Taiqing HUANG，Wanzhen DENG，Yanfei HUANG

Agricultural Resource and Environment Research Institute，Guangxi Academy of Agricultural Sciences，Nanning 530007，China

Abstract [Objectives] The paper was to explore the effects of different mulching methods on soil moisture content and water movement in citrus orchards，and to provide the theoretical basis for improving water and weed management level in orchards. [Methods] Three ground mulching treatments including spraying herbicide (CK)，grass-proof cloth cover (GPC) and natural grass mowing (NGM) were set up to analyze the soil moisture content and water flux characteristics of soil profile in the soil layers of 5，20，40 and 60 cm under different mulching methods. [Results] The GPC and NGM treatments significantly increased the soil moisture content in the soil layer of 0-60 cm at the young fruit stage and fruit expansion stage，which inhibited soil water evaporation and effectively improved soil water holding capacity，thus reducing irrigation water consumption and saving water resources. During the expansion stage of citrus fruits，the soil water flux in the soil layer of 0-60 cm in NGM and CK treatments was upward，and the upward soil water flux in NGM treatment was larger，which could mobilize more upward movement of deep soil moisture for uptake by citrus roots. However，the soil water flux in the soil layer of 0-60 cm in GPC treatment was downward，and the soil moisture conditions in the upper and middle layers were already sufficient for citrus growth. [Conclusions] Both GPC and NGM treatments can increase the overall soil moisture content. In the dry season，the soil moisture content in the upper layer treated by GPC is always relatively high，while more soil water in the lower layer move to the upper layer in NGM treatment，which has met the water requirements for citrus growth.

Key words Soil moisture content，Soil water flux，Mulching pattern，Weed management，Guangxi citrus orchard

1 Introduction

Citrus belongs to the world’s first category fruits，mainly distributed in the earth’s low latitude humid and mild areas. China is the main producer of citrus in the world，and it has become the world’s largest producer of citrus since 2008. Guangxi is one of the important producing areas of citrus in China. In Guangxi citrus orchards，weed management is an important work of citrus planting management，and the main measures include chemical weeding，biological weeding (orchard breeding)，physical weeding (manual mowing or mulching) and planting green manure. Many studies have shown that different weed management methods in orchards have important impacts on soil hydrologic characteristics，thus affecting the yield and quality of fruits. Compared with clean tillage，grass mulching significantly improved the water holding capacity of the soil in a vineyard in Shaanxi Province，and the water holding capacity in the soil layer of 0-40 cm was increased by 32.3%-42.7%[1]. The soil moisture content among apple orchard rows was improved by 4 types of grass cultivation，especially in the soil layer of 20-40 cm[2]，because grass cultivation improved the soil structure and increased the number of soil pores and porosity，which made the soil looser and improved the soil infiltration of moisture and water holding capacity[3-4]. However，in water-deficient areas or severe drought periods，grass competes with fruit trees for soil moisture，thereby reducing soil moisture content[5-6]. For example，studies in pear orchard showed that grass cultivation in the rainy season improved the infiltration of rainfall and enhanced the water holding capacity of the soil，but in the continuous dry season grass cultivation reduced the soil moisture content in the soil layer of 0-30 cm[7]. Research on apple orchards in Shanxi Province showed that ground cloth cover effectively inhibited soil water evaporation，thus improving soil moisture content; meanwhile，grass-proof cloth increased the ability of rain collection，moisture conservation and drought resistance，and improved the utilization efficiency of soil moisture[8]. Because of good water and air permeability，ground cloth on the one hand reduced the compaction of the surface soil，improved the soil structure and enhanced the water holding capacity of the soil; on the other hand，it could increase the infiltration of morning dew and small amount of rainfall，and reduce the ineffective evaporation of soil moisture，thus maintaining soil moisture and increasing the soil moisture content[9]. However，the ability of ground cloth cover to increase soil moisture content varies with different fruit tree growth stages and soil layers. The effect of ground cloth cover on increasing soil moisture content at young fruit stage and fruit expansion stage was significantly better than that in other stages[10]. At the same time，the research results of ground fabric mulching and milk vetch grass mulching showed that the two mulching modes both improved soil capillary porosity，soil water content，soil water holding capacity and water storage capacity，and the milk vetch grass mulching mode was better than the ground fabric mulching mode[11]. Hence，the effect of different weed management methods on soil moisture in orchards is a complex process，which varies greatly with different orchard types，soil conditions and weed management methods. Although there are some studies on citrus orchards，the treatment is relatively simple，and the research results are also quite different[12-13]. In this study，different weed management methods were set up in orange orchards in typical red soil region of Guangxi，and the variation of soil moisture content in different soil layers of 0-60 cm and the migration characteristics in the profile were monitored annually. The results would provide the theoretical support for weed management and water management of citrus in Guangxi，so as to promote the citrus planting management level in Guangxi.

2 Materials and methods

2.1 General situation of survey areaThe test was conducted in the Lijian Scientific Research Base (23°14′ N，108°02′ E) of Guangxi Academy of Agricultural Sciences in Wuming District，Nanning City，Guangxi Province. The area is a subtropical monsoon climate zone with abundant light and heat resources. The average annual temperature is 21.7 ℃ and the average annual rainfall is 1 304.2 mm. The soil in the experimental area is red soil developed from quaternary red clay parent material. The basic properties of the soil at the initial stage of the test were: pH 5.8，organic matter 1.36%，total nitrogen 0.71 g/kg，total phosphorus 0.15 g/kg，total potassium 12.6 g/kg.

2.2 Experimental designA field plot test was conducted，and the citrus variety planted in the test was Orah，which blossomed in March，entered the rapid fruit expansion stage in July，and entered the fruit ripening stage in January of the next year. Citrus seedlings in the experimental site were colonized in March 2018，with a planting density of 85 plants/0.067 ha. Different weed management measures (ground cover treatment) started in April 2019. Three treatments were designed in the test，including spraying herbicide (CK)，grass-proof cloth cover (GPC) and natural grass mowing (NGM). Each treatment was replicated 3 times，with a total of 9 plots，covering an area of 0.015 3 ha (17 m×9 m). In the herbicide spraying area，herbicide was sprayed once when the natural weeds grew to 30-40 cm high. The grass-proof cloth cover area was fully covered with black polypropylene woven grass-proof cloth with water permeability and air permeability. After removing the malignant weeds with developed roots，tall plants and climbing habit in the natural grass mowing area，the weeds were mowed once when the weeds grew to 30-40 cm，and the residual grass was covered in the citrus rows. All citrus plants in the test were cultivated under unified fertilizer，irrigation and pest management.

2.3 Soil moisture content and rainfall monitoringThe SS-TRS401 soil moisture sensor manufactured in China (accuracy<±3%，resolution 0.1%) was used to continuously monitor 9 citrus planting plots at fixed points. A representative position in each plot was selected，and a 70 cm deep soil profile about 1 m away from citrus tree was dug out. The soil moisture monitoring sensor was respectively installed at 5，20，40 and 60 cm depth of the profile，with the probe of the moisture sensor pointing to the nearby fruit tree，and the data line was connected with the data collector. The data collector had the function of data acquisition，storage and transmission. After inserting sensors in each soil layer，the excavated profile was backfilled and compacted layer by layer，so as to monitor the soil moisture content at different soil depths. The trial operation would be carried out within 2 months after the installation of soil moisture monitoring sensor，during which the monitoring results of the instrument would be adjusted and calibrated according to the measured soil volumetric moisture content. If there was no problem，the long-term positioning monitoring of soil moisture content would be carried out formally. In this study，the data of soil moisture content were collected every 1 h，and the data were collected from February 1，2020 to February 28，2021，with a duration of 13 months. Meantime，the local rainfall conditions were monitored through the weather station of the base.

2.4 Soil water fluxSoil water flux refers to the volume of water passing through the soil per unit area per unit time，which can be analyzed and calculated by the principle of conservation of mass and Darcy’s law. In this test，the soil water flux of 0-70 cm thickness of soil layer was calculated. The absolute value of soil water flux was calculated according to Gan Leietal.[14]，and the formula is as follows:

(1)

The direction of soil water flow can be judged according to the following formula:

(2)

2.5 Data analysis and icon makingAll data were summarized，calculated and plotted using Excel 2019 software，and statistical analysis and significance test of difference were performed using SPSS 25 software.

3 Results and analysis

3.1 Rainfall characteristics of the test siteFig.1 shows the characteristics of daily rainfall and monthly statistical rainfall in the test area. In terms of the annual change from February 2020 to February 2021，it can be seen that the rainfall mainly occurred from February to October 2020，accounting for more than 90% of the total rainfall of the whole cycle，and the total rainfall was 1 064.5 mm，which was less than that of conventional years. There were 129 d of rainfall in the whole year; there were 2 d with the daily rainfall greater than 50 mm，which were 74 mm on March 3，2020 and 154 mm on June 25，2020，respectively; there were 7 d with the daily rainfall of 25-50 mm，which were distributed in March，April，June，September and October，respectively.

Fig.1 Distribution of average daily rainfall in the test area

3.2 Overall variation characteristics of soil moisture content

Fig.2 shows the dynamic changes of soil moisture content in different soil layers under three different weed treatments in the citrus orchard during the experimental period. The soil moisture content was intensively influenced by rainfall，and the surface soil moisture content had the most obvious response to rainfall. Even small rainfall events caused significant changes in surface soil moisture content，but with the increase of soil depth，the influence of rainfall on soil moisture content decreased，because small rainfall events basically could not cause changes in deep soil moisture content[15-16]. After the occurrence of rainfall，especially after a heavy rainfall，the soil moisture content in all soil layers under different treatments increased significantly. However，increases in soil moisture content were sometimes observed in the absence of rainfall as a result of integrated fertilization during citrus cultivation or irrigation during dry periods. On the whole，the soil moisture content in the soil layer of 5 cm under the three different mulching methods fluctuated greatly，while the soil moisture content in other soil layers fluctuated little，and the greater the soil depth，the smaller the fluctuation of soil moisture content. This was because the surface soil moisture had a good response relationship with meteorological factors such as light，temperature，precipitation and atmospheric evaporation[17]，but since the deep soil did not contact with the atmosphere，the correlation between soil moisture and these meteorological factors was weakened，and the stability of soil moisture increased with the increase of soil depth[18].

As shown in Fig.2，the soil moisture content in the soil layer of 5 cm in GPC treatment was slightly larger than those in NGM and CK treatments from February to June 2020，and the variation range of moisture content was CK>NGM>GPC. This may be because herbicide spraying treatment was not blocked by mulching，the surface soil was susceptible to external natural factors (such as direct solar radiation，etc.)，and matter and energy exchanged more frequently with the atmosphere. At the beginning of July 2020，the soil moisture content in the three different treatments increased rapidly，mainly because the heaviest rainfall in the whole year came on June 25，2020，which made the soil moisture content replenished quickly，resulting in a sudden increase in moisture content. From July 2020 to February 2021，it was obvious that the soil moisture content in GPC treatment was the largest，followed by NGM treatment，and that in CK treatment was the smallest. This may be because GPC and NGM treatments significantly increased the organic matter content of the soil compared with CK treatment. The increase of organic matter content can promote the formation of soil aggregates，increase the content of water-stable aggregates，and enrich soil pore space，thus better retaining soil water and increasing soil moisture content[19-21]. The black polypropylene narrow strip of grass-proof cloth applied in GPC treatment was woven vertically and horizontally，which had good air permeability and water permeability，and can avoid the water and soil erosion and soil compaction caused by large raindrops washing directly over the soil surface when the rainfall is heavy; when the rainfall is small，it can also penetrate water downward; when the sunshine duration is long，it can keep the soil out of direct sunlight and reduce ineffective evapotranspiration of soil water，so as to maintain soil moisture and increase soil moisture content[22-23].

In the soil layer of 20 cm，the soil moisture content in GPC and NGM treatments was basically equal from February to August 2020，but was slightly greater than that in CK treatment. The two mulching treatments increased the soil organic matter content，which was conducive to soil moisture conservation. In addition，mulching treatment could intercept water and reduce ineffective water loss，so the soil moisture content in mulching treatment was higher than that in non-mulching treatment. From September to December 2020，the soil moisture content in GPC treatment was higher than those in CK and NGM treatments，and the soil moisture content in CK and NGM treatments was approximately equal. The grass growth in natural grass mowing treatment also consumed some soil moisture[24]，while the roots of shallow rooted weeds retained in this study were mainly distributed in the soil layer of 0-20 cm，so the soil moisture content in this layer was lower than that in GPC treatment.

During the whole period，the soil moisture content in the soil layer of 40 cm was the highest in NGM treatment，followed by GPC treatment，and that in CK treatment was the smallest. This indicated that ground mulching inhibited soil water loss and had a good effect on soil water conservation，which is consistent with the research results of Tang Yingfangetal.[25]that different ground mulching could reduce inter-tree evaporation and increase soil moisture content.

In the soil layer of 60 cm，the soil moisture content in CK treatment was the smallest，and those in NGM and GPC treatments were basically equal except that the soil moisture content in NGM treatment was greater than that in GPC treatment in the rainy season with abundant rainfall，and the opposite was true in the dry season. Hao Wanglinetal.[1]found that in the vertical profile of soil，the soil water holding capacity of different mulching treatments was more prominent than that of the open field; the water holding capacity of grass mulching treatment in the deep soil was better than that in the shallow soil，while the opposite was true in film mulching treatment. Our findings are in agreement with this report. The water holding capacity of natural grass mowing was better in the middle and lower soil，which may be related to the water holding capacity of turf. The water storage capacity of grass-proof cloth mulching treatment in the middle and lower soil was not as good as that of natural grass mowing treatment，probably because grass-proof cloth mulching treatment mainly enhanced the downward infiltration of soil water by hindering the evaporation of soil water in the surface layer，but the further down the soil，the higher the soil compaction degree，and the more difficult the soil water infiltration.

3.3 Effects of different mulching methods on soil water flux

Fig.3 shows the changes of soil water flux in the soil layer of 0-60 cm under three different mulching methods. The breakpoint in the figure was due to abnormal monitoring of soil moisture probe at that point，which was removed. A positive value of soil water flux indicated an upward transport movement of soil moisture，while a negative value indicated a downward transport movement. In February 2020 (flower bud differentiation stage)，the soil water flux in the treatments of spraying herbicide (CK) and grass-proof cloth cover (GPC) fluctuated greatly，while that in natural grass mowing (NGM) fluctuated little，indicating that the soil moisture movement in CK and GPC treatments was more active.

Fig.2 Dynamic changes of soil moisture content under different mulching methods

Fig.3 Effects of different mulching methods on soil water flux

From March to June 2020 (flowering and young fruit stage)，the soil water flux in CK and NGM treatments was downward，while that in GPC treatment was upward. However，the fluctuation of soil water flux in the three treatments was relatively small. This indicated that there were more soil water infiltrations in CK and NGM treatments，while the soil moisture in GPC treatment was in the process of consumption. The rainfall from March to June 2020 was 672 mm，accounting for 60% of the annual rainfall. In the herbicide spraying treatment area，there was no cover on the soil surface，so water infiltrated quickly. Because of the cover of grass-proof cloth，water penetrated slowly into the deep soil in the GPC treatment area. The density of grass in the NGM treatment area was smaller than that in grass-proof cloth area，and the soil connectivity was better due to the root system of grass，so water could also penetrate quickly. During this period，although the rainfall was heavy，the sunshine duration was long and the sunshine intensity was relatively high. Moreover，because citrus roots were mainly concentrated in the soil depth of 10-50 cm and the water demand was large in the shoot flowering and young fruit stages，citrus roots absorbed more water from the middle and upper layers of soil，and there was little difference in the volume of water added and consumed. Eventually，the fluctuation range of soil water flux in the soil layer of 0-70 cm was small in the three mulching treatments.

From July to August 2020 (early fruit expansion period)，the experimental area was in hot summer with high sunshine intensity and long sunshine duration，and there were much rainfall (132.2 mm). The soil water flux in CK treatment fluctuated sharply，while the soil water flux in GPC and NGM treatments fluctuated little，indicating that CK treatment was greatly influenced by external factors. This was also caused by the fact that there was no cover and obstruction on the ground surface of CK treatment，so water and energy between soil and atmosphere exchanged frequently，with large evaporation，and rainfall supplement resulted in frequent up and down movement of moisture. The upward water flux in GPC and NGM treatment was much smaller than that in CK treatment，and the soil moisture content in both treatments was also higher than that in CK treatment，indicating that GPC and NGM treatments had better effect on soil moisture conservation than CK treatment，which was also proved by previous studies[11,26].

The period from September to November 2020 was the later period of citrus fruit expansion，during which citrus fruits expanded rapidly and required a huge volume of water. During this period，the soil water flux in CK and NGM treatments was upward on the whole，and the net water flux in CK treatment was greater than that in NGM treatment，probably because the moisture content in the middle and upper soil of CK and NGM treatments could not meet the water demand for soil evaporation and citrus growth，and it was necessary to mobilize the deep soil water to the upper soil for replenishment. Moreover，the upper soil in CK treatment required more water supplements. The soil water flux in GPC treatment was occasionally upward，but the overall was downward. These results indicated that the water content in the middle and upper layers in GPC treatment could meet the huge volume of water required in the expansion period of citrus fruits with the supplement of rainfall，because the soil water holding capacity of GPC treatment was good. Although the effect of NGM treatment on soil moisture conservation was also relatively good，surface soil moisture was also used for grass growth consumption in addition to soil evaporation and citrus growth consumption，so it is necessary to mobilize deep soil moisture content.

The period from December 2020 to February 2021 was the ripening period of citrus fruits，during which there was little natural rainfall. The water flux was occasionally downward in GPC treatment，but the soil water flux in the three different mulching methods was generally upward，and the upward water flux values were NGM > GPC > CK. During this period，because there was very little supplement of precipitation，the three different mulching treatments all needed to mobilize the deep soil moisture content for supplement. Because NGM treatment had the best effect on deep soil moisture conservation and the highest soil moisture content，it could mobilize the most deep moisture content. However，CK treatment had the worst effect on soil moisture conservation，and the soil moisture content was significantly lower than the other two mulching treatments，and there was no way to move more soil moisture upward.

As shown in Fig.3，the soil water flux in spraying herbicide (CK) treatment fluctuated most dramatically during the test period，while those in NGM and GPC treatments fluctuated little，indicating that the soil water activity in CK treatment was the strongest，with unstable water status，while the NGM and GPC treatments were less influenced by external conditions because the top layer of soil was blocked by mulch，resulting in more stable soil moisture status than CK treatment. The net soil water flux in three different mulching methods was upward，indicating that soil moisture in citrus planting in the experimental area moved upward as a whole，which was consumed by citrus roots，leaf transpiration and soil evaporation，and soil moisture was in the process of consumption. The upward soil water flux value in CK treatment was 135% and 142% higher than those in NGM and GPC treatments，respectively，probably because the soil surface evaporation in CK treatment was much higher than that in the other two mulching treatments. A large number of studies have shown that soil surface mulching treatment can reduce soil evapotranspiration[1,27-28].

4 Conclusions

Natural grass mowing (NGM) and grass-proof cloth (GPC) had better water retention and water holding capacity than spraying herbicide (CK)，and could reduce soil evapotranspiration and increase soil moisture content. On the whole，GPC treatment had a better effect on improving soil moisture content in the middle and upper layers，and NGM treatment had a better effect on improving soil moisture content in the middle and lower layers. NGM and GPC treatments avoided direct sunlight and reduced soil moisture loss，thus helping to retain soil moisture. In the expansion stage of citrus fruits，the soil water flux in NGM and GPC treatments was upward，and the upward soil water flux in NGM treatment was larger，which could mobilize more upward movement of deep soil moisture for uptake by citrus roots，while the deep soil in CK treatment did not have so much water to mobilize. However，the soil water flux in GPC treatment was downward，and the upper soil water condition could meet the growth demand of citrus without mobilizing the deep soil moisture content. During the whole period，the soil water flux in CK treatment fluctuated most sharply，mainly because the soil water holding capacity in herbicide spraying treatment was relatively weak and the soil water evaporation was large，and there were more alternation between dry and wet due to replenishment of rainfall infiltration，resulting in more intense soil moisture movement. In conclusion，both GPC and NGM treatments can effectively increase soil moisture content and water absorption and utilization of citrus，which is conducive to reducing irrigation water，thus achieving water-saving effect.