Dynamic of soil chemical parameters in shifting agriculture in the Eastern Amazon

Abstract

In Northeast Pará, Brazil, shifting agriculture by land settlers has been practiced for around 100 years. After a common cropping period of 2 years the fields are left fallow for 4 to 8 years. Changes in extractable cations and C, N, and P of soils were studied on six fields which were under different phases of the rotational cycle. The chemical composition of soil solutions was also monitored for a period of 19 months. In topsoils, the differences in C and N contents and extractable Ca and Al could be related to the landuse history expressed as time elapsed since last burn. Repeated sampling on two ‘slash and burn’ plots, showed significant increases in pH, CEC, extractable K, Ca and Mg, but decreases in extractable Na and Al, C and N content in the plots from 7-year old fallow to the first-year cropping field. Soil solution chemistry at 105 cm depth did not change with increasing fallow age. On the ‘slash and burn’ field, big variations in solute composition at 10 cm depth were observed during the initial months of the cropping period. In the soil solutions after burning, Ca concentrations ranged from 4.8 to 30.1 mg l −1, but decreased to values between 1.2 and 2.7 mg l −1 within 2 months. Slash burning and subsequent cropping lead to increases of Cl − and NO − 3 in the soil solution at 105 cm depth. A small dose of NPK fertilizer (11 kg N ha −1, 13 kg P ha −1 and 17 kg K ha −1) did not increase NO − 3 concentration and chloride concentrations at this depth increased to a small extent. After the harvest of cassava, high nitrate concentrations of 11 mg N l −1 were observed. NO − 3 and Cl − concentrations on the cropping fields were significantly correlated ( n = 350) with the sum of the concentration of the M b cations (Na, K, Ca, Mg), (NO − 3M b, r = 0.82; Cl −M b, r = 0.81). Nitrate also showed significant correlations with protons ( r = 0.59) and Al ( r = 0.47) concentrations. Ion concentrations in the soil solution can be reduced by agricultural management. During harvest, the aboveground vegetation should not be removed completely in one single step. If clear cutting cannot be avoided, the timing should be chosen as to keep some vegetation with high uptake potential at the beginning of the rainy season. A small dose of NPK-fertilizer stimulated the nutrient uptake by the vegetation and did not enhance nutrient concentrations in the seepage water.