A lab study to quantify the effect of fresh and degraded crop residues on soil hydraulic properties

Mulching and incorporation of crop residues (CR) into soils is a common strategy to sustain soil carbon stocks and to regulate water losses via bare soil evaporation. To date, implementing the effect of mulching strategies into soil-plant- atmosphere models remain challenging due to limited information about their effect on hydraulic properties (HP), namely the water retention and unsaturated hydraulic conductivity curve and the temporal dynamics of the process.

In this laboratory study, we measured the HP of a loamy soil mixed with maize CR to different contents (0, 2, and 5 weight-%) and a mulch layer (100 weight-% CR) from saturation to oven dryness. We differentiated between leaves and roots CR and adapted the simplified evaporation method to measure the hydraulic properties of 100 % CR layer. The experiments run as triplicates and were repeated after three weeks of incubation under optimum condition  (30 °C, 90 % RH) to simulate organic matter degradation after harvest. Comparing the HP before and after incubation provided information about the temporal effect of CR on soil HP.  

Compared to the control, water retention was systematically increasing about 2 to 5 vol.-%  for the CR-soil mixtures and up to 50 vol.-%  for the 100 % CR samples over a broad suction range from pF 0 to pF 3. The effect was most pronounced for leaves. The unsaturated hydraulic conductivity of all CR-soil mixtures was not affected. In contrast, the 100 % CR samples provided measurements of unsaturated hydraulic conductivity around pF 1 which were by an order of magnitude lower compared to the CR-soil mixtures. Incubation of the samples significantly reduced the carbon content of the samples and changed the structure of the CR but surprisingly, a positive effect on the soil water retention curve was still measurable.

The study shows that the beneficial effect of CR incorporation on the soil HP of a loamy soil increases with the amount of CR and that the effect lasts for a period of at least one month after harvest. This period is crucial to define the starting condition of the following crop. In addition, the lower unsaturated hydraulic conductivity of a 100 % CR layer confirmed field observations where a mulch layer reduces water losses through bare soil evaporation.