The effect of C:N ratios on the fate of carbon from straw and green manure in soil

Abstract

SummaryStraw and green manures are among the most important soil amendments used worldwide to provide agricultural soil with organic matter (OM). The reported improvement in soil organic matter content varies greatly; therefore, there is a need for mechanistic approaches to evaluate substrate turnover in the soil. This study compares the results of a field and an incubation experiment to follow the fate of carbon from organic fertilizers in soil as a function of the C:N ratio of the amendments. The amendments were straw (small‐N, Hordeum vulgare L.; large‐N, Pisum sativum L.) or green manure (Sinapis alba (M)) with additional organic or mineral fertilizers to alter the C:N ratio of the soil–substrate mixture. In the 100‐day field study, C respiration was measured at defined intervals at three locations with different soil types (Luvisol, Fluvisol and Arenosol). In the incubation experiment, C changes over 300 days were quantified based on the measurement of C respiration and C amounts in the soil–substrate mixture at the start and end of the experiment. Furthermore, microbial biomass was quantified at the end of the experiment to enable an assessment of microbial C‐use efficiency. In the field experiment, a significant correlation between N supply and C fate occurred at one site only (Luvisol) where retention of C from organic substrates was larger with increased N supply. In the incubation experiment, N supply and C fate were correlated significantly for both soils under study (Arenosol and Fluvisol). Furthermore, C‐use efficiency was positively correlated with C retention from organic inputs to the Fluvisol in the incubation experiment, whereas the opposite situation occurred with the Arenosol. This indicated that C retention from organic inputs was not dependent on carbon‐use efficiency by the microbial community under these conditions. We conclude that stoichiometric implications apply to carbon retention in soil that depend on N availability for soil microbes. In the early stages of organic matter turnover, the outcome of N availability might be concealed by the effect of chemical recalcitrance of the substrates. The length of this phase will depend on environmental conditions for microbial activity. The effect of the C:N ratio on the fate of C from organic amendments must be considered in the assessment of C management strategies in arable farming.Highlights Organic inputs to soil with a large C:N ratio lose more C in turnover than amendments with a small C:N ratio. Retention of C from organic amendments in soil appears to depend on microbial C-use efficiency and availability of nitrogen. The soil type and related environmental conditions affect CUE and the magnitude of C retention. Negative correlation between C:N ratio of organic amendments and C fate in soil must be considered in carbon management strategies.