Net N2O production from soil particle size fractions and its response to changing temperature
Graphical abstract
Introduction
Nitrous oxide (N2O) is a potent greenhouse gas with global warming potential 297-fold greater than CO2 within a time horizon of 100 years (Forster et al., 2007). Nitrous oxide is also an important precursor to compounds contributing to ozone depletion (Ravishankara et al., 2009). N2O emissions from agricultural soils due to nitrogen (N) fertilization and from natural soils are the dominant sources of global N2O (Stehfest and Bouwman, 2006; Syakila and Kroeze, 2011). Actually, N2O emission (hereafter called net N2O production) from the soil surface is the net result of production and consumption processes (Chapuis-Lardy et al., 2007). It has been widely reported that soil net N2O production increases with rising temperature (Maag and Vinther, 1996), and thus provides a positive feedback for climate warming (Smith, 1997).
Soil contains mineral particles, which are usually classified as sand, silt, and clay. These particles have different adsorption affinities for organic matter due to different types of soil minerals and specific surface areas, with generally stronger affinity in the finer particle fraction (Baldock and Skjemstad, 2000). The sand fraction usually contains a large proportion of particulate organic matter (POM), whereas the silt and clay fractions mainly contain mineral-associated organic matter. Previous studies consistently reported that carbon (C) mineralization is more rapid within the sand fraction than within the silt and clay fractions (Christensen, 1987; Ding et al., 2014, Ding et al., 2018), and N mineralization rates per unit of soil N followed a consistent order clay > silt > sand (Christensen and Olesen, 1998; Patra et al., 1999; Parfitt and Salt, 2001). Accordingly, particle-size separation provides a useful approach to differentiate soil C or N stability within discrete fractions obtained from a complex soil matrix (Bimüller et al., 2014). The rates of CO2 emission have been repeatedly investigated from these particle fractions (Bimüller et al., 2014; Ding et al., 2014, Ding et al., 2018). However, there has been no study to explore the difference in the rates of net N2O production from these particle fractions, although it is known that soil N2O and CO2 emissions are tightly linked for bulk soil (Huang et al., 2004; Zou et al., 2004). The finer soil particles, with larger volume of small pores, may be more favorable for denitrification and production of N2O (Sessitsch et al., 2001). Therefore, a reasonable assumption is that net N2O production rates may be greater from the clay and silt fractions than from the sand fraction. This generally agrees with previous reports that fine-textured soils have greater N2O emission than coarse-textured soils (Chantigny et al., 2010; Pelster et al., 2012).
Moreover, whether the temperature sensitivities of net N2O production are distinct among these soil fractions is unclear, although our previous studies reported that CO2 emission was more sensitive to warming in fine soil particles than coarse particles (Ding et al., 2014, Ding et al., 2018). The small unconnected pores in the clay fractions may have a great opportunity to create anaerobic zones as temperature rises, since the increased C mineralization consumes a larger amount of O2 (Pelster et al., 2012). This provides a microenvironment where strong denitrification will likely occur and promote N2O production. Therefore, a reasonable assumption is that the response of net N2O production to warming will be greater in the clay fraction than in the silt and sand fractions. This assumption generally agree with previous findings that Q10 values (the proportionate increase in the rate for a warming of 10 °C) for N2O emissions or denitrification are higher in fine-textured than in coarse-textured soils (Maag and Vinther, 1996).
Our previous studies demonstrated a potential common phenomenon that finer soil particle fractions have smaller rates of CO2 emission but larger Q10 values (Ding et al., 2014, Ding et al., 2018). To test whether there is a regular pattern for the rate and Q10 of net N2O production across soil particle size fractions, we carried out a short-term incubation experiments with sand (>50 μm), silt (2–50 μm) and clay (<2 μm) fractions from soils of a grassland, a forest, and two croplands (one upland and one paddy, both with organic and inorganic fertilizer treatments) under a series of increasing and decreasing temperatures between 5 °C and 30 °C with 5 °C intervals. We measured soil net N2O production rates at each temperature. The Q10 value was estimated from the dynamics of the rates of net N2O production under the various temperatures. We also explored the relation between N2O and CO2 emission by comparing N2O measurements with the CO2 emission data from our previous studies (Ding et al., 2014, Ding et al., 2018). The objectives were to test the two hypotheses: 1) net N2O production rates are greater from the clay and silt fractions than from the sand fraction; and 2) Q10 values of net N2O production are greater in the clay fraction than in the silt and sand fractions.
Section snippets
Sites and sampling
Soil samples were collected from a grassland, a forest, and two croplands in China. The grassland site was at the Duolun Restoration Ecology Experimentation and Demonstration Station, Inner Mongolia, and the forest site was at the Beijing Forest Ecosystem Research Station. The two croplands included an upland at an Eco-farming Research Station, Pingyi County, Shandong Province, and a paddy field (submerged during the year for rice cultivation) at a long-term monitoring station of the Ministry
Net N2O production rates from soil particle size fractions
Organic C and total N concentrations were both greater in the clay fraction than in the silt and sand fractions (Table 2). Similarly, net N2O production per mass of soil was significantly larger from the clay fraction than from the other two fractions (P < 0.05) for all the soils except for the paddy soils and the upland soil with NPK treatment (Fig. 2a). In the paddy soils, the sand fraction had the greatest net N2O production among the three soil fractions (P < 0.05). Net N2O production rates
Net N2O production rates differ among soil particle fractions
Our study investigated net N2O production rates from soil particle size fractions. The clay fraction had larger net N2O production rates per unit of soil mass than the silt and sand fractions for the grassland and forest soils, and the upland soil under OM treatment (Fig. 2a), which we attribute to the greater total N and organic C concentrations in the clay fraction (Fig. 5, Table 2). When net N2O production expressed as the rate per unit of soil N (Fig. 2b), it roughly stands for emission
Conclusions
In our study, we investigated net N2O production from different particle size fractions of a soil. We demonstrated that for all the soils, the lowest net N2O production rate per unit of soil N was from the median size particle, i.e., the silt fraction. This was generally similar with the pattern of CO2 emission across these fractions. Our hypothesis that Q10 values for net N2O production would be larger in the clay fraction than in the silt and sand fractions was only supported in paddy soils
Acknowledgements
We thank Prof. Chengli Tong of the Institute of Subtropical Agriculture, Chinese Academy of Science, and Prof. Gaoming Jiang of the Institute of Botany, Chinese Academy of Sciences for providing experiment platforms for soil sampling. We thank Prof. Markus Flury of Washington State University for editing and polishing the language. We also thank three anonymous reviewers for their helpful and constructive comments on an earlier version. This work was supported by the National Science Foundation
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