Grass and Plastic Film Mulching Pattern Improve Soil Organic Carbon Pool, Physical Properties, Fertility and Fruit Quality of Ponkan Orchards

Aims: Special fertilizer and soil mulching have been used to improve crop yield worldwide. However, the effects of special fertilizer with mulching on soil characteristics of citrus orchards were not yet fully understood. This study aimed at assessing the effects of different mulch patterns including plastic mulching and grass mulching on improving fruit quality of Ponkan and providing a new insight of mulching development in citrus orchards. Methods: In this study, a two-year eld experiment was conducted to investigate the effects of citrus special fertilizer (FR), grass mulching (RGM), and plastic mulching (RPM) on fruit quality, soil organic carbon fractions, physicochemical properties, and plant nutrition in Ponkan citrus orchard. Results: The study resulted showed that special fertilizer treatment and grass mulching treatment increased Total soluble solid contents by 6.76% and 3.97%, while plastic mulching decreased Titratable acid contents by 19.44%, resulting in increases of fruit TSS: TA by 6.14%, 3.61%, and 22.76%, respectively. Correlation analyses showed that citrus fruit quality was associated with soil bulk density, Total porosity, Capillary porosit, Aeration porosity, Total organic carbon, Readily oxidized organic carbon, Microbial biomass carbon and Soil available phosphorus. Soil physical properties were improved by RPM, but FR and RGM had better effects on soil nutrients and organic carbon, as evidenced by the results of principal component analysis and loading matrix plot. Conclusions: This study suggested that the different effects of special fertilizer, plastic mulching, and grass mulching on improving fruit quality were associated with their diverse effects on improving soil characteristics.


Introduction
Soil mulching is an important part of agronomic practices which conserves soil moisture and reduces nutrient loss from soil. The effects of soil mulching management techniques on increasing production of crops such as wheat, maize, winter oilseed rape, and cotton have widely reported in arid and semi-arid regions of the world (Xiao-rui et al. 2002;Zhang et al. 2005;Naudin et al. 2010;Gu et al. 2017;He et al. 2017;Hu et al. 2019;Yadav et al. 2019). Mulching separates the soil and air-environment to conserve soil moisture, and reduces weed infestation and modify soil physical properties, as well as avoids nutrition loss and soil erosion caused by agricultural irrigation and rainfall (Aarstad and Miller 1981;Khatibu et al. 1984;Gholami et al. 2013;Li et al. 2013;Qin et al. 2013;Mo et al. 2016). On the other hand, earlier studies showed that mulching increased soil organic carbon of topsoil by 41% in the fourth year and by 52% in the eleventh year (Saroa and Lal 2003). A nine-year effects of soil mulching on soil C and soil bacterial diversity of dryland winter wheat showed that straw mulching increased soil C content, and plastic lm mulching enhanced fungal diversity (Fu et al. 2019). Wang et al. (2019) compared the effects of straw mulching, and plastic lm mulching on maize growth, and concluded that both straw mulching and plastic lm mulching could increase maize grain yield and above-ground biomass, and straw mulching had great effects on soil intermediate C fractions. Recently, the bene ts of soil mulching in regulating soil characteristics including organic carbon fractions, soil physicochemical properties and fertility attracted attentions of fruit growers.
Citrus is a perennial woody plant which prone to face soil deterioration and subsequent root damage, ultimately resulting in reduced citrus fruit yield and quality. China is one of the most important production countries, and the citrus industry has been the pillar industry of some city's economy. However, citrus orchards in China are mostly located in hilly areas where the ecosystem is very fragile and short of a welldeveloped irrigation system. Although building of small-scale storage or impounding reservoirs could partly alleviate water shortage, while nutrition loss and soil erosion caused by agricultural irrigation and rainfall are unavoidable (Zhou et al. 2010;Tang et al. 2016;Bagagiolo et al. 2018). Additionally, citrus orchards are mostly established on acidic soils, such as red soil, purple soil, and yellow soil, where the long-term and excessive application of chemical fertilizer aiming at increasing yield not only decrease fertilizer utilization, but also soil fertility (Zhang et al. 2009). Soil surface management including straw mulching and plastic lm mulching could signi cantly control runoff, conserve soil nutrition, and increase fruit yield (Liu et al. 2012). Moreover, plastic lm mulching enhanced the activities of acid invertase, sucrose synthase, and reduce activities of cyt-Aco and cyt-IDH, affecting citrus fruit quality (Jiang et al. 2014). A three-year eld trial result showed that grass mulching promoted root growth, increased fruit yield, and provided higher species diversity of citrus orchards (Homma et al. 2012). The total soluble solids and titratable acid of citrus fruit play an important role in fruit avor and market partiality. The fruit avor is in uenced by plant nutrients (Zhou et al. 2018), soil characteristics (Zheng et al. 2015), water stress (Navarro et al. 2010), temperatures, and illumination (Lombard et al. 1965). However, it was obscure how the variations of soil organic carbon pool, physicochemical properties, and soil fertility, caused by soil mulching, affected citrus fruit acidity and sweetness. Here, we compared the effects of grass mulching, plastic lm mulching, and special fertilizer on total soluble solids and titratable acid of citrus fruit, soil organic carbon pool, physicochemical properties, and fertility, as well as plant nutrition to address the above issues. We further discussed the potential mechanisms of improvement of fruit quality caused by soil mulching, aiming at providing a new insight to help mulching development in citrus orchards.

Materials And Methods
Plant material and experimental design available P, 13.50 mg kg −1 and available K, 53.00 mg kg −1 . Four treatments were designed as follows: CK (control or local customary fertilization); FR (special fertilizer with 25% less total NPK nutrition); RGM (special fertilizer with 25% less total NPK nutrition, and grass mulching); RPM (based on special fertilizer with 25% less total NPK nutrition, and plastic lm mulching), with three replicates, each treatment, and three Ponkan trees for every replicate. K 2 SO 4 (K 2 SO 4 ≥ 51%),, NH 4 H 2 PO 4 (NH 4 H 2 PO 4 ≥ 48.5%), CO(NH 2 ) 2 (CO(NH 2 ) 2 ≥ 46.4%) were used as fertilizers resource of CK, and special fertilizer for citrus (CO(NH 2 ) 2 ≥ 15%: NaH 2 PO 4 ≥ 7%, K 2 SO 4 ≥ 13%, with OM ≥ 10 %) was used as chemical fertilizers resource of FR, On November 20, 2018, we drilled three soil holes that 1m-1.2m away from trunk (Fig 1) per tree and collected 0-20 cm and 20-40 cm soil samples, then repeated this process and mixed three trees of the same replication to create one composite sample for nutrients and carbon fractions determination. The soil parameters were measured according to the soil physicochemical analysis handbook (Bao 2000). After removing the stones and organic materials and root debris, every soil sample was divided into two parts, one part was saved at 4 °C for measurement of soil organic carbon pool, the other part was crushed with a wooden roller after air-drying to pass through a 1 mm sieve for soil available N, P, K determination.
The alkaline hydrolysis diffusion method was used to determine the soil-available N. Available P was determined by Olsen's method. Available K was extracted with 1 mol L −1 NH 4 OAC and determined by ame photometer (FP6410, INESA, China). Soil total organic carbon (TOC), microbial biomass carbon (MBC), and readily oxidized organic carbon (ROC) were determined by K 2 CrO 7 -H 2 SO 4 wet oxidation method (Walkley and Black 1934), CHCl 3 fumigation-extraction method (Vance et al. 1987), and Centrifugal method (Blair et al. 1995), respectively. Field-moist soil samples (5.0 g) was used to extract soil dissolved organic carbon (DOC) with 50 mL deionized water, by shaking for two hours at 180 rpm, and then measured DOC by a Vario TOC select analyzer (Elementar). Soil samples were collected from 1m-1.2m away from trunk (Fig 1) for determination of physical properties with stainless steel ring-knife and measured as described by Klute and Hartage (1986). Fruit samples were collected in November 2018 which were bagged separately for the determination of fruit quality. Each replication consisted of 24 fruits from three trees were all located at the middle part of the tree crown. The content of fruit total soluble solids (TSS), and titrate acid (TA) were measured as described by Zhang (2014).

Statistical analysis
Unless otherwise noted, the results are given as means ± standard error (SE). Data were analyzed using analysis of variance (ANOVA) and the differences between means were determined by the least signi cant difference (LSD) test at p < 0.05. The SPSS PASW Statistics 18.0 analytical software package, origin Pro 2018, and Rstudio were used for all statistical analyses and picture production, respectively.

Effects of special fertilizer and mulching patterns on fruit qualities
There were no signi cant differences in fruit TSS (total soluble solids) contents but remarkable variations in TA (titratable acid) and TSS: TA among four treatments (Table 2). Compared with the CK treatment (the control), the FR treatment increased fruit TSS by 6.76%, resulted in a rise of 6.14% of TSS: TA. The results of RGM showed almost the same TSS, TA, and TSS: TA with FR, but RPM treatment decreased fruit TA content by 19.44%, resulted in 15.66% higher TSS: TA than FR treatment. The results suggested that special fertilizer could increase TSS content to improve fruit quality and plastic mulching decreased TA content to signi cantly increase fruit TSS: TA.
Effects of special fertilizer and mulching patterns on plant N, P, K nutrition Table 3 shows the effects of special fertilizer and mulching patterns on plant N, P, K nutrition. The plant N nutrition was signi cantly related to application period, and plant K nutrition was not only signi cantly related to years' change but also with treatment, while the plant P nutrition was neither related to treatment nor years' change. There were no signi cant differences in leaf N, P, and K contents among four treatments in 2017, while remarkable variations of leaf K contents were observed in 2018 (Table 3). Compared with control, the FR, RGM, and RPM treatment increased leaf K content by 8.86%, 27.91%, and 9.30%, respectively. It suggested that special fertilizer and mulching patterns could increase plant K nutrition with increasing application period, especially grass mulching patterns.
Effects of special fertilizer and mulching patterns on soil organic carbon pool of orchard Correlations between fruit quality and soil characteristics There was a signi cant relationship between fruit quality and soil characteristics ( g 5). The TA contents were negatively correlated with soil available P of 20-40 cm and SBD but positively correlated with AP, TP, and TOC of 20-40 cm. Signi cant correlations were observed between fruit TSS and AP, CP, and TOC40.
The TSS/TA ratio showed a signi cant relationship with soil characteristics including soil physical, soil organic carbon pool, and nutrition characteristics. There were dramatically positive correlations between TSS: TA and SBD, SP of 20-40 cm, MBC of 20-40 cm, ROC of 20-40 cm but negative correlations between TSS: TA and TP. Collectively, it indicated that better fruit quality caused by mulching patterns, (particularly the decreased content of TA), was associated with the changes of soil fertility, physicochemical properties, and organic carbon pool.

Principal component analysis of orchard soil organic carbon pool and physicochemical properties
The principal component analysis was carried out for every soil characteristic to re ect the effects of special fertilizer and mulching patterns ( g 6). It was observed that PC1 and PC2 accounted for 40.40% and 23.56% of the total variance, respectively. It was apparent that the CK treatment and RGM treatment were separated from each other on PC1, while the CK treatment and RPM treatment were separated from each other on PC2. The FR treatment did not separate from CK, RGM, and RPM. It suggested that special fertilizer with soil mulching was superior to special fertilizer alone in affecting soil characteristics. Grass mulching and plastic lm mulching separated from the FR treatment on PC1 and PC2 dimensions, respectively. The loading matrix plot showed that the main contributors to PC1 were DOC20, MBC20, ROC20, SP20, SP40, MBC40, DOC40, AW, and ROC40, mainly representing soil fertility and organic carbon while the main contributors to PC1 were the AP, CP, SBD, TP, mainly representing soil physical characteristics. It suggested that mulching led a fundamental role in two different aspects for the improvement of soil characteristics; one thing, affected soil physical characteristic, for another, affected soil fertility and organic carbon.

Discussion
Special fertilizer and mulching patterns improved citrus fruit quality The present study was performed to estimate the effects of special fertilizer and mulching patterns on soil organic carbon pool, soil physicochemical properties, soil fertility, plant nutrition, and fruit quality of Ponkan orchards. Compared to the control, special fertilizer and mulching patterns improved fruit quality.
The FR and grass mulching increased TSS content while plastic mulching decreased TA content to increase fruit TSS: TA (Table 2). In our study, the citrus special fertilizer modi ed the proportion of nitrogen, phosphorus and potassium and added organic matter, basing on the investigation of soil fertility of Hunan Province orchards (Liang 2017). Previous studies demonstrated that balanced fertilization with the 4R Nutrient steward concept as the core was considered to cut the cost and increase fruit yield and quality (Srivastava and Malhotra 2014). Moreover, organic cultivation including organic manures and biofertilizers of citrus could bring a positive effect on fruit quality production (Canali et al. 2012;Toselli et al. 2020). The FR treatment decreased 25% of total NPK nutrition which seemed to improve nutrient use e ciency and fruit quality. Grass mulching showed almost the same TSS, TA, and TSS: TA of fruit, while plastic mulching signi cantly decreased TA to improve fruit quality, compared with the FR (table 2). Similarly, a three-year mulching experiment conducted in the experimental farm of National Research for Citrus, Nagpur studied the effects of 5 mulches including plastic mulching, grass mulching, and straw mulching suggested that plastic mulching could decrease fruit acidity (Shirgure et al. 2003). Abouziena et al. (2014) also found the fruit quality with white plastic mulching was the best and the yield with black plastic mulching was the highest, compared with rice straw mulching, maize straw mulching, and crop weed. However, the reported effects of mulching on citrus yield and quality sometimes differed between studies, likely due to variations in citrus varieties, climatic, cultivation pattern, and soil characteristics (Qin et al. 2015). For example, some studies suggested grass mulching could produce lower fruit acidity (Panigrahi et al. 2017) and plastic mulching may increase fruit TA content (Rno et al., 2002). In our research, results of principal component analysis and loading matrix plot suggested that plastic mulching played an important role in improving soil physical characteristics while grass mulching had betterimproving effects on soil nutrients and organic carbon ( g 6). Correlation analyses showed that citrus fruit TA was associated with SBD, TP, CP, AP, TOC, and SP, mainly respecting soil physical characteristics. One possible mechanism was that the improvement in different aspects of citrus fruit could be associated with the different effects of special fertilizer and mulching patterns on soil characteristics.
Effects of special fertilizer and mulching patterns on soil characteristics It could be ascertained that fertilizer and mulching were associated with the changes of soil characteristics. In the present study, the principal component analysis had big differences of soil characteristics between the CK treatment and the other three treatments. PC1 separated the FR from CK, and the main contributors were DOC20, MBC20, ROC20, SP20, SP40, MBC40, DOC40, AW, SK40, and SN20 ( g 6). Special fertilizer seemed to improve soil characteristics by affecting soil fertility and organic carbon. Special fertilizer signi cantly increased soil available-P and TOC contents and tended to increase soil available-N and available-K contents ( g 3). The special fertilizer modi ed the proportion of nitrogen, phosphorus, and potassium as well as added organic matter basing on the investigation of soil fertility of Hunan Province orchards. More fertilizer application was instrumental in increasing soil available nutrients concentrations but also increasing the higher risk of loss, particularly soil available-N and soil available-K (Legaz et al. 1995;Yuan et al. 2002). On the one hand, the special fertilizer increased soil available N, P, K concentrations and improved soil fertility of citrus orchards by balanced fertilizer application. On the other hand, organic matter in special fertilizer treatment played a role not only in increasing soil total carbon but also in increasing soil available nutrients. Long-term eld experiments conducted from 1995 evaluated the effects of organic manures on soil nutrients, and results showed that organic manure application could increase soil available-P, soil available-K, soil available-Zn, soil available-Fe, soil available-Mn, soil available-Cu and soil available-Zn contents (Antil and Singh 2007). Soil organic carbon was related to increasing soil available nutrients more than improving water-holding capacity (Kimble et al. 1998). Different organic composts including green waste compost and urban organic waste compost increased soil nitrogen, phosphorous, and potassium (Legaz et al. 1995). Similarly, our results showed that soil organic carbon fractions such as ROC, MBC, and DOC were signi cantly associated with soil nutrients ( g 5). Our study also showed the effects of mulching on soil characteristics. The principal component analysis showed that CK treatment did not separate from FR treatment, but completely separate from FR treatments with mulching ( g 6). Soil physical characteristics were the main contributors to separate plastic lm mulching treatment from the FR treatment, while soil fertility and organic carbon were the main contributors to separate grass mulching treatment from the FR treatment. Similarly, a four-year eld experiment conducted in Xiaofuling demonstrated that grass mulching could increase soil organic carbon fractions such as dissolved organic carbon, easily oxidizable organic carbon, light organic carbon and particulate organic carbon (Gu et al. 2016). For the most soils, the effects of mulching on soil physical characteristics varied due to the proportion of solid, liquid, and vapor phases. Plastic mulching effectively reduced soil evaporation but increased topsoil temperatures and soil moisture changing soil aggregation, resulting in soil physical characteristics variations (Li et al. 2000). Favorable effects of grass mulching on soil organic carbon and prevention of available P, K, Ca and Mg loss have been widely reported (Rees et al. 2002;Saroa and Lal 2003). Mechanistic studies suggested that green manuring not only improved soil fertility by increasing soil organic matter and nutrients mineralization potential but also preserve biodiversity and the environment (Hwang et al. 2015;Pittelkow et al. 2015).

Conclusion
There were no signi cant differences in fruit TSS (total soluble solids) contents but remarkable variations in TA (titratable acid) and TSS: TA among four treatments. Special fertilizer and grass mulching increased TSS contents by 6.76% and 3.97%, while plastic mulching decreased TA contents by 19.44%, resulting in improvement of fruit TSS: TA. Soil characteristics were improved by special fertilizer and mulching, and plastic mulching played an important role in improving soil physical characteristics while grass mulching had better-improving effects on soil nutrients and organic carbon. Correlation analyses implied that soil characteristics affected by special fertilizer and mulching patterns may play an important role in the improvement of fruit qualities. Therefore, the better improvement of fruit quality of Ponkan by special fertilizer and soil mulching were associated with the changes in soil characteristics.

Date Availability statement
The data that support the ndings of this study are in the article itself.  Schematic diagram of the experimental design.

Figure 2
Effects of special fertilizer and mulching patterns on soil organic carbon pool of orchard Note: TOC: total organic carbon; ROC: readily oxidized organic carbon; DOC: dissolved organic carbon; MBC: microbial biomass carbon; Different lowercase letters indicate signi cant differences among the treatments by Duncan-test (P<0.05, n = 3).

Figure 3
Effects of special fertilizer and mulching patterns on soil N, P, K nutrition of orchard Note: Different lowercase letters indicate signi cant differences among the treatments by Duncan-test (P<0.05, n = 3).

Figure 4
Effects of special fertilizer and mulching patterns on soil physicochemical properties of orchard Note: AW: Absolute water content; FM: Field moisture capacity; RW: Relative water; SBD: soil bulk density; TP: Total porosity; CP: Capillary porosity; AP: Aeration porosity; Different lowercase letters indicate signi cant differences among the treatments by Duncan-test (P<0.05, n = 3).