SAINS TANAH – Journal of Soil Science and Agroclimatology, 18(2), 2021, 177-187 STJSSA, p-ISSN 1412-3606 e-ISSN 2356-1424 http://dx.doi.org/10.20961/stjssa.v18i2.44476 SAINS TANAH – Journal of Soil Science and Agroclimatology Journal homepage: http://jurnal.uns.ac.id/tanah Rice-husk biochar effects on organic carbon, aggregate stability and nitrogen-fertility of coarse-textured Ultisols evaluated using Celosia argentea growth Nancy E. Ebido, Ifeoma G. Edeh, Benedict O. Unagwu, Ogorchukwu V. Ozongwu, Sunday E. Obalum*, Charles A. Igwe Department of Soil Science, Faculty of Agriculture, University of Nigeria, Nsukka 410001, Enugu State, Nigeria ARTICLE INFO ABSTRACT Keywords: Low-fertility tropical soils Organic amendment Incubation length Soil quality Short-duration leafy vegetables Article history Submitted: 2021-11-06 Accepted: 2021-12-12 Available online: 2021-12-31 Published regularly: December 2021 * Corresponding Author Email address: sunday.obalum@unn.edu.ng There are insufficient data supporting the enormous potential of biochar in highly weathered tropical soils. This glasshouse study assessed rice-husk biochar (RHB) effects on soil organic carbon, aggregate stability and nitrogen fertility of sandy-loam Ultisols which were evaluated using spinach (Celosia argentea) growth. Five RHB rates 0, 5, 10, 20, and 40 g per two-kg-soil (0, 7.5, 15, 30 and 60 t ha −1 , respectively) were studied under 0, 4, 8, and 12 weeks of incubation (WOI). Batched potting of treatments enabled sowing on one date. Treatment effects on soil quality were assessed at sowing and spinach growth six weeks later. Soil organic carbon generally increased with RHB rate, with the greatest increments (37%) in maximum rate relative to no-biochar control for 8 WOI. Aggregate stability also generally increased with RHB rate, the range being 7.21%-17.21% for 8 WOI, beyond which it decreased in 10 and 20 but not 40 g pot –1 . Total nitrogen was always highest in maximum rate, increasing with rate only for 8 WOI. Treatment affected plant height more clearly than leaf count. Optimum rates were 5 or 10 g pot –1 for 8 and 4 WOI, respectively (plant height) and 10 g pot –1 for 8 WOI (leaf count). Soil organic carbon influenced soil aggregate stability (R 2 = 0.505) which in turn was quadratically related to plant height (R 2 = 0.517), indicating stability threshold for spinach. Adding RHB at 40 g pot –1 (≈ 60 t ha −1 ) to coarse-textured tropical soils is suggested to sustain its soil aggregating effect beyond the growth phase of short-cycle leafy vegetables which require a lower rate (10 g pot –1 ) 8 weeks before sowing. The observed role of soil aggregate stability in spinach growth rather than the overall effects of RHB should guide further search for edapho- agronomic optimum rate of RHB. How to Cite: Ebido, N.E., Edeh, I.G., Unagwu, B.O., Ozongwu, O.V., Obalum, S.E., Igwe, C.A. (2021). Rice-husk biochar effects on organic carbon, aggregate stability and nitrogen-fertility of coarse-textured Ultisols evaluated using Celosia argentea growth. Sains Tanah Journal of Soil Science and Agroclimatology, 18(2): 177-187. https://dx.doi.org/10.20961/stjssa.v18i2.56330 1. INTRODUCTION Some agricultural and industrial wastes can be used as organic soil amendments of diverse forms to overcome the challenges of low fertility status of most soils of the tropics and improve agricultural production in the region. Owing to the coarse nature of their texture, the soils of tropical Africa are generally ‘porous’ (Obalum & Obi, 2014). This situation, coupled with the prevailing tropical climate, implies that the soil resources for agriculture in this region are highly leached of base-forming nutrients, rendering them acidic. The low soil fertility and the adverse impact on food production that is characteristic of tropical African agriculture is epitomized in southern Nigeria (Adubasim et al., 2017; Obalum et al., 2011; Uzoh et al., 2015). The use of organic amendments has been proposed to improve the overall quality of these soils (Unagwu, 2019), including their soil organic carbon (SOC), aggregate stability and nitrogen (N)-fertility (Ogunezi et al., 2019; Umeugokwe et al., 2021). Because of the role of N in vegetative growth of plants, organic amendments have also been used to enhance the yields of particularly vegetable crops in these soils (Nwite et al., 2013; Nwite, Ogbodo, et al., 2012; Ogunezi et al., 2019). Biochars are organic materials which are charred at temperatures between 300 and 700 °C with little or no oxygen concentrations (Nwajiaku et al., 2018). They have diverse properties depending on the method of pyrolysis and the feedstock used. Biochars improve specific surface area,