and the
all-electron double-numeric plus polarization (DNP) functional [52 ]. The adsorption capacity between soil and water, as well as nutrient solutes, was
evaluated using calculated adsorption energy (ΔE
ads ).
∆
E
ads
=
E
ab
−
E
a
+
E
b
where Esw
is the total energy of the molecular models for the mixed system of SiO2 –H2 O and H2 O-solutes, the Ea
and Eb
indicate the total energy of the independent molecular models for SiO2 , H2 O, and solutes, respectively. The electronic transfer between H2 O and nutrient solutes, such as NO3
− and PO4
3− , was investigated using the electron density difference (EDD) method, with a self-consistent
field (SCF) tolerance level set at 1 × 10−6 eV atom−1 .
Bibliographical Record Debo He, Yunfeng Wang, Yinlin Zang, Tao Wang, Bo Zhu. Optimizing Irrigation and Fertilization
Contributes to Mitigating Nutrients Leaching While Improving Crop Yield: Insights
From a Field Experiment and Density Functional Theory Calculation. Sustainability
& Circularity NOW 2024; 01: a24196203. DOI: 10.1055/a-2419-6203