title: Aggregate size distribution in a biochar-amended tropical Ultisol under conventional hand-hoe tillage.
abstract: Biochar (or pyrogenic organic matter) is increasingly proposed as a soil amendment for improving fertility, carbon sequestration and reduction of greenhouse gas emissions. However, little is known about its effects on aggregation, an important indicator of soil quality and functioning. The aim of this study was to assess the effect of Eucalyptus wood biochar (B, pyrolyzed at 550 °C, at 0 or 2.5 t ha-1), green manure (T, fromTithonia diversifoliaat 0, 2.5 or 5.0 t ha-1) and mineral nitrogen (U, urea, at 0, or 120 kg N ha-1) on soil respiration, aggregate size distribution and SOC in these aggregate size fractions in a 2-year field experiment on a low-fertility Ultisol in western Kenya under conventional hand-hoe tillage. Air-dry 2-mm sieved soils were divided into four fractions by wet sieving: Large Macro-aggregates (LM; >1000 μm); Small Macro-aggregates (SM, 250-1000 μm); Micro-aggregates (M, 250-53 μm) and Silt + Clay (S + C, < 53 μm). We found that biochar alone did not affect a mean weight diameter (MWD) but combined application with eitherT. diversifolia(BT) or urea (BU) increased MWD by 34 ± 5.2 μm (8%) and 55 ± 5.4 μm (13%), respectively, compared to the control (P=0.023; n = 36). The B + T + U combination increased the proportion of the LM and SM by 7.0 ± 0.8%, but reduced the S + C fraction by 5.2 ± 0.23%. SOC was 30%, 25% and 23% in S + C, M and LM/SM fractions, and increased by 9.6 ± 1.0, 5.7 ± 0.8, 6.3 ± 1.1 and 4.2 ± 0.9 g kg-1for LM, SM, M and S + C, respectively. MWD was not related to either soil respiration or soil moisture but decreased with higher SOC (R2=0.37, P=0.014, n=26) and increased with greater biomass production (R2=0.11, P=0.045, n=33). Our data suggest that within the timeframe of the study, biochar is stored predominantly as free particulate OC in the silt and clay fraction and promoted a movement of native SOC from larger-size aggregates to the smaller-sized fraction in the short-term (2 years).
authors.affiliation: CGIAR Research Program on Climate Change, Agriculture and Food Security (CCAFS), World Agroforestry Center (ICRAF), P. O. Box 30667, UN Avenue-Gigiri, Nairobi, Kenya; Institute for Biodiversity and Ecosystem Dynamics (IBED), Faculty of Science, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands; National Agricultural Research Organization (NARO), P. O. Box 1752, Kampala, Uganda.
authors.forename: Bernard
authors.initials: B
authors.lastname: Fungo
authors.affiliation: Soil and Crop Sciences, Cornell University, Bradfield Hall, Ithaca, NY 14853, USA.
authors.forename: Johannes
authors.initials: J
authors.lastname: Lehmann
authors.affiliation: Institute for Biodiversity and Ecosystem Dynamics (IBED), Faculty of Science, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands; Soil Resources and Land Use, Institute of Soil Science and Site Ecology, Dresden University of Technology, Pienner Strasse 19, 01737 Tharandt, Germany.
authors.forename: Karsten
authors.initials: K
authors.lastname: Kalbitz
authors.affiliation: CGIAR Research Program on Climate Change, Agriculture and Food Security (CCAFS), World Agroforestry Center (ICRAF), P. O. Box 30667, UN Avenue-Gigiri, Nairobi, Kenya.
authors.forename: Margaret
authors.initials: M
authors.lastname: Thionģo
authors.affiliation: CGIAR Research Program on Climate Change, Agriculture and Food Security (CCAFS), World Agroforestry Center (ICRAF), P. O. Box 30667, UN Avenue-Gigiri, Nairobi, Kenya.
authors.forename: Irene
authors.initials: I
authors.lastname: Okeyo
authors.affiliation: College of Agricultural and Environmental Sciences, Makerere University, P. O. Box 7062, Kampala, Uganda.
authors.forename: Moses
authors.initials: M
authors.lastname: Tenywa
authors.affiliation: CGIAR Research Program on Climate Change, Agriculture and Food Security (CCAFS), World Agroforestry Center (ICRAF), P. O. Box 30667, UN Avenue-Gigiri, Nairobi, Kenya.
authors.forename: Henry
authors.initials: H
authors.lastname: Neufeldt
keywords: Biochar; Hand-hoe tillage; Soil aggregation; Soil organic carbon; Soil respiration; Ultisol