Soil Carbon and Nitrogen Responses to Long-Term Flood Irrigation and Fertilization in High-Elevation Hay Meadows

Abstract

Rangelands are an important grazing resource worldwide, but in many regions, supplemental forage is needed to support grazing during parts of the year. Mountain irrigated hay meadows are an integral hay-producing component of livestock operations in the Intermountain Western United States (US), but productive capacity is made challenging by short growing season (elevation >2000 m) and inefficient flood irrigation strategies. Further, these factors have reduced residue decomposition and led to the development of an organic horizon at the soil surface which stores large amounts of soil organic matter (SOM) and nitrogen (N). Better understanding of N-cycling and SOM dynamics is necessary to improve N management and sustainability of forage production in meadows. A field study was conducted at four ranches in southern Wyoming and northern Colorado, US, in 2021 to evaluate the effect of long-term flood irrigation and N fertilization on soil carbon (C) and N pools. Soil pits were excavated to 1.5-m depth in three management systems: unirrigated rangeland, irrigated-unfertilized meadow, and irrigated-fertilized meadow. In each pit, genetic soil horizons were sampled and analyzed for total organic C (TOC), total N (TN), dissolved organic C (DOC), and N (DON), microbial biomass C (MBC), and N (MBN), potentially mineralizable C (PMC), and N (PMN), nitrate, and ammonium. Irrigation dramatically increased TOC concentration at the soil surface, and also increased TOC in the entire profile. However, N fertilization negated the effects of irrigation, as fertilized meadow stabile and labile C and N pools more closely resembled those of rangelands. Fertilized and unfertilized meadows had less labile C and N per unit of stable C and N compared to rangelands, and lower PMN, suggesting lower SOM mineralization or loss to leaching. Alternative irrigation strategies that reduce soil waterlogging will likely improve SOM cycling and forage production in hay meadows but should be balanced with SOM sequestration and other ecosystem services.