30-YEAR CROP ROTATIONS AND MANAGEMENT-PRACTICES EFFECTS ON SOIL AND AMINO NITROGEN

Methods of assessing how agronomic practices influence soil productivity and sustainability are required. The effects of various agronomic practices on soil organic N and hydrolyzable amino N in a Udic Haploboroll were determined in a long-term (30-yr) crop rotation study at Indian Head, Saskatchewan. Practices assessed included fertilization, fallowing frequency, green manuring, and inclusion of a grass-legume forage crop in predominantly spring wheat (Triticum aestivum L.) systems. Fertilization, green manuring, or inclusion of a grass-legume hay crop significantly (P < 0.10) increased soil and amino N (amino acid N plus amino sugar N), particularly as following frequency was reduced. When the relative molar distribution of the amino acids was used to assess changes in soil organic-N quality, significant and consistent increases in aspartic acid and decreases in arginine and leucine were observed; however, the differences were mainly associated with the 6-yr mixed and fertilized continuous wheat (Cont W) rotations. Comparison of our N-concentration results with an estimate of the initial (1958) N concentration suggests that all fallow-containing monoculture wheat rotations and unfertilized Cont W have failed to maintain soil N, the unfertilized 3-yr legume green manure-wheat-wheat system has maintained soil N, while the 6-yr fallow-cereal-hay (grass-legume) and fertilized Cont W systems have increased soil N. Hydrolyzable amino N was only marginally more effective than Kjeldahl N for describing changes in soil organic-matter quality.