Cyanobacterial inoculation of heated soils: effect on microorganisms of C and N cycles and on chemical composition in soil surface

Abstract

Physiological groups of soil microorganisms, total C and N and available nutrients were investigated in four heated (350 °C, 1 h) soils (one Ortic Podsol over sandstone and three Humic Cambisol over granite, schist or limestone) inoculated (1.5 μg chlorophyll a g −1 soil or 3.0 μg chlorophyll a g −1 soil) with four cyanobacterial strains of the genus Oscillatoria, Nostoc or Scytonema and a mixture of them. Cyanobacterial inoculation promoted the formation of microbiotic crusts which contained a relatively high number of NH 4 +-producers (7.4×10 9 g −1 crust), starch-mineralizing microbes (1.7×10 8 g −1 crust), cellulose-mineralizing microbes (1.4×10 6 g −1 crust) and NO 2 − and NO 3 − producers (6.9×10 4 and 7.3×10 3 g −1 crust, respectively). These crusts showed a wide range of C and N contents with an average of 293 g C kg −1 crust and 50 g N kg −1 crust, respectively. In general, Ca was the most abundant available nutrient (804 mg kg −1 crust), followed by Mg (269 mg kg −1 crust), K (173 mg kg −1 crust), Na (164 mg kg −1 crust) and P (129 mg kg −1 crust). There were close positive correlations among all the biotic and abiotic components of the crusts. Biofertilization with cyanobacteria induced great microbial proliferation as well as high increases in organic matter and nutrients in the surface of the heated soils. In general, cellulolytics were increased by four logarithmic units, amylolytics and ammonifiers by three logarithmic units and nitrifiers by more than two logarithmic units. C and N contents rose an average of 275 g C kg −1 soil and 50 g N kg −1 soil while the C:N ratio decreased up to 7 units. Among the available nutrients the highest increase was for Ca (315 mg kg −1 soil) followed by Mg (189 mg kg −1 soil), K (111 mg kg −1 soil), Na (109 mg kg −1 soil) and P (89 mg kg −1 soil). Fluctuations of the microbial groups as well as those of organic matter and nutrients were positively correlated. The efficacy of inoculation depended on both the type of soil and the class of inoculum. The best treatment was the mixture of the four strains and, whatever the inoculum used, the soil over lime showed the most developed crust followed by the soils over schist, granite and sandstone. In the medium term there were not significant differences between the two inocula amounts tested. These results showed that inoculation of burned soils with alien N 2-fixing cyanobacteria may be a biotechnological means of promoting microbiotic crust formation, enhancing C and N cycling microorganisms and increasing organic matter and nutrient contents in heated soils.