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In the highlands of Western Kenya, we investigated the reversibility of soil productivity decline with increasing length of continuous maize cultivation over 100 years (corresponding to decreasing soil organic carbon (SOC) and nutrient contents) using organic matter additions of differing quality and stability as a function of soil texture and inorganic nitrogen (N) additions. The ability of additions of labile organic matter (green and animal manure) to improve productivity primarily by enhanced nutrient availability was contrasted with the ability of stable organic matter (biochar and sawdust) to improve productivity by enhancing SOC. Maize productivity declined by 66% during the first 35 years of continuous cropping after forest clearing. Productivity remained at a low level of 3.0 t grain ha -1 across the chronosequence stretching up to 105 years of continuous cultivation despite full N-phosphorus (P)-potassium (K) fertilization (120-100-100 kg ha )1 ). Application of organic resources reversed the productivity decline by increasing yields by 57-167%, whereby responses to nutrient-rich green manure were 110% greater than those from nutrient-poor sawdust. Productivity at the most degraded sites (80-105 years since forest clearing) increased in response to green manure to a greater extent than the yields at the least degraded sites (5 years since forest clearing), both with full N-P-K fertilization. Biochar additions at the most degraded sites doubled maize yield (equaling responses to green manure additions in some instances) that were not fully explained by nutrient availability, suggesting improvement of factors other than plant nutrition. There was no detectable influence of texture (soils with either 11-14 or 45-49% clay) when low quality organic matter was applied (sawdust, biochar), whereas productivity was 8, 15, and 39% greater (P < 0.05) on sandier than heavier textured soils with high quality organic matter (green and animal manure) or only inorganic nutrient additions, respectively. Across the entire degradation range, organic matter additions decreased the need for additional inorganic fertilizer N irrespective of the quality of the organic matter. For low quality organic resources (biochar and sawdust), crop yields were increasingly responsive to inorganic N fertilization with increasing soil degradation. On the other hand, fertilizer N additions did not 726improve soil productivity when high quality organic inputs were applied. Even with the tested full N-P-K fertilization, adding organic matter to soil was required for restoring soil productivity and most effective in the most degraded sites through both nutrient delivery (with green manure) and improvement of SOC (with biochar).

Abstract. Due to its recalcitrance against microbial degradation, biochar is very stable in soil compared to other organic matter additions, making its application to soils a suitable approach for the build-up of soil organic carbon (SOC). The net effects of such biochar addition also depend on its interactions with existing organic matter in soils. A study was established to investigate how the status of pre-existing soil organic matter influences biochar stabilisation in soil in comparison to labile organic additions. Carbon loss was greater in the C-rich sites (C content 58.0 g C/kg) than C-poor soils (C content 21.0-24.0 g C/kg), regardless of the quality of the applied organic resource. Biochar-applied, C-rich soil showed greater C losses, by >0.5 kg/m 2 .year, than biochar-applied C-poor soil, whereas the difference was only 0.1 kg/m 2 .year with Tithonia diversifolia green manure. Biochar application reduced the rate of CO 2 -C loss by 27%, and T. diversifolia increased CO 2 -C losses by 22% in the C-poor soils. With biochar application, a greater proportion of C (6.8 times) was found in the intraaggregate fraction per unit C respired than with green manure, indicating a more efficient stabilisation in addition to the chemical recalcitrance of biochar. In SOC-poor soils, biochar application enriched aromatic-C, carboxyl-C, and traces of ketones and esters mainly in unprotected organic matter and within aggregates, as determined by Fourier-transform infrared spectroscopy. In contrast, additions of T. diversifolia biomass enriched conjugated carbonyl-C such as ketones and quinones, as well as CH deformations of aliphatic-C mainly in the intra-aggregate fraction. The data indicate that not only the stability but also the stabilisation of biochar exceeds that of a labile organic matter addition such as green manure.

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