Green manuring inhibits nitrification in a typical paddy soil by changing the contributions of ammonia-oxidizing archaea and bacteria
Introduction
Green manuring offers a potentially valuable approach to supporting green agriculture. Utilization of green manures can contribute to high and stable yields (Zhou et al., 2016a), improve soil fertility (Tejada et al., 2008), and benefit soil carbon and nitrogen cycles (León Castro and Whalen, 2019; Yao et al., 2019; Zhang et al., 2019a). Green manuring is also an effective way to reduce chemical N application (Zhu et al., 2014). After being incorporated, green manures can release available N and reduce the dependence on mineral fertilizers for subsequent plants. A previous study reported that the N released from green manure provides approximately 17% of the crop N requirement during the growing season (Gardner and Drinkwater, 2009). In south China, milk vetch is the most traditional green manure in rice cropping system. Many studies have proved that the application of milk vetch in rice cropping systems can substitute for 20%–40% of mineral fertilizer nitrogen and maintain high rice yields (Xie et al., 2016; Xie et al., 2018; Yang et al., 2019a; Zhou et al., 2016a; Zhu et al., 2014). The use of leguminous crops in rice cropping system is currently of interest because of their ability to provide an N input by biological N fixation. The legume green manure milk vetch may have more influence than other green manure varieties on soil N cycling processes, but the mechanisms underlying the effects need further investigation.
Nitrification, a key process in N cycling controlling the transformation between ammonium and the more mobile nitrate, which is regulated by nitrifiers. The conversion of ammonium to nitrite is the first and rate-limiting step in nitrification (Stein and Klotz, 2016). Although complete nitrification by Nitrospira bacteria has been reported (Daims et al., 2015), most studies still focus on ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) (Carey et al., 2016). In agricultural soils, the abundances and communities of AOA and AOB respond differently because of varied soil properties and management practices (Carey et al., 2016; Gubry-Rangin et al., 2017; Liu et al., 2017; Ouyang et al., 2017). The responses of AOA and AOB altered by different soil types and management practices. Carey et al. (2016) revealed that AOB respond more strongly to N addition than AOA and that elevated N supply may enhance soil nitrification potential by increasing AOB. While Leininger et al. (2006) reported that AOA are quantitatively dominant in many soils from diverse ecosystems.
Soil nitrification generates mobile nitrate ions and thus may increase the risk of nitrate leaching (Adair and Schwartz, 2008). Nitrate also acts as an electron acceptor in the denitrification process leading to nitrous oxide N2O formation. Beeckman et al. (2018) reported that the nitrate leaching and N2O emissions following nitrification account for up to 50% loss of available N for the plant and that inhibition of nitrification may provide significant environmental benefits. Nowadays, the suppression of nitrate leaching associated with nitrification requires active practices that limit the activity of ammonia oxidizers. The mechanism of protein synthesis in AOA differs from that in AOB, resulting in different contributions to nitrification by AOA and AOB due to varied C and N sources applied in soils (Taylor et al., 2010). In green manure - rice cropping systems, the utilization of green manure and application of chemical N fertilizer are the two main N sources, which could have different effects on the soil nitrification process and the performance of AOA and AOB. Former studies have indicated that the utilization of green manure can reduce N leaching and improve N usage and management in agro-ecosystems (Thorup-Kristensen et al., 2003; Valkama et al., 2015; Zhang et al., 2019b). The incorporation of green manure brings large amounts of fresh organic matter into the soil and thus changes soil C and N status and that of the microbial community. Compared with chemical N fertilization, the changes in the soil environment and nutrient status after green manure application maybe one of the main influences on soil N cycling. Green manure in paddy soil has been shown to change soil microbial communities and influence microbial-related biogeochemical cycle (Gao et al., 2015). But the effects of green manuring on soil nitrification and relative contributions of AOA and AOB to nitrification remain unclear in specific agricultural soils, and the roles of AOA and AOB need further understanding. A pot experiment was conducted to explore the underlying processes responsible for soil nitrification and how they are affected by green manuring and N fertilization. We hypothesized that (1) the application of green manure may reduce nitrification potential and alter the contributions of AOA and AOB; and (2) the relative contributions of AOA and AOB may responds differently by green manuring and N fertilization.
Section snippets
Experimental design
Six treatments were established in the pot experiment: (1) winter fallow without N fertilizer (N0), (2) winter fallow with 60% N fertilizer (N60), (3) winter fallow with 100% N fertilizer (N100), (4) milk vetch without N fertilizer (MN0), (5) milk vetch with 60% N fertilizer (MN60), and (6) milk vetch with 100% N fertilizer (MN100). Each treatment had five replications, and the pots were arranged in a randomized block design. The soil used in this study was a purple alluvial soil (classified by
Yields and nutrition uptake of early and late rice
The grain yield of early rice was higher than that of late rice, and the trends within treatments were the same in early and late rice (Fig. 1). The grain yields increased significantly with increasing N fertilizer amount, and the highest one in early rice was found in MN100 (44.56 g pot−1). The grain yields did not significantly differ between MN60 and N100, indicating that the utilization of milk vetch could substitute 40% of the N fertilizer and maintain the grain yields of rice. Nutrient
Driving factors in soil nitrification as affected by green manuring
A previous study reported that the leguminous green manure effectively increases N use efficiency in the major rice production region of south China (Zhu et al., 2014). Xie et al. (2018) showed that nitrogen from organic sources, such as green manure residues, can improve the availability of soil total N and inorganic N in rice cropping systems. In the present study, no significant difference in the grain yield and N uptake of early rice was found between MN60 and N100, indicating that 40% of
Conclusion
The application of synthetic N fertilizers enhanced soil nitrification potential, which was consistent with the variation in patterns of soil nitrate concentration that resulted from N application. By contrast, the addition of green manure led to no nitrate accumulation and hence resulted in a decrease in the soil nitrification potential. The recovery of nitrification potential (RNP) and the contribution of AOB to RNP were significantly increased by N fertilizer, whereas the contribution of AOA
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by China Agriculture Research System - Green Manure (CARS-22); China National Crop Germplasm Resources Platform for Green Manure (NICGR-2019-19) and Jiangsu Science and Technology Department (SBK2019042713).
References (48)
- et al.
Nitrification in agricultural soils: impact, actors and mitigation
Curr. Opin. Biotechnol.
(2018) - et al.
Chloroform fumigation and the release of soil nitrogen: a rapid direct extraction method to measure microbial biomass nitrogen in soil
Soil Biol. Biochem.
(1985) - et al.
Meta-analysis reveals ammonia-oxidizing bacteria respond more strongly to nitrogen addition than ammonia-oxidizing archaea
Soil Biol. Biochem.
(2016) - et al.
Long-term rice-rice-green manure rotation changing the microbial communities in typical red paddy soil in South China
J. Integr. Agric.
(2015) - et al.
Archaea are the predominant and responsive ammonia oxidizing prokaryotes in a red paddy soil receiving green manures
Eur. J. Soil Biol.
(2018) - et al.
Temperature responses of soil ammonia-oxidising archaea depend on pH
Soil Biol. Biochem.
(2017) - et al.
Nitrification and nitrifiers in acidic soils
Soil Biol. Biochem.
(2018) - et al.
Responses of soil ammonia oxidation and ammonia-oxidizing communities to land-use conversion and fertilization in an acidic red soil of southern China
Eur. J. Soil Biol.
(2017) - et al.
Substrate availability drives spatial patterns in richness of ammonia-oxidizing bacteria and archaea in temperate forest soils
Soil Biol. Biochem.
(2016) - et al.
Ammonia-oxidizing bacteria are more responsive than archaea to nitrogen source in an agricultural soil
Soil Biol. Biochem.
(2016)