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Carbon inputs to soil from roots of two contrasting pasture swards

The soil carbon (C) pool is one of the largest pools of C in the terrestrial environment. Soil management strategies to increase soil C are of interest globally to reduce atmospheric CO₂ concentrations which are contributing to climate change. Changes in land use and land management can result in gains, or losses of soil C, and consequently change in atmospheric CO₂ concentrations. Increasing the C inputs to soil by either increasing plant root mass has been suggested as one method to increase soil C. In New Zealand, pastoral agriculture is dominated by the use of perennial ryegrass and white clover pastures which are typically shallow rooting species. The use of more diverse pastures including additional species such as lucerne, chicory and plantain, may increase root mass or rooting depth. However, there is limited data on root mass and C inputs from roots to soil under ryegrass-clover pasture systems and no data on more diverse pasture swards in New Zealand pasture systems. The objective of this research was to investigate whether more diverse pasture swards had greater root mass and C inputs to soil compared to ryegrassclover and whether diverse pastures offered scope to increase soil C. Root mass was measured under a moderately diverse and a ryegrass-clover pasture in an existing plant diversity trial at a research dairy farm (Scott Farm, DairyNZ) containing 6 replicate paddocks of each pasture type. Soil cores were collected seasonally over one year to 300 mm depth and divided into three 100 mm depth sections (0 – 100, 100 – 200, 200 – 300 mm) and root mass measured after soil was washed off. There was greater root mass (0 – 300 mm) under the moderately diverse pasture (5320-9350 kg ha⁻¹ ) compared to the ryegrass-clover pasture (3810-5700 kg ha⁻¹) for all seasons. Additionally, there was greater root mass lower in the soil profile (100 – 200, 200 – 300 mm) in the moderately diverse pasture. The increased root mass in the moderately diverse pasture resulted in an estimated additional C input to soil of about 1203 kg C ha⁻¹ y⁻¹ (0 – 300 mm) but this estimate did not include contributions from root exudates. Root trait measurements of individual plant species also demonstrated a greater diversity of root traits (specific root length, surface area, and diameter) in the moderately diverse pasture, which may be important for the C input and C stabilisation processes in soil. Root turnover and C input to soil was measured on three replicate paddocks each of moderately diverse and ryegrass-clover pastures, also at Scott Farm. An isotope (¹³CO₂) pulse labelling method was used whereby clear chambers (1 m² ) were placed over pasture and ¹³CO₂ taken up following photosynthesis. Labelling was carried out once weekly for a period of five weeks, giving a total of 5 labelling events. Soil cores were collected at regular intervals following isotope labelling for up to 138 days and δ13C measured in the roots (0 – 100, 100 – 200 mm depth) and soil (0 – 100 mm depth) to calculate root turnover and C input. There was no difference in root turnover rates between the moderately diverse (298 days) and ryegrass-clover (260 days) pastures, with a combined root turnover rate of 276 days for both pastures. However, large variability in data meant that the ability to detect differences between pasture swards was low. The average C input to soil for both pastures was 58 kg C ha⁻¹ d⁻¹ over an 88 day period which was greater than other reported studies in New Zealand. A likely cause of this high C input from roots to soil was the severe drought conditions during the study that may have increased root death and C inputs from roots to soil. While previous work determined similar root turnover and C inputs under both conventional and moderately diverse pastures, a final experiment focussed on whether pasture renewal of a ryegrass-clover pasture could result in increased root turnover, and therefore, greater C input to soil. Root turnover and C input to soil under ryegrass-clover pasture with and without pasture renewal was measured using an isotope pulse labelling method. Pastures (paired plots in 3 replicate paddocks) were labelled with ¹³CO₂ daily for five days within clear chambers (1 m² ) before one replicate in each pair was sprayed with herbicide and seed direct drilled. Soil cores were collected (0 – 100, 100 – 200 mm depths) at regular intervals over a 89 day period following isotope labelling and root turnover and C input measured by following the decline in ¹³C in extracted roots. Following herbicide application, there was an initial rapid increase in root turnover (17 days) followed by a more similar turnover (524 days) compared to the unsprayed treatment (585 days). The increased root turnover following the use of herbicide resulted in increased C input to the soil in the sprayed treatment (3238 kg C ha⁻¹ ) compared to the unsprayed ryegrass-clover treatment (1726 kg C ha⁻¹ ). This suggested that during pasture renewal there is a large input of C. However, the proportion of this C that is stabilised in soil requires further investigation. This research demonstrated there is potential to increase soil C by using more diverse pastures through increased root mass and rooting depth. This work also provided the first measurements of root mass and C input to soil under moderately diverse pastures in New Zealand and adds to the limited information on the root mass, root turnover and C input under ryegrass-clover pasture systems. Furthermore, this work provided the first measurements of root turnover and C input to soil during a pasture renewal event involving herbicide. The data from this research will contribute better information for use in modelling and increase the knowledge and understanding of soil C under grazed pasture systems in New Zealand. Further research on investigating the root dynamics under more diverse pastures with respect to root traits such as diameter, surface area and specific root length within these pastures and how these traits influence the root turnover and C input to soil would be beneficial. Improving the understanding on the quantity of C that is stabilised and the C stabilisation processes in these pasture systems is also important in order to achieve meaningful reductions in atmospheric CO₂ concentrations.
Type of thesis
McNally, S. R. (2016). Carbon inputs to soil from roots of two contrasting pasture swards (Thesis, Doctor of Philosophy (PhD)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/9934
University of Waikato
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