JIA-2019-11

2613 CHEN Xu et al. Journal of Integrative Agriculture 2019, 18(11): 2605–2618 protected biomass to grazing and predation (Tisdall and Oades 2012). The analysis of fungal PLFA indicated that an increase in readily decomposable C benefited fungal growth after aggregate-size reduction (Gregorich et al. 1989), with abundances averaging 18% higher in RAD than that in IAD soils. However, some previous studies indicated that aggregate disruption did not affect the avaiable C in long-term cultivated soils ( Elliott 1986; Drury et al. 2004; Tian et al. 2015). Cultivated soils have smaller aggregates, so less C is protected, which was consistent with our finding from FL soils. The PLFA profiles did not differ significantly between IAD and RAD for the BF or FL soils, perhaps for two reasons. Firstly the aggregate-size reduction was less important in FL (by 12.05 g 100 g –1 soil) and BF (by 7.6 g 100 g –1 soil) compared to GL (by 25.99 g 100 g –1 soil), indicating that the quantity of macroaggregates that were reduced to microaggregates determined the response of aggregate-associated microbial community composition (Blagodatskaya et al. 2007; Bimueller et al. 2016). Secondly the released C was not sufficient for sustaining or altering microbial activity and structure compared with GL (Elliott 1986; Cambardella and Elliott 1994). The location of fungi within the pore network is a key factor controlling their survival and activity, with fungal abundance consistently higher in larger pores of macroaggregates (Strickland and Rousk 2010). The reduction of aggregate size in the WSA >0.25mm fraction to produce the WSA <0.25mm fraction, however, did not decrease fungal abundance in GL. The fungal hyphae and spores may have benefited from the release of available C following size reduction (Blagodatskaya and Kuzyakov 2008; van der Wal and de Boer 2017). Also, 60-day incubation may not have been long enough to exhaust the available substrates or the aggregate-size reduction was not high enough to reduce pore sizes because fungi have a pore size exclusion of around 10 µm (Chenu et al . 2001), which would decrease fungal abundance in the community over the long term. The released C could also activate previously dormant microorganisms (spores and cysts), promote microbial succession and increase the turnover of microbial biomass (Blagodatskaya and Kuzyakov 2008). Fungi could still grow in the WSA 2–0.25mm fraction, even though the WSA >2 mm fraction had been disturbed. The RAD soils also had significantly more actinomycetic PLFAs, peaking at day 15. This finding supports the idea that actinomycetes can typically degrade more recalcitrant organic compounds with higher C/N ratios (Cheshire et al . 1999), which are typically a portion of the C protected in larger aggregates (Elliott 1986; Six et al . 1999). Overall, the PLFA abundance declined because of the substrate depletion (Moreno-Cornejo et al . 2015). 4.2. Responses of the aggregate-associatedmicrobial communities to substrates We observed the PLFA abundance from added substrates within 60 days in all treatments and soils. The abundances of total, bacterial, fungal and actinomycetic PLFAs were the strongest within 30 days of typical lab incubations (Zhang et al. 2013) as seen in our results. Zhang et al. (2013) reported that the highest bacterial PLFAs were observed on day 7 after substrate additions. The additions of glucose and alanine significantly influenced PLFA abundance in FL and BF, but not in GL (Fig. 2 and Appendix C). The input of available C can support microbial growth, which may increase PLFA abundance, but the input of inorganic N alone Table 2 Results of linear mixed-effects model on the effects of soil management, substrate, aggregate distribution and their interactions on microbial parameters 1) Soil management Substrate Aggregate distribution Soil management× Substrate Soil management× Aggregate distribution Substrate× Aggregate distribution Soil management× Substrate×Aggregate distribution Total PLFA 4 324.11 *** 13.15 *** 40.01 *** 6.92 *** 49.30 *** 0.22 0.46 Bacteria 4 549.41 *** 18.98 *** 44.35 *** 6.06 *** 53.95 *** 0.35 0.57 Fungi 5 755.66 *** 7.78 *** 58.74 *** 4.99 *** 61.19 *** 0.54 1.03 Actinomycetes 1 986.39 *** 4.63 ** 11.77 *** 1.95 13.79 *** 0.26 0.21 G + 3 740.05 *** 28.53 *** 35.68 *** 13.97 *** 43.10 *** 0.32 0.66 G – 3 718.95 *** 12.50 *** 28.69 *** 0.41 43.70 *** 0.49 0.43 NAG 2 131.58 *** 34.04 *** 17.16 *** 2.22 * 3.52 * 3.65 * 2.13 * BG 2 823.13 *** 3.40 * 29.77 *** 1.84 7.77 *** 3.58 * 1.92 Fungi/Bacteria 188.88 *** 16.81 *** 2.23 3.87 ** 0.08 2.37 3.33 ** G + /G – 254.31 *** 5.36 ** 3.38 5.45 *** 5.20 ** 0.44 2.05 1) PLFA, phospholipid fatty acid; G + , Gram-positive bacteria; G – , Gram-negative bacteria; NAG, N-acetyl-β-glucosaminidase; BG, β-glucosidase. * , P <0.05; ** , P <0.01; *** , P <0.001.