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 influenc e soil quality. We sampled soil from the 0- to 7.5-cm depth of two long-ter m 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 t imes in 1995 and 9 times in 1996. The soil is a silt loam, Orthic Brown Che rnozem 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 a nd LFN), mineralizable C (C-min) and N (N-min), and water-soluble organic C (WSOC). All biochemical attributes, except MBC, showed higher values for C ent W than for F-W, reflecting the historically higher crop residue inputs, less frequent tillage, and drier conditions of Cent W. Based on the season al mean values for 1996, we concluded that, after 29 yr, F-W has degraded s oil organic C and total N by about 15% compared to Cent W. In the same peri od it has degraded the labile attributes, except MBC, much more. For exampl e, 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 an d N inputs are small compared to the total soil C or N. All the labile attr ibutes varied markedly throughout the seasons. We explained most of the sea sonal variability in soil biochemical attributes in terms of C and N inputs from crop residues and rhizodeposition, and the influences of soil moistur e, precipitation and temperature. Using multiple regression, we related the biochemical attributes to soil moisture and the weather variables, account ing for 20% of the variability in MBC, 27% of that of N-min, 29% for LFC, 5 2% for C-min, and 66% for WSOC. In all cases the biochemical attributes wer e negatively related to precipitation, soil moisture, temperature and their interactions. We interpreted this to mean that conditions favouring decomp osition of organic matter in situ result in decreases in these attributes w hen they are measured subsequently under laboratory conditions. We conclude d that when assessing changes in OC or TN over years, measurements can be m ade at any time during a year. However, if assessing changes in the labile soil attributes, several measurements should be made during a season or, me asurements be made near the same time each year.