Effect of organic and conventional farming systems on nitrogen use efficiency of potato, maize and vegetables in the Central highlands of Kenya

Increased per capita food production in the tropics is closely tied to soil organic matter and water management, timely nitrogen (N) supply and crop N use efficiency (NUE) which are influenced by farming systems. However, there is lack of data on the effect of organic farming systems on NUE and how this compares to conventional farming systems under tropical conditions. Therefore, the objectives of this study were to determine the effect of conventional and organic farming systems at low and high management intensities on N uptake and N use efficiency of potato (Solanum tuberosum L.), maize (Zea mays L.), cabbage (Brassica oleracea var. Capitata), kale (Brassica oleracea var. Acephala) and Swiss chard (Beta vulgaris sub sp. Cicla). The organic high input (Org-High) and conventional high input (Conv-High) farming systems are managed as recommended by research institutions while organic low input (Org-Low) and conventional low input (Conv-Low) farming systems are managed as practiced by small scale farmers in the Central highlands of Kenya. The study was conducted during three cropping seasons between October 2012 and March 2014 in an ongoing long-term trial established since 2007 at Chuka and at Thika, Kenya. Synthetic N-based fertilizer and cattle manure were applied at ∼225 kg N ha−1 yr−1 for Conv-High and at ∼50 kg N ha−1 yr−1 for the Conv-Low. Composts and other organic inputs were applied at similar N rates for Org-High and Org-Low. Nitrogen uptake efficiency (NUpE) of potato was highest in Conv-Low and Org-Low at Thika and lowest in Org-High and Org-Low at Chuka site where late blight disease affected potato performance. In contrast, the NUpE of maize was similar in all systems at Chuka site, but was significantly higher in Conv-High and Org-High compared to the low input systems at Thika site. The NUpE of cabbage was similar in Conv-High and Org-High while the NUpE of kale and Swiss chard were similar in the low input systems. Potato N utilization efficiencies (NUtE) and agronomic efficiencies of N use (AEN) in Conv-Low and Conv-High were 11-21 % and 1.4-3.4 times higher than those from Org-Low and Org-High, respectively. The AEN of maize was similar in all the systems at Chuka but was 3.2 times higher in the high input systems compared to the low input systems at the Thika site. The AEN of vegetables under conventional systems were similar to those from organic systems. Nitrogen harvest index (NHI) of potato was similar between Conv-High and Org-High and between Conv-Low and Org-Low. N partitioned into maize grain was similar in all the system at Chuka, but significantly lower (P < 0.001) in Conv-low and Org-Low at Thika site. The NHI of cabbage in Org-High was 24 % higher than that of Conv-High. The study concluded that for maize and vegetables, conventional and organic farming systems had similar effects on NUpE, AEN, NUtE and NHI, while for potato conventional systems improved NUE compared to organic systems. The study recommends that management practices for potato production in organic systems should be improved for a more efficient NUE.