Department of Soil Science

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Browsing Department of Soil Science by Subject "Agricultural ecology"

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    Long-term effect of tillage and crop rotation practices on soil C and N in the Swartland, Western Cape, South Africa
    (Stellenbosch : Stellenbosch University, 2017-03) Cooper, Glen David; Hardie-Pieters, Ailsa G.; Labuschagne, J. A.; Strauss, J. A.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
    ENGLISH ABSTRACT: Soil Organic Matter (SOM) is an important indicator of soil quality influencing nutrient availability, water infiltration and retention and soil biological activity. The loss of SOM due to intensive cultivation is a growing concern worldwide. The Swartland is an important small grain production region in South Africa. It is situated in a semi-arid Mediterranean climate and as such has low SOM content (0.75 - 1.5 %). Conservation agriculture is the implementation of reduced tillage and diverse crop rotations and is seen as a possible solution to declining SOM in agricultural soils. The purpose of this study is to observe the effect of three commonly practiced tillage treatments and five different crop rotations on soil C and N stocks in the soil and the two major soil organic matter functional pools, namely, Mineral bound (MB) and Particulate Organic Matter (POM). The study was conducted on two long term trials on the Langgewens Research Farm, situated near Moorreesberg, Western Cape, South Africa (33°16’34.41” S, 18°45’51.28” E). The climate is semi-arid Mediterranean with an average rainfall of between 275-400 mm with 80% falling in the winter months (April – August). The soils in this region are mainly derived from Malmesbury shale and tend to be shallow and stony. The first trial site (Site A) was a long term tillage study in its 8th year and consisted of three different 4-year crop rotation systems each under three different tillage practices. The three crop rotations included two 100 % crop treatments: Wheat monoculture (WWWW); Wheat-Canola-Wheat-Lupin (WCWL); and one 50 % crop-50 % pasture treatment: Wheat-Medic-Wheat-Medic (WMWM). These treatments were planted under three tillage treatments: No tillage (NT); Minimum tillage (MT); Conventional tillage (CT). The second trial site (Site B) was a long term soil quality trial in its 19th year and consisted of four 4-year crop rotation systems under no tillage conditions. The four crop rotation systems included one 100 % crop system: Wheat monoculture (WWWW); and three 50 % crop-50 % pasture systems: Wheat-Medic-Wheat-Medic (WMWM); Wheat-Medic/Clover-Wheat-Medic/Clover (WMc); Wheat-Medic/Clover-Wheat-Medic/Clover with supplementary grazing on Salt Bush (WMc SB). No tillage had the highest total C stocks (0-40 cm) under both WWWW and WMWM, 31 Mg C ha-1 and 30 Mg C ha-1. These were significantly greater than both the MT, 28 Mg C ha-1 and 27 Mg C ha-1 respectively, and CT, 22 Mg C ha-1 and 21 Mg C ha-1, treatments under the same respective crop rotations. The effect under WCWL differed in that MT (28 Mg C ha-1) preformed significantly better than both NT (22 Mg C ha-1) and CT (13 Mg C ha-1). Conventional tillage under WCWL had the lowest total C stocks by a significant amount, 15 Mg C ha-1 lower than that of MT under the same crop. The two high biomass rotations, WWWW and WMWM have significantly greater total C stocks than that of WCWL. This is evident under both the CT (WWWW, 22 Mg C ha-1; WMWM 21 Mg C ha-1) and the NT (WWWWW 30 Mg C ha-1; WMWMW Mg C ha-1), where WCWL has a lower C stock of 13 22 Mg C ha-1 and 22 Mg C ha-1 respectively. WCWL however is able to accumulate a much higher total C stock under MT (28 Mg C ha-1), with there being no significant difference between it and WWWWW (28 Mg C ha-1) and WMWM (27 Mg C ha-1). The majority (55-95 %) of soil C at all sites were found in the MB fraction, while POM contributes a significantly smaller percentage. Under all treatments we can observe the trend of POM-C contribution to total C decreases with depth. There was very little difference found between the MB-C of all tillage and crop rotation treatments. However, there was great variation in the POM-C content of the treatments. Under WMWM, CT had significantly greater POM-C than NT at the 10-20 cm profile, 5.80 g kg-1 and 4.92 g kg-1 respectively, likely due to deeper incorporation of surface residues under CT. Under WWWW, NT had significantly greater POM-C than CT in the 5-10 cm profile at 2.18 g kg-1 and 1.10 g kg-1, respectively. The effect of crop rotation was similarly undefined, there was little significant difference between treatments in the MB-C while the POM-C showed great variation. Under NT in the 5-10 cm profile, WCWL had the largest POM-C, 3.76 g kg-1, significantly greater than both WMWM with 2.91 g kg-1, and WWWW with 1.81 g kg-1. However at the 10-20 cm profile WWWW with 2.18 g kg-1, was significantly larger than both WMWM and WCWL, with 0.75 g kg-1 and 0.89 g kg-1 respectively. Tillage was found to have the strongest influence on soil C stocks, with NT having the largest C stocks followed by MT, both being significantly greater than CT. Crop rotation had a lesser, but still significant influence on C stocks, but a larger role in N stocks. WWWW and WMWM had the greatest C stocks, while the reduced grazing on WMc SB also led to greater C stocks. The inclusion of a legume pasture (Medic and Medic-Clover) had a significant increase in N stocks while WCWL had the lowest N stock. The data gathered from this study, highlights the benefits of conservation agriculture through the usage of reduced tillage and high biomass producing leguminous pastures. WMWM and WMc SB under NT had excellent SOM accumulation and provide a diversified production system and would be recommended for this region for these reasons.
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    Mapping soil organic carbon stocks by combining NIR spectroscopy and stochastic vertical distribution models : a case study in the Mvoti River Catchment, KZN, South Africa
    (Stellenbosch : Stellenbosch University, 2019-03) Wiese, Liesl; Rozanov, Andrei Borisovich; De Clercq, W. P.; Seifert, Thomas; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
    ENGLISH ABSTRACT: The agricultural and environmental importance of maintaining and increasing soil organic carbon (SOC) has been increasingly recognized globally. To a large extent, this recognition can be attributed to soil being the largest terrestrial carbon pool, as well as to soil’s responsiveness to land use and management. Land use and land use change are major factors affecting SOC levels with changes from natural vegetation (forests, grasslands and wetlands) to croplands, for example, causing significant SOC losses. The topsoil (0-30 cm depth) is especially sensitive to changes in land use and management and the highest variation in SOC levels is observed in this zone. In this study SOC stocks in the first meter of soil were quantified and mapped under different land uses and management systems using a vertical SOC distribution model, applying near-infrared (NIR) spectroscopy for SOC analysis and estimating the uncertainty of the maps created using different approaches. The study area was chosen as a quaternary catchment of 317 km-2 south and southeast of Greytown in the Midlands area of KwaZulu-Natal, South Africa. The catchment exhibits complex topography and predominantly shale and dolerite parent material. Soils in the area have high organic carbon content ranging from 0.08 to 22.85 % (mean = 3.48 %), with clay content ranging from 3 to 49 % (mean = 14.7 % clay) and pH(H20) between 3.3 and 6.7 (mean pH(H20) = 4.5). Vertical SOC distribution functions were developed for 69 soil profiles sampled from different land uses (mainly forestry plantations, grasslands and croplands) in and around the study catchment. Bulk density samples were taken at 2.5, 7.5, 12.5, 17.5, 30, 40, 50, 75 and 100 cm depths. The aim was to reduce the number of soil observations required for SOC accounting to one point close to the soil surface by applying negative exponential vertical depth functions of SOC distribution. To achieve this, the exponential functions were normalized using the volumetric SOC content observed close to the surface and grouped as a function of land use and soil types. Normalization reduced the number of model parameters and enabled the multiplication of the exponential decline curve characteristics with the SOC content value observed at the surface to present an adequately represented value of soil carbon distribution to 1 m at that observation point. The integral of the exponential function was used to calculate the soil carbon storage to 1 m. The vertical SOC distribution functions were refined for soils under maize production systems using reduced tillage and conventional tillage. In these soils, the vertical SOC distributions are described by piecewise, but still continuous functions where the distribution within the cultivated layer (0-30 cm) is a linear decline under reduced tillage or a constant value under conventional tillage, followed by an exponential decline to 1 m (30-100 cm). The value of predicting SOC concentrations in soil samples using wet oxidation (WalkleyBlack method) and dry near-infrared (NIR) spectrometry was assessed by comparing them to the dry combustion method. NIR spectrometry is considered to be an especially promising method, since it may be used in both proximal and remote sensing applications. In addition, the effect of using paired samples with single SOC determination versus paired samples with replicated (three times) analysis by all (reference and test) methods was tested. It was shown that the use of paired tests without replication dramatically decreases the precision of SOC predictions of all methods, possibly due to high variability of SOC content in reference values analysed by dry combustion. While reasonable figures of merit were obtained for all the methods, the analysis of non-replicated paired samples has shown that the relative RMSE for the SOC NIR method only falls below 10 % for values above ~8 % SOC. For the corrected SOC Walkley Black method the relative RMSE practically never falls below 10 %, rendering this method as semi-quantitative across the range. It was concluded that for method comparison of soil analysis, it is essential that reference sample analysis be replicated for all methods (reference and test methods) to determine the “true” value of analyte as the mean value analysed using the reference method. Finally, the above elements of vertical SOC distribution models as a function of land use and soil type, predicting SOC stocks to 1 m using only a surface (0-5 cm) sample, and the use of NIR spectroscopy as SOC analysis method were combined to assess the changes in SOC stock prediction errors through mapping. Results indicated a dramatic improvement in precision of SOC stock predictions with increasing detail in the input parameters using vertical SOC distribution functions differentiated by land use and soil grouping. Still, the relative error mostly exceeded 20 % which may be seen as unacceptably high for carbon accounting, trade and tax purposes, and the SOC stock accuracy decreased in terms of map R 2 and RMSE. The results were generally positive in terms of the progressive increase in complexity associated with SOC stock predictions and showed the need for a substantial increase in sampling density to maintain or increase map accuracy while increasing precision. This would include an increase both in surface samples for the prediction of SOC stocks using the vertical SOC distribution models, as well as an increase in the sampling of profiles to include more soil types and increase the profile density per land use to improve the vertical SOC prediction models.