Irrigating grazed pasture decreases soil carbon and nitrogen stocks

Paul L. Mudge, Francis M. Kelliher, Trevor L. Knight, Denis O'Connell, Scott Fraser and Louis A. Schipper - Global Change Biology

Type: Research Paper
Knowledge level: Advanced

Farm Table says:

This research article focuses on irrigating grazed pasture and looks at decreases in soil carbon and nitrogen stocks. I was surprised to learn that increasing the area of temperate pasture land under irrigation would result in more CO2 in the atmosphere and may, directly and indirectly, increase nitrogen leaching to groundwater.

What is the problem?

Sustainably meeting increasing global food demand is challenging because the area of suitable agricultural land is constrained, and plant growth on existing agricultural land is often limited by water and nutrients, To reduce the water limitation of plant growth in drought-prone areas, soils have been irrigated.

This study aim was to determine whether soil C and N stocks differed between irrigated and unirrigated pastures in New Zealand. We sampled closely matched soils from 34 paired irrigated pastures and adjacent unirrigated pastures.

What did the research involve?

We sampled soils under paired irrigated and unirrigated pastures at 34 sites in four regions of New Zealand where irrigation is common:

  • Canterbury (14)
  •  Manawatu (7)
  • Bay of Plenty (7)
  • Otago (6)

At each paired site, two 10 m 9 10 m sampling areas were randomly located within each of the irrigated and unirrigated areas. Sampling areas were not positioned within 5 m of fences or water troughs, and there was at least a 10-m buffer between irrigated and unirrigated areas.

Soils were air-dried and sieved through a 2-mm sieve and then weighed.

Statistical analysis was applied to the average values of the two sample plots obtained from the irrigated and unirrigated pastures at each paired site.

What were the key findings?

Averaged across all 34 paired sites, soil C concentrations were significantly lower in the irrigated pastures for five of the six depth increments, while soil N concentrations were significantly lower for four of the six depth increments.

There were no significant relationships between differences in soil C and N (concentrations, stocks and cumulative stocks) and irrigation duration.

On average across 34 paired sites from four regions, irrigated pastures had significantly less soil carbon (C) and nitrogen (N) than adjacent unirrigated pastures, and these differences were sufficiently large to be of interest for C accounting and soil quality (e.g. the cumulative difference in C to 0.6 m was 9.86 t ha 1

• Based on the sampling and analysis conducted in this study we cannot resolve causal mechanisms for the observed differences in C between irrigated and unirrigated pastures.

• Globally, there are few studies comparing soil C and N stocks under irrigated and adjacent dryland pastures (or even agriculture in general).

• We had hypothesized that any ‘irrigation effect’ would increase with increasing irrigation duration (the number of years the pasture had been irrigated).

• Nitrogen lost from soil organic matter could have been taken up by plants, consumed by grazing animals and a portion removed from the system in milk or meat. Alternatively, N may have also been leached below the sampled depth or lost via gaseous pathways.

Final Comment

Given the large and increasing area of land being irrigated both in NZ and on a global scale, there is an urgent need to determine whether the results found in this study are also applicable in other regions and under different land management systems.

2016 - New Zealand - Paul L. Mudge, Francis M. Kelliher, Trevor L. Knight, Denis O'Connell, Scott Fraser and Louis A. Schipper - Global Change Biology
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