CO2 emission in a subtropical red paddy soil (Ultisol) as affected by straw and N-fertilizer applications: A case study in Southern China
Introduction
Soil respiration is one of the primary fluxes of C between soils and the atmosphere, with a global release of 75 Pg C per year (Schlesinger and Andrews, 2000). Understanding controls on soil respiration is critical because relatively small changes in respiration rates may dramatically alter atmospheric concentrations of CO2 as well as rates of soil C sequestration. It is expected to reduce CO2 emission from soils and/or to increase sequestration of atmospheric CO2 in soils. Accordingly, characterization of soil CO2 emission is increasingly important. Soil CO2 emission integrates all components of soil CO2 production, including rhizosphere respiration and soil microbial respiration.
Variations of soil CO2 flux are affected by agronomic management practices such as organic and inorganic fertilization (Ding et al., 2006). Agricultural management practices affect soil CO2 flux by changing the soil environment such as soil aeration, soil pH, soil moisture, soil temperature, C/N ratio of substances, etc. These soil environmental characteristics can have a significant impact on soil microbial activity and the decomposition processes that transform plant-derived C to soil organic matter (SOM) and CO2 (Franzluebbers et al., 1995). Previous research has shown that soil CO2 flux rates are strongly related to soil temperature and soil moisture conditions (Franzluebbers et al., 1995, Ren et al., 2007, Iqbal et al., 2008, Liu et al., 2008).
Rhizosphere respiration has been estimated to be 25–45% of gross primary productivity and accounts for 15–71% of ecosystem respiration (Rochette et al., 1999). Similarly, nitrogen fertilizers applied to soils influence soil CO2 emission, though their actual effects vary (Lee et al., 2007). The relative benefits of balanced fertilizer using crop residues, organic manures and green manuring in maintaining the organic C levels in arable soils are of increasing concern (Ladd et al., 1994). While chemical fertilizers are increasingly applied to paddies in Asia (FAOSTAT, 2005), the effect of chemical fertilizers or combined applications of organic and chemical fertilizers is particularly crucial for predicting the future trend of CO2 emission from Asian paddies and possible approaches to mitigate climatic change by agricultural practices. However, there is a lack of data on extensive crops, particularly in the south region of China. Red soil, one of the important typical soils in subtropical regions of China, which can be classified as Ultisols in the Soil Taxonomy System of the USA and Acrisols and Ferralsols in the FAO legend (FAO/UNESCO, 1974), cover about 1.13 million km2 or 11.8% of the country land surface, produces 80% of the rice (Oryza sativa L.) and supports 22.5% of the population (Zhao, 2002). In southern China, including 15 provinces, red soil covers 0.28 million km2 of cultivated land (Zhao, 2002). However, with the rapid economic and social development, red soils are subject to degradation as characterized by low organic carbon content and low crop productivity. Therefore, it is necessary to investigate soil CO2 evolution from red soils for better understanding the mechanisms that regulate C storage and loss processes in the extensively cultivated paddy field. Furthermore, the effects of N fertilization and rice growth on variation in CO2 emission under anaerobic conditions from paddy soils are not well known.
In China in the 1980s, approximately 60 Pg year−1 of straw were produced from 100 million ha of cultivated soil, of which 80% was burned either in the field or for cooking (Cheng and He, 1990). The use of straw is becoming less common due to increasing environmental concerns and the availability of fossil fuels in rural areas. Applying of straw in combination with inorganic fertilizer is an attractive alternative to burning because it can provide essential nutrients for crops (Edmeades, 2003), while also reducing C release to the atmosphere. However, Ajwa and Tabatbai (1994) found that as much as 27–58% of added organic C in corn and alfalfa residues was released as CO2 during the 30 d incubation period. In contrast, less organic C in wheat (Triticum aestivum L.) straw was decomposed during a similar period (Ghidey and Alberts, 1993). Decomposition rates, therefore, vary with plant type (Kirchmann et al., 2004). In China, current practice with rapeseed (Brassica napus L.) straw includes grinding and application directly to soils, when the crops are being harvested by machine. Using rapeseed straw as a fertilizer, however, presents a substantial challenge. Nevertheless, knowledge on the effect of fertilization on CO2 emission under anaerobic conditions from paddy soils is still insufficient. The present study was conducted to demonstrate the variation of CO2 emission under intensively cultivated paddy soil. The objectives of the study were to: (1) investigate rice-, rhizosphere and N-induced CO2 emission; (2) evaluate the influence of application of rapeseed straw in combination with inorganic fertilizer on soil CO2 emission from intensively cultivated paddy soil; and to (3) approximate the effect of nitrogen fertilization on the potential of carbon sequestration from the paddy field.
Section snippets
Site description
The field experiment was conducted on a well drained paddy soil located at the experimental station of Heshengqiao, Southern China (29°02′–30°18′N, 133°31′–144°58′E). The selected site was representative of the regional features of land use in Southern China. Altitude ranges from 86 to 147 m above sea level. The mean annual sunlight hours are 1857 h and the mean annual wind is 1.5 m s−1. This region has a typical subtropical monsoon climate with an annual mean temperature of 16.8 °C and an annual
Crop yield and biomass
Crop grain yield and total above ground biomass were significantly different among treatments (Fig. 1). The highest grain yield and total biomass were observed in N2 and N2 + S treatment, respectively, while lowest values were observed from CK treatment. Addition of straw with nitrogen fertilizer (N2 + S) increased the total biomass by 22% and decreased the grain yield by 0.3% as compared to N2 treatment. While addition of straw without nitrogen fertilizer (N0 + S) increased the total biomass by 19%
Soil CO2 fluxes
There was a clear seasonal variation in soil CO2 fluxes, depending on the soil temperature/growth stage of rice (Fig. 2). For row and inter-row soil, CO2 flux variation can be attributed to soil temperature/growth stage of rice. Maximum CO2 flux from row and inter-row was observed on panicle excertion and flowering stage of rice, respectively. The CO2 flux started to increase up to panicle excertion/flowering stage from the start of the measurements made after transplanting, and then, declined
Conclusion
N fertilization has the potential to deplete soil C pool. Greatest net C sequestration following inorganic fertilization would be expected in highly productive sites. However, combined use of inorganic and organic fertilizer may help to sequester C from productive to marginal sites.
On the whole, this field experiment has shown the key importance of the impact of N addition on the C sink efficiency of the paddy ecosystem. For better estimation of C sequestration capacities of the paddy
Acknowledgements
This research was supported by National Natural Science Foundation (No. 40471131), and National Science and Technology key projects (No. 2008BADA7B01) of China. The authors are grateful to the president and other staff members of the experimental station of Heshengqiao, located in Xianning, Hubei province, Southern China, for assistance during the field investigations. We also thank two anonymous reviewers for their constructive comments on the manuscript.
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