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.