Trade-off between soil enzyme activities and hotspots area depends on long-term fertilization: In situ field zymography.

Mineral fertilizers and livestock manure have been found to impact soil enzyme activities and distributions, but their trade-off and subsequent effects on soil functioning related to nutrient cycling are rarely evaluated. Here, we investigated the long-term effects of manure and mineral fertilization on the spatial distribution of enzyme activities related to carbon, nitrogen, and phosphorus cycling under field-grown maize. We found that the legacy of mineral fertilizers increased the rhizosphere extension for β-glucosidase and N-acetylglucosaminidase by 16-170 %, and the hotspots area by 37-151 %, compared to manure. The legacy of manure, especially combined with mineral fertilizers, increased enzyme activities and formed non-rhizosphere hotspots. Furthermore, we found a trade-off between hotspots area and enzyme activities under the legacy effect of long-term fertilization. This suggested that plants and microorganisms regulate nutrient investments by altering spatial distribution of enzyme activities. The positive correlation between hotspots area and nutrient contents highlights the importance of non-rhizosphere hotspots induced by manure in maintaining soil fertility. Compared to mineral fertilization, the legacy effect of manure expanded the soil functions for nutrient cycling in both rhizosphere and non-rhizosphere by >1.7 times. In conclusion, the legacy of manure expands non-rhizosphere hotspots and enhances soil functioning, while mineral fertilization expands rhizosphere extension and intensifies hotspots area for nutrient exploitation.