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\begin{document}

\title{Long-term trajectories of nutrient budgets and changes in soils of two
counties with contrasting land use changes: cereals vs fruit orchards}



\author[1]{Shimao Wang}%
\author[1]{Xiaowei Yu}%
\author[1]{Yucheng Xia}%
\author[1]{Jingbo Gao}%
\author[1]{Zhujun Chen}%
\author[2]{Gurpal S. Toor}%
\author[1]{Jianbin Zhou}%
\affil[1]{Northwest A\&F University College of Natural Resources and Environment}%
\affil[2]{University of Maryland Department of Environmental Science and Technology}%


\vspace{-1em}



  
  \date{June 10, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Land use change (LUC) is a key factor affecting nutrient budgets in
agricultural soils. Comparing the long-term trajectories of nutrient
budgets and changes in soils at a regional scale with contrasting LUC is
critical to optimizing nutrient management and minimizing adverse
environmental effects. We investigated the nutrient budgets and changes
in soils in two counties in the south Loess Plateau, China, with
contrasting LUCs from 1992 to 2017. Wugong County has cereals as the
main crop, whereas Meixian County has the main cereal crops changed to
kiwi orchards. We found that nitrogen (N), phosphorus (P), and potassium
(K) inputs in two counties increased rapidly, and the nutrient outputs
by crop harvests remained relatively stable. This resulted in increasing
nutrient surpluses in the soils of two counties. Nutrient surplus in the
orchard-dominated county was higher than that of the cereal-dominated
county, and nutrient use efficiencies were contrasting. High N surplus
in the orchard-dominated county resulted in high nitrate-N accumulation
in deeper soil profiles of orchards. High P and K surpluses in the two
counties significantly increased available P and K in 0-20 cm depth.
Soil available P and K in the orchard-dominated county were
significantly higher than in the cereal-dominated county, which was also
significantly higher than the threshold values of available P and K
contents in soils. We conclude that comprehensive measures should be
taken to control nutrient surpluses, which will help to balance nutrient
inputs and outputs and minimize nutrient losses in intensive
horticultural crop systems.%
\end{abstract}%



\sloppy


\textbf{Cover page}

\textbf{Type of contribution:} Research paper

\textbf{Date of preparation:} Mar 2, 2024

\textbf{Number of text pages:} 26

\textbf{Number of tables:} 0

\textbf{Number of figures:} 6

\textbf{Title:}

\textbf{Long-term trajectories of nutrient budgets and changes in soils
of two counties with contrasting land use changes: cereals vs fruit
orchards}

Shimao Wang \textsuperscript{1, 2}, Xiaowei Yu \textsuperscript{1, 2},
Yucheng Xia \textsuperscript{1, 2}, Jingbo Gao \textsuperscript{1, 2,
3}, Zhujun Chen \textsuperscript{1, 2}, Gurpal S.
Toor\textsuperscript{4}, Jianbin Zhou \textsuperscript{1, 2*}

1 College of Natural Resources and Environment, Northwest A\&F
University, Yangling, Shaanxi 712100, China

2 Key Laboratory of Plant Nutrition and the Agri-Environment in
Northwest China, MOA, Yangling, Shaanxi 712100, China

3 College of Agronomy and Life Sciences, Shanxi Datong University,
Datong Shanxi 037009, China

4 Nutrient Management and Water Quality Group, Department of
Environmental Science and Technology, University of Maryland, College
Park, MD, 20742, USA

\textbf{*Corresponding author:}

Jianbin Zhou

Email: \emph{jbzhou@nwsuaf.edu.cn}



\textbf{Abstract}

Land use change (LUC) is a key factor affecting nutrient budgets in
agricultural soils. Comparing the long-term trajectories of nutrient
budgets and changes in soils at a regional scale with contrasting LUC is
critical to optimizing nutrient management and minimizing adverse
environmental effects. We investigated the nutrient budgets and changes
in soils in two counties in the south Loess Plateau, China, with
contrasting LUCs from 1992 to 2017. Wugong County has cereals as the
main crop, whereas Meixian County has the main cereal crops changed to
kiwi orchards. We found that nitrogen (N), phosphorus (P), and potassium
(K) inputs in two counties increased rapidly, and the nutrient outputs
by crop harvests remained relatively stable. This resulted in increasing
nutrient surpluses in the soils of two counties. Nutrient surplus in the
orchard-dominated county was higher than that of the cereal-dominated
county, and nutrient use efficiencies were contrasting. High N surplus
in the orchard-dominated county resulted in high nitrate-N accumulation
in deeper soil profiles of orchards. High P and K surpluses in the two
counties significantly increased available P and K in 0-20 cm depth.
Soil available P and K in the orchard-dominated county were
significantly higher than in the cereal-dominated county, which was also
significantly higher than the threshold values of available P and K
contents in soils. We conclude that comprehensive measures should be
taken to control nutrient surpluses, which will help to balance nutrient
inputs and outputs and minimize nutrient losses in intensive
horticultural crop systems.

\textbf{Keywords:} Land use change; Nutrition use efficiency; Nutrient
surplus; Nitrate accumulation; Soil nutrients; Kiwifruit orchards;



\textbf{1. Introduction}

Synthetic fertilizers are indispensable in feeding the rapidly
increasing global population (Erisman et al., 2008) and dramatically
changing the global biogeochemical cycles of nitrogen (N) and phosphorus
(P) (Britz and Hertel, 2011; Steffen et al., 2015; Penuelas et al.,
2020). For example, N inputs in global cropping systems increased
fivefold from 1961 to 2015 (Galloway et al., 2008; Lassaletta et al.,
2023). High nutrient surpluses (the differences between nutrient inputs
and outputs) lead to a rapid decline in nutrient use efficiency (NUE)
(Ma et al., 2012; Chadwick et al., 2015), resulting in a series of
environmental problems, such as eutrophication, soil acidification,
biodiversity loss, and global warming (Bouwman et al., 2009; Mu et al.,
2016).

The application of mineral fertilizers increased rapidly in Europe and
North America after World War II (Krausmann et al., 2013; Billen et al.,
2021), which resulted in increased food production at the cost of
polluting surface and groundwater (Haygarth et al., 2014; Ilampooranan
et al., 2019). The adverse effects on water quality of the application
of nutrients were first recognized in Western European countries (Rivett
et al., 2008; Smith et al., 2017; Sabo et al., 2019; Sanchez et al.,
2023). For example, 80\% of European surface waters exceeded the
European Commission's drinking water standard (50 mg
NO\textsubscript{3}L\textsuperscript{-1}) (Mol\selectlanguage{ngerman}énat and Gascuel Odoux,
2002). In England and Wales, 28\% of rivers exceeded the N concentration
of 30 mg NO\textsubscript{3} L\textsuperscript{-1}, and 52\% of the
total river length exceeded the P concentration of 0.1 mg
L\textsuperscript{-1}(Cherry et al., 2008). To reduce nutrient surpluses
and minimize environmental pollution, a series of regulations and laws
have been introduced, such as the Clean Water Act in the United States
(EPA, 1972), the Nitrate Directive (1991/676/EC) and the Water Framework
Directive (2000/60/EC) in Europe (Grant and Blicher-Mathiesen, 2004;
Zhao et al., 2022). Implementing these regulations and laws has
successfully curbed net N input by reducing fertilizer usage and
nutrient surpluses, increasing NUE (Lutz et al., 2022). The average
gross soil N balance of the Tagus Nitrate Vulnerable Zone in Portugal
decreased significantly from 125 kg ha\textsuperscript{-1} in 1989 to 63
kg ha\textsuperscript{-1} in 2016 (Cameira et al., 2019). However,
implementing such mitigation regulations does not always lead to
immediate or clear responses in improving surface water and groundwater
quality (Tian et al., 2020; Nguyen et al., 2022). One of the reasons
behind such failure is considered the legacy effect from historical
nutrient accumulation in soils and aquifers. Simply managing yearly
nutrient balances is unlikely to improve water quality; rather, many
factors must be considered, including soil and groundwater storage
capacity and gaseous loss pathways (Metson et al., 2020). Therefore,
understanding the long-term trajectories of nutrient budgets in soils
and their fates at the regional scales is key for managing nutrients
efficiently and minimizing their negative environmental impacts.

Applying mineral fertilizer in emerging economies, such as China and
India, is about two to three decades later than in developed countries
(Lu and Tian, 2017; Yu et al., 2019; Bijay, 2023). As expected, the
negative effects of nutrient losses from fertilizers, livestock, and
domestic sources on water quality also occurred late (Zhang et al.,
1995; Liu et al., 2001). It is reported that nitrate in more than half
of the groundwater sampling sites in northern China Plain exceeded 50 mg
L\textsuperscript{-1} in the 1990s (Zhang et al., 1996). Meanwhile,
anomalously higher values of nitrate than the background in the Indian
watershed (Pawar and Shaikh, 1995). In recent years, some emerging
countries have taken measures to control environmental pollution caused
by nitrate pollution (Xin et al., 2021). However, the mitigation
regulations to control fertilizer usage and nutrient surpluses in
emerging economies are still in the early stages. The population in
developing countries is still high. Therefore, optimizing the beneficial
role of nutrients in sustainable crop production to feed the population
and minimizing environmental risks of excessive accumulation of soil
nutrients is a major challenge.

Nutrient budgeting approaches is a useful and simple tool for assessing
potential environmental risks from nutrient pollution by identifying
primary sources of nutrient loading as a means to develop more targeted
nutrient management practices (Chen et al., 2021; Kuestermann et al.,
2010; Grieger and Harrison, 2021; Lim et al., 2021). Land use change
(LUC) is one of the key forces driving the nutrient inputs (Zhi and Li,
2020; Suennemann et al., 2021). Dramatic LUCs have occurred across
mainland China since the early 1990s (Zhang et al., 2009; Gao et al.,
2021). Large areas of cereal lands have been converted into orchards
because of lucrative economic benefits. Compared with cereals, the
problem of excessive fertilization of orchards is particularly
prominent, resulting in a large accumulation of soil nutrients (Gao et
al., 2012; Zhou et al., 2016; Wang et al., 2017; Min et al., 2018). Soil
nitrate storage within 6 m depth ranged from 5039-14,693 kg
ha\textsuperscript{-1}, 4.7-11.7-fold higher than under cereal on field
scale on the Loess Plateau of China (Ren et al., 2023). The total
nitrate stock in the 0-10 m soil profile of the orchards was 3.7 times
higher than that of the croplands in the Guanzhong Plain (Niu et al.,
2022). These studies focused on nutrient accumulation at the field
scales. The nutrient accumulation at the county scale studies indicated
that the nitrate accumulation in soil profiles is the main fate of N
surplus at both Luochuan and Liquan counties dominated by orchards on
the Loess Plateau (Zhu et al., 2021; Miao et al., 2023). However, no
study has compared LUC from cereals to orchard systems at the county
scale. Comparative research in different cropping systems at a regional
scale will provide a clearer understanding of the impact of LUC on
differences in nutrient surplus and their fates in different
agricultural systems.

Therefore, we chose two neighboring counties (Wugong and Meixian) in the
southern part of the Loess Plateau in Shaanxi, with different LUCs since
the 1990s. The main crops in Wugong County are still cereals (winter
wheat and summer maize), whereas land use in Meixian County has changed
from cereals to kiwifruit orchards since the 1990s. This provides an
ideal case study to compare the nutrient surpluses and their fates at
the county scale between traditional cereal-dominant and
horticultural-dominant counties. Our objectives were to (1) quantify the
changes in the annual nutrient inputs and surpluses at the county scale,
(2) determine the changes in soil surface nutrients under different
land-use patterns, (3) investigate soil nitrate-N accumulation under
different tree ages, and N fate under different land use patterns in two
counties over the past decades.

\textbf{2. Materials and methods}

\textbf{2.1 Study Region}

The two study counties, Wugong (34°12\selectlanguage{english}'-34deg26\selectlanguage{english}' N, 108deg0\selectlanguage{english}'-108deg26\selectlanguage{english}' E) and
Meixian (33deg59\selectlanguage{english}'-34deg19\selectlanguage{english}' N,107deg39\selectlanguage{english}'-108deg00\selectlanguage{english}' E) in Shaanxi, are located in
the western Guanzhong Basin, the south edge of Loess Plateau in China.
The region is typical of a warm-temperate sub-humid continental monsoon
climate with a mean annual temperature of 13 \selectlanguage{english} and average annual
precipitation of 630 mm, with 60-80\% occurring between July and
September.

The study area has a more than 2,000 years-long agricultural history and
is a very intensive agricultural area in China now. The main crops in
Wugong and Meixian counties before 1992 were winter wheat and summer
maize, with two harvests each year (Fig. 1). Kiwifruit production
developed rapidly in Meixian County since 1995. In 2017, the kiwi
orchard area increased to 15688 ha, accounting for 64 \% of the total
cropland area. The dominant soil textures include loam, sandy loam, and
loamy sand (Gao et al\emph{.} , 2021).

Winter wheat and summer maize are still the main crops in Wugong. It
provided an ideal case to study the effects of LUCs on nutrient
surpluses and accumulations in soils at the county scale. The total
arable land of Wugong County is 31369 ha. The topography of Wugong
County consists of three types: the frontal zone of the piedmont
alluvial fan, the loess platform and valley alluvial terraces. There are
six soil types in the county: cinnamon soil, loess soil, flavor-aquic
soil, swamp soil, paddy soil, and silt soil. High-quality soils such as
loess soil, account for 94.3\%.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Fig. 1} Changes of agricultural land area in various crops from
1992 to 2017 in Wugong and Meixian counties.

\textbf{2.2 Study methods}

To estimate the input and output of nutrient content, the nutrient
system is regarded as a black box. Annual statistics for Meixian and
Wugong have been published since 1990 by the Agriculture Bureaus of both
counties. Data from the yearbooks from 1992 to 2017 were used, including
different crops in these land uses (e.g., kiwi, wheat, maize, beans,
rapeseed, vegetables, melon, and apple), synthetic N, P, and K
fertilizer inputs, numbers of livestock, and population.

\textbf{2.2.1 Fertilizer input}

The nutrient input in the chemical fertilizer is converted according to
the pure nutrient. Fertilizer inputs include elemental fertilizer and
compound fertilizer, the proportion of compound fertilizer referenced in
Li and Jin. (2011) included the proportion of the northwest region.

The fertilizer input is calculated as follows:

Input\textsubscript{fer} = \selectlanguage{greek}α \selectlanguage{ngerman}× F\textsubscript{ef} + \selectlanguage{greek}β \selectlanguage{ngerman}×
F\textsubscript{cf} (1)

Where

Input\textsubscript{fer} pure nutrient input of fertilizer for the
county (kg);

\selectlanguage{greek}α \selectlanguage{english}nutrient content of elemental fertilizer (\%);

F\textsubscript{ef} inputs of elemental fertilizer N, P or K (kg);

\selectlanguage{greek}β \selectlanguage{english}N, P or K ratio of compound fertilizer (\%);

F\textsubscript{cf} inputs of compound fertilizer N, P or K (kg).

\textbf{2.2.2Manure input}

Manure inputs, which we define as wastes excreted during livestock and
human production. Manure inputs were calculated based on the reported
number of heads of livestock and head per capita, population, and the
product of the manure content produced per head of livestock per year.
Nutrients in human feces are estimated by adult numbers, and the total
population multiplied by 0.85 equals the number of adults. Additionally,
we considered the collection utilization of excreta (pigs, cattle,
sheep, poultry and humans) (Le No\selectlanguage{ngerman}ë et al\emph{.} , 2018). See Table S1
for additional details and parameters used.

The manure input is calculated as follows:

Where

Input\textsubscript{man} = i × E\textsubscript{d} × N\textsubscript{d} ×
C\textsubscript{d} + NL\textsubscript{i} × E\textsubscript{u} ×
N\textsubscript{u} × C\textsubscript{u} (2)

Input\textsubscript{man} pure nutrient input of manure for the county
(kg);

i species of livestock;

NL\textsubscript{i} number of livestock and poultry (head);

E\textsubscript{d}, E\textsubscript{u} fecal excretion and urine
excretion (kg head\textsuperscript{-1} d\textsuperscript{-1});

N\textsubscript{d}, N\textsubscript{u} Fecal and urine nutrient content
(\%);

C\textsubscript{d}, C\textsubscript{u} Collection utilization (\%).

\textbf{2.2.3 Wet, dry atmospheric deposition and symbiotic
N\textsubscript{2} fixation}

Atmospheric N deposition from agricultural land was estimated by
multiplying the sown area by the atmospheric N deposition rate of the
corresponding area (Hong et al., 2013). Two main data sets were used to
calculate N deposition over the study period 1992-2017: (1) the linear
relationship between N deposition rate and year in Northwest China from
1992 to 2010 (Liu et al., 2013); (2) we calculated the atmospheric
deposition of N in Northwest China from 2010-2017 following the
long-term trends identified by (Fu et al., 2020) and (Wen et al., 2020).
Symbiotic N fixation in cropland was estimated based on the relationship
between N fixation and grain yield. (Anglade et al., 2015).

Atmospheric P and K deposition was based on Li and Jin. (2011). We
assumed a mean distribution across geographic regions and used the same
P and K deposition rates throughout the study period.

\textbf{2.2.4 Straw return}

The straw-returned crops included wheat, maize, soybean and rapeseed.
The annual straw returned to the field was calculated based on the
annual production of different crops, the ratio of straw to grain
production and the rate of straw returned to the field, and then
multiplied by its N, P and K content to calculate the inputs of N, P and
K from straw returned to the field in the crop production system during
the survey period (1992-2017). For additional details and parameters
used in the calculations, see Table S2. The straw return input is
calculated as follows:

Input\textsubscript{sr} = C\textsubscript{y} × S\textsubscript{g} ×
S\textsubscript{n} × R (3)

Where

Input\textsubscript{sr} nutrient input of straw return for the county
(kg);

C\textsubscript{y} crop yields (kg);

S\textsubscript{g} straw-to-grain yield ratios (\%);

S\textsubscript{n} nutrient content in straw (g kg\textsuperscript{-1});

R ratio of straw to field (\%).

\textbf{2.2.5 Crop uptake}

Total nutrients were estimated by multiplying crop production data by
crop-specific nutrient content (Li and Jin, 2011) for the following
crops: maize, wheat, beans, rapeseed, vegetables, melon, kiwi and apple.
We assumed that the nutrient content of each crop remained constant over
time. Table S3 lists the values of the parameters used in this study to
convert crop yield data to crop N, P, and K uptake. The crop uptake
output was calculated as follows:

Output\textsubscript{c} = C\textsubscript{y} × p (4)

Where

Output\textsubscript{c} nutrient output of crop uptake (kg
ha\textsuperscript{-1} yr\textsuperscript{-1});

C\textsubscript{y} different annual crop yields (kg);

p nutrient content in N, P, K of the different crops (\%).

\textbf{2.2.6 N, P and K balances}

The balance was defined as the sum of N inputs through mineral
fertilizers, manure, wet and dry atmospheric deposition, symbiotic
N\textsubscript{2} fixation, straw return and irrigation, minus N output
through crop uptake. The N, P and K balances in cropland are calculated
as follows:

B\textsubscript{c} = Input - Output

Efficiency (\%) = (nutrient output) / (nutrient input) × 100 (5)

Where

B\textsubscript{c} nutrient balance in cropland (kg
ha\textsuperscript{-1} yr\textsuperscript{-1});

Input include fertilizers, manure, wet, dry atmospheric deposition,
symbiotic N\textsubscript{2} fixation and straw return and irrigation
(kg ha\textsuperscript{-1} yr\textsuperscript{-1});

Output include crop uptake (kg
ha\textsuperscript{-1}yr\textsuperscript{-1}).

\textbf{2.3 Estimating nutrient accumulation in soil}

\textbf{2.3.1 Soil sampling and analysis}

Soils were sampled in two different years. Soil nutrient data (0-20 cm)
in 1980 were obtained from the Second National Soil Survey. The number
of sampling data points for total N, available P, and available K are
350, 367, and 112 in Wugong County and 412, 407, and 210 in Meixian
County, respectively. The soil nutrient data for Wugong County was
currently obtained from research studies with others (Li et al., 2016;
Qi et al., 2014). These soil samples were collected in 2018 and 2019 in
Meixian County, consisting of 0-2 m profiles from 10 cereal fields and
38 kiwi orchards. In addition, 22 0-5 m profiles (20 cm intervals) from
orchards were collected in October 2018. For soil sampling from each
field, three sites were randomly selected with relatively flat terraces,
whereas for sloping land, the three sites were selected from the upper,
central, and bottom parts of the field (Lu et al., 2016). Their average
value of three sites for each field was used to represent the sampling
site. The triplicate samples were mixed to obtain a composite sample
from each soil depth and placed in labeled plastic bags, sealed, and
stored immediately at 4 \selectlanguage{english}. In addition, 79 soil samples in Meixian
County were collected at 0-20 cm to determine available P, available K,
and total N.

The soil moisture was determined with the oven-drying method at
105\selectlanguage{english}\textsuperscript{*}C. Soil samples were extracted with 1M KCl (1:10
soil: solution), shaken for 1 h, and then filtered. The nitrate-N
concentrations in the KCl extract samples were determined by a
continuous flow N analyzer (Bran and Luebbe AA3, Norderstedt, Germany).
Soil was air-dried, ground, and passed through 1 mm and 0.15 mm sieves
to measure soil organic matter (SOM), total N (TN), available P and
available K. SOM was measured with the
K\textsubscript{2}Cr\textsubscript{2}O\textsubscript{7} oxidation
volumetric method, TN by the Kjeldahl method, available P (Olsen-P) was
extracted with 0.5 M NaHCO\textsubscript{3}, and available K was
extracted with 1M NH\textsubscript{4}OA\textsubscript{C}.

\textbf{2.3.2 Calculation of nitrate accumulation in soil profile}

The nitrate accumulation in the soil profit was calculated as follow:

Soil nitrate accumulation (kg N ha\textsuperscript{-1}) = (BD x t x c) /
10 (6)

Where

BD soil bulk density (g cm\textsuperscript{-3});

t thickness of the soil layer (cm);

c nitrate content in the soil (mg N kg\textsuperscript{-1});

10 conversion coefficient.

The soil bulk density (BD) of the top 60 cm was obtained by Lu et al.,
(2016) in the same study region; the soil bulk density of the 0-20,
20-40, and 40-60 cm soil layers in the kiwi fruit orchard and cereal
fields were 1.28, 1.37, and 1.38 and 1.37, 1.45, and 1.47 g
cm\textsuperscript{-3}, respectively. The BD of the deep soil layers
below 60 cm were the same as those of 40-60 cm depth because of small
variations in deeper layers (Gao et al., 2021).

The total nitrate storage within soil depths of 0-2 m and 0-5 m over the
entire cereal-orchard system in Meixian and Wugong counties was
calculated. The cumulative amount of nitrate in the county was
calculated by multiplying the cumulative amount of nitrate by the
cultivated area of different crops. The nitrate accumulation is the
average value of soil nitrate accumulation within a depth of 0-2 m or
0-5 m (kg N ha\textsuperscript{-1}). The area of kiwi fruit orchards of
different ages was calculated by cereals converted to kiwi orchards each
year from 1992 to 2017 in Meixian County. The cumulative amount of
nitrate in the county in 1992 was calculated based on 2017.

\textbf{2.4 Statistical analysis}

The significance of differences was evaluated by analysis of variance
using SPSS 17.0 (IBM Corp, Armonk, NY, USA), and the least-significant
difference test was performed to compare the average values at the 1\%
and 5\% levels. All figures were drawn in Origin Pro 8.0 (Origin Lab
Corporation, MA, USA).

\textbf{3 Results}

\textbf{3.1 Nutrient inputs, outputs and surpluses}

Nutrient inputs to croplands of the two counties have increased
gradually since 1992 (Fig. 2), with a greater increase in Meixian than
in Wugong County. In Wugong County, total N, P, and K increased from
303, 77, and 118 kg ha\textsuperscript{-1} in 1992 to 526, 136 and 240
kg ha\textsuperscript{-1} in 2017, with growth rates of 74, 77, and
103\%, respectively. Whereas in Meixian County, total N, P, and K
increased from 464, 71, and 118 kg ha\textsuperscript{-1} to 887, 216,
and 552 kg ha\textsuperscript{-1}, with growth rates of 91, 204, and
368\%. Among various input sources, nutrients from synthetic fertilizers
are the main contributors. Nutrient outputs in the two counties over the
years remained relatively stable, resulting in high nutrient surpluses,
especially in Meixian County.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Fig. 2} Changes in the input, output and surplus of total N, P
and K in Wugong and Meixian counties from 1992 to 2017. The left panel
is for Wugong County and the right panel is for Meixian County.

\textbf{3.2 Nutrient use efficiencies (NUE)}

The NUEs of N, P, and K in the two counties decreased from 1992 to 2007
(Fig. 3). The NUE decrease in Meixian County was higher than that in
Wugong County. NUE of N and P was 45-70\% and 32-49\% in Wugong and
19-35\% and 20-47\% in Mexian, respectively. While the NUE of K in
Wugong and Meixian was 74-137\% and 31-96\%, respectively.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Fig. 3} Changes in NUEs of total N, P and K in Wugong and
Meixian counties from 1992 to 2017.

\textbf{3.3 Total N, available P and available K in soils}

Contents of soil total N, available P and available K in the two
counties increased, and the increase was higher in Meixian County (Fig.
4). Total N in soils of Wugong and Meixian counties increased from 0.8 g
kg\textsuperscript{-1} and 0.9 g kg\textsuperscript{-1} in 1980 to 0.95
g kg\textsuperscript{-1} and 1.21 g kg\textsuperscript{-1} in 2017,
respectively. Available P in Wugong County increased 3.1 times from 9.6
mg kg\textsuperscript{-1} in 1980 to 30 mg kg\textsuperscript{-1} in
2017, whereas in Meixian County, available P increased from 4.9 mg
kg\textsuperscript{-1} in 1980 to 111 mg kg\textsuperscript{-1} in 2017.
Available K in Wugong County increased slowly from 1980 to 2017, from
112 mg kg\textsuperscript{-1} to 159 mg kg\textsuperscript{-1}. For
Meixian County, the available K increased rapidly, from 107 mg
kg\textsuperscript{-1} to 509 mg kg\textsuperscript{-1}.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Fig. 4} Changes of soil nutrients at 0-20 cm soil depth of
Wugong and Meixian counties from 1980 to 2017.

\textbf{3.4 Nitrate accumulation in soil profiles}

Nitrate contents in soil profiles of orchards were significantly higher
than those in cereal fields. The average nitrate contents in 0-2 m soil
profiles of kiwifruit orchards of Meixian County was higher than that in
cereal fields of Wugong and Meixian counties (Fig. 5). The nitrate
accumulation in the 0-2 m soil profiles significantly increased with the
increase in stand ages of kiwi orchards. The average nitrate
accumulation in 0-2 m soil profiles of 1-10-yr and 11-20-yr kiwi
orchards were 591 and 909 kg N ha\textsuperscript{-1}, which were
greater than the cereal fields (308 and 194 kg N
ha\textsuperscript{-1}in Wugong and Meixian County). The nitrate
accumulation in the 0-5 m soil profiles of kiwi orchards with 11-20-yr
was as high as 3071 kg N ha\textsuperscript{-1}.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Fig. 5} Nitrate-N content (left) and accumulation (right) in
Wugong and Meixian counties.

\textbf{3.5 Nutrient balances and the fates of surplus nutrient}

Over 25 years, the combined total N inputs at the county scale were much
higher in Meixian (16881 kg ha\textsuperscript{-1}) than in Wugong (9880
kg ha\textsuperscript{-1}) (Fig. 6). N fertilizer input in Wugong County
is 6279 kg ha\textsuperscript{-1}, while the N input in Meixian County
was 13452 kg ha\textsuperscript{-1}, resulting in a higher N surplus in
Meixian than in Wugong County. Regarding the fate of N inputs, N outputs
by harvesting crops were 5256 kg ha\textsuperscript{-1} (53.2\% of the
total N inputs) in Wugong and 4236 (25.1\%) kg ha\textsuperscript{-1} in
Meixian. This resulted in the accumulation of 40.7\% and 9.9\% of the
total N surplus as nitrate in 0-2 m soil profiles in Wugong and Meixian,
respectively, and 34.7\% of the total N surplus accumulation in 0-5 m
soil profiles in Meixian County. N stored in kiwi vines in Meixian
County was 2.4\% of the total N surplus. About 59.3\% and 62.9\% of the
total N surplus was lost through NH\textsubscript{3},
N\textsubscript{2}, and N\textsubscript{2}O emissions and into deeper
soil and groundwater in Wugong and Meixian counties, respectively.

Total P inputs over 25 years in Wugong and Meixian counties were 2788
and 3456 kg ha\textsuperscript{-1}, including synthetic fertilizers
(2182 and 2954 kg ha\textsuperscript{-1}), manure (546 and 447 kg
ha\textsuperscript{-1}), straw return (27 and 23 kg
ha\textsuperscript{-1}), irrigation (22 and 22 kg
ha\textsuperscript{-1}) and wet atmospheric deposition (11 and 12 kg
ha\textsuperscript{-1}). P surplus was 57.8 \% and 72.4 \% of the total
P inputs, and crop uptake was 42.2 \% and 27.6 \% of the total P inputs
in Wugong and Meixian counties, respectively.

Total K input over 25 years in Wugong and Meixian counties were 4300 and
7416 kg ha\textsuperscript{-1}. Regarding the fate of K inputs, 92.8\%
and 46.3\% of the total K inputs were removed by harvested crops. Unlike
the N and P balance, the K surplus was 7.2\% of input in Wugong County,
but Meixian County K surplus was 53.7\%.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Fig. 6} Percentages (\%) of the net soil nutrient storage, total
N, P and K surplus and total N, P and K crop uptake out of total N, P, K
input during the past 25 years in Wugong and Meixian counties. Note: 0-2
m nitrate means nitrate accumulated in the 0-2 m soil layers; 2-5 m
nitrate means nitrate accumulated in the 2-5 m soil layers; Other N
means N losses through NH\textsubscript{3}, N\textsubscript{2} and
N\textsubscript{2}O emissions and into deep soil profiles and
groundwater.

\textbf{4 Discussion}

\textbf{4.1 High surplus and low NUE at intensive orchard region}

Our study showed that nutrient surpluses in the soil of both two
counties have increased significantly since 1992 (Fig. 2) due to the
rapidly increasing application of synthetic fertilizers in China since
the 1980s (Bellarby et al., 2017; Sun et al., 2020; Yu et al., 2020). N
and P surpluses of the two counties are higher than that of other
studies. For example, the annual average N surplus (272 kg N
ha\textsuperscript{-1} in Wugong) was much higher than that of cereal
lands in the Sanjiang Plain (22.2 kg N ha\textsuperscript{-1}) and Haihe
basin (159 kg N ha\textsuperscript{-1}) (Zheng et al., 2015; Yang et
al., 2018). The nutrient surpluses in Meixian County were significantly
higher than that of Wugong County which is consistent with other studies
(Sun et al., 2008). This suggests that both the rapidly increasing
application of synthetic fertilizers and LUC from cereal to intensively
horticultural crops contributed to the high nutrient surpluses in soils.

It is expected that the high nutrient surpluses resulted in a
significant decrease in NUEs from 1992 to 2017 in two counties (Fig. 3),
which is consistent with previous studies (Ma et al., 2012). NUEs of
Meixian (19-31\%) was significantly lower than that of Wugong (30-54\%),
which could be explained by the high nutrient surpluses at Meixian (Fig.
3). The reasons for high nutrient surpluses and low NUEs in the
intensive horticultural crop regions are as follows: (i) The incomes of
intensively horticultural crops are usually much higher than that of
cereals. The smallholder farmers are usual with poor knowledge of better
fertilizer management. They tend to overfertilize as an ``insurance'' to
avoid yield losses (Yin et al., 2021). (ii) Unlike in Europe and North
America, there are still no regulations or laws in most regions of China
to limit nutrient surpluses in the soil to protect the environment (Hou
et al., 2021; Xin et al., 2021). It is also the situation in other
developing economies (Morseletto, 2019). Therefore, more urgent
attention should be paid to the issues of high nutrient surpluses and
low NUEs in intensively horticultural crop regions.

\textbf{4.2 Different fates of the surpluses of N, P and K}

The percentages of P and K uptakes by crops in Meixian were
significantly lower than that of Wugong (Fig. 6), due to their excessive
application in kiwifruit orchards. High P and K surpluses in the two
counties from 1992 to 2017 significantly increased available P and K in
soil (Fig. 4). The soil available P and K in Meixian County were 7.3
times and 3.4 times higher than that in Wugong County due to the higher
P and K surpluses in the intensively horticultural crop region (Fig. 3).
This is consistent with the studies of others, i.e., LUC is an important
cause for the high soil nutrient levels (Luz et al., 2019; Roobroeck et
al., 2021; Kim et al., 2023; Mulimbi et al., 2023). The average
available P and K in 0-20 cm soils Meixian in 2017 were 111 mg
kg\textsuperscript{-1} and 509 mg kg\textsuperscript{-1} (Fig.4), which
are much higher than the optimum thresholds of available P and K in
soils (Liu et al., 2021; Tian et al., 2021; Kochiieru et al., 2022). The
excessive accumulation of P and K in the soil results in a waste of
resources and environmental pollution.

Unlike P and K, the fates of surplus-N in the soil is more complex. N
uptakes by crops was the main fate of N input in the Wugong; however,
for Meixian, surplus-N in soil was significantly higher than crop uptake
(Fig. 6). Nitrate accumulation in soil profiles of kiwifruit orchards
was higher than that of cereal fields, which also increased with the
increase in stand ages of the orchards (Fig. 5). The nitrate contents in
deep soil profiles of kiwifruit orchard were high (Fig. 5), however, the
roots of kiwifruit vines are shallow (Bai et al., 2022; Dutta et al.,
2023; Wu et al., 2023), which will limit the nitrate uptake from the
deep soil profiles. The average nitrate accumulation in 0-5 m soil
profiles of kiwifruit orchards in stand ages of 11-20 years was as high
as 3071 kg N ha\textsuperscript{-1} (Fig. 5). The nitrate content in the
4.8-5 m layer was as high as 64.8 kg N ha\textsuperscript{-1},
indicating that the surplus N leached into the deeper soil layers or
groundwater. Assuming nitrate in the soil profile of 5-10 m is the same
as our studies in the region (Gao et al., 2021), the proportion of
nitrate in the soil profile of 0-10 m to the surplus N is 73.5 \% in
Meixian County. This value is higher than the results (40.30\%) from (Gu
et al., 2015) and 18.5-24.8\% from Sun et al., (2020).

Denitrification is usually considered the main process for eliminating
nitrate in the deep vadose zone. Contrary to subtropical and tropical
regions with strong denitrification in deep soil profiles, low content
of soil organic carbon and higher dissolved oxygen that denitrification
is limited (Wang et al., 2024), which leads to more nitrate accumulation
in kiwifruit production regions such as Meixian County.

\textbf{4.3 Implications and uncertainties}

Our study showed that long-term heavy fertilization caused high nutrient
surpluses in fields and nitrate accumulation in vadose zones, especially
in intensive horticultural cropping regions. Fertilizer inputs account
for more than 80\% of total inputs in the region, while nutrient output
is only 10\% of inputs (Lu et al., 2016). This calls for the
establishment and education effective nutrient management practices for
the horticultural production regions in China. Some strategies may
include using the 4R (right time, right place, right, right source, and
right rate) approach to fertilization, selecting farmers or cooperatives
with large planting areas for promotion and demonstration, and upgrading
smallholder education. The experiences of Europe and North America on
nutrient management since the 1980s indicate that comprehensive measures
combining the technological, economic, and social aspects are needed to
reduce nutrient surpluses and improve NUE (Billen and Garnier, 2021;
Christensen et al., 2021; van Grinsven et al., 2012). Such an approach
is urgently needed in agricultural production in China to curb high
nutrient surpluses in soils to protect the environment.

Excessive nitrate accumulation in soil threatens groundwater quality
(Ascott et al., 2016). To address the N related problems, the Nitrates
Directive has been implemented across Europe, resulting in the
designation of Nitrate Vulnerable Zones (NVZs) (Lutz et al., 2022).
However, there is still no similar regulation in China to control N
surplus in intensive agricultural regions. Although different water
quality mitigation measures (such as the EU Water Framework Directive
(Directive2000/60/EC)) have been implemented in the EU, it still
observes nitrate levels above the regulatory threshold in several
water-bodies across Europe due to the presence of N legacies (Batool et
al., 2022). Therefore, appropriate measures and models combining
climatic, soil and anthropogenic management factors must be developed to
fully understand the potential fate of nitrate in deep soils to avoid
future problems in intensive agricultural systems.

The research compared long-term nutrient surpluses and their fates in
regions with different LUCs. Despite considerable efforts to provide
reliable results, some uncertainties remained in this study. First, to
minimize the uncertainty of our analysis, we used official statistics
when calculating nutrient input and output because official statistics
are widely recognized as the best available data sources in China.
However, there is uncertainty in estimating nutrient budgets at the
county scale due to the large number of data sources over a long period.
Second, although we tried our best to select parameters used for
calculation nutrient budgets from the same region and our previous
studies, many parameters were still obtained mainly from literatures.
Thirdly, the complexity of the landform types in this area, close to the
Qinling Mountains and the Wei River, resulted in differences in
accumulation at different sampling sites. Although it was impossible to
sample every area, we collected as many soil samples as possible to
minimize uncertainties. A more systematic study is needed to reveal the
differences in nutrient accumulation at different sampling sites and the
factors affecting them in the future.

\textbf{5 Conclusions}

Our study indicates that both the rapidly increasing application of
synthetic fertilizers and LUC from cereal to intensively horticultural
crops since the early 1990s in China contributed to the high nutrient
surpluses in soils, resulting in low NUE of N, P, and K. Compared to the
cereal-dominated county, nutrient surpluses in soils in the
orchard-dominated county are very high, which reduced the NUE. The high
N surplus in the orchard-dominated county increased high nitrate
accumulation in deeper soil profiles of orchards; and nitrate
accumulation in soil profiles increased with the stand ages of orchards.
Unlike N, high P and K surpluses in the two counties from 1992 to 2017
significantly increased available P and K in 0-20 cm soil. The soil
available P and K in the orchard-dominated county is significantly
higher than the cereal-dominated county. This highlights that
comprehensive measures should be taken to reduce nutrient surpluses to
minimize nutrient loss and environmental pollution in China, especially
in the intensive horticultural crop systems.

\textbf{Acknowledgments}

We thank the National Natural Science Foundation of China (42277343),
the National Key R \& D Program of China (No. 2017YFD0200106), and the
111 Project (No. B12007) for their support.

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