Tracking the Release of Soil Nitrate and Labile C in A Legume-Maize Rotation in Zimbabwe

Jiri, Obert; Mafongoya, Paramu

doi:10.5539/sar.v7n1p92

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“…Considering the low nutrient holding capacity of the loamy sand soil used in our study, a large portion of N mineralized from leguminous residues was probably leached before soil samples were collected at the end of each growing season. This explanation is supported by previous research in Zimbabwe that showed soil nitrate concentrations from 0 to 1.2 m depth declined by 77% from the beginning of the rainy season to 1 week after planting maize (a 6-week span) in fields planted to vining mucuna the previous growing season (Jiri and Mafongoya, 2018).…”

Section: Discussionsupporting

confidence: 79%

Continuous cropping legumes in semi-arid Southern Africa: Legume productivity and soil health implications

et al. 2022

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Legume agronomic research in Southern Africa has often focused on integrating legumes into smallholder cereal cropping systems, but there is limited information available on the feasibility and soil health implications of continuous cropping legumes in the region. Continuous legumes may be suitable in areas with large livestock populations where a premium is placed on high-quality forage, or where efforts are underway to reclaim degraded cropland. Our objectives in this study were to (i) evaluate the performance of diverse legumes under continuous cropping and conservation tillage management with no fertility inputs and (ii) assess the response of soil health parameters to continuous legumes in a semi-arid environment. A 4-year study was conducted in Limpopo, South Africa beginning in the 2011–2012 growing season in which 10 legume and fallow treatments were imposed in the same plots for 4 growing seasons. All legumes responded negatively in varying degrees to continuous cropping in terms of biomass and nutrient accumulation. Lablab (Lablab purpureus L.) was the top-performing legume in the study and accumulated 4.5–13 Mg ha−1 of biomass and 153–345, 11–34, and 75–286 kg ha−1 of N, P, and K, respectively. Lablab often outperformed natural fallow, while other legumes generally performed as well as or inferior to natural fallow, depending on species and growing season. Cowpea (Vigna unguiculata Walp) was especially incompatible with continuous cropping and averaged less than 252 kg ha−1 and 2.1 Mg ha−1 of grain and biomass, respectively, from 2012–2013 to 2014–2015. Continuous cropping did not lead to sustained improvements in soil health. By 2014–2015, soil organic matter for all treatments had either declined or resembled baseline values. Rates of potentially mineralizable N in cowpea, lablab, vining mucuna (Mucuna pruriens var. Utilis), natural fallow, and bare ground plots fell by 70–96% during the study. There was also evidence for lower recovery of leached K by legumes compared to natural fallow species. In conclusion, legumes, such as lablab, should be considered as continuous forages on marginal land in areas where high-quality forage is in demand, but continuous cropping legumes without fertility inputs are not an effective strategy for improving soil health on degraded cropland in this semi-arid region of Southern Africa. Future research efforts may focus on the grazing strategies and baling frequencies required to optimize annual biomass accumulation of continuous lablab to meet livestock demand and support smallholder livelihoods.

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Section: Discussionsupporting

confidence: 79%