Spatial distribution of enzyme activities in rhizosphere soil under long-term mulching of dryland with in-situ zymography

Enzymes produced by roots and soil microorganisms have been the biological driving factors of organic matter decomposition to nutrient cycling in soil. However, the enzyme investigation remains a technical question, due to the heterogeneity of spatial distribution around the roots in the rhizosphere. Soil in-situ zymogram can directly display the distribution enzyme activity by fluorescence mapping. This study aims to investigate the spatial distribution of soil enzyme activities in the rhizosphere of spring maize with different soil surface mulching in Weibei dryland. Taking the spring maize "Xianyu 335" as the research object, a field experiment was carried out without damaging the crop roots by soil in-situ zymogram in June 2020 at the Loess Plateau agroecological experimental station of the Chinese Academy of Sciences. A complete randomized block design was employed with three treatments of Straw Mulching (SM), Plastic Film Mulching (FM), and no-mulching (CK). The in-situ zymography of the root profile was obtained at the silking stage of spring maize. The reaction parameters were calculated with the Michaelis-Menten equation after the enzyme kinetic experiment and simulation. Besides, the correlations between the concentration of enzyme and the soil chemical/physical properties variables were determined for the driving factors of enzyme spatial distribution with the different mulching treatments. The results show that there was the maximum distribution area of β- Glucosidase Activity (BG) in the rhizosphere of SM treatment, which was 1.9 times that of plastic film mulching, and 8.1 times that of CK treatment. The longest distance (2.5 mm) along the roots of β- glucosidase distribution was found with the SM treatment, which was 0.7 mm further than that with the FM, and 1.5mm further than that with CK. Correspondingly, the kinetic fitting showed that there were the maximum enzyme kinetic parameters Vmax and km of SM treatment, which was the fastest turnover time of soil substrate. The total distribution area of Leucine Aminopeptidase Activity (LAP) with the FM was the largest, which was 1.8 times that of SM, and 6.4 times that of CK. The longest distribution distance of enzyme activity with FM treatment was 4.5 mm, 1.7 and 2.1 mm further than those with the SM, and CK treatment, respectively. There were the maximum enzyme kinetic parameters with the FM treatment, all the same as the turnover of the substrate. The CK treatment presented the smallest hot spot area and kinetic parameters and the longest substrate turnover time, compared with the rest. The SM treatment increased the distribution area and extent of BG activity, indicating the fastest turnover rate of soil substrate, using the input of C, water storage, and soil N contents improvement. The FM treatment increased the distribution area and extent of LAP activity, with the fastest turnover rate of soil substrate using the soil temperature and water improvement. The CK treatment presented the smallest distribution area and extent of BG and LAP activities, where there was no change in the C input and soil temperature. As such, an enzyme kinetic mechanism was achieved to regulate the spatial distribution of enzyme activity in the crop rhizosphere soil for more nutrient uptake, particularly for the high yield and high nutrients utilization efficiency of dryland maize.