Oxidation Communications 38, No 1A, 357–366 (2015) Biochemical and biological oxidation processes – ecology, soil respiration, high temperature effect on rice yield, trace metals
SOIL RESPIRATION AND NITRIFICATION– DENITRIFICATION IN MAIZE/SOYBEAN INTERCROPPING SYSTEM YANG GAOa, XINQIANG LIa, XIAOJUN SHENa, JINGSHENG SUNa, AIWANG DUANb* Key Laboratory of Crop Water Use and Regulation, Ministry of Agriculture, 453 003 Xinxiang, China b Farmland Irrigation Research Institute, Chinese Academy of Agricultural Sciences, 453 003 Xinxiang, China E-mail:
[email protected] a
ABSTRACT In an experimental field with maize/soybean intercropping, soil respiration, nitrification, and denitrification in the intercropping system with two nitrogen treatments (N0 – 0 kg N ha–1 and N1 – 112.5 kg N ha–1) were investigated using the Barometric Process Separation (BaPS) method. Results indicated that root mass density in the rhizosphere in N1 was greater than that in N0. There was no significant difference in root mass density in the non-rhizosphere between two N treatments. Soil respiration rate in the rhizosphere in N1 was 1.1 times that in N0, while the difference in soil respiration in the non-rhizosphere between two N treatments was not significant. Soil nitrification rate in maize and soybean strips in N1 was 1.71 and 1.82 times that in N0, respectively. For two N treatments, soil nitrification in the rhizosphere was greater than that in the non-rhizosphere in intercropping, mainly because of the difference in root density. Soil denitrification rate was only measured in the late growth stage, indicating that denitrification was not the main way of nitrogen losses in maize/soybean intercropping in the experimental site. Results indicated that soil nitrification was the main way of N2O emission in intercropped filed in the experimental zone. Keywords: barometric process separation (BaPS), denitrification rate, intercropping, nitrification rate, soil respiration.
*
For correspondence.
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AIMS AND BACKGROUND Nitrification and denitrification represent two of the main biological processes involved in the N cycle and are the main biological sources of N2O emissions from soils1–4. Soil respiration is a key ecosystem process that releases carbon from the soil in the form of CO2, and is recognised as one of the largest fluxes in the global C cycle5. Agricultural practices have a strong influence on the C and N balance of ecosystems. Intercropping is usually defined as the growing together of two or more crops on the same area of land at the same time, and can make better use of environmental resources6. In legumes/cereals intercropping systems, due to a stronger competitive ability for soil N of cereals than of legumes7, cereals may benefit from having access to a more than proportionate share of the soil N sources8. A reduction in the soil inorganic N concentration, due to a fast uptake of inorganic N by cereals, could improve conditions for N2 fixation8. For now, most of the studies on the C and N cycles in intercropping have focused on the N fixation efficiency and N transfer between legumes and nonlegumes, while the study on the C cycle in intercropping is inadequate. Knowledge of gross N-turnover, nitrification and denitrification in intercropping is necessary for managing both the N supply and the potential N losses to the environment. However, there are no available data on soil nitrification and denitrification activities in intercropping. In order to understand the C and N cycles in intercropping systems, we evaluated the dynamics of soil respiration, gross nitrification and denitrification in maize/soybean strip intercropping. EXPERIMENTAL Sites and experimental design. The field experiments were conducted at Guangli Irrigation District Experimental Station (35o07′ N, 112o92′ E, elevation 150 m) from April to September in 2012. The station is located in the north Henan province in a semi-humid, temperate and monsoon climate zone. The annual mean temperature is 14.5°C, annual accumulated temperature above 10°C is about 5000°C, average annual sunshine duration 2460 h, annual frost-free days are 220 days, annual mean precipitation 593 mm, and the mean potential evaporation (measured with 20 cm pan) 1790 mm. Soil is sandy clay with mean bulk density of 1.35 g cm–3, mean field capacity of 26% (gravitational content) in 0–200 cm profile. Soil available N, P, and K contents of the cultivated horizon (0–30 cm) were 0.68, 12.5, and 72.6 mg kg–1, respectively. Soil organic matter content was 12.3 g kg–1 and pH was 7.1. The intercropping consisted of two rows of maize (Zea mays L. ‘Xianyu 335’) flanked by three rows of soybean (Glycine max ‘Lindou 10’) on either side. The field experiment comprised two treatments each with three replicates. Fertiliser nitrogen rates of the two treatments were 0 and 112.5 kg N ha–1 (N0 and N1), respectively. Each replicate comprised a plot of 6 m wide and 10 m long, with the crop rows oriented in a north–south direction. The maize strip was separated from the soybean strips by 50
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cm on either side; the inter-row spacing for soybean and maize was 50 cm; and the inter-plant spacing in both was also 50 cm. Both maize and soybean were sown on 7 April. Maize was harvested on 24 August, and soybean harvested on 4 September. Before sowing, a P at 48 kg ha–1 as calcium superphosphate, and K at 30 kg ha–1 as potassium sulphate were applied for all plots. For the treatment N1, a basal N at 112.5 kg ha–1 as ammonium nitrate was applied before sowing. Irrigation, weed control and other field management practices were kept the same for all the treatments and replicates. Soil temperature and moisture measurements. Soil temperature in the 0–5 cm soil layer was measured using JL-04 soil temperature recorders at 10 min intervals. Soil water potential in the 0–5 cm was measured using 253-L soil water potential sensors (Campell, USA). The data were recorded using a CX-1000 data logger (Compell, USA) at 10 min intervals. Soil water potential was converted to soil moisture with the water retention curve. In each plot, there were three measurement sites for soil temperature and water content, i.e. the internal row of maize strips, adjacent row between maize and soybean strips, and internal row of soybean strips, respectively. Three soil temperature sensors and three soil water potential sensors were buried in each measurement site, respectively. Then, measurements of the three sites were averaged as the value of soil temperature and moisture for the plot. Soil sampling. Soil samples were collected from the rhizosphere zone and the nonrhizosphere, respectively. The rhizosphere zone means the zone quite close to crop plants, and the non-rhizosphere means the area in the middle of rows. Therefore, there were 8 sampling sites, i.e. the rhizosphere zone of maize strips in N0 and N1 (N0-M-Rhi, N1-M-Rhi), the non-rhizosphere zone of maize strips in N0 and N1 (N0M-Non-Rhi, N1-M-Non-Rhi), the rhizosphere zone of soybean strips in N1 and N1 (N0-B-Rhi, N1-B-Rhi), and the non-rhizosphere zone of soybean strips in N0 and N1 (N0-B-Non-Rhi, N1-B-Non-Rhi), respectively. 7 intact soil cores (5.6 cm diameter, 4 cm depth) from the uppermost 5 cm were collected at each sampling site. The soil samples were cooled with freezer blocks and return to the laboratory in insulated boxes, then stored at 4°C in a refrigerator. Determination of soil respiration, nitrification and denitrification rates using BaPS system. The BaPS instrument from Umweltanalytische Meβ-Systeme (UMS) GmbH (Munich, Germany) was introduced to determine gross nitrification and denitrification rates and soil respiration. The instrument has a incubation chamber holding a maximum of 7 soil cores (diameter 5.6 cm, height 4 cm). Seven intact soil samples were filled into the BaPS instrument and the system was closed gastight and incubated for 24 h at observed in situ temperatures. Measurements with the BaPS system were performed and evaluated using the UMS BaPS evaluation software (version 2.2.4) (Ref. 9). After the determination, 7 soil cores were mixed. Then, the soil samples were soaked in tap water for about 6 h, and then the soil suspension was poured through a
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sieve (mesh size 0.2 mm2, diameter 25 cm, and depth 8 cm). Root samples were dried at 75°C, and root weight was recorded. RESULTS AND DISCUSSION Soil temperature, soil moisture, and root mass density in the top soil. Changes in soil temperature and moisture in the top soil from May to August, 2012 are shown in Fig. 1. Average soil temperature in the 0–5 cm layer from May to August was 25.17, 26.04, 27.79, and 29.80°C, respectively; and the mean value of soil water content was 19.12, 19.13, 19.25, and 21.88%, respectively.
Fig. 1. Variation of soil temperature and moisture in the 0–5 cm layer from May to August, 2012
Changes in root mass density in the rhizosphere and the non-rhizosphere in maize and soybean strips in intercropping are presented in Fig. 2. Root mass density in the rhizosphere increased at first and then decreased. Changes in root mass density in the non-rhizosphere during the growing season were not obvious, and the values were very low. For maize and soybean strips, root mass density in the rhizosphere was about 27 and 31 times that in the non-rhizosphere, respectively. For the two N treatments, root mass density in the rhizosphere in maize strips was 3.1 times that in soybean strips. Root mass density in the rhizosphere in N1 was greater than that in N0. However, there was small difference in root mass density in the non-rhizosphere between two N treatments.
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Fig. 2. Dynamics of root mass density in rhizosphere and non-rhizosphere in maize (a) and soybean (b) strips in intercropping with two nitrogen treatments in 2012 season
Soil respiration in the rhizosphere and the non-rhizosphere. Dynamics of soil respiration in the rhizosphere and the non-rhizosphere in maize/soybean intercropping with two N treatments is shown in Fig. 3. In the early growing period, as lower values of root mass and soil temperature in the top soil, soil respiration was very weak. Thereafter, soil respiration increased gradually with the increase in root mass and soil temperature in the top soil. In the middle of August, soil respiration rate decreased as the decrease in root mass. As root mass was small in the non-rhizosphere, soil respiration in the non-rhizosphere was very low during the growing season. Soil nutrient supply has significant effects on soil respiration. Soil respiration rate in the rhizosphere in maize and soybean strips in N1 was about 1.1 times that in N0, while the difference in soil respiration in the non-rhizosphere between two N treatments was not significant. Soil respiration was increased with fertilisation by microbial decomposition of applied compost, increased decomposition rate of soil organic matter, and increased root respiration10. Several studies also have shown that basal activity and microbial biomass are closely related to the availability of nutrients and soil organic matter11,12. 361
Fig. 3. Dynamics of soil respiration rates in rhizosphere and non-rhizosphere in maize (a) and soybean (b) strips in intercropping with two nitrogen treatments in 2012 season
Values of the ratio of soil respiration in the rhizosphere to the non-rhizosphere in maize strips of N0 and N1 treatments were 2.65 and 2.80, respectively; and for soybean strips, the values were 2.54 and 2.55, respectively. We defined the difference of soil respiration in the rhizosphere and in the non-rhizosphere as the root-derived respiration. Results indicated that for N0 and N1 treatments, the root-derived respiration accounted for 62 and 64% of the total soil respiration in the rhizosphere zone in maize strips, respectively; and for soybean strips, the values were 60 and 61%, respectively. Several studies suggested that the contribution of plant roots to CO2 fluxes from soils varied between 35 and 60% (Refs 13–15). Hanson et al.16 also indicated that the percent root contribution to total soil respiration throughout an entire year or growing season was 60.4% for nonforest vegetation. Kuzyakov et al.17 suggested that root respiration amounted on average to about 48% of the root-derived CO2 for cereal plants. Liu et al.18 found that the percentage of the root-derived respiration of maize (69%) was higher that of soybean (50%). The main reason for the difference between this study and previous investigations was the difference in cropping patterns. In intercropping, interspecific interactions can influence roots growth, microbial activity, and decomposition of organic matters, thus soil respiration19–21. However, more 362
research is needed into the mechanism of effects of interspecific interactions on soil respiration in intercropping.
Fig. 4. Dynamics of nitrification rates in rhizosphere and non-rhizosphere in maize/soybean intercropping system with two nitrogen treatments: (a) maize strips, (b) soybean strips
Nitrification and denitrification in the rhizosphere and the non-rhizosphere. Nitrification is a nitrogen transformation process, in which microorganisms consume carbon and nitrogen nutrition22. Accordingly, nitrogen fertilisers have significant influence on soil nitrification. Results indicated that soil nitrification in N1 was greater than that in N0, whether the rhizosphere or the non-rhizosphere (Fig. 4). Moreover, soil nitrification was also determined by root biomass and microbial activity of rhizosphere23,24. Carbon released by root can increase microbial biomass and numbers in the rhizosphere25. For maize and soybean strips, soil nitrification rate in N1 was 1.71 and 1.82 times that in N0, respectively (Fig. 4). Nitrification is influenced by rhizo-deposition such as root secretion, and root is an important factor driving soil nitrification22. Therefore, soil nitrification rate in the rhizosphere is usually higher than that in the non-rhizosphere. Soil nitrification rate in the rhizosphere in maize strips in N0 and N1 treatments was 1.14 and 1.10 times 363
that in the non-rhizosphere, respectively. Soil nitrification rate in the rhizosphere in soybean strips in N0 and N1 treatments was 1.24 and 1.31 times that in the nonrhizosphere, respectively. Soil denitrification of the first three measurements was very weak (17 May, 11 June, and 13 July, respectively), while soil denitrification increased significantly on 17 Aug. At one level of nitrogen, soil denitrification rate in the rhizosphere was higher than that in the non-rhizosphere (Fig. 5). There were three reasons for the lower values of soil denitrification in the early and middle growth stages. Firstly, gas exchange between soil and atmosphere was very frequent in the soil surface. Therefore, the anaerobic condition needed for denitrification can not be maintained in the top soil. Secondly, soil temperature in the top soil was 20–32oC from May to July (Fig. 1), which was suitable for nitrifying bacteria, but not suitable for denitrifying bacteria (the optimum temperature for denitrifying bacteria is 30–67oC) (Ref. 26). Lastly, soil moisture in the top soil was 60–70% of the field capacity, which was not suitable for denitrifying bacteria, the optimum soil moisture for denitrification was near or above the field capacity27.
Fig. 5. Dynamics of denitrification rates in rhizosphere and non-rhizosphere in maize/soybean intercropping system with two nitrogen treatments (16 August 2012)
As soil denitrification was very weak, soil denitrification was not the main way of nitrogen fertiliser losses of the intercropping in the experimental region. N2O is mainly produced in soils by nitrification and denitrification28. Denitrification was considered to be the major source of N2O under most situations, while nitrification was reported to make a substantial contribution to N2O emissions under aerobic conditions29,30. Results indicated nitrification was the main way of N2O emission in intercropped filed in the experimental zone. Further work is needed to determine which process between nitrification and denitrification is actually responsible for N2O emission in intercropping.
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