Do interactions between application rate and native soil organic matter content determine the degradation of exogenous organic carbon?

Although the amendment of various forms of exogenous organic matter (EOM) is a common practice in cropland production, it is to date not clear if its mineralisation in soil depends on application rate. Previous research suggested that spatial concentration of EOM in soil positively impacts its degradability. Here, we seek to test these reports and furthermore to investigate if an interactive effect exists with native soil organic carbon (SOC) concentration. We hypothesised that at a low EOM dose, more EOM remains undecomposed in soil and that this effect becomes stronger at lower SOC level. Moreover, as a secondary goal, we explored if priming of native SOC mineralisation depends on EOM dose. Therefore, we set up seventy-day soil incubation experiments with six varying C4-maize residue (δ13C = −12.7‰) doses (0–6 g kg−1) in soil mesocosms of loamy sand subsoils (δ13C = −25.7‰) with three different native SOC levels (i.e. 0.1, 0.5, and 1.0%). Soil CO2 emissions and isotopic signature measurements of CO2 allowed to discern EOM and SOC mineralisation. We found that EOM-derived C mineralisation increased proportionally to EOM dose, refuting the hypothesised relative undecomposed EOM at low concentration. Volumes of larger pore neck size diameter classes (60–100 and >300 μm) almost doubled at high EOM dose, demonstrating formation of macropores. Yet, this apparently did not impact EOM mineralisation, probably because O2 supply was always sufficient to allow unlimited activity of EOM degraders, even at higher EOM doses, as indicated by generally higher measured redox potential. With EOM dose, fungal marker PLFA abundances increased in the 1% SOC soil and protozoan abundances increased in all three soils, but apparently these shifts did not result in an enhanced relative degradation of the EOM. Increasing EOM doses induced negative priming, e.g. EOM ≥1 g kg−1 reduced SOC mineralisation by >43% and >24% compared to the control in the 0.1 and 0.5% SOC soils, respectively; whereas no priming occurred in soil with 1% SOC. These results were largely explained by the amount of added C relative to microbial biomass carbon, and the theorised switch of slow decomposers (so called K-strategists and involved in recalcitrant compound decomposition) from SOM to preferentially decompose EOM at higher doses. We also postulate that at low SOC %, the obvious increased O2 consumption with higher EOM dose more readily results in local anaerobic conditions in finer pores, i.e. where SOC is located and mineralised. We conclude that on the short term, agricultural management for SOM thus does not need to consider EOM doses but only the total amount of EOM.