CO2 emission in a subtropical red paddy soil (Ultisol) as affected by straw and N-fertilizer applications: A case study in Southern China

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Abstract

Soil sequestration of atmospheric CO2 through land application of inorganic N fertilizer along with organic residues may have beneficial effects as a strategy to offset the increase in the concentration of CO2 in the atmosphere. A field study was conducted to assess the effect of application of N fertilizer and rapeseed (Brassica napus L.) straw in a paddy field. To understand rice-, rhizosphere- and N-induced CO2 flux, CO2 flux was measured during the growth stages of rice (Oryza sativa L.) from row, inter-row and bare soil at the experimental station of Heshengqiao at Xianning, Hubei, China.

The study included seven treatments: (CK) control, (N0) fertilizer PK, (N1) fertilizer NPK (50% N), (N2) fertilizer NPK (100% N), (N3) fertilizer NPK (200% N), (N0 + S) fertilizer NP + traw, (N2 + S) fertilizer NPK (100% N) + straw. There was a distinct variation in soil CO2 fluxes, with the higher values being observed during the reproductive stage of crop growth while the lower fluxes were observed during the maturity stage. Soil CO2 fluxes from row (797–1214 g C m−2 season−1) were significantly higher than from inter-row (289–403 g C m−2 season−1) and bare soil (148–241 g C m−2 season−1), due to the contribution of rhizosphere respiration. Among different treatments, N fertilization significantly increased the CO2 flux from row with the highest being observed from N2 + S and lowest from N0 + S treatment. No significant differences among different treatments were observed from inter-row and bare soil. From bare soil, soil CO2 flux was decreased in response to N fertilizer application; this suggested suppression in microbial activity in response to increased N fertilizer application.

Soil temperature accounted for 68 and 38% of CO2 flux variability from row and inter-row, respectively, while no significant correlation was found from bare soil. Soil temperature explained 69% of N-induced CO2 flux variability from row, while no effect was observed from inter-row and bare soil. Soil temperature was also significantly correlated with rice- and rhizosphere-induced CO2 flux accounting for 42 and 31% of CO2 flux variability, respectively.

The amount of soil carbon sequestration was estimated by taking the difference between net primary production (NPP) and the amount of carbon in harvested rice. The values ranged from −176 to −89 g C m−2 season−1 with the highest value observed from N2 + S treatment; this suggested that N fertilizer application with straw has the potential to mitigate the global carbon budget. The current findings indicate that N addition increases the CO2 flux. However, integrated use of N fertilizer along with rapeseed straw may be a preferred strategy in sequestering C in red soil.

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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