Accumulation and translocation of selenium in a soil–rice system in Guangxi, China

Figures & data

Figure 1. Overview of the study area and sampling locations. The sampling points from the southwest to the northeast are numbered 1–16

Table 1. The description and determination of the Se types in paddy soil

Se fractions	Describe	Method*
W-Se	W-Se mainly includes Se (VI), a small amount of Se (IV) and soluble organic Se. W-Se is the easiest selenium to be absorbed by plants, and it is considered as bioavailable selenium in agriculture (Ge et al. 2000).	Twenty five mL of dd H2O was added to the sample, ultrasonic extraction for 30 mins, centrifuga at 4000 rpm for 8 mins, the supernatant was filtered through a 0.45 μm filter membrane. Samples need to be filtered (0.45 μm) before each determination of Se concentration.
L-Se	L-Se mainly refers to the tetravalent selenite ion, which is related to water-containing oxides and the surface adsorption of clay and humus (Zhang et al. 2014b).	Add 8 mL of 0.1 M KH2PO4 solution to the test tube containing residual solids in the previous step, shake vigorously for 3 mins, shake at 200 rpm for 4 h, centrifuge at 4000 rpm for 20 mins, and transfer the supernatant to a new test tube, the Se concentration was determined.
O-Se	Se species that are bound to SOM (Zhang et al. 2014b).	Add 6 ml of 0.1 M NaHO solution to the test tube containing the residual solids in the previous step. Ultrasound for 30 mins, heat in a water bath at 90°C for 2 h, cool to room temperature, and centrifuge at 4000 rpm for 15 mins. Add 5 mL of HCl, dilute to 25 mL, and analyze the Se concentration.
R-Se	R-Se mainly exists in silicate minerals, which is not easily destroyed by acid-base reaction and is not available to plants (Ge et al. 2000).	Add 10 mL of HNO3 and 5 mL of HClO3, heat (170°C) until the sample turns white, and dilute the supernatant to 25 mL with HCl before testing.

* method was modified from Qin, Zhu, and Su (2012)

Table 2. Comparison of soil characteristics between this study and other selenium-rich regions

Area	pH	CaCO3 (g/kg)	SOM (%)	Total sulfur (mg/kg)	TSe (mg/kg)	Crop type	Reference
Guangxi, China	5.19–6.22	13.33–59.33	1.21–3.92	110.03–591.54	0.11–0.91	Rice	This study
Hubei (Enshi), China	4.80–4.06		2.76–4.64	130.00–590.00	0.85–11.46	Rice	Chang et al. (2019)
Shaanxi (Ziyang), China	5.80–6.00	5.20–6.30	1.50–2.00		9.8–19.8	Maize	Wang et al. (2012)
Guangdong, China	5.17–5.48		0.65–2.75		0.28–0.41		Zhang et al. (2008)
Japan	3.96–7.29				0.05–2.80		Yamada et al. (2009)
Punjab, India*	8.05	22.90–46.80	0.53–0.97		1.31–2.41	wheat, rice	Dhillon and Dhillon (2000)
California, USA	6.90–8.80		1.03–1.55		0.07–1.17		Abrams, Burau, and Zasoski (1990)
Malawi	5.20–7.90		1.08–2.64		0.12–0.36	Maize	Chilimba et al. (2012)
Iran					0.11–0.45	wheat, date, pistachio	Nazemi et al. (2012)

* only refers to the soil characteristics of the 0–30 cm soil layer

Table 3. Relationship between the distribution of Se content in different parts of rice plant tissues and soil

ratio	ADC	range	CV	SE
root/soil	0.41	0.04–1.41	0.81	0.02
stem/root	0.43	0.16–0.84	0.53	0.01
leaf/stem	0.54	0.18–0.78	0.27	0.01
grain/stem	0.74	0.47–1.04	0.21	0.01

ADC: average distribution coefficient; CV: coefficient of variation; SE: standard error

Figure 2. Distribution and concentration of Se in rice plant tissues. IQR: interquartile range

Table 4. Pearson correlation matrix of soil properties, Se concentration in rice plant tissues, and soil Se type in Guangxi

	Total S	pH	CaCO3	TSe	O-Se	L-Se	W-Se	R-Se	Root	Stem	Leaf	Grain
SOM	0.360	0.649**	0.068	0.701**	0.726**	−0.393	−0.310	0.188	0.487	0.457	0.403	0.650**
Total S		−0.168	−0.273	−0.006	0.086	−0.208	−0.403	−0.122	0.196	0.005	−0.049	0.135
pH			0.209	0.633**	0.584*	−0.090	−0.051	0.257	0.473	0.350	0.449	0.440
CaCO3				0.191	0.335	0.053	0.354	−0.212	0.032	−0.307	−0.200	−0.322
TSe					0.868**	−0.111	0.059	0.492	0.430	0.466	0.506*	0.428
O-Se						−0.052	0.222	−0.001	0.629**	0.545*	0.534*	0.468
L-Se							0.440	−0.249	−0.042	−0.240	−0.285	−0.424
W-Se								−0.339	0.198	0.125	0.167	−0.227
R-Se									−0.228	0.008	0.108	0.093
Root										0.577*	0.588*	0.532*
Stem											0.865**	0.833**
Leaf												0.792**

* significant at the 0.05 level; ** significant at the 0.01 level

Figure 3. SOM and content of each type of Se in paddy soil of the 16 sampling sites

Figure 4. (A) Relationship between TSe, SOM, and O-Se in paddy soil. (B) Relationship between Se content in leaf, grain and stem in rice tissues

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