Responses of soil bacterial community and enzyme activity to organic matter components under long-term fertilization on the Loess Plateau of China
Introduction
Inorganic fertilizers have been widely used to improve soil fertility and crop yields across the world. However, long-term fertilizer application has become a significant environmental challenge because it increased soil acidity, nutrient runoff and leaching, and greenhouse gas emissions (Liu et al., 2013; Dou et al., 2016). The overuse of inorganic fertilizers in intensive agriculture is often associated with low nutrient use efficiency and high off-site environmental impacts (Fan et al., 2012). Studies show that excess nitrogen fertilizer that can not be retained in soil or utilized by a crop is lost to the atmosphere as NH3, N2O, NOx, and N2, or lost through leaching and runoff as NO3−-N (Robertson et al., 2013). Although inorganic fertilizers increase the labile fractions of soil organic matter in short-term (Brar et al., 2013; Sainju et al., 2000), long-term soil carbon storage may decrease due to the increased microbial activity and carbon mineralization (Manna et al., 2007; Jiang et al., 2014). Studies also reported neutral or adverse effects of inorganic fertilizers on microbial biomass and functional diversity (Lupwayi et al., 2012; Wang et al., 2019).
Compared to inorganic fertilizers, manure and compost applications can increase soil carbon and nitrogen sequestration (Guo et al., 2015) and lead to increased microbial biomass, bacterial abundance, and enzyme activities (Böhme et al., 2005; Ge et al., 2010; Pauline et al., 2011; Zhang et al., 2015; Gai et al., 2018). The addition of organic residues such as green manure, compost, and biosolids application maintained a higher soil carbon level than a control system that involved no organic residue addition (Ghimire et al., 2018; Wuest and Gollany, 2013). Compost addition also increased the microbial biomass, abundance of cultivable microorganisms, soil respiration, and enzyme activities (Zhen et al., 2014). Using manure and compost alone may not produce sustainable high crop yields. Therefore, in subsistence agriculture in developing countries that relies on small landholding and involves crop-livestock integrated systems, inorganic fertilizers are often applied to supplement the compost. Combining organic and inorganic fertilizers improve soil fertility and increase crop yields and soil extracellular enzyme activities compared to inorganic fertilizer alone (Cai and Qin, 2006; Miao et al., 2019). However, there is inconsistency in response to fertilizer and compost application because of inherent variability in the nutrient content of composts.
Long-term fertilization experiments are established to evaluate the agronomic and environmental benefits of fertilizer management. For example, a long-term N fertilization study reported the benefits of N addition on nutrient use and conservation, soil C accumulation, and crop production (Khan et al., 2007). According to Brar et al. (2013), the addition of manure and inorganic fertilizers can enhance soil C fractions and microbial community up to a 60 cm depth. Crop root depth can exceed 1 m, and root residue and exudates can enhance microbial community structure and functions throughout the soil profile. However, most of the studies discussing the influence of fertility management practices on soil microbial community, enzyme activities, and nutrient cycling are limited to the surface soil layer (Coonan et al., 2019). Comparing soil carobon and nitrogen in surface and sub-surface soils, Ghimire et al. (2018) highlighted the role of subsoil fertility in improving crop yields. Information on the impact of fertilization strategy on microbial dynamics at subsurface layers will further advance our understanding of subsoil carbon and nutrients cycling.
This study aimed to evaluate the impacts of long-term fertilization on soil health, specifically, soil bacterial community responses, enzyme activities, and organic matter components on the Loess Plateau of China. We measured soil carbon and nitrogen fractions, enzyme activities, and bacterial community composition at 0–15, 15–30, and 30–60 cm soil depths after 34-yr annual application of inorganic fertilizers, organic manure, and the combination of them under winter wheat monoculture system. A mixture of manure and inorganic fertilizers was expected to enhance soil carbon and nitrogen fractions, enzyme activity, and bacterial abundance compared to those under inorganic fertilizers alone or the control without fertilization.
Section snippets
Site description
A long-term field experiment was established in 1984 at the Shaanxi Changwu Agro-Ecological Station of China (107°44.70′E, 35°12.79 N), aiming to test the effects of fertilization and rotation on soil fertility and crop production under dryland cropping systems (Guo et al., 2008). The site has a sub-humid temperate climate, and the mean annual temperature and precipitation were 9.1 °C and 584 mm, respectively. The soil is Heilutu silt loam (Calcarid Regosol, FAO World Reference Base Soil
Soil carbon and nitrogen fractions and crop yield
Soil carbon and nitrogen fractions were significant with treatments, soil depth, and treatment × depth interaction. At 0–15 cm, SOC, MBC, PCM, STN, PNM, and NH4+-N were greater with M, NM, and NPM than CK and N (Table 1). The POC and PON were greater with M than NP and CK. The MBN was greater with NM and NPM than other treatments, and NO3−-N was greater with NP than N and CK. At 15–30 cm, SOC, POC, PCM, STN, PON, and NO3−-N were greater with NM and NPM than most other treatments. The MBC was
Long-term fertilization effect on soil organic carbon and nitrogen
Maintenance of soil organic carbon and nutrients in agroecosystems depends on carbon and nitrogen inputs and soil biogeochemical cycling. This study revealed an increase in soil carbon and nitrogen storage with manure application because they enhance SOC, STN, and POC. Manure alone or in combination with inorganic fertilizers increased soil carbon and nitrogen fractions compared to inorganic fertilizer alone, possibly due to increased organic matter inputs from organic manure. Organic manures
Conclusions
This study evaluating soil carbon and nitrogen fractions, enzyme activity, and bacterial community structure in long-term fertility management plots reinforced the importance of organic manure and integrated nutrient management in maintaining carbon and nitrogen in the soil profile and microbial growth. Long-term application of chemical fertilizers decreased most of soil carbon and nitrogen fractions, extracellular enzyme activities, and soil bacterial abundance. Manure alone or manure plus
Declaration of competing interest
We state that all authors in the manuscript have agreed to submit to your journal, and there is no conflict of interest for all authors.
Acknowledgments
This study was supported by the CAS “Light of West China” Program for Introduced Talent in the West, the National Natural Science Foundation of China (Grant No. 31570440, 31270484), and the Key International Scientific and Technological Cooperation and Exchange Project of Shaanxi Province, China (2020KWZ-010).
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