Carbon Balance in an Irrigated Corn Field after Inorganic Fertilizer or Manure Application.

Little is known about inorganic fertilizer or manure effects on organic carbon (OC) and inorganic C (IC) losses from a furrow irrigated field, particularly in the context of other system C gains or losses.  In 2003 and 2004, we measured dissolved organic and inorganic C (DOC, DIC), particulate OC and IC (POC, PIC) concentrations in irrigation inflow, runoff, and percolation waters (6-7 irrigations y^-1); C inputs from soil amendments and crop biomass; harvested C; and gaseous C emissions from field plots cropped to silage corn (Zea mays L.) in southern Idaho.  Annual treatments included:  (M) 13 (y 1) and 34 Mg ha^-1 (y 2) stockpiled dairy manure; (F) 78 (y 1) and 195 kg N ha^-1 (y 2) inorganic N fertilizer; or (NA) no amendment--control.  The mean annual total C input was 15.7, 10.8, and 10.4 Mg ha^-1 for M, F, and NA, respectively, while total C outputs for the three treatments were similar, averaging 12.2 Mg ha^-1.  Manure plots ended each growing season with a mean net gain of 3.3 Mg C ha^-1 (a positive net C flux) vs. a net loss for F and NA (-1.6 and -1.5 Mg C ha^-1, respectively).  The C added to M was ~1.5x that added to F or NA, yet relative to F, M increased gaseous C emissions only 1.18x, increased runoff DOC losses only 1.04x, decreased particulate runoff total C 19%, and decreased percolate DOC 32%.  Increased C gas emissions from manure (relative to fertilizer) was less when silage was removed than when retained (1.18x vs. 2x reported in other studies).  This suggests a means by which manure applications to corn crops can be managed to minimize C emissions.  Amendments had both direct and indirect influences on individual C components, e.g. the losses of DIC and POC in runoff and DOC in percolation water, producing temporally complex outcomes which may depend on environmental conditions external to the field.