The decomposition of sugar beet residues: mineralization versus immobilization in contrasting soil types

Abstract

The leaves and crowns from ¹⁵N-labelled sugar beets were incubated in either a silty clay loam or sand soil for almost one year. Four additions of fresh, chopped residues mixed with soil were tested: ¹⁵N-labelled leaves alone, ¹⁵N-labelled leaves plus unlabelled crowns, unlabelled leaves plus ¹⁵N-labelled crowns, and ¹⁵N-labelled crowns alone; a control with no addition was also incubated. The C:N ratio of the leaves was 11 and that of the crowns 40. Incubations were carried out in pots kept at 20 °C and optimal moisture conditions. The leaves mineralized N from the start of the experiment but the addition of crowns to soil at first caused immobilization of nitrogen followed eventually by mineralization after 6 or 12 weeks depending on soil type. The extra amounts of mineral N found in soil at the end of the experiment where additions were made corresponded to the sum of the background mineralization and the addition; no priming effects were encountered. Very slight differences only were found between the initial rates of mineralization of C in all of the treatments. Although there was also little difference between the sand and silty clay loam soils in the direct mineralization of nitrogen from the sugar beet leaves, where N was first immobilized (i.e. from crowns or a mixture) re-release of N took place more quickly in the sand soil. The total recovery of¹⁵ N found in soils after 24 weeks incubation ranged from 70% to 90% with least being lost from the sugar-rich but N-deficient crowns. Where leaves plus crowns were incubated together both residues contributed to the microbial biomass N.

In practice, immobilization of this magnitude and duration (expressed as a temperature sum) could exceed the growth period of a spring sown crop. The actual immobilization found in any one field is likely to depend on the C:N ratio of the residues and could account for much of the variation in the residual benefit of sugar beet residues reported in the literature.