Cropping systems change the concentration of microelements in soils

Keywords:

• Cropping systems
• ICP-MS
• Microelements

Introduction

Connections of microelements with human health, diseases and nutrition gain more and more recognition from various scientists in the domains of medicine, nutrition, etc. (He et al., 2004). The deficiency of elements such as Mn, Fe and Mo could result in some cancers (Wang et al., 1989), and it is well known that levels of Cr, Zn, Se and I are related to some diseases. Selenium, an essential element of the human body, has the function of enhancing immune ability, is anti-oxidant and anti-tumour, and many health foods depend on the levels of Se (Jing et al., 2003). Thus, it is important for human health to control the concentration of Se in food, especially the natural concentrations in food. Cr (III) can enhance resistance to diabetes, and I can be used for the prevention of thyroid gland disease.

Crops, as a direct neighbour of the soil, can influence soil physical and chemical characters, plant growth, quality and quantity of stubble, and soil microbial biomass and its activity (Huang et al., 2003).

There are many methods to detect microelements, such as atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) (Yang et al., 2005) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) (Xu et al., 2001), etc. Because many elements can be detected at the same time, with higher precision (Yang et al., 2005), ICP-MS has greater application than other methods.

Materials and methods

Materials (Hu et al., 2004)

Soils

Daxing district, located in southern Beijing, belongs to the North China Plain, and is a production base for cereals, fruits and vegetables for the Beijing market. Orthometric elevation ranges from 15 to 45 m; slope, 0.8‰∼1.0‰; soil types, fluro-aquic. The total area of Daxing county is 1039 km². Longitude, 116°13′∼116°43′E; latitude, 39°26′∼39°51′N. The climate reflects a warm temperate zone, semi-humid, mean annual temperature 11.5°C, mean annual precipitation 568.19 mm.

Sampling method

Seventy samples were obtained by the GPS positioning system. Three to five samples were selected at every locus, and mixed, to a sample size of 1.5 kg, by the point-centered quarter method, depth 0–20 cm. All 70 samples were classified into four cropping systems.

Cropping systems (Table I)

Apparatus

Inductively Coupled Plasma Mass Spectrometry Apparatus (PE-Sciex DRC), detecting Li, Cr, Mn, Fe, Zn, Se, Mo and I.

Table I. The crops of the four cropping systems from 1980 to 2000.

Samples	1980–1985	1986–1990	1991–1995	1996–2000
1	–	–	–	–
2	Wheat-Maize	Wheat-Maize	Wheat-Maize	Wheat-Maize
3	Wheat-Rice	Wheat-Rice	Vegetables	Vegetables
4	Fruit Trees	Fruit Trees	Fruit Trees	Fruit Trees

Table II. The comparison of microelements in apple peel and pulp (µg/g).

Microelements	1	2	3	4
Li	6.97	15.86	3.50	9.09
Cr (III)	7.32	6.05	1.20	12.55
Mn	126.54	337.13	64.33	365.26
Fe	9.42	18.24	4.38	7.72
Zn	16.72	32.81	8.62	35.90
Se	0.25	0.50	0.11	0.48
Mo	0.20	0.30	0.08	0.34
I	1.25	7.04	1.18	3.88

Methods

Treatments of the samples were by the Yang method (Yang et al., 2005).

Parameters of apparatus

Parameters of Inductively Coupled Plasma, related to the Huang method (Huang et al., 2003): power, 1100 W; rate of flow of cooling gas (Ar), 15.0 l/min; rate of flow of supplemental gas (Ar), 1.80 l/min; rate of flow of carried gas (Ar), 0.95 l/min. Parameters of Mass Spectrometry: vacuum of analysis room, 5.89×10⁻⁶ Tor r; impulse voltage, 1050 V. Parameters of detection: resolution (10% peak height), 0.8 amu (Nor), 0.6 amu (H); retention period, 100 ms; times of replication, 5; times of circulation, 10; mode of analysis, scanning of mass, period of analysis, 72 s; rate of sample, 1 ml/min.

Results and discussion

Research has shown that farming can change the concentration of microelements in soil, which indicated an increase of concentration of microelements (Zhao et al., 2005; Huiling et al., 2005), and some research showed that long-term cropping could change the forms and limit the availability of micronutrients (Wei et al., 2005). All the research considered that the change was a result of fertilization, but they did not compare the differences of every cropping system, so it is important to study the concentration in soils from different cropping systems.

The concentrations of these microelements in soils from four different cropping systems ranged from 0.08 µg/g to 365.26 µg/g. The hierarchy of concentration is Mn > Zn > Fe > Li > Cr (III) > I>Se > Mo in soils, and the concentrations of Cr, Mn, Zn and Mo were higher in soils from fruit tree cropping systems than others. The concentrations of Fe, Li, I and Se were higher in soil from a wheat-maize cropping system; the concentrations of all the eight microelements in soils from a vegetable copping system were the lowest. Therefore, more microelements or organic fertilizers should be applied to soils with vegetables to enhance production and promote the quality of farm produce.

All the data showed that the concentrations of the microelements were considerably different in the soils studied, which indicated that the concentrations of these microelements could be changed by, and reflect, the cropping systems, but the change of concentrations of these microelements in vegetable soils was in contrast to former research (Huiling et al., 2005), which was carried out in greenhouse conditions where more fertilizers are used than in the field (Jiang et al., 2005).

Acknowledgements

This research was supported by the Program for Changjiang Scholars and Innovative Research Team in the university (IRT0511), and Natural Science Foundation of China, No.70573111.