Soil microbe and soil enzyme activities are closely related to the fertilizer inputs. Our objective was to explore the dynamic change of soil microbe and soil enzyme activities in paddy field during early and late rice (Oryza sativa L.) main growth stages with different long−term fertilizer managements in the double−cropping rice system. we analyzed the soil microbe and soil enzyme activities with mineral fertilizer alone (MF), rice residues and mineral fertilizer (RF), 30% organic matter and 70% mineral fertilizer (LOM), 60% organic matter and 40% mineral fertilizer (HOM), and without fertilizer (CK) basis on long−term fertilizer experiment. The results showed that the soil enzyme activities were increased by application of mineral fertilizer along with manure or rice residues at the main growth stages of early and late rice. The soil urease activities for different fertilizer managements was RF>HOM>LOM>MF>CK at the main growth stages of early and late rice. The soil catalase activities with HOM, LOM treatments was highest, and was significantly higher than that of RF, MF, CK treatments at the main growth stages of rice. The soil invertase and dehydrogenase activities with HOM treatment were highest, and were significantly higher than that of CK at the main growth stages of rice. And the soil cellobiohyrolase activities with RF treatment were highest, and were significantly higher than that of MF, CK treatments at the main growth stages of rice. Meanwhile, the results indicated that different fertilization managements were significantly affected some physiological function soil microbial quantity. The amount of nitrifying and denitrification bacteria in soil for different fertilizer managements was HOM>LOM>RF>MF>CK at the main growth stages of early and late rice. Meanwhile, the amount of ammonifiers and cellulose−decomposing bacteria in soil for different fertilizer managements was MF>RF>LOM>HOM>CK and LOM>RF>MF>HOM>CK at the main growth stages of rice, respectively. The amount of azotobacteria in soil with RF treatment was highest, and was significantly higher than that of MF, CK treatments at the main growth stages of rice. As a result, the soil microbe and soil enzyme activities were increased by applied of fertilizer practices, the effect of mineral fertilizer combined with manure or rice residues is better than that of only mineral fertilizer.
Keywords: Rice; paddy field; long−term fertilizer management; soil microbe; soil enzyme activity.
Enzymes is an index of soil microbial activity and fertility for that it was respond to changes in soil management more quickly than other soil variables, and they play an important role in the cycling of nutrients in nature. Therefore, it was useful as early indicators of biological changes 1. It was involved in energy transfer, environmental quality and crop productivity 2,3. The number and activities of soil enzymes were influenced by soil microorganisms, plants and animals, and is closely related to environmental and ecological factors. Among the different enzymes in soils, the soil urease, catalase, invertase, dehydrogenase and cellobiohyrolase activities were important for the soil nutrients, for that they are play a key role in C, N, P cycling in ecological systems.
Soil microbial communities also play a critical role in the cycling of carbon and nutrients in terrestrial ecosystems, as well as regulating plant productivity and diversity 4. Therefore, there has a closely relationship between soil functions and its community structure, activity 5,6. Meanwhile, the communities and activities of soil bacterial were affected by the physicochemical properties of the soil, such as the pH 7, water content 8, mineral composition, fertilizer managements 9 and planting systems 10.
In recent years, many studies have indicated that the enzyme activities and microbial communities of soil are known to be affected by the field managements, including the vegetation types and crop rotations 11, soil tillage 12, fertilization regime 13,14 and so on. Meanwhile, fertilization is an important factor that influences the soil enzyme activities and diversity and growth of microorganisms in agricultural soils.
However, relatively few studies have investigated how different fertilizer managements affect the soil enzyme activities and soil microbes in double−cropping rice system paddy field in southern China. It is a traditional practice that application with organic manure used as the main nutrient source for rice production in China. In recently yeas, it was commonly accepted that application of mineral fertilizer along with rice residues practices in rice production systems enhance the soil quality 15. Therefore, the rice residues returning to the field practices is another important source of nutrients during rice production 16. However, in recent years, there has been a large increase in the use of mineral fertilizer and decrease in the use of organic fertilizer in rice production. Therefore, the soil quality was decreased in this fertilizer regime, such as decrease in soil organic matter (SOM) content and soil microbial community 17. Rather than applying with mineral fertilizer alone, the fertilizer regime of application mineral fertilizer with manure or rice straw is benefit for maintain both soil quality and high grain yields in double-cropping production systems.
The double-cropping production systems is the main crop rotation in southern China, and the fertilizer regime are very important for rice production and the paddy agroecosystems, however, limited information about the changes of soil enzyme activities and soil microbe under different long-term fertilization schemes in the double-cropping rice systems in southern China. Therefore, the objective of the present research was to study the soil enzyme activities and soil microbe at the early and late rice main growth stages as affected by long-term fertilization managements.
MATERIALS AND METHODS
Sites and cropping system
The experiment was established in 1986. It was located in Ningxiang County (28°07′ N, 112°18′ E) of Hunan Province, China. Under a continental monsoon climate, the annual mean precipitation is 1553 mm and potential evapotranspiration of 1354 mm. The monthly mean temperature is 17.2°C. Soil texture in the plough layer (0–20 cm) was silt clay loam with 13.71% sand and 57.73% silt. At the beginning of the study, the surface soil characteristics (0–20 cm) were as follows: soil organic carbon (SOC) 29.4 g/kg, total N 2.0 g/kg, available N 144.1 mg/kg, total phosphorous (P) 0.59 g/kg, available P 12.87 mg/kg, total potassium (K) 20.6 g/kg, and available K 33.0 mg/kg. There were three crops in a year, barley (Hordeum vulgare L.), early rice and late rice (Oryza sativa L.). Barley was sown in the middle of November and harvested in early May of the following year. Early rice was then transplanted and harvested in the middle of July. The growing season of late transplanted rice lasted from late July to the end of October.
The experiment had five treatments: control (without fertilizer input, CK), mineral fertilizer (MF), rice residue and mineral fertilizer (RF), low manure rate and mineral fertilizer (LOM), and high manure rate and mineral fertilizer (HOM). The design ensured all fertilized treatments received the same amount of N, phosphorus pentoxide (P2O5), potassium oxide (K2O) (the amount of N, P2O5, K2O in mineral fertilizer plus that from rice residue or manure) during the early and late rice growing season, respectively. The mineral fertilizers included urea, ordinary superphosphate and potassium chloride. Details about the fertilizer managements are listed in Table 1. Before transplanting rice seedlings, air-dried rice residue was manually spread onto the soil surface and incorporated into the soil at a cultivation depth of 20 cm. For early and late cropped rice, 70% and 60%, respectively, of mineral N fertilizer was applied at seedling and the remaining N fertilizer was applied by top dressing (7–10 days after transplanting) during crop growth. All the P and K fertilizers were applied at seedling. There were three replications and each plot size was 66.7 m2 (10 × 6.67 m).
Table 1. Nutrient supply from rice straw, manure and mineral fertilizer under different fertilizer treatments
|Treatment||Early rice||Late rice||Total|
MF: mineral fertilizer alone; RF: rice residues and mineral fertilizer; LOM: 30% organic matter and 70% mineral fertilizer; HOM: 60% organic matter and 40% mineral fertilizer; CK: without fertilizer.
* Input from mineral fertilizer + input from rice residue or manure. The numbers are in kg/ha.
For the RF treatment, rice straw return rate (air dry) was 2780, 3600 kg/ha for early and late rice.
For the LOM treatment, manure application rate (decomposed) was 2625.0, 2670.0 kg/ha for early and late rice.
For the HOM treatment, manure application rate (decomposed) was 5250.0, 5340.0 kg/ha for early and late rice.
The N, P, and K content of air-dry early rice straw was 6.5 g/kg, 1.3 g/kg, and 8.9 g/kg, N, P, and K content of air-dry late rice straw was 6.8 g/kg, 1.5 g/kg, and 9.1 g/kg, respectively, and N, P, and K content of decomposed chicken manure was 17.7 g/kg, 8.0 g/kg, and 11.2 g/kg, respectively.
Soil samples were collected from the plow layer (0–20 cm) at seedling stage, tillering stage, jointing stage, heading stage, and mature stage of the early and late rice growing season in 2016, respectively. Three soil samples were taken from each plot at the main growth stages of early and late rice. The soil samples were passed through a 2-mm sieve and kept moist in a refrigerator at 4℃ until use.
The urease and invertase activity was measured according to Kandeler et al.(2006) 18, and the invertase activity results were expressed as mg/(g soil•h) glucose released. Urease activity is calculated by the conversion of NH4-N after 24 h and colorimetric assessment at 690 nm.
Catalase activity was measured using the method of Roberge(1978) 19. Based on the H2O2transformation efficiency, the residual H2O2 was determined by titration with potassium permanganate, and catalase activity was expressed as mL KMnO4 consumed per gram per hour. Dehydrogenase activity was measured using triphenyl tetrazolium chloride (TTC) colorimetric analysis 3. The enzyme substrate TTC is degraded into triphenyl formazan (TPF), which is quantified colorimetrically at 485 nm.
Soil cellobiohyrolase was measured by 3, 5-dinitrosalicylic acid regent. Briefly, 5 g of air-dried soil (<1 mm), 15 mL of 8% sucrose, and 5-mL phosphate buffer at pH 5.5 were added to a 50-mL conical flask with 0.2-mL toluene. After shocking, the suspensions were incubated for 24 h at 37℃. Following filtration, 1-mL filtrate and 3-mL 3,5-dinitrosalicylic acid were added to a 50-mL flask and heated for 5 min at 100℃. The solution was immediately cooled and diluted to 50 mL with distilled water. Absorbance was then measured at 508 nm using a spectrophotometer 20.
Soil microbial populations
The number of culturable nitrifying and denitrification bacteria, ammonifiers bacteria and azotobacteria in a sample was determined by counting colony forming units (CFU). The number of culturable nitrifying and denitrification bacteria, ammonifiers bacteria and azotobacteria were measured using the method of Xu and Zheng (2006) 21. The density of culturable cellulose-decomposing bacteria was determined as colony forming units (CFU) using a soil dilution plate-count technique 22.
All data were expressed as mean ± standard error. The data were analyzed as a randomized complete block, using the PROC ANOVA procedure of SAS 23. Mean values were compared using the least significant difference (LSD) test, and a probability value of 0.05 was considered to indicate statistical significance.
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