Effect of the conversion to irrigation on mid-term soil organic C dynamics in a semiarid Mediterranean agrosystem. An approach using C natural isotopes.

Mr Rodrigo Antón1, Dr Iñigo Virto1, Dr Henar Urmeneta3, Dr Alberto Enrique1, Dra Delphine Derrien2, Dr Gregory van der Heijden2

1Departamento de Ciencias-ISFOOD, Universidad Pública de Navarra, Pamplona, Spain, 2Biogéochimie des Ecosystèmes Forestiers, INRA-Nancy, Champenoux, France, 3Departamento de Estadística e Investigación Operativa, Universidad Pública de Navarra, Pamplona, Spain

The conversion of rainfed agricultural land to irrigation is a widespread strategy for increasing yields and the variety of crops cultivated. It can also have other repercussions in the agroecosystems. From the soil functioning perspective, changes in soil organic carbon (SOC) can be expected, as not only inputs from crop residues can change, but also the conditions for incorporation and mineralization of these inputs can be affected. The objective of this study was to identify and quantify the changes induced by this land-use change in SOC dynamics in a semi-arid Mediterranean region.

A 7-years C3-C4 chronosequence on a calcareous Mediterranean soil was used for this study. Maize and wheat are grown under two different management conditions: irrigated and rainfed. The dataset includes data on SOC stock (0-30 cm), crop residues inputs and d13C signatures of total SOC and the particulate organic carbon fraction (POC, >53mm). A two-compartment model (fast and slow cycling pools) was fit on the data to provide quantitative estimates of the turnover rates of SOC in the irrigated and non-irrigated maize plots

Results show that irrigation increases SOC stocks, both total and POC fraction. This increase is related to a raise by 72±9% of plant productivity and a faster turnover rate in the fast cycling pool. Contrary to what is commonly acknowledged for acidic or neutral soil, POC is not a good proxy for the fast-cycling C pool in the studied calcareous soil. This questions the suitability of analytical methodologies to isolate POC, and calls for a revision of POC correspondence with the soil labile C pool under certain conditions.

Biography: PhD student at the Science Department-ISFOOD at Universidad Pública de Navarra (UPNA). He joined UPNA in 2013 after completing his postgraduate education at the Institut Agronomique Méditerranéen de Montpellier. Since then, he has participated in a number of projects related to soil quality and management.

Differental responses of the components of ecosystem carbon exchange to irrigation frequency in mesocosms with a C4 grassland

Ms Yuan Li1,2, Prof. Tim J Clough1, Dr. Gabriel Y K Moinet2, Dr. John E Hunt2, Dr. David Whitehead2

1Lincoln University, Chirstchurch, New Zealand, 2Manaaki Whenua – Landcare Research, Lincoln, New Zealand

Conversion of non-irrigated grasslands to high-intensity farm systems with irrigation is a major land-use change in dryland areas of New Zealand. The conversion has the potential to cause major changes in ecosystem carbon cycling but differences in the sensitivities of the components of net ecosystem carbon dioxide (CO2) exchange (FN) to water availability are not clear. Here, we partitioned the components of FN for the C4 plant Bermuda grass (Cynodon dactylon L.) growing in mesocosms irrigated every 1, 3, and 6 days (I1, I3, and I6, respectively) with a constant intensity (14.8 mm). FN decreased linearly with increasing soil water deficit (W), indicating increasing ecosystem CO2 losses with decreasing irrigation frequency. Over the 30 days of the experiment, FN reached 657 ± 44 (mean ± standard deviation, n = 4), 604 ± 29, and 552 ± 47 g C m−2 for I1, I3, and I6, respectively. This was due to a linear decrease in ecosystem gross primary productivity with increasing W that exceeded the decrease in ecosystem respiration (RE). RE declined non-linearly with increasing W with the aboveground component increasing and the below-ground component (soil respiration rate, RS) decreasing non-linearly with increasing W. Using a 13C natural abundance isotopic technique, RS was partitioned into autotrophic (RA) and heterotrophic respiration (RH) at the end of the experiment, when differences in W bewteen the treatments were greatest. RH contributed 41 ± 17, 23 ± 21, and 30 ± 14% of RS for I1, I3, and I6, respectively. Increasing irrigation frequency resulted in increased ecosystem CO2 uptake as a result of diverging responses of gross primary production and respiration components. However, care in interpreting the implications for changes in soil carbon stocks is warranted as RH was also highest for the I1 treatment with highest CO2¬ uptake, suggesting possible positive priming.

Biography: Yuan Li is currently studying for his PhD in the field of Soil Science at Lincoln University. Professor Timothy Clough and Dr David Whitehead are supervising this research. Currently I am doing a PhD project about “linking carbon and nitrogen dynamics to mitigate carbon dioxide and nitrous oxide emissions in grazed grasslands”. This research aims to determine how irrigation drives the interactions between carbon and nitrogen cycles and the regulation of soil organic matter decomposition and nitrous oxide emissions, and identify the potential for farm management practices to mitigate soil organic matter losses and reduce nitrous oxide emissions.



7th International Symposium
Soil Organic Matter

6 – 11 October 2019

Hilton Adelaide

Adelaide, South Australia


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