Differences in carbon and nitrogen mineralization in soils of differing initial pH induced by electrokinesis and receiving crop residue amendments

Initial soil pH has been reported to have a great impact on the decomposition of added organic materials and hence to determine the direction and magnitude of subsequent soil pH changes. However, most previous investigations have been conducted on different soil types differing in initial pH and other soil properties. Here, we investigated the effects of initial soil pH on the subsequent soil pH and N mineralization changes caused by addition of crop residues to two soils (a Paleudalf and a Plinthudult) with different pHs induced by treatment with direct electric current. This produced pH gradients of 6.50-3.20 and 6.74-3.81, respectively. Three typical field crop residues differing in C/N ratio, i.e. rice straw C/N = 42, canola residue C/N = 36 and Chinese milk vetch C/N = 14 (vetch), were incubated with the soils for 102 days. With both soils, total CO2 fluxes differed between the type of added crop residues, with vetch > canola residue > rice straw, and decreased with decreasing initial soil pH. The incorporation of crop residues into the two soils at all pHs increased soil pH except for the Paleudalf at pH 3.81 and 4.25 amended with rice straw and canola residue, where pH slightly decreased. As expected, vetch incorporation caused a greater pH increase than the two non-legume residues (rice straw and canola residue), but this effect was transient in soils of higher initial pH. The pH declined rapidly with time in soils of initial pH ≥ 4.40 treated with vetch, while addition of rice straw and canola residue maintained stable pHs. Irrespective of crop residue addition, soil pH continuously increased or remained steady over the 102-day incubation period in the highly acidic soils (pH ≤ 4.25). While NH4+ accumulated in highly acidic soils, NO3− accumulated in higher pH soils. Contrasting effects on nitrification and subsequent pH changes occurred between soil pH ≤ 4.25 and pH ≥ 4.40. However, no clear relationship between net N mineralization and soil pH was found. This study demonstrated that low soil pH greatly inhibited nitrification, while net N mineralization was generally less affected. This, in turn, affected the direction and extent of soil pH changes after addition of crop residues.