Characteristics of Non-Point Source Pollution Under Different Land Use

sustainability

Article Characteristics of Non-Point Source Pollution under Diﬀerent Land Use Types

Jun Yang 1,2, Jiping Liang 1,2, Gaihe Yang 1,2,*, Yongzhong Feng 1,2,*, Guangxin Ren 1,2 , Chengjie Ren 1,2, Xinhui Han 1,2 and Xiaojiao Wang 1,2

1 College of Agronomy, Northwest A&F University, Xianyang 712100, China; [email protected] (J.Y.); [email protected] (J.L.); [email protected] (G.R.); [email protected] (C.R.); [email protected] (X.H.); [email protected] (X.W.) 2 Shaanxi Engineering Research Center of Circular Agriculture, Xianyang 712100, China * Correspondence: [email protected] (G.Y.); [email protected] (Y.F.); Tel./Fax: +86-029-8708-2317 (G.Y.); +86-029-8709-2265 (Y.F.) Received: 14 February 2020; Accepted: 2 March 2020; Published: 6 March 2020

Abstract: Non-point source pollution (NPSP) is a major challenge for current global water resources. The output characteristics of pollutants under different land use types are very important for controlling NPSP. In this study, long-term positioning monitoring and an analysis of rainfall runoff from different land use types were used to evaluate a typical watershed in the water source area of the middle route of the South-to-North Water Diversion Project (MR-SNWDP). The results show significant differences in nitrogen and phosphorus content in the runoff water bodies of various land use types. The nitrogen and phosphorus content in the MR-SNWDP was directly related to rainfall intensity and the fertilization period in the runoff following fertilization of farmland and vegetable plots. This nitrogen and phosphorus content was also observed to be significantly higher in the fertilization period than in other periods. The loss of nitrogen and phosphorus in forestland was greatly affected by rainfall intensity. Nitrogen in runoff comes primarily from farmland and vegetable fields, where its main form is nitrate nitrogen (NN). Vegetable fields are the main source of 3 phosphorus, where its primary form is soluble phosphate (PO4 −-P). Nitrogen and phosphorus have a defined incubation period during the dry season. Farmland and vegetable fields receive less rainfall during the dry season and it is difficult to form effective runoff; this allows nitrogen and phosphorus deposition. The runoff formed by the first rainfall at the beginning of the flood season (April or May) will carry a large amount of nitrogen and phosphorus from the soil into water bodies. Therefore, it is crucial to pay careful attention to the season when attempting to control NPSP.

Keywords: land use; south-to-north water diversion project; non-point source pollution; nitrogen; phosphorus

1. Introduction The middle route of the South-to-North Water Diversion Project (MR-SNWDP) is the largest water diversion project in the world. Currently, non-point source pollution (NPSP) is the most prominent issue affecting water quality in the reservoir area of this project [1]. NPSP can be formed at any time and place, especially when chemical fertilizers and farm manures are used in production and daily life [2]. Because of the insufficient absorption and utilization by plants and improper planting methods in some areas, runoff generated by pollutants from rainfall can bring these pollutants into a water body, seriously affecting the water quality. The 1977 Amendment to the Clean Water Act of the United States defines NPSPs as pollutants that enter the surface and groundwater in a wide, dispersed, and microscopic form. NPSP is regarded as a major contributor to water quality deterioration and has

Sustainability 2020, 12, 2012; doi:10.3390/su12052012 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 2012 2 of 13 become a major global and regional environmental problem. According to statistics, 30–50% of water sources on the earth’s surface have been affected by NPSP [3]. Approximately 60% of water pollution in the United States is caused by NPSP [4], and 89% of the nitrogen load in the Gulf of Mexico each year comes from NPSP. Eutrophication of European water bodies is also a serious problem. Nitrogen and phosphorus produced by agricultural production activities and imported into the North Sea estuary account for 60% and 25% of the total load, respectively [5]. In the Netherlands, total nitrogen and total phosphorus produced by agricultural NPSP accounts for 50–60% and 40–50% of total water pollution, respectively [6]. In China, water pollution has always been a very serious ecological and environmental problem. Total of 38% of reservoirs and 65% of lakes in China were identified as eutrophic water bodies in 2010 [7]. Controlling NPSP through river basins has become an important task for water environment management research [8]. Li Chongwei et al. [9] showed that the spatial difference of NPSP in the Qiao Reservoir can be attributed to differences in landforms and vegetation. The key to controlling NPSP is to change the spatial distribution of vegetation cover. Different land use types have different effects on NPSPs. Some landscapes can inhibit or absorb pollutants, including woodland and unused land [10–12]. Tong et al. [13] studied the hydrological effects of land use in Ohio, and found that different land use types have a significant influence on nitrogen and phosphorus in river water. Kibena et al. [14] analyzed the impact of land use activities on water quality in the upper Manieme River using hydrological response units and buffer analysis to determine the main land use types that affect water quality. A change in land use type will change the landscape pattern under certain conditions. In turn, this affects the entire ecological environment. Differences in the soil nutrient conservation capacity of different land use types and in the absorption and utilization of nitrogen, phosphorus, and other nutrients by vegetation, means that it is important to study the relationship between land use types and runoff water quality. It is also important to consider the source of NPSP in order to control it more effectively. The state of water resources in the water source area of the MR-SNWDP not only affects the residents in the water source area but also affects the Beijing-Tianjin-Hebei region. It is unclear what kind of land use methods are in the water source area and what the loss characteristics of non-point source pollution are. Therefore, this paper has conducted long-term positioning monitoring and analysis of nitrogen and phosphorus in rainfall runoff of different land use types in a typical watershed in the water source area of the MR-SNWDP in China. This study provides a theoretical basis for the loss of groundwater sources and pollution prevention methods.

2. Materials and Methods

2.1. Study Area This study was conducted in Maotang Township, Xichuan County, Henan Province, China (Figure1). Xichuan County is located at the junction of Shaanxi, Hubei, and Henan. The southwest part is adjacent to the low mountains and hills of northern Hubei, and the northeast part belongs to the Nanyang Basin. The total population of the county is about 740,000. The county’s land area is 2820 square kilometers, of which the core water source area of the South-to-North Water Diversion Middle Line area is 2616 square kilometers, accounting for 92.8% of the county’s total land area. There are obvious diﬀerences in the distribution of soil types in the water source area. The yellow-brown loam is mainly composed of ten soil types such as lime soil, paddy soil, mortar black soil, and purple soil. The soil layer thickness is generally 30 cm. Land use types in the reservoir area are mainly grassland, shrubs, and woodland, which are 70% of the total area of the reservoir area, and agricultural land accounts for 22% of the total area of the reservoir area. The land use in the reservoir area has the characteristics of a distinct mountainous area, with less farmland, more slopes, more arable land, less mountainous grasslands, moderate soil pH, but poor soil fertility, and agricultural production is more dependent on the chemical fertilizers. Most of the Maotang Township is located in a deep mountainous area. The land use ratio is similar to the rest of the county. The township has 20,000 acres of arable land, 180,000 acres Sustainability 2020, 12, 2012 3 of 13

of forest land, and a forest coverage rate of more than 50%. The runoff monitoring point of this study Sustainabilitydid not have 2020, any12, x humanFOR PEER intervention REVIEW in the plantation rows in the forestland, farmland, or vegetable3 of 13 gardens. Furthermore, the planting types, fertilization amounts, and fertilization methods were carried rowsout in according the forestland, to the localfarmland, farmers’ or vegetable historical garden plantings. Furthermore, habits [15–17 ].the Therefore, planting thetypes, establishment fertilization of amounts,runoff communities and fertilization reflects methods the true were situation carried of NPSPout according in the sub-watersheds to the local offarmers’ the reservoir historical area. plantingThe types habits of runo[15–17].ff monitoring Therefore, stationsthe establishment in this study of runoff included communities farmland reflects runoff ,the woodland true situation runo ff, ofvegetable NPSP in the field sub-watersheds runoff, well water, of the and reservoir river water area. stations. The types The of monitoringrunoff monitoring time was stations from February in this study2017 included to September farmland 2018, runoff, and each woodland monitoring runoff, station vegetabl only testede field therunoff, water well quality water, from and water river thatwater can stations.form runo Theff .monitoring time was from February 2017 to September 2018, and each monitoring station only tested the water quality from water that can form runoff.

FigureFigure 1. 1.LocationLocation of ofthe the study study area. area.

TheThe farmland farmland planting planting pattern pattern in inthe the township township consists consists of ofa awinter winter wheat wheat and and a asummer summer corn corn rotation.rotation. Corn Corn planting planting occurs occurs in inearly early May, May, and and wh wheateat planting planting occurs occurs in inearly early October. October. The The fertilizer fertilizer appliedapplied to toplanting planting corn corn is compound is compound fertilizer fertilizer and and phosphate phosphate fertilizer, fertilizer, and andthe fertilizer the fertilizer applied applied to wheatto wheat is compound is compound fertilizer. fertilizer. Urea Urea is used is used as top as top dressing dressing fertil fertilizer.izer. Vegetable Vegetable fields ﬁelds are are planted planted throughoutthroughout the the year, year, and and primarily primarily consist consist of of ga garlicrlic seedlings, seedlings, lettuce, lettuce, cabbage, cabbage, and and other other common common vegetables.vegetables. The The fertilizer fertilizer applicationapplication toto these these ﬁelds fields contains contains farm farm manure manure (human (human and animal and animal manure). manure).The woodland The woodland is comprised is comprise of a compoundd of a compound forest of shrubs forest andof shrubs trees. Welland trees. water Well comes water directly comes from directlyfarmers, from and farmers, the river and water the originatesriver water from origin theates Suohe from River the Suohe in Maotang River Township.in Maotang Township.

2.2. Data Sources 2.2. Data Sources Rainfall data for this study came from the China Meteorological Data Network (http://data.cma.cn/) Rainfall data for this study came from the China Meteorological Data Network surface climate data Days China dataset (V3.0), monitoring site for Xixia station (No. 57156). Rainfall (http://data.cma.cn/) surface climate data Days China dataset (V3.0), monitoring site for Xixia station (No. 57156). Rainfall levels are classified using runoff monitoring according to rainfall level distribution issued by the National Meteorological Administration (Table 1). The runoff data and water quality monitoring data are measured using fixed-point monitoring. The size of the runoff field is a rectangle of 5 m × 10 m. The slope of the forest land is 30°, and the slope of the farmland and Sustainability 2020, 12, 2012 4 of 13 levels are classified using runoff monitoring according to rainfall level distribution issued by the National Meteorological Administration (Table1). The runo ff data and water quality monitoring data are measured using fixed-point monitoring. The size of the runoff field is a rectangle of 5 m 10 m. × The slope of the forest land is 30◦, and the slope of the farmland and vegetable land is 10◦. The total rainfall in 2017 was 1106.1 mm, of which the rainfall during the high water period from June to October was 884.2 mm. The rainfall during this period accounted for 79.9% of the annual precipitation. The total number of runoff monitoring samples used was 122.

Table 1. The basic conditions of runoﬀ monitoring in sub-basins.

Rainfall Level Sample Category Longitude Latitude Gradient Number I II III IV (<10 mm) (10–24.9 mm) (25–49.9 mm) (50–99.9 mm)

Farmland 21 111.3914 33.2011 10◦ 4 8 6 2 Vegetable ﬁeld 20 111.3935 33.2024 10◦ 4 11 7 3 Forestland 25 111.3971 33.2022 30◦ 2 9 7 3 River 28 111.3896 33.2061 5◦ 6 12 7 3 Well water 28 111.3945 33.2015 0◦ 6 12 7 3

In this study, a rainwater collection device was placed at the outlet of the runoff monitoring station. The collection method consisted of sampling farmland runoff, woodland runoff, vegetable field runoff, well water, and river water after each rainfall runoff. This was done using a polyethylene bottle (500 mL) to collect a total of two bottles of water when sampling. Sampling time, rainfall, and runoff were all observed. The collected samples were then frozen and brought back to the laboratory within 48 h to complete the test analysis. The analysis method was the “Water and Wastewater Monitoring and Analysis Method (Fourth Edition)” [18]. Water analysis indicators included total nitrogen (TN), total particulate nitrogen (PN), dissolved total nitrogen (DN), nitrate nitrogen (NN), ammonium nitrogen (AN), total phosphorus (TP), dissolved total phosphorus (DTP), dissolubility organic phosphorus 3 (DOP), soluble phosphorus (PO4 −-P), and particulate phosphorus (PP). One of the water samples needed to be filtered through a 0.45 µm filter. Two bottles of water samples were measured each 3- for NN, AN, TN, DN, TP, DTP, and PO4 -P using a nitrogen and phosphorus analysis module in a continuous flow analyzer (AutoAnalyzer-3, Norderstedt, Germany). DN, DOP, and PP were calculated 3 using the following equations: DON = DN-AN-NN, DOP = DTP-PO4 −-P, and PP = TP-DTP [18–20].

2.3. Analytical Methods In this study, SPSS 19.0 was used for data analysis and Origin 2016 was used for drawing. This research includes five sets of data including farmland, forest land, vegetable field, surface water, and ground water. To study whether there are differences in these data, multiple comparisons should be made between multiple samples. The least significant difference method (LSD) is the simplest method of multiple comparisons to test the mean difference between two groups. LSD used t-test to complete the matching comparison of questions for each group and the test was highly sensitive. Small differences in the mean values of each level might also be detected. Therefore, the LSD of SPSS19.0 was used for a significance test.

3. Results and Discussion

3.1. Spatiotemporal Changes of Nitrogen and Phosphorus at Diﬀerent Monitoring Sample Points

3.1.1. Changes of Nitrogen and Phosphorus in Farmland Runoff Plot Soils in agricultural plots are susceptible to oxidation under dry farming conditions, which facilitates nitrification. During natural rainfall, nutrients in soil and crop residues migrate to surface runoff. In terms of nitrogen structure, the presence of NN is consistent with the change in total soluble nitrogen. In this study, nitrogen and phosphorus losses in farmland were affected by Sustainability 2020, 12, 2012 5 of 13 rainfall in different periods (Figure2). Among these, the nitrogen concentration in the water body on 23 May 2017 and 7 May 2018 exhibited the maximum values for the year. The rainfall on 23 May 2017 Sustainabilityhad a high 2020 impact, 12, xon FOR the PEER nitrogen REVIEW content in the farmland soil. The TN content in the water reached5 of 13 116.9 mg/L, and the total soluble nitrogen (DN) within the TN reached 88.19 mg/L. The water-soluble fertilizer NN is the main contributor to nitrogen lo lossss in TN in nitrogen, accounting for 69.8% of TN content. This This is is likely likely because because the the spring spring corn corn has has just just been been planted planted in in the the farmland farmland in in early early May, May, and the fertilizers applied by farmers have not yet been used by thethe vegetation.vegetation. With With the occurrence of rainfall, the nitrogen content in the farmland runoff runoff water continued to decrease until mid-July. The nitrogen in farmland increased with rainfall from mid-July to August.August. Subsequently, nitrogen loss decreased andand stabilized.stabilized. ThisThis is is because because local local farmers farmers will will apply apply quick-acting quick-acting nitrogen nitrogen fertilizer fertilizer to the to theearly early stage stage of grain of grain filling filling in order in order to ensure to ensure an acceptablean acceptable yield yield at aat later a later date date [21 [21,22].,22]. As As a result a result of ofsupplementation supplementation with with external external nitrogen nitrogen elements, elements, rainfall rainfall will cause will thecause continuous the continuous loss of nitrogen. loss of nitrogen.During the During dry season the dry at se theason beginning at the beginning of 2018, there of 2018, was there no runo wasff nodata runoff prior data to May. prior This to May. was This due wasto the due fact to that the rainfall fact that did rainfall not form did an not eff formective an runo effectiveff on farmland runoff on at farmland this time [at23 ,this24]. time Heavy [23,24]. rains Heavyoccurred rains in earlyoccurred May in and early mid-May, May and causing mid-May, underutilized causing underutilized nitrogen fertilizer nitrogen in the fertilizer wheat field in the of wheatthe farmland field of tothe be farmland discharged to be into discharged the river withinto runothe riverff. Consequently, with runoff. Co thensequently, nitrogen content the nitrogen in the contentfarmland in decreasedthe farmland sharply. decreased sharply.

Figure 2. The change of nitrogen and phosphorus with rainfall in farmland. Figure 2. The change of nitrogen and phosphorus with rainfall in farmland. Phosphorus and nitrogen in farmland display similar trends. Most of the fertilizers applied to winterPhosphorus wheat in theand study nitrogen area in were farmland phosphate display fertilizers, similar andtrends. a large Most amount of the fertilizers of nitrogen applied fertilizer to waswinter used wheat as topdressing in the study in area the later were stage. phosphate On 29 Septemberfertilizers, and 2017, a winterlarge amount wheat had of nitrogen just been fertilizer planted. Thewas subsequentused as topdressing heavy rain in the caused later astage. large On amount 29 September of various 2017, types winter of phosphate wheat had fertilizers just been to planted. be lost Thein the subsequent soil, and theheavy maximum rain caused value a oflarge total amount phosphorus of various in the types water of phosphate reached 6.34 fertilizers mg/L. There to be willlost inalways the soil, be aand certain the maximum amount of value phosphorus of total lossphosphorus in the runo in theff caused water reached by rainfall, 6.34 and mg/L. the There main will loss 3 alwayscomponent be a ofcertain phosphorus amount is of dissolved phosphorus phosphate loss in (POthe 4runoff−-P). Duringcaused by the rainfall, key period and ofthe fertilization main loss componentin this study, of thephosphorus rainfall intensity is dissolved was phosphate found to be (PO both43−-P). significant During the and key positively period of correlated fertilization with in thisthe amountstudy, the of nitrogenrainfall intensity and phosphorus was found lost. to be The both results significant of this studyand positively are consistent correlated with with those the of wangjiagouamount of nitrogen agricultural and sub-basin phosphorus in the lost. Three-Gorges The results reservoir of this study area [25 are]. consistent with those of wangjiagou agricultural sub-basin in the Three-Gorges reservoir area [25]. 3.1.2. Changes of Nitrogen and Phosphorus in Vegetable Field Runoff Area 3.1.2.The Changes runo ffofformed Nitrogen during and Phosphorus initial monitoring in Vegetable in mid-to-late Field Runoff May Area 2017 showed that the TN concentrationThe runoff was formed 43.76 mgduring/L, and initial the TPmonitoring concentration in mid-to-late was 6.37 mgMay/L 2017 (Figure showed3). These that were the theTN concentrationhighest values observedwas 43.76 during mg/L, theand monitoring the TP concentration period. The mainwas 6.37 forms mg/L of nitrogen (Figure and 3). phosphorusThese were lossthe werehighest AN values and DTP, observed accounting during for th 91.5%e monitoring of total period. nitrogen The and main 84.2% forms of total of nitrogen phosphorus, and phosphorus respectively. lossDuring were the AN dry season,and DTP, it is accounting difficult for for rainfall 91.5% to formof total an enitrogenffective runoandff 84.2%. This causesof total a largephosphorus, amount respectively. During the dry season, it is difficult for rainfall to form an effective runoff. This causes a large amount of nitrogen and phosphorus to accumulate in the soil. At the same time, the addition of farm manure applied in early spring makes the nitrogen and phosphorus content in rainfall runoff highest in May. In this study, the repeated occurrence of rainfall caused the nitrogen and phosphorus content of vegetable field runoff to decrease to its lowest point around mid-June. In mid-to-late June, Sustainability 2020, 12, 2012 6 of 13 of nitrogen and phosphorus to accumulate in the soil. At the same time, the addition of farm manure applied in early spring makes the nitrogen and phosphorus content in rainfall runoff highest in May. In thisSustainability study, the 2020 repeated, 12, x FOR occurrence PEER REVIEW of rainfall caused the nitrogen and phosphorus content of vegetable6 of 13 field runoff to decrease to its lowest point around mid-June. In mid-to-late June, 7 August, 6 September, 297 August, September, 6 September, and 22 May 29 2018,September, nitrogen and and 22 phosphorusMay 2018, nitrogen concentrations and phosphorus in the water concentrations body formed in significantthe water body peaks formed and troughs. significant The crestpeaks period and troughs. of rainfall The can crest be seenperiod after of fertilizationrainfall can ofbe vegetableseen after fields.fertilization Figure 3of shows vegetable that the fields. change Figure in nitrogen 3 shows and that phosphorus the change concentration in nitrogen within and the phosphorus vegetable fieldconcentration runoff is significantlywithin the vegetable correlated field with runoff the is fertilization significantly period correlated and rainfall,with the andfertilization the change period of nitrogenand rainfall, and phosphorusand the change concentration of nitrogen are and significantly phosphorus consistent concentration with eachare significantly other in terms consistent of time. Therefore,with each runootherff waterin terms quality of time. in the Therefore, vegetable fieldrunoff is alsowater significantly quality in correlatedthe vegetable with rainfallfield is andalso fertilizationsignificantly [26 correlated]. with rainfall and fertilization [26].

Figure 3. The change of nitrogen and phosphorus with rainfall in vegetable field. Figure 3. The change of nitrogen and phosphorus with rainfall in vegetable field. Vegetable gardens are an indispensable part of the life for rural residents in China. Most of the nutrientsVegetable used for gardens planting are are an derived indispensable from farm part manure. of the life Therefore, for rural the residents pollution in caused China. byMost runo offf thein vegetablenutrients gardensused for canplanting be used are as derived a representation from farm ofmanure. rural animal Therefore, husbandry the pollution and domestic caused pollutionby runoff sources.in vegetable Since gardens there is can no clearbe used special as a period represen fortation the fertilization of rural animal of vegetable husbandry fields, and the domestic amount ofpollution fertilizer sources. applied Since has a there direct is relationshipno clear special with period the growth for the cycle fertilization and replacement of vegetable of vegetables. fields, the Studyingamount of vegetable fertilizer fields applied can helphas determininga direct relationship the extent with of pollution the growth caused cycle by rainfall,and replacement aquaculture, of andvegetables. daily life Studying during the vegeta runobleff period fields ofcan rainfall help determining in the reservoir the exte area.nt The of pollution pollutant caused load in by the rainfall, runoff changesaquaculture, with and seasonal daily characteristics life during the and runoff is also period affected of rainfall by the in dual the e ffreservoirects of human area. The agricultural pollutant activitiesload in the and runoff natural changes climate. with seasonal characteristics and is also affected by the dual effects of human agricultural activities and natural climate. 3.1.3. Changes of Nitrogen and Phosphorus in Forest Runoff Plots 3.1.3.The Changes total nitrogen of Nitrogen concentration and Phosphorus of woodland in Forest in this Runoff study Plots exhibited four peaks during 2017–2018 (FigureThe4). total The highestnitrogen value concentration appeared of on woodland 28 June 2017, in this and study its value exhibited reached four 23.14 peaks mg during/L. TN, 2017– DN, and2018 AN (Figure displayed 4). The a consistenthighest value change appeared trend, andon 28 the June main 2017, output and formits value was reached observed 23.14 to be mg/L. DN. TheTN, changeDN, and of AN phosphorus displayed concentration a consistent inchange woodland trend, had an threed the highmain peaks output during form thewas rainfall. observed The to highest be DN. primaryThe change phosphorus of phosphorus concentration concentration was on in 29woodland September had 2017, three and high the peaks highest during value the reached rainfall. was The 3 6.56highest mg/ L.primary The trends phosphorus for TP, PP, concentration and PO4 −-P was are on similar. 29 September The main 2017, output and modethe highest of phosphorus value reached was 3 POwas4 −6.56-P. Figuremg/L. 4The shows trends strong for TP, correlation PP, and PO between43−-P are total similar. nitrogen The main concentration output mode in forestland of phosphorus and rainfall,was PO4 and3−-P. the Figure formation 4 shows of strong runoff correlationthat comprises between the peaktotal andnitrogen valley concentration of nitrogen andin forestland phosphorus and concentrationrainfall, and the in theformation water quality of runoff of forestlandthat comprises monitoring the peak plots and can valley also of be nitrogen seen. When and thephosphorus slope is greaterconcentration than 17 ◦in, thethe amountwater quality of soil of erosion forestland with moni rainfalltoring runo plotsff will can increase also be [ 27seen.]. When the slope is greater than 17°, the amount of soil erosion with rainfall runoff will increase [27]. Sustainability 2020, 12, 2012 7 of 13 Sustainability 2020, 12, x FOR PEER REVIEW 7 of 13

Figure 4. The change of nitrogen and phosphorus with rainfall in forestland. Figure 4. The change of nitrogen and phosphorus with rainfall in forestland. Forest land is used as a “sink” for NPSP in most areas, and this causes degradation, buffering, and otherForest e fflandects is on used NPSPs. as a The“sink” main for external NPSP in sources most areas, of nutrients and this such causes as nitrogen degradation, and phosphorus buffering, andin woodland other effects runo onff NPSPs.communities The main include external the sources decay of of leaves,nutrients vegetation, such as nitrogen and litter and by phosphorus long-term inmicroorganisms. woodland runoff When communities rainfall forms include runo theff, it decay erodes of the leaves, forest vegetation, soil. Subsequently, and litter the by surface long-term soil microorganisms.migrates with the When runoff rainfall, causing forms the lossrunoff, of nutrients it erodes suchthe forest as nitrogen soil. Subsequently, and phosphorus. the surface Therefore, soil migratesforestland with can the also runoff, be used causing as a “source” the loss of of pollution, nutrients assuch it transports as nitrogen nutrients and phosphorus. such as nitrogen Therefore, and phosphorusforestland can to thealso outside be used world. as a “source” Forest slope of pollution, and rainfall as it conditions transports (rainfall nutrients level, such intensity, as nitrogen duration, and phosphorusetc.,) affect soil to nutrientthe outside loss andworld. loss Forest patterns. slope A greater and rainfall runoff strengthensconditions the(rainfall effect level, of soil intensity, erosion. duration, etc.,) affect soil nutrient loss and loss patterns. A greater runoff strengthens the effect of soil erosion.3.1.4. Changes of Nitrogen and Phosphorus in Groundwater Most of the nutrients in well water are formed by the infiltration of surface water into the ground. 3.1.4. Changes of Nitrogen and Phosphorus in Groundwater Except for the high nitrogen content on 20 August 2017 (Figure5), the nitrogen content concentration was relativelyMost of the stable nutrients at other in times well in water this study. are Theformed average by the values infiltration of TN, DN, of andsurface NN werewater 7.03 into mg the/L, ground.5.85 mg/ L,Except 6.11 mgfor/ L,the respectively. high nitrogen NN content is the main on 20 component August 2017 ofTN (Figure dissolved 5), the in nitrogen water, and content there concentrationare large differences was relatively in the concentration stable at other of times different in this kinds study. of phosphorus The average in values well water. of TN, The DN, main and phosphorusNN were 7.03 component mg/L, 5.85 in mg/L, water 6.11 is DTP. mg/L, With respecti the formationvely. NN of is rainfall,the main the co phosphorusmponent of contentTN dissolved in the waterin water, fluctuates and there greatly. are large DTP differences in surface in runo theff conceasilyentration enters theof different water body kinds through of phosphorus infiltration in afterwell water.rainfall, The and main the phosphorus phosphorus composition component and in rainfall water intensityis DTP. changeWith the significantly formation as of a consequence.rainfall, the Thephosphorus change characteristicscontent in the water of nitrogen fluctuates and greatly. phosphorus DTP in in surface groundwater runoff easily in this enters study the are water similar body to throughthose of manyinfiltration groundwater after rainfall, studies and [28,29 the]. Wellphosph waterorus is formedcomposition by surface and waterrainfall penetrating intensity intochange the significantlyground, meaning as a that consequence. changes in theThe nitrogen change and ch phosphorusaracteristics contentof nitrogen of well and water phosphorus can reflect thein groundwaterstate of surface in water this study after filtration.are similar to those of many groundwater studies [28,29]. Well water is formed by surface water penetrating into the ground, meaning that changes in the nitrogen and phosphorus content of well water can reflect the state of surface water after filtration. Sustainability 2020, 12, x FOR PEER REVIEW 8 of 13

Sustainability 2020, 12, 2012 8 of 13 Sustainability 2020, 12, x FOR PEER REVIEW 8 of 13

Figure 5. The change of nitrogen and phosphorus with rainfall in groundwater. Figure 5. The change of nitrogen and phosphorus with rainfall in groundwater. Figure 5. The change of nitrogen and phosphorus with rainfall in groundwater. 3.1.5. Changes of Nitrogen and Phosphorus in Surface Water 3.1.5.The Changes process of of Nitrogen surface andrunoff runo Phosphorusff caused by in rainfall Surface ca causes Wateruses the loss loss of of nitrogen nitrogen and and phosphorus phosphorus [30]. [30]. River water in this study is primarily derived from the collection of precipitation in the surrounding RiverThe water process in this of studysurface is runoff primarily caused derived by rainfall from the causes collection the loss of of precipitation nitrogen and in phosphorus the surrounding [30]. woodland and farmland. The water body itself has a self-purification self-purification effect, effect, meaning that the change Riverof nitrogen water and in this phosphorus study is primarily in river water derived analyzed from the is also collection the state of ofprecipitation nitrogen and in phosphorus the surrounding after woodlandof nitrogen and and farmland. phosphorus The in water river body water itself analyzed has a isself-purification also the state of effect, nitrogen meaning and phosphorus that the change after self-purification.self-purification. The results in Figure 66 indicateindicate thatthat nitrogennitrogen inin thethe riverriver showedshowed anan extremelyextremely highhigh ofvalue nitrogen in mid-to-late and phosphorus May at thein river beginning water ofanalyzed the rainfall, is also and the then state fluctuated of nitrogen while and alsophosphorus remaining after at self-purification.value in mid-to-late The May results at the in Figure beginning 6 indicate of the rainfall,that nitrogen and then in the fluctuated river showed while an also extremely remaining high at low levels. The The contents contents of of NN NN and and PN PN are are higher higher th thanan other indicators, with average concentrations valueof 2.79 in mg/L mid-to-late and 1.42 May mg/L, at the respectively. beginning Theof the cont rainfall,ents of and each then component fluctuated fl uctuatedwhile also greatly remaining during at lowof 2.79 levels. mg/ TheL and contents 1.42 mg of/L, NN respectively. and PN are The higher contents than other of each indicators, component with fluctuated average concentrations greatly during the initial rainfall, but changed smoothly at other times. This This is is likely likely due due to to a a lack lack of of effective effective runoff runoff offormed 2.79 mg/L during and the 1.42 dry mg/L, season, respectively. as most of the The pollutants contents flowedof each intocomponent the water fluctuated body through greatly the during initial theformed initial during rainfall, the but dry changed season, as smoothly most of theat other pollutants times. flowed This is intolikely the due water to a body lack of through effective the runoff initial runorunoff.ff. However, the initial rainfall was small, limiting the digestion of pollutants and exceeding the formedself-purification during the capacity dry season, of the as water most body. of the Therefore, pollutants the flowed first rainfallinto the likelywater caused body through the surface the initialwater runoff.self-purification However, capacity the initial of therainfall water was body. small, Therefore, limiting the the first digestion rainfall of likely pollutants caused and the exceeding surface water the nutrients to reach extremely high values.values. In subsequent rainfalls, the dual effects effects of rainfall and the self-purificationrainfall cycle were capacity likely ofresponsible the water forbody. keeping Therefore, the value the first of nitrogen rainfall likely in a stable caused state. the surfaceAccording water to nutrientsrainfall cycle to reach were extremely likely responsible high values. for keepingIn subsequent the value rainfalls, of nitrogen the dual in aeffects stable of state. rainfall According and the tothese these observations, observations, it appears it appears that thatphosphorus phosphorus in surface in surface water waterhas a strong has a strongrelationship relationship with rainfall with rainfalland seasonal cycle werechanges. likely Similarly, responsible other for studies keeping have the shown value ofthat nitrogen the surface in a nitrogenstable state. and According phosphorus to theserainfall observations, and seasonal it appears changes. that Similarly, phosphorus other in studiessurface water have shownhas a strong that therelationship surface nitrogen with rainfall and phosphorusoutput pathways output can pathways be attributed can be to attributed surface torunoff, surface and runo theff ,seasonality and the seasonality of the output of the load output is andsignificant seasonal [31,32]. changes. Similarly, other studies have shown that the surface nitrogen and phosphorus outputload is significantpathways [can31,32 be]. attributed to surface runoff, and the seasonality of the output load is significant [31,32].

Figure 6. The change of nitrogen and phosphorus with rainfall in river. Figure 6. The change of nitrogen and phosphorus with rainfall in river.

Figure 6. The change of nitrogen and phosphorus with rainfall in river. SustainabilitySustainability 20202020, ,1212, ,x 2012 FOR PEER REVIEW 99 of of 13 13

3.2. Comparative Study of Water Quality under Different Land Use Types 3.2. Comparative Study of Water Quality under Different Land Use Types 3.2.1. Comparison of Nitrogen Content in Different Land Use Types 3.2.1. Comparison of Nitrogen Content in Different Land Use Types In this study, the farmland water body, vegetable fields, woodland, well water, and river water In this study, the farmland water body, vegetable fields, woodland, well water, and river water were monitored by long-term runoff in the sub-watershed. The nitrogen concentration of the different were monitored by long-term runoff in the sub-watershed. The nitrogen concentration of the different types of samples exhibited significant differences (Figure 7). The overall nitrogen concentration levels types of samples exhibited significant differences (Figure7). The overall nitrogen concentration levels are ranked as farmland > vegetable field > forestland. This shows that total nitrogen output forms at are ranked as farmland > vegetable field > forestland. This shows that total nitrogen output forms different points, and is strongly affected by the types of sample points. Different measures should be at different points, and is strongly affected by the types of sample points. Different measures should taken for NPSP source pollution control. The average value of TN in farmland in this study was 18.26 be taken for NPSP source pollution control. The average value of TN in farmland in this study mg/L, and the concentration of TN in farmland runoff was significantly different from TN was 18.26 mg/L, and the concentration of TN in farmland runoff was significantly different from TN concentration in forestland, well water, and river water. The average value of vegetable field TN was concentration in forestland, well water, and river water. The average value of vegetable field TN was 13.50 mg/L. This concentration was observed to be significantly different from that of forestland, well 13.50 mg/L. This concentration was observed to be significantly different from that of forestland, well water, and river water. Key differences were observed in nitrogen concentrations of different types water, and river water. Key differences were observed in nitrogen concentrations of different types in the five runoff monitoring sample points. When compared with vegetable fields and farmland, in the five runoff monitoring sample points. When compared with vegetable fields and farmland, forestland was observed to be more conducive to the buffering, adsorption, and fixation of nitrogen. forestland was observed to be more conducive to the buffering, adsorption, and fixation of nitrogen. An appropriate increase in crop cover in farmland and vegetable fields could effectively reduce the An appropriate increase in crop cover in farmland and vegetable fields could effectively reduce the amount of nitrogen and phosphorus easily absorbed on eroded soil in surface runoff. amount of nitrogen and phosphorus easily absorbed on eroded soil in surface runoff.

Figure 7. Comparison of water quality nitrogen under different runoff monitoring types. Figure 7. Comparison of water quality nitrogen under different runoff monitoring types. The main source of nitrogen in farmland is the application of chemical fertilizers. In vegetable The main source of nitrogen in farmland is the application of chemical fertilizers. In vegetable fields, it is livestock manure and domestic sewage. The main source of nitrogen in forest land is fields, it is livestock manure and domestic sewage. The main source of nitrogen in forest land is self- self-replacement in the ecosystem. Nitrogen output from well water and river water is primarily replacement in the ecosystem. Nitrogen output from well water and river water is primarily derived derived from infiltration and runoff. As previously mentioned, the output forms of nitrogen are mainly from infiltration and runoff. As previously mentioned, the output forms of nitrogen are mainly DN DN and NN. The high NN concentration of well water is due to the precipitation of nitrogen-containing and NN. The high NN concentration of well water is due to the precipitation of nitrogen-containing groundwater into the ground through soil pores and the poor self-purification ability of the groundwater groundwater into the ground through soil pores and the poor self-purification ability of the body. This is consistent with the results of significantly higher groundwater nitrogen content in groundwater body. This is consistent with the results of significantly higher groundwater nitrogen cultivated areas studied by Hallberg [33] and others. AN is unstable and volatile, and it is easy for content in cultivated areas studied by Hallberg [33] and others. AN is unstable and volatile, and it is vegetation to absorb and use AN. As a result, the AN concentration in runoff is typically low. PN is easy for vegetation to absorb and use AN. As a result, the AN concentration in runoff is typically low. easily adsorbed on the surface of soil and organic matter, so its content in runoff is also usually low. PN is easily adsorbed on the surface of soil and organic matter, so its content in runoff is also usually low.

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3.2.2. Comparison of Phosphorus Content in Different Land Use Types 3.2.2. Comparison of Phosphorus Content in Different Land Use Types The phosphorus and nitrogen concentration in farmland, vegetable land, forestland, well water The phosphorus and nitrogen concentration in farmland, vegetable land, forestland, well water and river water runoff in the sub-watershed are quite different (Figure 8). The phosphorus content in and river water runoff in the sub-watershed are quite different (Figure8). The phosphorus content in these land use types are ranked as vegetable land > forest land > farmland. The concentrations of these land use types are ranked as vegetable land > forest land > farmland. The concentrations of 3− DOP, PP, PO3 4 -P, DTP, and TP in river water and well water are much lower than those in vegetable, DOP, PP, PO4 −-P, DTP, and TP in river water and well water are much lower than those in vegetable, woodland, and farmland runoff. The overall TP, DTP, and PO43−-P of vegetable field runoff are also woodland, and farmland runoff. The overall TP, DTP, and PO 3 -P of vegetable field runoff are also much higher than those of woodland and farmland runoff. The4 − average TP of vegetable field runoff much higher than those of woodland and farmland runoff. The average TP of vegetable field runoff was 2.23 mg/L, and the TP content of vegetable field was 2 times and 4.82 times that of woodland was 2.23 mg/L, and the TP content of vegetable field was 2 times and 4.82 times that of woodland and farmland runoff, respectively. DTP and PO43−-P in vegetable plots reached 1.91 mg/L and 1.73 and farmland runo , respectively. DTP and PO 3 -P in vegetable plots reached 1.91 mg L and 1.73 ff 3− 4 − / mg/L, respectively. DTP and PO34 -P in crop plots were only 0.39 mg/L and 0.32 mg/L, respectively. mg/L, respectively. DTP and PO4 3−− -P in crop plots were only 0.39 mg3/−L and 0.32 mg/L, respectively. A medium level of DTP and PO43 -P was observed in forestland. PO4 -P3 in vegetable, forestland, and A medium level of DTP and PO4 −-P was observed in forestland. PO4 −-P in vegetable, forestland, farmland accounted for 77.7%, 63.5%, and 69.3% of TP, respectively. Therefore, PO43−-P is the main and farmland accounted for 77.7%, 63.5%, and 69.3% of TP, respectively. Therefore, PO 3 -P is the output method of phosphorus and has an important impact on eutrophication of water4 bodies.− This main output method of phosphorus and has an important impact on eutrophication of water bodies. study is similar to the migration characteristics of phosphorus in the xitiao river along with the This study is similar to the migration characteristics of phosphorus in the xitiao river along with the amount of rainstorm runoff [34]. amount of rainstorm runoff [34].

Figure 8. Figure 8.Comparison Comparison of of phosphorus phosphorus in in water water under under di differentfferent runo runoffff monitoring monitoring types. types. 4. Conclusions 4. Conclusions The output of NPSP has a strong relationship with different land use types, and rainfall has a large impactThe output on the of output NPSP ofhas NPSP. a strong In this relationship study, the relationshipwith different between land use nitrogen, types, phosphorus,and rainfall andhas a rainfalllarge impact in different on the runo outputff communities of NPSP. In and this the stud typesy, ofthe output relationship were analyzed, between andnitrogen, the nitrogen phosphorus, and phosphorusand rainfall output in different of di ffrunofferent landcommunities use types and were the compared.types of output The were results analyzed, of our study and the led nitrogen to the followingand phosphorus conclusions: output of different land use types were compared. The results of our study led to the (1)following Nitrogen conclusions: and phosphorus loss in runoff plots of different land use types are affected by the rainfall(1) Nitrogen intensity and and phosphorus fertilization loss periods. in runoff Nitrogen plots of and different phosphorus land use content types inare runo affectedff formed by the immediatelyrainfall intensity after fertilization and fertilization in farmland periods. and Nitrogen vegetable and plots, phosphorus and displayed content a significant in runoff increase. formed immediately after fertilization in farmland and vegetable plots, and displayed a significant increase. The loss of nitrogen and phosphorus in forestland is directly affected by rainfall intensity. Greater The loss of nitrogen and phosphorus in forestland is directly affected by rainfall intensity. Greater amounts of rainfall result in higher losses of nitrogen and phosphorus. amounts of rainfall result in higher losses of nitrogen and phosphorus. (2) The main sources of nitrogen in NPSP are farmland and vegetable fields, and the main source (2) The main sources of nitrogen in NPSP are farmland and vegetable fields, and the main source of phosphorus is vegetable fields. NN is the primary form of nitrogen loss in NPSP, and NN within of phosphorus is vegetable fields. NN is the primary form of nitrogen loss in NPSP, and NN within farmland runoff accounts for 69.8% of the total nitrogen content. The main component of phosphorus Sustainability 2020, 12, 2012 11 of 13 farmland runoff accounts for 69.8% of the total nitrogen content. The main component of phosphorus 3 loss in NPSP is PO4 −-P, and vegetable fields are the main source of this. The TP content in vegetable fields is 2 times and 4.82 times that of forestland and farmland runoff, respectively. This shows that the nitrogen in NPSP is primarily derived from planting, while most phosphorus originates from domestic sewage and livestock breeding. Overall, agriculture has the greatest impact on water quality. (3) Nitrogen and phosphorus pollution have a defined incubation period. In the dry season of the reservoir area (November–April), it is difficult for rainfall to form effective runoff. Because of this, underutilized nutrients such as nitrogen and phosphorus will settle in the soil. When the rainy season arrives, nitrogen and phosphorus deposited on the soil will be taken away following the first rainfall runoff. Therefore, controlling NPSP requires effective management of the first runoff generated by rainfall.

Author Contributions: Conceptualization, J.Y. and J.L.; Methodology, J.Y.; Supervision, G.Y.; Validation, Y.F.; Writing—original draft, J.Y. and J.L.; Writing—review and editing, J.Y., G.R., C.R., X.H. and X.W. All authors have read the paper and agreed to publish this version of manuscript. Funding: This research was funded by National Major Science and Technology Projects of China, grant number 2015FY110400-3. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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