Irrigation changes the distribution of new photo-assimilated carbon into various soil size fractions

Ms Carmen Rosa Medina Carmona1,2, Sam R.  McNally2, Mike H. Beare2, Tim Clough1, John e. Hunt3, Craig S. Tregurtha2

1Lincoln University-Deparment of Soil and Physical Sciences, Lincoln, New Zealand, 2The New Zealand Institute for Plant and Food Research , Lincoln, New Zealand, 3Manaaki Whenua – Landcare Research, Lincoln, New Zealand

Globally, the need for irrigation to increase dry matter production (DMP) on grazed pastures has been steadily increasing to support the growing demand for meat and dairy products. However, research on the effects of irrigation on the dynamics of soil carbon (SOC) in these pastures has been scarce and produced contradictory results. Our objective was to quantify the effects of irrigation on the storage and stability of new photo-assimilated carbon (C) in a plant-soil system.

The experiment consisted of two soil moisture treatments (simulating either Dryland or Irrigated conditions) applied to soil mesocosms established with perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) pasture. Plants were continuously pulse labelled with 13CO2 between December 2016 and March 2017 (e.g. an irrigation season) by growing within a sealed plant growth chamber. Following the labelling phase, the soil moisture regimes, for both treatments, were adjusted to simulate autumn/winter conditions. The mesocosms were destructively harvested at 1, 12, 125, 237 and 349 days after the last labelling event. The partitioning of new photo-assimilated C was determined through 13C analysis of herbage, roots, bulk soil, rhizosphere soil and soil size fractions (>250 µm, 53–250 µm, 20–53 µm, 5–20 µm and <5 µm). The net storage and subsequent loss of new photo-assimilated C in soil was monitored through time.

Our results showed that despite irrigation causing a significant increase in DMP (additional ~2000 kg DM ha-1) compared to the dryland treatment, the net input of new photo-assimilated C into the soil during the labelling phase was similar for both treatments (~2500 kg C ha-1). However, irrigation did effect the distribution of new C into certain fractions (53–250 µm, 5–20 µm and <5 µm). Our results did not support the common perception that increasing production via irrigation would result in increased SOM.

Globally, the need for irrigation to increase dry matter production (DMP) on grazed pastures has been steadily increasing to support the growing demand for meat and dairy products. However, research on the effects of irrigation on the dynamics of soil carbon (SOC) in these pastures has been scarce and produced contradictory results. Our objective was to quantify the effects of irrigation on the storage and stability of new photo-assimilated carbon (C) in a plant-soil system.

The experiment consisted of two soil moisture treatments (simulating either Dryland or Irrigated conditions) applied to soil mesocosms established with perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) pasture. Plants were continuously pulse labelled with 13CO2 between December 2016 and March 2017 (e.g. an irrigation season) by growing within a sealed plant growth chamber. Following the labelling phase, the soil moisture regimes, for both treatments, were adjusted to simulate autumn/winter conditions. The mesocosms were destructively harvested at 1, 12, 125, 237 and 349 days after the last labelling event. The partitioning of new photo-assimilated C was determined through 13C analysis of herbage, roots, bulk soil, rhizosphere soil and soil size fractions (>250 µm, 53–250 µm, 20–53 µm, 5–20 µm and <5 µm). The net storage and subsequent loss of new photo-assimilated C in soil was monitored through time.

Our results showed that despite irrigation causing a significant increase in DMP (additional ~2000 kg DM ha-1) compared to the dryland treatment, the net input of new photo-assimilated C into the soil during the labelling phase was similar for both treatments (~2500 kg C ha-1). However, irrigation did effect the distribution of new C into certain fractions (53–250 µm, 5–20 µm and <5 µm). Our results did not support the common perception that increasing production via irrigation would result in increased SOM.


Biography: Carmen Medina is currently a PhD student at Lincoln University in partnership with The New Zealand Institute for Plant and Food Research. Carmen thesis is focused on measuring the effects of irrigation on soil carbon dynamics in grazed pastures. Her research interests lie in the area of agroecology and sustainable food production.

SOIL ORGANIC MATTER

7th International Symposium
Soil Organic Matter

6 – 11 October 2019

Hilton Adelaide

Adelaide, South Australia

Australia

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