Trends in wheat yields and soil organic carbon in the Permanent Rotation Trial at the Waite Agricultural Research Institute, South Australia

Abstract

The Permanent Rotation Trial at the Waite Agricultural Research Institute in South Australia was established on a red-brown earth in 1925, with predominately cereal-long fallow rotations on 34 adjacent plots. The trial was upgraded in 1948 to include a greater proportion of pasture leys in the rotations and currently contains 11 treatments. The trial is unreplicated; however, each phase of a sequence is represented each year. Seven of the original rotations have remained in an unbroken sequence since 1925: continuous wheat (W), wheat-fallow (WF), wheat-peas (WPe), wheat-pasture-fallow (WPaF), wheatoats- fallow (WOF), wheat-barley-peas (WBPe), wheat-oats-pasture-fallow (WOPaF). For the 11 rotations, soil organic carbon (SOC) in the top 10 cm declined from 2.75% in 1925 to a mean value of 1.56% in 1993. One plot, which had reverted to permanent pasture in 1950, showed the smallest decline with an SOC content of 2.46% in 1993. The greatest declines in SOC were in the 4 original rotations that included fallow phases in the sequence (mean value of 1.22%). In the WF rotation the SOC content had declined from 2.75 to 1.04% during 68 years of cropping. Associated yield decreases showed that the treatment could not sustain production. Soil organic C declined linearly with increasing frequency of fallows and decreasing frequency of pasture in the rotations. Average grain yields (1925-93) in the 7 original sequences ranged from 2.64 t/ha in WOPaF to 0.89 t/ha in the continuous W plot. The linear decline in yields for WBPe, WPaF, WPe, and WOF treatments indicate a convergence in the 1990s under current management, with an average yield of 1.54 t/ha in 1993 and average SOC in the top 10 cm of 1.32%. We hypothesise that the gradual increase in grain yields from the continuous W plot since the 1960s is the result of a gradual build-up of light fraction organic material, which assists in the maintainence of structure and nutrient availability.

Greater soil carbon stocks and faster turnover rates with increasing agricultural productivity

“…The soil is classified as a Rhodoxeralf (USDA; Soil Survey Staff, 1999) or Chromic Luvisol (WRB; FAO, 1998) with a fine sandy loam texture in the upper soil horizons. Soil in the upper 10 cm had an average pH (H 2 O) of 5.9 and a clay content of 18 % (Grace et al, 1995). As most Australian soils are typically deficient in phosphorus (Richardson et al, 2009), a basal dressing of superphosphate fertilizer was applied annually across all managements.…”

“…No nitrogenous fertilizers were used in the trial. Cultivars, cultivation and stubble management were consistent with typical district practice throughout the trial and have been detailed in Grace et al (1995). Full agronomic production and climatic data records can be downloaded from the CSIRO Data Access Portal (doi:10.4225/08/55E5165EC0D29).…”

“…where C dm is the carbon content of plant residues estimated as 450 mg C g −1 ; StubRet is the stubble retention factor, which was set to 0.40 for both crops and pasture phases based on agronomic records (Grace et al, 1995); RtS is the root-to-shoot ratio, which was set to 0.40 for wheat and oats and to 0.55 for pasture as adopted in the Australian National GHG Accounting system (Skjemstad et al, 2004); and F root is the fraction of total root biomass found in the upper 10 cm, which was estimated at 0.65 for crops and 0.63 for improved pastures based on root biomass distribution data of similar varieties in similar soil conditions (Siddique et al, 1990;Lodge and Murphy, 2006).…”

Review: Redesigning Canadian prairie cropping systems for profitability, sustainability, and resilience

“…Self-regenerating cover crops, which reseed themselves, have the potential to address this issue. Adaptations of the ley farming system of Australia, where self-regenerating subterranean clover (Trifolium subterraneum L.) and annual medic are grown in pasture-grain systems (Grace et al 1995), are being explored in both fallow-based and continuous grain production systems, with and without livestock, in the NGP (Carr et al 2005a, b;May et al 2010).…”

Potential for Conservation Agriculture in the Dry Marginal Zone of Central Syria: A Preliminary Assessment

“…Thus, we were unable to ascertain the full impact of the various treatments on soil biological, chemical, hydrological and physical properties and on the cropping system and land productivity and resilience over time, but were buoyed by initial results, which were encouraging. Although the need to replicate the trials should also be considered in future research, a number of published on-farm managed trials have been unreplicated yet have yielded useful insights (see, e.g., [39]). Moreover, other authors have noted that a trial design with no replication on a farmer's field simplifies the demonstration, thereby making it easier for farmers to understand and evaluate the technology [40].…”