Long-term effects of tillage practices on biological indicators of a soil cropped annually to wheat
Clayton, Hannah Gudrun
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Soil sustainability is a long-term goal. Although physical and chemical properties of soil have been utilized extensively to evaluate soil quality, the application of biological indicators is becoming more important. In order to assess soil quality, soil enzymes and other biological parameters need to be considered. In semi-arid Bethlehem, South Africa, samples were taken at a wheat (Triticum aestivum L.) monoculture trial which was established in 1979 by the Agricultural Research Council-Small Grain Institute. The treatments were: no-tillage (NT), stubble-mulch (SM), and conventional tillage (CT); all paired with chemical weed control, the absence of burning residues, and 40 kg nitrogen ha-1 as limestone ammonium nitrate with single superphosphate as the fertilizer sources. The study period lasted from October 2010 to October 2011 with eight sampling times conducted over this year and two depths sampled (0-5 cm, 5-10 cm). Oat (Avena sativa L.) was growing in the plots from the start of the study until December 2010 when it was harvested. A fallow period then lasted until the planting of wheat in August 2011 which was harvested after the end of the study period. Potential enzyme activities were assayed for β-glucosidase, urease, acid- and alkalinephosphatase, and dehydrogenase at all eight sampling times, along with soil texture, total carbon, total nitrogen, Olsen-extractable phosphorus, and pH. Whole microbial community profiling using BIOLOG EcoPlatesTM was employed at the first sampling time and phospholipid fatty acid (PLFA) analysis for the first, third, and fifth sampling times. It was found that NT and SM had higher values than CT across all enzymes except alkaline phosphatase, which ranked NT higher than both SM and CT. BIOLOG EcoPlatesTM and PLFA showed similar results across tillage treatments. Microbial biomass, estimated from both potential dehydrogenase activities and PLFA values, was higher in NT and SM than in CT. Over the study period the values for all parameters varied but the average ranking of tillage treatments stayed consistent. In comparing the two soil depths, soil quality was easily shown to be higher in NT and SM in the 0-5 cm depth, but often in the 5-10 cm depth the differences faded. Potential acid phosphatase activity was the only measured parameter which was consistently higher in the 5-10 cm depth. If the parameters can be used as an index of soil quality, then it can be accepted that NT has higher quality than CT and often SM has higher quality than CT, but is not at the same level as NT; it can then be recommended that in semi-arid South Africa, NT will enhance soil quality under a monoculture cropping practice.