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Identification of the productivity-limiting nutrients of Xuxiang kiwifruit ( Actiniadia chinensis ) in China's central Shaanxi province by analyzing soil fertility and leaf elements

A survey was initiated to identify the productivity-limiting nutrients of Xuxiang kiwifruit ( Actiniadia chinensis Lindl.) orchards in central Shaanxi province in China during 2016-2017. For this purpose 149 kiwifruit orchards were selected for leaf sampling and 59 of them were randomly selected for soil analysis. These investigated orchards were divided into two subpopulations: high-productivity subpopulation (21 orchards) and low-productivity subpopulation (128 orchards) according to the fruit yield and appearance as well as vine growth performance. The nutrient concentrations in leaves of high-productivity subpopulation were used to compute the norms of the diagnosis and recommendation integrated system (DRIS), and in low-productivity orchards the order of nutrient requirement was found to be Zn>Mn>K>Fe>Cl>P>B>Ca>Mg>Cu>N. Among them, the deficient nutrients were Zn, Mn and K, while the excessive ones were N, Cu and Mg. However, the limiting elements in various locations and orchard ages differed. These results obtained from leaf analysis were further validated by soil fertility evaluation using sufficiency range, 94.9%, 83.1%, 76.3%, 32.2% and 23.7% of the orchards were high in pH, Ca, Mg, nitrate N and Cu respectively. By contrast, approximately 70% of the soils were low in Fe, Mn, Zn and Cl, and over 30% low in K, P and OM. Results obtained could be used for guiding the sustainably-integrated nutrient management for kiwifruit orchards in central Shaanxi and other regions with similar environmental conditions.

Kiwifruit (Actiniadia chinensis Lindl.) is one of the most important recently domesticated fruit crops in the world. In 2015, the kiwifruit yield and harvested area in China represented 2187867 tons and 181900 ha, and accounted for 52.7% and 69.0% of the world's yield and area respectively (FAO 2018). Among them, 47.1% and 24.3% of China's yield and area come from central Shaanxi (Zhang 2016, FAO 2018. However, the average yield of kiwifruit is much lower in China (12.0 tonnes/ha) than the whole world (15.7 ton/ha), especially in the second and third largest kiwifruit-producing countries, Italy (21.9 tonnes/ha) and New Zealand (34.0 tonnes/ha, FAO 2018). The poor yield is often attributed to inadequate information on plant nutrition and soil fertility (Liu et al. 2002, Song et al. 2003, Wang 2008. Indeed, nutrient disorders of kiwifruit orchards in central Shaanxi province (e.g. N excess and hidden hunger of Fe, K, Cl and P) occur frequently, and result in a substantial loss of fruit quantity and quality (Zhang et al. 2001, Lai 2011, Tran et al. 2012, Lu et al. 2016a. Nutritional diagnosis is an important tool for increasing the fruit yield and quality as well as improving the plant growth through efficient fertilization. Several approaches for interpreting the results of leaf nutrient analysis have been proposed, such as critical values (CV) or sufficiency ranges (SR) (Bates 1971), diagnosis and recommendation integrated system (DRIS; Beaufils 1973), deviation from optimum percentage (DOP; Montañes et al. 1993), and compositional nutrient diagnosis (CND; Parent and Dafir 1992). Compared with the former two methods, the latter two were less employed due to the lack of useful references and field trials to ensure validity and accuracy. SR method is widely used for plant nutrient diagnosis owing to its easy assess and practice. Another widespread method is DRIS, and this method possesses some additional advantages over SR: (1) to reflect the nutrient balance or interaction, (2) to identify the order of nutrient requirements or limiting nutrients, and (3) to reduce the effects of sampling time and plant position (Walworth and Sumner 1987). Thus, DRIS has sightly higher diagnostic precision than SR in many cases (Walworth and Sumner 1987). To date, DRIS has been used 53 https://doi.org/10.56093/ijas.v89i4.88845 a depth of 0 to 30 cm and a distance of 1.0 to 1.2 m from the main vines with a soil auger of 5 cm interior diameter. Each soil sample was composed of five random soil cores per orchard, stored in double-lined plastic bags, and brought into the laboratory for soil property analysis. The samples were air-dried, ground by a rolling pin to pass through a 1.00-mm sieve (half of the soil sub-samples were passed through a 0.15-mm sieve for organic matter analysis), and stored in sealed plastic bags until analysis. Soil chemical properties (i.e. pH, organic matter (OM), salt, sodium (Na), P, K, Ca, Mg, Fe, Mn, Cu, Zn, B and Cl) were analyzed according to the methods described by Bao (2000), and N was determined according to the method described by Mohammed et al. (2017). Briefly, soil pH was measured with a glass electrode in a 1:5 suspension of soil to water by mass, and electrical conductivity (ec) was measured in the same extract and the total salt concentration was calculated by EC. OM content was determined by the potassium dichromate volumetric method of external heating. Mineral N (including ammonium N and nitrate N) was extracted with 1.0 mol/L KCl and determined using a continuous flowing analyzer. Available soil P was determined with the molybdate blue method after extraction with NaHCO 3 . Exchangeable cations (K + , Ca 2+ , Mg 2+ , and Na + ) were extracted with 1.0 mol/L NH 4 OAc, and most available micronutrients (Fe 2+ , Mn 2+ , Cu 2+ and Zn 2+ ) were extracted with 0.05 mol/L diethylenetriaminepentaacetic acid (DTPA) reagent, and all determined by atomic absorption spectrophotometer (AAS). Available B was extracted by boiling water and determined by ICP-OES. Soil Cl was extracted from a 1:2.5 soil-water mixture and determined with a discontinuous analyzer.
When taking leaf and soil samples, data on the yield and cultivation-related information of each orchard were obtained from the growers and reassessed by local kiwifruit experts to guarantee the reliability of the orchard information.

Procedure of DRIS norms
To establish DRIS norms, the population was divided into two subpopulations: high-productivity subpopulation and low-productivity subpopulation. The high-productivity subpopulation was defined as those orchards with a fruit yield equal to or above 2000 kg/667m 2 (fruit quantity), uniform and large-sized fruits (fruit quality), and healthy green leaves (the capability to successively bear fruits in following years) at harvest stage. The mean value, standard deviation (SDs) and coefficient of variation (CVs) of leaf nutrient concentrations and ratios in high-productivity subpopulation were considered nutritionally balanced or a reference for diagnostic norms and listed in Table S1. The subpopulation with high productivity was represented by 21 orchards (14.1% of the orchards) and low productivity by 128 orchards (85.9% of the orchards).
The DRIS provides a means of ordering nutrient ratios into a meaningful expression referred as the DRIS index (Walworth and Sumner 1987). The values of nutrient ratios with relatively large variance ratios (variance of successfully for a wide range of cash crops, such as orange (Srivastava and Singh 2008), apple (Nachtigall and Dechen 2007), banana (Wairegi and Asten 2011), guava (Hundal et al. 2007), sugarcane (Barłóg 2016) and oil palm (Behera et al. 2016). Nevertheless, the DRIS norms for 'Xuxiang', the currently predominant kiwifruit cultivar in central Shaanxi, have not been developed.
The objectives of this study were: (i) to develop the DRIS norms of leaf nutrients for 'Xuxiang' kiwifruit in central Shaanxi province, and (ii) to evaluate the nutritional status of these kiwifruit orchards through leaf analysis and soil testing.

Site description
The research was performed in the central region of Shaanxi province, China (34°7'~35°10'N, 106°12'~108°58'E, evaluation 451~831 m). This area has sufficient sunshine and a warm temperate continental monsoon climate with hot, wet summers, and cold, dry winters. The average annual temperature and precipitation are 11 to 13°C and 500 to 800 mm, respectively. About 60% of the precipitation is received between June and September. The majority of the studied soils are classified as earth-cumuli-orthic anthrosols or cultivated loessial soils according to China Soil Taxonomy (Xiong and Li 1987).

Sampling and analysis
A total of 149 leaf samples were collected from kiwifruit (Actinidia chinensis var. deliciosa 'Xuxiang') orchards in late August and early September during 2016-2017. Among them, 109 orchards were from Meixian, 5 from Qishan, 5 from Wugong, 6 from Yangling and 24 from Zhouzhi. Leaf samples were taken from the second leaf past the final fruit cluster on a fruiting lateral, and each sample consisted of 40 leaves (blade plus petiole) from 20 vines with 2 leaves per vine (Sher 2008). The samples were washed in sequence with tap water, 0.5% detergent solution, 0.1% hydrochloric acid (HCl) solution and deionized water. Then, the leaf samples were oven-dried at 65°C to a constant weight, ground in a mill to pass through a 20-mesh sieve, and stored in sealed plastic bags for analysis. The concentration of phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), and boron (B) in plant leaves was determined by an inductively coupled plasma optical emission spectrometry (ICP-OES, IRIS-Advan type, Thermo, USA), and chlorine (Cl) by a discontinuous analyzer after dry-ashed (with the addition of CaO only for Cl measurement) and dissolved in 0.5 mol/L HNO 3 solution (Zhou et al. 2014). The nitrogen (N) concentration in leaf tissue was quantified using Kjeldahl methods (Bao 2000).
A total of 59 soil samples were taken from the randomly selected leaf-sampling orchards. Among them, 35 orchards were from Meixian, 5 from Qishan, 5 from Wugong, 6 from Yangling and 8 from Zhouzhi. Soil samples were taken at low productivity subpopulation (S 2 l )/variance of high productivity subpopulation (S 2 h ); i.e. F value) were selected to calculate the DRIS indices (Serra et al. 2013). The DRIS indices for nutrient A through N were computed using the following equations (Beaufils 1973, Walworth andSumner 1987): where when A/B is larger or equal to a/b, where Index A, DRIS index; f(A/B), DRIS functions; z, the number of DRIS functions; A/B, nutrient ratio of the samples diagnosed in low productivity subpopulation; a/b, mean value of nutrient ratio in high-productivity subpopulation; CV, coefficient of variation of nutrient ratio in high-productivity subpopulation.
The more negative the DRIS index, the more shortage would be the nutrient relative to others; and vice versa. The DRIS indices were assessed by comparing DRIS indices with the mean value of nutrient imbalance index (NIIm), which was equal to the sum of the absolute value of each DRIS index divided by the number of DRIS indices (Nachtigall and Dechen 2007): Deficient = Ix<0 and |Ix|>NIIm, Normal = |Ix|≤NIIm, and Excessive = Ix>0 and |Ix|>NIIm. All data were computed using Microsoft Office Excel 2007.

Leaf nutrient status of kiwifruit orchards
We assessed the nutritional status of kiwifruit leaves in low-productivity subpopulation according to the DRIS norms established by high-productivity subpopulation (Table 3), and found that the nutrient requirement of Xuxiang kiwifruit in central Shaanxi followed the order: Zn>Mn>K>Fe>Cl>P>B>Ca>Mg>Cu>N (Table 1). Among them, the deficient elements were recorded for Zn, Mn and K while the excessive ones for N, Cu and Mg (Table 1). However, the limiting nutrients in various locations differed. Leaf samples from both Meixian and Zhouzhi were deficient in Zn and Mn, Qishan deficient in P, Wugong deficient in K, and Yangling deficient in Cl, P and Mn (Table 1). By contrast, the orchards in Meixian were excessive in N and Mg, Zhouzhi excessive in N, Ca and B, Qishan and Yangling excessive in Cu, and Wugong excessive in Mg, Cu and Fe (Table 1). Moreover, kiwifruit nutrient requirement varied considerably with plant age (Table 1). Mn deficiency occurred across all orchard ages, and other deficiency elements shifted from Zn and K through Zn to Mg and Cl with increasing kiwi-vine age ( Table 1). The orchards older than 12 years were excessive in Ca, Zn and K, whereas those equal to or younger than 12 years were leaf spray of Cu-containing fungicides or N-containing foliar fertilizers and (2) ionic selective absorption by plant roots (Marschner 2012). In addition, Lu et al. (2016aLu et al. ( , 2016b reported that N overfertilization was found in the Yujiahe catchments of Zhouzhi and changing arable crops to kiwifruit orchards increased the environmental burden of this region, and thus was unfavorable to the sustainability of kiwifruit industry (Müller et al. 2015, Cui et al. 2018). Consequently, rational N fertilization is necessary for high-productivity kiwifruit orchards in central Shaanxi. Nutrient disorders are commonly observed in kiwifruit cultivation (Smith et al. 1987a, Liu et al. 2002, Tran et al. 2012. Results indicate that the investigated region was hidden-hungry in Fe and Cl, besides Zn, Mn and K (Table 1 and Table 2). Fe deficiency is generally related to the phenomenon of kiwifruit leaf chlorosis (Tran et al. 2012), which may be induced by several factors, such as high soil pH, high CaCO 3 content, excessive fruit loading in previous year, and damages to root system (Tagliavini and Rombolà 2001). Here we suggested that Fe deficiency in central Shaanxi was mainly caused by high pH and Ca in calcareous soils (Table 2), and leaf chlorosis might be a combined effect of several chlorophyll-related nutrient deficiencies, such as Fe, Zn and Mn (Table 1 and Table 2). In central Shaanxi, Cl deficiency has been well documented 17 year ago (Zhang et al. 2001), and the increasing kiwifruit yield by applying Cl-containing fertilizers was reported recently (Yang 2016). However, the increase of kiwifruit yield diminished with increasing the amount of Cl-containing fertilizers (Yang 2016). Therefore, Cl in central Shaanxi may be in hidden hunger. The reasons leading to potential Cl excessive in Cu, N and Mg (Table 1). This may be due to the nutritional status of fruit crops influenced by a variety of factors, such as soil type, climate, cultivar, and plant age as well as nutrient reserves in plant tissues (Mourão Filho 2004, Vajari et al. 2018. Additionally, the kiwifruit yield decreased with NIIm increased (Table 1), indicating that NIIm is able to reflect the nutrient balance well.

Soil fertility status of kiwifruit orchards
To validate the DRIS results from kiwifruit leaves, we further randomly selected 59 orchards from the investigated orchards to assess soil fertility. 94.9% and 83.1% of the orchards were excessive in pH and Ca respectively ( Table  2), indicating that the soils are calcareous. Moreover, 76.3%, 32.2% and 23.7% of the soil samples were high in Mg, nitrate N and Cu (Table 2). On the other hand, approximately 70% of the kiwifruit orchards were low in Fe, Mn, Zn and Cl (Table 2). Over 30% of the soils were low in K, P and OM (Table 2). Additionally, more than 70% of the orchards were in optimum range for soil B, Na and salt (Table 2). These results agreed well with the DRIS results from leaf analysis (Table 1 and Table 2). Taken together, our results showed that both nutrient deficiency and excess existed in kiwifruit orchards in central Shaanxi, and a better soil management is highly needed to improve soil fertility and thus to enhance the kiwifruit productivity (Lago et al. 2015, Peticila et al. 2015, Zhao et al. 2017. Noteworthy, the excessive percentages of soil nitrate N and Cu were not as high as soil Ca and Mg (Table 2), but those of leaf N and Cu seemed to be higher than leaf Ca and Mg (Table 1). This may be associated with (1) frequent Optimum range is summed from Li et al. (1996), Bao (2000), Sher (2008), Tong (2011), Huang et al. (2014 and Yu (2017).

PRODUCTIVITY LIMITING NUTRIENTS IN KIWIFRUIT
deficiency include (1) low Cl content in soils (Table 2), (2) high Cl requirement of kiwifruit plant (Smith et al. 1987b), and (3) the large amount of Cl removal by harvested fruit, summer and winter prunings, and fallen leaves (Wang 2008, Vajari et al. 2018.
In sum, the nutritional status of 'Xuxiang' kiwifruit orchards in central Shaanxi was characterized by Zn, Mn and K deficiency, as well as N, Cu and Mg excess. Moreover, the limiting nutrients varied across locations and orchard ages. These results provide a useful reference for rational fertilization of kiwifruit orchards in calcareous soils.