Carbon and nitrogen addition related to stocks of these elements in soil and corn yield under management systems

Long-term experiments are essential for research about organic matter dynamics of soils. This paper reports results of an experiment installed in September 1985 on a Paleudult soil degraded under inadequate management for 16 y ears. The experiment was conducted at the Experimental Station of the Federal University of Rio Grande do Sul (Brazil) in Eldorado do Sul county. Three soil tillage methods (conventional tillage-CT, reduced tillage-RT and no-tillage-NT), three crop systems (oat /corn-O / C, vetch /corn-V / C and oat + vetch / corn + cowpea-O + V / C + Cp), and two N rates applied to corn as urea (0 and 139 kg ha) were tested. A split-plot design arranged in randomized blocks with three replications was used with soil tillage as main plot, the cropping systems as sub-plots and nitrogen rates as sub-blocks. Carbon and nitrogen addition by crops was estimated for the experimental period of 13 years. In September 1998, the soil was sampled in six layers down to 0.30 m depth, and total organic carbon (TOC) and total nitrogen (TN) contents were determined in each layer and then calculated for the 0-0.175 m and 0.175-0.30 m layers. Legumes were responsible for the greatest annual soil C and N increase by crops (4.17 to 8.39 Mg ha and -21 to 178 kg ha, respectively). The highest C and N additions were positively related to these elements stocks in the soil under NT (0-0.175 m layer), and promoted gradual corn yield increase in all soil tillage treatments. The annual rate of C addition (A) necessary to maintain de initial condition (dC / dt = zero) was estimated at 4.2 Mg ha for NT, 7.3 Mg ha for RT and 8.9 Mg ha for CT. Similarly, the annual rate of N addition required to maintain the initial condition (dN / dt = zero) was 5 kg ha for NT, 31 kg ha for RT, and 94 kg ha for CT. An estimation by the angular coefficient of the linear regression, which relates the annual C addition rates and annual TOC stock variation in the surface layer, calculated that 12.9 % for NT, 8.1 % for RT, and 11.5 % for CT of the C added to the soil was retained in the soil organic matter, representing approximately the humification coefficient (k). Likewise, it was estimated that 49.7 % of the net annual N addition under NT, 21.0 % under RT, and 33.1 % under CT was retained as TN in the soil. The TOC loss coefficient from the soil (k), calculated for the condition dC / dt = zero, was 0.0166 yr under NT, 0.0182 yr under RT, and 0.0314 yr under CT. No-tillage systems with the reduction of TOC loss (k), and legume based crop systems with the high C and N additions are good alternatives to recover the soil TOC and TN stocks and increase corn yield in the subtropical region of southern Brazil.