Adição de carbono e nitrogênio e sua relação com os estoques no solo e com o rendimento do milho em sistemas de manejo

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 years. 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-1) 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-1 and -21 to 178 kg ha-1, 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-1 for NT, 7.3 Mg ha-1 for RT and 8.9 Mg ha-1 for CT. Similarly, the annual rate of N addition required to maintain the initial condition (dN/dt = zero) was 5 kg ha-1 for NT, 31 kg ha-1 for RT, and 94 kg ha-1 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 (k1). 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 (k2), calculated for the condition dC/dt = zero, was 0.0166 yr-1 under NT, 0.0182 yr-1 under RT, and 0.0314 yr-1 under CT. No-tillage systems with the reduction of TOC loss (k2), 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.

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