Conservation agriculture effects on soil organic matter on a Haplic Cambisol after four years of maizeaoat and maizeagrazing vetch rotations in South Africa

• Published in: Soil & tillage research 2012-07, Vol.123, p.21-28
• Main Authors: Dube, E, Chiduza, C, Muchaonyerwa, P
• Format: Article
• Language: English
• Subjects: Agrochemicals; Avena sativa
• ISSN: 0167-1987
• DOI: 10.1016/j.still.2012.02.008

A study was conducted to determine the effects of oat (Avena sativa) and grazing vetch (Vicia dasycapa) winter cover crops and fertilization regimes on soil organic matter (SOM) in an irrigated maize-based (Zea mays L.) conservation agriculture (CA) system following four years of continuous practice. Separate plots of oat and grazing vetch cover crops were grown in winter and then maize was planted in all plots in the following summer season. The four fertilization regimes used were: (i) fertilizer applied to the cover crops and the maize crop (F1), (ii) fertilizer applied to cover crops only (F2), (iii) fertilizer applied to the maize crop only (F3) and (iv) no fertilizer applied (F4). Control plots (weedy fallows) were included and the treatments were laid out in a randomized complete block design with three replications. Soil samples from 0a5, 5a20 and 20a50 cm depths were analyzed for total SOM, particulate organic matter (POM) fractions, hot water soluble C (HWC) and C-associated with water stable macro- and micro-aggregates (WSAC). While total SOM was more concentrated in the 0a5 cm soil depth across treatments, a lack of maize fertilization (F2 and F4 regimes) significantly (P < 0.05) reduced the stratification ratio. Oat and grazing vetch rotations produced significantly higher (P < 0.05) fine POM, coarse POM and HWC than weedy fallow rotations at 0a5 and 5a20 cm. When fertilized, oat was better able to support SOM sequestration in water stable aggregates at 0a20 cm while grazing vetch was more effective at 20a50 cm. The F3 regime had similar SOM levels as the F2. When no fertilizer was applied (F4-regime), there were significant (P < 0.01) reductions in biomass input and total SOM on the oatamaize and weedy fallowamaize rotations whereas the grazing vetchamaize rotation did not respond, both at 0a5 and 5a20 cm. The findings suggested that in the low fertilizer input CA system, targeting fertilizer to the winter cover crop as opposed to the maize crop could give similar SOM response, with less fertilizer invested and that grazing vetch cover crops may be better suited to low N input CA systems for SOM improvement.