# Effects of different agronomic practices on the selective soil properties and nitrogen leaching of black soil in Northeast China

Sep 10, 2020

### General situation of the research area

The research area was conducted at Liufangzi village, Gongzhuling city, Jilin Province (N43°34′10″, E124°52′55″), as shown in Fig. 8. The area has a continental monsoon climate in the humid area of the middle temperate zone, with an average annual precipitation of 594.8 mm, which is mainly concentrated in June and August. The average annual temperature is 5.6 °C, and the daily average temperature drops to 0 °C in November of each year, with a freezing period of up to five months. Corn is one of the main commodity crops in the area, with a sowing date in early May and a harvest date in early October.

The soil of the site is a silty loam black soil, which had been planted with monoculture corn with no tillage for 5 years. On October 5, 2018 (after the autumn harvest), a flat field was selected to set up the experiment. Soil samples were collected using the zigzag sampling method, and selective physical and chemical properties of soil were determined, including pH (5.48), organic matter (26.4 g kg−1), clay (29.12%), and soil bulk density (1.21 g cm−3 in 5–10 cm and 1.53 g cm−3 in 20–25 cm).

### Reagents and instruments

#### Reagents

The main raw material of the added impervious agent was corn starch and acrylic compound, which was entrusted to Jilin Yida Chemical Co., Ltd. The added urea was an analytical reagent, and the reagents used for analysis included H2SO4, H3PO4, NaOH, NH4OH, NH4Cl, K2S2O8, Na2B4O7, KNO3, KNO2, K2Cr2O7, FeSO4, sulfonamide, and naphthalene ethylenediamine hydrochloride; these were all analytical reagents provided by Beijing Chemical Plant.

Instruments laboratory-built soil leaching column; continuous flow injection analyser (SKALAR SA++, Netherlands).

### Test plot setup and agronomic practices

The experimental plots were maintained in the field consisting of (1) CK (no-tillage control treatment, with corn straw removed and soil left under no-till management); (2) ploughing treatment (corn straw was removed and then mouldboard ploughed to a 30 cm depth); (3) straw returning treatment (corn straw (25.32% moist) was incorporated into the soil on October 5, 2018 (after autumn harvest), with an application amount of 1.25 kg m−2. Briefly, corn straw was chopped into small pieces (0.5 cm length), evenly placed on the soil surface, and then incorporated into the soil with ploughing (the depth of 30 cm)); and (4) impervious agent addition treatment (the impervious agent mentioned previously evenly laid on the soil surface at the amount of 15 g m−2 and then incorporated into the 0–30 cm soil by mouldboard ploughing). The abovementioned field operations were conducted after corn harvest in the fall of 2018 with a testing area of 10 m × 50 m for each plot and three replicates for each treatment. In the following spring (2019), grain corn was planted in all treatment plots with a planting density of 65,000 plants ha−1. All plots were managed in the same way with a one-time fertilization application of 200–90-90 kg (N-P-K) ha−1 and 2,4-d spray as weed control.

For all the above treatments (including the control treatment), undisturbed soils (0–30 cm layer) were collected with an undisturbed soil column (refer to Fig. 9) for the leaching experiment on September 25, 2019 (before autumn harvest, after 350 days of straw returning to the field); soil samples of 0–15 cm were collected for determination of soil organic matter and adsorption experiment of nitrogen in the soil; and soil samples of 5–10 cm and 20–25 cm layers were collected for determination of soil bulk density. In addition, for the straw returning treatment, one sampling was added on May 25, 2019 (one month after sowing, 230 days after straw returning), for the determination of soil organic matter content and soil bulk density, nitrogen adsorption and leaching experiment in soil.

The soil samples used for soil organic matter determination and nitrogen absorption testing were air dried, sieved through a 2-mm sieve and visible plant debris and stones were removed, and then stored.

### Experiment of nitrogen adsorption in soil

Ten parts of the soil samples (air-dried, < 2 mm and 1.00 g per part) were weighed into a triangular flask (numbered 1 to 10), and then 20.00 mL of urea solution with different nitrogen concentrations of 0, 25, 50, 75, 100, 125, 150, 175, 200, and 300 mg L−1 was added to each flask numbered 1 to 10, respectively. The flasks were placed in an incubator with oscillator shaking for 4 h at a constant temperature of 25 ± 1 °C, and then the supernatant was collected to determine the total nitrogen in the solution. This test was performed with 3 replicates for each treatment.

### Nitrogen leaching test with undisturbed soil column

#### The structure of the leaching column

The leaching column was made of stainless with an inner diameter of 10 cm and three parts: part I (the main body of the leaching column), part II (the soil cutter, used for soil core collection) and part III (the collector of leaching solution, used for leachate collection) (see Fig. 9). The height of the leaching column was 40 cm, with the top 10 cm empty for holding leaching solution and the bottom 30 cm was filled with undisturbed soil.

#### Collection of undisturbed soil columns

The bottom of part I was connected with part II, and the undisturbed soil column was collected by pressing the handles vertically. When the soil surface of the soil column was 10 cm away from the upper end of the column, the sampling was finished. To reduce the soil disturbance and resistance during the sampling process, the soil surrounding the targeted soil column was first removed carefully with a spade (manually) and the diameter of the soil column was left slightly larger than the inner diameter of the leaching column, watered evenly and slowly; then, the leaching column was carefully placed on top of the soil column and gently pressed down to the required depth.

#### Leaching method of nitrogen in undisturbed soil column

1. (1)

Pre-leaching and exogenous nitrogen addition The soil in the collected undisturbed soil column of part II was scraped off, part II was removed, and part III was connected to part I (see Fig. 9b). The flow control valve at the three diversion ports was connected at the upper end of component I to simulate the surface runoff under the field conditions, and the leaching speed was controlled through the flow valve at the lower end (part III). Before the nitrogen leaching experiment, the soil column was pre-leached with 3,000 mL deionized water to remove soil residual nitrogen. The eluent was discarded, and the soil column was placed for 48 h before the start of the leaching experiment.

The top 1 cm of undisturbed soil was collected as the covering soil, and then 2 cm of additional undisturbed soil was taken and mixed thoroughly with 4.0 g of urea. The soil-urea mixture was installed back into the leaching column, covered again with the covering soil, and then placed for 4 h before the leaching test.

2. (2)

Leachate collection The leaching test was conducted by adding deionized water to the soil leaching column. The leaching process was carried out continuously until the projected volume of leachate was achieved. The total volume of projected leachate collection was 4,000 mL, roughly equivalent to the average annual precipitation (510 mm) in the study area. In total, 10 leachate samples were collected from each column, and the sample volumes varied from 250 mL each for the 3rd to 6th samples and 500 mL each for the rest of the samples. To eliminate the influence of temperature on the leaching process, the leaching test was carried out in a constant temperature chamber at 25 ± 1 °C.

### Analysis method

Soil bulk density was determined with the ring knife method. Soil organic matter and total nitrogen contents were analysed using the potassium dichromate volumetric method and N-(1-naphthyl) ethylene diamine dihydrochloride spectrophotometry, respectively36.

### Data processing methods

The amounts of nitrogen adsorbed by the soils were determined using the following equation (Eq. 1):

$$Q = (c_{0} – c_{e} )V/m$$

(1)

where Q (mg L−1) is the adsorption capacity, c0 (mg L−1) denotes the initial solution concentration of the adsorption test, ce (mg L−1) represents the concentrations of the adsorption equilibrium solution, and “m” refers to the soil quantity (g)27.

Two isothermal models were used to calculate the nitrogen adsorption capacity (Eqs. 2 and 3):

$${text{Langmuir}};{text{equation}}:;Q = k_{L} Q_{e} c_{e} /(1 + k_{L} c_{e} )$$

(2)

$${text{Freundlich}};{text{equation}}:;Q = k_{F} c_{e}^{1/n}$$

(3)

where Q (mg L−1) is the adsorption capacity, ce (mg L−1) represents the concentration of the adsorption equilibrium solution, Qe (mg L−1) refers to the saturated adsorption, and kL, kF, and n are constants related to adsorption capacity27.

Microsoft Excel 2010 was used for data processing and statistical analysis, and Origin 8.5 software was used to fit the data. T-test was used to test the difference between the two groups.