Abstract
Organic amendments can reportedly sustain and increase lowland rice productivity in smallholder systems. Few studies have assessed locally-available substrates in hydrologically variable floodplain environments. We investigated the effects of green and farmyard manures on rice yields, and total soil C and N in the Kilombero floodplain, Tanzania. At both the fringe and the middle positions, five treatments were applied in 2016 and 2017, comprising (1) non-amended control, (2) farmyard manure, (3) pre-rice legumes, (4) post-rice legumes and (5) a combination of green and farmyard manures. Residual treatment effects were assessed in 2018 when rice plots were uniformly non-amended. Depending on the year and the position, organic amendments increased rice grain yields by 0.7–3.1 Mg ha–1 above the non-amended control. Sole green and farmyard manure applications had similar effects on grain yield, while a combination of green and farmyard manure led to a significant increase in grain yield above both the control and sole applications of organic amendments in both years. The contribution from biological N2 fixation by legumes ranged from 4 to 61 kg N ha–1. Despite partial N balances being mostly negative, we observed positive residual effects on the yield of the non-amended rice in the third year. Such effects reached up to 4 Mg ha−1 and were largest with post-rice legumes, sole or combined with farmyard manure. Irrespective of the position in the floodplain, manures significantly increased soil C and N contents after two years, hence enhancing soil fertility and resulting in increased rice grain yields. Comparable benefits may be obtained along the hydrological gradients of other large river floodplains of the region and beyond.
Highlights
Tanzania is one of the largest rice producers in East Africa, accounting for about 50% of the total regional output [1]
Above-ground biomass accumulation, N content, N accumulation and the amounts of N derived from biological N2 fixation by green manures differed between legume species, position and cropping season (Table 2)
Biomass accumulations by lablab and cowpea were comparable in both years and positions, the biomass of stylosanthes was much higher in 2016 (3.1–5.2 Mg ha−1 ) than in 2015 (1.9–2.5 Mg ha−1 ) and differed between positions
Summary
Tanzania is one of the largest rice producers in East Africa, accounting for about 50% of the total regional output [1]. Rice grain yields are generally low, ranging between 0.5 to 2.1 Mg ha−1 across all rice-growing environments [2]. Tanzania will need to double its current rice production by 2030 to meet the rapidly-growing domestic demand [3,4]. Most rice is produced in rainfed lowland floodplain environments and it is predominantly grown by smallholder farmers. Rice yields and total annual production in floodplains are highly variable, partly due to erratic rainfall and unpredictable soil submergence regimes and because of low and variable soil fertility and poor management practices [7,8]. While mineral fertilizers are still considered being the primary option for Agronomy 2020, 10, 1280; doi:10.3390/agronomy10091280 www.mdpi.com/journal/agronomy
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