Effects of different returning method combined with decomposer on decomposition of organic components of straw and soil fertility

Site description

The experimental was conducted in Gengzhuang Town, Haicheng(40° 48′ N, 122° 37′ E), Liaoning Province from 2019 to 2020. This area is belonged to the continental monsoon climate zone of warm temperate zone, the annual average temperature is above 10 °C, the annual accumulated temperature is 3000–3100 °C, the frost-free period is about 170 days, and the annual rainfall is 600–800 mm. The soil at the experimental site is classified as brown earth²⁹. Before the experiment, this test field had been in rotary tillage mode each year, with no straw return. The concentration of soil organic carbon, total nitrogen, available nitrogen, available phosphorus, available potassium, and soil bulk density in 0–20 cm surface layer were 12.50 g kg⁻¹, 0.89 g kg⁻¹, 129.6 mg kg⁻¹, 25.96 mg kg⁻¹, 117.94 mg kg⁻¹, and 1.53 g cm⁻³, respectively. The components and nutrient contents of corn straw were shown in Table 1, The average temperature and precipitation from May 2019 to May 2020 are shown in Fig. 1.

Experimental design and management

We adopted a split plot design, with the main plot as the cultivation method, and with three cultivation options: no-tillage, deep loosening + deep rotary tillage and rotary tillage. Then, when adding straw as a decomposer, two methods were used: adding straw decomposer and not adding it. Our experimental approach included six treatments: No-tillage and straw mulching to the field + straw decomposer (NT + S); no-tillage and straw mulching to the field + no straw decomposer (NT); rotary tillage and straw mixed into the soil + straw decomposer (RT + S); rotary tillage and straw mixed into the soil + no straw decomposer (RT); deep loosening + deep rotary tillage and straw return to the field + straw decomposer (PT + S); and deep loosening + deep rotary tillage and straw return to the field + no straw decomposer (PT). Each treatment was replicated 3 times, located in random blocks, with a total plot area of 68.4 m². At the same time as the previous year’s corn harvest, straw was returned to the field, crushed to about 10 cm long, spread evenly on the ground. No-tillage mulching and straw return to the field is the direct no-tillage maize sowing operation in spring; In the deep loosening + deep rotary tillage treatment, the soil is turned using a subsoiler to a depth of 35 cm, and then the straw is mixed into the soil through deep rotary tillage (a depth of 30 cm); and rotary tillage involves mixing straw into the soil with a rotary tiller to a depth of 20 cm and then raking it flat.

Using the nylon net bag method (mesh bags were 5 cm × 6 cm, small; 15 cm × 20 cm, medium; and large, 25 cm × 35 cm; each size with an aperture of 100 mesh), we simulated three return modes. Soil added to the net bags was taken from the top 0–20 cm prior to sowing in 2019, in the corresponding plots of each treatment. Corn stalks were added at a ratio of 5:4 per stem and leaf (dry weight of stem and leaf of corn stalks in mature stage), and crushed to 2 cm long. In no-tillage treatment, 10 g straw was added to the medium mesh bag, in the rotary tillage and deep loosening + deep rotary tillage treatment, 10 g straw was evenly divided into five parts and put into five small net bags, then the five small net bags were evenly mixed into the soil of the outer large net bag and sealed, the weight of soil added to each large net bag is 2 kg, and the compactness between the inner net bag and the soil in the outer net bag was adjusted.

Net bag layout was determined according to different treatment tillage patterns, and bags were placed in the field on seeding day in 2019. Deep loosening + deep rotary tillage was achieved by ploughing furrows 30 cm long, 15 cm wide, and 35 cm deep between corn rows in corresponding plots, large net bags were buried vertically in the furrows, filled with soil and compacted, so that return depth and straw distribution were basically the same as deep loosening + deep rotary tillage in the field. Rotary tillage mode was achieved by ploughing furrows 30 cm long, 15 cm wide, and 20 cm deep between corn rows in the corresponding treatment plot, the packed net bags were tilted in the furrows, filled with soil and moderately compacted, the top end of the net bags was level with the ground surface, which is basically consistent with the return depth of rotary tillage and straw distribution in actual field production. No-tillage mulching treatments involved laying the net bags containing straw on the ground and covering the four corners with soil to prevent the net bag from being blown away by the wind. The decomposer addition treatments involved evenly spraying c. 6.5 ml straw decomposer on the straw surface before bagging, in the treatment without decomposer, 6.5 ml water was sprayed on the surface of straw to maintain the same water content.

In all treatments we applied the same amount of N, P and K (N 240 kg hm⁻², P₂O₅ 74 kg hm⁻² and K₂O 89 kg hm⁻²). The nitrogen fertilizer was urea, the phosphate fertilizer superphosphate, and the potassium fertilizer, potassium chloride. The brand of straw decomposing agent is Gainby and the model number is d-68 (created by NORDOX company and produced by Beijing Shifang Biotechnology Co., Ltd.). Straw decomposer dosage was 1.5 kg hm⁻², diluted with water 100 times, and the effective viable bacteria number was ≥ 50 million g⁻¹. The effective bacteria in the decomposer include: Bacillus licheniformis, Aspergillus niger and Saccharomyces cerevisiae and so on.

Sampling and analysis methods

On the 15th, 35th, 55th, 75th, 95th, 145th and 365th day after the nylon net bags were placed in the field plots, 3 bags were randomly sampled from each plot. For each net bag, we first washed the surface soil off with tap water, then washed the sample with distilled water 3 times, dried it at 60 °C, weighed it and then ground it to deter-mine the decomposition rate of straw and its components. At the same time, in the no-tillage treatments, 200 g soil was taken from 0 to 5 cm below the straw net bag, in rotary tillage and deep loosening + deep rotary tillage treatments, 200 g soil from net bag was taken for the determination of soil SOC, MBC and DOC. Content of cellulose, hemicellulose and lignin in straw were determined following Van’s method³⁰, using a SLQ-6A semi-automatic crude fiber analyzer (Shanghai Fiber Testing Instrument Co., Ltd.).

The following formula was used to calculate decomposition rate of straw and its components. M₀ is the initial straw or cellulose (hemicellulose, lignin) mass, g, and M_t is the straw or cellulose (hemicellulose, lignin) mass at time t, g.

The following formula was used to calculate the straw carbon release proportion. C₀ is the initial straw carbon content, g, C_t is the straw carbon content at time t, g.

The following formula was used to calculate the straw and its components decomposition rate. M₃₆₅ is the quality of straw or cellulose (hemicellulose, lignin) mass on the 365th day, mg day⁻¹.

The relationship of the straw decomposition proportion (%) changes over time was fitted as follows:

where y_t is the proportion of the straw decomposition proportion at time t, %; t is the decomposition time of straw; k is the decomposition rate constant calculated using the least-squares method; a and b are constants.

SOC concentrations (g kg⁻¹) was determined using the K₂Cr₂O₇–H₂SO₄ digestion method³¹. Soil MBC content was determined using the Chloroform fumigation extraction method³². Two fresh soil samples were weighed, and then one of them was placed in a vacuum dryer with chloroform added, and pumped until the chloroform boiled violently, and after a period of time, the dryer cover was opened, the container containing chloroform removed, and the lid replaced. Another portion of soil was placed in a vacuum dryer without chloroform as a control. Then, 20 g each of fumigated and unfumigated soil samples were weighed, 50 mL 0.5 mg L⁻¹ K₂SO₄ was added, extracted by vibration for 0.5 h, filtrate was pumped by 0.45 μm organic filter membrane, and then the filtrate was directly analyzed and detected using a TOC organic carbon analyzer. Based on the difference of organic C content between fumigated and unfumigated soil extracts, the microbial biomass carbon was obtained by multiplying the coefficient by 2.64. For the determination of soil DOC content, we used a slightly modified method of Jones³³ and Hu Haiqing³⁴. We made a leaching solution with 0.5 mol L K₂SO₄, weighed 10 g over 2 mm sieve of air dried soil, added the soil to the leaching solution to create a soil mass ratio of 2.5:1, and then applied a shock temperature for 1 h (220 r min⁻¹). Then, after filtering, the filtrate was centrifuged for 20 min (3800 r min⁻¹), filtered with a 0.45 μm organic membrane, and the filtrate subjected to TOC organic carbon analysis meter tests.

Data analysis

In this experiment, Excel 2016 (Microsoft Corporation, New Mexico, USA) software was used to collate and analyze the data, and SPSS 19.0 (SPSS Inc., Chicago, Illinois, USA) statistical software was used to conduct variance analysis, LSD multiple analysis comparison and nonlinear regression analysis on the data. Duncan’s multiple range test was used to compare the treatment means at a 95% confidence level. Graphs were drawn using Origin 9.0 (Originlab, Northampton, USA).

Source: Ecology - nature.com