Evolution of water‐extractable copper in soil with time as a function of organic matter amendments and aeration

Abstract

SummaryVariations in soil conditions can influence the speciation and hence availability of copper. We have followed the evolution of water‐extractability of both native and added copper in a clay‐loam topsoil over 5 months. The effects of addition of organic amendments, either leaf compost or lucerne straw, were studied. Soils were incubated under controlled conditions either moist and well‐aerated, or flooded. Other chemical properties were monitored (pH, redox potential, water‐extractable organic carbon, iron and manganese). Freshly added Cu was initially more soluble than native Cu, but the contrast decreased with time. Solubility was greater in flooded soils than in aerated samples. Solubility also increased with increasing pH, contrary to often‐reported trends. Although the extractability of added Cu increased with increasing dissolved organic carbon, as expected from the formation of complexes in solution, that of native soil Cu did not. Large, short‐lived flushes of Cu were associated with flushes of dissolved organic matter, particularly from decomposition of lucerne straw, but these relations were not always statistically significant when the entire data set was considered. The total amount of dissolved organic carbon is not a sufficiently sensitive probe to assess the extent to which Cu is solubilized by the formation of complexes in soil solution. The changing composition of soil and soil solution during incubation led to 10‐fold differences in the proportion of Cu that was water‐extractable. Single measurements of extractability can therefore never give an accurate estimation of mobility and availability of copper in soil.