Long-term irrigation effects on soil organic matter under temperate grazed pasture

Irrigation of grazed pasture significantly increases plant and animal production, which may in turn increase soil organic carbon (SOC), depending on the balance between primary production and below-ground allocation of C on the one hand, and the decomposition and export of C from the soil on the other. To evaluate the effect of irrigation on SOC we sampled a grazed pasture field experiment maintained under different irrigation treatments for 62years. The dry-land treatment in this experiment only received rainfall at an average of 740mmyear-1. The 10 and 20% irrigation treatments involved application of 100mm of irrigation when the soil reached 10 and 20% gravimetric moisture content, respectively. The 10 and 20% irrigation treatments received average total annual irrigation inputs of 260 and 770mmyear-1, respectively. The 10 and 20% irrigation treatments increased pasture production by 44 and 74%, respectively, compared with that from the dry-land. Analysis of soils taken to 1-m depth revealed that amounts of SOC were not significantly different between the dry-land (125.5Mgha-1) and 10% irrigation (117.8Mgha-1) treatments, but these were significantly greater than the 20% irrigation treatment (93.0Mgha-1). At 50-100cm, SOC was also less (34%) for the 20% irrigation treatment than for the 10% irrigation treatment. The relative quantities of carbon (C) and nitrogen (N) in the light fraction (LF) at all soil depths decreased successively from dry-land to the 20% irrigation treatment, suggesting that wetter soil conditions accelerated decomposition of the LF fraction, a comparatively labile SOC fraction. The C-to-N ratio of the bulk soil was also less for the 20% irrigation treatment, indicating more decomposed SOM in the irrigated than in the dry-land treatment. There were no significant differences in the microbial biomass between the three different irrigation treatments, but the respiration rate (CO2 production) of soil organisms in the 20% irrigation treatment was consistently greater than in the other two treatments. It was concluded that large increases in plant productivity as a result of irrigation had either no effect or significantly reduced SOC stocks under grazed pasture. The reduced SOC content observed in the 20% irrigation treatment was attributed to a combination of increased C losses in animal products and drainage associated with greater stocking, together with accelerated decomposition of organic C resulting from elevated soil moisture maintained throughout the growing season. © 2014 British Society of Soil Science.