Fresh and degraded maize shoot and root residues temporarily change soil hydraulic properties

Abstract. Mulching and incorporation of crop residues (CR) into soils are common strategies to sustain soil carbon stocks, return nutrients, and regulate water losses through bare soil evaporation. To date, implementing the effect of mulching strategies into soil-plant-atmosphere models remains challenging due to limited information on their influence on soil hydraulic properties (SHP) as well as on the persistence of these effects over time. We hypothesized that increasing amounts of incorporated maize CR benefits water retention and reduces unsaturated hydraulic conductivity, and that the quality of the CR would determine the persistence of the effects, i.e., that mulching with relatively fast decaying shoot residues would be less persistent than incorporating root residues.

In a laboratory study, we quantified the effect of maize CR in various concentrations (0, 2, and 5 wt.%) on the SHP of a loam soil and additionally measured the SHP of a mulch layer (100 wt.% CR) from saturation to oven dryness. We differentiated between shoot and root CR to quantify the effect of biomass quality and adapted the simplified evaporation method to measure the hydraulic properties of 100 % CR layer. The experiments were run in triplicate and repeated after three weeks of incubation under optimal conditions for biological activity (30 °C, 90 % RH) to simulate organic matter degradation after harvest. Comparing the SHP before and after incubation provided information about the temporal dynamics of CR effects on SHP.

Compared to the control, water retention increased systematically by 2 to 5 vol.-% across the CR-soil mixtures from saturation to field capacity while the unsaturated hydraulic conductivity was slightly reduced. Incubation reduced carbon mass about 46–50 % in 100 % CR layers, 7–15 % in root-soil mixtures and 21–27 % in shoot–soil mixtures, simultaneously altering residue carbon and nitrogen concentrations. Despite this degradation, a positive effect on the soil water retention curve persisted, with water content increasing on average by 1.45 vol.% per gram of carbon per kilogram of soil. However, soil hydraulic properties measured after three weeks showed that much of the beneficial effect had diminished, most notably for shoot residues, which decomposed most rapidly.

Overall, the study demonstrates that the beneficial effects of CR incorporation on the SHP of a loam soil increase with CR amount and persist for at least one month after harvest. In agroecosystems, this post‑harvest period and the mulching process are crucial for defining the initial soil conditions for the subsequent crop. Furthermore, the reduced unsaturated hydraulic conductivity of the 100 % CR layer confirms field observations that mulch layers can effectively reduce water losses through bare soil evaporation.