Organic fractions, N, P and S changes in an Argentine semiarid Haplustoll under different crop sequences

Crop sequences play an important role in changing soil properties. Knowledge of the chemical composition, turnover, dynamics and biological effects of different fractions of soil organic carbon (SOC), nitrogen (N), phosphorus (P) and sulfur (S) components will result in a better understanding of soil productivity. The objective of this research was to compare the effects of crop rotation on SOC, N, P and S evolution in two granulometric fractions of an Entic Haplustoll. Rotations of mixed pasture (5.5 years)-annual crops (4.5 years) (Pa-C), and wheat (Triticum aestivum L.)-sunflower (Helianthus annus L.) (W-S), and a reference plot (Ref) which was located between them, were studied. Fine (<100 mu m, FF) and coarse (100-2000 mu m, CF) soil granulometric fractions were separated by wet sieving. In each fraction, total N and S, and several SOC and P forms were determined. Similar contents (5.23-6.07 Mg ha(-1)) of humic acid (HA) carbon were found in the three situations. The Pa-C rotation maintained the SOC level at 17.33 Mg ha(-1) in the fine fraction during a 10-year period. Conversely, SOC was much lower (11.16 Mg ha(-1)) in the same fraction in the W-S treatment. Losses of SOC, N, P and S were highest in the coarse fraction, thus showing the dynamics of this soil fraction and its important role in plant nutrient turnover and availability to growing crops. (C) 1997 Elsevier Science B.V.