Irrigating grazed pasture decreases soil carbon and nitrogen stocks

Abstract

The sustainability of using irrigation to produce food depends not only on the availability of sufficient water, but also on the soil's 'response' to irrigation. Stocks of carbon (C) and nitrogen (N) are key components of soil organic matter (SOM), which is important for sustainable agricultural production. While there is some information about the effects of irrigation on soil C stocks in cropping systems, there is a paucity of such studies in pastoral food production systems. For this study, we sampled soils from 34 paired, irrigated and unirrigated pasture sites across New Zealand (NZ) and analysed these for total C and N. On average, irrigated pastures had significantly (P < 0.05) less soil carbon (C) and nitrogen (N) than adjacent unirrigated pastures, with differences of 6.99 t C ha and 0.58 t N ha in the uppermost 0.3 m. Differences in C and N tended to occur throughout the soil profile, so the cumulative differences increased with depth, and the proportion of the soil C lost from deeper horizons was large. There were no relationships between differences in soil C and N stocks and the length of time under irrigation. This study suggests SOM will decrease when pastures under a temperate climate are irrigated. On this basis, increasing the area of temperate pasture land under irrigation would result in more CO in the atmosphere and may directly and indirectly increase N leaching to groundwater. 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 (e.g. arable).

Exacerbated nitrogen limitation ends transient stimulation of grassland productivity by increased precipitation

“…3,4), by stimulating soil microbial activity (e.g., N mineralization, N nitrification, and litter decomposition; Aerts 1997, Wang et al 2006, Schaeffer et al 2013. However, water addition could diminishingly stimulate productivity over time if water addition results in decreased soil N availability because water addition likely increases N losses (Yahdjian and Sala 2010), e.g., through denitrification and leaching (Mudge et al 2016). Our experiment included the combined addition of water and N, and showed that productivity was persistently increased by water only when N was also added (Fig.…”

Soil carbon sequestration potential of permanent pasture and continuous cropping soils in New Zealand

“…The SOC associated with the fine fraction is relatively resistant to changes in management (Campbell, Zentner, Bowren, Townley‐Smith, & Schnitzer, ; Chung, Grove, & Six, ; Skjemstad, Spouncer, Cowie, & Swift, ) and is characterized by long turnover times (Balesdent, Besnard, Arrouays, & Chenu, ; Buyanovsky, Aslam, & Wagner, ). However, intensive management can result in losses of C (Mudge et al., ; Schipper et al., ) from the fine fraction (Gregorich et al., ) leaving a deficit of stable C that may be filled with the use of appropriate sequestration practices.…”

The effect of irrigation on cadmium, uranium, and phosphorus contents in agricultural soils

“…One site in the Canterbury region was sampled to only 0.3 m due to high stone content lower in the profile. The selection criteria considered when choosing sampling locations have been fully described by Mudge et al (2016) along with further information on irrigation duration (ranging 3-20 years), soil orders, type of irrigation and long-term climate data for each sampling site. The irrigated and unir rigated areas within a field were identified using local knowledge, Google Earth imagery, Irrigation Resource Consents and help from rural professionals (Mudge et al, 2016).…”

“…Soils and topography were the same between irrigated and unirrigated areas, and 4. Farm management (such as fertilizer application) was the same in irrigated and unirrigated areas (expect for irrigation) (Mudge et al, 2016).…”

“…The objective of this study was to determine the potential for loss of Cd, U, and P due to irrigation. We took advantage of the wide scale soil sampling campaign of adjacent irrigated, and unirrigated pastures, conducted by Mudge et al (2016) that demonstrated losses of carbon and nitrogen stocks under irrigation. On each farm, both irrigated and unirrigated areas were sampled in the same field to ensure uniform fertilizer history and therefore Cd, U, and P inputs.…”