[Response of the Soil N2O Emission and Ammonia-oxidizing Microorganism Community to the Maize Straw Return with Reducing Fertilizer in Purple Soil]

Crop straw is an important agricultural source, which can replace chemical fertilizers. A field experiment with six different amounts of fertilization combined with maize straw residues was carried out in purple soil, including the control (CK), conventional fertilizing (F), straw return with conventional fertilizing (100FS), straw return with 70% conventional fertilizing (70FS), straw return with 60% conventional fertilizing (60FS), and straw return with 50% regular fertilizing (50FS), to determine the response of the soil N₂O emission and ammonia-oxidizing microorganism community distribution to straw return with reducing fertilizer. The dynamic characteristics of the N₂O emission in purple soil were observed using an in situ closed chamber and gas chromatography-based system. The ammonia-oxidizing microorganism community distribution was analyzed with multiple molecular techniques (DNA-based clone library and qPCR) linked to physical-chemical soil properties. The results show that the combination of straw with fertilizer increases the N₂O emission and cumulative N₂O emission. The highest N₂O emission[57.59-6238.02 μg·(m²·h)^-1]and cumulative N₂O emission (60.76 kg·hm^-2) were observed for the 100FS treatment. Compared with the F treatment, the soil ammonium nitrogen and nitrate nitrogen contents are reduced and the soil organic matter increases after crop straw return with chemical fertilizer. However, significant changes of the soil total nitrogen and pH were not observed. The bacterial ammonia oxidizer (AOB) amoA gene abundance is higher than that of the archaeal ammonia oxidizer (AOA). The AOA amoA gene abundance during F treatment (50.9×10³ copies·g^-1) is significantly higher than that of others, while the AOB amoA abundance gene of the F treatment is the lowest (1.36×10⁵ copies·g^-1). The 100FS reduces the community diversity and Pielou index of AOA and AOB amoA gene. Their amoA gene abundance significantly declines during 100FS treatment. However, the increment of the AOA and AOB amoA gene diversity and dominant increment of AOB amoA gene abundance are significant when applying straw with reducing fertilizer. The specific AOA indicator OTU1 may be most important with respect to the direct and indirect production of N₂O in purple soil. The redundancy analysis (RDA) shows that the community structure of AOA is remarkably relevant to the soil ammonium nitrogen, organic matter, and available phosphorus (P<0.05) and that the community structure of AOB is remarkably relevant to the soil dissolved organic nitrogen, total nitrogen, available potassium, and available phosphorus (P<0.05). The tolerance to different environments and ecological niches of AOB is weaker than that of AOA. Our results illustrate that the maize straw return with 60%-70% regular fertilizing dramatically increases the community diversity and abundance of the AOA and AOB amoA genes and partly mitigates the soil N₂O emission without significantly decreasing the vegetable yields.

[Effects of Straw in Combination with Reducing Fertilization Rate on Soil Nutrients and Enzyme Activity in the Paddy-Vegetable Rotation Soils]

The effects of straw returning combined the reducing application of chemical fertilizer for crop yield, soil nutrients and enzyme activity were studied in a typical southwestern hilly area of China in a rice-vegetable (Brassica juncea var. gemmifera Lin.) cropping system. The purple soil was selected as the target type of soil, and the Pioneer Town, Jiangjin District, Chongqing, China was selected as the typical southwestern hilly area during 2013 and 2014.Scientific basis based on the optimized fertilization strategy and the recyclable utilization of straw was provided through a field in-situ experiment in this study. There were five treatments in the field experiment: F (conventional fertilizer), 90% F+AS (100% straw with 90% conventional fertilizer), 80% F+AS (100% straw with 80% conventional fertilizer), 70% F+AS (100% straw with 70% conventional fertilizer), 50% F+DS (200% straw with 50% conventional fertilizer), and the 100% straw was 7500 kg·hm^-2. The results showed that the yields of rice and vegetable were all increased in straw with reduced fertilizer treatments (3.0%-17.9% in rice yield and 12.2%-36.4% in vegetable yield) compared with conventional fertilizer (F) treatment. Moreover, the yield of rice in the second season was also increased by 820-1240 kg·hm^-2. Soil pH values in straw with reduced fertilizer treatments raised by 0.06-0.55 compared with F treatment, especially in straw with 70% and 80% of conventional fertilizer dosage which increased by 6.74-6.88 and 6.52-6.84, respectively. The highest content of soil organic matter was 41.01 g·kg^-1 in straw with 80% of conventional fertilizer treatment. For the aspect of soil available nutrients, straw with 80% of conventional fertilizer treatment increased 110-178 mg·kg^-1 content of soil available nitrogen and 31.3-64.0 mg·kg^-1 content of soil available phosphorus. However, the excessive application of straw had negative effect on the accumulation of soil available phosphorus. In the same cultivation period, the contents of soil urease increased significantly in straw with 70% and 80% of conventional fertilizer treatments, which increased by 13.6%-76.4% and 20.1%-75.0% compared with F treatment. The contents of soil catalase in straw with reduced fertilizer treatments were significantly higher than F treatment; in first two seasons, soil catalase contents increased by 0.37 and 0.31 mL·(h·g)^-1 relative to F treatment in straw with reduced fertilizer treatment (80% of conventional fertilizer). With the increasing time of planting, the soil phosphatase content in the third season was higher than those in first two seasons. Straw with 70% and 80% of fertilizer treatments increased the activity of soil phosphatase by 45.2% and 48.2% compared with F treatment. It was concluded that straw application with 70% or 80% dosage of conventional fertilizer could benefit the rice-vegetable rotation in the southwest hilly area of China.