Temporal changes in mineralization and immobilization of N during degradation of plant material: implications for the plant N supply and nitrogen losses

Abstract

Economical and ecological criteria for optimal N fertilization are inherently in conflict, largely due to lack of precise predictions of nitrogen availability as a function of agronomic practice and weather conditions. This problem is particularly pronounced in organic farming, where plant N supply depends on microbial mineralization of organic N such as ‘green manure’. In the present paper, N dynamics after incorporation of a mixture of a green manure material (undersown white clover) and barley straw was investigated in laboratory and field experiments. Nitrogen flows in the soil–plant system were further analyzed with simulation models (COUP: heat and water transport model, SOILN_NO: soil nitrogen model). Our laboratory experiments showed that the incorporation of a mixture of N-rich white clover material and N-poor barley straw created a transient accumulation of clover derived inorganic N during the first period. Thereafter, microbial N demand during straw C utilization resulted in net immobilization of most of the clover derived inorganic N. As a consequence, the synchronization between net N mineralization and plant N uptake would be better by spring than autumn incorporation. This was largely confirmed both by measurements and simulations. However, the expected benefit of spring incorporation (ploughing) was counteracted by a large loss (36%) of clover material due to freeze/thaw damage and subsequent surface runoff during the winter. Such winter loss is likely to be a rare phenomenon, however.The results supported our hypothesis that net inorganic N accumulation due to degradation of a clover–straw mixture is transient and that spring incorporation of undersown clover gives a better coincidence of N availability and N uptake by a subsequent crop than does autumn incorporation. Model scenarios showed that ploughing as late as possible in autumn appears to be a good alternative to spring ploughing with regard to N leaching, but early autumn ploughing should be avoided.The study shows that the timing of agronomic operations are important, not only for determining the fraction of inorganic N lost by leaching, but also for the fraction assimilated by microorganisms. In effect, the plants and the microorganisms compete for the same nitrogen, and the success of the plants depend on a proper synchronization.