Tillage - fertilizer changes: Effect on some soil quality attributes under long-term crop rotations in a thin Black Chernozem

Abstract

A crop rotation experiment initiated in 1958 on a thin Black Chernozemic clay at Indian Head, Saskatchewan, was managed using conventional tillage until 1989 and changed to zero-tillage in 1990. We soil sampled in 1987 and 1997 to determine management effects on selected soil biochemical characteristics, and the change in some of the more labile soil quality attributes relative to the change in soil organic C and total N. Rotations examined were: fallow-wheat (Triticum aestivum L.) (F-W), fallow-wheat-wheat (F-W-W), continuous wheat (Cont W), legume green manure (GM)-W-W, and F-W-W-hay (legume-grass)-hay-hay (F-W-W-H-H-H). The monoculture cereal rotations were either fertilized with N and P based on soil tests or unfertilized, while the legume-containing systems were unfertilized. There was also a F-W-W (N + P) treatment, in which about 20% of the straw was harvested each crop year. With the change to zero-tillage management in 1990 and in anticipation of greater soil water storage, higher rates of N were added thereafter. This resulted in an upward trend in stubble-crop yields and a positive yield response of wheat grown on fallow, where before the change wheat grown on fallow did not respond to fertilizer. The corresponding increase in crop residue production and residue C inputs resulted in all fertilized systems gaining organic C and total N in the 0- to 15-cm depth between 1987 and 1997, while the unfertilized systems remained unchanged. Soil organic C and total N, microbial biomass C (MBC), light fraction organic C and N (LFC and LFN), mineralizable N (Nmin) and wet aggregate stability (WAS), generally had positive responses to fertilization, to increased cropping frequency, and to the inclusion of legume green manure or legume hay crops in cereal-based rotations. Straw harvesting did not influence grain yields, nor did it influence the soil biochemical characteristics, though it tended to render the soil more prone to erosion. Response to cropping frequency was apparent only in the fertilized systems, where the more labile soil quality attributes, (i.e., MBC, LFC, LFN, and Nmin) were more sensitive than organic C or total N. However, gains in LFC and MBC in response to fertilizer did not account for a significant fraction of the gain in total organic C. During the period 1987 to 1997, MBC in the 0- to 15-cm depth increased by 40% in absolute value and by 33% relative to organic C (3.6% of organic C in 1997 vs. 2.7% in 1987). The same was true for LFN in the fertilized treatments and in the green manured system (1.46% of total N in 1997 vs. 1.15% in 1987). However, LFC hardly changed over this period. Relative to total N, Nmin decreased in 1997 compared with 1987, likely due to higher immobilization. Wet aggregate stability was generally greater in 1997 compared with 1991, reflecting greater crop residue inputs and less soil disturbance under zero-tillage management. Key words: Microbial biomass, Light fraction C and N, aggregate stability, Mineralizable N, yields