2.3.1 Experimental site selection and description
Three pairs of paddy-vegetable soils were collected (PS1-VS1, PS2-VS2 and PS3-VS3) from different cities in the Tailake region in Jiangsu Province (Fig 2.2 a). All soils are evolved from same pathway and classified as the same soil type. The vegetable fields where soil was sampled are alongside paddy fields (Fig 2.2 b) and were used for paddy cultivation for hundreds of years but have been converted to vegetable cultivation in the past 10 years. An investigation was carried out before sampling to study information from the crops planted in each field. The details are presented in Table 2.1.
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(a) Sampling pairs in satellite photos (b) Picture of sampling place for pair 2
Fig. 2.2 Locations of the three pairs (paddy soil & vegetable soil) of soil samples collected. VS means the place for sampling the vegetable soil. PS means the place for sampling the paddy soil. The number after VS and PS references the pair the soils belonged to.
Tailake
Paddy
Vegetable
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Table 2.1 Descriptions of the sampling at the experimental sites
Pair 1 Pair 2 Pair 3
PS1 VS1 PS2 VS2 PS3 VS3
longitude and latitude
31°31'35"N 120°06'35"E
31°17'18"N 119°54'13"E
31°24'28"N 120°25'05"E Village Hudai town,
Wuxi city
Dingshu Town, Yixing city
Wangting town, Suzhou city Climate Sub-tropical monsoon climate.
Annual mean air temperatures are 15.5 ℃ in Hudai, 15.7 ℃ in Dingshu and 16.6 ℃ in Wangting.
Annual mean rainfalls are 1068 mm in Hudai, 1177 mm in Dingshu and 1038 mm in Wangting.
Soil type Gleyed paddy soil
Evolved from lacustrine deposits Classified as Hydragric Anthrosols Surface soil layer : 0-20 cm
Cultivated pattern (recent years)
Fields of PS1 and PS2 are planted with the rotation of summer paddy (June-October) and winter wheat (November- following May).
Fields of PS3 are planted with the rotation of summer paddy (June- October) and rape (November- following May).
Fields used for vegetable production of VS1, VS2 and VS3 are all planted with different vegetables rotations. The last year before soil was sampled, tomato, celery and lettuce were successively planted in VS1 field; pakchoi cabbage，tomato, cucumber and celery were successively planted in VS2 field; chilli, tomato and spinach were successively planted in VS3 field。
Annual fertilizer application (kg/ha)
N 500 1500 500 1000 500 900
P 120 360 120 360 160 300
K 150 450 90 450 90 400
Duration >100
years 5 years >100
years 10 years >100
years 8 years
2.3.2 Soil sampling
Soil was sampled from the 3 experiment sites between June and July 2015. Each soil sample (20 kg) was collected from the top 0–20 cm by a combination of 10 pores for each field.
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After thorough mixing, the soil was divided into two portions. One portion of the sample weight was used for testing the soil’s chemical properties. The other portion was air-dried and pulverized for the pot experiment.
2.3.3 Pot experiment and plant sampling
A pot experiment was carried out in the greenhouse at Jiangsu Agricultural Academy of Science (32°2'14"N, 118°51'58"N), Jiangsu, China. The greenhouse was maintained at a temperature between 15℃-30℃. Six soils from three paddy-vegetable pair fields were used for pakchoi (Brassica chinensis L.) planting. One pot was considered as an individual unit and each soil was used for planting with three replicates (in three respective pots). Each pot, measuring 28 cm in height, 15 cm in width and 10 cm in depth, was filled with 2 kg air-dried soil. Before the seed was sown, 85 mg P/kg and 120 mg K/kg chemical fertilizers were applied as base fertilizers to each pot at the beginning of each growing season. The seeds weighed 0.2 g (about 50-60 seeds) and were sown into the soil of each pot. The germination time was calculated from planting date until more than 35 seeds had germinated (3-5 days after sowing).
The water- holding capacities of six soil samples were examined before the pot experiment, and soil moisture in each pot was maintained at 60% water-holding capacity of each soil sample by irrigating twice a week during the whole period of experiment. The water added to each pot was determined by its soil moisture, tested by a soil moisture content analyser (Spectrum, USA).
Pakchoi was harvested 35 days after germination time. After that, the soil in the pot was turned over and air-dried for 14 days. Then a new growing season began with the same process mentioned above. The experiment was conducted over three growing seasons without any nitrogen fertilizer application from the beginning of September 2015 until January 2016. The planting and management schedule is presented in Table 2.2.
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Table 2.2 Dates of cultivation and management for the pot experiment
Growing seasons Activity Date
1st season Sowing and basal P,K fertilization 11 September, 2015
Germination 14 September, 2015
Four times over irrigation for simulating leaching
20 September, 2015 27 September, 2015 4 October, 2015 11 October, 2015 Harvest and air drying soil 18 October, 2015 2nd season Sowing and basal P,K fertilization 27 October, 2015
Germination 30 October, 2015
Four times over irrigation for simulating leaching
6 November, 2015 13 November, 2015 20 November, 2015 27 November, 2015 Harvest and air drying soil 4 December, 2015 3rd season Sowing and basal P,K fertilization 17 December, 2015
Germination 21 December, 2015
Four times over irrigation for simulating leaching
28 December, 2015 4 January, 2016 11 January, 2016 18 January, 2016 Harvest and air drying soil 25 January, 2016
2.3.4 Simulation of leaching and leachate sampling
The collection of leachates from the six soils was done by placing a tray under each pot to collect all the leachate flowing out from the three pores at the bottom of pots (Fig.2.3).
Leachate from the six soils was collected via a tray placed under each pot, gathering the liquid flowing out of the three pores at the base of the pot.
Increased irrigation was implemented four times (at the 7th, 14th, 21st and 28th days) in every growing season to simulate natural rainfall conditions in the glasshouse. The amount of deionized water was 1.05 L for every excessive irrigation (4.2 L every season) for each pot (equivalent to 25 mm precipitation per time and 100 mm precipitation in one growing season).
Deionized water was added several times and in small quantities to avoid water flowing down
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along the inner wall. The process of irrigation continued over a period of two hours and all leachate was collected from the trays 6 hours later.
Fig. 2.3 Pot experiments leachate collecting
2.4 Sample analysis