Combined effects of labile and recalcitrant carbon on short-term availability of nitrogen in intensified arable soil.

In sustainable agriculture, returns of organic materials to soil have a major impact on carbon (C) and nitrogen ( N) turnover, but their effect on the final availability of N to crops is difficult to quantify, and the nature of the C source is usually neglected in predictive models. We investigated the individual and combined effects of labile sugars and phenolic acids on soil microbial processes related to the N cycle. Gross rates of N transformations were measured by using ¹⁵N isotope dilution after the addition to the soil of glucose, phenols and of an extract of the common catch crop Lolium multiflorum. Whatever the nature of the organic C added, a decrease in the availability of the soil ammonium pool was observed. This result was a response to two contrasting mechanisms: (i) a fuelling of microbial growth after the addition of glucose and (ii) a decline in microbial growth and a tendency toward depression of gross N transformations after the addition of phenols. We also observed a suppression of nitrification potential with the addition of Lolium extract and with phenolics added in combination with glucose. These mechanisms may reduce nitrate loss, but may have an adverse short-term effect on N-availability. We conclude that all carbon additions help to retain inorganic N in the short term, but the timing of N release to the following crop depends on the final outcome of the divergent microbial dynamics that may come into play. [ABSTRACT FROM AUTHOR]