Irrigation System and Tillage Effects on Soil Carbon and Nitrogen Fractions

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Abstract
Irrigation and tillage systems may affect surface residue and 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 and 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 (fine, smectitic, frigid Vertic Argiustolls) in Sidney, MT. Soil C and N fractions were soil organic carbon (SOC) and total nitrogen (STN), particulate organic carbon and nitrogen (POC and PON), microbial biomass carbon and nitrogen (MBC and MBN), potential carbon and nitrogen mineralization (PCM and PNM), NH 4 –N, and NO 3 –N. While crop biomass across treatments increased from 2004 to 2007, surface residue was greater with ST than with CT from 2005 to 2007. The NH 4 –N and NO 3 –N contents at 5 to 10 and 10 to 20 cm in 2005 and STN at 0 to 5 cm in 2007 were greater with ST than with CT, but SOC at 5 to 10 and 10 to 20 cm, POC and MBN at 5 to 10 cm, and PNM at 0 to 5 cm in 2007 were greater with CT than with ST. The MBC at 0 to 5 cm and MBN at 10 to 20 cm were greater in LEPA than in MESA. The PCM at 10 to 20 cm was greater with CT than with ST in LEPA. While ST increased surface soil residue and N storage, residue incorporation to a greater depth in CT increased soil C storage, microbial activity, and N mineralization. Slow rate of water application near the soil surface increased microbial biomass in LEPA.

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