JIA-2019-11

2614 CHEN Xu et al. Journal of Integrative Agriculture 2019, 18(11): 2605–2618 often has a limited effect or no effect (Ai et al. 2012). The relatively small increases in total and labile SOC contents were presumably insufficient to support a substantial growth of microorganisms under the level of N added in our study (Geisseler and Scow 2014). The availability of labile C was likely the main factor limiting microbial growth (Bremer and Vankessel 1992). Moreover, microorganism would tend to utilize SOC rather than labile substrates after addition of easily degradable substrates, but responded widely by different soils (van der Wal and de Boer 2017). Under low available C, the dependence of microorganism on SOC increased in the presence of labile substrates (Tian et al. 2016). Fontaine et al . (2003) put forward the concept of preferential substrate utilization, which is the competition for energy and nutrient between microorganisms specialized in decomposing labile inputs and stable SOC. The more microbial diversity in GL may have led to this type of competition to build PLFAabundance resulting in less of the C inputs being utilized. The FL and BF, though, lacked new plant C inputs for over 30 years, leading to the deficiencies of C and nutrients for soil microbes, with its SOC being in a decomposition state and likely more stable compared to GL, tended to utilize labile substrates to support microbial growth. In general, G – bacteria preferentially use fresh organic materials as C sources, whereas G + bacteria favour older and more microbially processed organic matter (Fierer et al. 2003). We did not observe a significant increase in G + /G – ratios with the substrate additions in GL during the entire incubation, regardless of aggregate treatment, likely because the G – bacteria in GL were less sensitive to lower levels of available C and N addition (Appendix E). In contrast, the inorganic-N (alanine and ammonium sulfate) treatments had larger effects on G + /G – ratios after 15 days for BF-RAD than BF-IAD, suggesting that aggregate-size reduction with inorganic-N addition provided fewer available resources, leading to a shift to a more oligotrophic condition where G + bacteria thrive (Inglett et al. 2011). The fungi/ bacteria ratio generally increases with the C/N ratio, because fungi have a lower demand for N and use C more efficiently than bacteria (Fierer et al. 2003). The abundances of fungi and bacteria, however, tended to be similar throughout the incubation, producing similar fungi/bacteria ratio between the RAD and IAD treatments, suggesting that the fungi/ bacteria ratio was not an important factor contributing to the patterns of microbial PLFAs in GL (Smith et al. 2014). The dominance of bacteria in microaggregates also suggests that bacteria may compete with fungi for substrates or Fig. 4 Principal components analysis of the phospholipid fatty acids in grassland, farmland and bare fallow with various substrate additions. The triangle, square and circle symbols indicate grassland, farmland and bare fallow soils with the various substrate additions. Solid and open symbols represent IAD and RAD. The arrows indicate the microbial parameters. IAD, intact aggregate distribution; RAD, reduced aggregate distribution; FL, farmland; G, glucose; A, alanine; N, inorganic nitrogen; GL, grassland; F/B, fungi/bacteria ratio; BF, bare fallow; BG, β-glucosidase; NAG, N-acetyl-β-D-glucosaminidase;G+, gram-positive bacteria; G – ,gram- negative bacteria; TPLFA, total phospholipid fatty acid. –1.3 1.3 –1.0 1.0 TPLFA Bacteria Fungi Actinomycete G + G – F/B G + /G – NAG BG SOC GL-IAD GL-RAD GL-IAD+N GL-RAD+N GL-IAD+G GL-RAD+G GL-IAD+A GL-RAD+A FL-IAD FL-RAD FL-IAD+N FL-RAD+N FL-IAD+G FL-RAD+G FL-IAD+A FL-RAD+A BF-IAD BF-RAD BF-IAD+N BF-RAD+N BF-IAD+G BF-RAD+G BF-IAD+A BF-RAD+A PC1 (97.6%) PC2 (1.5%) Farmland-IAD Bare fallow-RAD Bare fallow-IAD Farmland-RAD Grassland-IAD Grassland-RAD