Crop yield, plant nutrient uptake and soil physicochemical properties under organic soil amendments and nitrogen fertilization on Nitisols

Sustaining soil fertility and enhancing food production on smallholder farms is a great challenge in sub-Saharan Africa. The effects of organic amendments and nitrogen fertilizer on soil physicochemical properties and barley yield were investigated in the central Ethiopian highlands. The treatments were factorial combinations of no organic amendment (control), 10 t ha−1 biochar only (B), 10 t ha−1 compost only (Com), 10 t Com ha−1 + 2 t B ha−1 and 10 t ha−1 co-composted biochar-compost (COMBI) as main plots, and five N fertilizer levels (0, 23, 46, 69 and 92 kg ha−1) as sub-plots, with three replicates at two sites (Holetta and Robgebeya) both on Nitisols in the 2014 cropping season. Application of organic amendments and N fertilizer all significantly improved soil fertility and barley yield. The highest yield, chlorophyll content, number of productive tillers and nutrient uptake were obtained from the Com + B soil amendment at Holetta and from Com at Robgebeya. Mean grain yield responses of barley to the organic amendments were 30–49% at Holetta and 51–78% at Robgebeya, compared to the control. Fertilizer N significantly increased grain yield, chlorophyll content and N uptake at both locations. The highest grain yield obtained was at 69 kg N ha−1 at Holetta and at 92 kg ha−1 at Robgebeya. The organic amendment by N fertilizer interaction significantly influenced grain yield at both sites. Com + B and 69 kg N ha−1 addition resulted in the highest grain yield (5381 kg ha−1) at Holetta, whereas Com and 92 kg N ha−1 resulted in the highest grain yield (4598 kg ha−1) at Robgebeya. Organic amendments significantly improved soil properties through increases in soil water content, soil organic carbon (SOC), cation exchange capacity (CEC) and pH (0–20 cm depth). Addition of B, Com and B + Com increased SOC and CEC by 23–27% and 20–24% at Holetta and 26–34% and 19–23% at Robgebeya compared to their respective initial values. Soil pH increased from the initial value of 5.0 to 5.6 at Holetta and from 4.8 to 5.4 at Robgebeya at harvest due to biochar soil amendment. Grain yield was significantly correlated with total biomass, number of productive tillers, SOC and CEC. We conclude that application of organic amendments optimizes soil physicochemical properties and will help sustain barley yields in the Ethiopian highlands. The use of B, Com or Com + B may substantially reduce the amount of mineral fertilizer required for the sustainable production of barley in the long term