Seasonal trends in selected soil biochemical attributes: Effects of crop rotation in the semiarid prairie

Measurements of seasonal changes in soil biochemical attributes can provide valuable information on how crop management and weather variables influence soil quality. We sampled soil from the 0- to 7.5-cm depth of two long-term crop rotations [continuous wheat (Cont W) and both phases of fallow-wheat (F-W)] at Swift Current, Saskatchewan, from early May to mid-October, 11 times in 1995 and 9 times in 1996. The soil is a silt loam, Orthic Brown Chernozem with pH 6.0, in dilute CaCl2. We monitored changes in organic C (OC) and total N (TN), microbial biomass C (MBC), light fraction C and N (LFC and LFN), mineralizable C (C-min) and N (N-min), and water-soluble organic C (WSOC). All biochemical attributes, except MBC, showed higher values for Cent W than for F-W, reflecting the historically higher crop residue inputs, less frequent tillage, and drier conditions of Cent W. Based on the seasonal mean values for 1996, we concluded that, after 29 yr, F-W has degraded soil organic C and total N by about 15% compared to Cent W. In the same period it has degraded the labile attributes, except MBC, much more. For example, WSOC is degraded by 22%, C-min and N-min by 45% and LFC and LFN by 60-75%. Organic C and TN were constant during the season because one year's C and N inputs are small compared to the total soil C or N. All the labile attributes varied markedly throughout the seasons. We explained most of the seasonal variability in soil biochemical attributes in terms of C and N inputs from crop residues and rhizodeposition, and the influences of soil moisture, precipitation and temperature. Using multiple regression, we related the biochemical attributes to soil moisture and the weather variables, accounting for 20% of the variability in MBC, 27% of that of N-min, 29% for LFC, 52% for C-min, and 66% for WSOC. In all cases the biochemical attributes were negatively related to precipitation, soil moisture, temperature and their interactions. We interpreted this to mean that conditions favouring decomposition of organic matter in situ result in decreases in these attributes when they are measured subsequently under laboratory conditions. We concluded that when assessing changes in OC or TN over years, measurements can be made at any time during a year. However, if assessing changes in the labile soil attributes, several measurements should be made during a season or, measurements be made near the same time each year.