Response of soil N2O emission and nitrogen utilization to organic matter in the wheat and maize rotation system

Study site

The study site (N 38° 49′, E 115° 26′) is located in the Guanzhuang Village in Baoding City of Hebei Province, China, in the humid temperate and monsoon climatic zone with the average annual air temperature of 13 °C, annual rainfall of 500 mm, and frost-free period is 210 days. Although the experiment was a one-year, the distribution of precipitation (488.50 mm) and temperature (13.45 °C) during the experimental period (2014–2015) were close to the the latest 10-year averages (2005–2015) (500.19 mm and 13.61 °C) (Fig. 1). Determine the basic nutrient indexes of the 0–20 cm surface soil in the test plot. The soil type is silty loam, consists with 22.55% sand, 71.09% silt and 6.36% clay. Analysis of soil basic characteristics showed that it has a pH of 8.3, and its content of organic matter, total N, available phosphorus (P) and available potassium (K) was 11.27 g kg⁻¹, 1.47 g kg⁻¹, 25.49 mg kg⁻¹ and 127.43 mg kg⁻¹, respectively.

Experiment materials

The planting mode of the experimental site was a winter wheat-summer maize rotation, the winter wheat variety was ‘Jinnong 6’ that verage thousand weight was 47.6 g. The summer maize variety was ‘Zhengdan 958′ that average thousand weight 330 g.

Test fertilizers include inorganic fertilizer, organic fertilizer, soil conditioner, compound bateria, amino acid liquid fertilizer and nutrient agent. Inorganic N, P and K in the tested fertilizers were provided by urea (N 46%), superphosphate (P₂O₅,16%) and potassium chloride (K₂O, 54%), respectively, as well as in the form of zinc and humic acid urea which is mainly a combination of N with humic acid (N 46% and HA 1.2%). The organic fertilizer used in the experiment was mainly decomposed chicken manure. But the N content in the chicken manure is 1.32% in wheat season and 4.48% in maize sason. Soil conditioner mainly containing calcium (Ca) and magnesium (Mn), Compound bacteria could fix N potentially, promote root growth, decompose cellulose lignin and thus rapidly to degrade. The number of living bacteria reached 2 billion per gram. Amino acid liquid fertilizer and nutrient agent sprayed according to crop growth to provide the required amino acids and trace elements for plant growth.

Experiment design

Field experiment consisted of five treatments with 3 replicates. The experiment uses a completely randomized block setting, the plot size was 79.2 m² (13.2 m × 6 m). Before the experiment, no crops were planted in the area and it was idle for more than one year. The five treatments were: CK (zero N), FN (farmers’ traditional inorganic N rate, through mass surveys on actual production), RN (recommended inorganic-N rate, according to the experimental results of many scholars, combined with the local soil N supply, crop straw returning in the previous season as well as wheat or maize N demand for target yield in the current season)^22,23, HAN (zinc and humic acid urea, the N supply same as RN), RN40% + HOM (40% inorganic N rate of RN (RN40%) with homemade organic matters (HOM). HOM was an organic control measure, it including organic fertilizer, soil conditioner compound bacteria, amino acid liquid fertilizer and nutrient agent. these constituents and amount according to Shu et al²⁴ (Table 1).

For wheat, N fertilizer was broadcast for ratio of 4:3 (basal to topdressing) in RN40% + HOM, whereas for the rest N treatments the ratio was 1:1. During maize planting, N ratio (basal to topdressing) for all treatments was 2:3.The N, P and K fertilizers for wheat were applied in the form of urea, single superphosphate and potassium chloride, respectively. The amount of N fertilizer applied in different treatments of different crops is different, specific application amount reference Table 1. Except for treatment RN40% + HOM, all treatments have the same amount of single superphosphate and potassium chloride. Single superphosphate (120 kg P₂O₅ ha⁻¹)and potassium chloride (150 kg K₂O ha⁻¹)were used in winter wheat season. For maize, single superphosphate (90 kg P₂O₅ ha⁻¹) and potassium chloride (150 kg K₂O ha⁻¹) were used. P and K fertilizers were applied once before sowing. For the doses of P and K in RN40% + HOM brought by organic fertilizer were firstly assessed (48.4 kg P₂O₅ ha⁻¹ and 149.3 kg K₂O ha⁻¹ for winter wheat; 87.2 P₂O₅ ha⁻¹ and 19.1 kg K₂O ha⁻¹ for maize), remaining amounts were supplemented with chemical P and K fertilizers.

Wheat at a rate of 187.5 kg ha⁻¹ with a row space of 15 cm, was sown on 12 October 2014 and harvested at 7 June 2015. Then, at the same wheat plot, Maize of 37.5 kg ha⁻¹ with a row space of 57 cm, was sown on 18 June 2015 and harvested at 5 October 2015.

N₂O sampling and measurements

N₂O gas was collected using a closed static chamber from sowing to harvest of wheat and maize²⁵. The sampling box was divided into two parts and made by PVC material: a box body and a base. The upper part of the box body was provided with a gas sampling port sealed with a rubber plug, and a thermometer probe was arranged inside the box body to monitor the soil surface temperature. The box body is 15 cm high and the bottom diameter is 25 cm. The base was ring shaped, and buried into the soil. Gas collection was performed from 9:00 to10:00 am. A 30 mL of air sample was collected at 0, 8, 16 min after closure²⁶. The air samples were taken once at an interval of 7 days in general and subsequently continuous 5 days following fertilization or precipitation. Continue to collect gas samples from the beginning of the experiment. No gas samples are collected during the freezing of wheat field soil in February and March every year. At the same time, the air temperature was measured by a thermometer and the soil moisture in the 0–5 cm depth was measured by soil moisture tester (TK3-BASIC). Gas concentrations were analyzed by using a gas chromatography (Agilent 7890 A, USA), fitted with a 4 mm by 3 m stainless steel column packed with Porapack Q and N₂ was used as the carrier gas. The column and the detector temperatures were set at 70 °C and 300 °C, respectively. The standard N₂O was supplied from National Center of Standard Measurement.

N₂O flux was calculated using the following equation (Wang et al.²⁷).

where F is N₂O emission flux, ρ = m/22.414, ρ is the density of gas in airtight box, m is molecular weight, H is the height of the static chamber, T₀ is 273 K, c₁ and c₂ are the gas concentration in time of t₁and t₂, respectively, T₁and T₂ are gas temperatures, ∆t = t₂ − t₁ ,where t₂ and t₁are times.

Cumlative N₂O emissions were from the growth season was calculated by the equation:

where T is the total amount of N₂O emissions from the growth stage (kg N ha⁻¹), F_i and F_i+1 denote the N₂O flux of the i and i + 1 sub-sampling (μg N m⁻² h⁻¹); D_i and D_i+1 represent sampling days (d)²⁶.

N₂O emission coefficient (EF) was estimated with equation²⁸:

Soil sampling and measurements

At wheat and maize maturity, soil samples were collected from depths of 0–20, 20–40 and 40–60 cm with a hand probe from three places in central rows of each plot and mixed together. Fresh soil samples were sieved through a 2 mm, extracted with 1 mol L⁻¹ KCl and a soil-solution ratio of 1:10, and analyzed for inorganic N (mainly including NH₄⁺–N and NO₃⁻–N) contents with continuous flow analysis technique(AA3-HR, Germany)²². Soil moisture and density of each soil layer were measured simultaneously, and the soil residual N in 0–60 cm was calculated. Other soil sample was air-dried and sieved, organic matter and total N content were measured by agrochemical analysis method²⁹.

Plant harvest

For wheat, plants with double rows (1 m length) in each plot were harvested and 20 spikes were selected to count the numbers of effective spikes. All the harvested plant samples were separated into straw (including stem, leaves and remaining of ears) and grains, and the grain yield was calculated to 12.5% moisture content (PM-8188, Japan). Three samples were chosen from each plot and weighted to get the average 1000-grain weight.

For maize, two representative plants in each plot were harvested and separated into straw (including stems, leaves, tassels, husks, cobs) and grains in the central rows. Moreover, 20 ears were continuously selected to thresh and measured grain yield. Grain yield was calculated to 14% moisture content (PM-8188, Japan).

All harvested wheat and maize samples were dried, weighed, ground into powder to measure the total N content using H₂SO₄–H₂O₂ Kjeldahl digestion method²⁹.

N balance and N efficiencies

Total N input was comprised of N fertilizer, the initial inorganic N in soil before planting (including both NO₃⁻–N and NH₄⁺–N), pre-crop N straw return (no straw was returned when sowing wheat and the N uptake in maize stage was calculated from pre-wheat straw), N deposition from dry and wet atmosphere and mineralized N in soil. Atmospheric N deposition was derived from Research result by Liu et al.³⁰. N output was comprised of crop uptake, post-harvest residual soil N and apparent N loss. This study calculated soil N to a depth of 0–60 cm. Mineralized soil N, apparent N loss, Nitrogen production effiency (NPE) , Nitrogen agronomic effiency (NAE) and Nitrogen use efficiency (NUE) were calculated as follows:

Net income analyses

Prices of fertilizers and grains as well as other costs in Chinese Yuan (RMB: 1 USD = 6.71 RMB in the experiment year) were based on local prices. Net income was calculated by the equation:

where Fertilizer costs were composed of the prices of inorganic N (3.9 RMB kg⁻¹), P₂O₅ (5.65 RMB kg⁻¹), K₂O (6.5 RMB kg⁻¹), pure N in zinc and humic acid urea (5.0 RMB kg⁻¹), decomposed chicken manure (0.5 RMB kg⁻¹), soil conditioner (2.8 RMB kg⁻¹), compound bacteria, amino acid liquid fertilizer and nutrient agent together (30 RMB kg⁻¹). Other field management costs included seed, labor for fertilization, irrigation, mechanical sowing, etc. Grain prices of wheat and maize during the experiment were 2.2 and 1.8 RMB kg⁻¹, respectively.

Statistical analysis

This research adopted SPSS Statistics 20.0 software (SPSS Inc., Chicago, IL, USA) to date analysis. Through least significant differences (LSD) method, the statistically significant differences were calculated. The differences level was prominent when P < 0.05. Spearman method was used to analyze the correlation between measured variables and N₂O flux³¹. Besides, the difference level is considered extremely significant when P < 0.01. All the figures and statistical analyses were computed in Origin 9.0 (Origin Lab Ltd., Guangzhou, China).

Source: Ecology - nature.com