Irrigation System and Tillage Effects On Soil Carbon and Nitrogen Fractions.

Irrigation system and tillage may affect soil C and N fractions by influencing crop biomass yield, residue placement, and movement of water soluble C and N in the soil. We studied the effects of irrigation (mid-elevation spray application [MESA] and low energy precision application [LEPA]) and tillage (conventional [CT] and strip tillage [ST]) systems on crop biomass (stems + leaves) yield, surface residue, and soil C and N fractions at the 0- to 20-cm depth from 2004 to 2007 in a Savage clay loam in Sidney, MT. Soil C and N fractions were total C and N (STC and STN), particulate organic C and N (POC and PON), microbial biomass C and N (MBC and MBN), potential C and N mineralization (PCM and PNM), NH₄-N, and NO₃-N. While crop biomass increased from 2004 to 2007, surface residue was greater with ST than with CT from 2005 to 2007. The STC and PCM at 10 to 20 cm were greater with CT than with ST but STN at 0 to 5 cm was greater with ST than with CT in LEPA. The POC and MBN at 5 to 10 cm, PNM at 0 to 5 cm, and NO₃-N at 0 to 20 cm were greater with CT than with ST but MBC at 0 to 5 cm and NH₄-N at 0 to 20 cm were greater with ST than with CT. The MBN at 10 to 20 cm was also greater in LEPA than in MESA irrigation. While reduced tillage increased surface soil residue and N storage, conventional tillage followed by slower rate of water application near the soil surface increased subsoil C storage, microbial activity, and N mineralization. Increased N loss due to leaching, volatilization, and denitrification probably reduced soil NO₃-N content in reduced than in conventional tillage. Reduced tillage may increase surface soil residue and C and N storage and microbial biomass, especially in the LEPA irrigation system.