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Seasonal Variation of Potentially Mineralizable Nitrogen in Four Cropping Systems¹

ABSTRACT

The concept of potentially mineralizable N is discussed and data that support the identification of N₀ as an active fraction of soil organic N are presented. Aerobic medium-term incubations (13 weeks) were used to measure the potentially mineralizable nitrogen (N₀) in four cropping systems on four different occasions from early spring to late autumn. The cropping systems consisted of barley (Hordeum distichum L.) with no N fertilizers (B0), barley receiving 120 kg N ha^–1 yr^–1 (B120), meadow fescue (Festuca pratensis Hudson) ley receiving 120 + 80 kg N ha^–1 yr^–1 (GL), and lucerne (Medicago sativa L.) ley with no N additions (LL). The amounts of N mineralized during the 13-week incubations (37°C) ranged from 188 to 483 kg N ha^–1, and the cropping systems ranked in the order GL > LL > B120 > B0 and showed a steady decline in all systems from spring to harvest and a subsequent increase from harvest to autumn. The seasonal differences were as large as the differences between systems. Three related models were employed to describe the kinetics of N mineralization during the incubations: (i) first-order, (ii) two-component (sum of two first-order models), and (iii) a simplified special case of the two-component model. In all cases, the special case of the two-component model offered the best description of the curves of accumulated mineral N. Based on the mineralization course and an observed relationship between literature values of N₀ and the rate constant (k), it is hypothesized that the mineralization is mediated by two distinct populations: an opportunistic, mainly immobile, population (bacteria) and a generalistic, mobile one (fungi), and that the k value is influenced both by the amount of substrate and its quality. The fact that the amount of mineralizable N decreases during the growing season and increases in autumn as a result of organic matter input provides evidence for the existence of an active fraction of soil organic matter.

¹ Contribution from the project Ecology of Arable Land. The Role of Organisms in Nitrogen Cycling at the Swedish Univ. of Agricultural Sciences, Uppsala, Sweden.

² Graduate Research Associate and Professor, Dep. of Water in Environment and Society, Univ. of Linköping, S-581 83 Linköping, Sweden, respectively.

Received for publication March 11, 1986.

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