Changes in forms of soil zinc after 23 years of cropping following clearing of a boreal forest

Abstract

The long term effects of cropping systems on changes in forms of soil Zn following clearing of a poplar (Populus sp.) forest were determined by a sequential extraction procedure. Another objective was to determine from these changes the forms of available soil Zn. Four cropping systems were imposed on the broken land in 1968: (i) continuous barley, (ii) continuous bromegrass, (iii) continuous legume (alfalfa or red clover) and (iv) barley-hay rotation (3 yr each in barley and bromegrass-red clover hay). Only N and P fertilizers were applied annually. Soil (0–15 cm depth) samples taken in the spring of 1968 and in the autumn of 1977 and 1990 were extracted sequentially with (a) 1 M MgCl2, (b) 0.2 M CuCl2, (c) 0.1 M NH2OH.HCl (pH 3.0), (d) 0.2 M (COONH4)2 at pH 3.0 and (e) 0.2 M (COONH4)2 + 0.1 M ascorbic acid at 100 °C. The extracted Zn was assumed to be, respectively, exchangeable (ZnEX), chemisorbed by organic matter and hydrous oxides (ZnADS), associated with reducible Fe and Mn oxides (ZnRO), occluded by amorphous Fe and Al oxides (ZnAMO), and occluded by or coprecipitated with crystalline Fe and Al oxides (ZnCRO). The following changes in soil Zn were observed: (i) ZnEX and ZnADS decreased, (ii) ZnRO and especially ZnAMO increased, (iii) ZnCRO remained constant. Labile (DTPA) Zn decreased, but total extractable Zn increased. Zinc removed in the harvested crop was considerably less than the decrease in labile Zn. Therefore, it is concluded that (i) agricultural cropping mobilized soil Zn in the order forage legume > grass and cereal crops; (ii) only ZnEX and ZnADS were readily plant-available; and (iii) some available Zn changed to less available forms (oxide-associated). Key words: Available Zn, crop type, labile Zn, sequential extraction, Zn fractions