Dynamics of soluble organic C and C mineralization in cultivated soils with varying N fertilization

Laboratory studies have shown that N availability may affect C decomposition in soils. A field study was undertaken to determine to what extent water-soluble organic C (WSOC) and C mineralization could be affected by mineral N fertilizers. Soil cores were collected in the 1993 and 1994 growing seasons, from a sandy loam (loamy, mixed, frigid, Typic Haplorthod) and a sandy clay (fine, mixed, frigid, Typic Humaquept) under early-maturing maize (Zea mays L.). Soils were fertilized with NH4NO3 at reduced (10, 60, 120 kg N ha−1) or recommended (180 kg N ha−1) rates. For both soils, C mineralization rate increased significantly (P<0.05) with amount of N fertilizer only at two of the nine sampling dates. However, WSOC contents generally decreased (P<0.05) with increasing N fertilizer rate. Overall, a decrease in soil mineral N content was consistently associated with an increase in WSOC content. The relationship between soil WSOC and mineral N contents was logarithmic for both soils (r2=0.70 in the sandy loam and r2=0.89 in the sandy clay). For both soils, we estimated a critical level of N of about 60 mg kg−1, below which small decreases in mineral N were associated with large increases in WSOC contents.