DYNAMICS OF SOIL NITROGEN IN CEREAL-BASED CROPPING SYSTEMS IN THE NIGERIAN SAVANNA
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Author
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Department of Agricultural Science
ABSTRACT
PLEASE NOTE:
Two field trials were conducted in the 1993 and 1994 cropping seasons to develop strategies for efficient use of nitrogen in cereal-based cropping systems of the Northern Guinea Savanna of Nigeria. The objective of the first trial was to evaluate the contribution of root morphology to N use efficiency (kg grain per kg available soil N) and yield of five maize cultivars and one local sorghum cultivar planted early in the season at five levels of applied nitrogen. Root length density and root dry matter at 35 DAP in 1994 were significantly increased in 0-15 cm soil layer when N was increased from 0 to 60 kg ha" . Application of 120 kg N ha in splits of 60:60 at 2 and 5 WAP stimulated lateral root development in the 0-15cm soil layer at silking (64-69 DAP). In the subsoil £45cm root length density was lower (P<0.01) by 47 and 63 percent at 0 and 120 than at 30 kg N ha . Root length density, root dry matter, and N uptake were highest in Mokwa 87TZPB-SR in both in 1993 and 1994.. The most widely grown maize, cultivar TZBSR had the least root development. All the maize cultivars rooted to 90cm depth, but 30-45cm layer was the most active rooting depth for nutrients uptake, because it had the highest concentration of fine root system. Sorghum root production was higher, but more superficially located within the topsoil (0-30cm), than maize roots. Screening maize cultivars for improved root development in the subsoil (>30cm depth) late in the season was a better indicator of N uptake efficiency than screening for rapid early root growth. The hybrid maize cultivar 8644-27 gave the highest grain yield of 5.8 t ha and was the most N-use (18-30 kg grain per kg available soil N) efficient cultivar. It had higher sink capacities, including extended grain filling period, harvest index and 1000-kernel weight than the other cultivars. The most widely grown cultivar TZB-SR was the least efficient in utilizing available soil N for grain production (15-28 kg grain per kg available soil N), although it had the highest source capacity. The efficiency of local sorghum to convert available soil N to grain yield was relatively low, being only 9 as against 17-27 kg grain per kg available N, average for maize. i v i i i Marginal analyses showed that 60 kg N ha applied at 2 WAP was the most profitable rate under the conditions of the experiment. A farmer planning to invest on improved maize technology will receive higher return for his investment by growing an N-use efficient hybrid, cultivar 8644-27, than growing his popular open pollinated maize, cultivar TZB-SR. But, he will be better off growing the N-use efficient open pollinated cultivars than growing an N-use efficient hydrid, cultivar 8644-27. The second trial was conducted to evaluate the hypothesis that the contributions of an N-use efficient maize and legumes to N use efficiency can be improved by matching legume residue decomposition rate with soil nitrate release. Soybean (Glycine max cv. TGx 923-2E) and stylo (Stylosanthes hamata cv. Verano, a forage legume), and a control (maize cv. TZB-SR) were grown in four farmers' fields of different native fertility in Tsibiri village in 1993. At the end of the season, soybean grain and stover were harvested and stylo biomass was removed for fodder. Total available soil N in the 0-3 0cm layer at the beginning of 1994 season was 75, 52 and 44 kg ha" following previous soybean, stylo and maize, respectively. Total N uptake of maize was over 25 percent higher following legumes than following maize. A combination of legume rotation, 60 kg ha applied at 2 WAP, and an N-use efficient cultivar 8644-27 gave a yield advantage of 1.3t ha over a two-year sole crop maize, cultivar TZB-SR supplied with 60 kg N ha at 2 WAP. There was no effect of previous crops on sorghum yield Losses of N probably due to leaching, were lowest in plots previously planted with stylo. Soybean rotation system reduced Striga hermonthica incidence and severity compared to a two-year sole crop maize or a stylo rotation system.The highest incidence of Striga hermonthica was found in stylo rotation system. There was less Striga emergence in the N-use efficient cv. 8644-27 plots than in plots planted with cv. TZB-SR. Results indicate opportunities to integrate appropriate legume-based technologies into savanna farming system based on an identification of soil and legume N-release patterns.
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