Abstract
Abstract. Tillage induces decomposition and mineralisation of soil organic matter (SOM) by the disruption of macroaggregates and may increase soil CO2 efflux by respiration, but these processes are not well understood at the molecular level. We sampled three treatments (mineral fertiliser: MF; biogas digestate: BD; unfertilised control: CL) of a Stagnic Luvisol a few hours before and directly after tillage as well as 4 days later from a harvested maize field in northern Germany and investigated these samples by means of pyrolysis-field ionisation mass spectrometry (Py-FIMS) and hot-water extraction. Before tillage, the Py-FIMS mass spectra revealed differences in relative ion intensities of MF and CL compared to BD most likely attributable to the cattle manure used for the biogas feedstock and to relative enrichments during anaerobic fermentation. After tillage, the CO2 effluxes were increased in all treatments, but this increase was less pronounced in BD. We explain this by restricted availability of readily biodegradable carbon compounds and possibly an inhibitory effect of sterols from digestates. Significant changes in SOM composition were observed following tillage. In particular, lignin decomposition and increased proportions of N-containing compounds were detected in BD. In MF, lipid proportions increased at the expense of ammonia, ammonium, carbohydrates and peptides, indicating enhanced microbial activity. SOM composition in CL was unaffected by tillage. Our analyses provide strong evidence for significant short-term SOM changes due to tillage in fertilised soils.
Highlights
The influence of tillage on soil organic matter (SOM) is generally well understood
The soil of all treatments had similar C and hot-water-extractable organic C (HWC) contents, but differences appeared between MF and BD, where the N and HWN contents were slightly higher in MF, resulting in narrower C / N and HWC / HWN ratios in MF (8.5 and 5.9, respectively) compared to BD (9.0 and 8.5, respectively) (Table 1)
Combining pyrolysis-field ionisation mass spectrometry (Py-FIMS), as a sensitive technique to detect differences and alterations of specific compound classes of SOM, with classical methods like hot-water extraction and measurements of soil CO2 efflux allowed us to gain a better understanding of short-term SOM turnover after tillage operations
Summary
The influence of tillage on soil organic matter (SOM) is generally well understood. Tillage stimulates decomposition of SOM resulting in increased CO2 efflux (Dao, 1998), mostly by aeration and by the disruption of macro-aggregates, leading to release of protected SOM (Grandy and Robertson, 2007). Research on short-term effects of tillage on SOM has focused largely on CO2 efflux: several studies have recorded the dynamics of CO2 efflux immediately after tillage (see Table 5 in Fiedler et al, 2015) and some basic models have been developed that describe correlations between CO2 efflux and the turnover of soil organic carbon (SOC) after tillage by first-order kinetics (La Scala et al, 2008). These correlations do not causally explain which organic components are mineralised. SOM–CO2 efflux relationships are influenced by the type of soil amendment (Fiedler et al, 2015)
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