The objectives of this study were to evaluate if zero tillage had significantly altered the distribution of certain soil properties formally developed under the shallow cultivation (conventional tillage) soil management systems used on the Canadian prairies. To this end, concentration gradients of available plant nutrients, microbial biomass and mineralizable C and N, were determined in the Ap soil horizon from four locations representing zero and conventional (shallow) tillage systems of 2-, 4-, 12- and 16-yr duration. No significant change coud be detected in total soil organic C and N between tillages systems. Concentrations of plant-available P and K were slightly increased in the surface 0- to 2-cm depth after 16 yr of zero tillage. Except for the 2-yr tillage site, concentration gradients of potential microbial biomass C and N, and potential net mineralizable C and N were significantly greater in the surface soil under zero tillage in comparison to conventional tillage. The reverse situation was observed at the lower depth. The percentage of soil organic C and N that was in the microbial biomass also reflected the above trends. Accumulation of mineral N and calculated N mineralization potentials were closely correlated to both the initial microbial biomass N and the decrease in size of the latter during mineralization. The possible relationships of tillage induced change and redistribution in potential biological activity to N availability were discussed.

The organic fraction of soil is known to be composed of the soil biomass, partially decomposed plant and animal residues, and the materials commonly referred to as humic substances. Knowledge of the persistence of these fractions in soil is vital to the understanding of their contribution to soil fertility and soil genesis. Much information concerning the biochemistry of the humus materials also could be obtained through a knowledge of the mean residence times of the various organic fractions.The nature of the organic compounds and of the organo-mineral bonds varies from soil to soil; consequently, the relative stability of the organic matter of different soils should also vary (8, 16). Tyurin investigated the nature of the humus of the main soil types of the U.S.S.R. He found that the chernozemic humic acids were relatively complex with a preponderance of calcium humates, which were said to be very stable. In contrast, the humus of podzolic soils was associated primarily with nonsilicate forms of iron and aluminum and consisted of relatively simple organic structures. These were thought to be relatively mobile and unstable (7, 17).The availability of the carbon-dating technique has made it feasible to determine the mean residence time of soil humic fractions (2, 4, 11). Thus, the general postulations concerning the relative stability of the soil humic components can now be tested. Campbell and Paul (3) discussed the factors which affect the accuracy of the carbon-dating method of analysis when applied to soils. Isotopic fractionation of the carbon isotopes results in a fairly constant correction to each apparent mean residence time (m.r.t.). No large error was observed to result from the increase of nuclear-bomb-produced C14 in the atmosphere; other errors were relatively small. The present study was undertaken to utilize the natural C14 activity of the carbon in the various organic fractions to provide an accurate comparison of the relative stability of a number of soil organic matter fractions.

The gaseous losses of N from conventional-fill (CT) and zero-till (ZT) crop fields were 3 to 7 and 12 to 16 kg N ha ' y", respectively. In contrast, losses from CT and ZT fallow were severalfold higher, namely, 12 to 14 and 34 kg N ha' 1 , respectively. The more dense surface soil and consistently higher moisture content (lower air-filled porosity) were identified as major factors affecting increased denitrification under ZT. The potential denitrification rates were markedly higher under ZT, and the population of denitrifiers was up to six times higher than in CT soil samples.The contribution of lower soil horizons towards gaseous N losses was found to be low on both CT and ZT fields, and this finding was confirmed from a survey carried out on three other widely differing soils.Volumetric soil moisture and air temperature were the only two of several factors that accounted for a significant portion of the variations in gaseous N fluxes under field conditions. The average mole fraction of N,O ranged from almost 100% to as low as 28% of the total gaseous products and showed a negative relationship with soil moisture.

No abstract

. 1982. Gaseous nitrogen losses from cropped and summer-fallowed soils. Can. J. Soil A study designed to assess gaseous losses of N as N2O and N2 from soils of conventional till fields seeded to wheat in the Chernozemic