Soil nitrogen dynamics and spring wheat (Triticum aestivum) production in different cropping systems in the Swartland

[摘要] ENGLISH ABSTRACT:Protein and oilseed production in the winter rainfall region of South Africa(primarily the Southern and Western Cape) is well below the potential forthe area. One possible method of increasing production is to convinceproducers that the inclusion of protein and oilseed rotational crops willenhance the sustainability of wheat production systems. A project toinvestigate, quantify and elucidate the effect of canola (Brassica napus),lupins (Lupinus angustifolius) and medics (Medicago spp.) on subsequentwheat (Triticum aestivum) production as well as their influence on soilfertility in respect to nitrogen was initiated in 1999. The long- termrotational crop system experiment on Langgewens experimental farmnear Malmesbury, provided the infrastructure for the project.During the 1999 and 2000 wheat-growing seasons, 50m2 plots weredemarcated in wheat fields where wheat, lupins, canola or medics wereproduced in 1998 and 1999 respectively. Each plot was cleared of allvegetation and received no fertilizer N. In these fallow plots, soil sampleswere collected at three to four week intervals and the available nitrate-Nand ammonium-N content of the topsoil (0-15cm depth) and the subsoil(15-30cm depth) were determined.Additional soil samples were also taken from the topsoil on 09/03/2000after a summer fallow period. These samples were taken on plots thatwere subjected to a wheat crop in 1999, but were preceded by eitherwheat, canola, lupins or medics in 1998. The soil samples were thensubjected to a 60-day incubation period at 15°C while being maintained at75% of their water holding capacity. Mineral N was determined after 0, 3,7, 15, 30, 45 and 60 days of incubation. Large variation in the nitrogencontents observed made it difficult to always obtain significantdifferences. The results of the soil investigations however indicate thatthe inclusion of lupins and medics in wheat cropping systems in theSwartland could help to limit the decline of mineral N in the soil profile,experienced within a growing season. This will therefore enable farmersto reduce fertilizer inputs without facing soil mineral N depletion.Plant samples were also collected from wheat plots during the 1999growing season on three occasions (23/06/99 (growth stage 5), 5/08/99(growth stage 15), 14/09/99 (growth stage 23)). Plots were selected torepresent the same treatments as in the soil sampling procedure duringthe growing season of 1999 as described above. Plant and tiller numbersof a 0.25m2 sub sample from each plot were counted and leaf area wasdetermined, whereafter the dry material was weighed. The percentagenitrogen content in the leaves and stems were subsequently determinedusing Near Infra-Red Spectroscopy. In terms of vegetative growth, littledifference was observed. A significant increase in nitrogen content ofwheat plants growing in plots where wheat and canala were grown in1998 was observed in the second plant sampling (growth stage 15) andthis was attributed to higher nitrogen topdressing. However, on the thirdsampling date on 14/9/1999 (growth stage 23), the difference was nolonger evident. The most important conclusion to make from this study, isthat farmers can probably save on fertilizer inputs when including lupinsand medics in their wheat production systems, without risking poorerwheat growth.At growth stage 28, a 0.25m2 sub sample of wheat plants was removed ata randomly chosen point in each of the plots described above. Ears andgrains were subsequently counted. The wheat remaining on the plotswere then harvested at the end of October 1999 using a plot combine.Subsequently the yield, hectolitre mass, thousand grain mass and %crude protein was determined from samples of harvested grain from eachplot. The % flour and % bran extraction were then determined whereafterthe % flour protein was determined. Micro bread loafs were baked toestimate the loaf volume. Flour and dough properties were also testedusing the Falling Number System, mixograph and alveograph. Wheat incropping systems consisting of legume phases such as lupins andmedics, required less nitrogen fertilizer application to achieve statisticallythe same yield, flour and dough properties. These crop rotations cantherefore be considered as more ecologically sustainable andeconomically viable for the Swartland.This study was aimed at determining the effect of different crop rotationson soil fertility, and because clear soil fertility trends take time to form,this study was probably too short to obtain fully significant differences.

[发布机构] Stellenbosch University