Nitrogen dynamics in organic and conventional farming systems in the sub-humid highlands of central Kenya

Abstract

Nitrogen (N) deficit is one of the limiting factors to food security in most developing countries while the excessive use of N has resulted in environmental contamination. Timely N availability, at the right rate is crucial to improving crop yield and N use efficiency in farming systems. Therefore, understanding nitrogen dynamics under different farming systems is essential to improve N use and recovery efficiencies of crops and in addressing environmental impacts associated with increased use of inorganic and organic inputs. This study focused on N dynamics in conventional (Conv) and organic (Org) farming systems as practiced by small scale farmers (at ∼50 kg N ha−1 yr−1 , Low input) and at recommended levels of input (∼225 kg N ha−1 yr−1 , High input) for commercial use in the sub humid and humid regions of Central Kenya. Data was collected during three cropping seasons between October 2012 and March 2014 in an on-going long-term trial established since 2007 at Chuka and at Thika sites located in central highlands of Kenya. Mineral N-based fertilizer and cattle manure were applied in Conv-High and Conv-Low while composts and other organic inputs were applied at similar N rates for Org-High and Org-Low. Farming systems were laid down in a randomized complete block design with 4 and 5 replications at Chuka and Thika respectively. The trial follows a 2 season-three-year crop rotation envisaging maize, legumes, vegetables and potatoes.N mineralization was studied using a modified buried bag approach while N loss was measured using Self-Integrating Accumulator (SIA) cores. N synchrony was assessed using daily N flux differences constructed as daily N release minus daily N uptake at different stages of the crops.N uptake was assessed at various stages of the crop through destructive sampling while nitrogen use efficiency (NUE) was assessed at harvest. Surface N balances were constructed using N applied as inputs, N deposition via rainfall, biological N fixation and crop yield and biomass as outputsOut of the total N applied from inputs, only 61, 43 and 71 % was released during potato, maize and vegetable seasons respectively. Farming systems did not show a major impact in their influence on N synchrony, i.e. matching N supply to meet N demand. Rather the N synchrony varied with crop and N demand stages. Positive N flux differences were observed (higher N release compared to N demand) during the initial 20-30 days of incubation for all the farming systems, and negative N flux differences (higher N demand than release) at reproductive stages of the crops.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 where late blight disease affected potato performance. In contrast, 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 higher than those from Org-Low and Org-High, respectively. The AEN of maize was similar in all the systems at Chuka but was higher in the high input systems compared to the low input systems at the site in Thika. The AEN of vegetables under conventional systems were similar to those from organic systems. Both conventional and organic systems lost substantial amounts of mineral-N into lower soil horizons before crop establishment (0-26 days). Cumulative NO3 --N leached below 1 m was similar in all the farming systems but was higher at the more humid Chuka site compared to Thika site during the maize season. Significantly more N was leached during potato season compared to maize and vegetable seasons. When NO3 --N leached was expressed over total N applied, 63-68% more NO3 --N was leached from the low input systems compared to the high input systems. Org-High showed a positive partial N balance at both sites and in all the cropping systems except during the vegetable season at Chuka. All the other systems exhibited negative partial N balances for the three cropping seasons with exception of Conv-High during potato season and Conv-Low and Org-Low during vegetable season at Thika site.In summary, organic and conventional had similar effects on N release, synchrony and N loss through leaching. Furthermore, more N was leached (when expressed as a fraction of N applied) during potato and vegetables cropping seasons in the low input systems compared to the high input systems. In addition, conventional and organic farming systems had similar effects on NUpE, AEN, NUtE and NHI for maize and vegetables, while conventional systems improved NUE of potato compared to organic systems. The research therefore concludes that organic and conventional farming systems at high input level are viable options of increasing food security in sub-Saharan Africa (SSA) for maize and vegetables as demonstrated by similar yields, NUE, N supply and loss. Ability to meet food security in conventional and organic system at low input is hampered by high N losses, negative N balances coupled with low productivity due to biotic and abiotic stresses. In both conventional and organic systems, there is a need to reduce N application at planting and increase N applied at reproductive stages to minimize potential loss during the initial 20-30 days after application and improve N supply midseason when crop demand is high. Since organic systems depend on organic inputs, there is a critical need to improve the quality of manure, composts and other organic inputs to improve N supply and availability