Long-term effect of tillage and crop rotation practices on soil C and N in the Swartland, Western Cape, South Africa

Cooper, Glen David (2017-03)

Thesis (MScAgric)--Stellenbosch University, 2017.

Thesis

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.

AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar

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