Microbial dynamics in hierarchical aggregates of tropical soil amended with bambara groundnut seed residue biochar
Chinyere Blessing Okebalama1,2, Isaac Asirifi2, Bernd Marschner2
1University of Nigeria, Dept. of Soil Science, Nigeria
Application of Bambara groundnut seed residue biochar (BSB) in vegetable production has shown to affect soil fertility, especially carbon (C) and nitrogen (N). However, there is a lack of knowledge on the microbial responses to repeated applications of BSB and NPK fertilisers in different soil aggregates. For this purpose, three replicates of four-treated soil samples (control, NPK, BSB, and NPK+BSB) arranged in Randomised Complete Block Design were taken after four years of continuous cultivation of cucumber crops at Nsukka Nigeria. The samples were mechanically separated into four dry-stable aggregate fractions of 4.75-2.00, 1.00-2.00, < 0.25-1.00, and < 0.25 mm. In addition to bulk soil < 2.00 mm, these five soil aggregate hierarchies were evaluated, with the aim of determining the effect of treatments on soil microbial nutrient cycling in a 20-day incubation experiment. The cumulative CO2 emission in soil hierarchies at day 20 was relatively low in the control and BSB soils, ranging from 0.10 to 0.21 and 0.14 to 0.26 mg CO2-C soil-1, respectively. In general, increased labile C utilisation was significantly higher in the < 0.25 mm aggregate than in other soil fractions in all the amended soils. Nonetheless, cumulative CO2 respiration in all the soil hierarchies increased significantly in the NPK and NPK+BSB amended soils compared to the BSB and control soils. There is a small pool of labile C that explains the insignificant effect of the BSB application on the basal respiration of the soil aggregates. The NPK+BSB amendment increased microbial C in 1.00-2.00 mm aggregate and < 2.00 mm soil by 40 % and 70 %, respectively, and microbial biomass N in 4.75-2.00 and 1.00-2.00 mm aggregates, and in < 2.00 mm soil by 106 %, 89 % and 341 %, respectively. The β-glucosidase activity was significantly reduced in the NPK and NPK + BSB amended soils compared to the BSB soil, whereas the reverse was the case with the reduced N-acetyl-β-glucosaminidase enzyme. Increased tyrosine aminopeptidase enzyme activity was evident in the NPK and NPK+BSB amended soils and in all aggregate hierarchies, but the C-cycle enzyme activities were mostly maximal in the < 0.25 mm aggregate fraction. The study highlights that NPK+BSB amendment can induce greater C mineralisation in < 0.25mm aggregate fractions. Moreover, the substantial increase in microbial C and N with the NPK+BSB amendment indicates the significant association of the > 1.00 mm macro-aggregates to the improvement of soil quality.
Keywords: Biochar amendment, C mineralisation, dry-stable aggregates, enzyme activity, microbial biomass
Contact Address: Chinyere Blessing Okebalama, University of Nigeria, Dept. of Soil Science, Faculty of Agriclture, 410001 Nsukka, Nigeria, e-mail: chinyere.okebalamaunn.edu.ng